Fast ionic conduction achieved through the design and synthesis of ceramic heterointerfaces

Abstract

Lithium (Li) chloride and iron oxychloride (FeOCl), typically nonconductive, were combined to form a [Li_1+δCl]^δ+/[FeOCl]^δ− heterointerface composite material (LFH), achieving ionic conductivities of >1 mS cm⁻¹. Analysis techniques (scanning transmission electron microscopy [STEM] and electron energy-loss spectroscopy [EELS]) indicated that the microstructure of LFH consisted of an amorphous LiCl-based shell surrounding a crystalline FeOCl-based core. Electrochemical measurements alongside solid-state ^6,7Li nuclear magnetic resonance (NMR) and molecular dynamic simulations revealed Li⁺ as the sole conductive species, with a diffusion barrier of ∼0.25 eV. X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) results further supported interstitial Li⁺ diffusion at the heterointerface and within the LiCl phase, made possible by the heterointerface. Despite susceptibility to electronic conductivity, iron’s defects and multivalency (Fe³⁺, Fe²⁺) enable the Fe–O–Cl framework to accept Cl⁻, facilitating Li⁺ ionic conduction. A prototype solid-state cell (showing 97% Coulombic efficiency) demonstrated the viability of this heterointerface design for applications in energy storage.