With the increasing importance of wireless microelectronic devices the need for on-board power supplies is evidently also increasing. Possible candidates for microenergy storage devices are planar all-solid-state Li-ion microbatteries, which are currently under development by several start-up companies. However, to increase the energy density of these microbatteries further and to ensure a high power delivery, three-dimensional (3D) designs are essential. Therefore, several concepts have been proposed for the design of 3D microbatteries and these are reviewed. In addition, an overview is given of the various electrode and electrolyte materials that are suitable for 3D all-solid-state microbatteries. Furthermore, methods are presented to produce films of these materials on a nano- and microscale.
Electronic devices play a continuously increasing role in our daily life and the number of wireless devices is nowadays rapidly growing. Many efforts are devoted to develop autonomous wireless devices, which can be used for smart building control, smart medicine and other ambient technologies. These devices need a power supply, often an energy scavenging device like a photovoltaic cell or a bio-fuel cell. To ensure a stable current supply or to be able to deliver high peak currents, on-board energy storage is essential. The most obvious devices that guarantee energy storage are batteries. Common rechargeable batteries are based on a liquid electrolyte, which implies that there are several restrictions for their design and size due to the available separators and liquid electrolytes. Secondly, these liquid electrolytes carry the inherent risk of leakage. Therefore the need for all-solid-state microbatteries arises, which can facilitate miniaturization, will create more flexibility for the design of stand-alone microelectronic devices and enhance the applicability in (medical) implants due to the avoided leakage risks. To enhance the power and energy density of these thin-film all-solid-state microbatteries significantly, new advanced concepts are proposed, which are all based on the exploration of the third dimension.
A few reviews have been published that highlight the research on solid-state microbatteries, which together give a nice overview of the research in this field from several viewpoints.1–6 In the present review an overview will be given of various aspects of thin-film solid-state batteries based on the 3D geometry. First a brief historical context will be outlined by illustrating the development of all-solid-state planar microbatteries. Secondly, the necessity of thin film solid-state batteries, the requirements for the design of these, and their advantages and drawbacks will be discussed. In the third part of this review various commonly used thin-film battery materials will be described and compared. In the subsequent section, several examples of three-dimensional battery concepts will be presented together with the methods to deposit thin-film battery materials. Although the present state-of-the-art of 3D microbatteries does not yet allow a full comparison of the various approaches as only very few working devices have been demonstrated, it is interesting to highlight the advances that were already made in a relatively short period of time and the remaining challenges that are still ahead. This may serve as source of inspiration for further research and development in this interesting new scientific field.
2. Historical Context of Li-Ion Solid-State Batteries
The development of solid-state lithium-ion batteries is very similar to that of their liquid-based counterparts, which has been described extensively in the available literature.7–9 Just as for liquid-based lithium-ion batteries, the first solid-state lithium-ion batteries were non-rechargeable. As early as 1972 the first results on primary solid-state lithium-ion batteries were published, based on metallic lithium anodes, (doped) lithium iodide solid-state electrolytes and a metal-iodide based cathode.10 These solid-state electrolytes were doped with small amounts of calcium iodide or ammonium iodide and had an ionic conductivity of the order of 2·10−6 S·cm−1 at room temperature.10 Since then, many different solid-state electrolytes were examined, for example lithium phosphates,11 lithium metal phosphates12 and polymer electrolytes.13 A more elaborate description of several lithium ion conductors will be given in section 3.3. Rechargeable all-solid-state lithium-ion batteries were intensively reported in the 1990s, when the group of Bates patented14 and published15–20 a planar thin film battery concept based on glassy solid-state electrolytes, for example, lithium phosphate, lithium phosphorus oxynitride and lithium phosphorus silicon oxide. This approach formed the basis for many all-solid-state planar thin-film microbatteries and also frequently served as a source of inspiration for the development of 3D integrated batteries.
3. All-Solid-State Microbatteries
Solid-state batteries do not principally differ in working mechanism from all other lithium-ion batteries: these also consist of two current collectors, two electrodes and an electrolyte in between (Figure1). The main difference between all-solid-state and liquid electrolyte-based batteries is that the size of the former battery system can be made smaller: where conventional liquid electrolyte-based batteries cannot be easily made smaller than typical coin cell dimensions, all-solid-state thin-film microbatteries can be manufactured with a footprint area of less than a few square millimeters. Also for the thickness much smaller scales can be achieved: solid-state electrolytes have a typical thickness of approximately 1 μm, whereas the separator in liquid electrolyte-based cells typically has a thickness of 20 μm.
All-solid-state microbatteries are usually deposited on a solid substrate making use of a large freedom of shape and material. The substrate can consist of virtually any material, varying from glasses, ceramics, metals to even polymers and paper. The main requirement for this material is that it should be able to withstand the conditions at which the battery layers are deposited and operated, and that the material does not show any interaction with the functional layers of the battery that are deposited on top of it. This last requirement means that the substrate material should be blocking for all chemical elements that might be harmful for the battery materials and interfaces, and should not allow lithium to diffuse out of the battery stack. If this substrate is conductive for lithium or other elements, an additional barrier layer is required. This barrier layer can, if sufficiently electronically conducting, also serve as current collector. On top of the first current collector, an active electrode (cathode or anode) is deposited. The requirements for a well-functioning electrode are a high volumetric charge density at a well-defined potential, and good electronic and ionic conductivities to ensure high power capabilities.
The next layer is the solid-state electrolyte, which should combine a high ionic conductivity with a negligibly low electronic conductivity. The number of electrons leaking through this layer per unit of time, or by-passing via pinholes, determines the battery self-discharge rate. Therefore it is important that the electrolyte layer is completely closed and covers the whole interface between the two electrodes. On top of this solid electrolyte a second electrode is positioned, which has similar requirements as the first electrode, but should have a sufficiently different voltage window. Indeed, the difference between the equilibrium potential of the two electrode materials should be large enough to yield a sufficiently high battery voltage. The top contact of the second electrode of the battery is formed by the second current collector. Another important requirement for the battery design is the packaging structure, which should provide protection against external chemical and physical influences and thus prevent parasitic reactions of the battery materials with air and moisture.
All-solid-state batteries have the obvious advantage that there are no liquids involved, which increases their intrinsic safety. The absence of organic solvents results in the absence of possible fluid leakage and also reduces the risk of fire and explosions. Since the solvents of the liquid electrolytes are often involved in the degradation mechanisms of traditional lithium-ion batteries,21 the cycle-life performance of solid-state batteries is reported to be much larger. The disadvantage of solid-state electrolytes is that the ionic conductivity is usually significantly lower than that in the liquid-based counterparts, resulting in a relatively higher voltage drop. This can, however, be circumvented by application of much thinner electrolyte layers, to keep the total voltage drop over the electrolyte within an acceptable range. This voltage drop can also be reduced by using lower current densities, which can be achieved by making use of the enlarged surface area of 3D microbatteries.
The use of solid-state materials also provides a large degree of freedom in the design of the batteries and can even be integrated in various types of devices. The application of thin film electrodes reduces the need of electronically conductive additives as the paths to the current collectors are relatively short. This ensures that the entire electrode has essentially the same potential and therefore adopts the same State-of-Charge (SoC), assuming that no ionic transport limitations occur inside the electrodes. This significantly reduces the risk of local over-(dis)charging.
3.1. Positive Electrode Materials
In this section several cathode materials will be described that can be applied in all-solid-state thin-film batteries. Attention will be paid to the preparation methods, the gravimetric and volumetric storage capacity of these electrode materials. The various electrochemical storage capacities are reviewed in Figure2.
Lithium cobalt oxide is one of the most frequently used cathode material in Li-ion batteries. Therefore it has been thoroughly studied in powder form and its material properties are well documented.22 In general, LiCoO2 exists in two crystal forms: a spinel low temperature form, generally abbreviated as LT-LiCoO2,23 and a high temperature layered hexagonal structure, HT-LiCoO2.24 This denomination is based on the formation of LiCoO2 using solid-state reactions, where HT-LiCoO2 is generally formed at significantly higher temperatures than the LT-LiCoO2 compound. However, the formation of HT-LiCoO2 has also been reported by the application of relatively low temperature wet-chemical processes.25 The same two polymorphs of LiCoO2 are also observed when depositing solid-state thin-films which can, amongst other methods, be achieved by RF sputtering,26–29 sol-gel deposition,27, 30–32 chemical vapor deposition33, 34 and pulsed laser deposition.29
HT-LiCoO2 is well-known for its layered hexagonal structure. Due to this layered structure, strong orientation effects play a large role in the performance of LiCoO2 films. Because only two-dimensional diffusion paths are available in the layered structure, lithium transport through the active electrode can be seriously hindered if a LiCoO2 film is deposited in a strongly preferred orientation.28, 29 Differences in orientation result in a variation in lithium ionic conductivity of a few orders of magnitude.35
When a polycrystalline HT-LiCoO2 film is randomly oriented, thereby providing lithium diffusion paths in all directions, or when it has a favorable orientation with respect to the electrolyte, it shows good rate capabilities. This makes it possible to reveal a very flat lithium intercalation/extraction plateau around 3.9 V vs. a Li/Li+ reference electrode, the standard reference electrode that will be used throughout this publication. This plateau corresponds to a lithium content of 0.75 < x < 0.93 in LixCoO2 and results from the two-phase coexistence of two different hexagonal phases. At higher potentials, between 4.1 and 4.2 V, a third monoclinic structure of LixCoO2 appears.36 It is generally accepted that LixCoO2 can be reversibly cycled between 0.5 < x < 1, bringing the gravimetric charge capacity of a LiCoO2 electrode at a theoretical maximum of 137 mAh·g−1. This is equivalent to a volumetric storage capacity of approximately 700 mAh·cm−3 for a 97 Å3 Li3Co3O6 unit cell.36, 37
LiNiO2 has a similar layered structure as LiCoO2, it also can exist in a cubic and a layered hexagonal structure of which only the layered hexagonal structure shows electrochemical activity.38, 39 LiNiO2 can also be prepared by similar methods as LiCoO2, although LiNiO2 is less elaborately reported in the available literature. That is especially the case for thin-films of LiNiO2. Lithium nickel oxides with different stoichiometries and crystal structures are often applied for their electrochromic behavior. Deposition of these films on transparent substrates is demonstrated using pulsed laser deposition,40 sputter deposition41 and sol-gel deposition.42 Deposition of the layered LiNiO2 that is suitable for thin-film batteries is reported by sputter deposition43 and electrostatic spray deposition44 but, similar to the electrochromic layers, also other deposition methods have been applied.
Layered hexagonal LixNiO2 undergoes several phase transitions upon (de-)lithiation, and presents therefore a less stable charge/discharge voltage than LiCoO2. The (dis)charge plateaus of LixNiO2 range from 2.7 to 4.1 V.45, 46 The first cycle of LiNiO2 always deviates from subsequent cycles. In the following cycles lithium can reversibly be cycled between 0.35 < x < 0.85,47 revealing a gravimetric capacity of around 140 mAh·g−1. When taking a Li2Ni2O4 unit cell of 68 Å3,48 the calculated maximum volumetric charge is slightly over 650 mAh·cm−3.
3.1.3. Spinel Structures
Spinel structures are cubic structures, which facilitate 3D diffusion. Therefore, the spinel structures do not suffer from any orientation effects that are encountered in layered structures like LiCoO2. A common spinel material for Li-ion application is LiMn2O4, which has recently drawn much attention as an alternative for commercial LiCoO2 cathodes.49 Also in thin film form, LiMn2O4 was successfully prepared using various techniques, including sol-gel deposition,50, 51 pulsed laser deposition,52 sputter deposition53, 54 and chemical vapor deposition.55
LixMn2O4 has its main electrochemical response at around 4 V vs. Li/Li+ where x varies between 0 < x < 1. When lithium manganese oxides are cycled in this region, the crystal lattice remains cubic and only a minor volumetric change occurs.56–58 Due to this small volume change, a long cycle-life can be obtained when cycling between 3.5 and 4.5 V. When going to lower potentials, a second (de)lithiation region exists, with 1 < x < 2 at 3 V. In this region a Jahn-Teller distortion of the crystal lattice occurs and the cubic framework transforms into a tetragonal symmetry.56, 58, 59 This deformation is so severe that structural degradation of the electrode structure will significantly reduce the cycle-life. Therefore, only the 4 V region can be applied for practical battery applications, which yields a reversible gravimetric capacity of 148 mAh·g−1. Using a spinel unit cell of Li8Mn16O32 with a volume of 550 Å3, a theoretical volumetric capacity of 650 mAh·cm−3 can be calculated.60
3.1.4. Lithium Metal Phosphates
Recently lithium metal phosphates have received a lot of attention as potential electrode material candidates. Materials of this class are nowadays often applied in commercial rechargeable batteries, due to their high gravimetric capacity and relatively low cost. The most frequently applied lithium metal phosphate is LiFePO4, but also LiMnPO4 and LiCoPO4 are reported as suitable electrode materials in the literature. In general, lithium metal phosphates form an olivine (orthorhombic) crystal structure.
LiFePO4 is applied in powder form in commercial batteries. It has also shown to be effective as thin film electrode, which can be deposited by pulsed laser deposition61, 62 and sputter deposition.63–65 Sol-gel techniques have been applied to form LiFePO4 powders,66–69 but this method can possibly be extended to thin film deposition.
LixFePO4 has a stable reversible (dis)charge voltage at 3.4 V, which is relatively low for cathode materials and it is usually cycled between 0.1 < x < 1. The theoretical value of the gravimetric storage capacity of LiFePO4 can be up to 152.9 mAh·g−1 due to the low molecular weight. The volumetric capacity is less exceptional: when an olivine unit cell is taken, consisting of Li4Fe4P4O16 with a unit-cell volume of 291 Å3,70 cycling all lithium gives a maximum obtainable volumetric capacity of 610 mAh·cm−3.
Other Lithium metal phosphates include LiCoPO4 and LiMnPO4, which adopt the same olivine crystal structure and can be prepared using similar methods. The deposition of LiCoPO4 films has been reported by RF sputtering71 and electrostatic spray deposition.72 Films of LiMnPO4 have been prepared by electrostatic spray deposition.73 Also sol-gel techniques were demonstrated, which yielded electrochemically active LiMnPO4 powders. The main benefit of LiCoPO4 over LiMnPO4 is its high (dis)charge voltage at 4.9 V. This high potential is combined with a relatively high maximum charge of 167 mAh·g−1 for cycling LixCoPO4 between 0 < x < 1, which yields a volumetric capacity of 620 mAh·cm−3 for a 289 Å3 Li4Co4P4O16 unit cell. When cycling LixMnPO4 between 0 < x < 1 around 4.1 V,74 a maximum capacity of 171 mAh·g−1 can be obtained. With a unit cell of 302.4 Å3,75 this leads to a maximum theoretic capacity of 590 mAh·cm−3.
3.1.5. Vanadium Oxides
Vanadium oxides exist in many oxidation states and received much attention during the last decades. Originally V2O5 was mostly investigated as electrode material and more than 30 years ago its electrochemical behavior with respect to lithium was already well documented.76 Since then, various vanadium oxides were synthesized, and investigated for lithium storage. The most popular include besides V2O5 also VO2, V3O8, V4O10 and V6O13.
VO2 and V2O5 have a layered structure, consisting of square-based VO5 pyramids. For VO2 these pyramids are sharing the edges of their bases with up-side-down pyramids, while V2O5 has some vacancies, resulting in partially edge- and corner-sharing pyramids.70 Except for these layered geometries, many other crystal structures have been described, which can, for example, be found in the extensive review by Zavalij and Whittingham.77
Various methods have been described for the preparation of vanadium oxides and many thin film deposition techniques have proven to be successful for the deposition of vanadium oxide thin films. V2O5 films were, for example, deposited by sol-gel methods,78, 79 RF sputter deposition,80 pulsed laser deposition,81, 82 chemical vapor deposition83 and atomic layer deposition.84 Similar methods could also be used for the deposition of VO2 (sol-gel coating,85 RF sputter deposition,86 pulsed laser deposition82 and chemical vapor deposition87).
When the most common vanadium oxide, i.e. V2O5, is cycled electrochemically, three main regions can be observed. At 3.2 and 3.4 V two sharp peaks are observed. The underlying electrochemical reactions can reversibly be applied for Li storage. In this region the lithium content of LixV2O5 varies between 0 < x < 0.8.88 When going to lower potentials, a third response is observed at 2.3 V. At this potential the lithium content is increased up to x = 2, but the phase transformation that takes place is irreversible.89 Therefore, LixV2O5 is generally not intercalated further than x = 0.8, which results in a theoretical maximum gravimetric capacity of 118 mAh·g−1 and a volumetric capacity of 400 mAh·cm−3, considering a volume of 90 Å3 per V2O5 unit.90
3.2. Solid-State Electrolytes
3.2.1. LiNbO3 and LiTaO3
LiNbO3 and LiTaO3 are widely applied for their optical and electrical properties. Therefore many methods have been described to form films of these materials, which include for LiNbO3 sol-gel deposition methods,91 pulsed laser deposition,92 chemical vapor deposition93 and sputter deposition.94 Also for LiTaO3 pulsed laser deposition,95 sol-gel deposition,96 chemical vapor deposition97 and sputter deposition98 were reported.
For the use of these materials as solid-state electrolytes several properties are important. Most important is the ionic conductivity. Reported values for the ionic conductivity of LiNbO3 vary between 2.2·10−9 99 and 8.4·10−7 S·cm−1 100 for crystalline films at room temperature, while for amorphous materials values up to 10−5 S·cm−1 were measured.101 Mixing LiNbO3 with, for instance, silicates can increase the conductivity for crystalline materials several orders of magnitude. 102 For LiTaO3 films values around 8·10−8 S·cm−1 are reported,103 but also in this case amorphous bulk materials have been prepared with a much higher ionic conductivity.101 A second important parameter is the electronic conductivity of these layers, this should be as low as possible, because this partially determines the self-discharge rate of the battery in which the electrolyte is used. The DC electronic conductivity for LiNbO3 and LiTaO3 is estimated to be lower than 10−11 S·cm−1, which is several orders of magnitude lower than the ionic conductivity.101, 103 These solid electrolytes have already been applied successfully in e.g. electrochromic “smart windows”,104 where these electrolytes are used in combination with various electrode materials which change color upon (de)lithiation.
3.2.2. Lithium Lanthanum Titanium Oxides (LLTO)
Materials belonging to the LLTO class have recently received increased attention, due to their high ionic conductivities, which can be as high as 10−3 S·cm−1.105 Due to this interesting property, some elaborate reviews have been published, evaluating the available literature on this class of materials.106, 107 LLTO with stoichiometry Li3xLa(2/3)-xTiO3 is amorphous or can adopt the perovskite crystalline structure.
Various deposition techniques have been demonstrated for LLTO films, although most of these films show a significantly lower ionic conductivity than bulk materials. Reported deposition techniques include e-beam evaporation108 and sol-gel deposition.109 Films with a higher ionic conductivity of up to 10−5 S·cm−1 were deposited with pulsed laser deposition.110, 111
LLTO has the benefit that it combines a high ionic conductivity with a good (electro)chemical stability. Half-cells based on LiCoO2 films covered with LLTO films deposited by pulsed laser deposition could be cycled for hundreds of cycles.111 The disadvantage of bulk LLTO is that it requires high annealing temperatures (often > 1000 °C107) to obtain this high ionic conductivity. Thin films might not withstand these annealing steps due to the formation of interfacial layers, internal stress and even the formation of cracks. Furthermore, Li2O is formed upon annealing and lithium is extracted from the LLTO phase, resulting in less control over the stoichiometry of the LLTO material and, consequently, the ionic conductivity.112 The electronic conductivity of LLTO is slightly higher (10−8 – 10−9 S·cm−1)105 than that of, for example, LiNbO3 or LiTaO3. Therefore, a thicker electrolyte layer is required in a solid-state battery to obtain a similar self-discharge rate. The benefits of the high ionic conductivity are then, however, (partially) lost.
3.2.3. Lithium Phosphate Based Electrolytes
Glassy amorphous lithium phosphates are a popular class of electrolytes due to their high ionic conductivity and relatively modest deposition conditions. Pure Li3PO4 has a relatively low ionic conductivity of 3·10−7 S·cm−1 in bulk form and 7·10−8 S·cm−1 has been reported for thin films.15 Therefore, many experiments were performed to evaluate the properties of mixed phosphates and to increase the ionic conductivity. Examples of these mixed phosphates are the Li2O-P2O5-SiO2113–115 and Li2O-P2O5-TiO2 system.116 Also a combination of these, sometimes with various metal oxides added, is frequently reported.117
In the early 1990’s Bates et al. reported that Li3PO4 thin films deposited by sputter deposition in the presence of nitrogen gas resulted in a nitrogen-doped lithium phosphate (LIPON), in which doubly and triply coordinated nitrogen atoms form cross-links between the phosphate chains. This material showed a high ionic conductivity of up to 3·10−6 S·cm−1.15, 118 Moreover, where most lithium phosphates were not stable in combination with a lithium metal plating anode, LIPON formed a (electro)chemically stable system. LIPON is therefore nowadays one of the most frequently employed electrolytes for all-solid-state planar micro-batteries.
LIPON films are usually prepared using reactive RF sputter deposition. In a nitrogen plasma, sputtering from a Li3PO4 target yields a LIPON film, of which the composition might vary with both the sputter power119 and the partial nitrogen pressure in the system.15 A second method for the formation of LIPON is pulsed laser deposition, in which also both the nitrogen partial pressure and the laser power influence the ionic conductivity.120 A third method that was reported to yield ionically conductive LIPON films is ion-beam assisted deposition (IBAD).121, 122
LIPON films have very high ionic conductivities compared to other inorganic solid-state electrolytes: up to 3·10−6 S·cm−1 (sputtered films),15 1.6·10−6 S·cm−1 (pulsed laser deposition)120 or 1.3·10−6 S·cm−1 (IBAD).121 LIPON also has a low electronic conductivity of 8·10−13 S·cm−1 (IBAD).121 In addition, it is stable within a wide voltage range and can be used in combination with many different electrode materials, varying from pure lithium to LiCoPO4.71
3.2.4. Polymer Electrolytes
Polymer electrolytes for solid-state batteries exist in different types. The first type is a polymer membrane impregnated with a lithium salt solution. This class of Hybrid Polymer Electrolytes (HPE), or gel-electrolytes, is already produced on a large scale and rechargeable macro-batteries based on this electrolyte type can already be found in state-of-the-art consumer electronics. The advantage of this class of electrolytes is that a high ionic conductivity can be obtained, comparable to electrolytic salt solutions. The disadvantage of these electrolytes is that many problems imposed by liquid electrolytes (solvents) are still present: the potential hazards due to leakage of the highly flammable organic solvents and the formation of a Solid Electrolyte Interphase (SEI).123
Therefore a second class of polymer electrolytes was developed, the Solid Polymer Electrolytes (SPE) that do not rely on liquid electrolyte solutions. A classic example of such a SPE is a complex of a lithium perchlorate salt with poly(ethylene oxide).124 Also, various more complex electrolytes were developed to achieve a higher ionic conductivity. Several extensive reviews give a more detailed description of the different types of polymer electrolytes and their development.125–127
Polymer electrolyte films can be produced using various methods. It was reported that electrolyte films could, for example, be prepared by casting from a polymer solution,128 evaporation methods,129 or in situ plasma polymerization techniques.130
The first dry solid-state electrolytes had an ionic conductivity of approximately 10−7 S·cm−1.127 It was discovered that ionic conductivity is mostly taking place in the amorphous zones of the polymer, so deposition methods that prevent crystallization improve the ionic conductivity. These methods may involve the addition of large side-groups to the polymer chains, cross-linking and the addition of plasticizers to keep the polymer chains mobile.125, 126 Using these improvements, the ionic conductivity could be increased by several orders of magnitude up to 10−3–10−4 S·cm−1.127
3.3. Negative Electrode Materials
3.3.1. Titanium Oxides
Titanium oxide has been investigated as interesting anode candidate for lithium-ion batteries. Titanium(IV)oxide (TiO2) is commonly encountered in three crystal structures: brookite, rutile and anatase, of which anatase shows the best electrochemical response.131 Anatase is a material commonly used for (photo-)electrochemical applications and various methods for the deposition of thin-films have been elaborately described. It can be formed using sol-gel deposition,132 sputter deposition,133, 134 pulsed laser deposition,135 chemical vapor deposition136 and atomic layer deposition.137
During electrochemical lithiation, around 1.8 V, the anatase LixTiO2 structure can be intercalated up to x = 0.5, which results in a theoretical gravimetric charge density of 168 mAh·g−1, which is equivalent to a volumetric capacity of 660 mAh·cm−3, considering an anatase unit cell (Ti4O8) of 135 Å3.138
Another lithium electrode based on titanium is Li4Ti5O12. This oxide has a spinel structure (Li[Li1/3Ti5/3]O4), which has the advantage that it undergoes only very minor volumetric changes (<0.2%) upon cycling.139 Therefore, no structural degradation is observed. Li4Ti5O12 can be deposited using several thin film deposition techniques. Successful deposition is reported using sol-gel techniques,140 magnetron sputtering141 and pulsed laser deposition.142 Li-ion intercalation in Li4Ti5O12 occurs via a two-phase coexistence process, resulting in a very stable (dis)charge voltage between 1.5 and 1.6 V. These high voltages imply that these titanium oxides can be used either as high-voltage anodes or low-voltage cathodes, depending on which electrode material these are combined within a battery. During the (dis)charging process the lithium content can be varied for LixTi5O12 between 4 < x < 7, resulting in a maximum theoretical gravimetric capacity of 175 mAh·g−1. The lattice parameter for a spinel unit cell (Li[Li1/3Ti5/3]O4)8 is 8.37 Å,139 which means that the maximum obtainable volumetric capacity is 610 mAh·cm−3.
3.3.2. IVb-Group Materials
Elements of the IVb group in the periodic table of the elements have always received a lot of attention to serve as anode materials in lithium-ion batteries. Carbon, of course, is a common anode material in its graphitic form, but it has been investigated in other forms as well, for instance, nano-tubes.143, 144 Moreover, the other elements in this column of the periodic table of elements show also very promising electrode properties: silicon can, for example, accommodate about 4 lithium atoms per atomic silicon by thermal alloying.145 And although the electrochemical behavior deviates from the thermally made alloys, electrochemical experiments show a high lithium uptake of 3.75 Li per Si.146, 147 Similarly, germanium, tin and lead are known to alloy large amounts of lithium as well (more than 4 lithium atoms per lattice atom).145 Unfortunately, a price has to be paid for these enormous storage capacities: during loading of these materials with lithium, a large volume expansion of about 300% is regularly observed, which is most detrimental for the structural integrity of the electrodes.148, 149 This will limit the cycle-life of these electrodes when fully charged/discharged. It is expected that this deterioration is much smaller when thin-film electrodes are applied: when these materials are deposited on a solid substrate that can accommodate the stress.149 Films of these materials can be deposited with several deposition techniques. Silicon films can, for example, be deposited using low pressure chemical vapor deposition150 and evaporation.151, 152
The group IVb elements have a very high lithium uptake: the lithium content can theoretically be increased up to approximately 4008 mAh·g−1 for Li21Si5, 1565 mAh·g−1 for Li21+3/16Ge5, 963 mAh·g−1 for Li21+5/16Sn5 and 556 mAh·g−1 for Li21+1/4Pb5. 145 These values are based on thermodynamically stable alloys. In electrochemical experiments, however, some differences may occur. The lithium content that can be reached electrochemically in Si and Ge is 3.75 Li per host atom, corresponding to 3579 mAh·g−1 and 1385 mAh·g−1, respectively. With the crystal structure refinements for thermally obtained alloys145 the volumetric charge capacity can be calculated, yielding (for the expanded lithiated structures) about 2300, 2000 and 1900 mAh·cm−3 for germanium, tin and lead, respectively. For Li15Si4 a volumetric charge capacity is estimated of almost 2200 mAh·cm−3 based on the lattice constant of approximately 10.7 Å that has been determined for the electrochemically lithiated structure.146, 147
3.3.3. Conversion Anodes
Lithium is stored by either intercalation reactions or alloying in the anode materials described above. A third different type of reaction mechanism takes place in conversion anodes. In these electrodes a metal oxide or a metal nitride is reduced to result in the formation of Li2O or Li3N following in case of a metal oxide the generalized reaction scheme153, 154
This type of mechanisms is frequently reported for transition metal oxides, for example, oxides of Co,153 Fe,153, 155 Ni,153 and Cu,153 but also for nitrides, such as Cu3N.156 A nice overview of various conversion electrode materials was given by Li et al.157
In some cases the formed metal is even reported to be active for further lithium storage as an alloying electrode, according to
SnO is an example of this class of conversion-alloying electrode materials, where the formed tin metal will further alloy with lithium until the composition of approximately Li21.3Sn5 is reversibly reached. The reversibility of reaction 2 (reaction 1 is in this case irreversible) is actually increased when compared to a metallic tin electrode. This effect was attributed to the formation of the Li2O framework that stabilizes the shrinking and expanding metal particles.158 Also several nitrides were reported to undergo a similar alloying storage reaction, such as SnNx,159 Zn3N2160 and Ge3N4.161
4. Three-Dimensional All-Solid-State Batteries
Planar solid-state thin-film batteries have a relatively low volumetric capacity, because relatively much volume is occupied by inactive materials as substrate and packaging. To increase the volumetric energy density of these batteries, 3D geometries can be applied, for which with almost the same amount of packaging and substrate material much higher energy storage capacities can be obtained. An additional advantage of these 3D batteries is that the internal surface area between cathode, electrolyte and anode is enlarged, which means that with similar internal current densities, a much higher total battery current can be obtained. This ensures a relatively high current- and power capability for 3D all-solid-state batteries.
Several concepts have been proposed for a 3D microbattery layout. However, most of these are only conceptual and most published results have only been focusing up till now on partial solid-state devices. These concepts, the accompanying deposition methods and reported intermediate results will be highlighted in this section.
4.1. Three-Dimensional Concepts
4.1.1. Three-Dimensional Substrates Based on Templated Deposition
To increase the active surface area of a solid-state battery, a three-dimensional structure can be formed onto a conventional planar substrate. A method to form such structure is the use of a template combined with classical solid-state deposition techniques. After removing the template, a 3D morphology is obtained which may serve either as support, current collector or active electrode in a 3D battery (Figure3).
For instance, when a membrane is pressed onto a conductive planar substrate, electrochemical deposition can be performed through the pores of the membrane. This process will result in selective deposition onto the surface of the conductive substrate inside the pores. In case of a membrane with straight pores, the deposited material will, in the ideal case, form a columnar structure. When subsequently the membrane is selectively dissolved, an array of free-standing nano- or microrods can be obtained.162, 163 Aluminum and copper are common materials for this method, and since these nano-rods will be low-ohmic, they can be used as current collectors for 3D batteries with a high surface area enlargement.162–165 The approach proceeds with the deposition of the battery stack, for instance, either by sol-gel, electrodeposition or atomic layer deposition.
Advantages of this technique are that large area enhancements can be achieved in a relatively simple way. Perre et al.162 calculated the surface area enhancement (A) for such structure to be
in which d and h are the diameter and height of the deposited columns, respectively, s is the spacing between the rods, measured between the centers of the rods, and Θ is the angle of the pattern in which the rods are positioned, to compensate if the pattern is not square. Using typical values of h = 10 μm, d = 200 nm, s = 500 nm and Θ = 60° it can be calculated that a surface area enhancement factor of 30 is obtained. Another advantage is that the size of the created micro or nano-rods can be tuned by the choice of the applied membrane. The amount of material to be deposited can be accurately controlled by monitoring the experimental electrochemical conditions. A disadvantage of this method is that, since most membranes have a very open structure, the resulting structure of rods will be very dense, which will significantly limit the possibilities for the deposition of the subsequent battery layers. In addition, the rods are rather fragile and these are highly sensitive to mechanical damage upon charging and discharging.
4.1.2. Arrays of Interdigitated Carbon microrods
microrods can also be constructed by means of photolithography and etching, using a top-down approach. This method was developed at the University of California and is schematically shown in Figure4. In this case a layer of PhotoResist (PR) is spin-coated onto a planar substrate and developed after a UV-mask illumination step. The developed photoresist structure is subsequently pyrolized to form conductive carbon rods. Using two illumination/development steps, a 3D structure can be obtained consisting of arrays of carbon microrods on top of contact fingers with an interdigitated layout (Figure 4).166 These interdigitated contact fingers are connected to two separate contact pads, which enables the use of half of these microrods as cathode current-collectors and the remaining rods as battery anode. Therefore an active cathode material can be prepared independently by electro-deposition on half of the rods, simply by contacting the current collector onto which deposition is required.167, 168
This method presents some strong advantages. Since the cathode as well as the anode sides are each connected to a current collector, both materials can be electrodeposited. Alternatively, the carbon rods themselves can serve as anode. Moreover, this technique offers a large degree of freedom for the size of the rods and their spacing in comparison with membrane template growth. However, there are still two major challenges: the solid-state electrolyte needs to fill the entire structure and make good contact with the entire surface. Most likely, this electrolyte needs to be deposited via another technique than electro-deposition. Secondly, even though the rows of rods are alternating cathode and anode rows, the charge transport in the rods and through the electrolyte will be inhomogeneous and local over(dis)charging will be a significant risk when high currents are applied. Theoretical studies show that this risk could be reduced (although not completely avoided) by the choice of different pillar geometries and optimized arrangement of positive and negative electrodes.169
Another drawback is that the surface area enhancement with the application of interdigitated arrays of microrods is limited when compared to the rods created in section 4.1.1, since only half of the rods are used for one electrode, and a part of the planar surface area cannot be used. Finally, if this system is applied as a completely filled solid-state structure, volume change of the electrodes upon (de)lithiation might give rise to mechanical problems.
4.1.3. Three-Dimensional Architectures Based on Aerogels
The three-dimensional structures described above have a regular structure. Another approach is to make use of an aerogel as a basis for a solid-state battery. An aerogel is a solid-state material with the structure of a gel: a randomly oriented solid-state network that consists for a large part of open volume, often as much as 75–99%.3 Aerogels are usually produced by sol-gel techniques including non-equilibrium evaporation of the solvent. This non-equilibrium condition is a prerequisite, since equilibrium evaporation would result in capillary forces by which the porous structure collapses. Supercritical conditions are therefore often applied for the production of aerogels, which ensure that the open volume is maintained. A special case of aerogels are the ambigels that do not rely on supercritical pressures: very similar effects can be obtained with the use of low surface-tension, non-polar, solvents.
When these aerogels consist of a material with adequate electronic and ionic conductivity, like manganese oxides, these can be used as a combined current collector and cathode in 3D microbatteries (Figure5a).3, 4 A benefit of these structures is that an extremely large surface-to-volume ratio can be formed. The relatively open structure also allows the expansion and shrinkage of the battery materials upon lithium ion insertion and extraction without damaging the structure. The next challenging step is to cover these types of structures with electrolyte, counter electrode and current collector. Although the volume is relatively open, which will promote the deposition into the three-dimensional framework, it is not straightforward to obtain a uniform deposition onto such a random morphology. A very open morphology also results, however, in a low volumetric energy density. Therefore an optimum in porosity should be found.
The aerogel approach has some extra requirements for battery materials. The electrolyte needs to be covering the random aerogel framework homogeneously with a closed layer (Figure 5b). Holes in this layer will create short-circuits between the cathode and anode and will, at best, result in a battery with a high self-discharge rate. Self-limiting electrodeposition processes, as described in section 4.2.2, are for example suitable for these structures. Also the electrode material that is deposited on top of the electrolyte needs to have a homogeneous distribution throughout the structure, although this layer does not need to be completely closed and could even consist of a percolated system of conductive nano-particles, as is suggested in the example of Figure 5c. These particles should, however, always form a continuous conductive phase or an additional current collector is required to connect individual “islands”.
Another wet-chemical technique to obtain high surface area materials is the use of surfactants that form micellar structures in a liquid phase. In principle any reaction that selectively yields a solid product inside or outside the micelles can subsequently be applied to form a three-dimensional solid meso-porous structure. With different reaction parameters the morphology can be well controlled. It has been shown that, for example, micellar spheres or rods can be obtained in this way as schematically represented in Figure6.170 Common parameters used to tune the morphology are the reaction temperature, and the concentration and chemistry of the surfactant. Generally, this procedure yields a nano-structured powder. If the liquid phase micelles are combined with an electrochemical technique, the formation of nano-structures can selectively occur on a conductive substrate. Application of this technique for high-surface area battery materials was proposed by Attard et al.,170 and was proven to be an attractive method to yield porous films of silica,171 platinum,170, 172 tin170, 173 and nickel oxides174, 175 with a significantly increased surface area. The pore size is in most cases very small, usually below 100 nm.
The advantage of this technique is that the chemistry is relatively straightforward and well-controllable. The feasibility of this method has been proven for various battery materials and a highly increased surface area can be obtained. The disadvantage of this method is that, since the typical dimensions are very small, homogeneous deposition of subsequent battery layers will be challenging.
4.1.4. 3D Batteries Based on Microchannel Plates
In the concepts described above freestanding 3D-batteries are proposed or 3D structures on top of a planar substrate. A second possibility is to use a 3D structure inside a substrate. This method has the advantage that a mechanically more robust 3D structure is formed. One method to significantly increase the active surface area of a thin film battery is, for example, to use a perforated material. Such perforated substrate can consist of silicon, but also various other substrate materials can be used, e.g. metals, glasses or even polymers. This approach was demonstrated by Peled et al.,176, 177 who employed plasma etching techniques to create microchannels in silicon or used commercially available microchannel glass plates. These channels have a typical radius between 15 and 50 μm. Subsequently, a stack comprising of a current collector, an active electrode, an electrolyte, a second electrode and, if required due to low conductivity of the second electrode, a second current collector are deposited by means of a combination of electrolytic and vacuum impregnation techniques.176, 177 When a conductive substrate is used, no additional bottom current collector needs to be deposited. A schematic representation of the formed microbattery is shown in Figure7.
With this approach a good surface area enhancement can be achieved and a device with a good structural stability can be obtained. Peled et al. calculated a surface area increase (A) with a simplified equation
in which d represents the microchannel diameter, s the interchannel spacing and t the substrate thickness. With typical values of d = 50 μm, s = 10 μm and t = 500 μm a surface area enhancement close to 23 is obtained, when compared to a single sided planar battery.177
The advantages of this production concept are that it consists of techniques which are reasonably cheap to apply, since microchannel substrates are readily available, and relatively simple preparation methods are applied. Another benefit of this concept is that the feasibility has already been proven and results for working devices have been published. Indeed reversible storage capacities of a factor 20–30 higher than equivalent planar stacks have been published.176 The main challenge for this approach is to find simple techniques and conditions for reproducible step-conformal 3D deposition. A second issue might be that, because both sides of the substrate are used for energy storage, device handling and integration into microelectronic or microelectromechanical devices become more complex than with single-sided devices. Also, the demonstrated devices are based on a polymer separator soaked with a liquid electrolyte. This has the benefit of a high ionic conductivity but also introduces the disadvantages of leakage and SEI- or dendrite-formation which might occur upon cycling, thereby potentially limiting the cycle-life of these devices.
4.1.5. 3D-Integrated All-Solid-State Batteries
Silicon is the most widely used material in the semiconductor industry. Notten et al. therefore proposed to use this material as substrate material for the integration of all-solid-state microbatteries (Figure8).178 The first step in this approach is to use an anisotropic etching method to create a three-dimensional structure in the substrate to enlarge the surface area. As etching techniques, either electrochemical179 or reactive ion etching180 can be applied. The etched 3D structure typically consists of trenches,178 but various other geometries can also be made, like pores or pillars.150 Commonly used dimensions for etched trench structures vary from 1 μm to 30 μm in width, and 10–100 μm in depth. The surface area enhancement can be calculated for a trench structure using
in which d is the trench depth, w the width, s the spacing between the trenches and L the total footprint length of the battery structure. 181 A surface area enhancement of, for example, 28 can be calculated using d = 135 μm, w = 5 μm and s = 5 μm for single-sided integration,181 and this value doubles for double-sided integration. Based on standard etching technology these dimensions are shown to be feasible. To prevent the battery from local over(dis)charging or self discharge, the cathode film, the electrolyte and the anode have to be deposited step-conformally. To meet this requirement, techniques such as Chemical Vapor Deposition (CVD), Atomic Layer Deposition (ALD) or electrodeposition can be employed to deposit the battery layers. These techniques, and the conditions for successful 3D deposition, will be described in more detail in section 4.2. The step conformality of the current collectors is less critical: these layers should be closed and need to provide a back contact with a low enough resistivity to prevent significant ohmic losses over the current collector. It is important to note that, as silicon is also known as highly promising anode material in lithium-ion batteries, it is essential to introduce a lithium barrier layer in between the silicon substrate and the silicon anode in order to prevent lithium ions to diffuse from the anode into the silicon substrate. Lithium ions in the substrate might even interfere with other functionalities of the electronic device in which the battery is integrated or result in expansion of the substrate.
The benefit of this approach is that standard microelectronic techniques can be used, which makes integration into autonomous devices more straightforward and allows a simpler transfer to existing production facilities. Also the use of a trench geometry is beneficial: the battery mainly consists of parallel planar films inside the trenches and on top of the substrate, and the risk for local overcharging is therefore reduced. The third advantage is that this concept introduces a solid-state electrolyte. Therefore a longer cycle-life is expected and no leakage risks are present, which widens the possible fields of application. Another advantage of a not completely filled structure is its mechanical stability. A disadvantage, in the other hand, is the relatively dense structure, which makes deposition of step-conformal battery layers challenging.
4.2. Deposition Techniques
To deposit thin films of battery materials, various deposition techniques can be applied. Important for 3D batteries is the possibility to obtain a good penetration into the 3D structure. Therefore, a comparison is made of several deposition techniques comparing their applicability for battery materials and possibility to deposit step-conformal films.
4.2.1. Sol-Gel Deposition
The sol-gel deposition method is a wet chemical technique often applied to produce films of metal oxides. The first step of this technique is to form a solution of the precursor, often a metal alkoxide, which is subsequently hydrolyzed. These hydrolyzed molecules are very reactive and will start to form a colloidal suspension, i.e. a sol. The viscosity of the sol will further increase when the reactions proceed and this will eventually result in the formation of a gel. When the solvent is evaporating from the gel, two processes can take place: either an aerogel- or ambigel-structure is formed or the gel collapses and forms a dense solid. This latter process will occur when the capillary forces of the evaporating liquid are sufficiently high. This technique can be used to form powders, but with the use of coating technologies, like spray coating, dip coating or spin coating, homogeneous thin films can also be obtained. Even a patterned film can be achieved when the sol-gel mechanism is combined with, for instance, inkjet printing.
Since many thin film battery materials are metal oxides and sol-gel is a common method to prepare such oxides, sol-gel techniques are often applied to form various lithium-ion battery electrode materials,182 such as lithium cobalt oxides,140, 183–185 lithium nickel oxides,186 lithium manganese oxides,183 various vanadium oxides,187, 188 tin oxide185 and titanium oxides.140 It is, on the one hand, remarkable that sol-gel is a relatively mature technique for the deposition of electrode materials but, on the other hand, that a relatively low number of publications are describing the results for complete battery stacks produced by this method. Sol-gel techniques have the advantage that they are low-cost, give a good stoichiometric flexibility and can be applied for various metal oxides. One disadvantage is that the morphology of the deposited film is not always predictable; many process parameters influence the deposition process. It is also difficult to control the pin-hole density in sol-gel films: the substrate as well as the precursor solution should ideally be completely free of particles. A third disadvantage is that the purity of the deposited films is often rather low due to various additives to give the sol the desired viscosity or reactivity.
Sol-gel coatings on a planar surface often form a 3D morphology and can therefore be used as basis for 3D batteries. Sol-gel techniques can also be used to coat 3D micro or nano-particles in suspension. This last procedure is, for example, reported for the coating of LiCoO2 microparticles with a layer of Al2O3,189 SnO2190 or ZnO191 to protect it against deterioration and to enhance the electrode cycle-life. The impregnation of a 3D substrate with a highly viscous sol is, however, a challenge: uneven wetting of the surface or trapping of gas bubbles inside pores will prevent formation of homogeneous coatings. A low viscosity sol is therefore required for deposition in 3D structures192 or a vacuum impregnation step needs to be applied in order to enhance the sol infiltration and therefore the 3D step coverage. However, to obtain better step conformalities in high aspect ratio structures, a self limiting technique or a more directive method is desired.
4.2.2. Electrochemical Deposition
Another liquid based deposition technique is electrodeposition or electroplating. By applying a potential between a counter electrode and a conductive surface on which the material deposition is desired, a redox-reaction can be initiated. This is a common method to apply a metallic protective film on industrial scale but it can also be applied at lab-scale for the production of various battery layers. It has been demonstrated that, for example, Cu2Sb,193 V2O5,194 MnO2,195 MoS2176, 177 and metallic anodes like Bi,196 Sn197 and Cu-Sn alloys198 can be deposited using electrodeposition.
Advantages of electrodeposition are that the amount of deposited material can be well-controlled, since this amount is proportional to the charge flowing through the electrodes. This technique is also capable of depositing step-conformal layers into 3D porous structures when the charge transfer reaction is kinetically controlled and is not limited by the diffusion of ionic species involved. Good step coverage can be achieved by limiting the current or by applying pulsed currents, which allows the concentration profile to reach an equilibrium state between the current pulses.
Electrodeposition is also possible in combination with a non-conductive template, as was described in section 4.1.1. In this case a current collector or a sufficiently electronically conducting electrode, is deposited selectively in the pores of the template, yielding a material with a large surface area enhancement factor. After selective removal of the template a free-standing microstructure of a current collector or electrode is created.
There are a few requirements for the electrodeposition process. There should be an electronically conducting back contact available, on top of which a material can be electroplated. Secondly, the material to be deposited should be sufficiently electronically conductive itself; otherwise the deposition process becomes self-limiting. This characteristic can, however, also be considered as an advantage for the coverage of 3D substrates since a self-limiting deposition process can induce homogeneous step conformal coatings over the entire structure. Self-limitation takes place when the deposited layer is sufficiently blocking electron-transport. This effect can, in principle, be exploited for the deposition of fully closed solid-state electrolytes as solid-state electrolytes have this intrinsic, non-conductive, material property: the electro-deposition reaction will be inhibited when the layer thickness is growing. Eventually no electron transport is possible, the charge transfer reaction stops and a closed electrolyte layer has been deposited. As an example, the deposition of polymer electrolytes, using electrodeposition, was presented a few years ago by Rhodes et al.199
A special case of electrodeposition is electroless plating, where a conductive back-contact and a counter electrode are not required. Electroless plating is, just as electrodeposition, dependent on a redox system. The difference is that electroless deposition is independent of an external current but dependent on reduction and oxidation reactions, taking place at the same surface. To promote deposition on a non-conductive substrate and to prevent the formation of solid particles in the electrolyte, the deposition should be catalyzed only by the substrate and the deposited layer itself. Therefore, this latter process is often referred to as autocatalytic. Electroless plating has the advantage that the substrate and the deposited layers are not required to be electronically conductive. The disadvantage is that, since no external current source is applied to control the deposition rate, it is more complicated to achieve a rate-limiting deposition process. It will therefore be more complex to achieve step-conformal 3D depositions.
Most results published on electroless deposition of thin films applicable for lithium-ion battery systems are the deposition of current collectors,200, 201 or the deposition of metal layers to protect an electrode against liquid electrolyte decomposition.202 Li et al. recently published the electroless deposition of a Sn anode onto a copper foam structure, which yielded a stable 3D electrode configuration that can be used in lithium-ion batteries.203
4.2.3. Physical Vapor Deposition
For all-solid-state thin-film microbatteries, often techniques are used that fall in the category Physical Vapor Deposition (PVD). In a PVD process a vapor is formed at very low pressures that subsequently condensates onto a substrate. PVD techniques include, amongst others, sputter-deposition, pulsed laser deposition, e-beam evaporation and other evaporation techniques.
The most simple evaporation technique involves the evaporation of a metal by resistive heating. A metal is used as a source and is heated until it releases a sufficient amount of vapor to condensate on the substrate. Another heating technique is used during e-beam evaporation, where a beam of electrons is used to heat the metal source and to evaporate the material. This evaporation technique is a line of sight deposition and the evaporated metal atoms have a relatively low energy (typically in the range of 0.04 to 0.3 eV).204 Therefore this technique offers a relatively low chance to damage the structure of the deposited and underlying layers. The disadvantage of evaporation techniques is that the source material should be sufficiently volatile for the applied power, so it is usually limited to metals and organic materials. Evaporation is, for example, commonly used to deposit a pure lithium metal anode layer in all-solid-state planar thin-film batteries.19, 26 Metal oxides have, on the other hand, a much lower vapor pressure and can therefore not readily be deposited by evaporation.
Pulsed Laser Deposition (PLD) is another PVD technique that is used in thin film battery research. Here a powerful laser is used to evaporate the material from a target (the source material). From the target molecular fragments, ions or ionic clusters are released, forming a plasma. These fragments will eventually recombine upon condensation onto the surface of the substrate. The advantage of pulsed laser deposition is that the highly focused energy beam ensures that even materials with a very low vapor pressure can be evaporated, such as several metal oxides. Consequently, the particles arriving at the substrate surface have a much higher energy (typically 10–100 eV)205 than those made by the evaporation method and can therefore induce surface modifications. Possible modifications include re-arrangement of the surface atoms or even ejection of these. However, these surface reconstructions can often be reduced by active cooling of the substrate holder. Examples of materials for thin-film solid-state batteries prepared by PLD include cathode materials like LiCoO2,29, 52, 206 LiMn2O452, 207 and V2O5.208 Also the PLD of solid-state electrolytes, such as LiPON209 and LLTO,110, 210 and even complete battery stacks211 are reported in the literature.
Another, more common technique to deposit low vapor pressure materials is sputter deposition. In this case the target is bombarded by high energy ions, either created by a plasma above the target or by an ion-gun. The target will subsequently release atomic or ionic fragments to the gas phase with an average kinetic energy in the range of 1–20 eV.212 These fragments are subsequently deposited when reaching a solid surface. Also electrons are released from the surface of the target, which are usually trapped in a magnetic field above the target. These trapped electrons will collide with the sputtering gas, resulting in the formation of new ions and therefore helping to sustain the plasma, increase the sputter rates and allow working at lower pressures. The gas mixtures used for sputtering can be inert (e.g. argon) but can also be reactive (e.g. N2 or O2) to be incorporated into the growing film.
Similar to PLD, high energy sputtering can be used to deposit low-volatility materials. Also in this case, the high energy of the deposited fragments can introduce structural modifications of the substrate and the deposited layer. Sputter deposition has another disadvantage that atoms from the gas phase can be incorporated in the deposited film. However, this can also be beneficial when a reactive gas is used to form, for instance, oxidic and nitridic materials.
Sputter deposition is frequently applied for planar thin-film solid-state Li-ion batteries and can, amongst others, be applied to deposit TiS2,19, 213 V2O5,19, 214 LiMn2O419, 54 and LiCoO226, 215 electrodes. It is also the most common method to deposit lithium phosphate18 and LiPON15, 19, 216 solid-state electrolytes. In general, PVD is performed at very low pressures, where the mean free path in the vapor phase can extend up to several meters. These line-of-sight techniques are, therefore, in principle unable to deposit materials onto 3D structures in a step-conformal way. There will in most cases be severe shadowing effects. This can, on the other hand, allow patterning: shadow masks can prevent deposition at places where it is not desired.
4.2.4. Chemical Vapor Deposition
Chemical Vapor Deposition (CVD) relies, just as PVD, on a vapor source. Transport in the gas phase of this technique is, however, not line-of-sight but is based on gas flow- and molecular diffusion mechanisms. The essential difference between PVD and CVD is that the formation of a layer is not based on a physical condensation process but on a chemical reaction mechanism. This combination of diffusive transport and reactive deposition makes CVD in principle more suitable for highly conformal 3D deposition. This is, however, only the case when the deposition rate is controlled by the chemical surface reaction and not by the flux of the chemical precursors to the surface. The supply rate can be increased by, for example, lowering the pressure in the reactor which increases the reactants diffusion rate.
The chemical process in the CVD reactor can be activated by several methods. This can be thermal activation but also plasmas or laser sources are frequently applied. A disadvantage of the latter two is that 3D penetration of a plasma or a laser is very limited, which makes step conformal deposition onto 3D substrates less likely: more reactive species are formed in the bulk gas phase, which influences the ratio of diffusive transport versus surface reaction rate. This makes it more likely that the overall process becomes diffusion-limited. When thermal activation is applied, activation can also occur inside the trenches, reducing this effect.
Different designs are also available for the vapor source, which can consist of classical bubblers but might also consist of mist-evaporators or liquid injection systems which have less severe requirements with respect to the vapor pressure of the precursor. With these different designs of vapor sources, also various types of precursors are commonly utilized. The simplest precursors are already in gaseous form, like silane, which is frequently used for the deposition of silicon films. For the deposition of metal oxides, mostly organometallic precursors are applied. These include amongst others metal alkyls, metal acetylacetonates and metal cyclopentadienyls. A third frequently applied precursor class consists of metal halogenides.
Although CVD is a relatively immature technique for the fabrication of thin film batteries, successful depositions of several thin film battery materials were demonstrated in the literature, such as lithium cobalt oxide,33, 34, 55, 217–219 vanadium oxides,220, 221 platinum current collectors222 and various forms of silicon.181, 223, 224 The deposition of common metal oxide layers like titanium dioxide225, 226 and tungsten- and molybdenum oxides,227, 228 that can be used as electrode materials for thin film batteries, have been studied thoroughly for several other applications. The first steps to deposit lithium cobalt oxide into a trench structure using liquid delivery metal-organic CVD have also been reported.33, 219
Advantages of CVD are that the deposition relies on relatively low kinetic energy particles, that a multitude of chemistries is available and that the deposition of materials into 3D structures is in principle possible. However, CVD is relatively unknown for lithium containing films and therefore many reaction mechanisms that control the deposition process still have to be elucidated. Moreover, the precursors have to fulfill stringent requirements: a sufficient supply of material in the gas-phase is required and the precursor chemistry should allow reaction rate controlled growth under the applied deposition conditions when homogeneous deposition into 3D structures is desired.
4.2.5. Atomic Layer Deposition
Atomic Layer Deposition (ALD) is a special case of CVD, where the deposition reactions are self-limiting. The deposition process involves cycles consisting of chemisorptions of the precursor onto the surface and separate re-activation of the newly formed surface. One typical re-activation mechanism is the removal of the precursor ligands. Because the ALD process is self-limiting, it is highly suitable for step-conformal deposition in porous structures. ALD requires similar activation methods to start the deposition processes as CVD, also in this case thermally activated- or plasma-assisted deposition methods are frequently applied.
There are only a few groups working on thin film microbatteries using ALD. Knoops et al. reported the deposition of planar and 3D films of TiN and TaN, which may serve as lithium diffusion barrier layers,224, 229, 230 and 3D layers of platinum that can be used as cathode current collector.231, 232 Also the deposition of active electrode materials like V2O5,233 TiO2164 and Co3O4234 have been reported. Recently, the first successful deposition of lithium containing films (Li2CO3 and lithium lanthanum titanate) was shown.235
ALD has the major advantage that it is a technique capable of step conformal deposition in high-aspect ratio 3D geometries. The main disadvantage is that the growth rate for ALD processes in general is relatively low (typically 0.05 nm/cycle). These cycles take longer in the case of high aspect ratio 3D structures, where the precursor dosing and purging steps are required to be somewhat longer. To increase the ALD deposition rate, new methods are investigated, of which spatial atomic layer deposition is an interesting example.236 This method does not rely on time separated dosing of different gasses, but the spatial separation of different gas phases through which the substrate moves. It can thus be concluded that ALD is a promising technique for 3D batteries consisting of nano-scale battery layers. In the future, when high deposition rate techniques become readily available, higher capacity electrochemical energy storage applications may benefit from the ALD deposition methods.
4.3. Recent Progress
In this section the recent progress in 3-dimensional thin-film microbattery research is described, in line with the concepts that were presented in section 4.1.
4.3.1. Three-Dimensional Substrates Based on Templated Deposition
The deposition of nano-rods through meso-porous membranes is a mature technique, which also has been explored for application in Li-ion rechargeable batteries. These nano-structures can be used in liquid electrolyte batteries to reduce the lithium ion transport distance through electrodes that have a low ionic conductivity, e.g. V2O5,237, 238 or to cope with the high volume change that occurs in metallic electrode materials, e.g. Sn.197 As described in section 4.1.1., these nano-rods can also serve as the starting point for 3D integrated solid-state microbatteries, as was proposed by Perre et al.162 The preparation of aluminum nano-rods, which are aimed to serve as cathode current collector, was achieved by pulsed electrodeposition of aluminum through an alumina membrane on an aluminum foil substrate.162 These nano-rods had a diameter of approximately 200 nm, a height of circa 2 μm and a spacing of 400 nm (Figure9). The calculated surface area enhancement was a factor 10. On these rods, a 17 nm anatase TiO2 film was grown with ALD, giving good step conformality. Testing this 3D TiO2 electrode in a liquid electrolyte indeed yielded a capacity that was one order of magnitude higher than for the equivalent planar geometry (Figure10).164 A similar experiment was published in which nano-rods of copper with similar dimensions as the afore-mentioned aluminum rods were formed via template electrodeposition and subsequently covered with an electrodeposited Fe2O3 electrode film.165 Although this approach was simply aiming at increasing the rate capabilities in liquid electrolyte batteries, it could also be applied for 3D batteries.
4.3.2. Arrays of Interdigitated Carbon Microrods
The formation of microrods based on pyrolized photoresist with an aspect ratio of up to 40 was demonstrated by Dunn et al.168 The microrods were not only demonstrated on a planar substrate but the preparation of aligned rods on a (interdigitated) patterned current collector has also been demonstrated in the literature. These rods are sufficiently electronically conductive and are therefore suitable as current collectors for thin film batteries.166 Since conductive current collectors are already present before preparation of both electrodes, the electrode layers can selectively be grown using electrodeposition. Since these rods consist of carbon it is, however, also possible to use these directly as an anode for Li-ion batteries.168
One electrodeposition process to cover rods with an active electrode layer is electropolymerization of polypyrrole doped with dodecylbenzylsulfonate (PPYDBS), a cathode material with a redox potential around 3 V. The published results cover only rods with a relatively low aspect ratio of up to 3.2. As the spacing between the rods was relatively large, a very low surface area enhancement was obtained in this case. Nonetheless, measurements in a liquid electrolyte with carbon rods as anode and rods covered with an electrodeposited PPYDBS film as cathode (Figure11) showed that both rod-type electrodes were electrochemically active and that a combination of two types of rods in an interdigitated geometry can be suitable as a battery. However, there is still a need for improvement: a very large self-discharge was observed resulting in a much lower discharge capacity than charge capacity (Figure12). Secondly, the internal resistance of the current collectors was relatively high.167 Also the energy density was low and these measurements were performed in a liquid electrolyte, so if this system would be applied for a solid-state battery, a method should still be developed to include a solid-state electrolyte.
An interdigitated pillar layout can also be obtained with template deposition. When a membrane is attached to a substrate with a patterned metal film, electrodeposition can be selectively performed in the pores of the membrane that ends at the connected metallic pattern.239 When the metallic rods are formed by this templated deposition to serve as current collectors for a solid state battery, the resistance of these current collectors is expected to be significantly lower than that of carbon microrods. The resulting electrochemical cell should therefore be capable of a much higher power delivery. Another method to create an interdigitated geometry is by using a 3D-structured solid-state electrolyte. This principle has very recently been demonstrated by Kanamura et al., who used a pre-shaped solid-state electrolyte with an interdigitated array of microcavities. This array was subsequently filled at each side with LiCoO2 and Li4Mn5O12 to produce a functioning 3D microbattery.240
4.3.3. Three-Dimensional Architectures Based on Aerogels
A lithium-ion battery based on an aerogel nano-architecture can employ several types of chemistry. One system, proposed by Rolison et al., is based on manganese oxide (MnO2) as electrode material. This MnO2 electrode can either be a self-supported aerogel (Figure13a),199 or electroless MnO2 plated onto a preformed carbon nano-foam, used to enhance the electronic conductivity of the electrode.241, 242 Important for the application of a coating onto a nanoporous structure is that the deposition process is self-limiting. Self-limiting deposition can be achieved by ALD but also by electrochemical techniques, like electro- and electroless deposition. Self-limiting electroless deposition was demonstrated for MnO2 but only within a certain pH range: under neutral conditions electroless deposition reaction was shown to be self-limiting. At lower pH the deposition process is not self-limiting, resulting in excessive deposition on the outer surface of the aerogel particles.241, 242
Also for the subsequent electrolyte layer, a self-limiting process is desired. For this film, Rolison et al. suggested the self-limiting electrodeposition of a poly(phenylene oxide) (PPO) film as separator. Even though this film is relatively thin (<25 nm), apparently a closed layer could be deposited that did prevent short-circuits between the cathode and anode (Figure 13b).199 After deposition this film was impregnated with a lithium perchlorate liquid electrolyte. The final step to complete the battery stack is the addition of a counter-electrode, which may e.g. consist of a nano-scale RuO2 colloid network3 or can involve filling of the pores with V2O5.4 A prototype based on this V2O5 electrode material was produced but the results revealed several major challenges: The electronic conductivity of the V2O5 was relatively low, thereby introducing a large ohmic drop in the operation voltage of the device. Secondly, the ionic transport through the PPO film was relatively slow and the overpotential losses were therefore increased further.4
In section 4.1.3 it was described that micelles of surfactants can also be used to create a 3D meso-porous framework. The use of this method is widespread for various applications but its use for all-solid-state microbatteries is limited. Owen etal. have reported that several materials prepared, based on micelle template deposition can be used as components for lithium-ion batteries. Platinum is an example of a prepared meso-porous material, which was suggested to be used as a current collector in batteries and various other applications.170, 172 This platinum was made in film form by using a solution containing a non-ionic surfactant, water and hexachloroplatinic acid (H2PtCl6). When electrodeposition from this solution was performed on a gold electrode, it yielded a platinum film with a specific surface area of approximately 22 m2·g−1, which is approximately five times higher than the surface area of a film deposited without the presence of a surfactant. Important to notice is that the top surface roughness is only approximately 20 nm and that the main surface area enhancement is achieved due to pores in the film. These pores were found to be only approximately 25 Å in diameter, which is a much smaller scale than the surface roughness. However, the authors suggested that the pore diameter could be controlled by varying the chain length of the surfactant. 170, 172 In its present form this current collector can serve as basis for a 3D-battery, although the surface area enhancement will become insignificant with the application of the first active electrode layer, which will be blocking the pore structure.
Tin is another material, prepared by Owen et al., that can be applied as a negative electrode material.170, 173 Similar as for platinum deposition the deposition of a meso-porous tin layer was also performed using a surfactant in combination with electroplating. Tin has the tendency to lose a large part of its storage capacity upon cycling, which has mainly been attributed to material pulverization. Tin has a large volumetric expansion upon lithiation, which leads to structural deformation and eventually to disintegration. It was found that surfactants could be used to form a meso-porous structure onto a copper foil substrate and that the disintegration effect upon cycling was delayed for several cycles. It was suggested that this stability increase was originating from the more porous structure, which can better accommodate the volume changes in the electrode.170, 173 However, due to their small pore size, these meso-porous structures do not seem to be applicable for 3D all-solid-state thin film batteries.
4.3.4. 3D Batteries Based on Microchannel Plates
Peled et al.177 used a soda-lime glass microchannel plate as basis for their 3D micro-battery. This plate had a thickness of 500 μm with pores through it, having an average diameter of 50 μm. The surface area enhancement of this 3D substrate is 20–30 compared to a single-sided planar device. As the substrate was non-conductive, the first process step consisted of electroless deposition of nickel as bottom current collector. A step conformal film of a few μm thickness was produced in this way.243 Subsequently, a cathode material was deposited. This material was prepared by electrodeposition and consisted of molybdenum oxysulfide.177 Alternative demonstrated cathode materials were electrodeposited copper-244 and iron sulfide.245
The electrolyte in this device consisted of a hybrid polymer electrolyte: first the cathode was coated with a polymer separator using successive impregnation and evacuation steps. The polymer precursor was a commercial Poly(vinylidene fluoride) (PVDF) mixed with a solvent and SiO2 nanopowder. This combination was previously found to yield high ionic conductivities when impregnated with liquid electrolytes: ionic conductivities up to 2·10−3 S·cm−1 were reported.123
The anode precursor consisted of a slurry, comprising meso-carbon microbeads (MCMB), which was also deposited by consecutive spincoating and evacuation steps. The coating evacuation cycle was repeated until the microchannels were completely filled. The polymer membrane film and the MCMB anode were simultaneously soaked under vacuum with a liquid electrolyte composed of LiPF6 or LiBF4 in ethyl carbonate/diethyl carbonate to obtain sufficient ionic conductivity. The last step to create an active battery stack was the lithiation of the graphite anode, which was obtained by placing a piece of lithium foil onto the graphite anode and allowing it to equilibrate for several hours.176, 177
A device based on this procedure was prepared and the reported SEM images clearly showed a well-covered 3D substrate (Figure14). Electrochemical tests of samples mounted in coin cells were presented. When comparing the 3D device to a planar sample that was produced with the same procedure, an increase in the storage capacity of a factor 20–30 was indeed obtained, in agreement with the predicted surface area enhancement (Figure15).176, 177
These nice results demonstrate that a 3D Li-ion microbattery can indeed be prepared and that it is feasible to use these as power supply for microdevices. A disadvantage is that still a hybrid polymer electrolyte is used, soaked in a liquid electrolyte. To achieve full solid-state devices an inventory of possible solid-state polymer electrolytes has recently been made.246
4.3.5. 3D-Integrated All-Solid-State Batteries
When 3D thin-film batteries are integrated into a silicon substrate, a lithium diffusion barrier layer is essential to prevent the loss of lithium into the substrate. Lithium diffusion barrier layers are often metal-nitrides, of which titanium nitride (TiN) and tantalum nitride (TaN) show suitable properties. Thin films of these materials are commonly deposited using sputtering deposition techniques but since the goal of this approach is to make 3D geometries covered with step-conformal layers, ALD was investigated as potential technique to provide these barrier layers. Knoops et al.229, 230, 232 found a process for the ALD deposition of these layers and concluded that ALD TiN formed an even more promising barrier layer than its sputtered equivalent.150, 230 Since ALD is a self-limiting technique, it was expected to be capable of delivering step-conformal 3D barrier layers. This was indeed demonstrated: a film of approximately 60 nm thickness could be deposited into trenches step-conformally of 1 μm width and 20 μm depth (Figure16a and b).232
TiN is well electronically conductive, so apart from utilizing it as a barrier layer, it can also be used as current collector. When a higher electronic conductivity is required or when no barrier layer is needed, for example when the first deposited layer is a cathode film, an ALD film of platinum can be used as current collector. Metallic platinum can also be deposited using ALD, but it is known to form various silicides.247 This sillicide formation is detrimental for the adhesion and stability of the film. Therefore, a film of SiO2 followed by a film of TiO2 has been introduced before depositing the Pt layer, to provide a current collector stack that is stable under conditions at which the thin film battery is deposited and operated. Also for this stack, deposition was demonstrated in high aspect-ratio trenches. It is clearly visible in Figure 16c and d that the layers at the bottom of the trench are significantly thinner than the layers at the top: at the bottom the SiO2, the TiO2 and the Pt layers have a thickness of approximately 60, 50 and 30 nm, while their thicknesses at the top are 100, 60 and 55 nm, respectively. However, since the Pt layer is used as current collector, a uniform thickness is of less importance as long as the closed current collector stack is sufficient thick, providing good electronic conductivity.
For the electrochemically active electrode materials, the volume of the layer determines the storage capacity of the electrode. Usually a film of the order of μm thickness is required for the cathodes, whereas alloying anodes generally require a much lower film thickness. The low deposition rate of ALD is making this technique less favorable for the deposition of cathode layers. CVD techniques seem therefore more suitable.
One of the most common cathode materials for Li-ion thin-film batteries is LiCoO2. This film is usually produced by sputter-deposition, which is generally less suitable for deposition into 3D structures. Therefore the use of LPCVD was recently investigated as a method to deposit poly-crystalline LiCoO2 films. Conditions were found that delivered crystalline LiCoO2 films that showed a good electrochemical response in liquid electrolyte.217 As expected, sharp charge- and discharge peaks were observed at 3.9 V (Figure17). The cycle-life was negatively affected by the liquid electrolyte, which could be significantly prolonged by the application of a solid-state electrolyte layer.217, 248 Moreover, preliminary experiments showed that a good step coverage could be obtained for the deposition of LiCoO2 in low aspect ratio trenches of 10 μm wide and 30 μm deep (Figure 16e).
Si, Ge and Sn-based materials are suitable candidates to be applied as anode. These materials are known for their extremely high lithium uptake but suffer from a short lifetime in bulk materials due to structural disintegration in the first (dis)charge cycles. For silicon it was, for example, demonstrated that a mono-crystalline silicon wafer shows many cracks after only one full charge/discharge cycle.181 This expansion is, however, less detrimental for thin films, where large stresses can be more easily accommodated and mechanical integrity can be maintained. Experiments were performed on 60 nm poly-Si films deposited onto a TiN current collector/barrier layer and tested in liquid electrolyte. These tests demonstrated that the films kept their mechanical stability over more than 50 cycles. The cycle-life was also in this case limited by the liquid electrolyte: cycling up to full capacity was hindered after 30 cycles by the formation of a SEI layer, which could nicely be visualized by SEM and using single crystal wafers.150, 181 A Si film covered with a protective solid-state electrolyte, on the other hand, did not show any SEI formation and hence no capacity loss was found upon cycling, which indicates that Si layers are indeed very suitable candidates for 3D all-solid-state batteries.150, 151, 178, 181
Thin films are also of particular interest as these can be (dis)charged with very high rates: when delithiated at 100 C-rate (i.e. complete discharge in 36 seconds) Si thin films still deliver 90% of their original storage charge capacity.181 An advantage of silicon as anode is furthermore that it has already been successfully proven as electrode in commercial 3D-capacitors, deposited by LPCVD in high aspect ratio pores.249 Also for high aspect ratio 3D negative electrode stacks, silicon was successfully deposited and it showed a reversible electrochemical storage capacity that was significantly higher than that of planar films (Figure18).224
Other promising thin film anode materials for all-solid-state thin-film batteries are Ge- and Sn-based. These materials reveal several advantages, such as several orders of magnitude higher electronic and ionic conductivities, while the volumetric storage capacities are very similar to that of Si.159, 250
Integrated batteries are expected to play an increasing role in many interesting new applications in the near future. 3D solid-state batteries are ideal candidates for use in Small Autonomous Networked sensor Devices (SAND) as they combine a small volume with a high storage and power capability. The key features of these autonomous devices are wireless communication, on-board sensing function(s) as well as an energy harvesting capability to facilitate autonomy. Energy harvesting can be done by photovoltaic cells (Figure19a) but also piezoelectric, thermoelectric and electrostatic energy scavengers are proposed.251, 252 Since wireless communication often requires high peak currents, on-board energy storage is a necessity for these autonomous devices. When the overall energy consumption is relatively low, storage can ideally be offered by 3D solid-state microbatteries, which combine high current capabilities with relatively high energy densities.
Another field in which microbatteries will be applied is in medical systems, e.g. implants and electronic pills. E-pills can function as imaging devices and mechanical surgeons,253 or as autonomous functioning devices that can diagnose and accordingly control drug delivery.254, 255 For in vivo applications, different energy scavenging techniques have to be adopted. A possible source of electrical energy is, e.g. by making use of bio-fuel cells. The most straightforward and most examined bio-fuel cell for in vivo applications is nowadays based on glucose oxidation glucose and oxygen reduction. This can occur either microbially by active enzymes present in living cells, or enzymatically, mimicking biological power supply. More details of these types of bio-fuel cells are widely available in literature.256, 257 For in vivo applications more stringent safety requirements also hold for energy storage: there should be a complete absence of substances which can be harmful for living organisms and the leakage risk should be completely eliminated. This makes solid-state batteries promising candidates for the storage of energy in combination with bio-fuel cells (see artist impression in Figure 19b).
Thin-film all-solid-state battery research is a relatively new field of scientific interest. Many different materials and various deposition methods were applied to manufacture planar thin film batteries. To increase the battery storage capacity while keeping the same footprint area, various different 3D methodologies are proposed. In this review the most relevant of these approaches have been highlighted and some of the most recent advances have been shown. The discussed concepts for 3D batteries are based on a membrane template, on pyrolysed photoresist microrods, porous aerogels or micelle structures, microchannel plates and anisotropically etched micro-structures. All these approaches have their own advantages and disadvantages and all concepts are still in the very early stage of development. Only recently, the very first tests indicating the feasibility of these 3D batteries were published. Although several challenges still remain, it is shown that various research groups all over the world put a large effort into this new scientific field. It is therefore very likely that the appearance of the first practical application(s) of 3D-integrated solid-state microbatteries is to be expected shortly.
The authors gratefully would like to thank Frans Schraven, Harm Knoops, Merijn Donders, Rogier Niessen, Marcel Mulder, Erwin Kessels and Mart de Croon for their valuable contributions and many fruitful discussions leading to this work. AgentschapNL is kindly acknowledged for the financial support of this project.
Jos F. M. Oudenhoven studied Chemistry and Chemical Engineering at Eindhoven University of Technology (TU/e, The Netherlands). His master thesis research was done in the group Physical Chemistry of Surfaces in Catalysis where he studied the behavior of thin-film model catalysts. In 2006 he joined the Integrated Battery project of Prof. Notten as a PhD candidate, where his work focused on the deposition and evaluation of thin-film battery materials on 3D-patterned substrates, using low pressure chemical vapor deposition. Recently, he joined the micropower group at the Holst Centre/imec-NL (Eindhoven, The Netherlands).
Loïc Baggetto was educated in Materials Sciences and Electrochemistry at ENSEEG (École Nationale Supérieure d’Électrochimie et d’Électrométallurgie de Grenoble, France). After a master research project at Philips Research Laboratories (Eindhoven, The Netherlands) within the research group led by Prof. Notten, he joined the Integrated Battery project at TU/e in 2006 as a PhD candidate. He recently successfully defended his PhD thesis devoted to the characterization of Li-barrier layers and negative electrodes such as Si, Ge and Sn-based thin-film materials.
Peter H.L. Notten joined Philips Research Laboratories (Eindhoven, The Netherlands) from 1975 to 2010, where he investigated the electrochemistry of the etching of III-V semiconductors to obtain his PhD degree from TU/e in 1989. Since then he focused on energy storage research, including studies on hydrogen and lithium storage materials, new battery technologies, modeling of various electrochemical systems and design of battery-management algorithms. In 2000 he was appointed as part-time professor at TU/e where he now heads the group Energy Materials and Devices.