Point Defects in Layer-Structured Cathode Materials for Lithium-Ion Batteries

The structural characteristics related to point defects within layer-structured cathode materials for lithium-ion batteries were investigated. Crystal models containing certain types of defects were designed, and phase diagrams of Li-Co-O and Li-Ni-O systems were simulated by assuming these crystal models were independent phases based on first principle methods, enabling the thermodynamic examination of the stability and formation probability of point defects. From the formation energy and mixing entropy of a defect phase in the thermodynamically stable phase, a quantitative calculation equation was designed to predict the concentration of defects. By combining the equation with the simulated phase diagrams, the equilibrium concentrations of every defect in LiCoO2 and LiNiO2 systems were calculated. Point defects in LiCoO2 were predicted to form below 0.1%, whereas the formation of several percent of Li-deficiency and Li-Ni cation mixing appeared to be thermodynamically unavoidable in LiNiO2. The reliability of the theoretical study was confirmed by good agreement with experimental features, and thus this theoretical approach is expected to be utilized to interpret defect formation and related properties in various material systems.