Multicomponentization of a super-Na ionic conductor chloride NaTaCl6, enhancing ionic conductivity and electronic resistivity

Abstract

All-solid-state Na-ion batteries have attracted considerable attention because of their advantages such as high safety, high energy density, and low cost. Solid electrolytes used in these batteries require high Na-ion conductivity to minimize energy loss, high electronic resistivity to prevent self-discharge, and high oxidation resistance to enable the use of high-potential cathodes. Recently, NaTaCl₆ was reported to possess both high oxidation resistance and ionic conductivity, and its ionic conductivity improved with a decrease in its crystallinity. Therefore, in this study, we aimed to further reduce the crystallinity of NaTaCl₆ and improve its ionic conductivity and electronic resistivity through multicomponentization. The verified composition was Na₂Ta_0.625Zr_0.25Ga_0.125Cl_5.625(CO₃)_0.25(BO₃)_0.125, wherein polyatomic anions (CO₃²⁻ and BO₃³⁻) were expected to have inductive effects that maintained the high oxidation resistance of chloride. Through multicomponentization, the NaTaCl₆ phase transitioned to a low-crystallinity state, resulting in a significant improvement in the ionic conductivity (1.2 × 10⁻³ S cm⁻¹), which was approximately ten times higher than that of crystalline NaTaCl₆ (1.1 × 10⁻⁴ S cm⁻¹). The electronic resistivity of the low-crystallinity multicomponent was more than one order of magnitude higher than that of crystalline NaTaCl₆, effectively suppressing self-discharge and improving the energy-storage preservation properties of the material. Furthermore, the multicomponent NaTaCl₆ retained a high oxidation resistance with an oxidation limit of 4.6 V vs. Na/Na⁺. Thus, this multicomponentization strategy simultaneously retains high oxidation resistance while improving the ionic conductivity and electronic resistivity of the electrolyte material, thereby enabling the development of high-performance all-solid-state Na-ion batteries.