Optimizing the crystal structure and electrochemical properties of Na4MnCr(PO4)3 by trace La doping

Na 4 MnCr(PO 4 ) 3 , a manganese-based NASICON-type phosphate cathode material, is attracted considerable attention due to its high voltage and energy density. However, its practical application is hindered by poor electronic conductivity and inferior sodium ion diffusion kinetics. In this study, trace La 3+ is utilized to optimize the lattice structure of Na 4 MnCr(PO 4 ) 3 due to its unique energy level structure. La 3+ doping significantly enhances the local environment of the Mn-O bond and stabilizes the lattice structure by improving covalency, thereby mitigating Mn dissolution during charge–discharge cycles. Additionally, the large ionic radius of La 3+ promotes the sodium ion transport kinetics by expanding the cell volume and facilitating rapid sodium storage. As a result, the optimal Na 4 MnCr 0.995 La 0.005 (PO 4 ) 3 /C sample demonstrates an impressive initial discharge specific capacity of 124.5 mAh g −1 at 0.1C and superior cycling stability with a capacity retention of 70.2 % after 500 cycles at 5C. Furthermore, density functional theory calculations indicate that La 3+ doping can enhance conductivity and reduce the energy barrier for sodium ion diffusion. This study presents an effective and cost-efficient doping strategy for modifying high-energy density NASICON-type phosphate cathode materials, showing promising potential in the field of sodium ion batteries.