Solid-State Lithium and Li-Ion Batteries with Silica-Gel Solid Nanocomposite Electrolytes

Abstract

The transition to solid-state Li-ion batteries requires solid electrolytes with Li-ion conductivity exceeding 1 mS/cm. Solid composite electrolytes or 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 imec, we have developed a ternary nano-composite electrolyte composed of a non-volatile ionic liquid and organic Li-salt confined in mesoporous silica. The material is made by a one-step sol-gel process whereby the ionic liquid acts as template for the hydrolysis and polycondensation reaction leading to an aqueous gel. The process is similar to that for ionogels with that difference that no acid is used but water. The water and solvents are subsequently carefully removed to form a solid nano-composite electrolyte. The slow sol-gel reaction and drying allows the adsorption of an ordered molecular layer on the fully hydrolyzed silica surface. In this way, several nano-SCE with conductivities between 0.3 and 3 mS/cm have been synthesized, using TFSI-based ionic liquid electrolytes (ILE). We demonstrate that the Li-ion conductivity of nano-composites consisting of a mesoporous silica monolith with an ionic liquid electrolyte as filler can be several times higher than that of the pure ionic liquid electrolyte itself when the silica surface is appropriately hydroxylated. Interfacial ice layers induce strong adsorption and ordering of the ionic liquid molecules through H-bonding rendering it immobile and solid-like as for the interfacial ice layer itself. The dipole over the adsorbate results in solvation of the Li+ ions for enhanced conduction. We will show that the ice layer is electrochemically inactive, in contrast with water-in-salt electrolytes. Functional cells with LFP, LMO and NCA cathodes with Li, Li-alloy and LTO electrodes are demonstrated. As the ice-water layer was confirmed to be electrochemically inactive, it doesn’t cause degradation during cycling of the batteries. Furthermore, damage to the active electrode materials is avoided as our water-based sol-gel precursor does not contain corrosive acid compounds such as formic acid typically proposed as catalyst in literature. The cells are made by impregnation of the liquid sol-gel precursor solution inside the porous electrodes, very similar to how liquid cells are made. The sol-gel reaction and solidification is done in-situ inside the electrodes. By careful drying, the nano-SCE contracts the electrodes together and the electrolyte fills the spaces in the porous electrodes, providing an all around contact with the active material. As such, we have shown high capacity cells at C-rates up to 0.5C. C-rate and cycling performance of the solid-state cells with nano-SCE is shown. Figure caption: Functionality of nano-SCE as Li-ion electrolyte up to 4.3V: galvanic charge/discharge curves of Li/nano-SCE/ LiMn2O4 cell for 5 cycles at each C-rate of 1 C, 5 C, and 20 C. Figure 1