A Study into the Significant Improvement in Performance of Silicon Nanowire Electrodes through the Use of Electrolyte Additives

Si nanowires (NWs) are a promising anode material for next generation lithium-ion batteries due to their large maximum theoretical capacity (3579 mAh/g) when compared with conventional graphitic based materials (372 mAh/g). NWs also provide good electrical conductivity along their length, have a high interfacial area in contact with the electrolyte, have an optimal short diffusion distance for Li-ion transport and can be grown directly from current collectors, eliminating the need for binders and conductive additives. However before Si NWs can be realized as a viable alternative to graphite it is essential that suitable electrolytes are developed that result in a stable SEI layer formation over the initial cycles. Here we present a systematic study of the effect of common LIB electrolyte additives such as FEC and VC on the capacity retention, rate capability and SEI layer composition of binder-free Si NW electrodes. The efficacy of 5 different electrolyte additives in various combinations and used in conjunction with standard carbonate based electrolytes was investigated. We show that small quantities of additives can improve the cycle life and rate capability of the Si NWs significantly when compared with a standard 1M LiPF₆ in EC/DEC electrolyte. Using XPS and EIS techniques we elucidate the effect of each additive combination on the composition and electrical properties of the SEI layer and correlate these results with the improved performance. Furthermore, through an ex-situ high-resolution transmission electron microscopy and high-resolution scanning electron microscopy study we show that the additives can also affect a change in morphology of the active material with cycling, with the better performing additives inducing a change from a NW morphology to a stable porous network like structure over initial cycles.