Study on the electrochemical performance of rechargeable lithium batteries using functionalized SiO2 nanoparticles

Abstract

Rechargeable lithium secondary batteries dominate the power source market for portable electronic devices due to their high energy density and long cycle life, and new applications such as electric vehicles and energy storage systems are gradually emerging onto the market. In these batteries, proper selections of the electrolyte and separator are very important for achieving good battery performance and ensuring safety. The major purpose of this study is to improve the cycling performancce and safety of lithium secondary batteries. In the pursuit of this purpose, various type of reactive inorganic materials containing radical polymerizable groups were synthesized and applied to separators, electrode materials and composite polymer electrolytes. We synthesized not only various SiO2 particles with different size but also functionalized SiO2 particles with different reactive groups such as vinyl and methacryloxy. Furthermore, mesoporous inorganic materials were used in the lithium-ion polymer batteries to overcome the limitation of non-porous particles. In chapter 2, the core-shell structured SiO2 particles containing poly(lithium 4-styrenesulfonte) in their shell were synthesized and used as coating materials for the preparation of ceramic-coated separators for lithium-ion cells. The ceramic-coated separators exhibited good thermal stability and wettability for liquid electrolyte due to the presence of a heat-resistant silica with hydrophilic poly(lithium 4-styrenesulfonte). By using these ceramic-coated separators, lithium-ion cells composed of a carbon anode and a LiNi1/3Co1/3Mn1/3O2 cathode were assembled and their cycling performances were evaluated. The cells assembled with the ceramic-coated separators demonstrate superior cycling performance compared to those prepared with a polyethylene separator. In chapter 3, we tried to improve the cycling stability of spinel LiNi0.5Mn1.5O4 materials by their surface modification. The reactive SiO2 nanopaticles containing a vinyl group were synthesized and coated onto the LiNi0.5Mn1.5O4 electrode. The composite polymer electrolyte layer was formed on the surface of a LiNi0.5Mn1.5O4 material by in-situ radical polymerization between diethylene glycol diacrylate and reactive SiO2 nanoparticles. The protective composite polymer layer formed on the LiNi0.5Mn1.5O4 material suppressed the irreversible decomposition of the electrolyte at high voltages and reduced the dissolution of transition metals from the charged LiNi0.5Mn1.5O4 electrode into the electrolyte at elevated temperature, which resulted in more stable cycling characteristics than the pristine LiNi0.5Mn1.5O4 electrode. In chapter 4, vinyl-functionalized SiO2 nanoparticles were synthesized and uniformly dispersed on the surface of a fibrous polyacrylonitrile (PAN) membrane for use as an additional cross-linking sites. The composite polymer electrolyte was prepared by in-situ cross-linking between vinyl-functionalized SiO2 particles on the PAN membrane and an electrolyte precursor containing tri(ethylene glycol) diacrylate. The cross-linked composite polymer electrolyte effectively encapsulated electrolyte solution without leakage. It exhibited good thermal stability as well as favorable interfacial characteristics toward electrodes. Lithium-ion polymer cells composed of a graphite negative electrode and a LiNi0.8Co0.15Al0.05O2 positive electrode were assembled with the in-situ cross-linked composite polymer electrolyte. The cells with a cross-linked composite polymer electrolytes using fibrous PAN membrane and vinyl-functionalized SiO2 particles exhibited high discharge capacity and good capacity retention at both ambient and elevated temperatures. In chapter 5, the cross-linked composite gel polymer electrolyte was prepared and applied to lithium-ion polymer cells as more reliable electrolyte. Mesoporous SiO2 nanoparticles containing reactive methacryloxy groups as additional cross-linking sites were synthesized and dispersed into the fibrous PAN membrane. They directly reacted with gel electrolyte precursors containing tri(ethylene glycol) diacrylate, resulting in the formation of a cross-linked polymer electrolyte with high ionic conductivity and favorable interfacial characteristics. The mesoporous SiO2 particles also served as HF scavengers to reduce the HF content in the electrolyte at high temperature. As a result, the cycling performance of the lithium-ion polymer cells with cross-linked composite gel polymer electrolytes employing methacryloxy-functionalized mesoporous SiO2 nanoparticles was remarkably improved at elevated temperatures. In these studies, reactive SiO2 particles were applied to various systems for lithium-ion batteris, such as separators, electrode materials and polymer electrolytes. It was demonstrated that reactive SiO2 materials can be one of the promising approach to improve the thermal safety and of electrochemical performance of lithium secondary batteries.