Abstract
Layered oxides (AMO(2), 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 (x) CoO(2), O3-Na (x) CoO(2), and P2-Na (x) CoO(2). 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.