Studies on the functional electrolytes for high performance enhancement of lithium secondary batteries

Abstract

Lithium secondary batteries have become a commercial battery throughout the world. In spite of the successful commercialization, intensive efforts to increase energy densities and to improve safety have been done as the application of lithium secondary batteries expand. To increase the energy densities of lithium secondary batteries, from the capacity viewpoint, new material with high capacity such as silicon, tin, metal oxide, and newly introduced lithium metal oxide anode is preferable in replacement of graphite. Additionally, new type of lithium battery system like Li-S and $Li-O_2$ battery can be one way of improving the energy densities of lithium batteries. From the safety viewpoint, new material based on the olivine structure such as $LiFePO_4$ is considered as good candidate electrode material. On the other hand, operation condition is another factor of determining performance of a battery. Proper controlling of operation condition can enhance the cell performance. Especially, controlling the interphase between electrode material and electrolyte is quite important factor to improve the performance of battery systems. From this point of view, we investigate and controlling the solid-electrolyte interphase (SEI) of given battery system. In this thesis, we examine the interfacial characteristics of new electrode material designed for the high capacity and safety. Firstly, we handled with interfacial characteristics of new anode candidate material. Lithium vanadium oxide ($Li_1.1V_0.9O_2$, LVO), newly introduced as an alternative anode material. This material has 1.5 times larger volumetric energy density. However, relative low electrical conductivity, particle break-down during lithium ion insertion and decomposition of electrolyte induce poor cycle performance. To resolve such problems, we introduced fluoroethylene carbonate (FEC) to stabilize the SEI layer of $Li_1.1V_0.9O_2,$ and 3wt% of FEC showed enhanced cycle performance. From the surface analysi...