With the development of technology and the comprehensive popularization of mobile terminals, the widespread promotion of electric vehicles and the storage and utilization of resources are closely related to battery technology. Lithium-ion batteries are the mainstream technology in the field of energy storage and have extremely high development prospects. Graphene occupies an important position in lithium ion batteries due to its unique high electrical conductivity, large specific surface area and high chemical stability and other unique, excellent physical and chemical properties, and is widely used in positive and negative materials of lithium ion batteries .
1 Introduction
Lithium ion batteries have the characteristics of high energy density, large reversible capacity, large open circuit voltage, and long service life. With the development of the electronic information era, lithium-ion batteries have gradually become the main object of people's attention and research in the field of secondary battery development and application. In the research process of lithium ion battery electrode materials, some carbon allotropes and mixtures can be used as stable materials with excellent electrical conductivity, and are often used to develop new lithium ion battery anode materials.
2 Lithium battery overview
At present, lithium-ion batteries account for 66% of the scale of various energy storage technology devices in China. As the mainstream technology in the domestic energy storage market, lithium-ion batteries also have broad application prospects in mobile terminals and electric vehicles. The high energy density, large reversible capacity, large open voltage, and long service life are the unique characteristics of lithium ion batteries, which give it an unparalleled advantage over other batteries. Lithium-ion batteries include four main components: positive and negative electrodes, electrolyte and separator. The traditional lithium ion battery conductive additives are mainly composed of acetylene black and carbon black. Through the point-to-point method, the conductive additives and the positive and negative electrode active material particles are in contact with each other, which brings a large thermal resistance and high temperature The group brought great security risks. Therefore, the demand for new battery conductive additive materials that can provide efficient conductive networks has become extremely urgent. The new materials can not only reduce the amount of addition, but also greatly improve the conductivity of the positive and negative electrodes, reduce battery costs, and also improve Lithium ion battery rate and charge-discharge cycle performance.
3 Application of graphene in lithium ion batteries
3.1 Application of graphene in cathode materials
For lithium-ion batteries, the applicable cathode materials should meet the characteristics of large reversible capacity, high potential and stability, no toxicity, and low manufacturing cost. At present, the most common cathode materials for lithium-ion batteries are lithium iron phosphate (LiFePO4) materials, which have the characteristics of high specific capacity, non-toxicity, and low production cost. However, LiFePO4 has poor conductivity and low lithium ion mobility. If LiFePO4 material is combined with graphene, it can theoretically improve its electrical conductivity and rate performance. Due to the particularity of graphene materials, there are relatively few studies on graphene materials in terms of positive electrodes. Studies have shown that it is not ideal to improve the rate performance of composite materials made by directly covering graphene on the surface of LiFePO4 by hydrothermal method. The reason may be the stacking or destruction of graphene material structure. Wei et al. Found that the material formed by graphene half-encapsulating LiFePO4 can improve the conductivity of LiFePO4 material, but the ion transmission efficiency decreases after it is fully wrapped, and it may be because lithium ions cannot pass through the six-membered ring structure of graphene . Some researchers mixed LiFePO4 nanoparticles with graphite oxide ultrasonically to produce LiFePO4 / graphene composites with better microstructure. After further conventional carbon coating of the material, the specific capacity of lithium intercalation is greatly improved, and it can still be maintained at about 70mAh / g under the condition of 60C high rate.
3.2 Application of graphene in anode materials
Graphene is directly used as the negative electrode material of lithium ion batteries, and its specific capacity is 540mAh / g; due to the large specific surface area of graphene, scientists use graphene nanosheets as negative electrode materials for lithium batteries to increase their reversible capacity at all times. Later, its reversible capacity loss rate is lower. In addition, graphene can be used as a negative electrode material for lithium ion batteries after being compounded with metal oxides and alloy materials, such as tin-based and silicon-based oxides. To improve the characteristics of nano-materials and use the conductive properties and structural characteristics of graphene materials will improve the lithium ion transmission rate and the rate performance of lithium-ion batteries, so as to overcome the shortage of raw materials and many defects and reduce costs.
3.3 Other applications of graphene in lithium-ion batteries
In view of its excellent electrical conductivity, graphene materials can be used as conductive additives to optimize the electrical conductivity of the battery. Han et al. Added graphene materials to Si nanomaterials, and its modification effect is better than that of general conductive additives such as natural graphite. Its reversible specific capacity for the first cycle is as high as 2347mAh / g, and it can still reach 2041mAh / g after 20 cycles; Song et al. Added graphene as a conductive additive to the graphite material to optimize the conductive performance of the graphite material. The mechanism is that the graphene material is built between graphite in a layered structure, similar to building a "bridge" through which electrons pass. This material has a large contact area with graphite, avoiding the volume change similar to acetylene black particles after multiple cycles and reducing the performance area caused by the reduced contact area with the graphite material. In addition, graphene can also be used to improve the electrical conductivity of LiFePO4 nanoparticles.
In addition, graphene also plays a unique role in the preparation of highly deformable lithium-ion batteries due to its excellent mechanical strength and toughness. The composite material formed by He and the surface of ethylene terephthalate coated with graphene film has considerable flexibility, and reduces the density of the material, and optimizes its performance; Cheng and others attached the graphene material to the vacuum suction filter. On the surface of the filter paper, a graphene / cellulose composite material with superior mechanical properties and conductive properties is prepared. The above-mentioned and other flexible lithium-ion battery materials are mostly used for improving the environmental adaptability of lithium-ion batteries. With the development of research, such flexible electrode materials are also expected to be used in wearable electronic devices.