The Asymmetric Charge-Discharge Kinetics in Li_1-XNi_1+XO₂ from First Principles

The ever-increasing demand on Li-ion batteries requires the cathode materials to be inexpensive and environmentally friendly. LiNiO2 is such a promising Co-free cathode. However, the presence of Ni in the Li layer (NiLi) becomes more common in LiNiO2 than its cousin layered compounds, which limits its electrochemical performance. These excess Ni might randomly distribute in the bulk due to Li deficiency in synthesis, or/and form a surface densified phase due to oxygen loss in cycling. This study combines density functional theory (DFT), cluster expansion and kinetic Monte Carlo (KMC) simulations to identify the effects of these defects on Li transport. Both types of NiLi were found to impede Li transport at the end of charge and discharge, but not at the beginning. This asymmetry kinetics cannot be solely explained by the Li diffusivity as a function of Li contents but stems from the phase boundary between Li orderings. NiLi from synthesis smooths the voltage plateaus and contributes to the 1st cycle capacity loss. NiLi from degradation hinders Li transport more severely when the densified phase fully covers the particle surface. Interestingly, during charge the surface phase traps the last 25% Li for an extremely long time but shows little impedance when Li%>25%. Figure 1