Boosting sodium-ion battery performance by anion doping in NASICON Na₄MnCr(PO₄)₃ cathode

Na superionic conductor (NASICON)-structured Na₄MnCr(PO₄)₃ (NMCP) possessing unique three-electron transfer process renders admirable energy density for sodium ion batteries (SIBs). However, the current issues like its sluggish Na⁺ diffusion kinetics, deficient intrinsic conductivity, and unsatisfactory structural stability, hinder its practical application. Herein, a selective replacement of O elements in PO₄ group by Cl anions in the NMCP system was developed to significantly enhance its electrochemical performance. The results affirm that the enhanced performance of Cl doped samples can be attributed to the enlargement of cell size, the creation of Na vacancies and the weakness of Na2O bond after Cl doping. The as-prepared Na_3.85□_0.15MnCr(PO_3.95Cl_0.05)₃/C (NMCPC - 15/C) cathode delivers a high capacity (128.0 mAh/g at 50 mA g^-1) and excellent rate performance (73.0 mAh/g at 1000 mA g^-1) in contrast to NMCP/C that merely provides 105.2 mAh/g at 50 mA g^-1 and reduces to 47.4 mAh/g at 1000 mA g^-1. Meanwhile, NMCPC - 15/C shows a capacity retention of 60.7 % at 1000 mA g^-1 after 500 cycles, while only 37.1 % for NMCP/C in the same test conditions. Moreover, the satisfactory performance and energy density of NMCPC - 15/C||hard carbon (HC) full cell confirm the potential practicality of NMCPC - 15. Therefore, chloride ions doping into NMCP has practical application prospects in the preparation of high-performance cathode materials and our work also offers new inspiration to apply anion doping strategies in promoting the performance of the other NASICON-structured cathodes for SIBs.