Polymer-Keramik Hybridelektrolyte für Lithiumionen-Festkörperbatterien

  • Polymer-ceramic hybrid electrolytes for all-solid-state lithium-ion batteries

Wirtz, Maike; Eichel, Rüdiger-A. (Thesis advisor); Simon, Ulrich (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021

Abstract

Hybrid electrolytes are electrolytes for lithium ion batteries (LIB) with lithium anode meeting the requirements of safety, performance and processing. Hybrid electrolytes are defined as composites that combine two or more ionically conducting materials. In the scope of this thesis an organic polymer electrolyte poly(ethylene oxide) with lithium bis(trifluoromethanesulfonyl)imide (PEO with LiTFSI) and an inorganic solid-state electrolyte Li7-xLa3Zr2-xTaxO12 (Ta-LLZO) were used. Cubic Ta-LLZO possesses a higher ionic conductivity than the tetragonal phase of Ta-LLZO and was synthesized via a classical all-solid-state and a sol-gel route. For the application of Ta-LLZO in hybrid electrolytes, the powder was remilled after calcination reducing the average particle size. The composition of the hybrid electrolyte was optimized regarding the ionic conductivity. Influencing parameters such as Ta-LLZO particle properties, LiTFSI salt concentration, Ta-LLZO weight fraction and substitution with biopolymers were investigated. In comparison to a pure polymer electrolyte PEO12LiTFSI a higher ionic conductivity was measured for a hybrid electrolyte with 20 wt.-% Ta-LLZO in a PEO6LiTFSI polymer matrix. The substitution of PEO with biopolymers, specifically chitosan mesylate or cellulose acetate, was an approach for more sustainable materials in LIB, but resulted in a loss of performance. The applicability of hybrid electrolytes in Li-LiFePO4 (Li-LFP) cells was confirmed. LFP cathode sheets were adapted to the requirements of the hybrid electrolytes by using PEO6LiTFSI as binder and lithium ion conductor. Compared to the polymer electrolyte, an increase in the electrochemical stability of the hybrid electrolyte was obtained. The substitution of PEO by cellulose acetate further increased the electrochemical stability. The interfacial layer at the organic-inorganic interface was characterized on microscopic level using Infrared Spectroscopy and Electrochemical Strain Microscopy (ESM). Interactions of polymer and ceramic lead to an increase of mobile charge carriers. Moreover, ESM measurements visualized the possible formation of an interfacial layer in the polymer phase near the interface with the ceramic. This supports the assumption that interfacial percolation substantially influences the ionic conductivity of hybrid electrolytes on macroscopic level.

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