Study on the synthesis and electrochemical performance of nitrogen-doped double carbon-coated Na4VMn1-xCrx(PO4)3/NC@CNTs

Sodium-ion batteries (SIBs), as an emerging energy storage technology, have been recognized as one of the most promising alternatives to lithium-ion batteries (LIBs). Among various cathode materials, the NASICON-type Na 4 VMn(PO 4 ) 3 (NVMP) composite material has garnered considerable attention owing to its distinctive 3D framework structure, high redox potential, stable crystal structure, and rapid Na + diffusion rate. Nevertheless, the practical application of NVMP cathodes is constrained by several inherent limitations, including inherently low electronic conductivity, Mn 3+ Jahn–Teller distortion, and structural degradation mechanisms, which collectively contribute to compromised rate capability and limited cycling durability. To tackle this challenge, an N-doped dual nano-carbon coating was applied to the NVMP material to enhance the conductivity of the composite. Building upon this foundation, a series of Na 4 VMn 1- x Cr x (PO 4 ) 3 /NC@CNTs cathode materials (where x  = 0, 0.25, 0.5, 0.75, and 1) were successfully prepared using the sol–gel method, enabling targeted regulation of Mn/Cr ratios for enhanced electrochemical performance. The N-doped dual nano-carbon coating is capable of forming carbon layer defects and active sites, as well as constructing a 3D conductive network structure, which can enhance the diffusion rate of Na + ions and shorten their transport path. The results indicate that the Na 4 VMn 0.5 Cr 0.5 (PO 4 ) 3 /NC@CNTs cathode material demonstrates the highest discharge capacity, achieving 132.1 mAh·g −1 when x  = 0.5. Notably, the capacity decreases rapidly at high rates, potentially attributed to the collapse of the crystal structure and excessive polarization, resulting in a deterioration of the material’s electrochemical performance. The optimized Na 4 VCr(PO 4 ) 3 /NC@CNTs (when x  = 1) demonstrates exceptional rate performance with 91.2% capacity retention at 1C, accompanied by significantly reduced charge transfer resistance (Rct = 331.9 Ω). This study establishes a design paradigm for NASICON-type cathode materials through dual-transition metal synergistic doping and demonstrates the efficacy of Cr/Mn co-substitution in balancing ionic/electronic conductivity for next-generation high-power sodium-ion batteries.