The effects of alkali metal ions with different ionic radii substituting in Li sites on the electrochemical properties of Ni-Rich cathode materials

The influences of alkali metal ions with different ionic radii substituting in Li sites on structure and electrochemical performances of Ni-rich cathode are systematically investigated. Li_0.99M_0.01Ni_0.8Co_0.1Mn_0.1O₂ (M = Li, Na, K, Rb) are synthesized by calcining Ni_0.8Co_0.1Mn_0.1(OH)₂ with lithium carbonate and other alkali carbonates. Diffraction analysis shows doping with alkali metal ions will not change the host layered structure. Specifically, alkali metal ion with appropriate ionic radius (Na⁺) dopes in Li site can enlarge Li slab space, decrease cation mixing and stabilize crystal structure by acting as “pillar ions”; while, the K⁺ and Rb⁺ ions with larger ionic radii dope in Li sites will block Li⁺ diffusion and cause lattice distortion. The capacity retention of the bare, Na⁺, K⁺ and Rb⁺ doped samples after 200 cycles at 1C within 2.8–4.3 V is 93.2%, 96.8%, 88.7% and 92.9%, respectively. The experimental results and theoretical calculations reveal that doping ions with appropriate ionic radii in Li sites are in favor of Li⁺ diffusion, while doping ions with overlarge ionic radii will deteriorate the structure and electrochemical performances of the cathode. We believe this study will benefit the future works for the Li-site doping of layered materials.