Boron doping modulates the electrostatic interaction of Na–O bonds for inducing the P2/O3 biphasic structure in layered oxide cathodes

Abstract

Sodium-ion batteries (SIBs) are promising for large-scale energy storage, yet developing cathode materials with both high capacity and long-term stability remains challenging. Herein, the strategy of modulating the electrostatic interaction of Na–O bonds via the polarization effect through boron doping is proposed, enabling the precise construction of the P2/O3 biphasic structure. A series of Na_0.8Fe_1/3Ni_1/3Mn_1/3B_xO₂ (x = 0.00–0.08) cathode materials are synthesized. At low x doping levels, the material exhibits the O3 phase, while the P2/O3 biphasic structure emerges starting at x = 0.05. With increasing boron content, the Na–O bond electrostatic interaction weakens and the R value increases, which facilitate the emergence of the P2 phase. Among the samples, the Na_0.8Fe_1/3Ni_1/3Mn_1/3B_0.05O₂ cathode has the optimal biphasic ratio comprising 10.3% P2 and 89.7% O3, endowing it with as low as 1.38% volume variation during charge/discharge, as identified by in situ X-ray diffraction measurement. It achieves an optimal balance of electrochemical performance, delivering a remarkable cycling stability with a capacity retention of 92.24% after 200 cycles at 1C and a sufficient capacity of 100.2 mAh g⁻¹ at 0.1C. This work demonstrates that B³⁺ doping effectively tailors Na–O bond chemistry to construct a stable P2/O3 biphasic structure, offering a viable approach for designing high performance layered oxide cathodes for sodium-ion batteries.