Effects of copper trifluoromethanesulphonate as an additive to propylene carbonate-based electrolyte for lithium-ion batteries

Addition of copper trifluoromethanesulphonate (CuTF) to propylene carbonate (PC)-based electrolyte effectively suppresses the cointercalation and decomposition of PC in the mesocarbon microbeads (MCMB) electrodes during the first lithiation process. During the first charging cycle, copper ions are reduced at a higher potential (2 V versus Li/Li+) than the potential of PC cointercalation and decomposition (0.6-0.8 V versus Li/Li+), and predominately form a porous copper layer over the MCMB surface, thereby obstructing PC to cointercalate. An increase in reversible capacity can be achieved by increasing the amount of CuTF. However, above a critical value, the copper layer inhibits the intercalation of lithium ions and lowers the capacity. The AC impedance data reveal that the passivation film and the charge-transfer resistance are both increased when the deposited copper is in excess. An optimum result may be obtained when the addition is approximately 5 wt.%. CuTF is a possibility for PC-based electrolyte additive in lithium-ion batteries. (C) 2004 Elsevier Ltd. All rights reserved.