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

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., NaNi_1/3Fe_1/3Mn_1/3O₂) 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 NaNi_1/3Fe_1/3Mn_1/3O₂ 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.