For the development of next-generation lithium batteries, major research effort is made to enable a reversible lithium metal anode by the use of solid electrolytes. However, the fundamentals of the solid− solid interface and especially the processes that take place under current load are still not well characterized. By measuring pressure-dependent electrode kinetics, we explore the electrochemo-mechanical behavior of the lithium metal anode on the garnet electrolyte Li 6.25 Al 0.25 La 3 Zr 2 O 12 . Because of the stability against reduction in contact with the lithium metal, this serves as an optimal model system for kinetic studies without electrolyte degradation. We show that the interfacial resistance becomes negligibly small and converges to practically 0 Ω•cm 2 at high external pressures of several 100 MPa. To the best of our knowledge, this is the smallest reported interfacial resistance in the literature without the need for any interlayer. We interpret this observation by the concept of constriction resistance and show that the contact geometry in combination with the ionic transport in the solid electrolyte dominates the interfacial contributions for a clean interface in equilibrium. Furthermore, we show thatunder anodic operating conditionsthe vacancy diffusion limitation in the lithium metal restricts the rate capability of the lithium metal anode because of contact loss caused by vacancy accumulation and the resulting pore formation near the interface. Results of a kinetic model show that the interface remains morphologically stable only when the anodic load does not exceed a critical value of approximately 100 μA•cm −2 , which is not high enough for practical cell setups employing a planar geometry. We highlight that future research on lithium metal anodes on solid electrolytes needs to focus on the transport within and the morphological instability of the metal electrode. Overall, the results help to develop a deeper understanding of the lithium metal anode on solid electrolytes, and the major conclusions are not limited to the Li|Li 6.25 Al 0.25 La 3 Zr 2 O 12 interface.

Li 7 La 3 Zr 2 O 12 (LLZO)-based garnet materials are recently being investigated as suitable electrolytes for solid-state batteries with lithium-metal electrodes. Unfortunately, lithium-metal penetration through polycrystalline garnet-type electrolytes limits the electric current density during cell charging. In this study, we introduce an electrochemical operando approach that is well suited to get insights into the early stage of lithium-metal penetration that was yet only accessible with very elaborate neutron measurements. Combined with in situ as well as ex situ electron microscopic techniques, we investigate the morphological instability of the lithium-metal anode on garnet-type solid electrolytes under cathodic load and demonstrate the inter-relationship between microkinetic aspects and lithium-penetration susceptibility.

The charge transfer kinetics between a lithium metal electrode and an inorganic solid electrolyte is of key interest to assess the rate capability of future lithium metal solid state batteries. In an in situ microelectrode study run in a scanning electron microscope, it is demonstrated that—contrary to the prevailing opinion—the intrinsic charge transfer resistance of the Li|Li6.25Al0.25La3Zr2O12 (LLZO) interface is in the order of 10−1 Ω cm2 and thus negligibly small. The corresponding high exchange current density in combination with the single ion transport mechanism (t+ ≈ 1) of the inorganic solid electrolyte enables extremely fast plating kinetics without the occurrence of transport limitations. Local plating rates in the range of several A cm−2 are demonstrated at defect free and chemically clean Li|LLZO interfaces. Practically achievable current densities are limited by lateral growth of lithium along the surface as well as electro‐chemo‐mechanical‐induced fracture of the solid electrolyte. In combination with the lithium vacancy diffusion limitation during electrodissolution, these morphological instabilities are identified as the key fundamental limitations of the lithium metal electrode for solid‐state batteries with inorganic solid electrolytes.

Li7La3Zr2O12 (LLZO) garnet-based oxides are a promising class of solid electrolytes used as the separator in all-solid-state batteries (ASSBs). While LLZO is considered to have a wide electrochemical stability window, its intrinsic stability in contact with lithium metal is not sufficiently well understood, and there is still a debate on the key question of whether LLZO does or does not form passivation layers before and during cycling. Utilizing both in situ and operando X-ray photoelectron spectroscopy techniques, we reveal the presence of a kinetic barrier to the reduction of LLZO by Li metal, with the extent of oxygen-deficient interphase (ODI) formation depending sensitively on the energetics of Li metal arriving at the Li|LLZO interface. Despite the clear presence of a kinetic barrier to reduction, the electrochemical response of the Li|LLZO interface is unchanged by the presence of the ODI, indicating that ODI formation during electrochemical cycling does not hinder charge transfer across the Li|LLZO interface. Overall, these results reveal that the reactivity of LLZO with Li metal depends not only on the material properties of the adjoining phases (i.e., surface purity and active contact) and their resulting thermodynamic stability but also on the energy input at the interface and the resulting reaction kinetics. Furthermore, the presence of a kinetic barrier to reduction highlights the additional complexities governing the reactivity of solid-state interfaces in ASSBs and underscores the importance of operando characterization of interfacial stability to design more robust, high-performance protection strategies for solid electrolytes in contact with reactive electrodes.

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