Chemo-mechanical failure mechanisms of the silicon anode in solid-state batteries.

Silicon is a promising anode material due to its high theoretical specific capacity, low lithiation potential and low lithium dendrite risk. Yet, the electrochemical performance of silicon anodes in solid-state batteries is still poor (for example, low actual specific capacity and fast capacity decay), hindering practical applications. Here the chemo-mechanical failure mechanisms of composite Si/Li6PS5Cl and solid-electrolyte-free silicon anodes are revealed by combining structural and chemical characterizations with theoretical simulations. The growth of the solid electrolyte interphase at the Si|Li6PS5Cl interface causes severe resistance increase in composite anodes, explaining their fast capacity decay. Solid-electrolyte-free silicon anodes show sufficient ionic and electronic conductivities, enabling a high specific capacity. However, microscale void formation during delithiation causes larger mechanical stress at the two-dimensional interfaces of these anodes than in composite anodes. Understanding these chemo-mechanical failure mechanisms of different anode architectures and the role of interphase formation helps to provide guidelines for the design of improved electrode materials.

Toward a Fundamental Understanding of the Lithium Metal Anode in Solid-State Batteries-An Electrochemo-Mechanical Study on the Garnet-Type Solid Electrolyte Li6.25Al0.25La3Zr2O12.

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 Li6.25Al0.25La3Zr2O12. 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·cm2 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|Li6.25Al0.25La3Zr2O12 interface.