Coordination Chemistry Regulation Suppressing Voltage Hysteresis for Na3MnTi(PO4)3 in High-Rate Sodium-Ion Batteries

Abstract

As a natrium superionic conductor, NASICON-type Na₃MnTi(PO₄)₃ (NMTP) has garnered increasing attention for large-scale sodium-ion batteries due to its high stability and power densities. Nevertheless, it still suffers from an inferior rate capability and poor cycling longevity, arising from sluggish intrinsic kinetics and severe structural degradation. Herein, vanadium (V) is used as a dopant for equal substitution of manganese (Mn) and titanium (Ti) in NMTP to alleviate voltage hysteresis and enhance the cycling performance. V-doping regulates the local coordination chemistry of transition metals and reduces derivative antisite defect concentration upon cycling. Through density functional theory analysis, Na₃Mn_0.9V_0.2Ti_0.9(PO₄)₃ (NMTP-V0.2) demonstrates a lower bandgap and higher electronic conductivity. Additionally, V-doping significantly lowers the diffusion barrier of Na₂, leading to Na⁺ diffusivity that is approximately two orders of magnitude higher than that of NMTP during the Mn²⁺/Mn³⁺ redox process. The as-prepared NMTP-V0.2 delivers an excellent rate capability of 85.3 mAh g^–1 under 50 C and satisfactory cycling retention of 81% with a high capacity over 1400 cycles. Thus, the assembled NMTP-V0.2/hard carbon sodium-ion full cell achieves a high energy density of 292.3 Wh kg^–1 as well as outstanding capacity retention of 92% after 500 cycles under 10 C. This result not only provides an approach for suppressing voltage hysteresis in polyanion cathodes but also offers guidance for designing high-power SIBs.