Phosphorus doped Li2FeSiO4 thin films prepared by magnetron sputtering as high-performance thin-film cathode materials

Polycrystalline Li ₂FeSiO ₄ (LFS) and Li ₂FeP _xSi _1-xO ₄ (LFSP) thin film cathode materials tested in quasi-solid thin-film Li-ion cells have been synthesized using radio frequency magnetron sputtering in combination with rapid thermal annealing. The thin films were characterized for their phase and morphology using X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The charge transport properties and electrochemical performance of the films were evaluated through constant current charge-discharge tests, cyclic voltammetry, rate performance testing, electrochemical impedance spectroscopy (EIS), and galvanostatic intermittent titration technique (GITT). The research findings revealed that after the sputtering growth process, during rapid annealing at 850 °C, the as prepared amorphous Li ₂FeSiO ₄ thin film showed selective crystalline growth, while crystallized grains on the surface of Li ₂FeSi _0.95P _0.05O ₄ thin films exhibited a fusion like and relatively regular distribution. The electronic conductivity and ionic conductivity of Li ₂FeSi _0.95P _0.05O ₄ thin films were measured at 6.60 × 10 ⁻⁷ S·cm ⁻¹ and 4.87 × 10 ⁻⁷ S·cm ⁻¹, respectively. The Li ₂FeSi _0.95P _0.05O ₄ thin film displayed an initial discharge specific capacity of 224.05 mAh g ⁻¹ at a rate of 0.1 C, with a capacity retention of 73.5 % after 100 cycles in half-cell structure. The assembled Li ₂FeSi _0.95P _0.05O ₄/Li _1.3Al _0.3Ti _1.7(PO ₄) ₃/Li quasi-solid-state battery presented an initial discharge specific capacity of 195.62 mAh g ⁻¹ at 0.1 C, with a capacity retention of 55.03 % after 100 cycles. This study offers a valuable reference for the fabrication and doping modification of poly-anion-type Li ₂FeSiO ₄ cathode thin film materials, thus aiding the progress of next-generation thin-film quasi-solid-state lithium-ion batteries.