Solid-state batteries are considered to reach the high energy densities needed for our emerging sustainable energy society. Recently, solid electrolytes (SE) with ion conductivities equalling or even surpassing that of liquid electrolyte solution are already available. However, the rigid nature of the SE powder materials caused poor interface contacts between individual electrolyte particles and with the active electrode particles. To solve this issue, a solid composite electrolytes (SCE) composed of ionic liquid (IL) and Li-salt supported on a porous inorganic matrix have been explored. [1] These composite electrolytes were prepared by sol-gel methods. The low viscous liquid sol-gel precursor can be impregnated into the porous powder electrodes and gelled in the electrodes, thus resulting in good interfacial contacts between the active electrode and the electrolyte. Further drying and curing make the composite electrodes compact, which results in high energy density. However, the ionic conductivity was typically lower than that of pure ionic liquid electrolyte (ILE) used in SCE. This is due to the increase of the IL viscosity in the nano-sized pores, which is known as IL confinement effect. [2]

To overcome this problem, our group developed a new type of SCE with water-based sol-gel process, which is named “nano-SCE”. [3] We demonstrated systematic promotion of the Li-ion conductivity by controlling the IL molecules ordering on the silica pore surface with an functional adsorbed ice-water layer. The strong hydrogen bonding between the ice-water and the IL molecules induces the ordering of IL anions and cations on the silica surface. This interfacial interaction weakens the association between the Li-ion and its anion, thus enhancing the ion conductivity well beyond that of pure ILE. Further control of the porous structures will make the conductivity go higher, and thus can be a breakthrough technology to develop the high-conductive SEs.

For designing the ideal porous structure and control the synthesis process of nano-SCEs, the silica nanoporous structure must be clarified. However, so far, it is difficult to characterize the silica matrix structure because the silica structure collapsed by the surface tension when the ILE is removed by rinsing with solvents such as ethanol and acetone and after drying both in ambient and vacuum. In this paper, we applied the CO₂ supercritical drying method to extract ILE. A careful drying above CO₂ super critical point enabled to keep the original structure avoiding the collapse due to the surface tension. The porous silica matrix was successfully obtained without damage or shrinkage. The porous structures of nano-SCEs with different IL/SiO₂ material ratio were analysed by SEM, TEM, N₂ adsorption/desorption and positron annihilation spectroscopy (PALS) technique. SEM and N₂ adsorption/desorption analysis revealed that there are > 10 nm pores distributed, and their size and number were increased with ILE/SiO₂ molar ratio. Our measurements confirmed that the IL confinement effect, that is, the conductivity of nano-SCE with smaller pore-size was lower due to the increasing of the viscosity of ILE filled in pores. In contrast, the surface area was decreased with ILE/SiO₂ ratio due to the reduction of 5 nm size pores, which is characterized by PALS analysis. From the relationship between surface area and surface enhancement factor (s_ice-water/s_dried) in Fig. 1, the surface conduction was verified to be increased with the surface area, indicating the conduction promotion effect was clearly related with the surface chemistry. A nano-SCE with higher conductivity can be realized by designing the pore structure with high surface area and optimized pore size.

[1] J. Le Bideau, J. B. Ducros, P. Soudan and D. Guyomard, Solid-state electrode materials with ionic-liquid properties for energy storage: The lithium solid-state ionic-liquid concept, Adv. Funct. Mater., 2011, 21, 4073–4078.

[2] S. Zhang, J. Zhang, Y. Zhang and Y. Deng, Nanoconfined Ionic Liquids, Chem. Rev., 2017, 117, 6755–6833.

[3] 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, 2018, 470, MA2018-02.

Fig. 1 The relationship between surface enhancement factor of nano-SCEs and BET surface area of porous silica matrix. The surface enhancement factor was defined as the ratio of the conductivity with and without functional ice-water layer on the silica surface. Porous silica was obtained by removal of ILE from nano-SCE with ethanol soaking for 36 hours at 40 ^oC and subsequent drying by CO₂ super critical dryer.