Fast Charge Transfer between LLZO Solid Electrolyte and LCO Cathode Thin Films with a Modified Interface

Thin film deposition of solid electrolytes can be useful for the fabrication of better solid-state batteries by reducing the thickness of the electrolyte material to a few hundred nanometers. This facilitates ionic conductance for faster charge-discharge and reduces the total volume of inactive material, therefore increasing energy and power density. Thin film batteries can also be valuable model systems to investigate interface charge dynamics and to serve as playgrounds for interface engineering, without the complexity of bulk systems. Here we investigate the charge transfer properties between the lithium garnet Li7La3Zr2O12 (LLZO) solid electrolyte and the LiCoO2 cathode using a thin-film model system, with the aim of reducing the interface resistance and allowing high charge-discharge rates. Bulk LLZO electrolyte has come under the spotlight owing to its high ionic conductivities (10-4 - 10-3 S/cm) as well as a wide electrochemical stability window (against metallic lithium anode and high potential cathode materials). We developed a method for fabricating crystalline LLZO thin films using magnetron co-sputtering followed by an annealing step at 700°C (significantly below the standard processing temperatures). The resulting 500 nm-thick Ga-doped LLZO thin films show densities and ionic conductivities (2×10-4 S/cm) comparable to the values observed in bulk ceramic pellets.[1] Based on this thin film fabrication process, we fabricated an all-thin-film model system to investigate the LLZO / LCO cathode interface. This architecture provides an easy access to the interface for characterization, allowing one to identify the degradation processes taking place at the interface under high-temperature co-sintering. Introducing an in situ-lithiated Nb2O5 diffusion barrier at the interface, it was possible to lower the LLZO / LCO charge transfer resistance to about 50 Ω cm2, a 3-fold reduction with respect to previously reported values. The low interfacial resistance combined with the high conductance through the LLZO thin-film electrolyte allows charge transfer at high charge–discharge rates up to 40 C (0.9 mA/cm2).[2] References [1] Sastre, Jordi, et al. "Lithium Garnet Li7La3Zr2O12 Electrolyte for All‐Solid‐State Batteries: Closing the Gap between Bulk and Thin Film Li‐Ion Conductivities." Advanced Materials Interfaces (2020): 2000425. [2] Sastre, Jordi, et al. "Fast Charge Transfer across the Li7La3Zr2O12 Solid Electrolyte/LiCoO2 Cathode Interface Enabled by an Interphase-Engineered All-Thin-Film Architecture." ACS Applied Materials & Interfaces 12.32 (2020): 36196-36207.