Solid Nano-Composite Electrolytes (nano-SCE) with Ion Conductivity Promotion at an Interfacial Ice Layer on the Mesoporous Silica Matrix

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. Composite electrolyte materials with inorganic oxide matrixes such as silica, alumina or titania with, for example, inorganic or polymer electrolytes have indeed shown enhancements in ion conductivity, however, the total ion conductivity was still well below 1mS/cm due to the low conductivity of the starting individual electrolytes used. Also composites of mesoporous oxide matrix filled with non-volatile ionic liquid electrolyte (ILE) fillers have been explored as solid electrolyte option. The advantage is that ILE can already have quite high ionic conductivity as individual electrolyte component. However, simple confinement of an electrolyte solution in nanometer size pores results in lower conductivity as its effective viscosity increases. The decrease in ion conductivity is expected the worse for mesoporous channels (~10nm in diameter) where the ionic liquid can even turn solid. To achieve higher ion conductivity, interface enhancement has to exceed the decrease in conductivity by confinement. Monolithic nanoporous silica with ILE confined in the porous structures, also named “ionogels” have shown ion conductivities approaching that of the ILE bulk conductivity, indeed, indicating the presence of interface enhancement in these materials. However, so far, the ionic conductivity of the confined ILE in the nanoporous oxide has never exceeded that of the ILE conductivity itself. These ionogels are fabricated by a sol-gel process e.g. by hydrolysis-condensation reaction with tetra-ethylorthosilicate (TEOS) precursor and typically formic acid where the ILE is added in the solution as the template for the silica to grow around. In this paper 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. The existence of an interfacial mesophase layer is proven by Infrared and Raman spectroscopy. Higher Li-ion diffusion coefficients for the nanocomposite compared to the pure ionic liquid electrolyte reference is shown by Pulsed-Field-Gradient NMR. The principle of ion conduction enhancement is generic and could be applied to different ion systems. The concept also allows for further (nano)engineering towards specific properties of ion conduction, transport number, electrochemical window, safety and cost for future battery cell generations. The nano-SCE was fabricated in a similar way as the ionogels: a single-step sol-gel process with a TEOS precursor and with the ionic liquid electrolyte in the homogeneous precursor solution, except we did not use formic acid but water. We will focus on systematic study of our nano-SCE model system with (N-butyl- N-methyl pyrrolidinium bis(trifluoromethanesulfonyl) imide ([BMP]TFSI) and bis(trifluoromethanesulfonyl)imide lithium salt (LiTFSI). Comparison with ILE-based composite electrolytes with solid and mesoporous oxide particles will be made, showing that enhancement exist only in the monolith structures and not in the particle systems where percolation limits the conductivity.