Carbonate-modified siloxanes as solvents of electrolyte solutions for rechargeable lithium cells

Influence of mixing carbonate-modified siloxanes into LiPF₆-ethylene carbonate (EC)/ethylmethyl carbonate (EMC) (mixing volume ratio = 3:7) mixed solvent electrolytes on charge-discharge cycling properties of lithium was examined. As the solute, 1 M (M: mol L⁻¹) LiPF₆ was used. As siloxanes, 4-(2-trimethylsilyloxydimethylsilylethyl)-1,3-dioxolan-2-one and 4-(2-bis(trimethylsilyloxy)methylsilylethyl)-1,3-dioxolan-2-one were investigated. These siloxanes are derivatives of butylene cyclic carbonate or vinyl ethylene carbonate. Charge-discharge cycling efficiencies of lithium metal anodes improved and an impedance of anode/electrolyte interface decreased by mixing siloxanes, compared with those in 1 M LiPF₆-EC/MEC alone. Slightly better cycling behavior of natural graphite anode was obtained by adding siloxanes. Si-C/LiCoO₂ cells exhibited better anode utilization and good cycling performance by using 1 M LiPF₆-EC/MEC + siloxane electrolytes. Thermal behavior of electrolyte solutions toward graphite-lithium anodes was evaluated with a differential scanning calorimeter. By adding siloxanes, temperature starting the large heat-output of graphite-lithium anodes with 1 M LiPF₆-EC/MEC electrolyte solutions shifted to higher temperature about 100 °C. However, amount of heat-output did not decrease by adding siloxanes.