Li3–xZrx(Ho/Lu)1–xCl6 Solid Electrolytes Enable Ultrahigh-Loading Solid-State Batteries with a Prelithiated Si Anode

We report two new families of lithium metal chloride solid electrolytes Li_3–xZr_x(M)_1–xCl₆ (0 ≤ x ≤ 0.8; M = Ho or Lu) with ionic conductivities of up to 1.8 mS cm^–1 and a low activation energy of 0.34 eV. Structural elucidation via high-resolution neutron diffraction determines the Li ion distribution in trigonal Li₃HoCl₆, orthorhombic-I Li₃LuCl₆, and orthorhombic-II Li_2.4Zr_0.6(Ho/Lu)_0.4Cl₆. The last compound exhibits well-connected Li-ion pathways and abundant Li-ion carriers/vacancies to promote diffusion. All-solid-state batteries with Li_2.6Zr_0.4(Ho/Lu)_0.6Cl₆ solid electrolytes and NCM85 cathodes exhibit stable cycling up to 4.6 V vs Li⁺/Li, which is even preserved up to 4.8 V. Stable cathode interphases are formed for both electrolytes upon cycling to 4.3, 4.6, and 4.8 V cutoff potentials, as identified by a ToF-SIMS analysis. Solid-state cells with a prelithiated Li_0.7Si anode exhibit a significantly increased initial coulombic efficiency of 94.5% compared to Si and a high areal capacity of up to 16.3 mAh·cm^–2.