Atomistic Insights into the Phase Stability of High-Na-Content P2 Layered Cathodes

Abstract

P2-type layered oxides are promising Na-ion cathodes for their high rate capability and structural reversibility, yet their practical capacity is limited by low Na content (~2/3 per f.u.). Synthesizing Na-rich (> 0.8 per f.u.) P2 phases remains challenging due to thermodynamic phase competition from O3 counterparts. Herein, we leverage first-principles calculations to reveal that the stability of high-Na-content P2 is governed by the balance between Na electrostatic interaction and site preference energy. While distinct electronic structure in P2 results in enhanced Na-Na repulsion, this can be counterbalanced by populating high-energy Na sites, thereby optimizing the spatial distribution of Na ions. Moreover, we demonstrate that site energy of Na is tunable by tailoring transition metal composition. Specifically, incorporating Li, Ni, Mg effectively lowers the energy penalty of occupying high-energy Na sites, flattening the energy landscape to facilitate Na rearrangement. Therefore, we propose the concept of transition metal-modulated Na site preference energy as an effective descriptor that accurately predicts P2/O3 phase preference across diverse compositions, which is experimentally validated by the successful synthesis of P2-type Na 0.901 Li 0.12 Ni 0.24 Mn 0.64 O 2 with a high Na content of 0.9. Our work not only elucidates the atomicscale mechanism of P2 phase stability, but also provides an effective design strategy for the rational design of high-Na-content P2 cathodes.