Chloride electrode composed of ubiquitous elements for high-energy-density all-solid-state sodium-ion batteries

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 using NaFeCl₄, comprising ubiquitous elements, as an 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 Fe^2+/3+ redox reaction of the monoclinic NaFeCl₄ electrode has 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 exhibits unusually high deformability (99% of the relative density of the pellet) upon uniaxial pressing (382 MPa) at 298 K. NaFeCl₄ operates at 333 K in an electrode system containing no electrolyte, thereby realizing next-generation all-solid-state batteries with high safety. A high energy density per positive electrode of 281 Wh kg^-1 was achieved using only a simple powder press.