New insight into designing a thick-sintered cathode for Li-ion batteries: the impact of excess lithium in LiCoO₂ on its electrode performance

Increasing the capacity of Li-ion batteries is one of the critical issues that must be addressed. A thick and dense electrode using an active material sintered disk is expected to have a high capacity because the volume of the active material is 100% in the cathode. This study focused on LiCoO₂, the most well-known active material for the cathode, to improve the properties of the sintered cathode. We investigated the impact of excess Li on various properties. We found that the degree of c-axis orientation in the sintered disk decreased as excess Li increased. In addition, results of ⁷Li-MAS-NMR suggest the presence of defects resulting from excess Li when the Li excess reached 5.1% or more. The discharge capacity of the LiCoO₂ sintered cathode increased as the amount of excess Li increased, and a maximum discharge capacity of 11.2 mA h cm⁻² was obtained when the Li excess amount was 7.3%. This result was attributed to the significant improvement in the Li-ion conductivity of LiCoO₂ by both the decrease in the degree of c-axis orientation and the introduction of defects due to excess Li. Notably, introducing defects derived from excess Li enhances the Li-ion conductivity. Thus, tuning the amount of excess Li for the LiCoO₂ sintered cathode was crucial in enhancing its electrochemical performance as an electrode.