Relationship between carbonaceous materials and electrolyte in secondary lithium-ion batteries

We have examined the initial charge characteristics of the following prototype battery: (i) positive electrode: LiCoO2; (ii) negative electrode: meso-carbon micro-beads (MCMB) with heat treatment at 2800 °C; (iii) electrolyte: mixture of ethylene carbonate (EC) and other solvent(s), and mixture of EC+diethyl carbonate (DEC) and other solvents dissolved with LiPF6, and (iv) battery: cylinder-type battery, in which positive and negative electrodes are spirally wound. We found that electrolytes which contain methyl acetate (MA), methyl propionate (MP) and ethyl propionate (EP) showed good performances. In general, when the distance between carbon layers d(002) value became smaller, the charge/discharge capacity became higher when the carbon electrode was charged to 0 V versus metallic Li electrode. We prepared a cylinder-prototype battery by using 1.5 M LiPF6/EC+DEC+MP as electrolyte and examined its stability at conditions of regular use and found, when it was charged and stored at high temperature, the ester-exchange reaction product of the electrolyte occurred and methane gas at the negative electrode and gas mainly carbon monooxide (CO) and carbon dioxide (CO2) at the positive electrode were formed. In the case of an electrolyte with mixed solvents, it was assumed that the ester-exchange reaction and gas-evolution reaction by oxidation/reduction may have taken place.