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.
Figure 9. Aluminum nano-rods covered with an ALD layer of TiO2 to serve as battery electrode. The inset shows a TEM cross-section of a single nano-rod covered with ALD TiO2. Reproduced with permission.164 Copyright 2009, The American Chemical Society.
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Figure 10. Comparison of the storage capacity of a 3D electrode consisting of Al nano-rods covered with TiO2 (circles) with a corresponding planar layer (triangles). Reproduced with permission.164 Copyright 2009, The American Chemical Society.
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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.
Figure 11. Connected arrays of carbon and PPYDBS coated carbon microods. The panels show the same sample at various angles and magnifications. Reproduced with permission.167 Copyright 2008, Elsevier.
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Figure 12. (Dis)charging of the graphite anode (a) and PPYDBS cathode (b), both at 50 μA·cm−2. A combination of these was charged at 90 μA·cm−2 and discharged at 20 μA·cm−2 (c). Reproduced with permission.167 Copyright 2008, Elsevier.
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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
Figure 13. SEM images of a MnO2 nano-architecture (a) and a nano-architecture coated with PPO (b). Reproduced with permission.199 Copyright 2004, Elsevier.
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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
Figure 14. SEM images of several steps in the formation of a 3D microbattery based on a microchannel plate: nickel current collector on the microchannel plate (a), Molybdenum oxysulfide cathode on a nickel current collector (b) and a polymer membrane on the current collector-cathode stack (c). Reproduced with permission.176 Copyright 2006, Elsevier.
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Figure 15. Cycle-life of a 3D microbattery based on a microchannel plate compared to a corresponding single-sided planar 2D battery. Modified batteries were prepared with a poly(ethylyneoxide) additive in the deposition-electrolyte, which improved the electrochemical stability and activity of the electrodeposited molybdenum oxysulfide cathode. Reproduced with permission.176 Copyright 2006, Elsevier.
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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
Figure 16. Thin films deposited in trench structures: ALD TiN film at the top (a) and at the bottom (b) of a 1 × 20 μm trench 232. ALD-deposited SiO2, TiO2, Pt current collector stack at the top (c) and bottom (d) of a 1 × 20 μm trench. 232 400 nm thick LiCoO2 deposited by LPCVD in a 10 × 30 μm trench (e). (a,b,c,d: Reproduced with permission.232 Copyright 2009, The Electrochemical Society; e: unpublished result Notten group).
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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