Variable Energy Hard X-ray Photoemission Spectroscopy: A Non-destructive Tool to Analyze the Cathode-Solid State Electrolyte Interface.

All-solid-state batteries are expected to be promising next-generation energy storage systems with increased energy density compared to the state-of-the-art Li-ion batteries. Nonetheless, the electrochemical performances of the current all-solid-state batteries are currently limited by the high interfacial resistance between active electrode materials and solid-state electrolytes. In particular, elemental inter-diffusion and the formation of interlayers with low ionic conductivity are known to restrict battery performance. Herein, we apply a non-destructive variable energy hard X-ray photoemission spectroscopy to detect the elemental chemical states at the interface between cathode and solid state electrolyte, in comparison to the widely used angle resolved (variable angle) X-ray photoemission spectroscopy /X-ray absorption spectroscopy methods. The accuracy of variable energy hard X-ray photoemission spectroscopy is also verified with a focused-ion beam & high-resolution transmission electron microscopy. We also show the significant suppression of inter-diffusion by building an artificial layer via ALD at this interface.