Fabrication of Ceramic/Polymer Hybrid Solid Electrolytes for All-Solid-State Lithium Batteries

Abstract

The demand for solid-state lithium ion electrolytes has increased owing to the safety concerns regarding the use of lithium ion batteries, which contain flammable organic solvent electrolytes. The liquid electrolyte can give rise to serious problems such as leakage and gas explosions when the operating temperature rises. All solid-state batteries using organic or inorganic solid electrolytes have a higher energy density than lithium-ion batteries and can reduce the number of parts and packaging space, thereby saving weight and volume while using the same amount of power. To increase the size and, thus, the energy density of the lithium secondary battery while maintaining stability, the use of a solid electrolyte is required in place of the conventional flammable organic liquid electrolyte. NASICON-type Li1.3Al0.3Ti1.7(PO4)3 (LATP) ceramics are typical lithium ion conducting materials, with ion conductivities as high as 10-4 S·cm-1 at room temperature. In addition, LATP is chemically stable in humidified air or carbon dioxide and is expected to exhibit a high oxidative potential (4.21 V). In this regard, LATP has attracted significant attention as a promising solid electrolyte for all-solid-state lithium batteries. In addition to high ionic conductivity and chemical stability, the solid electrolyte needs to possess high strength and toughness to allow production of large area thin electrolyte sheets. The poor mechanical properties of ceramic-based solid electrolytes such as LATP or Li7La3Zr2O12 limit their practical application. Incorporating organic fillers with lithium ion conductivity in a ceramic-based electrolyte is an effective way to give not only flexibility but also enhanced mechanical properties. Dry polymer-based solid electrolytes present various advantages, such as excellent interfacial stability with lithium, the formation of complexes with lithium salts, and flexibility. Various types of polymer-based solid electrolytes exist, such as polyethylene oxide (PEO), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), and polyvinylidene fluoride (PVDF). Among them, the PEO-based polymer has attracted considerable attention, owing to its flexible skeleton and good interfacial stability with lithium electrodes. However, the commercialization of PEO is difficult because its room-temperature ionic conductivity is as low as 10−7–10−6 S·cm−1. Recently, we found that the solid polymer electrolyte with PEO:PMMA=8:1 and 8 wt% silica aerogel exhibited the high lithium-ion conductivity (1.35 × 10−4 S∙cm−1 at 30 °C) and good mechanical stability. In this study, a polymer blend of PEO and PMMA containing silica aerogel particles was incorporated to the LATP matrix. The PEO/PMMA solid polymer which was dispersed in the LATP skeleton permit flexibility and good bonding between the LATP. The LATP powder was synthesized by the sol-gel technique from lithium nitrate, aluminum phosphate, and titanium iso-propoxide to induce a rapid gelation reaction without further dissolution. The lithium ion conductivity of LATP/polymer composite electrolytes was measured from a symmetrical cell consisting of the LATP electrolyte and Pt blocking electrodes. The effects of the LATP:polymer ratio and the structure of the LATP skeleton such as porosity and grain size on the conductivity were investigated.