Electrolytic characteristics of asymmetric alkyl carbonates solvents for lithium batteries

Asymmetric alkyl carbonate solvents (ACS) have been used as components of liquid electrolyte systems designed for Li-ion cells. Four ACS were selected: methyl-propyl carbonate (MPC), ethyl-propyl carbonate (EPC), methyl-isopropyl carbonate (MiPC) and ethyl-isopropyl carbonate (EiPC). The common features of all these ACS are a low melting point and a low viscosity, enhancing electrolytes conductivity toward low temperatures. The viscosity and the conductivity (salt: LiPF6, 1 M) of the ACS and their mixtures with ethylene carbonate (EC, 50% v/v), were studied as a function of the temperature (7). Arrhenius types plots of the logarithm of the conductivity versus 1/T reveals that ACS and ACS/EC mixtures are vitreous at low temperatures. The electrochemical and cycling behaviors of a graphite anode and a LiCoO2 cathode have been evaluated using coin cells with a Li counter electrode. The charge and discharge capacities have been determined as a function of the cycle number. MiPC which builds an highly stable surface film on the graphite electrode, can be used as a single-solvent electrolyte with only a slight decrease in capacity of the LiCoO2 cathode. All ACS/EC mixtures exhibit good filming properties at the negative electrode and no capacity loose at the positive electrode. The ability of some of the electrodes-electrolyte systems to undergo increased rates of discharge (C/5 to C/2) has been also evaluated. (C) 2002 Elsevier Science B.V. All rights reserved.