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The understanding of cycling and electrochemical characteristics of Si particle anodes for Li-ion batteries has previously been hindered by very fast capacity fading. Optimizing the electrode architecture to significantly improve its stability up to the 1000mAh∕g charge-discharge level has made it possible to investigate these properties to a greater depth than before. The capacity fading and lithiation mechanisms of Si and C-coated Si particles have been studied in this paper by cycling test and electrochemical impedance spectroscopy (EIS) analysis. The capacity vs cycle number plot exhibits two regions of different fading rates, including an initial region of slow fading followed by accelerated decay. The latter may be associated with large-scale failure of the electrode structure. EIS revealed a core-shell lithiation mechanism of Si. C-coating not only exerts remarkable favorable effects against capacity fading, but also serves as a conduit for Li ions to the reaction with Si particles.

Porous NiSi-Si composite particles having homogeneously distributed intraparticle pores with the size distribution peaked at 200 nm and a porosity of ϳ40% have been synthesized by a novel method, which comprises steps of ballmilling induced reaction to form Ni/NiSi/Si preform particles and subsequent dissolution of unreacted Ni. Upon lithiation/delithiation cycling, the composite particle electrode exhibits much reduced thickness expansion and capacity fading rate, as compared with the pure Si particle electrode. The improvements have been attributed to the success in introducing the preset voids to partially accommodate volume expansion arising from Si lithiation. In situ synchrotron XRD further indicates that NiSi of the composite is active toward Li alloying, and it undergoes reversible transformation to/from Ni 2 Si and Li y Si. The reversible transformation between the silicides involves volume change in opposite to lithiation of Si, and is beneficial to stabilizing the composite electrode upon charge/ discharge cycling.

Addition of vinylene carbonate ͑VC͒ in electrolyte solution has been found to greatly improve the high-temperature ͑55°C͒ cycling performance of LiFePO 4-based Li-ion batteries. It has been established that the VC additive remarkably suppresses Fe dissolution from LiFePO 4 cathode and hence, subsequent Fe deposition on the anode side. Furthermore, the VC additive also significantly reduces formation of solid-electrolyte interface layers on both LiFePO 4 cathodes and mesocarbon microbead ͑MCMB͒ anodes. With VC addition, a 18650-type LiFePO 4 /MCMB cell has been shown to retain ϳ80% capacity after 980 cycles at 55°C under 1-3 C charge-discharge rates. This is in contrast with more than 25% capacity loss after merely 100 cycles when no VC is added.