Unraveling Electrochemical Stability and Reversible Redox of Y-Doped Li₂ZrCl₆ Solid Electrolytes

Lithium halide electrolytes show great potential in constructing high-energy-density solid-state batteries with high-voltage cathode materials due to their high electrochemical stability and wide voltage windows. However, the high cost and low conductivity of some compositions inhibit their applications. Moreover, the effect of electronic additives in the cathode mixture on the stability and capacity is unclear. Here, the Y³⁺ doping strategy is applied to enhance the conductivity of low-cost Li₂ZrCl₆ electrolytes. By tailoring the Y³⁺ dopant in the structure, the optimal Li_2.5Zr_0.5Y_0.5Cl₆ with high conductivity up to 1.19 × 10⁻³ S cm⁻¹ is obtained. Li_2.5Zr_0.5Y_0.5Cl₆@CNT/Li_2.5Zr_0.5Y_0.5Cl₆/Li_5.5PS_4.5Cl_1.5/In-Li solid-state batteries with different carbon nanotube (CNT) contents in the cathode are fabricated. The stability and electrochemical performances of the cathode mixture as a function of CNT content are studied. The cathode mixture containing 2% (wt.) CNT exhibits the highest stability and almost no discharge capacity, while the cathode mixture consisting of Li_2.5Zr_0.5Y_0.5Cl₆ and 10% (wt.) CNT delivers a high initial discharge capacity of 199.0 mAh g⁻¹ and reversible capacities in the following 100 cycles. Multiple characterizations are combined to unravel the working mechanism and confirm that the electrochemical reaction involves the 2-step reaction of Y³⁺/Y⁰, Zr⁴⁺/Zr⁰, and Cl⁻/Cl_x⁻ in the Li_2.5Zr_0.5Y_0.5Cl₆ electrolyte. This work provides insight into designing a lithium halide electrolyte-based cathode mixture with a high ionic/electronic conductive framework and good interfacial stability for solid-state batteries.