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

Depth-selective incorporation of B³⁺ into the interstitial site of P2-Na_0.75Ni_0.2Mn_0.7Co_0.1O₂, effectively remits the parasitic P2-P’2phase transition associated with its structural degradation, which is attributed to the synergistic effect coming from robust B─O bond and B─O─M in bulk-enriched BO₃ ³⁻and surficial enriched BO₄ ⁵⁻. This strategy underscores a milestone to innovate the structural stability of layered cathode materials for high energy-density and long lifespan NIB full cells.

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_2g ³ _eg ¹) during deep sodiation triggers serious structural degradation followed by capacity decay. Herein, the incorporation of borate-anion groups either into the bulk (BO₃ ³⁻) or on the surface (BO₄ ⁵⁻) 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 a pristine NCM analo. 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.