Preparation of Hybrid Solid Electrolyte by LiPF6 Liquid Electrolyte Infiltration into Nano-porous Na2O-SiO2-B2O3 glass membrane.

Abstract

Many researches to develop high energy density, lightweight, and shape-flexible lithium rechargeable batteries have carried out in the field of all-solid-state batteries based on solid electrolyte [1]. The solid state polymer electrolyte has been studies extensively to realized the solid state battery. However, the unstable physical and electrochemical material properties are hindering the real application of these materials [2]. As an alternative technique, the hybrid electrolytes are developed in the form of the organic-inorganic molecular hybridization and meso-scale hybridization by infiltration of liquid electrolyte or gel electrolyte into meso-porous membrane [3]. The inorganic glass electrolyte has long been studied as a candidate for solid state electrolyte. However, its major limitation have been recognized as the low lithium ion mobility. The application of the glass electrolyte to conventional bulk type battery, hence, is impractical in spite of its merits and the researches on this group of materials are diminishing recently. Despite of this limitation, the chemical and physical stability oxide glass is still very useful if it is utilized to a membrane. Sodium borosilicate glasses are well known as materials in which heat treatment can induce phase separation. The glasses in the Na2O-SiO2-B2O3 system are separated in the form of 3 dimensionally inter-connected nano-sized channels. The pore size distribution of which can be determined through variations in the initial concentration, time and temperature of the thermal treatment [4-5]. By successive acid leaching, a porous silica glass can be obtained. This structure can be utilized to fast ion conducting routes while maintaining the solid external shape and mechanical strength. In the present study, the oxide glass membrane of the systems Na2O-SiO2-B2O3 has been prepared by melt-quenching method and subsequently phase-separated to form nano-sized channels. In these channels, the conventional LiPF6 liquid electrolyte was infiltrated to fabricate hybrid solid electrolyte with high lithium ion conductivity. xNa2O-(1-x)(yB2O3-(1-y)SiO2) glasses were prepared by melt quenching technique. The starting materials were well-dried powders of reagent grade Na2O, B2O3 and SiO2, which were mixed in proportions appropriate to form 30g batches. Each batch was melted at 1000 oC for 6 h in Pt crucibles in a electrically heated furnace. Then, prepared each samples were crushed to obtaining annealing temperature(100~300mesh) and glass transition temperatures were measured using TG-DTA(Shimadzu, DTA-60H) at heating rates of 5 oC/min, in platinum crucible for Tg up to 1000 oC. To form nano-sized channel by phase separation, prepared samples were heated at 600 oC for 1 h and wet etched by HF solution. The microstructure of the cross-section of xNa2O-(1-x)(yB2O3-(1-y)SiO2) glass electrolytes were examined with a scanning electron microscope. Then, liquid electrolyte was infiltrated by dipping method into etched glasses electrolyte. Conductivity measurements were performed by complex impedance method using Solatron impedance analyzer with an ac voltage of 1V amplitude over the frequency range 1MHz to 1Hz at room temperature. Fig. 1 shows an example of the SEM image of wet-etched xNa2O-(1-x)(yB2O3-(1-y)SiO2) and nano-sized channels formed. One can see from this image that the size of 3 dimensionally inter-connected channels are around 100nm and the phase separation is successfully done. The ionic conductivities of liquid electrolyte infiltrated thin specimen were measured by impedance analyzer and the conductivity was comparable to that of liquid electrolyte. The result is somewhat expected since liquid electrolyte exhibits the normal ionic conduction along the 3 dimensionally inter-connected channels through solid skeleton formed by glass electrolyte. Therefore, the apparent conductivity of this solid electrolyte is reduced only by the reduction of effective area of ion conducting path.