Engineering Mixed Valency in LiVPO₄F Cathode Material: The Dual Role of Mn²⁺ 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.