Assessing Si-based anodes for Ca-ion batteries: Electrochemical decalciation of CaSi2

Density functional theory (DFT) calculations are used to investigate the basic electrochemical characteristics of Si-based anodes in calcium ion batteries (CIBs). The calculated average voltage of Ca alloying with fcc-Si to form the intermetallic CaxSi phases (0.5 b x ≤ 2) is of 0.4 V, with a volume variation of 306%. Decalciation of the lower Ca content phase, CaSi2, is predicted at an average voltage between 0.57 V (formation of Si-fcc, 65% volume variation) and 1.2 V (formation of metastable deinserted-Si phase, 29% volume variation). Experiments carried out in conventional alkyl carbonate electrolytes show evidence that electrochemical “decalciation” of CaSi2 is possible at moderate temperatures. The decalciation of CaSi2 is confirmed by different characterization techniques.
We are grateful to the Toyota Battery Research division at Higashi Fuji (M6) for financial support. Diffraction experiments were performed at MSPD beamline at ALBA Synchrotron and the collaboration of François Fauth (ALBA staff) is sincerely acknowledged. ICMAB authors acknowledge the Spanish Ministry of Economy and Competitiveness (“Severo Ochoa” Programme for Centres of Excellence in R&D (SEV- 2015-0496)).