Boosting the rate capability and working lifespan of K/Co co-doped Na3V2(PO4)3/C for sodium ion batteries

Abstract Na3V2(PO4)3 (NVP) has been deemed to be a prospective cathode for sodium ion batteries (SIBs) duo to its high structural stability and flat voltage platform. Herein, a series of K/Co co-doped NVP composites with optimized sodium storage property are produced via a feasible sol-gel method. Significantly, the introduced K+ has substituted of Na+ at Na1 site, strictly demonstrated by the rietveld refinement results. Meanwhile, this substitution has hardly influence on the reversible de/insertion of Na+ situated at Na2 site. Moreover, the beneficial K+ with larger ionic radius can act as the pillar ion to improve the structural stability efficiently. Furthermore, the replacement of V3+ by Co2+ facilitates to enlarging the interplanar spacing in crystal structure, resulting in accelerating the Na+ migration effectively. Moreover, substitution with low valence element (Co2+ vs. V3+) can generate the favorable holes to increase the intrinsic electronic conductivity. Notably, doping with appropriate content of K/Co elements favors to reduce the particle size and shorten the pathway for Na+ transformation, distinctly boosting the apparent ionic diffusivity. Specifically, K/Co co-doping induces to forming the unique mesoporous morphology, immensely enlarging the specific surface area of active grains and therefore facilitating the infiltration between the particles and electrolyte. Comprehensively, the optimized K0.1Co0.05-NVP/C exhibits an outstanding electrochemical performance. It shows a high initial capacity of 122.7 mA h g-1 at 0.1 C, which is higher than the theoretical value of NVP (117.6 mA h g-1). It can release an initial capacity of 100.9 mA h g-1 at 1 C and maintain 71.0 mA h g-1 after 500 cycles. Moreover, the reversible capacity can be achieved as 98.8 mA h g-1 at a high rate of 10 C and a favorable retention of 75.9% can be obtained after 400 cycles. Besides, the kinetic characteristics for the modified K0.1Co0.05-NVP/C cathode are superior than those of pristine NVP/C. Specifically, it reveals a relatively high diffusion coefficient of Na+ (0.53×10-10 cm2 s-1). Thus, the modified K/Co co-doped NVP/C sample can be a promising cathode material for SIBs.