Bridging Ionic Field Strength Disparity in O3-Type Layered Oxides for High-Voltage Sodium-Ion Batteries

O3-type layered oxides represent an attractive class of sodium-ion battery cathodes, combining high theoretical specific capacity and high pristine Na content. However, their deeply desodiated frameworks at high voltages (≥4.0 V) are prone to redox-coupled distortion of TMO6 octahedra within the transition metal (TM) slabs, leading to severe structural degradation. Here, we originally define an ionic field strength disparity coefficient (η), which enables quantitative evaluation of the local chemical-field heterogeneity. The η is derived from the ratio between the standard deviation and the average value of ionic field strength (Φ) of all TM ions, defined as Φ= Z/r2, where Z and r correspond to the ionic charge and ionic radius, respectively. Introducing compensatory cations with an intermediate Φ value between two cations featuring largely disparate Φ values effectively reconciles local chemical-field discrepancy and lowers the η parameter upon charge–discharge processes. Moreover, synchrotron X-ray absorption spectroscopy, strain mapping analyses, and first-principles calculations reveal that the low-η sample demonstrates the reduced lattice strain and enhanced structural reversibility. Consistently, electrochemical evaluation shows that it retains more than 80% of its capacity after 500 cycles at 10C, markedly outperforming the unmodified counterpart with only 22% retention. This work establishes the η as a quantitative descriptor for inactive dopant selection and provides a general strategy for developing highly stable layered oxide cathodes.