(Carl Wagner Memorial Award -and- Battery Division Research Award) Anodes for Lithium Ion Batteries Revisited: From Graphite to High-Capacity Alloying- and Conversion-Type Materials and Back Again
Regarding the need for high capacity anode materials in order to further increase the specific energy (Wh kg-1) and energy density (Wh L-1) of LIBs, silicon is one of the most promising candidates. Among different alloying elements, such as tin or germanium, silicon displays the highest reversible capacity of ca. 3,500 mAh g-1 and a relatively low lithiation potential of ca. 0.3 V vs. Li/Li+. However, drawbacks arise with the use of silicon, which are mainly related to the severe volume changes (up to 300%) during the lithium uptake/release process and the low intrinsic electronic conductivity.(3) The large stress and strain during lithiation/de-lithiation may lead to particle cracking and even particle pulverization resulting in a contact loss between the electrode components as well as into an ongoing formation of the solid electrolyte interphase (SEI) and an increase in the cell impedance.(3, 4)
Several strategies have been developed in order to reduce the detrimental effects of the large volume changes of silicon and to suppress the side reactions with the electrolyte and thus to improve the long-term cycling stability (Figure 1).(3) One main approach is to reduce the silicon particle size to the nanometer range. Another strategy is to design porous structures, e.g. macroporous (pore width ˃ 50 nm) or mesoporous materials (2 to 50 nm), that can offer sufficient local void space to absorb the volume expansion.(5)
A further strategy to improve the electrochemical performance of silicon is the formation of multiphase composite materials, i.e. by introducing components, which display no or only less volume changes as well as preferentially a high electronic conductivity. The idea of this approach is to use the host matrix to buffer the volume changes of silicon and thus to maintain the electrode integrity and electronic network. Examples are silicon-carbon composite materials and silicon-based alloys or composite heterostructures, such as Si/SiOx/carbon composites, or intermetallic compounds like Si/TiSi2 or NiSi alloys.(5, 6)
Figure 1. Strategies to improve high-capacity anode materials for LIBs.
Besides alloying-type anode materials, also conversion-type materials, such as transition metal oxides like Fe3O4 and MnO, which store charge via a conversion reaction (MOx + 2xLi ⇌ M + xLi2O), are of growing interest for next generation LIBs.(7) This is related to their high specific capacity of ca. 800 to 1200 mAh g-1 and the broad range of material combinations (oxides, sulfides, etc.). However, these materials typically suffer from a large irreversible capacity in the first charge/discharge cycle and from a relatively poor cycling stability.(7)
Here, we report on the improvement of graphite-based anode materials as well as high-capacity materials based on alloying and conversion mechanisms for application in LIBs. The latter ones include the optimization of group IV elements such as silicon and germanium (porous and core-shell structures) and conversion materials like ZnFe2O4, with a special focus on the metal-ion dissolution mechanism.
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