A Novel Material Class of Solid-State Nano-Composite Electrolytes with High Li-Ion Conductivity
Authors: Mees Maarten J. , Gandrud Knut Bjarne , Sagara Akihiko , Chen Xubin , Hendrickx Nathalie , Murata Mitsuhiro , Kaneko Yukihiro , Arase Hidekazu , Vereecken Philippe M. 
Published: February 2020

All-solid-state Li-ion cells are expected to become the next major cell technology. Besides the fact that they are intrinsically safer compared to today’s cells with a flammable liquid organic electrolyte, a solid-state electrolyte also enables the introduction of a metallic lithium anode, resulting in cell energy densities of >1000 Wh/L. This is significantly higher compared to the practical limit of 800 Wh/L of the liquid Li-ion cell technology. Applying metallic lithium electrodes with a liquid electrolyte is challenging due to the likelihood of dendrite formation during cell charging (i.e. during lithium plating) [1]. It is expected that a solid-state electrolyte is a better candidate to deal with the lithium dendrite formation problem, as it forms a physical barrier for the dendrites to grow [2].
For many years, it was considered practically impossible to engineer solid-state materials having ionic conductivities that are equally high and surpassing those of the liquid electrolytes, as ion diffusion in solids is typically much slower compared to diffusion in liquids. A high Li-ion conductivity through the electrolyte is essential for the Li-ion cells to be charged and discharged at high rate. We here present a novel class of nano-composite electrolytes (nano-SCEs) for which ion conductivity values are obtained higher than those seen in conventional liquid electrolytes. These nano-SCEs constitute a nano-porous SiO2 monolith that encloses an adsorbed ionic liquid electrolyte. Different from to so-called ionogel electrolytes – which follow a similar approach in that they confine an ILE in a porous oxide matrix [3] – our electrolytes feature the unique property that the ionic conductivity of the composite is higher than to that of the ionic liquid electrolyte [4]. The so-called enhancement factor reaches values of > 200 %, as shown in the Figure 1.
The high ion conductivity and enhancement factor were established by engineering the surface chemistry in the nano-SCE [4]. Along the pore walls of the oxide matrix, the ionic liquid electrolyte molecules are organizing themselves such that “highway” conduction paths are created for fast Li-ion transport. The exact nature of these highway conduction paths strongly depends on the chemistry of the ionic liquid electrolyte. Figure 1 show the influence of the ILE chemistry on the nano-SCE enhancement factor. Interestingly, the enhancement factor increases with increasing ILE viscosity.
In conclusion, a new material class of composite electrolytes is presented having the unique feature of enhanced ion conductivity. The conduction promotion arises from the chemical interaction between the ILE and the nano-porous SiO2 matrix. Furthermore, the enhancement factor can be controlled by engineering the ILE viscosity.

References

[1] M. D. Tikekar, S. Choudhury, Z. Tu and L. A. Archer, "Design principles for electrolytes and interfaces," Nature Energy, pp. 1-7, 2016.

[2] C. Monroe and J. Newman, "The Impact of Elastic Deformation on Deposition Kinetics," Journal of The Electrochemical Society, vol. 152, no. 2, pp. A396-A404, 2005.

[3] N. Chen, H. Zhang, L. Li, R. Chen and S. Guo, "Ionogel Electrolytes for High‐Performance Lithium Batteries: A Review," Advanced Energy Materials, vol. 8, p. 1702675, 2018.

[4] P. M. Vereecken, X. Chen, K. B. Gandrud, B. Put, A. Sagara, M. Murata, M. Tomiyama, Y. Kaneko, M. Shimada, J. Steele, M. Roeffaers, M. Debucquoy and M. J. Mees, "Mechanism Analysis of Enhanced Li-Ion Conductivity in Mesoporous Silica-Based Solid Nano-Composite Electrolytes," ECS Meeting Abstracts, vol. 470, pp. MA2018-02, 2018.

Figure 1