Composite Electrolyte Composed of Li1.5Al0.5Ti1.5(PO4)3 and PVDF-Based Gel Electrolyte Containing Highly Concentrated Electrolytes

Abstract

Li ion conducting inorganic solid electrolytes have been widely investigated because of their high thermal stability and single-ion conducting property. Recently, some of inorganic solid electrolytes were found to have high ionic conductivity in the order of 10-3−10-2 S/cm at room temperature, which are comparable to that of conventional liquid electrolytes. The use of a Li metal anode, which has a high specific capacity, can significantly increase the cell energy density. However, most of the inorganic solid electrolytes are known to be thermodynamically unstable against Li metal and form the decomposition phases, which cause high interfacial resistance between electrode | electrolyte and degrade the cell performance. Surface roughness and brittleness of inorganic solid materials, which make the poor interfacial contact, are known to cause the increase in the interfacial resistance between electrode and electrolyte. To address these issues, much effort has been devoted to developing composite electrolytes combining inorganic solid electrolytes with polymer electrolytes. Poly(ethylene oxide) (PEO) is often used in fabricating composite electrolytes because of its advantageous characteristics such as processability, flexibility and Li ion solvation ability. However, there are intrinsic problems such as low Li transference number of ~0.2, low ionic conductivity in the order of 10-5 mS/cm at room temperature, and poor oxidative stability. Polymer gel electrolytes composed of ionic liquids and polymer network are self-standing and flexible while inheriting the properties of ionic liquids such as high ionic conductivity, electrochemical stability, and nonflammability.In the present study, we prepared a flexible composite electrolyte by combining a NASICON type Li1.5Al0.5Ti1.5(PO4)3 (LATP) fillers and a polymer gel electrolyte. The gel electrolyte was composed of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) and a liquid electrolyte of LiN(SO2CF3)2 (LiTFSA)/sulfolane (SL). The molar ratio of [LiTFSA]/[SL] was controlled to be 1/2. The gel electrolyte (without LATP) was prepared by a solution casting method, and the obtained gel was flexible. The electrochemical properties of the gel electrolyte are comparable to that of the liquid electrolyte of [LiTFSA]/[SL] = 1/2, which has a relatively high transference number of ~0.64. The composite electrolytes composed of the gel and LATP mixed in various ratios were prepared. The solid-state 6Li magic-angle spinning (MAS) NMR measurements revealed that the Li ion exchange reaction occurred at the interface between LATP and the gel electrolyte, suggesting that both LATP and gel phases contribute to the Li ion conduction in the composite electrolytes. However, AC impedance measurement for the interface between LATP and gel electrolyte suggested that the resistance for the Li-ion transfer at the interface causes the increase in resistance of the composite electrolyte, resulting in the decrease in ionic conductivity. We will present the effects of the LATP/gel electrolyte interface on the physicochemical properties of the composite electrolyte such as ionic conductivity and Li-ion transference number. We will report the charge-discharge performance of lithium batteries using the composite electrolytes.