Interdiffusion Induced Cathode-Electrolyte Interface Instability of Single-Crystalline LLTO Solid Electrolyte

All-solid-state battery using solid electrolyte is promising for next generation energy storage device due to its potential for stability and high energy density. In particular, Li_3<i>x</i>La_2/3<i>-x</i>TiO₃ (LLTO) (0.05 &lt; <i>x </i>&lt; 0.167), a perovskite type solid electrolyte, has shown an ionic conductivity as high as 10^-3 S/cm at a <i>x</i> of 0.11 as well as air-stability. However, a high interfacial resistance when in contact with an electrode and interfacial instability between the electrode and solid electrolyte caused by electrochemical degradation make it difficult to put LLTO into practical use. Although various efforts including adding an artificial solid electrolyte interface layers such as Al₂O₃ have been employed to mostly mitigate the interface of lithium-metal anodes, the degradation issue at the electrolyte-cathode interface still remains. The interdiffusion of metal-ions between polycrystalline LLTO pellet and LiCoO₂ (LCO) has been reported, however the bulk diffusion without grain boundaries is desirable to evaluate the interdiffusion phenomena and investigate its effects on the Li-ion transport properties of LLTO.<br/>In this study, we investigate the instability of the single crystalline LLTO electrolyte-cathode interface induced by the interdiffusion of metal cations in conventional cathode materials, LCO and LiNi_0.8Co_0.1Mn_0.1O₂ (NCM). We deposit single crystalline LLTO (100) solid electrolyte thin films to exclude the effect of grain boundaries and structural defects of LLTO. Single crystalline LLTO (001) thin-films are deposited by using pulsed laser deposition (PLD) technique epitaxially on SrTiO₃ (001) substrates having a lattice mismatch of 0.8%. The LCO or NCM thin-films are also stacked on LLTO/STO. The structure and the diffusion dynamics between the cathode and the electrolyte are characterized by scanning transmission electron microscopy and X-ray photoelectron spectroscopy with a depth profiling. The interdiffusion of metal cations at the LLTO-LCO interface are measured to be more rapid than that of the LLTO-NCM interface, even with lower deposition temperature. The interfacial resistance of the resulting interfaces from each cathode and the electrolyte are measured through electrochemical impedance spectroscopy. Furthermore, approaches using diffusion barrier layers to mitigate the interdiffusion of ion conductors and cathodes are also discussed.