All-solid-state batteries (ASSBs) are currently attracting much attention as next generation energy storage systems, with potentially both higher energy and power densities than state-of-the-art lithium-ion batteries (LIBs). The latter may be realized by combining layered Ni-rich oxide cathode materials such as Li_1+x(Ni_1–y–zCo_yMn_z)_1–xO₂ (NCM) and thiophosphate-based solid electrolytes (SEs). As for SEs, in particular argyrodites, Li₆PS₅X (X = Cl, Br), show great promise since they exhibit sufficient Li-ion conductivity (~2 mS/cm) and because of the simplicity of preparation.¹ However, their practical implementation is hindered by poor electrochemical stability at the interface, in particular with positive electrode materials.^2,3 In order to pave the way for rational interface design, processes occurring between cathode and electrolyte need to be better understood. Following these guiding principles, we herein report our recent efforts in implementing various in situ and ex situ techniques (X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, differential electrochemical mass spectrometry etc.) to gain mechanistic insight into interfacial SE degradation when in direct contact with NCM622 (60% Ni). We reveal the potential resolved formation of degradation products, which are found to be responsible in part for irreversible capacities and performance decay of ASSB cells. However, we also demonstrate that by using protective oxide coatings such as LiNbO₃, these detrimental side reactions can be suppressed to a large extent. In summary, we reveal the importance of developing in situ characterization techniques in the area of ASSBs and provide valuable insight into the cathode/SE interfacial chemistry.

References:

[1] Bachman, J. C.; Muy, S.; Grimaud, A.; Chang, H.-H.; Pour, N.; Lux, S. F.; Paschos, O.; Maglia, F.; Lupart, S.; Lamp, P.; et al. Inorganic Solid-State Electrolytes for Lithium Batteries: Mechanisms and Properties Governing Ion Conduction. Chem. Rev. 2016, 116 (1), 140–162.

[2] Richards, W. D.; Miara, L. J.; Wang, Y.; Kim, J. C.; Ceder, G. Interface Stability in Solid-State Batteries. Chem. Mater. 2016, 28 (1), 266–273.

[3] Auvergniot, J.; Cassel, A.; Ledeuil, J.-B.; Viallet, V.; Seznec, V.; Dedryvère, R. Interface Stability of Argyrodite Li₆PS₅Cl toward LiCoO₂, LiNi_1/3Co_1/3Mn_1/3O₂ , and LiMn₂O₄ in Bulk All-Solid-State Batteries. Chem. Mater. 2017, 29 (9), 3883–3890.