Interfacial Stability Study of Argyrodite and Nb₂O₅-coated Ni-Rich NMC Under Electrochemical Cycling by Transmission Electron Microscopy

Interfacial phenomena in all-solid-state lithium-ion batteries are extensively studied since they are considered as the main limiting factor for performances and lifetime [1]. Among all electrolytes, Li₆PS₅Cl argyrodite receives great attention due to its high ionic conductivity and its convenience in technological integration [2]. On the other hand, high Ni content in NMC layered oxide cathode materials are known to offer best specific energy density and higher lithium utilization despite higher surface reactivity [3]. Hence, coating materials have been developed in order to inhibit surface degradation and lithium loss through stable SEI formation, such as Niobium-based coatings. However, all solid-state composite electrode based on argyrodite and cathode materials present complex chemical and structural changes that are not yet fully understood. Several approaches have been adopted to enhance comprehension of argyrodite reactivity such as X-ray photoelectron spectroscopy (XPS) [4], electrochemical impedance spectroscopy (EIS) [5], time-of-flight secondary ion mass spectrometry (TOF-SIMS) [6] and ultimately, Transmission Electron Microscopy (TEM) studies have excelled to provide comprehensive and spatially resolved chemical and structural data.<br/> <br/>In this context the spontaneous and the electrochemical reactivity of argyrodite Li₆PS₅Cl electrolyte material toward single-crystal Ni-rich NMC cathode material has been investigated. The impact of the Nb₂O₅ coating over the cathode material has also been deeply studied. Most of the characterizations have been led with the help of state of the art TEM-related technics, such as energy-dispersive X-ray spectroscopy (EDX), electron energy loss spectroscopy (EELS) and selected area electron diffraction (SAED). Great care has been taken to work in conditions in which the materials are not altered.<br/> <br/>Preliminary study of Nb₂O₅ coating provides details on its crystallinity, thickness, coverage on cathode material, and stability toward lithium diffusion. Cathode material / coating interface has been notably observed on a thin specimen with high resolution imaging coupled with EDX and remarkable details on lithium diffusion through a few nanometers in the coating have been recorded on spectroscopic measurements. With and without this coating, NMC / argyrodite mixture does not show any spontaneous interface generation. However, under electrochemical solicitation, several species have been identified and characterized such as LiCl, P₂S₅, P₂O₅, Li₃PS₄, NiS, and S⁰. The correlation with XPS allowed to point out unrevealed subtleties until now and to discuss degradation mechanisms. The segregation of LiCl toward Li₃PS₄ has been directly observed, validating the theoretical predictions of argyrodite degradation depending on stability window.<br/> <br/>Moreover, a novel <i>operando</i> TEM characterization technique has been developed to study the interfacial modifications of argyrodite / NMC cells during electrochemical cycling within the TEM. This is the first time such original setup is proposed inside the electron microscopy community: it allows the direct <i>operando</i> characterization of the battery while keeping it always prevented from air exposure from the device preparation to the characterization inside the TEM.<br/> <br/>The authors gratefully acknowledge the financial support of Umicore for this work and for material providing.<br/> <br/> <br/>[1] K. Nie et al., Front. Chem. 6 (2018) 616<br/>[2] N. Kamaya et al., Nature Mater. 10 (2011) 682–686<br/>[3] W. Liu et al., Angew. Chem. Int. Ed. 54 (2015) 4440 – 4457<br/>[4] J. Auvergniot et al., Solid State Ionics 300 (2017) 78–85<br/>[5] D.H.S Tan et al., ACS Energy Lett. 4 (2019) 2418−2427<br/>[6] F. Walther et al., Chem. Mater. 31 (2019) 3745−3755