Enhancement of LiVPO$_4$F cathode materials via manganese doping: Boosting energy density and rate capability for high-voltage lithium-ion batteries

High-voltage fluorophosphate cathode materials have emerged as promising candidates for next-generation lithium-ion batteries, offering enhanced safety, energy density, and stability compared to conventional oxide-based materials. In this study we investigate the effects of manganese doping on the structural and electrochemical properties of LiVPO4F cathode material synthesized via an optimized and simple sol-gel method. A series of LiV$_{1–2y/3}$Mn$_y$PO$_4$F/C (y = 0, 0.03, 0.09, 0.15) compositions was successfully prepared and characterized to highlight the impact of Mn substitution on structural stability and electrochemical performance. X-ray diffraction analysis confirmed the preservation of the initial crystal structure of the undoped material for manganese content up to y = 0.09, while higher concentration (y = 0.15) leads to the appearance of secondary phases. The successful incorporation of Mn was confirmed by systematic shifts in diffraction peaks and changes in the unit cell parameters. Electrochemical characterization revealed that the optimized composition (y = 0.09) delivered a remarkable discharge capacity of 150 mAh/g at C/5, approaching the theoretical capacity of 153 mAh/g, with 98 % capacity retention after 100 cycles. ... mehrNotably, all compositions demonstrated excellent thermal stability at 50 °C. Once again, the composition corresponding to y = 0.09 shows the best performance as it maintains a discharge capacity of 149 mAh/g with minimal capacity fade (≈0.7 %) after 50 cycles. Rate capability tests showed enhanced performance for Mn-doped samples, particularly at higher C-rates, attributed to improved charge transfer kinetics and structural stability. The enhanced performance is attributed to the synergistic effects of successful Mn incorporation and the uniform carbon coating derived from the in-situ carbonization of citric acid. These results demonstrate the effectiveness of our synthetic approach in developing high-performance cathode materials for advanced lithium-ion batteries.