Structurally controlled Na4VMn(PO4)3 cathodes via alkali metal cation substitution for high-performance sodium-ion batteries

The Na superionic conductor (NASICON)-type Na₄VMn(PO₄)₃ (NVMP), with its three-dimensional framework and high operating voltage, has demonstrated great potential as a cathode material for Sodium-ion batteries (SIBs). However, its performance is hindered by the Jahn-Teller effect from Mn³⁺ and slow Na⁺ diffusion kinetics, leading to capacity fade and structural instability. In this study, Li⁺ and K⁺ cations were separately doped into the NVMP structure to improve electrochemical performance. Li-doping enhanced the V³⁺/V⁴⁺ redox activity and significantly reduced the Jahn-Teller effect, contributing to a high-capacity retention of 97.7 % after 100 cycles at 1 C and 96.1 % after 1000 cycles at 10 C. This high-rate stability highlights the role of Li-doping in maintaining structural integrity and efficient ion transport under fast cycling conditions. Meanwhile, K-doping improved the structural stability by stabilizing V³⁺ oxidation states, leading to a capacity retention of 110.3 mAh g⁻¹ after 100 cycles at 1 C and a discharge capacity of 99 mAh g⁻¹ at 10 C. X-ray photoelectron spectroscopy (XPS) further confirmed that both Li⁺ and K⁺ doping suppressed the Jahn-Teller effect by regulating the Mn³⁺ environment, which contributed to the improved cycling stability and Na⁺ cation diffusion kinetics. These findings highlight the effectiveness of alkali metal cations doping as a strategy to enhance the cycling performance of NVMP cathodes for high-performance SIBs.