Electrochemical performance of overlithiated Li1+xNi0.8Co0.2O2: structural and oxidation state studies

Pure, layered compounds of overlithiated Li_1+xNi_0.8Co_0.2O₂ (x = 0.05 and 0.1) were successfully prepared by a modified combustion method. XRD studies showed that cell parameters of the material decreased with increasing the lithium content. SEM revealed that the morphology of particles changed from rounded polyhedral-like crystallites to sharp-edged polyhedral crystals with more doped lithium. EDX showed that the stoichiometries of Ni and Co agrees with calculated synthesized values. Electrochemical studies revealed the overlithiated samples have improved capacities as well as cycling behavior. The sample with x = 0.05 shows the best performance with a specific capacity of 113.29 mA?h?g^-1 and the best capacity retention of 92.2% over 10 cycles. XPS results showed that the binding energy of Li 1s is decreased for the Li doped samples with the smallest value for the x = 0.05 sample, implying that Li⁺ ions can be extracted more easily from Li_1.05Ni_0.8Co_0.2O₂ than the other stoichiometries accounting for the improved performance of the material. Considerations of core level XPS peaks for transition metals reveal the existence in several oxidation states. However, the percentage of the+3 oxidation state of transition metals for the when x = 0.1 is the highest and the availability for charge transition from the+3 to+4 state of the transition metal during deintercalation is more readily available.