High-Entropy Rare Earth Halide Electrolytes with Improved Cycling Stability and Cost Effectiveness

Rare earth halide solid state electrolytes (SSEs) have been attracting wide interest recently. However, the high cost of rare earth resources and poor cycling performance in all-solid-state batteries (ASSBs) hinder their practical applications. In this work, two novel high-entropy rare earth halide electrolytes, Li_2.6(Y,Ho,Er,Tm,Yb)Cl_5.6 (HEE-1) and Li_2.6(Y,Ho,Er,Tm,Yb)_0.5Zr_0.5Cl_6.1 (HEE-2), were rationally designed. Both electrolytes use five specially selected rare earth elements to achieve good cost effectiveness. These two electrolytes show high ionic conductivities of 0.52 mS·cm^-1 and 1.46 mS·cm^-1 at 25 °C, respectively. Lab-scale ASSBs incorporating HEE-1 and HEE-2 exhibit superior high-voltage compatibility and long-cycle stability. The capacity retentions reach 96.2% and 88.1% after 200 cycles, respectively, when charged to 4.5 V (vs Li⁺/Li) at a rate of 0.3C. Meanwhile, the cell with HEE-2 maintains 91.4% capacity after 1000 cycles between 2.5 and 4.2 V (vs Li⁺/Li) at a rate of 3C. The high-entropy design enables rare earth halide SSEs with better commercial potentials.