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

A series of LiV$_{1−x}$Mn$_x$PO$_4$F/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 LiVPO$_4$F. The XAS measurements reveal partial oxidation of vanadium, forming a mixed V$^{3+}$/V$^{4+}$ state to compensate for aliovalent Mn$^{2+}$ 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 Mn$^{2+}$ doping is an effective strategy to tune the redox environment and transport properties of LiVPO$_4$F, thereby enhancing its viability for high-performance lithium-ion battery applications.