Capacity fade of Li-ion cells cycled at different temperatures

Cycling characteristics of Sony 18650 cells with LiCoO₂ as the positive electrode material have been studied at different temperatures with DC charging protocol. The capacity fade of lithium ion cells was found to increase with increase in temperature. Cells cycled at RT and 45°C showed a capacity fade of 30% and 36% respectively after 800 cycles, while the cell cycled at 55°C failed to continue beyond 500 cycles. The rate capability of the cells continues to decrease with cycling. This can be attributed to increased resistance at both electrodes. Impedance measurements for both full and half-cells show an overall increase in the cell resistance with cycling and temperature. Charge-discharge studies on individual pellet electrodes show a reduced tendency for lithiation for both LiCoO₂ and carbon. XRD studies of the positive electrode up to 300 cycles shows a decrease in the lithium stoichiometry with cycling and temperature. Capacity fade in Sony 18650 cells is attributed to oxidation of cathode (LiCoO₂). Both primary (Li⁺) and secondary active material (LiCoO₂/C) is lost during cycling