Functional Electrolytes for Improving Electrochemical Performances of Carbon-Coated Porous Silicon Anodes in Lithium-Ion Batteries

Abstract

Today, people use many electric devices, such as mobile phones and laptops, in our life. Not only that, hybrid electric vehicles are commercial and the need of electric vehicles is expected to increase. As the amounts of using these products are increased, portable electricity should be increased. Therefore, batteries which can store electricity are inevitable, especially secondary batteries. Among of rechargeable batteries, lithium ion batteries are receiving great attention because of its high energy density. Lithium ion batteries have high operating voltage compared to other batteries and show long cycle life. Graphite, carbon-based material, has been commonly used as anode for commercial lithium ion batteries. Graphite anode shows good stability in terms of cycle life. However, its relative low theoretical capacity of 372 mAh/g, is not enough for electric vehicle or energy storage system. Therefore, other new anode materials have been investigated for using in Li ion battery system, such as Li metal, Sn, Al, Sb and Si. Among of them, silicon has been studied extensively from many researchers because of its high gravimetric capacity and low operating voltage. However, Si anode has a serious problem for using as anode material, which is large volume expansion about 400%. Dramatic volume change causes serious capacity fade of Si anode, because volume expansion looses contacts between active materials and electrode, resulting in the mechanical instability. During cycling, electrolytes experience decomposition at cathode and anode. Especially, large electrolyte decomposition happens at anode because of its low operating voltage. Due to electrolyte decomposition, electrodes would be covered by surface film. This surface film is called by solid electrolyte interphase (SEI) layer. The SEI layer is important in battery system because SEI layer protects electrode from further electrolyte decomposition. Typically, graphite anodes make surface film at 0.5-0.7 V by ethylene carbonate (EC) decomposition, which is main component of electrolyte. This graphite SEI layer remains on the anode surface during cycling. It is one of the reason that graphite anode shows cycle stability. However, such SEI layer would be problem in the case of silicon anode. Due to large volume expansion, SEI layer of Si anode would be broken. Therefore, new active site would be disclosed and cause an increase in the side-reaction with electrolyte. During repeated cycling, lithium ions and electrons are consumed to make SEI layer. That is why Si anode experience serious capacity fade. Many researchers have tried to solve volume expansion of Si anode. Many approaches have focused changing morphology and reducing size of Si particles to buffer the mechanical stress caused by volume change. For example, porous structure provides space to accommodate volume expansion and nano-size particles better stand volume expansion compared to bulk structure. Another strategy is using coating layer. Coating layer is able to increase conductivity of electrode and inhibit direct contact electrolyte and active material. Other example is using electrolyte additives to make stable SEI layer. Because SEI layer is mainly formed by electrolyte decomposition, electrolyte additives are important materials for making stable SEI layer. Additives make SEI layer instead of organic solvents. Fluoroethylene carbonate (FEC) is known for effective additive to make stable SEI layer on the silicon anode. Then, we can expect that better cycle life of silicon anode can be obtained by using FEC as co-solvent instead of ethylene carbonate (EC), which is conventional solvent of liquid electrolyte. From the experiment result, better cycle retention and higher coulombic efficiency (>99%) are obtained.