The Sodium Metal Electrode and its Interface with Inorganic Solid Electrolytes for Solid-State Batteries

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2024

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In the pursuit of the decarbonization of modern societies, the demand for energy storage solutions such as rechargeable batteries is rapidly increasing. Although conventional lithium-ion batteries are approaching their physicochemical limits, solid-state batteries (SSBs) are emerging as a promising alternative. Using a solid electrolyte (SE) instead of a liquid electrolyte, SSBs may enable the use of metal electrodes, which offer higher energy and power densities than traditional carbon-based anode materials. Concurrently, the importance of sustainability and the availability of resources is increasing, which is why sodium-based batteries are becoming the focus of research. These considerations raise the question of whether sodium-based SSBs, particularly reservoir-free battery designs, could be viable alternatives compared to LIBs. However, the implementation of metal electrodes (MEs) such as lithium or sodium in SSBs is complicated by challenges such as chemical degradation and morphological changes at the Me|SE interface. To overcome these challenges requires a fundamental understanding of the underlying processes that occur at the Me|SE interface during electrochemical reactions to enable the use of a sodium metal electrode (SME) in sodium-based SSBs.
This doctoral thesis focuses on characterizing the interfacial kinetics and morphological changes upon anodic dissolution and cathodic deposition of a SME in contact with a Na3.4Zr2Si2.4P0.6O12 (NZSP) SE. The results indicate that the fundamental charge transfer process is inherently fast and chemical degradation has a minimal effect on the interfacial kinetics. Instead, the current constriction phenomenon was identified as the rate-determining process at the Na|NZSP interface. Based on this result, the evolution of interfacial morphology was investigated by impedance spectroscopy and electron microscopy. During anodic dissolution, pore formation was found to be the primary cause of contact loss and interfacial polarization. In view of reservoir-free cells (RFCs), the formation of an SME by cathodic deposition of sodium at a current collector|SE interface was examined. Independent of deposition parameters (current density and stack pressure), a dense sodium layer of micrometer thickness was observed, with impedance analysis showing no evidence of dendrite formation. Furthermore, increasing the current density during deposition improved sodium coverage across the electrode area, while increased the stack pressure only had a minor effect. These findings suggest that reservoir-free sodium SSBs are feasible from a physicochemical perspective.
In addition, the effect of metal microstructure on the electrochemical performance of SMEs was investigated. A reliable workflow was developed to visualize the microstructure of alkali metals using electron backscatter diffraction (EBSD). Sodium metal foils exhibit grain sizes on the order of several hundred micrometers, while no significant grain growth was observed during storage at room temperature. Electrochemically deposited sodium exhibits a columnar structure with a smaller grain size compared to mechanically prepared foils. In situ EBSD experiments revealed lateral grain growth during electrodeposition, attributed to the movement of grain boundaries. During anodic dissolution, pore formation was preferentially observed at the Na|NZSP interface within the interior of grains.
In summary, the findings presented in this doctoral thesis improve the understanding of interfacial kinetics and morphological evolution of SMEs in contact with SEs upon cycling. Furthermore, the systematic analysis of the Na|NZSP interface provides a fundamental framework for evaluating impedance data and offers guidance for the characterization of other SME|SE combinations. Additionally, EBSD has been established as a technique to visualize the microstructure of electrode materials, allowing the assessment of electrochemical properties from a microstructural perspective. This approach may facilitate the development of tailored solutions to optimize the Na|SE interface for the development of reservoir-free sodium SSBs.

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