Contact mechanics and electrochemical properties of the Li6.5La3Zr1.5Ta0.5O12| Li2.25Zr0.75Fe0.25Cl6 hetero-electrolyte interface in a low-pressure regime

While there are numerous solid electrolytes (SE), not one simultaneously meets all the criteria to enable practical application of lithium (Li) metal solid-state batteries. The multilayer electrolyte configuration has garnered significant attention where one electrolyte is used in contact with the anode and another one as a catholyte, creating a hetero-electrolyte interface between the two SEs. Studies of hetero-electrolyte interfaces, especially at low pressures (< 5 MPa), are limited. This work investigates the contact mechanics and interfacial resistance of the hetero-electrolyte interface between the high voltage stable halide Li_2.25Zr_0.75Fe_0.25Cl₆ (LZFC) and the oxide Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZTO), which is stable with Li. We report that the surface roughness of either SE strongly affects the LLZTO|LZFC interfacial resistance (R_int) below 5 MPa. By controlling the interface roughness, R_int was decreased by a factor of 5 – from 3280 Ωcm² to 648 Ωcm². We demonstrate that warm pressing of the LLZTO|LZFC interface at 80 °C promotes creep of the LZFC to assist in forming more contact at low pressures, such that R_int decreased by 50 %. Furthermore, acid washing the LLZTO with HCl or H₃PO₄ imparts additional roughness by 2-6× and leads to an interfacial instability between HCl treated LLZTO and LZFC. Nevertheless, after warm pressing, a LLZTO|LZFC|LLZTO symmetric cell was cycled for 50 cycles, passing 1 mAh/cm² of charge per half cycle with no increase in resistance regardless of the LLZTO surface treatment, indicating a passivated LLZTO|LZFC interface. This work provides insight into the design and construction of multilayer SSBs at low stack pressures.