Capacity fade studies on spinel based Li-ion cells

The performance of Cell-Batt((R)) Li-ion cells using non-stoichiometric 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. For all charge currents, the resistance of both the electrodes does not vary significantly with cycling. 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 XRD patterns of the spinel electrode that were charged and discharged for several cycles, it can be seen that apart from the non-stoichiometric 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. EDAX analysis 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.