Thermodynamically Driven Phase Separation and Heterogeneous Interfacial Fast-Ion Conduction in the NTC-LTC Electrolyte

Halide solid electrolytes have emerged as promising candidates for all-solid-state batteries (ASSBs) owing to their high oxidative stability, mechanical deformability, and continuously improving ionic conductivity. Notably, amorphous halide electrolytes often exhibit superior ionic transport compared to their crystalline counterparts, benefiting from the absence of grain boundaries, flexible local coordination environments, and decoupled ion–matrix dynamics. However, their practical development is hindered by the intrinsic thermodynamic preference of halides for crystallization, which necessitates prolonged high-energy mechanical milling to achieve amorphization. Here, we propose a thermodynamics-driven strategy to promote amorphization through mixed-cation design. Using the NaTaCl 6 –LiTaCl 6 (NTC–LTC) system as a model, we demonstrate that the introduction of mixed alkali cations destabilizes the crystalline phase, leading to a positive mixing enthalpy and spontaneous phase separation into NTC and LTC. This thermodynamic instability significantly lowers the energy barrier for amorphization, enabling rapid formation of an amorphous phase. Remarkably, substantial amorphization is achieved after only 5 h of high-energy ball milling, in stark contrast to the 40 h and 120 h required for NTC and LTC, respectively. The resulting phase-separated electrolyte exhibits a high ionic conductivity of 3.11 mS·cm⁻ 1 . Furthermore, NMR relaxation analysis reveals that the system predominantly conducts Na⁺ ions, suggesting preferential amorphization of the Na-rich phase and enhanced ion transport across heterointerfaces. This work establishes a generalizable thermodynamic design principle for developing easily amorphizable halide electrolytes. By leveraging composition-induced phase instability, this approach reduces processing requirements while simultaneously generating beneficial heterostructures, offering a scalable pathway toward high-performance amorphous halide electrolytes for next-generation ASSBs.