Research progress of metal oxides as negative electrodes for lithium ion capacitors

With the rapid consumption of fuel and the increase of environmental pollution, people are increasingly demanding energy storage equipment. Traditional energy storage devices can no longer meet the needs of economic development. Lithium-ion capacitors (LIC) are a new energy storage device between supercapacitors and lithium-ion batteries, which has attracted widespread attention in recent years. Combining the energy storage mechanism of electric double layer capacitors and lithium ion batteries, the positive electrode relies on the absorption and desorption of anions in the electrolyte to achieve rapid energy storage and release, while the negative electrode relies on the insertion and extraction process of Li+ in the electrolyte to undergo redox reaction storage. Charge.

Lithium-ion capacitors combine high energy density, high power density, and long cycle life, and are expected to be widely used in electric vehicles, rail transit, smart grids, and mobile electronic devices. At present, the main factor affecting the performance of lithium-ion capacitors is the mismatch between the positive and negative kinetic processes. The electrode material plays a key role in finding electrode materials with high specific capacity, good rate performance and high cycle stability to meet high energy density and high power. Lithium-ion capacitors with density and long cycle life are imminent. At present, most of the research work focuses on selecting suitable positive and negative materials and performance optimization. The positive electrode material of lithium ion capacitor is porous carbon material for electric energy storage of electric double layer capacitor. The research of positive electrode has large specific surface area and high electrical conductivity. The main carbon materials, such as activated carbon. The negative electrode materials of lithium ion capacitors currently studied are mainly anode materials of lithium ion batteries, which can be divided into two types according to materials. One is the conventional carbon anode materials such as graphite, hard carbon and soft carbon, which pass Li+ between the pores of carbon materials. Repeated insertion and removal of Li+ for energy transfer; second, metal oxide electrode materials such as SnO2, TiO2, CoO, etc. According to the energy storage principle of metal oxide anode materials, it can be divided into three types: 1 alloyed lithium intercalation material, which forms alloy with lithium, and can store and release Li+ during charge and discharge, mainly tin-based and silicon-based materials. 2 zero-strain lithium intercalation material, Li+ is intercalated and deintercalated between materials during charge and discharge, and the crystal structure of the material does not change during charge and discharge, such as TiO2, Li4Ti5O12 and other titanium-based materials; 3 conversion type lithium intercalation material For example, transition metal oxides such as MoO2 and CoO have a redox reaction during charge and discharge, and there is a change in valence state to transfer energy.

Compared with the anode material of the traditional lithium ion capacitor, the metal oxide has excellent deintercalation/intercalation ability, and its theoretical specific capacity is generally high, up to 500-1000 mA•h/g, and it is rich in natural resources and environmentally friendly. However, its electronic conductivity is poor, irreversible volume distortion during Li+ de-intercalation, causing crystal pulverization and shedding, which greatly affects the cycle stability of lithium-ion capacitors. Therefore, metal oxides are rarely used as lithium-ion capacitors. Electrode material. In view of the above advantages and disadvantages, the researchers obtained various kinds of composite materials by modifying or modifying the surface of the metal oxide with carbon materials, and the synergistic effect between the two materials was used to improve the comprehensive performance of the electrode materials. The high electronic conductivity of the material provides a good conductive network and surface coating, improves the conductivity of the material, and buffers the lattice expansion of the metal oxide during charge and discharge, improving cycle stability. At present, the main carbon materials used are graphene, carbon nanotubes, carbon fibers, etc., and the metal oxides form a composite material through a certain synthesis method and a preparation process, and exhibit good electrochemical performance. This paper introduces the main problems of metal oxides as negative electrodes for lithium ion capacitors and recent research progress, and prospects for its future development.