In situ Observation of Lithium Stripping and Plating Process in an Open-cell All-solid-state Lithium Metal Battery

All-solid-state batteries hold great promise to electric vehicles and commercial appliances with high energy density and improved safety concerns [1]. The superior ionic conductive solid-state electrolytes utilized in the solid-state battery not only improve the safety of the battery system but also significantly boost the energy density when coupled with Li metal anode, which enables extraordinarily high capacity (3860 mAh g−1) and lowest redox potential (-3.04 V vs. standard hydrogen potential) [2]. However, realizing high-performance solid-state battery remains challenging due to the solid-solid contact between Li metal anode and solid electrolytes. Poor interfacial contact is caused by the intrinsic surface roughness of both the Li metal and the electrolyte, which produces an increase in interfacial impedance and serves as a precursor for Li dendritic growth [3]. The formation of voids during Li stripping on the Li metal anode degrades the metal–electrolyte contact further [4]. These interfacial phenomena, involving the dynamics of the load and the evolution of microstructures, are critical to understanding the electrochemical nucleation behavior. Coupled interactions at Li-electrolyte interface necessitate a quantitative and multi-physics measurements. Previous efforts have been limited to characterizing individual component materials and have relied on post-mortem approaches that focus on major end products. The critical intermediate and metastable microstructural and/or electrolyte decomposition products were not clear through conventional approaches. In this study, we have for the first time performed real-time in operando observation of Li stripping and plating process at the Li-electrolyte interface using an in situ testing vessel from Solid Design Instruments, LLC. Figure 1 shows the schematic of vessel and the cell specimen setup. The Ar+-polished cross-sectional cell surface provided information on the spatial distribution of the process. Figure 2a shows a stripping and plating cycle on a realistic, fully operational solid-state cell (NMC | Li6PS5Cl | Li) as monitored by an optical microscope (Micromanipulator 1844 B&L Manual Probe Station, 20X). The stripping experiment was performed on the Li metal anode (Figure 2b-d) by applying a constant current density of 1.57 mA cm-2 with the opposite polarity for 1 hour. 1.69 mAh cm-2 of Li was stripped, corresponding to a thickness reduction of ∼7.2 µm on the Li metal anode (theoretically, the reduction should be ∼8.4 µm, assuming uniform Li stripping, as shown in Figure 2h). During the subsequent 1 hour of Li plating, a capacity of 1.45 mAh cm-2 was produced, with a matching deposition thickness of ∼4.08 µm on Li metal anode was measured (theoretically, the deposition should be ∼7.25 µm, assuming uniform Li deposition, as shown in Figure 2i). The significant loss of Li during the plating may be attributed to the lithium metal creep behavior [5, 6], which will be quantified in future studies. Moreover, the vessel is compatible with advanced characterization instruments such as SEM/FIB, Raman spectroscope, XRD, ToF-SIMS, and more, in addition to the optical microscope. This enables a direct examination of structural and property changes at the Li-electrolyte interface that are responsible for crucial electrochemical performance. In the future, we will report the results of these diagnostic tests performed with superior spatial and spectral resolution analytical toolsets. Experimental setup of the in situ testing vessel. Real-time monitored lithium stripping and plating process. (a) Current density, pressure, voltage profiles in a one cycle stripping and plating experiment. (b-d) Stripping. (e-g) Plating. (h) Theoretical and actual thickness of stripped Li. (i) Theoretical and actual thickness of plated Li.