Improvement in potassium storage performance of magnesium-doped K3V3(PO4)4/C cathode materials

Potassium vanadium phosphate (K 3 V 3 (PO 4 ) 4 ) has emerged as one of the most promising cathode candidates for potassium-ion batteries because of exceptionally stable structural framework, yet suffers from intrinsic poor electronic conductivity characteristic of polyanionic compounds. Herein, a strategic Mg 2+ doping approach was adopted to prepare Mg-doped K 3 V 3 (PO 4 ) 4 /C (K 3+x V 3-x Mg x (PO 4 ) 4 /C, x = 0.1, 0.2 and 0.3) composites, where larger Mg 2+ (0.72 Å) substitution for V 3+ (0.64 Å) effectively widens K + diffusion pathways. Optimal doping of Mg 2+ significantly elevates K + diffusion, rate capability, and cyclability beyond undoped K 3 V 3 (PO 4 ) 4 /C. The optimal K 3.2 V 2.8 Mg 0.2 (PO 4 ) 4 /C (Mg0.2-KVP/C) demonstrates comprehensive performance enhancements over pristine K 3 V 3 (PO 4 ) 4 /C, exhibiting 16.5 % higher initial discharge capacity (67.0 vs. 57.5 mAh g −1 ) at 1C with significantly improved cycling stability (85.8 % vs. 65.5 % capacity retention after 150 cycles). Notably, Mg0.2-KVP/C maintains 63.8 % larger capacity at 5C rate (53.4 vs. 32.6 mAh g −1 ) while achieving capacity retention of 75.2 % after 1000 cycles-representing a 50 % improvement in long-term cycle stability compared to the undoped K 3 V 3 (PO 4 ) 4 /C. The significant improvement in electrochemical performance of K 3 V 3 (PO 4 ) 4 /C upon Mg doping suggests that strategic ionic doping is an effective strategy for improving potassium storage performance of K 3 V 3 (PO 4 ) 4 /C and other polyanionic compounds for potassium-ion batteries. • Mg 2+ is introduced into V sites to form K 3+x V 3-x Mg x (PO 4 ) 4 (x = 0.1–0.3). • Mg 2+ doping lowers K + diffusion activation energy, enhancing fast K + diffusion. • Mg 2+ doping expands K 3 V 3 (PO 4 ) 4 cell and increases diffusion coefficients of K + . • The optimal Mg-doped sample is K 3.2 V 2.8 Mg 0.2 (PO 4 ) 4 /C (Mg0.2-KVP/C). • Mg0.2-KVP/C has superior capacity, rate capacity and cyclability to K 3 V 3 (PO 4 ) 4 /C.