Conformal Hetero-Electrolyte Interface between Soft Oxyhalides and Garnet Enables Low-Pressure Lithium Reservoir-Free Solid-State Batteries

Abstract

Operation of solid-state batteries with a lithium metal anode and a high voltage cathode requires solid electrolytes (SE) to be chemically stable with lithium, have a wide electrochemical window, and accommodate volume changes of the electrodes. Unfortunately, no SE exhibits satisfactory mechanical and electrochemical properties that fit these requirements to date. Dual solid electrolyte systems that use a different SE for the anolyte and catholyte present a viable solution. Here, we focus on oxyhalide SEs that demonstrate superior ionic conductivity and cathode compatibility, and where their lithium metal reactivity and poor reduction stability can be resolved using a lithium garnet (Li6.5La3Zr1.5Ta0.5O12, LLZTO) separator. Nonetheless, this imposes a new hetero-electrolyte (H-E) interface at the anolyte|catholyte contact that defines ion transport across the boundary. We report its promising properties, deconvoluted from the electrical measurements of bilayer symmetric cells, for three representative oxyhalide catholytes - LiNbOCl4, LiTaOCl4, and Li3Al3O2Cl8. Pressure-dependent measurements reveal that the relative softness of the oxyhalides (≤ 0.4 GPa) enable H-E resistances lower than 150 Ω cm2 at 2-3 MPa. Mesoscale modeling reveals that the transfer-active interface of oxyhalides with LLZTO is about 2-3 fold higher compared to the argyrodite Li6PS5Cl. The low H-E resistance of the LiNbOCl4|Li6.5La3Zr1.5Ta0.5O12 dual electrolyte enables cycling of a LiNi0.82Mn0.07Co0.11O2|Li full cell with a high discharge capacity (200 mAh g-1) at 60 °C and ~7 MPa.