Hybrid Solid Electrolyte for All-Solid-State Batteries
Authors: Keller Marlou , Appetecchi Giovanni Battista , Kim Guk-Tae , Sharova Varvara , Passerini Stefano 
Published: February 2020

The public awareness of climate change and depletion of fossil fuels urges to prepare for a post-petrol future. The transportation sector is still one of the major consumers of this limited and pollutive resource. On the other hand, electric mobility concepts have already been introduced in the mid-19thcentury and very recently gained tremendous interest. Electric vehicles (EVs) powered by electricity from renewable energies are predicted to replace the internal combustion engines for an emission-free future of transportation.

The major drawbacks of EVs to be overcome are limited range and safety issues. A technology jump from the commercial state-of-the-art battery to all-solid-state batteries is necessary to dramatically boost the battery performance. This is due to high safety risks of liquid organic based electrolytes such as leakage and thermal runaway upon short circuiting due to dendrite formation if lithium metal is used. The integration of a solid electrolyte suppresses these safety issues. Therefore, this novel technology can solve both problems as lithium metal can safely be used as anode which allows high energy densities and, thus, enables an extended driving range.

Garnet materials like Li7La3Zr2O12 (LLZO) show high ionic conductivities and very good electrochemical stability.[1] However, the integration of hard and brittle ceramic solid electrolytes in an all-solid-state battery remains particularly complex. Fine cracks can favor dendrite growth and short circuits. Additionally, poor interfacial contact between the solid electrolyte and electrodes increases the cell impedance.[2] The manufacturing of thin membranes for large scale production meets challenges like high energy consumption and mechanical instability.

The integration of lithium-ion conducting solid electrolytes in a polymer matrix is a promising approach to obtain flexible hybrid solid electrolytes with high ionic conductivities and thermal stability. Previous studies use exclusively the tape-casting method with toxic acetonitrile as solvent to process the membranes.[3–5] This process is rather unfavorable for industrial application as the solvent recovery is time consuming and costly. Also, solvent residues are known to increase the interfacial resistance on the lithium metal. Hence, in this work, a solvent-free hot-pressing method is applied.[6]

This procedure allows to investigate the effects due to LLZO aging, such as water uptake and the formation of insulating Li2CO3 layer onto the LLZO particles, on the electrochemical performance of the hybrid solid electrolyte. The hybrid solid electrolyte is composed of Li7La3Zr2O12 lithium-ion conductor (70% in weight) integrated in a P(EO)15LiTFSI solid polymer electrolyte matrix. In this study, the purity of the LLZO material is investigated by thermogravimetric analysis, SEM and XRD. Finally, the hybrid solid electrolyte is electrochemically examined in terms of ionic conductivity, long-time compatibility with lithium metal electrodes and Li stripping/plating behavior.

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Acknowledgment

The research leading to these results has received funding from the German Federal Ministry of Education and Research under grant agreement no. 03XP0026F. LLZO was kindly provided by Schott AG.