Silicon oxycarbide/nano-silicon composite anodes for Li-ion batteries: Considerable influence of nano-crystalline vs. nano-amorphous silicon embedment on the electrochemical properties

Silicon oxycarbide/nano-silicon composites (SiOC/nSi) are prepared by mixing of nano-sized silicon, either crystalline (nSi_c) or amorphous (nSi_a), with commercially available polyorganosiloxane RD-684a and subsequent pyrolysis. The influence of the type of nano-silicon, namely crystalline vs. amorphous, on the electrochemical properties and performance is analyzed and correlated with the corresponding composite microstructure. In the case of crystalline nano-silicon, a high reversible capacity of 905 mAh g−1 is registered, whereas that for amorphous nano-silicon embedment reaches 704 mAh g−1. However, regarding the cycling stability, SiOC/nSi_c shows a significant capacity fading upon continuous cycling, related to SiOC matrix failure. The host phase is not able to accommodate the arising mechanical stresses upon Si grain expansion and contraction when alloying/dealloying with Li. SiOC/nSi_a on the contrary, demonstrates a stable cycling performance for up to 100 cycles. This excellent performance is explained by the enhanced matrix integrity of the compound, rationalized by a smaller size of the embedded crystallized Si grains and an intrinsically enhanced electrical conductivity due to the formation of SiC.