코어-쉘 구조의 SiO2(Li+) 입자를 포함하는 고분자 복합막을 이용한 리튬폴리머전지의 전기화학적 특성 연구

Abstract

Lithium-ion batteries have rapidly become the dominant power source for portable electronic devices, electric vehicles, and other energy storage systems due to their high energy density and long cycle life. However, safety issues surrounding these batteries must be solved before they can be widely utilized in large-scale applications, such as electric vehicles and energy storage application. Furthermore, safety component of lithium-ion battery has led to a quest for polymer electrolyte systems to replace the liquid electrolyte, which is currently being used in lithium-ion batteries. Most of the recent research works have been directed to the preparation and characterization of gel polymer electrolytes that exhibit higher ionic conductivity and non-leakage characteristics at ambient temperature. For preparing gel polymer electrolytes, polyacrylonitrile (PAN), poly(vinylidene fluoride) (PVdF), poly(vinylidene fluoride-co-hexafluoro propylene) (P(VdF-co-HFP)) and poly(methyl methacrylate) (PMMA) have been widely studied as host polymers. Also, ceramic fillers such as SiO2, Al2O3, TiO2, BaTiO3 and ZrO2 have been incorporated along with the host polymer to fabricate composite electrolytes. The addition of inorganic filler is a useful tool for improving the electrical and mechanical properties of gel polymer electrolytes, and the characteristics of the ceramic filler play a vital role on the electrochemical properties of the polymer electrolytes. However, theses ceramic fillers do not involve in the lithium transport process and it is of main interest to introduce the inorganic materials containing lithium ions, which aids in lithium ion transport, as fillers in the composite. To overcome these problems, the hybrid composite materials are to be designed with fillers that exhibit ion transport function in gel polymer electrolyte systems. The major goal of this study is to synthesize lithium ions containing the core-shell structured organic-inorganic composite material as gel polymer electrolyte filler to develop lithium batteries with high uptake, high ionic conductivity, high lithium transport ability, high rate capability, and reliable cycling performance. In chapter 2, synthesis of monodispersed core-shell structured SiO2(Li+) particles containing lithium ions in the shell are presented. The silica core particles prepared by the hydrolysis and condensation of vinyltrimethoxysilane (VTMS) in aqueous solution, and the shell layer was synthesized by varying the content of 4-styrenesulfonic acid sodium salt monomer in the polymerization with SiO2 core particles. Then, the sodium ions (Na+) in the shell of the SiO2 particles were exchanged to lithium ions (Li+). Finally, the core-shell structured SiO2(Li+) particles were incorporated into the P(VdF-co-HFP) polymer matrix to form the composite membrane. The composite gel polymer electrolytes prepared with core-shell structured SiO2(Li+) particles exhibited higher electrolyte solution uptake and ionic conductivities than those prepared with fumed SiO2. Lithium-ion polymer batteries composed of graphite anode and LiCoO2 cathode were assembled with composite gel polymer electrolyte, and their charge /discharge cycling performances were evaluated. In the composite gel polymer electrolyte, the hydrophilic shell of core-shell structured SiO2(Li+) particles can hold the solvent effectively and the reactivity between the organic solvent and the electrodes is decreased, which can limit the growth of the resistive layer on the electrodes. Lithium-ion polymer cell assembled with the composite gel polymer electrolyte containing 20 wt.% SiO2(Li+) particles exhibited good capacity retention (95 % after 100 cycles) and excellent high rate performance (131 mAh•g-1 @ 5C rate). In chapter 3, the successful synthesis of monodispersed functional core-shell silica particles with different core sizes as inorganic fillers in gel polymer electrolyte along with the effect of core particle size on the performance of the cells are presented. Three different sizes of particles with uniform spherical shape of 260 nm, 420 nm, and 860 nm diameters were synthesized by controlling the feed ratio and the reaction time, and the shell thicknesses of all the particles were kept at 200 nm. Lithium polymer cells composed of graphite anode and LiFePO4 cathode were assembled with gel polymer electrolyte containing core-shell structured SiO2(Li+) particles of different sized core, and their cycling performances were evaluated. Lithium-ion polymer cell with composite gel polymer electrolyte containing SiO2(Li+) particles of 260 nm core diameter exhibited excellent electrochemical performance as well as good capacity retention. The core-shell structured SiO2(Li+) particles of smaller core diameter shown that the ability to retain the fast ion transport in the composite gel polymer electrolyte was supported by 260 nm core particles of increasing ion transport number and higher ionic conductivity. In chapter 4, core-shell structured SiO2(Li+) particles with different shell thicknesses were synthesized and used as fillers in Li+ conducting composite polymer electrolyte. A detailed investigation on the effect of shell thickness was made to prepare the composite polymer electrolytes with high ionic conductivity, good mechanical strength and favorable interfacial characteristics. The composite polymer electrolytes were applied in the lithium-ion polymer batteries composed of a graphite anode and a LiNi1/3Co1/3Mn1/3O2 cathode. The results demonstrate that the composite polymer electrolytes containing core-shell SiO2(Li+) particles with an optimized core diameter of 200 nm and a shell thickness of 320 nm are very promising electrolyte materials for high electrochemical performance lithium-ion polymer batteries. The ionic conductivity and lithium transport number increase with increasing shell thickness, which reduces the concentration polarization in the cell during cycling and provides high discharge capacity at high current rates. In chapter 5, the composite gel polymer electrolyte containing core-shell structured SiO2(Li+) particles was applied in cells with lithium powder as anode and lithium vanadate as cathode to demonstrate its applicability in a wide variety of electrode combinations. By using the composite gel polymer electrolytes, the lithium powder anode and a layered lithium vanadate (LiV3O8) cathode were assembled and their cycling performance was evaluated. The cycling efficiency of lithium powder electrode was higher than that of lithium foil electrode. The resulting lithium powder polymer batteries delivered a high discharge capacity of 264 mAh•g-1 at room temperature and exhibited good capacity retention even at high current rates. The morphological analysis of the lithium powder anode revealed that the dendrite growth during cycling was effectively suppressed by using the composite gel polymer electrolytes. In these studies, as lithium ion sources of a single ion conductor, the core-shell structured SiO2(Li+) particles with uniform spherical shape were synthesized and used as functional fillers in the composite gel polymer electrolytes. We showed that the novel composite gel polymer electrolytes exhibiting high ionic conductivity and good mechanical stability were prepared. Also, their improved electrochemical properties enhanced cell performance during cycling test.