Leidenfrost‐Assisted Synthesis of Indium‐Substituted Mixed Phosphate Cathodes with Superior Cycling Stability and Enhanced Sodium Storage Kinetics

The unavailability of high-performance cathodes hinders large-scale adoption of sodium-ion batteries (NIBs). In this work, we report for the first time a Leidenfrost-assisted synthesis as a low-cost and scalable approach for designing In³⁺-doped mixed phosphate ([PO₄]^3--[P₂O₇]^4-) cathodes. The strategic substitution of In³⁺ at the Fe site induces lattice expansion, thereby facilitating enhanced Na⁺ diffusion and improved electrochemical performance. The optimized cathode composition, Na₄Fe_2.97In_0.03(PO₄)₂P₂O₇ (NFIPP03), exhibits exceptional electrochemical performance, with a specific capacity of 129.3 mAh g^-1 at 0.05 C, corresponding to an energy density of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mo>∼</mml:mo> <mml:annotation>$\sim $</mml:annotation></mml:semantics> </mml:math> 359 Wh kg^-1, and a stable cycling performance more than 10 000 cycles at 20 C. Temperature-dependent magnetic susceptibility (M-T) and electron paramagnetic resonance (EPR) measurements reveal an enhanced spin state in NFIPP03 compared to the pristine sample, as well as improved electrical conductivity. Ex situ XRD and XPS analyses confirm excellent structural and chemical stability during (de)sodiation. Furthermore, density functional theory (DFT) calculations indicate significantly widened Na⁺ migration pathways, reduced activation energy barrier, and an attenuated bandgap in NFIPP03 which corroborates our experimental observations. Our findings highlight the synthesis for developing cost-effective, high-performance iron-based mixed-phosphate cathodes, advancing the sustainability and scalability of NIB technology.