Potassium regulated Na4MnV(PO4)3 microspheres cathode towards robust sodium ion batteries

The dual strategy of K regulation and microsphere morphology tailoring for Na4MnV(PO4)3 (NMVP) is designed in this work. By introducing potassium ion within the NMVP structure, the sodium superionic conductor (NASICON)-type framework can be well stabilized. Potassium ions can act as "pillars" within the NASICON framework, slightly expanding the lattice and thereafter promoting faster sodium ion diffusion. Simultaneously, the microsphere morphological design is tailored to optimize the electrochemical performance by decreasing ion diffusion pathways and facilitating a more efficient ion exchange between the electrolyte and the active material, improving the kinetics of Na+ ion upon cycling. As a proof of concept, DFT computations corroborate that K selectively replaces Na1 sites in NMVP, which effectively ameliorates electronic conductivity and diminishes Na diffusion barrier. On the other hand, the heightened ionicity between Mn/V and O, induced by K-doping regulation, arouses a higher redox voltage in K-doped NMVP due to enhanced charge localization. As expected, the optimized NMVP-K1 delivers satisfied capacities of 103.5 mAh and 86.1 mAh g- 1 at 0.1 and 10 C, retaining 79.8 % of its initial capacity undergoing 2000 circulations at 10 C. Moreover, when used in a full SIB, NMVPK1//HC reflects a high energy density of 256.8 Wh kg- 1 at 321 W kg- 1.