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Lithium metal anodes have attracted extensive attention owing to their high theoretical specific capacity. However, the notorious reactivity of lithium prevents their practical applications, as evidenced by the undesired lithium dendrite growth and unstable solid electrolyte interphase formation. Here, we develop a facile, cost-effective and one-step approach to create an artificial lithium metal/electrolyte interphase by treating the lithium anode with a tin-containing electrolyte. As a result, an artificial solid electrolyte interphase composed of lithium fluoride, tin, and the tin-lithium alloy is formed, which not only ensures fast lithium-ion diffusion and suppresses lithium dendrite growth but also brings a synergistic effect of storing lithium via a reversible tin-lithium alloy formation and enabling lithium plating underneath it. With such an artificial solid electrolyte interphase, lithium symmetrical cells show outstanding plating/stripping cycles, and the full cell exhibits remarkably better cycling stability and capacity retention as well as capacity utilization at high rates compared to bare lithium.

Interphase Free Download

Engineering ex situ protective layers have attracted much attention for Li metal batteries (LMBs). Improving the modulus properties and ionic conductivity of the interphase by various strategies have been reported13,14. The poor Li contact between these interfacial layers and bulk Li could lead to an increase in both interfacial and overall cell resistance. The low wettability of interphase towards nonaqueous electrolyte also leads to sluggish Li-ion transport. Differing from surface engineering of the artificial interphase layer, the use of various electrolyte additives7,15 provides an alternative pathway, where a more intimate contact could be ensured.

MFiX-TFM (Two-Fluid Model) is an Eulerian-Eulerian model which supports a broad range of capabilities for dense, reacting, multiphase flows by representing the fluid and solids as interpenetrating continua. This is the most mature MFIX model and is capable of modeling multiphase reactors ranging in size from bench top to industry-scale. Approximation of the solid phase as a continuum typically allows for faster simulation time than Lagrangian techniques, however it also introduces the need for accurate mathematical models to capture realistic solids phase behavior. This includes transport properties, heterogeneous reaction kinetics, and constitutive relations for interaction between fluid and solid phases, e.g., solids phase drag and interphase heat transfer.

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Create citation alert 1945-7111/168/7/070557 Abstract Sulfide solid electrolytes (SEs) show promise for Li metal solid-state batteries due to their high ionic conductivities and relative ease of manufacturing. However, many sulfide SEs suffer from limited electrochemical stability against Li metal electrodes. In this work, we use a suite of operando analytical techniques to investigate the dynamics of solid electrolyte interphase (SEI) formation and the associated effects on Li plating. We contrast a sulfide SE that forms an electrically insulating SEI (Li6PS5Cl) with an SE that forms an SEI with electrically conducting phases present (Li10GeP2S12). Using anode-free cell configurations, where the Li/SE interface is formed against a current collector, we perform complimentary operando video microscopy and operando X-ray photoelectron spectroscopy (XPS) experiments. The combination of these techniques allows for the interpretation of electrochemical voltage traces during Li plating. The electrically insulating nature of the SEI in Li6PS5Cl facilitates Li metal nucleation and plating after the initial SEI formation. In contrast, in cells that form an electronically conducting SEI, the onset of Li plating is suppressed, which is attributed to a low Faradaic efficiency from continuous SE decomposition. The insights in this study reveal how interphase dynamics control the transition from SEI formation to plating in anode-free solid-state batteries.

In this work, we investigate the role of sulfide SE stability during Li plating on LPSCl (electrically insulating SEI) and LGPS (electrically conducting SEI) electrolytes in anode-free cells. We apply a multi-modal approach that combines operando analysis with post-mortem characterization of SEI formation across multiple length scales, ranging from bulk cells to nanoscale surface analysis. The electrochemical signatures of nucleation at a current collector/SE interface are compared, indicating a stark difference in the Faradaic efficiency for Li plating between the SE materials. Specifically, Li nucleation and growth occur rapidly in the LPSCl system, while the Faradaic yield is dominated by continuous SEI formation in LGPS.

The mechanisms behind these differences are analyzed using operando video microscopy and operando X-ray photoelectron spectroscopy (XPS) to probe the chemical evolution of the SEI and the initial nucleation of Li metal. Operando imaging of the anode-free current collector is synchronized with the cell voltage traces, which allows for observation of SEI formation and Li metal plating and direct correlation of these reaction pathways with the corresponding electrochemical signatures. This is complemented by operando XPS analysis, which reveals chemical differences on the SE surface during dynamic SEI evolution, and highlights the evolution of distinct SEI components in the two SE systems. The XPS analysis also provides chemical confirmation of the earlier onset of Li metal nucleation on the SE that forms a stable SEI (LPSCl). Together, these results help elucidate the mechanisms behind SEI growth in anode-free SSBs by directly contrasting the differences between stable and unstable SE systems.

Figure 1. Cell configurations. In each configuration Li is plated from the counter electrode up to the anode-free surface at the top of the cell. (A) Bulk cell with a Cu foil current collector at the anode-free interface. When studying the anode-free interface in LGPS, a thin layer of LPSCl was added at the Li counter electrode interface. (B) Operando video microscopy cell with a sputtered Mo current collector at the anode-free interface. (C) Operando XPS cell using an electron gun as a virtual electrode.

In contrast to LPSCl, Li/LGPS/Li symmetric cells that were cycled showed increasing polarization during cycling (Fig. S2), which is attributed to continued electrolyte decomposition in LGPS. 30 Therefore, in order to avoid convolution in the voltage trace of the (anode-free) working electrode with side-reactions at the bulk Li foil counter electrode, an LPSCl interlayer was used between the Li and LGPS (Fig. 1A). LGPS pellets were fabricated in a similar manner to the LPSCl samples. 54 mg of LGPS powder (1 mm) was pressed at 100 MPa and then the Cu current collector was added to one side. 25 mg (0.5 mm) of LPSCl powder was then added to the opposite side of the LGPS pellet (the Li metal side, Fig. 1A). The whole stack was then pressed to 520 MPa to densify the SE and adhere the Cu to the anode-free surface. 30

To complement the optical visualization experiments, operando XPS was performed to probe the chemical evolution of the anode-free surface during SEI formation. In the operando XPS technique employed here, a "virtual electrode" is formed by using an electron gun to provide a flux of electrons to the SE surface (Fig. 1B). 15,30,37 Similar to a cell with a physical current collector, as charge builds up on the SE surface, a potential gradient is generated across the cell. Li+ ions are pulled from the grounded counter electrode and drawn up through the SE to the exposed anode-free surface. At the exposed surface, the Li+ ions combine with the electrons from the electron gun, driving reduction reactions. Initially, the reduction products are composed of SEI species, and after a sufficient amount of charge is passed, the reaction pathway transitions to plating of metallic Li. Therefore, operando XPS provides insight not only into the dynamic evolution of SEI formation in anode-free cells but also into the chemical reaction pathways that determine the Faradaic efficiency of Li plating.

In summary, the operando XPS analysis provides complementary information to the bulk electrochemistry and the operando video microscopy experiments. In particular, the low Faradaic efficiency in the LGPS samples is shown to correspond with continuous SEI formation throughout the experiment. In contrast, in the LPSCl samples the interface stabilizes following initial SEI formation, after which Li metal plates out at the anode-free surface.

Figure 7. Li Reaction Pathways. (A) Equivalent circuit model illustrating three parallel reaction pathways at the anode-free interface. (B) Schematic of SEI formation in LPSCl, followed by (C) a transition in reaction pathways to Li nucleation when the impedance of the SEI formation pathway becomes sufficiently large. (D) Schematic of SEI formation in LGPS, showing the formation of an electrically conductive Li-Ge constituent. (E) Electron conduction through the Li-Ge pathways in the SEI results in the continuous reduction of "fresh" LGPS, and the reaction pathway never transitions to Li nucleation and growth.

In this study, a multi-modal operando analysis was employed to provide a direct comparison of the differences in SEI formation and Li plating between a sulfide SE that forms an electrically conducting SEI (LGPS) and one that forms an electrically insulating SEI (LPSCl). Anode-free cell configurations were probed using ex situ optical microscopy, operando video microscopy, and operando XPS. For LPSCl SEs, after initial SEI formation, the interface stabilizes and Li metal plating initiates. In contrast, for LGPS SEs, the interface does not stabilize and the SEI continues to grow throughout the charging process. Operando XPS analysis of the SEI components demonstrates the dynamic evolution of SEI formation in the two SEs. Specifically, the formation of LiCl in LPSCl and a Li-Ge alloy in LGPS result in dramatic differences in SEI formation dynamics. By integrating the observations from this multi-modal analysis, it can be concluded that the transition in reaction pathways from SEI formation to Li plating determines the Faradaic efficiency in anode-free SSBs. 041b061a72


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