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 NaFeCl, comprising ubiquitous elements, as an electrode material for all-solid-state sodium-ion batteries. Monoclinic NaFeCl, expected to be the most resource-attractive Fe redox material, is also thermodynamically stable. The Fe redox reaction of the monoclinic NaFeCl electrode has a higher potential (3.45 V vs. Na/Na) than conventional oxide electrodes (e.g., FeO with 1.5 V vs. Na/Na) because of the noble properties of chlorine. Additionally, NaFeCl exhibits unusually high deformability (99% of the relative density of the pellet) upon uniaxial pressing (382 MPa) at 298 K. NaFeCl 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 was achieved using only a simple powder press.