Functionalized Silica Particle Based Single Ion Conducting Hybrid Electrolyte for Li-Ion Battery

Abstract

For advanced energy storage (batteries and supercapacitors), energy conversion (fuel cells and solar cells), and electromechanical transduction devices (electroactive ionic actuators and sensors), it is vital to utilize single-ion polymer conductors (ionomers) that have been designed by covalently attaching anions (or cations) to the polymer chains. This allows the high lithium transference number in lithium polymer batteries as well as the fast response time under low voltage operating conditions in ionic polymer actuators. However, current single-ion conductors exhibit relatively low conductivities, precluding use of these materials in potential applications. This is predominantly due to relatively slow polymer segmental motion, resulting in decreased ion mobility, and large ion dissociation energy, lowering the number of simultaneous charge carries. In order to overcome the issues, we have synthesized porous silica-based lithium single-ion conducting nanoparticles containing anions with lithium counter ions. These nanoparticles are then introduced into poly(ethylene oxide) matrix, allowing us to prepare nanocomposite polymer electrolytes. Anion functionalized mesoporous silica (FMS-TFSISPE) nanoparticles were prepared by the two-step selective functionalization method. Poly(ethylene glycol) group was attached using sol-gel method on mesoporous silica particles. Trifluoromethanesulfonimide group functionalized to silica particles for binding lithium cation. The particles are uniformly distributed with an average size of 50 nm. HRTEM measurement further suggests that the clear mesoporous structure is observed. In order to extensively investigate the structural information on FMS-TFSISPE, the N2 adsorption/desorption isotherm is obtained as shown, where FMS-TFSISPE shows type IV isotherms of typical mesoporous materials, and the BET surface area is 995 m2 g- 1. The average pore size of the FMS-TFSISPE nanoparticles is about 3 nm which is close to that observed by the HRTEM measurement. The small angle XRD analysis shows three resolved peaks from which the pore structure corresponds to a 2D hexagonal (P6mm) structure. From the structure analysis, we successfully synthesized the FMS-TFSISPE nanoparticles with well-ordered hexagonal arrays of mesopore channels and exceptionally high surface area and pore volume. It is apparent that FMS-TFSISPE nanoparticles have a considerable effect on the enhancement of the ionic conductivity of these electrolytes, and their conductivities increase monotonically with increasing FMS-TFSISPE content. The maximum enhancement in conductivity is found for the nanohybrid electrolyte containing 30 wt % FMS-TFSISPE and order of sDC ~ 10- 3 S/cm at 25 oC, 10 times higher than the minimum practical requirement for single-ion conductors. We develop a novel nanohybrid single-ion conductor based porous silica structure. These systems display exceptional ionic conductivity and lithium transference number. It is shown that such nanohybrid electrolytes significantly immobilize the anion movement and pave the way for fast lithium ion-only conducting pathway. Provided that the optimization of the nanohybrid electrolytes with high-energy and -power electrodes is realized, the use of our highly conducting single-ion conductor in practical polymer electrolyte batteries will be envisaged.