Influence of glyme-based nonaqueous electrolyte solutions on electrochemical properties of Si-based anodes for rechargeable lithium cells

Influence of electrolyte solutions on charge-discharge properties of Si-based anodes were examined. As Si-based anodes, electrodes prepared from (i) a mixture of Si and carbon (Si + Q and (ii) a mixture of a Si-SiO2-C composite and carbon (Si-C + Q were used. As electrolyte solutions, ethylene carbonate (EC)/methylethyl carbonate (MEC)/n-glyme ternary mixed solvent with I M (M: mol L-1) LiPF6 was used. Poly(ethylene glycol) dimethyl ethers [(CH3O(CH2CH2O)(n)CH3, n = 1, 2, 3 and 4)] are generally known as "glymes". In case of Li/Si + C cells, by mixing glymes to I M LiPF6-EC/MEC (30:70 in mixing volume percentage; EM), discharge capacity tended to become larger than that with EM alone. Cycle life of Li/Si + C cells was approximately the same when different electrolyte solutions were used. Primary reason for the degradation of charge-discharge cycling life of Li/Si + C cells is a physical breakdown and loosing electronic conductive path of Si electrode. This phenomenon was resulted from a large volume change of electrode by expansion and shrinking of Si during charge-discharge of lithium in Si. Also, reactivity of electrolyte solutions toward lithium affected the degradation of charge-discharge capacity. Discharge capacity depended on a reduction potential (E-red) of glymes, not on an electrolyte conductivity (kappa). In case of Li/Si-C + C cells, by mixing n-glymes to EM, the discharge capacity tended to become larger than that with EM alone. Cycling life of Li/Si-C + C cells improved, compared with that of Li/Si + C cells. This was due to a suppression of physical breakdown by Si-C structure with a thin carbon surface layer on it. Influence of reactivity of electrolyte solutions toward lithium on the degradation of charge-discharge capacity of Li/Si-C + C cells was much smaller than that of Li/Si + C cells. Discharge capacity of Li/Si-C + C cells depended on kappa, not on Ered. (c) 2006 Elsevier B.V. All rights reserved.