Sol–Gel Auto-Combustion Synthesis of Co-Substituted LiFePO4: Roles of Xerogel-Processing Parameters on Crystal Evolution, Magnetic Behaviours and Iron Valence States

Olivine-type LiFePO4 is a promising cathode candidate for lithium-ion batteries. In this study, undoped and a series of Co-substituted LixCoᵧFePO4 (x = 0.85–1.00, y = 0–0.15) nanocomposites were synthesised by sol–gel auto-combustion using organic–inorganic hybrid xerogel precursors. The effects of sol pH, calcination temperature, calcination holding time and Co2+-doping content on crystal structure, phase composition, crystallinity, particle morphology, magnetic-hysteresis performance and Fe valence-state distribution were systematically explored using TG-DTA, XRD, FT-IR, SEM, VSM and Mössbauer spectroscopy. XRD analyses reveal that intermediate-pH conditions (pH 6.5–7.5) readily produce crystalline impurities including LiFeO2 and Fe2O3, whereas high-pH environments (pH 8.5–9.5) facilitate the formation of pure-phase olivine-structured products. Diffraction-peak sharpness increases with rising calcination temperature, indicating improved crystallinity, and the samples treated at 800 °C exhibit the sharpest diffraction signals. As the doping level increases, the (311) diffraction peak first shifts toward lower and then higher angles; correspondingly, the unit-cell volume first increases to a maximum and then declines, whereas the crystallite size varies non-monotonically, confirming that Co doping predominantly modifies the lattice geometry. For the heavily doped Li0.85Co0.15FePO4, carbothermal reduction induced by excessive thermal treatment (T > 700 °C or extended holding) yields a secondary phosphide phase that is tentatively indexed as Co2P from two weak reflections. Within the Co-substituted series, the 700 °C—10 h condition gives a single-phase product with the highest saturation magnetisation (Ms = 3.45 emu·g−1), which decreases to 2.07 emu·g−1 at 800 °C. VSM results demonstrate a decoupling between the saturation magnetisation (Ms) and the coercivity (Hc); the weak ferromagnetic hysteresis observed in all samples signals trace ferromagnetic secondary phases—most plausibly amorphous or nanoscale products of xerogel thermal decomposition that lie below the XRD detection limit—rather than indicating phase purity; this attribution is indirect and is critically assessed below. Mössbauer spectroscopy validates the coexistence of dominant high-spin Fe2+ and minor Fe3+ species within the samples. High-level Co substitution (y = 0.15) lowers the relative proportion of the Fe3+ impurity species to 7.2% and alters the local Fe coordination environments without generating magnetically ordered sextet components. This work demonstrates that the sol–gel-derived hybrid xerogel is a tunable platform for fabricating phosphate-based battery nanocomposites and provides insights for the gel-precursor-directed design of energy-storage functional materials.