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 4 , comprising ubiquitous elements, as an electrode material for all-solid-state sodium-ion batteries. Monoclinic NaFeCl 4 , expected to be the most resource-attractive Fe redox material, is also thermodynamically stable. The Fe 2+/3+ redox reaction of the monoclinic NaFeCl 4 electrode has a higher potential (3.45 V vs. Na/Na + ) than conventional oxide electrodes (e.g., Fe 2 O 3 with 1.5 V vs. Na/Na + ) because of the noble properties of chlorine. Additionally, NaFeCl 4 exhibits unusually high deformability (99% of the relative density of the pellet) upon uniaxial pressing (382 MPa) at 298 K. NaFeCl 4 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.