All-solid-state lithium ion batteries (SLiB) represent a promising battery technology thanks to the replacement of the volatile and flammable state-of-the-art liquid electrolyte by a non-flammable solid electrolyte (SE).[1] Recent progress in the development of SE with high ionic conductivity moves the battery performance, i.e. energy and power density closer to the state-of-the-art Li-ion technology.[2] Despite the significant improvement of the bulk materials properties, degradation phenomena at the electrode/electrolyte interface remain to be understood in order to ensure long-term cycling stability of the batteries. Post-mortem XPS has proved to be a powerful and appropriate technique for monitoring the chemical states of elements at the interfaces between the SE, the active materials, and conductive carbon.[3] However, it still suffers from experimental limitations to explore the full picture of the surface and interface reaction mechanisms.

We present here operando X-ray photoemission spectroscopy (XPS) as an advanced methodology to assess the interfacial (electro-) chemical reactivity between active materials (AM), conductive carbon and SE during battery cycling leading to the formation of interphases at the nanoscale which critically affects the ion transport within the SLiB. Starting from the working principles of electron spectroscopy, we derive a model to assign the observed binding energy (BE) shift to different electronic properties of the materials. This way, in addition to the observed chemical reactions in real-time, we gain further knowledge on the surface potential by analyzing the voltage dependent spectra evolution. Through an efficient experimental implementation, this method enables simultaneous monitoring of the chemical and electronic properties of the electrode-electrolyte interphases during battery operation. These fundamental knowledge will be applied and demonstrated in several case studies: i) using a mixture of amorphous (Li₂S)₃-P₂S₅ (LPS) as SE with conductive carbon, the redox reaction mechanism of the SE is fully revealed, ii) using LiCoO₂ and amorphous (Li₂S)₃-P₂S₅ as SE, this methodology allows to determine precisely the oxidation potential of the SE, the interfacial reaction products as well as the cross reaction mechanisms between LCO and LPS and iii) using a mixture of In, LPS and carbon, the chemical and electrochemical reaction within a commonly used counter electrode composite is elucidated.

References:

[1] Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba, R. Kanno, Nature Energy, 1 (2016) 16030.

[2] N. Kamaya, K. Homma, Y. Yamakawa, M. Hirayama, R. Kanno, M. Yonemura, T. Kamiyama, Y. Kato, S. Hama, K. Kawamoto, A. Mitsui, Nat Mater, 10 (2011) 682-686.

[3] J. Auvergniot, A. Cassel, J.-B. Ledeuil, V. Viallet, V. Seznec, R. Dedryvère, Chemistry of Materials, 29 (2017) 3883-3890.