Electrochemical and Surface Chemistry Analysis of Lithium Lanthanum Zirconium Tantalum Oxide (LLZTO)/Liquid Electrolyte (LE) Interfaces

A stable solid electrolyte/liquid electrolyte (SE/LE) interface is essential to enable hybrid electrolytes, where the SE protects Li metal anodes and the LE improves SE/cathode interfacial kinetics. However, previous reports have shown that the promising SE Lithium Lanthanum Zirconium Tantalum Oxide (LLZTO) reacts with LiPF6-based state-of-the-art LEs. This work aims to identify the source of this instability and propose approaches to ameliorate it. This is achieved by studying and isolating the effects of organic solvents and Li salts present in the LE on LLZTO. The results indicate that the reactions with the typical solvents are not as detrimental as with the corresponding salts. For example, LiPF6 salt reacts with LLZTO to form LiF, LaF3 and ZrF4, which leads to an increased SE/LE interfacial resistance (R-interface). The R-interface after reacting with LiPF6 and LiBOB salts is similar to 120 and 2000 Ohms.cm(2), respectively. However, LiTFSI salt is compatible with LLZTO with R-interface similar to 55 Ohms.cm(2). Furthermore, optimization of the LiTFSI salt concentration (3 M) results in an Rinterface of similar to 30 Ohms.cm(2). We demonstrate preliminary cycling of Li/LLZTO/LE/NCA cells with similar to 100% utilization and 98.5% capacity retention over 10 cycles. Overall, this study provides a step towards the successful implementation of hybrid electrolytes in Li metal batteries.