Engineering Mixed Valency in LiVPO$_4$F Cathode Material: The Dual Role of Mn$^{2+}$ in Enhancing Electronic and Ionic Transport

A series of LiV1−xMnxPO4F/C (x = 0, 0.03, 0.09) materials were synthesized via a sol−gel method and thoroughly characterized to understand the structural, electronic, and electrochemical modifications induced by manganese incorporation. Structural analysis confirms that Mn doping preserves the triclinic lattice of LiVPO4F. The XAS measurements reveal partial oxidation of vanadium, forming a mixed V3+/V4+ state to compensate for aliovalent Mn2+ insertion. The impedance spectroscopy indicates improved lithium-ion mobility and reduced charge-transfer resistance in Mn-substituted samples. Among them, the V−Mn9 (x = 0.09) composition exhibits the best performance, delivering a high reversible capacity of 147 mAh g−1 with excellent cycling stability (99.3% capacity retention over 200 cycles at 1C). These results demonstrate that Mn2+ doping is an effective strategy to tune the redox environment and transport properties of LiVPO4F, thereby enhancing its viability for high-performance lithium-ion battery applications.