Polymer-based solid electrolytes for lithium secondary batteries with enhanced safety

Abstract

In recent years, enhancement of safety in battery is the biggest challenge for energy storage industry. Compared with organic liquid electrolyte, gel polymer electrolyte and solid electrolytes offer an attractive option due to their potential in improving the safety and achieving high energy density. Despite extensive research efforts, the development of safer batteries is still struggling because of the lack of suitable candidate materials for the electrolyte required for practical applications. In this dissertation, I report some methodologies for improving safety and electrochemical performance of lithium polymer batteries and solid-state batteries. In chapter 2, the solvent-free hybrid solid electrolytes composed of lithium aluminum germanium phosphate (LAGP) and poly(ethylene oxide) (PEO) are prepared in the form of flexible film, and their electrochemical characteristics are investigated. The addition of LAGP powder into PEO-based solid polymer electrolyte improves the electrical and mechanical properties of the solid electrolytes due to ionic conductive nature of LAGP powder. For the hybrid solid electrolytes under this study, the optimum composition of LAGP is found to be about 70 wt.%. The solid-state Li/LiFePO4 cell assembled with hybrid solid electrolyte containing 70 wt.% LAGP delivers an initial discharge capacity of 135.1 mAh g-1 and exhibits excellent cycling stability at 55℃. In chapter 3, flexible ceramic separators based on Li+-conducting lithium lanthanum zirconium oxide (LLZ) are prepared as thin films using polymeric binder and directly applied onto negative electrode to produce a separator-electrode assembly with good interfacial adhesion and low interfacial resistances. The ceramic separators show an excellent thermal stability and high ionic conductivity as compared to conventional polypropylene separator. The lithium-ion batteries assembled with graphite negative electrode, Li+-conducting ceramic separator and LiCoO2 positive electrode exhibit good cycling performance in terms of discharge capacity, capacity retention and rate capability. It is also demonstrated that the use of a ceramic separator can greatly improve safety over cells employing a polypropylene separator, which is highly desirable for lithium-ion batteries with enhanced safety. In chapter 4, poly(ε-caprolactone)-(PCL)-based solid polymer electrolytes were prepared, and their characteristics and electrochemical properties were investigated. The optimized polymer electrolyte exhibited ionic conductivities of 2.5×10-5 S/cm and 2.2×10-4 S/cm at room temperature and 55℃, respectively, and good electrochemical stability. In order to improve the dimensional stability, the PCL-based polymer electrolyte was infiltrated in porous electrospun polyacrylonitrile (PAN) and non-woven poly(propylene) membranes. A solid-state Li/LiNi0.6Co0.2Mn0.2O2 cell assembled with the PCL-based polymer electrolyte in the electrospun PAN membrane exhibited an initial discharge capacity of 148.7 mAh/g and good capacity retention. The good cycling performance of the cell was attributed to the high ionic conductivity and lithium transference number of the polymer electrolyte and its good interfacial contacts with the electrodes. In chapter 5, amorphous poly(ethylene ether carbonate) (PEEC), alternating copolymer of ethylene oxide and ethylene carbonate, was synthesized by ring opening polymerization of ethylene carbonate. Solid polymer electrolytes were prepared with PEEC and lithium bis(trifluoromethane) sulfonylmide, and their electrochemical characteristics were investigated. Ethylene carbonate unit in PEEC improved the ionic conductivity, electrochemical stability and lithium transference number of the polymer electrolytes, as compared to those of poly(ethylene oxide)-based polymer electrolytes. A cross-linked solid polymer electrolyte was synthesized by photo-cross-linking reaction using PEEC and tetraethyleneglycol diacrylate as a cross-linking agent, in the form of flexible thin film. The solid-state Li/LiNi0.6Co0.2Mn0.2O2 cell assembled with solid polymer electrolyte based on cross-linked PEEC delivered a high initial discharge capacity of 141.4 mAhg-1 and exhibited good capacity retention at 25℃, demonstrating feasibility of solid polymer electrolyte for all solid-state lithium batteries that can operate at ambient temperature. Several studies have successfully replaced liquid electrolytes with polymer-based electrolyte materials. Polymer-based electrolytes improved the safety and electrochemical properties of the batteries compared to liquid electrolytes.