Manipulating Na occupation and constructing protective film of P2-Na0.67Ni0.33Mn0.67O2 as long-term cycle stability cathode for sodium-ion batteries

Abstract

P2-Na_0.67Ni_0.33Mn_0.67O₂ (NNMO) is promising cathode material for sodium-ion batteries (SIBs) due to its high specific capacity and fast Na⁺ diffusion rate. Nonetheless, the irreversible P2-O2 phase transformation, Na⁺/vacancy ordering, and transition metal (TM) dissolution seriously damage its cycling stability and restrict its commercialization process. Herein, Na occupation manipulation and interface stabilization are proposed to strengthen the phase structure of NNMO by synergistic Zn/Ti co-doping and introducing lithium difluorophosp (LiPO₂F₂) film-forming electrolyte additive. The Zn/Ti co-doping regulates the occupancy ratio of Na_e/Na_f at Na sites and disorganizes the Na⁺/vacancy ordering, resulting in a faster Na⁺ diffusion kinetics and reversible P2-Z phase transition for P2-Na_0.67Ni_0.28Zn_0.05Mn_0.62Ti_0.05O₂ (NNZMTO). Meanwhile, the LiPO₂F₂ additive can form homogeneous and ultrathin cathode-electrolyte interphase (CEI) on NNZMTO surface, which can stabilize the NNZMTO-electrolyte interface to prevent TM dissolution, surface structure transformation, and micro-crack generation. Combination studies of in situ and ex situ characterizations and theoretical calculations were used to elucidate the storage mechanism of NNZMTO with LiPO₂F₂ additive. As a result, the NNZMTO displays outstanding capacity retention of 94.44% after 500 cycles at 1C with 0.3 wt% LiPO₂F₂, excellent rate performance of 92.5 mA h g⁻¹ at 8C with 0.1 wt% LiPO₂F₂, and remarkable full cell capability. This work highlights the important role of manipulating Na occupation and constructing protective film in the design of layered materials, which provides a promising direction for developing high-performance cathodes for SIBs.