Revisiting the ion dynamics in LixCoO2 and NaxCoO2

Layered oxides (AMO₂, where A = Li or Na and M = transition metal) are essential positive electrode materials for lithium- and sodium-ion batteries. A fundamental question in ion transport is whether Li⁺ or Na⁺ diffuses faster in these materials; however, distinguishing intrinsic diffusion properties from the effects of particle size and electrode composition is challenging. Using operando muon spin spectroscopy and molecular dynamics simulations, we determined the Li⁺ and Na⁺ self-diffusion coefficients in O3-Li_xCoO₂, O3-Na_xCoO₂, and P2-Na_xCoO₂. Our findings revealed that Na⁺ diffusion is higher in the P2-type structure than in the O3-type structure primarily due to weaker electrostatic interactions. In the O3-type structure, Li⁺ diffuses faster than Na⁺, whose larger ionic size hinders mobility. These insights clarify the ion transport mechanisms and advance the design of next-generation battery materials.