Study on functional additives to improve electrochemical performances of overlithiated layered oxides

Abstract

The lithium ion battery is introduced for small electronic appliances at first. After several decades, sustainable energy sources are required to replace existing fossil fuel. One of renewable energy sources, electricity is suggested as candidates for available energy sources because it has no air pollution and high efficiency. Due to this benefits, it is considered to replace internal combustion engine of car and adapt to energy storage system (ESS). However, large scale batteries used in ESS or vehicle which need high capacity and high voltage is not able to be realized with existing cathode. The lithium cobalt oxide (LCO) is used for common lithium ion batteries. It is viable option for small appliances because it doesn’t require high spec. The structure of LCO is being unstable when it release more than half of its lithium sources. Also, it cannot bear high voltage circumstances above 4.2V because it react with electrolyte easily. Many researchers attempt to solve this problem. They use coating on cathode surface to protect it from side reactions and structural changes at high voltages instead of changing cathode. It has two type. One is physical method directly spraying coating materials and the other is chemical method which use electrochemical reaction. The chemical method is modifying solid electrolyte interphase (SEI) layer by additives. Those additives less than 5% of total electrolyte solution make ideal SEI layer which protects cathode material from side reactions with bulk electrolyte and selectively transfers lithium ions. Furthermore, they search cathode materials fit to industrial requirements. It is necessary to develop new cathode because existing electrode has limit even we use coating method. The overlithiated layered oxides (OLO), lithium rich layered oxides, is one of promising candidates for high voltage, high capacity cathodes. It is denoted as Li1+xM1-xO2 (0<x<1, M=Co, Mn, Ni) which is composed of two type phase represented as zLi2MnO3-(1-z)LiMO2 (0<z<1, M=Mn, Ni, Co). It is able to fulfill high capacity and high voltage condition, but its phase transformation during cycling is problem because it increases irreversible capacity. In this work, we use Li1.17Ni0.17Mn0.5Co0.17O2 (0.4Li2MnO3-0.6LiNi1/3Co1/3Mn1/3O2) as cathode and additives to improve its stability at severe conditions. The additive is selected from salt type additives. The salt type is composed of lithium ion and anion pair like other lithium salts. The oxalate group forms durable carbonate components and core elements helps to ion transfer. It shows better electrochemical performances than reference. It regulate insulating materials like lithium fluoride caused by decomposition of lithium salt when building layer on surface and construct durable and good ionic permeable membrane which is able to endure high temperature and high power condition.