Surficial and Interior Incorporation of Borates Mitigating the Inherent Jahn–Teller Distortion in a P2 Mn-Rich Layered Cathode for Na-Ion Batteries

Abstract

Layered Mn-rich materials are regarded as a promising cathode candidate for Na-ion batteries (NIBs) owing to its environmentally friendly nature, decent theoretical capacities, and relatively low cost. However, the irreversible phase transition originating from the Jahn–Teller distortion attributed to high-spin Mn³⁺ (t_2g³_eg¹) during deep sodiation triggers serious structural degradation followed by capacity decay. Herein, the incorporation of borate-anion groups either into the bulk (BO₃³⁻) or on the surface (BO₄⁵⁻) successfully modulates the local-structure environment of the P2-type layered cathode, changing the lattice parameters and valence states of the transition metals inside. The optimized Na_0.734Ni_0.207Mn_0.694Co_0.098(B_0.063O_x)O_2-x (B-NCM) can remit a P2-P’2 phase transition by mitigating the inherent Jahn–Teller distortion of MnO₆ octahedra, allowing a reversible phase transition with reduced strain even after deep sodiation to 1.5 V. The B-NCM cathode exhibits excellent capacity retention, reaching 82.02% after 200 cycles. In addition, the modulated local structure inside B-NCM helps to relieve Na⁺/vacancy ordering, enhancing Na⁺ diffusivity and rate capability compared to pristine NCM homologues. This work demonstrates a novel approach based on the incorporation of glassy anion groups into both surface and bulk to improve the electrochemical properties of layered Mn-rich cathode materials.