Fundamental Studies on the Impedance of Lithium All-Solid-State Batteries

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The impedance of a lithium all-solid-state battery (ASSB) depends critically on the contacts between the solid particles (Li intercalating active material particles and solid electrolyte particles) inside the battery and thus on the applied stack pressure. The impedance spectra of this battery type consist of different contributions, which could not yet be attributed in an unambiguous fashion to processes in the anode, in the cathode, and at interfaces. In this project, a systematic approach for the analysis and interpretation of the impedance spectra of ASSBs will be developed with the aim to achieve a clear-cut attribution of impedance contributions to processes in the ASSBs. The ASSBs will consist of an In-Li anode, a solid electrolyte separator and a composite cathode. In the composite cathode, active material particles (single-crystalline and poly-crystalline LiNi0.6Mn0.2Co0.2O2, respectively), solid electrolyte particles (Li5.3PS4.3ClBr0.7 and Li2.5Y0.5Zr0.5Cl6, respectively) and carbon black will be mixed in different volume ratios. The ASSBs will be cycled in a specifically designed measuring system with active pressure and tempertur control, and the impedance spectra will be collected at different stack pressures and states of charge. For the analysis and interpretation of the impedance spectra of the ASSBs, the following approach will be used: (i) Impedance measurements on symmetric cells (anode | solid electrolyte | anode), (cathode | solid electrolyte | cathode), and (anode | solid electrolyte | cathode | solid electrolyte | anode) will provide information on the impedance contributions of anode, cathode and interfaces; (ii) Impedance measurements on ASSBs with different cathode thickness will be important for differentiating between ion transport resistances of the solid particles and charge transfer resistance at the active material | solid electrolyte interfaces. The analysis of the impedance contribution of the cathode will be based on a suitable transmission-line model. Values for the charge transfer resistance inside the cathode will be obtained in dependence of the material combination, stack pressure, and state of charge. These values will be compared to published values for composite cathodes of conventional lithium-ion batteries containing a liquid electrolyte.