Solid-State Lithium and Li-Ion Batteries with Solid Nano-Composite Electrolytes

Abstract

Solid electrolytes with a Li-ion conductivity exceeding 1 mS/cm are a prerequisite to enable solid-state Li-ion batteries. In solid composite electrolytes (SCEs), the interface between an ionic conductor and a dielectric matrix can be engineered to enhance this ionic conductivity. The solid composite electrolyte developed at imec contains a monolithic, mesoporous, silica matrix filled with a non-volatile ionic liquid and an organic Li-salt. This composite material is made by a sol-gel process, similar to that for ionogels, with that difference that no acid is used but water. The resulting aqueous gel is carefully dried from water and solvents, resulting in the solid nano-composite electrolyte where the ionic liquid and the lithium salt are confined in the pores and channels of the mesoporous silica matrix. The slow sol-gel reaction and drying allows the adsorption of an ordered molecular layer on the fully hydrolyzed silica surface. Interfacial ice layers induce strong adsorption and ordering of the ionic liquid molecules through H-bonding, rendering them 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 demonstrate that when the silica surface is appropriately hydroxylated, the Li-ion conductivity of the nano-SCE can be several times higher than that of the pure ionic liquid electrolyte itself. By this process solid nano-SCEs with conductivities between 0.4 and 8 mS/cm have been synthesized, using TFSI- and FSI-based ionic liquid electrolytes. Battery cells can be made by impregnating the liquid sol-gel precursor solution inside the powder-based electrodes, very similar to the application of liquid electrolytes. As our precursor solution does not contain corrosive acid compounds such as formic acid, which is typically proposed as catalyst in literature recipes, we can deposit the solution directly on and into the porous electrodes. The sol-gel reaction and drying take place in-situ inside the electrodes allowing the nano-SCE to fill the empty spaces and providing an all-around contact with the active material. As such, we are able to realize functional solid-state cells with the TFSI- and FSI-based nano-SCE. Not only low voltage cells with LFP and LTO electrodes, but also high voltage cells with NCA and Li metal or Li alloy electrodes are demonstrated (Fig 1a and b). C-rate and cycling performance of these solid-state cells with the nano-SCE are given. Figure 1