The transition to solid-state Li-ion batteries will enable progress toward energy densities of 1000 W·hour/liter and beyond. Composites of a mesoporous oxide matrix filled with nonvolatile ionic liquid electrolyte fillers have been explored as a solid electrolyte option. However, the simple confinement of electrolyte solutions inside nanometersized pores leads to lower ion conductivity as viscosity increases. Here, we demonstrate that the Li-ion conductivity of nanocomposites consisting of a mesoporous silica monolith with an ionic liquid electrolyte filler can be several times higher than that of the pure ionic liquid electrolyte through the introduction of an interfacial ice layer. Strong adsorption and ordering of the ionic liquid molecules render them immobile and solid-like as for the interfacial ice layer itself. The dipole over the adsorbate mesophase layer results in solvation of the Li + ions for enhanced conduction. The demonstrated principle of ion conduction enhancement can be applied to different ion systems. Mees, P. M. Vereecken, Silica gel solid nanocomposite electrolytes with interfacial conductivity promotion exceeding the bulk Li-ion conductivity of the ionic liquid electrolyte filler. Sci. Adv. 6, eaav3400 (2020).

The development of solid composite electrolytes or solid composite electrolytes (SCEs) consisting of an ionic conductor and a dielectric matrix offers an elegant strategy to enhance the ionic conductivity of electrolytes by engineering the interface conduction. At the conductor/matrix interface, the ionic conductivity can be enhanced by the enriched charge carrier concentration and/or the changed molecular structure of the ionic conductor. This review deals with the interfacial ion conduction mechanisms of the inorganic particle‐based SCE, polymer‐SCE, and ionic liquid electrolyte‐based SCE (ILE‐SCE). In the first part of this review, an overview is given of the space‐charge theory developed to describe the increased vacancy concentration at the interface of inorganic particle‐based SCE. In the second part, the proposed interface interactions and structural changes associated with interface conduction for the polymer‐SCE and ILE‐SCE are reviewed. For the ILE‐SCE, the preparation methods and interface characterization are discussed together with the proposed conductivity models. The use of mesoporous matrix materials with high internal surface area for ILE‐SCE is reviewed here for the first time.

A nanocomposite electrolyte composed of a non-volatile ionic liquid, organic Li-salt and porous-inorganic material can be a promising option as a solid electrolyte material. We present a high-rate performance in solid-state lithium metal and Li-ion batteries using a silica-gel solid nanocomposite electrolyte (nano-SCE) made by the sol-gel method with a bis(fluorosulfonyl)imide (FSI)-based ionic liquid. The nano-SCE, composed of 1-ethyl-3-methylimidazolium bis(fluorosulfonyl) imide (EMI-FSI) and Li-FSI confined in the mesoporous silica matrix, exhibits an ionic conductivity of 6.2 mS cm−1 at room temperature. The capacity of the Li-LiFePO4 cell using the EMI-FSI based nano-SCE reaches 150 mAh g−1 at 0.1C and 113 mAh g−1 at 1C, which is higher than that achieved by the other reported batteries that use a similar composite electrolyte. The C-rate performance of the prepared solid batteries is comparable to that of cells with the conventional lithium hexafluorophosphate (LiPF6) electrolyte. Our results show that impregnation of a liquid precursor is an efficient approach for an excellent electrode/electrolyte interface contact in the solid composite electrode as the reaction kinetics at the interface of the active mass and nano-SCE are sufficiently fast, and thus is advantageous compared with the other types of solid electrolytes.

Solid nanocomposite electrolytes (nano-SCEs) that exhibit higher ionic conductivity than the individual confined electrolyte were investigated for high-performance solid-state batteries. Understanding the behavior of Li-ion conduction through the pores is important to design ideal nanoporous structures for nano-SCEs, which are composed of an ionic liquid electrolyte (ILE) in a highly porous (∼90%) silica matrix. To establish the relationship between the pore structure of the silica matrix and the ionic conductivity of the solid nanocomposite, the liquid electrolyte fraction was successfully extracted from the nano-SCE to reveal the fragile porous silica matrix. A careful drying using the CO 2 supercritical drying method helps in sustaining the original structure, preventing its collapse due to surface tension. The pore size distribution, Brunauer−Emmett−Teller (BET) surface area, and porosity were characterized using scanning electron microscopy, transmission electron microscopy, and N 2 adsorption/desorption techniques. Our results revealed a wide size distribution of macropores and mesopores in the silica matrix. The pore size increased and the effective surface area decreased with increasing ILE/SiO 2 molar ratio. The interface conductivity enhancement was found to increase with the thickness of the adsorbed (ice-like) bound-water layer on the silica surface, confirming that the strong hydrogen bonding of the adsorbed ionic liquid molecules on the bound-water layer causes the conduction promotion effect in the nano-SCE. In addition, a large number of small pores lead to a severe pore confinement effect that results in a decreased conductivity due to the increasing viscosity of the ILE filling the pores. The conductivity can be improved by realizing a nano-SCE with an optimized pore size to minimize the pore confinement effect.

A thin‐film composite electrolyte (CE) with thickness of only 100 nm is fabricated for the first time. The conductivity of the solid electrolyte is 2 × 10−7 S cm−1 which corresponds to a cell resistance of only 50 Ω cm2 at this thickness. The electrolyte is composed of a nanoporous thin film, a lithium salt, and a polymer, and is fabricated by a two‐step approach. First, a porous SiOxCyHz film is deposited on the electrode substrate, then the porous structure is filled with a polyethylene glycol (PEG)‐LiX mixture by spin‐coating. The filling ratio of the mesoporous SiOxCyHz is as high as 90% with less than 1% remaining porosity, as determined from ellipsometric porosimetry. The effect of the anion, X−, of the LiX salt as well as the molecular weight of the PEG on the Li+ ion conductivity of the CE is investigated. The CE with smaller anion and with lower molecular weight PEG yields the highest conductivity. A functional Li/CE/TiO2 battery is built for demonstration purposes.

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