Studies on capacity fade of spinel-based Li-ion batteries

The performance of Cell-Batt(R) Li-ion cells using nonstoichiometric spinel as the positive electrode material has been studied at different charging rates. The capacity of the cell was optimized based on varying the charging current and the end potential. Subsequent to this, the capacity fade of these batteries was studied at different charge currents. During cycling, cells were opened at intermittent cycles and extensive material and electrochemical characterization was done on the active material at both electrodes. For all charge currents, the resistance of both the electrodes does not vary significantly with cycling. This result is in contrast with cells made with LiCoO2 cathode where the increase in cathode resistance with cycling causes the fade in capacity. Comparison of cyclic voltammograms of spinel and carbon electrode before and after 800 cycles reveals a decrease in capacity with cycling. Low rate charge-discharge studies confirmed this loss in capacity. The capacity loss was approximately equally distributed between both electrodes. On analyzing the X-ray diffraction patterns of the spinel electrode that were charged and discharged for several cycles, it can be seen that apart from the nonstoichiometric spinel phase, an additional phase slowly starts accumulating with cycling. This is attributed to the formation of defect spinel product lambda -MnO2 according to a chemical reaction, which also leads to MnO dissolution in the electrolyte. Energy dispersive analysis by X-ray of the carbon samples shows an increase in Mn content with cycling. These studies indicate that capacity fade of spinel-based Li-ion cells can be attributed to (i) structural degradation at the cathode and (ii) loss of active materials at both electrodes due to electrolyte oxidation. (C) 2001 The Electrochemical Society.