Conflicting Roles of Anion Doping on the Electrochemical Performance of Li-Ion Battery Cathode Materials

Anion doping is one of the most widely adopted strategies to improve the electrochemical performance of cathode materials for Li-ion batteries. However, undesirable side effects are often observed together with enhanced electrochemical properties, leading to an unsatisfactory overall performance. In order to develop an anion doping strategy which enhances the positive effects and suppresses undesirable side effects, the understanding of their origin at the atomic scale is a crucial step. In this work, using density functional theory (DFT), we report a systematic study on the effects of three common anion dopants (F, S, Cl) on a wide range of properties of a model cathode material, LiNiO2, including redox potential, ionic conductivity, Li/Ni exchange, lattice distortion, and Ni migration upon delithiation. The results show that the dopants improve certain properties but worsen others, revealing some distance-dependent features. Overall, our work shows conflicting roles of anion doping on the battery voltage, rate performance, and structural stability of the cathode material. By identifying the origins of the different roles, we propose a rational anion doping strategy for the optimization of the overall electrochemical performance of the cathode material. These results for LiNiO2 can also promote anion doping studies and improved materials design in other Ni-rich layered oxide cathode materials.