The emergence of layered structure materials has positioned itself as a focal point in the progress of potassium-ion battery (KIBs) cathode materials , as it faces challenges concerning cycling-induced structural stability. In this particular investigation, the successful synthesis of a carbon-coated polyanionic layered composite denoted as K 3 V 3 (PO 4 ) 4 /C has been achieved. K 3 V 3 (PO 4 ) 4 /C was subsequently employed as a novel positive electrode material for KIBs. Upon subjecting it to 100 cycles at a current density of 200 mA g −1 , the discharge specific capacity exhibited a remarkable value of 37 mAh g −1 , thus showcasing a capacity retention of 88%. Impressively, even after undergoing 300 cycles at a high current density of 400 mA g −1 , the capacity retention remained constant at 85%. The superior long-term cycling performance and high capacity retention observed in K 3 V 3 (PO 4 ) 4 /C can be attributed to the inherent stability of the polyanionic layered structure and the dual contributions derived from the carbon coating. Furthermore, the application of characterization techniques such as synchrotron X-ray absorption near-edge structure spectroscopy (XANES) revealed a V valence state of + 3 within the synthesized sample. By establishing a structural model for K 3 V 3 (PO 4 ) 4 /C and conducting first-principles calculations on its band structure and density of states, it was confirmed that the material facilitates electron transfer . An investigation into potassium ion migration and diffusion elucidated the involvement of tetrahedral site hopping (TSH). The successful synthesis of K 3 V 3 (PO 4 ) 4 /C material not only provides valuable insights but also serves as a reference for the development of KIBs 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.
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