Li3–x Zr x (Ho/Lu)1–x Cl6 Solid Electrolytes Enable Ultrahigh-Loading Solid-State Batteries with a Prelithiated Si Anode

We report two new families of lithium metal chloride solid electrolytes Li3–x Zr x (M)1–x Cl6 (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 Li2.4Zr0.6(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 Li2.6Zr0.4(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 Li0.7Si 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.