Formation of Silicon Local Environments upon Annealing for Silicon Anodes: A 29Si Solid State NMR Study

Abstract

29Si magic angle spinning nuclear magnetic resonance (MAS NMR) has been used to characterize the binding interactions and interfacial phases formed between silicon and copper in a series of all-inorganic negative electrodes. The phases identified were correlated with electrochemical performance of the electrode. The electrode materials were prepared by copper deposition on silicon particles with varying Cu:Si ratios followed by heat-treatment at varying temperatures. Initial studies of the electrochemical performance of the anode materials indicated that samples with a 1:4 copper to silicon ratios and annealed at temperatures below 700°C had the best cycling performance. Utilizing 29Si MAS NMR we have been able to show that the best electrodes possess a single silicon local environment (from elemental silicon) and very little Cu3Si intermetallic formation. Electrodes with the compositions Cu:6Si and Cu:8Si annealed at certain conditions showed additional silicon resonances at −75, −92, −107 and 750 ppm and poor electrochemical performance. Combination of 29Si Single Pulse and 1H/29Si Cross Polarization techniques were performed to study the nature of these silicon local environments. Preliminary assignments indicate that the presence of surface oxidation and hydration (from annealing process) are key variables that limit the electrochemical activity of the electrode.