Ionic-Size-Dependent Reversible Interlayer Cation Migration and Voltage Hysteresis in P2-Type Sodium Layered Cathodes

Voltage hysteresis in layered oxide cathodes of Li/Na-ion batteries, a significant barrier to energy efficiency, is commonly attributed to reversible cation migration. This is particularly prevalent in materials utilizing anionic redox for high capacity, yet the strong coupling between cation migration and complex anionic processes (such as O–O dimerization, oxygen loss, and charge transfer) has obscured a definitive causal link. To decouple these phenomena, we designed a series of P2-type Na_2/3Ni_0.1M_0.1Mn_0.8O₂ compounds, where M is an electrochemically inactive trivalent cation (Al³⁺, Ga³⁺, Sc³⁺) of increasing ionic radius (0.535 Å, 0.62 Å, 0.745 Å). Interestingly, these systems exhibit negligible anionic redox activity but display increasing voltage hysteresis that scales with the size of the M³⁺ cations. Through a combination of experimental and theoretical analyses, we correlate this hysteresis with an enhanced tendency for reversible interlayer cation migration of M³⁺, which is facilitated by a larger cationic size that is more adaptive to the large Na interlayer spacing. By successfully isolating cation migration from anionic redox, this work establishes its direct role in driving voltage hysteresis, providing fundamental insights into the design of cathode materials having less cation migration and reduced voltage hysteresis for Na-ion batteries.