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Keywords:

  • lithium;
  • Li-ion;
  • high capacity;
  • battery;
  • review

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Considerations of Electrode Parameters
  5. 3 Material Parameters
  6. 4 Alloying Elements—Group IV
  7. 5 Conversion Electrodes
  8. 6 Alloying Elements—Others
  9. 7 Nanostructured Carbon
  10. 8 Impact of the Electrode or Particle Architecture
  11. 9 Conclusions and Outlook
  12. Acknowledgements
  13. Biographies

Growing market demand for portable energy storage has triggered significant research on high-capacity lithium-ion (Li-ion) battery anodes. Various elements have been utilized in innovative structures to enable these anodes, which can potentially increase the energy density and decrease the cost of Li-ion batteries. In this review, electrode and material parameters are considered in anode fabrication. The periodic table is then used to explore how the choice of anode material affects rate performance, cycle stability, Li-ion insertion/extraction potentials, voltage hysteresis, volumetric and specific capacities, and other critical parameters. Silicon (Si), germanium (Ge), and tin (Sn) anodes receive more attention in literature and in this review, but other elements, such as antimony (Sb), lead (Pb), magnesium (Mg), aluminum (Al), gallium (Ga), phosphorus (P), arsenic (As), bismuth (Bi), and zinc (Zn) are also discussed. Among conversion anodes focus is placed on oxides, nitrides, phosphides, and hydrides. Nanostructured carbon (C) receives separate consideration. Issues in high- capacity research, such as volume change, insufficient coulombic efficiency, and solid electrolyte interphase (SEI) layer stability are elucidated. Finally, advanced carbon composites utilizing carbon nanotubes (CNT), graphene, and size preserving external shells are discussed, including high mass loading (thick) electrodes and electrodes capable of providing load-bearing properties.

1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Considerations of Electrode Parameters
  5. 3 Material Parameters
  6. 4 Alloying Elements—Group IV
  7. 5 Conversion Electrodes
  8. 6 Alloying Elements—Others
  9. 7 Nanostructured Carbon
  10. 8 Impact of the Electrode or Particle Architecture
  11. 9 Conclusions and Outlook
  12. Acknowledgements
  13. Biographies

Significant research effort has been devoted to novel electrodes for Li-ion batteries in recent years due to its high potential impact on energy storage for electric vehicles (EVs) and portable electronics. Li-ion batteries have already become the battery of choice for portable electronics and EVs due to their high energy density and decreasing cost. However, they are also often a limiting factor. Li-ion batteries often make up a large portion of the mass and volume of portable electronics and still limit their available energy, thus requiring frequent recharging. Currently, battery-powered EVs either offer a small driving range or are expensive due to the high price tag of Li-ion batteries. An increase in battery energy density, particularly volumetric energy density, can greatly improve and expand the possibilities of portable electronics. If similar production expenses per unit cell, and thus a lower cost per unit energy, is achieved at the same time, higher range and more affordable EVs can be produced.

In order to address the market needs of reduced cost and improved energy density, many researchers have aimed to improve volumetric capacity of anodes and cathodes. High-capacity anode research, in particular, has been extremely active, and materials such as silicon (Si), silicon oxide (SiOx), tin (Sn), and tin oxides (SnOx) have received enormous attention, while many other, sometimes more exotic materials, have appeared in literature as well. This review is an attempt to provide a general overview of high-capacity anode materials and to develop a broader understanding of the scientific explorations in the field of high-capacity Li-ion anodes. A detailed and comprehensive review is not attempted, and there are no doubt topics and groups of publications, which we will inevitably overlook. However, the greatest effort is made to cover the entire periodic table, and to review the various innovations in electrode architecture and chemistry, which can dramatically improve anode capacity.

A traditional high-capacity Li-ion battery is made from a lithium cobalt oxide (LiCoO2 often called LCO) cathode and a graphite (C) anode. Both electrodes are produced from active (Li-ion storing) powders mixed with a small content (3–5 wt%) of a polymer binder (mostly polyvinylidene fluoride, PVDF) and a small content (1–5 wt%) of conductive carbon additives (mostly carbon black, but on occasion, vapor grown carbon fibers or multi-walled carbon nanotubes, MWCNTs) and casted on both sides of metal current collector foils (an aluminum, Al, foil for a cathode and a copper, Cu, foil for an anode). A typical thickness of an electrode layer ranges from 60 to 100 μm on each side of a foil. In a battery, the electrodes are separated with a porous electrically insulated membrane with a typical thickness of 15–25 μm. By using higher capacity active materials and/or designing a structure/material that obviates/reduces the need for a separator membrane, binders, conductive additives, or current collectors, the overall battery energy density can be increased.

Historically, the most commercially successful electrode chemistry has been intercalation-type electrodes: transition metal oxide cathodes and graphite anodes. Intercalation electrodes are capable of providing rapid Li ion transport by having Li conductive 1D paths or 2D planes within relatively large individual particles (commonly >1 μm in diameter cathode particles and >5 μm in diameter anode particles). The advantage of low-volume expansion on lithiation and de-lithiation (commonly <7%), results in good mechanical and electrochemical stability of intercalation electrodes. The low-volume changes in the graphite anode particles are particularly important because of the need to maintain a low strain within a solid electrolyte interphase (SEI) layer, which otherwise may form electrolyte solvent-permeable defects and continuously grow upon electrolyte reduction at a low anode potential, irreversibly consuming Li from the cell and reducing its capacity with cycling. However, intercalation electrodes explored thus far tend to have much lower theoretical capacity compared with other alloy-type or conversion-type active materials. In the case of a graphite, solvent intercalation and exfoliation also limits the choice of electrolytes.

Electrodes capable of electrochemically alloying with Li often have high charge capacity, due to the relatively large stoichiometric ratio of Li that such active materials can commonly accommodate. However, the low mobility of Li within alloying electrodes, combined with their tendency for high volumetric expansion on lithiation often results in low rate capability and limited cycle life when tested in both half cells, (against a Li foil) and more importantly, full cells (against a Li-containing cathode).

Finally, in conversion/displacement/substitution electrodes (hereon called conversion electrodes), Li replaces the reducer (often a transition metal) in a compound upon lithiation.[1] A prominent example of a conversion electrode is MnO, which can be reversibly lithiated to form Mn and LixO. However, such electrodes have so far exhibited lower capacity than alloying electrodes and suffer from similar issues to the alloying electrodes, having a tendency for low Li mobility, high volumetric expansion, and coarsening of active material upon repeated lithiation/delithiation. Moreover, conversion electrodes tend to have large polarization, and require a large voltage window for lithiation/delithiation, ultimately reducing the energy density.

Nanoscale features have been the big enabler of the latter two chemistries. Nanostructured electrodes can compensate for slow electron and Li ion transport in the active material by decreasing electron/Li ion diffusion length, enabling higher rate performance and higher capacity utilization. Nanoscale dimensions can additionally prevent the build-up of internal stresses during volume expansion and contraction, which may otherwise lead to the formation of cracks within high-capacity active particles. Furthermore, formation of nanostructured porous composite particles may limit the volumetric expansion of the active component of the composite during charging and discharging of a cell. This may further improve the anode structural stability and greatly stabilize the anode SEI. Finally, various fibrous nanostructures (mostly MWCNT-based composites) have shown great potential for the formation of multifunctional electrodes with load-bearing capabilities, which may reduce the weight of energy storage on a system level. Such MWCNT-based nanostructures often eliminate or reduce the relative volume and mass fraction of the current collector. Unfortunately, nanoscale geometries often require advanced processing methods, which can result in increased anode powder costs. In addition, the processing of many fibrous nanostructures into battery electrodes and their incorporation into cells require tools that are incompatible with common battery assembling equipment. Therefore, only high-capacity drop-in replacements for graphite particles will likely be adopted by industry in the near future. While electronic industry is less sensitive to battery costs, EV industry will likely only adopt novel high-capacity anode solutions offering reduced cost per energy ($ Wh−1).

2 Considerations of Electrode Parameters

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Considerations of Electrode Parameters
  5. 3 Material Parameters
  6. 4 Alloying Elements—Group IV
  7. 5 Conversion Electrodes
  8. 6 Alloying Elements—Others
  9. 7 Nanostructured Carbon
  10. 8 Impact of the Electrode or Particle Architecture
  11. 9 Conclusions and Outlook
  12. Acknowledgements
  13. Biographies

In order to understand and evaluate new electrode materials which operate by different mechanisms, key parameters need to be identified and clearly defined. For example, “capacity,” which is the main parameter of interest in this review, can be defined in several ways. “Capacity” generally refers to specific capacity, denoting charge per mass (mAh g−1). This value is often cited because it can be calculated easily with reasonable accuracy and is an important parameter for some weight-sensitive applications. To be precise, the most relevant value is the de-lithiation capacity (i.e., reversible capacity), and a note should always be made when the mass of the active material is used instead of the entire electrode, which may additionally include significant content of polymer binders and conductive additives. By convention, the current collector foil is never included in the calculation of mass because the electrode material properties are largely independent of the foil thickness.

However, high specific capacity on its own does not guarantee a usefulness of the electrode for practical applications. First, an electrode must show good performance at a sufficiently high mass loading, or more relevantly, high capacity loading, which are measurements of electrode mass per unit area (mg cm−2) and electrode capacity per unit area (mAh cm−2). Electrodes in commercial cells commonly show capacity loadings of 2.5–3.5 mAh cm−2. A low mass loading undesirably increases the relative weight, volume, and costs of metal current collector foils and electrolyte-filled separator. A higher mass and capacity loading makes it challenging to attain a combination of good uniformity and mechanical stability (for crack- and defect-free electrodes), good adhesion, and high ionic and electronic conductivities in commercial electrodes produced using an industry standard slurry casting and drying technique. As such, one may expect that new materials should be tested at similar capacity loadings to demonstrate their performance at comparable conditions.

Here we note that the vast majority of electrode tests reported in scientific literature are performed in half cells (against a Li foil counter electrode). But the performance of half cells can be limited by both the working electrode and a Li counter electrode. The rapid growth of Li dendrites and increase in the Li SEI resistance with cycling is often experimentally observed at current densities above 0.5–1 mA cm−2. Therefore, half-cell cycle life testing should be conducted in conditions where the current density stays below this limit, to ensure that the performance is limited by the working electrode, and not the Li foil. If the cycle life tests are conducted at a “C/2” (charge/discharge within 2 h) the mass loading of the working electrode should, thus, be limited to 1–2 mAh cm−2 or below. However, very small capacity loading may hide important material limitations, such as high electrical and ionic resistance or low structural stability, all of which are affected by the electrode thickness. High-capacity anodes are particularly sensitive to electrode thickness, and commonly suffer from low ionic and electronic conductivity, uncontrolled SEI growth, and internal stresses generated during cycling. Therefore, we recommend the preparation of anodes with capacity loading of ≈1 mAh cm−2 for cycle life tests at the rates of C/2 or 1C in half cells. We would like to emphasize that very stable performance of materials cast to thin electrodes with a capacity loading of, say, 0.1–0.2 mAh cm−2 or below does not guarantee their stability when tested at higher loadings.

Second, in the majority of applications, the volumetric capacity becomes more important than the gravimetric capacity. This is measured in charge per volume (mAh cm−3) and can be calculated from specific capacity, mass loading, and the thickness of the electrode. Industrial electrode fabrication commonly include a pressure rolling (calendaring) step to densify the electrode. Not only does this increase the electrode density and its volumetric capacity, but it also improves electrical conductivity and, quite often, electrode and battery cycle life. Commercial anodes based on graphite particles show volumetric capacities in the range from 400–470 mAh cm−3. Highly porous geometries are occasionally utilized to increase Li ion transport and reduce internal stresses within high-capacity anodes being studied, but they come at a tradeoff against volumetric capacity.

Thirdly, the charge/discharge rate of the electrode is critical for commercial viability, in particular to satisfy a strong market need for rapid charging of electronic devices and EVs. The parameter conventionally reported is the “C-rate” (we have already used such a notation in this section), related to the theoretical number of hours required for a full charge. For example, C/2 denotes a charge rate at which the electrode will be charged from 0 to full capacity in 2 h, while 2C denotes that the electrode is charged in 1/2 of an hour. We shall note, however, that battery cycling tests are often run galvanostatically with voltage limits, rather than capacity limits. This results in varying capacities, even from cycle to cycle, and current rates are often set experimentally in current per unit mass (mA g−1) or unit area (mA cm−2) of the electrode.

Cycle life and electrode potential are also of critical importance. For an electrode to be commercially viable, it must be able to sustain a stable capacity for at least 100–200 charge/discharge cycles for wearable computing and 300–1000 cycles for portable electronics. Full EVs and industrial applications require a cycle life on the order of thousands of cycles. The electrode's potential, measured in a half-cell with Li foil counter electrode during electrochemical testing, determines both safety and energy density characteristics. For anodes, lower potential results in higher voltage of a full Li-ion battery cell, which means higher energy density. On the other hand, if the electrode potential is brought too close to 0 V versus Li/Li+, it may lead to Li dendrite formation. Conversely, potential higher than 0.8–1.0 V versus Li/Li+ can prevent the electrochemical reduction of the electrolyte with the SEI formation, and can result in a long lasting anode with high rate capabilities, such as the lithium titanate (Li4Ti5O12, also called LTO) anode.

Coulombic efficiency (CE) (the ratio of delithiation to lithiation capacities) is an often overlooked but critical parameter when considering practical applications. When a full Li-ion cell is assembled, the Li ions are hosted in a cathode, as this configuration is stable in air. However, some of this Li is permanently lost to irreversible reactions on the anode side, such as electrolyte reduction and SEI formation.[2] Commercial graphite anodes with a specific surface area of a few m2 g−1 or less exhibit good first cycle CE on the order of 90–95% (first cycle irreversible losses of 5–10%). Furthermore, due to the low volume changes in graphite anode particles and high SEI stability, a graphite CE in subsequent cycles approaches 99.99% or even higher. However, the majority of testing equipment is not capable of measuring CE with such a high precision. As a result, CE is often estimated by the fade rate of a matched full cell with a very stable counter electrode having the same capacity loading. Many high-capacity nanostructured anode materials have very large specific surface area available for the SEI formation, which leads to low first-cycle CE. To compensate for the loss of Li made inaccessible by SEI formation, one will need to provide excess of Li in the cathode, which may reduce the cathode capacity available for subsequent cycling and thus limit the full cell energy density. Furthermore, many high-capacity anode materials suffer from volume expansion during battery cycling. Such volume changes may reduce SEI stability and cause continuous consumption of Li ions and rapid degradation of a full cell (where amount of Li in the cathode is limited). This SEI growth process is manifested by a low anode CE (<99.5%) when an anode is tested in half cells.

Other parameters of interest include the effect of temperature on charging kinetics and electrode stability. Qualitative features such as affordability and safety considerations should also be taken into account, particularly for large-scale applications where such issues become more problematic.

3 Material Parameters

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Considerations of Electrode Parameters
  5. 3 Material Parameters
  6. 4 Alloying Elements—Group IV
  7. 5 Conversion Electrodes
  8. 6 Alloying Elements—Others
  9. 7 Nanostructured Carbon
  10. 8 Impact of the Electrode or Particle Architecture
  11. 9 Conclusions and Outlook
  12. Acknowledgements
  13. Biographies

Many characteristics of the anodes are naturally determined by the characteristics of their active materials. Figure 1 shows a periodic table of elements which have previously been used to form Li alloy phases or as the metallic compound in substitution reactions, to our knowledge. Average values in the last 5 years are reported for the price of commodity metals from primary sources in their smelted form.[3] The theoretical capacities of the Li alloys with pure elements are also shown with respect to the volume of the fully lithiated phases and the mass of the fully delithiated phase (as conventional batteries are assembled with un-lithiated anodes). Volumetric capacity was calculated using density values from the Materials Project.[4] In the case of Al and Si, LiAl and Li15Si4 were assumed to be the maximum lithiated phased due to experimental evidence (Li21Si5 is only formed at high temperature).[5] Our values otherwise agree with the Materials Project.

image

Figure 1. Elements investigated for high-capacity Li-ion battery anode material applications. The above chart shows the elements and the average commodity cost of metals in USD/lb in the past 5 years. For Li alloying phases, the specific capacity of each element (mAh g−1) is shown above, per mass of unlithiated (Li-free) material, and the volumetric capacity (mAh cm−3) is shown below, per volume of fully lithiated material.[3-5] For conversion anodes, a metal-forming element of the conversion compounds (oxides, nitrides, phosphides, sulfides, antimonides, and others) is specified.

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The theoretical maximums for the capacities of the alloying phases can be significantly higher than that of graphitic carbon, but decrease at higher atomic numbers. In Groups IV and V, higher atomic mass imposes significant penalties on specific capacity, but very interestingly, the volumetric capacity remains largely the same. Transition metals and Group III elements do not appear to follow this trend. Elements lower in the table also tend to have higher conductivity.

Lithiation and delithiaton potentials of alloying electrodes are shown in Table 1. So called “average” potentials can be found from the Materials Project[4] website, but differ experimentally based on parameters such as mechanical strain and surface energy. Representative values of experimental lithiation/delithiation peaks have been gathered from literature for comparative purposes. Lithiation potentials generally increase to the right of the periodic table, as can be expected from the increase in electronegativity. Overpotentials required to drive the alloying reactions are on the order of 0.1 V, which are generally greater than that for intercalation electrodes.

Table 1. Major lithiation/delithiation peak potentials from representative experimental data for various pure elements and average potentials from the Battery Explorer,[156] the Materials Project[4]
ElementLithiation potential [V]Delithiation potential [V]DescriptionAverage Materials Project [V][4, 156]
C0.07, 0.10, 0.190.1, 0.14, 0.23Graphite (SFG6)[157] 
 0.08, 0.11, 0.200.11, 0.15, 0.22Graphite (SFG44)[157] 
Si0.07, 0.220.3, 0.49Hollow nanospheres[158]0.27
 0.05, 0.210.31, 0.47Nanoparticles[159] 
Ge0.2, 0.3, 0.50.5, 0.62Nanowires[160]0.41
Sn0.4, 0.57, 0.690.58, 0.7, 0.78<10 μm powder[61a]0.43
Pb   0.35
P0.6, 0.78, 0.80.99, 1.16Carbon composite[115e]0.93
As   0.95
Sb0.78, 0.830.99Carbon composite[127]0.84
Bi0.73, 0.780.85Thin film[161]0.71
Al0.190.45Thin film[140a]0.23
Ga0.25, 0.52, 0.820.25, 0.73, 0.90Prelithiated[142b]0.45
Zn0.06, 0.19, 0.210.16, 0.22, 0.25Carbon composite[146a]0.42
Ag0.04, 0.120.04, 0.1, 0.26, 0.38Nanoparticles[162]0.42
Mg0.050.24Li-Mg alloy[138] 

Li transport properties in the electrode materials are very difficult to measure, as we are mostly interested in the lithiated phase(s). Transport properties additionally change with the degree of lithiation and cycling history. Measurements are also complicated by the difficulty in obtaining an accurate measurement of the active area. Ideally, measurements should be made on flat thin-film samples, while preventing crack formation. Table 2 shows ranges of chemical diffusion coefficients in Li containing phases that form in Li-ion batteries. The numbers reported are obtained using the most reasonable and widely accepted values, where there is a choice.

Table 2. The chemical diffusion coefficient of Li in various lithiated phases at room temperature
Li alloyD [cm2 s−1]Reference
β-LiAl10−11–10−9[163]
LiC6 (along basal plane)10−7–10−6[164]
LiC6 (across basal plane)≈10−11[164]
LixC (graphite particles)10−11–10−7[149, 165]
LixSi (amorphous)10−13–10−11[9e],[16],[21]
LixGe10−12–10−10[166]
LixSn10−16–10−13[62]
Li2O (amorphous)5 × 10−12–5 × 10−10[167]

Combining such parameters with availability considerations, it is no mystery why Si and Sn have been investigated the most extensively. High theoretical capacity, high availability, acceptable conductivity, low toxicity, and low potential render these materials the seemingly optimal choices for Li-ion batteries. However, various issues still face the full utilization of such materials in Li-ion batteries, and therefore, no definitive conclusion can be made. As will be seen, many other materials also exhibit very favorable and interesting properties.

4 Alloying Elements—Group IV

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Considerations of Electrode Parameters
  5. 3 Material Parameters
  6. 4 Alloying Elements—Group IV
  7. 5 Conversion Electrodes
  8. 6 Alloying Elements—Others
  9. 7 Nanostructured Carbon
  10. 8 Impact of the Electrode or Particle Architecture
  11. 9 Conclusions and Outlook
  12. Acknowledgements
  13. Biographies

4.1 Silicon

Si is the most widely studied anode material, and entire reviews have been published on specific types of Si anodes.[6] This is partly due to the extremely large theoretical capacity of elemental Si. The practical limit of Si anode capacity is 3579 mAh g−1 in the pure phase.[7] Along with such impressive theoretical values, Si processing technology is also advanced due to the extensive Si use in a semiconductor (including photovoltaic) industry. Si anode research has thus developed at a rapid speed. The performance advantages and limitations of Si anodes as well as their operational and failure mechanisms have been studied extensively and the knowledge gained may serve as a basis to understand other types of alloying electrodes.

Upon lithiation, both amorphous Si (a-Si) and crystalline Si (c-Si) anodes first become amorphous and may later form a metastable Li15Si4 crystalline phase below ≈0.05 V versus Li.[8] If initially crystalline, Si will expand anistropically, primarily in the <110> direction as seen in Figure 2.[9] This initial lithiation is thought to be limited by the reaction rate at the interface between the crystalline Si and amorphous lithiated phases.[10] However, as the lithiated phase becomes thicker, the large volume change can cause GPa levels of stress to build up,[11] especially at high charge rates.[12] Nanostructures are therefore necessary to relieve the stress at the surfaces and provide necessary void space for expansion.[9f] Without such measures, stress will cause crack formation and limit capacity by inducing a large polarization. Ex situ experiments have shown that cracks become less frequent for c-Si pillars of diameters 240–360 nm diameter,[9c] and in situ experiments showed no cracks for particles below ≈150 nm in spite of the very high (minutes or less) lithiation rates.[13]

image

Figure 2. Lithiation of Si pillars, showing their anisotropic growth in the <110> directions. Reproduced with permission.[9a] Copyright 2011, American Chemical Society.

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In large dimensions or continuous films, a-Si initial material may be preferable over c-Si due to its isotropic lithiation and volume expansion. Experimental evidence agrees for thin-film experiments,[14] and for nanowires, cycle life was improved by lithiating a CVD deposited a-Si layer over the as-grown c-Si nanowires.[15] a-Si is rate limited by Li transport through the alloyed phase, assuming that the surface reaction is not hindered by a thick SEI.[16] SEI growth is facilitated by the formation of solvent-permeable defects and fractures in the SEI layer caused in turn by large-volume changes upon Li insertion and extraction.[10b]

Volume change in a-Si has been measured to proceed almost linearly with degree of lithiation, after about 0.8 Li per Si have been inserted.[17] Based on measurements of curvature in a thin-film anode, Si anodes have been observed to undergo an initially elastic buildup of stress before plastic flow can occur.[11] Independent measurements of the start of plastic flow and the start of linear volume expansion roughly align as shown in Figure 3, supporting this theory.[17b,[18]] The comparison shows volume expansion occurring after the start of plastic strain because volume measurements were done with a thicker film (210 nm versus 50 nm for stress measurement), causing greater stress buildup. Also, a linear volume change agrees with the densities of crystalline Li alloys with Group IV elements.[19] It should be noted, however, that mechanical properties are geometry dependent, and the onset of plastic flow also depends on the charge rate.[20]

image

Figure 3. Stresses and volume changes within Si thin films upon Li insertion: a) voltage profile and stress for 50 nm Si thin-film electrode, showing the transition from elastic to plastic strain; b) voltage profile and relative volume for 210 nm Si thin-film electrode. The start of volume change is abrupt, indicating the transition from elastic to plastic strain[17, 18]

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The chemical diffusion coefficient of lithiated Si is in the range of 10−13–10−11cm2 s−1.[9e,[21]] Such sluggish transport is associated with the breaking and forming of chemical bonds upon electrochemical alloying of Li with Si, and may cause a buildup of high Li concentration near the surface of Si. The high Li concentration, in turn, causes large stress and volume change, resulting in surface fracture similar to c-Si.[12, 20] Nanosized dimensions therefore prevent localized stress by reducing the diffusion length.[18] Geometry and charge rate also plays a role, as discussed earlier. Experimentally, even a 150-nm thick amorphous thin-film Si deposited on a rigid metal substrate and cycled at a C/2 (1800 mA g−1) rate has been observed to crack, although it did maintain contact with the substrate.[20]

In turn, Li mobility also changes with composition and stress. Therefore, Li mobility changes with time and space, depending on geometry and charge rate. The strain induced by high Li composition should aid diffusion,[22] while an overall PITT measurement using a polymer electrolyte to correct for crack formation in thin films suggest a slightly rising chemical diffusion coefficient with increase in Li content.[21b] The measurement of diffusion coefficients is further complicated by the limitations of various electrochemical methods used. Understanding this diffusion process is critical to understanding the operation of Si anodes. However, a detailed study is beyond the scope of this review.

Outside of the Si-active material, Li transport can also be limited by the SEI layer build-up. Structurally stable nanostructured electrodes may fail due to high impedance in a thick SEI layer that plug voids between individual particles, particularly if the electrode thickness and mass loading are reasonably large.[23] Thin electrodes with low mass and capacity loadings, in contrast, may show excellent stability in half cells because Li transport within the SEI-plugged anode may still be sufficiently fast to achieve high capacity utilization.

Although the SEI composition is largely controlled by the components in the electrolyte, the native oxide on Si also contributes a layer of the SEI closest to the active material. The oxide layer is irreversibly lithiated in the first cycle, and a slight decrease in CE is observed.[24] XPS (X-ray photoelectron spectroscopy) analysis shows that SiO2 decomposes to form SiOxFy if left to age in contact with standard LiPF6 and ethylene carbonate (EC)-based electrolyte. However, if a formation step is done immediately, LixSiOy is formed instead, with Li2O at the immediate interface between the active material and the SEI. An SEI layer composed of organic and fluorinated compounds, similar to what is found on graphite, then covers these inner layers.[25] As with standard graphite anodes, commercially available and widely used additives such as lithium bis(oxatlato)borate (LiBOB),[26] fluoroethylene carbonate (FEC),[27] and vinylene carbonate (VC)[28] reduce the SEI layer thickness, improve its stability, and greatly enhance overall performance of high-capacity anodes.

Finally, the bulk of the high-profile research conducted in the field of Si anodes has been in the design of fabrication methods. Various researchers have created nanostructures that are well spaced[29] and sized to allow good transport and prevent sintering[30] and cracking.[9c] Because of the large theoretical capacity of Si, this has been much more successful for Si than for any other material. In 1999, Si nanoparticles were used to assemble a high-capacity anode, although this degraded very quickly.[31] Later studies showed significantly improved stability of electrodes with Si nanoparticles,[32] some with over a thousand stable cycles.[33] High-rate capability has been demonstrated with Si anodes at low mass loadings. For example, a porous nanowire structure even achieved a 18 A g−1 rate at >1000 mAh g−1 for 250 cycles, at low loading of 0.3 mg cm−2.[34] Many other electrodes have also achieved cycle life on the order of 1000+ cycles.[33, 35]

Various carbon-silicon (C-Si) composite particles have been used to enhance Si anode performance. For example, 2D layers of graphite/Si hundreds of nm in thickness have been used to achieve 840 mAh g−1, 1.1 mg cm−2, C/3.4, and 100 cycles,[36] while C-coated Si nanoparticles [37] and porous Si-C composite spheres[38] achieved similar performance at higher rates. More complex electrodes have achieved high capacities and excellent stability at ultra-high mass and capacity loadings[39] without major compromises in other properties. Such results show that well-designed composite Si anodes with proper dimensions can, in principle, achieve most of the necessary characteristics of a well performing high-capacity anode. The remaining major challenge of Si and other high-capacity anodes is the low CE, which is due to their high specific surface area and insufficient SEI stability. How composite architecture and geometry may overcome this challenging problem will be discussed in a separate section at the end of this review.

Pre-lithiation of anodes before their assembling and/or cycling may reduce the impact of volume expansion, while also reducing first-cycle irreversible capacity loss. The concept has been utilized for other systems where irreversible capacity is an issue. Ma et al.[40] used this method to improve the capacity and cycle stability of Si. However, about 1/3 of the capacity was lost after 20 cycles, possibly due to large particle size (up to 1 μm in diameter) and insufficient electrode fabrication control combined with still significant volume changes within individual particles and the resulting SEI growth. In addition, due to their extremely reductive potential, Li-Si alloys are very reactive with air, particularly if produced as nanoparticles, and need to be handled with extreme care in an Ar-filled glovebox environment. The use of Ar increases the cost of battery fabrication significantly, which creates a major obstacle for the widespread adoption of lithiated anodes by industry.

Binders keep particles in electrical contact, and are critical to assembling high-quality electrodes, regardless of the particle size. Polyvinylidene fluoride (PVDF) is the standard binder used in Li-ion batteries, but has been shown to be inferior to carboxymethylcellulose (CMC), polyacrylic acid (PAA), and alginate, which have become the three binders of choice for Si anodes. All three of these binders (and their derivatives) have higher strength and elastic moduli than PVDF, and have carboxyl groups to form strong chemical bonds to the Si surface.[33, 41] In contrast to PAA, CMC, and alginate, PVDF also swells in electrolyte and experiences a dramatic reduction to its already inferior elastic modulus.[41b] While PVDF withstands greater strain before failure than CMC,[42] it becomes very soft and incapable of holding expanded Si particles in electrical contact upon delithiation and the resulting particle contraction. In addition, permeation of the solvent through PVDF leads to the formation of the SEI between the Si and the binder, which further weakens the binder–Si interface.[33]

Alternatively, the use of electrically conductive binders may improve the electrode's ability to maintain electrically conductive network between individual particles. Such polymer binders have been also shown to achieve high capacity and long cycle life in half cells, but tend to reduce CE in the initial cycles.[35c,d,[43]] Conversely, binder-less anodes have also been fabricated using 1D geometries,[44] and those utilizing carbon nanotubes have achieved perhaps the most promising results.[45] Carbon nanotubes can also be used to replace not only the binder and conductive additive but also the current collector, which comprises a much larger portion of the battery's volume and mass. This results in even greater energy density and is possible due to the high conductivity and strength of carbon nanotubes.[46] The various benefits of such a system are discussed in further detail later in this review.

In spite of the significant progress in the field of Si anodes, no commercial anodes are currently based on Si or Si-containing composites, except in a few very recent designs utilizing a small quantity of small carbon-coated silicon oxide (SiOx)-containing particles added to graphite anodes to slightly increase their gravimetric capacity or pulse-power performance.[47] The authors are optimistic, though, and believe that Si-based anode technology will likely be commercialized within the next 2–4 years. This will require further technology development and overcoming the main challenge of maintaining a constant particle volume and size, and doing so with particles synthesized on a large commercial scale.

4.2 Germanium

Despite its very high cost (prohibitively high in most applications), Ge has recently received increasing interest due to its superior conductivity and Li diffusivity compared with Si. Interestingly, Ge lithiation can result in intermediate lithiated crystalline phases[48] until Li15Ge4 is ultimately formed,[49] while its delithiation results in the formation of a porous amorphous phase.[50] Unlike Si, the entire process is isotropic and does not lead to crack formation, even at a high rate and particle size of ≈620 nm.[50c,[51]] An in situ study by Liang et al. exemplifies this difference by comparing the lithiation process of similarly sized particles (Figure 4). Furthermore, thin-film experiments have shown reversible lithiation at rates as high as 1000 C.[48a] A recent paper analyzing Si–Ge alloys aptly demonstrates Ge's superior rate capabilities (Figure 5), showing the gradually increasing rate capability as the transition is made from Si to Ge.[52]

image

Figure 4. In situ lithiation of Si and Ge: a–c) Initial lithiation and crack formation of a crystalline Si particle, d–f) Initial lithiation without crack formation of a crystalline Ge particle. Both particles are ≈160 nm in diameter. Reproduced with permission.[51] Copyright 2013, American Chemical Society.

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image

Figure 5. Impact of Ge content on the rate performance of Si–Ge alloy Li-ion battery anode. Reproduced with permission.[52] Copyright 2013, American Chemical Society.

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The properties of Ge allow for using particle sizes larger than that of Si. This is a major advantage over Si, as nanoscale particles and the resulting high specific surface area result in the formation of excessive SEI, resulting in the build-up of irreversible capacity and impedance. Indeed, even 500 nm Ge particles have been reported to exhibit stable cycle life, albeit at 150 mA g−1 rate and 800 mAh g−1 capacity.[53] 200 nm sheets of Ge on graphene have been shown to exhibit <20% loss in capacity over 400 cycles, at nearly theoretical capacity although with only 45 wt% Ge.[54] However, to date, most studies have focused on nano-Ge with dimensions often <100 nm,[55] and even as small as 3 nm.[56] This suggests that the size of Ge particles has yet to be fully optimized. Nevertheless, truly impressive rate capability has been achieved, and even 40 C rate capability has been reported after 400 cycles at 1000 mAh g−1 and 15 mg cm−2 mass loading with a LiFePO4 cathode as a counter electrode.[50b]

4.3 Tin

Sn anode research is different from Si and Ge anodes in that much focus has been placed on oxides, which showed significantly improved cycle stability. In the fully lithiated state, Sn forms Li22Sn5, which places its theoretical limit at 990 mAh gSn−1. While Sn specific capacity is significantly inferior to Si, its volumetric capacity is comparable (Figure 1). However, the theoretical capacity of pure Sn has not been achieved with stable cycling, possibly due to the brittleness of the fully lithiated phase.[57] A recent ex situ study showed that 10 nm particles decrease in size and increase in size distribution after lithiation and delithiation, indicating that fractures occur even at such small particle sizes.[58] Indeed, embedded Sn anodes have shown slightly higher stable capacities,[59] and tin oxide has been suggested to be superior very early on due to the oxide phase “gluing” or holding the Sn grains together.[60]

One would assume that the change in properties would be continuous from Si to Ge to Sn. However, the Sn lithiation process follows the formation of crystallographic phases (also see Figure 6):[61]

  • display math(1)
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Figure 6. Formation of crystalline phases upon electrochemical Li insertion to and extraction from a Sn anode. Reproduced with permission.[61a] Copyright 1998, American Physical Society.

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Sn is different from Ge and Si in that it has an extremely low melting point, so much that room temperature is already 60% of the melting point. Sn atoms are therefore much more mobile than in Ge and Si, and crystallize more easily. Crystallization helps to explain the easy fracturing of lithiated Sn, as well as the 5–10 orders of magnitude difference in the measured chemical diffusion coefficient of Li between electrochemically[62] and chemically[63] lithiated Li-Sn alloys. In the former case, the brittle Li17Sn4 phase easily forms on exterior regions of a particle. In the latter case, chemically lithiated Sn most likely has large grain size and a uniform degree of lithiation. Taking such properties into consideration, Sn anodes appear to require composite structures to help maintain electrical contact and Li diffusivity.

Tin oxide composites received heightened attention in the late 1990s due to its commercial development by Fujifilm and a high-profile publication by Idota et al.[64] Based on their achieved capacities, many researchers have traditionally viewed tin oxide composites to form an electrochemically inert irreversible Li2O, which hold together Sn phase regions.[60, 65] Such claims were bolstered by the observation that Sn-phase regions could be observed to lithiate/delithiate by in situ XRD.[60, 66] However, more recent advanced nanostructures have improved uniform electron transport and surpassed the theoretical capacity of SnO2 by reversing the Li2O formation reaction, although at a higher Li extraction potentials.[67] This reaction is believed to lithiate at ≈0.8 V and delithiate at ≈1.3 V,[65a,67b,67e] with extremely broad peaks.[68] Unfortunately, as the overall cell voltage is reduced, this extra capacity is generally of lower value. Tin oxide anodes are also believed to degrade due to the coarsening of Sn particles,[65a,[69]] and oxidizing the Li2O phase may accelerate this process. Even so, Zhou et al.[67g] have reported high capacity of 1000–1400 mAh g−1, good cycle life of 500 cycles (at a high mass loading of ≈10 mg cm−2), and good rate capability, although at the expense of low CE due to the use of high-surface area graphene oxide.[2a,[70]]

Lastly, a review of Sn anodes would not be complete without visiting the Sn-Co-C anodes used in the Nexelion battery initially commercialized and advertised by Sony. Although the battery had a relatively short life, it boasted a 20% improvement in overall volumetric capacity.[71] Lithiated Sn–Co alloys may at first form a Li-Sn-Co phase, but eventually separates into two phases to form a new Li-Sn phase.[72] Then, if sufficient Co is present (20% is sufficient), an amorphous Co phase can be formed as well.[73] When the anode is delithiated, the Sn then re-alloys with the Co instead of forming its own Sn phase.[74] This process is believed to be the cause of the superior cycle life of Sn–Co alloys over pure Sn, perhaps preventing fracture. Other Sn-transition metal alloys also appear to have similar properties.[75]

4.4 Lead

Perhaps due to toxicity and low specific capacity, studies in Pb alloying Li-ion batteries have been limited. However, high volumetric capacity and high availability make Pb an attractive material. To date, studies of Pb have been mostly limited to oxides, which show relatively low performance in terms of specific capacity,[76] although they were improved by the use of low mass loading[77] and carbon composites.[78] More recently, a SiC-Pb-C composite was constructed, and achieved higher performance than the former oxides.[79] Even so, the cyclable specific capacity achieved is still under half of its theoretical value, and further studies are necessary to give a fair evaluation of this material.

5 Conversion Electrodes

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Considerations of Electrode Parameters
  5. 3 Material Parameters
  6. 4 Alloying Elements—Group IV
  7. 5 Conversion Electrodes
  8. 6 Alloying Elements—Others
  9. 7 Nanostructured Carbon
  10. 8 Impact of the Electrode or Particle Architecture
  11. 9 Conclusions and Outlook
  12. Acknowledgements
  13. Biographies

Conversion electrodes can have specific capacities much higher than graphitic carbon through reactions generalized by:[80]

  • display math(2)

Often times, M is a transition metal, while X is an anion (most commonly O in the case of anodes). The definition can be broadened to cases where M is an alloying metal such a Group IV intermetallic, but the mechanism is unique in that it can combine abundant but non-alloying transition metals with also abundant but gaseous non-metals (O, N, F) to create new high-capacity solid electrode materials. In general, this concept opens up the possibility to many new material combinations, which could potentially serve as high-capacity Li-ion electrode materials. An in depth review by several prolific researchers in the field highlights a multitude of elemental combinations tried for such types of electrodes.[80]

However, the Achilles' heel of conversion electrodes is the extremely large overpotential generally required to drive the reaction. A simple comparison of conversion electrodes with cobalt by Oumellal et al.[81] shows a trend in overpotentials in the order of fluroide > oxide > sulfide > nitride > phosphide (> hydride) (the hydride used for comparison is MgH, not CoH). The authors have generally observed the same trend, as seen in Figure 7. However, it should be noted that polarization is also heavily influenced by electrode construction, and that all of the electrodes exhibit overpotentials on the order of 0.1–1 V.

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Figure 7. Voltage profile hysteresis for selected conversion-type anodes: difference between charge and discharge curves for similar cycle conditions: a) NiO,[168] Ni3N,[169] Ni2P,[170] NiP2[171] and b) Co3O4,[172] Co3N,[173] CoP[174]

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Cabana et al.[80] point to high temperature[82] and nanoscale[83] studies to propose that this overpotential originates not only in the kinetics but also the thermodynamics of the system. Indeed, the electrode potentials are known to be affected by the cycling history of the electrode. FeF3 conversion cathodes have been observed to have a 280 mV hysteresis even after 72 h of rest,[84] or about 1/4 of the overpotential observed at C/60. An ab-initio calculation suggests that for FeF3 electrodes, this is due to differing reaction pathways during lithiation/delithiation caused by the disparity in Li and Fe diffusion rates.[85] Thus thermodynamic causes of overpotential should not be neglected.

In a practical sense, a large overpotential has serious consequences to the capacity and energy efficiency of a Li-ion battery. Conversion anodes are often tested in a large voltage range versus Li, reducing the voltage difference between the anode and cathode during discharge and thereby reducing the overall energy density and efficiency of the battery. Also, the large required overpotential results in a large first-cycle irreversible capacity, as not all the LinX can be oxidized in the anodic scan. The energy density of the battery is therefore reduced in both voltage and capacity, raising a critical issue for the viability of such electrodes.

Creative attempts have been made to address the issue of irreversible capacity.[86] Each of these strategies can overcome the irreversible capacity, but raise additional safety considerations. Chemical pre-lithiation is one option, but as with Si, this can result in a highly reactive material, which is difficult to handle. Addition of sacrificial salts to the cathode has also been proposed, but results in gas evolution.[87] FMC Corporation has developed a stabilized Li powder, which can be processed in dry air, but requires a change in slurry solvent.[88]

None of these requirements are necessarily prohibitive with further development. However, they have yet to result in well-performing high-capacity anodes. The issues of large required overpotential and irreversible capacity of conversion anodes remain unsolved, and require further research and development to produce a viable high-capacity anode. Furthermore, it should be noted that these problems are in addition to the problems alloying electrodes face with irreversible capacity losses resulting from high surface area and SEI instability.

5.1 Oxides

Depending on the current density and particle size, Fe2O3[89] and Co3O4[90] have been observed to form ternary lithiated phases during the initial lithiation. (TiO2[91] also forms a ternary phase, but this is a special case and occurs even at high rates.[92]) As lithiation proceeds, a metal phase then precipitates in a sea of Li2O for all oxides. The transition metal precipitates are of nanometer dimensions and are also separated only by a few nm of Li2O.[93] Often times, this initial lithiation step is different from the latter cycles due to kinetics. Delithiation often results in a different, more kinetically favorable metal oxide phase, perhaps due to the large polarization. For example, cobalt oxides have been observed to convert to CoO.[90, 94] Similarly copper oxides can reach the 2+ oxidation state at high voltages,[93b] but tend to form a relatively amorphous Cu2O phase.[95]

To be exact, however, particle sizes and separation distances in the lithiated electrode are dependent on transport properties. For instance, Sn phases are observed by in situ transmission electron microscopy (TEM) to extrude from the surface of the Li2O phase and form particles on the order of 100 nm if lithiation proceeds at a slow rate.[69b] This indicates that Sn has a high mobility in the Li2O/SnO phases when compared with transition metals. Kinetically, Li2O and metal oxide phases are not ideal for lithium/electron/hole transport, and changes in the lithiation/delithiation mechanism can be observed for different rates, electrode constructions, and temperatures.[90]

The nanoscale dimensions,[83a] conductive additives, and the choice of the metal ion are therefore critical in reducing the polarization but do not solve the issue. MnO, for example, has been shown to have overall hysteresis between lithiation/delithiation of <1 V in a thin-film study, which is fairly small for a transition metal oxide.[96] However, this overpotential increases rapidly with increasing current density,[97] and even high surface area electrodes with dimensions <10 nm do not significantly reduce this effect.[98] High overpotentials have also been seen for nanoparticles of iron oxide[99] and cobalt oxide.[100]

Electrode polarization is also seen to suddenly increase at the end of delithiation in MnO galvanostatic intermittent titration technique (GITT) studies.[[96],98a,[101]] Advanced composites with carbon have shown to remove this sudden increase in polarization.[98a,101b] Meanwhile, favorable electronic conductivity is observed for the fully lithiated Ni/Li2O and Fe/Li2O matrices, in comparison to their corresponding metal oxide phase,[102] and a NiO/graphene composite has also been observed not to show a sudden increase in polarization.[103] All this evidence suggests that the sudden jump in polarization seen in MnO is due to its low electrical conductivity, and that this may be common to other metal oxides as well. Also, the fact that this polarization only occurs during the delithiation step indicates a different mechanism in lithiation versus delithiation, as has been predicted for the Li-Fe-F system.[85]

First principles calculations have also been attempted to understand the voltage hysteresis in oxide electrodes, but are limited to interfacial effects.[104] Asymmetric interfacial polarization has been suggested to be a cause of the asymmetric polarization in lithiation versus delithiation.[104a] However, these studies do not include mechanical stress or Li/electrical conductivity effects. As such properties become better known, it will be interesting to see the root causes of the large overpotential required to cycle conversion oxide anodes.

5.2 Nitrides

Nitrides are much less studied than oxides, but are interesting as Li3N is known to be a Li ion conductor,[105] as is true of all Group V elements.[106] In many cases, nitrides can have high capacity but require a large voltage range as with oxide anodes, even in the thin film[107] or nanoparticle[108] electrodes. However, exceptions exist.[109]

In particular, chemically pre-lithiated lithium transition metal nitrides/pnictides have been known to have attractive properties for a variety of applications,[105b,[110]] including their application as anodes in Li-ion batteries. If the voltage range is limited, such electrodes can potentially operate as an intercalation electrode system with low polarization and good Li transport properties.[111] However, it is unlikely that long-range order can be maintained for a high-capacity anode. The highest performance has so far been achieved with Fe, Co, Ni, and/or Cu as the transition metal, and result in a loss of long-range structure.[112] In particular, Shodai et al.[112c] achieved 750–950 mAh g−1 (>1500 mAh cm−3) for 50 cycles at 0.5 mAcm−2 in the 0–1.4 V range.

5.3 Phosphides and Phosphorus

Phosphorus is an extremely intriguing material due to its position in the periodic table. It is a Group V element and therefore may benefit from the improved Li conductivity, while it is also in Period III, allowing it to have multiple valencies and have sufficient atomic mass to be in the solid state at room temperature. Thus, it has been suggested that the phosphide ion plays a more critical role in the redox reactions governing phosphorus-based anodes.[113] Indeed, phosphide anodes generally have similarly positioned voltammetry peaks to that of phosphorus anodes, while Sn4P3 has been shown to form LiP as an intermediate before Li3P by NMR.[114]

The major reversible lithiation peaks for phosphorus occur in the 0.6–0.9 V range and ≈0.1 V, while major delithiation peaks occur in the 1.0–1.2 V range.[115] Black phosphorus is generally thought to be the best allotrope for phosphorus anodes, as lithiation is hypothesized to proceed via an intercalation-type mechanism.[115a,115d] However, a reaction mechanism via the formation of various LixP phases has also been proposed.[115d] In either case, the final product of full lithiation is Li3P.[115a-d] Phosphide anodes are also generally observed to form Li3P at the end of lithiation, and almost always exhibit voltammetry peaks in similar positions.

However, computational studies for LixMPn4 (M = V, Ti; Pn = P, As) suggest that for Group V elements, the covalent nature of the metal-pnictide bond allows lithiation without destruction of short-range order.[116] In other words, LixP phases are not necessarily formed immediately on lithiation of the active material. The unit cell is also calculated to only expand by several percent for such a mechanism, while the localization of electrons on the metal-pnictide bonds allows good transport for Li. Furthermore, the strong covalent bonds also allow for a much larger variation in Li content than standard intercalation metal oxide electrodes.

Experimental evidence shows up to 9 Li can be lithiated/delithiated per transition metal in such structures.[116b] VP4 has also been shown to never form metallic V by X-ray absorption near edge structure (XANES).[117] Although Li3P does still form, greater than 1000 mAh g−1 of reversible capacity has been reported for the first two cycles. MnP4 has also been observed to have such ternary structures, and GITT has shown the thermodynamic voltage difference between lithiation/delithiation to be only 0.03 V.[118] A TiP2/P mixture has also been seen to form Li10.5TiP4 upon full lithiation and have stable cycling at 600 mAh g−1 at C/12.[119] Meanwhile, other phosphides, such as MnP,[120] FePx (x = 1,2,4),[121] Cu3P,[122] and Ni5P4[123] have also been suggested to exhibit ternary phases in both the lithiation and delithiation processes, based on varying degrees of experimental evidence. Chemically pre-lithiated Li2CuP has been observed by X-ray diffraction (XRD) to regain its original crystalline structure even after full delithiation.[124] Ni2P does not exhibit ternary phases, but XRD spectra show intermediate Ni12P5 and Ni5P2 phases.[125]

There is a plenty of evidence to suggest that the strong covalent transition metal phosphide bond could possibly result in a high-capacity anode with stable cycle stability, low polarization, and low-volume changes. Even so, electrodes assembled thus far have generally required a low rate or large voltage range in order to achieve higher gravimetric capacity than graphite. To our knowledge, the best lithium metal phosphide anode constructed so far to take advantage of a stable ternary structure (i.e., no Li3P formation) is a Li5.5Mn2.5P4 anode by Gillot et al.,[126] which achieved 700–1000 mAh g−1 for 30 cycles at ≈C/15.

The best performing phosphorus-based anode to the knowledge of the authors is an amorphous phosphorus/carbon composite anode, which had 1950–2200 mAh g−1 for 100 cycles at 1000 mA g−1 current density and a mass loading of 3 mg cm−2, measured per mass of P (Figure 8).[115e] Although approximately half of the electrode was composed of carbon or binder material, this is an example of some of the very best anodes described in this review. The specific capacity is also very close to the theoretical limit of phosphorus and is only achievable due to the absence of relatively massive transition metals in the electrode.

image

Figure 8. Electrochemical properties of the amorphous P/C composites: a) cyclic voltammetry curve recorded at a scan rate of 0.02 mV s−1; b) Charge–discharge profiles at various current densities from 250 mA g−1 to 8000 mA g−1; c) Cycling performances at very high rates. Mass is calculated per mass of P, which was 56% of the total electrode mass. Reproduced with permission.[115e] Copyright 2012, Royal Society of Chemistry.

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Finally, the safety characteristics of phosphorus should also be taken into consideration. Alkali and alkali earth metal phosphides are known to chemically react to form phosphine, a highly flammable and toxic gas. Considering the safety concerns already surrounding Li-ion batteries, phosphorus-based anodes may require additional safety developments for consumer applications.

5.4 Arsenides, Arsenic, Antimonides, Antimony, Bismides, and Bismuth

As-, Sb-, and Bi-based anode materials have several major disadvantages. First, As and Sb are both known to be dangerous carcinogens, while Bi is flammable. Second, they are not very abundant, especially when compared to lighter elements. Their higher atomic mass decreases their specific capacity as well.

Even so, Pnictogens have intriguing Li transport properties, as discussed earlier with nitride and phosphide anodes. Furthermore, As, Sb, and Bi have higher electronic conductivity than lighter Group V elements. At the very least, As-, Sb-, and Bi-based anodes can serve as model cases for a generalized analysis of Pnictogens.

Sb-based anodes appear to have a similar set of characteristics as P-based anodes discussed in the previous section. Pure Sb appears to lithiate to Li2Sb, then Li3Sb.[127] Antimonides sometimes appear to reversibly form ternary phases,[128] and initially high specific capacity (≈700–800 mAh g−1) has been reported for CoSb3.[129] Similar capacity has also been reported for antimonides forming binary phases in which both elements can individually alloy with Li.[130] To date, the best performance has been reported from TiSnSb, which achieved ≈550 mAh g−1 for 240 cycles at 4 C.[131]

Literature on As and Bi is less common, but also appear to follow the same trend. For As, 1000 mAh g−1 initial capacity has been reported.[132] FeAs has also been tried, but has so far resulted in much lower capacity[133] and rate capability.[116a] Bi clearly has a two-step lithiation in which LiBi is formed before Li3Bi,[134] but it is only interesting due to its high (relative to graphite) volumetric capacity. As of now, experimental evidence has not shown electronic conductivity to be a major advantage as carbon composites have been necessary for all three elements.

5.5 Hydrides

In 2008, Oumellal et al.[23] showed MgH2 to be a high-capacity anode material. Greater than 500 mAh g−1 were reported for 50 cycles with mass loadings of 7–10 mg cm−2, although at a very slow rate. Furthermore, the theoretical capacity of MgH2 is 2037 mAh g−1, and the material exhibits a much smaller potential difference between lithiation/delithiation than most other substitution electrode materials. Several publications have followed this initial publication, confirming that metal hydrides do indeed react to form a metal phase and, most likely, a LiH phase.[135] However, reversible capacity has not been reported for other materials. Metal hydride electrodes also evolve hydrogen at high temperatures, which may trigger safety concerns.

6 Alloying Elements—Others

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Considerations of Electrode Parameters
  5. 3 Material Parameters
  6. 4 Alloying Elements—Group IV
  7. 5 Conversion Electrodes
  8. 6 Alloying Elements—Others
  9. 7 Nanostructured Carbon
  10. 8 Impact of the Electrode or Particle Architecture
  11. 9 Conclusions and Outlook
  12. Acknowledgements
  13. Biographies

6.1 Magnesium

Only several publications exist on the performance of Mg anodes for Li-ion batteries. Although the phase diagram for the Li-Mg system shows no known binary phases, there is high solubility for either element in the other phase.[136] Unfortunately, most of the solubility is for Mg in the Li phase. At 25 °C, up to ≈15% (atom) of Li is predicted to be soluble in the Mg phase (based on high temperature data), which corresponds to only ≈195 mAh g−1. Although extremely high initial capacity has been reported for Mg by Sohn and co-workers, this has very sluggish kinetics and the mechanism of the lithiation is unknown.[137] A separate study conducted CV at 1 mV s−1, and showed that negative current persists even after the anodic scan has begun, again suggesting slow kinetics.[138] In our own studies, we achieved specific capacity of over 450 mAh g−1 for ≈100 cycles, which does exceed the solubility limit of Li in Mg and may suggest a reversible formation of Mg-containing Li phase. However, it does appear that Mg has sluggish kinetics, possibly due to more resistive SEI, and this issue must be addressed for practical applications.

6.2 Aluminum

Al anodes have been long investigated due to their low potential versus Li, high capacity, low cost, and abundance, but did not receive much attention due to it slow performance. Table 2 shows that the lithiated phase of Al has a higher diffusion coefficient than Si, indicating that the kinetics of the Al system should be superior to that of Si. Yet despite all its advantages, Al anodes fail due to pulverization, even for a nanowire ≈50 nm in diameter.[139]Figure 9 compares this in in situ TEM study with that of a Ge nanowire, which stays intact despite being of greater diameter.[50c] Any attempt to utilize Al as an active anode material should therefore encapsulate or embed the Al in order to maintain electrical contact. The native oxide may assist in this process, as it is lithiated first, and forms an irreversible sheath, which does not break even when the Al is fully lithiated. The Al2O3 layer is clearly beneficial to the performance of the electrode and can be applied to other electrode materials as well. However, at the present, studies have not shown stable reversibly cycling using Al as an active anode material.[140]

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Figure 9. Impact of the Li alloying element of a nanowire on its structural integrity (resistance to pulverization) during electrochemical Li insertion and extraction: a) delithiated Al nanowire, initially 40 nm in diameter, showing a stable Li-Al-O sheath around the pulverized Al nanoparticles;[139] b) delithiated Ge nanowire, initially 40–125 nm in diameter, showing formation of pores but no mechanical disintegration (no pulverization).[50c] Reproduced with permission.[50c,[139]] Copyright 2011, American Chemical Society.

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6.3 Gallium

Despite its high price, Gallium is uniquely interesting as an anode material due to its very low melting point (29.8 °C) in its pure form.[141] It has been hypothesized that Ga could possibly be used as a “self-healing” anode. Even if a fracture forms within the anode during lithiation, it could be electrochemically melted by delithiation as long as a fracture does not induce a loss of an electrical contact and prevent electrochemical reactions. Ga also has a high theoretical specific and volumetric capacity under 1 V versus Li.

Lithiation/delithiation of Ga near room temperature proceeds via the reversible formation of crystalline phases in the order Ga[LEFT RIGHT ARROW]Li2Ga7[LEFT RIGHT ARROW]LiGa[LEFT RIGHT ARROW]Li2Ga.[142] Ga alloys with Cu,[143] which improves the delithiation rate, but reduces the lithiation rate.[144] Studies attempting to assemble a conventional electrode with Ga have prelithiated[142b] or encapsulated it in carbon,[[145]] but have yet to show high-capacity stable cycling.

6.4 Zinc

Lithiation/delithiation of Zn proceeds through the formation of crystalline phases of its many line compounds, ultimately forming LiZn.[146] The theoretical capacity of Zn, calculated based on the LiZn, indicates that Zn has a relatively high volumetric capacity. Relative abundance and high availability also make it a favorable choice. However, electrochemical results indicate that Zn has low rate capability, and stable cycling has yet to be achieved.[146a,[147]]

7 Nanostructured Carbon

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Considerations of Electrode Parameters
  5. 3 Material Parameters
  6. 4 Alloying Elements—Group IV
  7. 5 Conversion Electrodes
  8. 6 Alloying Elements—Others
  9. 7 Nanostructured Carbon
  10. 8 Impact of the Electrode or Particle Architecture
  11. 9 Conclusions and Outlook
  12. Acknowledgements
  13. Biographies

Numerous publications have reported high capacities for nanostructured carbon material.[148] However, the mechanism for the high capacity is still unclear[149] and the results may potentially be erroneous. Furthermore, high-capacity carbon anodes commonly suffer from high irreversible capacity, which may be due to their high specific surface area, surface functional groups, and water adsorption in nanopores.[2a] These electrodes further suffer from low density and thus offer limited volumetric capacities. One of the best performances reported on nanostructured carbon anodes, 1100 mAh g−1 under 2 V after 50 cycles at C/3 rate with a mass loading of 0.65 mg cm−2, was achieved by the Al2O3-coated carbon nanotubes.[150] This electrode also reported good rate capability, achieving similar capacity at C/1 rate. As with other high-capacity carbon anodes, the first-cycle CE for this electrode was ≈50%. In our own studies, we have never observed reversible delithiation specific capacity of any pure carbon materials (porous or not, with or without external coatings) to exceed that of the graphite, when tested in half cells in the voltage range of 0.01–1 V versus Li/Li+. Furthermore, the volumetric capacity of various carbons we tested was always significantly inferior to that of the graphite. We, therefore, remain rather skeptical regarding promises of pure nanostructured carbons for high energy density cells.

8 Impact of the Electrode or Particle Architecture

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Considerations of Electrode Parameters
  5. 3 Material Parameters
  6. 4 Alloying Elements—Group IV
  7. 5 Conversion Electrodes
  8. 6 Alloying Elements—Others
  9. 7 Nanostructured Carbon
  10. 8 Impact of the Electrode or Particle Architecture
  11. 9 Conclusions and Outlook
  12. Acknowledgements
  13. Biographies

Several recent innovations in the composite particle synthesis may allow one either to overcome the key limitations of high-capacity anode powders or to increase the energy density of an energy storage system on a full cell or a full system level by using various chemistries of active materials, including conventional intercalation chemistry.

As previously discussed, one of the main challenges in the successful development of high-capacity anode technology is achieving small changes in the size and volume of the high-capacity anode particles or, at least, achieving small changes in the surface area of these particles in direct contact with the electrolyte. Electrolyte decomposition on the surface of low-potential anode powder results in SEI formation. Repeated expansion and contraction of the high-capacity anode particles, such as Si, results in the continuous formation of solvent-permeable defects within the SEI, which leads to undesirable SEI growth (Figure 10a). Hertzberg et al.[39d] were the first to offer a solution that combats this problem by designing porous composite particles that experience little changes in dimensions during charge and discharge. In their initial design and a proof-of-concept study, they used cylindrical particle geometry of a thin Si tube confined by a Li-ion permeable carbon shell layer and directly attached to a current collector (Figure 10b–d). During insertion of Li, the rigid carbon shell prevents the inner tube's outward expansion, forcing the Si–Li alloy material to expand inward (Figure 10b). If the bonding between the Si-Li alloy and the outer C shell surface is weak, a partial delamination of the inner Si tube may take place (Figure 10b). Interestingly, such a partial delamination was found to preserve an electrical contact between the Si and the outer C shell. A very stable performance (over 300 cycles, Figure 10e) was achieved at a high (for Si anodes) mass loading of 5–8 mg cm−2. The size-preserving external rigid carbon shell allowed for the very high value of the CE (in excess of 99.9% for cycles 50–300) to be achieved. Several groups world-wide followed up with this idea and developed their own versions of porous Si particles with shape-preserving shells,[151] although tested at significantly smaller (sometimes as small as 0.02-0.1 mg cm−2) anode mass loadings.

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Figure 10. Schematic use of an external shape-preserving shell around porous high-capacity particles for SEI stabilization: a) schematic of the SEI growth cause by the volume changes during charge and discharge, b) schematic of a stable SEI formation in the case of a shape-preserving particle, c,d) SEM images of the Si-in-C tubes utilized in the original experiments and e) cycle stability of the produced electrode shown in (d) Reproduced with permission.[175] Copyright 2010, American Chemical Society.

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Three-dimensional (3D) particles composed of 2D layers, first proposed by Evanoff et al.,[39b] may offer an alternative solution for the highly needed SEI stabilization. Previous studies on thin-film anodes[35b,[152]] by other groups have shown that during Li insertion into high-capacity (such as Si) thin films, the volume changes can be accommodated via changes in film thickness. Therefore, the changes in the external surface area during cycling can be minimized and, thus, formation of a stable SEI could be achieved. The authors utilized exfoliated graphite (multi-walled graphene) as a carbon scaffold for the deposition of high-capacity active material films. While only Si-based particles were demonstrated, the proposed morphology is applicable for a variety of other high-capacity materials.

Multifunctional materials capable of providing energy storage coupled with a load-bearing ability may reduce the overall mass and volume of the equipment or system even if traditional intercalation-type battery materials are utilized.[153] In a recent report, Evanoff et al.[46b] showed the opportunity to develop flexible load-bearing anodes exhibiting both good mechanical properties and high specific and volumetric capacities (Figure 11). In their study, researchers coated a strong and flexible MWCNT current collector (Figure 11a–d) with a thin uniform layer of active material (such as Si). Compared with an electrode composed of graphite coated on a heavy Cu foil current collector, the new anode design offered over 2.5 times higher specific capacity (Figure 11f).[46b] More importantly, the anodes demonstrated high strength (Figure 11g,h) with specific strength higher than that of many structural materials, including Cu, cast iron, and even selected Al alloys and structural steels (Figure 11h).[46b] A study using standard electrode materials by Wang et al. shows that the load-bearing improvements are further compounded by the superior adhesion of the electrode to carbon materials.[46c] Further development of multifunctional electrode technology may be attractive for space and mass-sensitive (and less cost-sensitive) applications, such as aerospace.

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Figure 11. Multifunctional load-bearing flexible battery technology: a) schematic of a flexible battery with MWCNT fabric current collectors; b–e) SEM and optical images of the CNT and Si-coated MWCNT fabrics; f) cycle stability of the Si-MWCNT fabric; g) a typical stress–strain curve of the Si-MWCNT fabric and h) comparison of its specific strength with that of selected structural materials. Reproduced with permission.[46b] Copyright 2012, American Chemical Society.

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Finally, the development of new routes to increase the thickness of electrodes effectively reduces the volume and weight fraction of inactive battery components, such as electrolyte-filled separator and metal current collector foils. Achieving high-electrode mass loadings (calendered thickness of each side of an electrode above 90–100 μm) using conventional slurry-based processing is challenging. Thick slurry-produced electrodes commonly suffer from poor control over the resulting porosity, inhomogeneity,[154] tortuous electrolyte diffusion paths, and high electrical and, more importantly, high thermal resistance caused by point contacts between individual particles. These issues become particularly critical for nanopowder-based electrodes. In addition, thicker electrodes often suffer from the formation of cracks during the slurry drying process. This is caused by the uneven solvent evaporation (drying from the top), poor adhesion to the metal current collectors, and brittle behavior of thick electrodes. As a result, the overall progress on the development of thick electrodes has been slow,[154] although leading Japanese and Korean manufacturers are working in this direction. Several companies are also trying to develop disruptive dry powder electrode processing techniques to avoid the use of a binder solvent and various issues associated with the time and cost consuming electrode drying step. However, these novel technologies are not yet commercially available. One recent publication proposed the formation of thick electrodes using tall (0.5–5 mm) vertically aligned MWCNT arrays (Figure 12),[45b] which can be uniformly coated with active material and compressed in a cell to minimize the fraction of the remaining porosity. The recent developments in the MWCNT growth[155] allow low-cost formation of such long MWCNTs within minutes. Such architecture may offer unique benefits, such as straight and aligned pores for rapid ion transport, high thermal and electrical conductivity for long cycle life (since most degradation processes are thermally activated), high uniformity and control over the dimensions of individual coated CNTs for predictable and reproducible performance, very smooth electrode (which allows for a safer use of thinner separator membranes), and the absence of the binder for higher capacity. Evanoff et al. used Si as an active material (Figure 12a) and demonstrated good stability in spite of the extreme electrode thickness (Figure 12b). Moreover, the electrodes did indeed demonstrate several orders of magnitude higher electrical (Figure 12c) and thermal (Figure 12d) conductivities compared with fully dense electrodes of similar material composition.

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Figure 12. Vertically aligned MWCNT-based thick electrode technology; a) SEM images of the Si-coated MWCNT electrodes, b) their cycle stability as well as c) resistivity and d) thermal conductivities in comparison with that of conventionally produced electrodes after high-pressure compaction [45b]

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9 Conclusions and Outlook

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Considerations of Electrode Parameters
  5. 3 Material Parameters
  6. 4 Alloying Elements—Group IV
  7. 5 Conversion Electrodes
  8. 6 Alloying Elements—Others
  9. 7 Nanostructured Carbon
  10. 8 Impact of the Electrode or Particle Architecture
  11. 9 Conclusions and Outlook
  12. Acknowledgements
  13. Biographies

This brief overview of the available anode materials showcases the impressive developments in the field of high-capacity anode materials in the past years, as well as the ample room left for improvement. Si anodes have especially been shown to exhibit very favorable properties, but other materials, such as P and Ge, also show high measured specific and volumetric capacities and good rate capability. The very low melting point of Ga makes it an interesting material as well. Other materials, such as transition metal oxides and hydrides, Mg, Al, and Sn, have some favorable characteristics, but face challenges, such as polarization and pulverization. Structural materials innovations show great promise, replacing, or obviating the standard binder, conductive additives, and even the current collectors. In addition, novel anode architecture may allow fabrication of multifunctional batteries with load-bearing capabilities, which can further improve energy density on a system level.

In the view of the authors, the largest challenge to overcome for the successful development and commercialization of high-capacity anodes for EV and portable electronics applications is achieving high CE and good SEI stability (comparable to or better than that of graphite anodes) and, ideally, doing so using abundant low-cost materials. The technology of various novel anodes are far from mature, and still requires significant improvements and innovations on various levels, from basic science all the way to large-scale manufacturing. With continued advanced in situ and ex situ analyses, theoretical simulations, and persistently innovative electrode and cell designs, the growing collective understanding of the inter-related chemical, electrical, mechanical, and electro-chemomechanical phenomena is bound to continue producing better performing and more viable high-capacity anodes. The recent rapid developments in the field are very encouraging, and bode well for the future of high-capacity anode research.

Acknowledgements

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Considerations of Electrode Parameters
  5. 3 Material Parameters
  6. 4 Alloying Elements—Group IV
  7. 5 Conversion Electrodes
  8. 6 Alloying Elements—Others
  9. 7 Nanostructured Carbon
  10. 8 Impact of the Electrode or Particle Architecture
  11. 9 Conclusions and Outlook
  12. Acknowledgements
  13. Biographies

This work was partially supported by the US Air Force Office of Scientific Research (AFOSR) (grant FA9550-13-1-0054) and by the Energy Efficiency & Resources program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Korea government Ministry of Knowledge Economy (grants 20118510010030). Naoki Nitta acknowledges a doctoral fellowship supported by the US NSF IGERT NESAC program.

Biographies

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Considerations of Electrode Parameters
  5. 3 Material Parameters
  6. 4 Alloying Elements—Group IV
  7. 5 Conversion Electrodes
  8. 6 Alloying Elements—Others
  9. 7 Nanostructured Carbon
  10. 8 Impact of the Electrode or Particle Architecture
  11. 9 Conclusions and Outlook
  12. Acknowledgements
  13. Biographies
  • Image of creator

    Naoki Nitta received his B.S. in Chemical and Biomolecular Engineering from Rice University in 2011 and is currently pursuing his Ph.D. in Materials Science and Engineering at Georgia Tech. His current research involves anode materials for lithium-ion batteries and XPS analysis of the SEI on various electrode materials. He is interested broadly in electrochemical energy storage and its applications.

  • Image of creator

    Prof. Gleb Yushin received his Ph.D. in Materials Science from North Carolina State University, and M.S. in Physics from Saint-Petersburg Technical University. He is an associate professor in the School of Materials Science and Engineering and director of the Center for Nanostructured Materials for Energy Storage at Georgia Tech. Current research in his laboratory is focused on electrochemical energy storage with an emphasis on nanostructured and nanocomposite materials for use in advanced batteries and supercapacitors.