Chloride Electrode Composed of Ubiquitous Elements for High-Energy-Density All-Solid-State Sodium-Ion Batteries

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Inexpensive and safe energy-storage batteries with high energy densities are in high demand (e.g., for electric vehicles and grid-level renewable energy storage). This study focused on NaFeCl4, comprising ubiquitous elements, as electrode material for all-solid-state sodium-ion batteries. Monoclinic NaFeCl4, expected to be the most resource-attractive Fe redox material, is also thermodynamically stable. The Fe2+/3+ redox reaction of the monoclinic NaFeCl4 electrode, had a higher potential (3.45 V vs. Na/Na+) than conventional oxide electrodes (e.g., Fe2O3 with 1.5 V vs. Na/Na+) because of the noble properties of chlorine. Additionally, NaFeCl4 exhibited unusually high deformability (99% of the relative density of the pellet) upon uniaxial pressing (382 MPa) at 298 K. This material operates at 333 K in an electrode system without an electrolyte, which is realizable in next-generation all-solid-state batteries with high safety. High energy density per positive electrode of 311 Wh kg-1 is achieved using only a simple powder press.