Lithium-ion batteries are the energy storage devices of choice not only for portable devices but electric vehicles and grid energy storage as well due to their high gravimetric and volumetric energy density. Their ancestors, the lithium metal batteries offer way higher capacities but suffered severely from safety issues due to the dendrite formation. Solid state electrolytes; e.g. LISICON, garnets, perovskites, etc.; may mitigate this risk but suffer from low ionic conductivity (<10^-4 S/cm). ^[1] The interfacial resistance between the electrode and the solid electrolyte is also very challenging.

Hybrid solid state electrolytes can address these issues and allow for an easier implementation into conventional battery technology. The hybrid solid state electrolyte combines the desirable properties of both liquid and solid electrolytes by confining liquid electrolytes within a (meso-)porous solid framework.^[2,3] This approach allows to maintain the high ionic conductivity and intimate contact with the electrodes while offering the safety of solid electrolytes.

Our group developed the eutectogels in which a deep eutectic electrolyte (binary mixture of N-methylacetamide and LiTFSI) is confined within a porous silica framework designed via a facile one-pot non-aqeous sol-gel route.^[4] This hybrid electrolyte can act as a greener and cheaper alternative to the well-known ionogels.^[5] The eutectogels offer a broad electrochemical window, with a beneficial anodic stability limit of up to 4.8 V vs Li⁺/Li while impedance spectroscopy revealed ionic conductivities of up to 1.46 mS/cm at room temperature for the optimal composition. LiFePO₄ (LFP) demonstrator half-cells were assembled with these hybrid solid state electrolyte membranes and displayed a highly reversible capacity for over 100 cycles. Elaborate electrochemical characterization reveals the promising nature of this newest member of the hybrid solid electrolyte family.

In the current presentation, we will show the expansion of the eutectogel family via two pathways. First, new deep eutectic electrolytes with higher thermal stability have been designed and confined within a silica matrix. The compositions have been optimized and fully (electro-)chemically characterized for their potential in electrochemical cells. Another route is replacing the silica framework with a polymeric cage. This route allows the further simplification of the synthesis while fully harnessing the potential of the confined deep eutectic electrolyte. Both routes are evaluated for their potential to formulate the design principles for the development of novel eutectogels.

Figures

Figure 1: (Left) Synthesis route for the eutectogels, optical photograph of a membrane (12 mm diameter) with schematic representation of the nanostructure. (Right) Arrhenius plots of the ionic conductivities for several compositions.

Acknowledgements

B. Joos is a PhD fellow of the Research Foundation – Flanders (FWO Vlaanderen). This project receives the support of the European Union, the European Regional Development Fund ERDF, Flanders Innovation & Entrepreneurship and the Province of Limburg (project number EFRO936). The authors would like to thank other group members for their assistance.

References

[1] C. Cao, Z. Li, X. Wang, X Zhao, W. Han, Frontiers in Energy Research, 2 (2014), 1-10.

[2] G. Tan, F. Wu, C. Zhan, J. Wang, D. Mu, J. Lu, K. Amide, Nano Lett., 16 (2016), 1960-1968.

[3] X. Li, S. Li, Z. Zhang, J. Huang, L. Yang, S. Hiranob, J. Mater. Chem. A., 4 (2016), 13822-13829.

[4] B. Joos, T. Vranken, W. Marchal, M. Safari, M. K. Van Bael, A. T. Hardy, Chemistry of Materials, 30 (2018), 655.

[5] J. Le Bideau, J.-B. Ducros, P. Soudan, D. Guyomard, Advanced Functional Materials, 21 (2011), 4073.