Nanoscale Characterization of Composite Electrodes for All-Solid-State Batteries by means of Electrochemical AFM Methods

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Bulk-type all-solid-state batteries exhibit potentially enhanced energy density and enhanced safety as compared to start-of-the-art lithium-ion batteries and are thus considered as promising electrochemical energy storage devices for future applications in the field of electromobility. In the case of Li all-solid-state batteries, the composite electrodes consist of active material particles, which reversibly intercalate Li+ ions, and of solid electrolyte particles. The mean size of both types of particles is in the micro-m to sub-micro-m range. In order to achieve a better understanding of the electrochemical processes in such electrodes, a spatially resolved characterization with a resolution in the sub-micro-m is desirable. AFM-based electrochemical methods are well suited for this purpose. In this project, AFM-based methods, which were partly developed in a previous project and are partly known from the literature, will be used for the electrochemical characterization of composite electrodes. To this end, six composite electrodes will be prepared, which differ in the thermodynamic and kinetic stability of the interfaces between the active material particles and the solid electrolyte particles. In all composite electrodes, the sulfide-based solid electrolyte Li5,3PS4,3ClBr0,7 (LPSClBr) with an ionic conductivity of about 5 mS/cm will be used. The following active material particles will be applied: (i) LiCoO2 uncoated, (ii) LiCoO2 with LiNbO3 coating (iii) LiNi0,6Mn0,2Co0,2O2 uncoated; (iv) LiNi0,6Mn0,2Co0,2O2 with LiNbO3 coating (v) LiFePO4 uncoated and (vi) Li4Ti5O12 uncoated. For the preparation of an ASSB, the respective composite cathode will be combined with a solid-electrolyte separator and a Li metal counter electrode. For the characterization of the composite electrodes, conductive AFM, Kelvin probe force microscopy and electrochemical strain microsocopy will be combined with a FIB/SEM/EDX-based analysis of surface topography and chemical composition. The results will provide new fundamental insights into the local electrochemical properties of single particles in composite electrodes as well as into the local properties of particle/particle interfaces. The following aspects are in the focus of interest: (a) State-of-charge-dependent chemical potentials of Li+ ions and electrons in single active material particles; (b) State-of-charge-dependent ambipolar Li diffusion coefficients in single active material particles; (c) Vegard strains in single active material particles; (d) Space charge layers and resistive interphases at the interfaces between active material particles and solid electrolyte particles.