Deformable, Crack-free, and Low-Cost NaFeCl₄ Cathode with Highly Structural Reversibility for Stable All-Solid-State Sodium Batteries

All-solid-state sodium batteries (ASSSBs) with a working voltage of approximately 3.5 V hold great potential for safe, sustainable, and high-energy-density electrochemical energy storage. However, 3.5 V oxide-type cathodes (e.g., NaNi1/3Fe1/3Mn1/3O2) may suffer from interfacial contact loss with solid electrolytes due to their intrinsic rigidity, resulting in irreversible capacity degradation during cycling. Herein, we report a highly deformable chloride cathode, NaFeCl4, displaying Young's modulus of only 1.33 GPa and excellent structural reversibility. Benefiting from the soft lattice, the NaFeCl4 cathode maintains a crack-free morphology despite significant changes in structural units between tetrahedra [Fe3+Cl4] and octahedra [Fe2+Cl6] upon repeated Na+ insertion/extraction, achieving a high capacity retention of 84.4% over 500 cycles. Moreover, with only 5.2% of the cost of the NaNi1/3Fe1/3Mn1/3O2 and 3.7% of the cost of Na3V2(PO4)3 cathode, NaFeCl4 can deliver a working voltage of ∼3.45 V vs Na+/Na and energy density of ∼405 Wh kg-1. This chloride cathode offers new insight into solving the mechanical issues in solid-state systems, guiding future developments of low-cost, stable, and high-voltage cathodes for ASSSBs.