Graphene has a completely open double-surface structure, which can undergo a series of organic reactions, and can be combined with other materials to improve its mechanical properties and electrical and thermal conductivity. If graphene is modified with functional groups, its chemical activity can be more abundant. This structural property of graphene also makes it ideal for synthesizing composites with electrochemically active materials for improving the performance of electrode materials such as lithium-ion batteries or supercapacitors.
1. Development progress of graphene industry
The preparation and application of graphene is a key investment project in countries all over the world. For example, the US Department of Advanced Research Projects announced in July 2008 a carbon electronic radio frequency application project with a total investment of 22 million US dollars, which is mainly used to develop ultra-high speed. and ultra-low energy graphene-based radio frequency circuits used to make computer chips and transistors. Subsequently, in May 2009, the National Science Foundation of the United States launched the graphene-based composite supercapacitor project, which was researched and implemented by the University of Texas at Austin. Other application projects include the commercial production of nano-graphene composite electrodes in lithium-ion batteries, which was funded by the Ohio State Research and Commercialization Funding Project and funded by Nanotek Instruments.
On the EU side, the EU FP7 framework plan announced the graphene-based nanoelectronic device project plan in January 2008. The participating institutions include Germany AMO Co., Ltd., the Nanoelectronics Research Group of the University of Italy, and the Semiconductor Physics Group of the University of Cambridge, UK. The important direction of the research is "Beyond CMOS". In July 2009, the German Science Foundation announced the launch of a 6-year graphene emerging frontier research project. The goal is to better understand and apply the properties of graphene, so as to facilitate the development of new graphene-based electronics with better properties. product.
In 2007, the Japan Academic Promotion Agency started the technical research and development of graphene-silicon materials/devices, and the responsible institution is Tohoku University, Japan. The important research and development direction of this project is "graphene silicon" material/process technology, and based on this technology, the products of advanced auxiliary switching devices and plasmon resonance Hertz devices will be developed. This research will advance the realization of time-free, ultra-high-speed, large-scale integrated device technology for charge transport.
2. Research progress of graphene and its composite materials in lithium-ion batteries
1. Principle and introduction of lithium-ion battery
Lithium ions have the characteristics of intercalating carbon materials or metal oxides. This process is quickly reversible. Using this characteristic, two materials that can reversibly intercalate and deintercalate lithium ions are used as positive and negative electrodes. The rechargeable battery is called. as a lithium-ion battery. When the battery is charged, lithium ions are deintercalated from the positive electrode and embedded in the negative electrode, and vice versa when discharging.
The positive electrode material should be in the state of lithium intercalation before charging. Common materials include lithium cobalt oxide (LiCoO2), lithium manganate (LiMn2O4), lithium nickel oxide (LiNiO2), and the common ternary material Li (NiCoMn) O2 and so on.
Commonly used negative electrode materials are various carbon materials, including: graphite, activated carbon, mesophase spherical carbon, multi-component composite carbon materials, graphene, carbon nanotubes and metal oxides.
2. Development of cathode materials
Lithium cobalt oxide (LiCoO2) is used as a cathode material for lithium-ion batteries, with an electronic conductivity of 10-4S/cm, relatively large specific energy, high open circuit voltage, long cycle charge and discharge life, and can withstand relatively fast charge and discharge, but It is easy to heat up and has poor safety. Therefore, it has not been applied to power lithium-ion batteries. LiNiO2 is cheaper than LiCoO2, and its performance is comparable to LiCoO2, but it is difficult to prepare and difficult to mass-produce. Lithium manganese oxide LiMn2O4 has an electronic conductivity of 10-6S/cm, which is cheaper than LiNiO2, relatively simple to prepare, and has good overcharge resistance, but its capacity is low, and its structure is unstable during charge and discharge. The problem of (Mn2+) dissolving into the electrolyte is also more difficult to solve. The cathode material widely used in power lithium-ion batteries is lithium iron phosphate (LiFePO4), which is safer and more stable in cycle charge and discharge than traditional cathode materials. The electronic conductivity (10−9S/cm) is poor. There are other cathode materials such as Li3V2(PO4)3, which have a higher operating voltage than LiFePO4 and have an electronic conductivity of 2.4×10−7S/cm. These materials have relatively low electrical conductivity, which often affects the capacity of lithium-ion batteries. Therefore, adding electronic conductive agents to improve the electrochemical performance is now a very common and convenient method to improve the electrochemical performance of lithium-ion batteries.
In recent years, there have been more and more studies on the composites of graphene and some cathode materials. Table 1 summarizes some graphene-containing cathode materials and their preparation methods. The literature also pointed out that adding conductive agents such as carbon black or glucose-derived carbon to graphene composites can make it have better electrochemical performance. At present, the research on LiCoO2/graphene composites has not been reported so far. Most of the graphene used in these cathode materials is reduced graphene oxide. These graphene oxides are usually prepared by the Hummer method and the offeman method or some improved methods based on it. The carbon sp2 bond network of these graphene oxides is disrupted and thus becomes insulating, to reduce these materials. A more common case is when at least a portion of the graphene oxide is reduced and mixed with the precursor. Commonly, graphene oxide nanosheets are used instead of graphene because of its strong hydrophilicity and thus easy mixing with nanoparticles of cathode materials.
(1) Specific surface area and morphology of cathode materials
According to the literature, graphene can increase the surface area of the electrode. Although the surface area of reduced graphene oxide (420-684m2/g) is much smaller than the theoretical value of 2630m2/g.
The surface area of Li3V2(PO4)3/C/rGO (156m2/g) electrode material is still much larger than that of Li3V2(PO4)3/C (9.0～27m2/g), Li3V2(PO4)3/rGO (16.8m2/g) and Li3V2(PO4)3 (3.2m2/g) surface area without any other material added. In terms of morphology, graphene can form a 3D electronically conductive network in the cathode composite. There is no specific research on how to obtain a fully mixed or attached graphene composite cathode, but the hydrophilic oxygen-containing groups (epoxy compounds, metal hydroxides, carboxylic acid groups) on graphene oxide can be As attachment points, the nanoparticles are attached to the surface and edges of graphene oxide. Therefore, compared with pure graphene, graphene oxide and reduced graphene oxide are more likely to form an attached structure rather than a mixed structure. The reaction between LiNi0.33Co0.33Mn0.33O2(NCM) NCMs and functionalized multi-walled carbon nanotubes forms an active particle layer, which hinders the insertion and de-intercalation of Li, and ultimately leads to poor battery capacity. Therefore, it can be understood that the composite structure is not only dependent on the degree of oxidation, but also affected by the active cathode material.
(2) Electrochemical properties of cathode materials
The properties of LiFePO4/graphene composite cathode materials are shown in Table 2.
Due to the addition of graphene, the electronic conductivity is improved, and the LiFePO4/graphene composite exhibits better rate capability. In particular, the charge and discharge capacity can be significantly increased at a large discharge rate (to 50C). Impedance tests show that graphene can reduce charge transfer resistance. It should be noted that replacing a part of graphene with some carbon materials with high electronic conductivity can reduce this charge transfer resistance more effectively. Besides graphene, adding a small amount of amorphous carbon can also add new rate capability. Furthermore, the researchers found that replacing graphene with a small amount of glucosamine-derived carbon could improve the electrochemical performance of the electrode material, because glucose-derived carbon could prevent graphene sheets from stacking on top of each other during synthesis. It has been pointed out that the composite material of 2% graphene and LiFePO4 has better charging capacity than the LiFePO4 composite material containing 1% or 4% graphene. Compared with LiFePO4, the composite of graphene and LiFePO4 showed better cycle charge-discharge life. In addition to the introduction of graphene, particle size reduction is also a necessary method to increase rate capability and charge capacity, as well as addition of electrochemical additives to improve electronic conductivity and doping, mixing technology and size of electronic conductive agents, graphene nanomaterials The conductivity and distribution of the sheet can affect the electrochemical performance of the material.
3. Development of anode materials
Common anode materials for lithium-ion batteries include graphite, soft carbon, mesocarbon microspheres, hard carbon, carbon nanotubes, and fullerene (C60). The molecular formula of the lithium intercalated graphite ionic compound is LiC6. According to reports, Japan's Honda Research and Development company uses PPP-700 as a negative electrode, and the reversible capacity is as high as 680mAh/g. The PPP-700 lithium storage capacity developed by MIT in the United States can reach 1170mA·h/g. In lithium-ion batteries, carbon materials as negative electrodes have problems such as voltage hysteresis and gradual decrease in cycle capacity, that is, the lithium intercalation reaction occurs between 0 and 0.25V (phase-related Li+/Li), while the delithiation reaction occurs at 1V (phase-related Li+/Li). About Li+/Li), after repeated charge and discharge, the pore structure of the carbon material collapses, and the capacity decreases significantly. Therefore, the preparation of anode materials with high cycle life, high purity and similar potential to Li+/Li has always been the research direction of researchers.
Transition metal oxides have now become another anode material that can replace carbon materials. They have very high Li storage capacity. Among these metal oxides, ferric oxide (Fe2O3) has attracted the attention of many researchers and manufacturers for its high theoretical capacity (924mAh/g), low cost and low environmental impact. However, Fe2O3, as an anode material in Li-ion batteries, has very poor cyclic charge-discharge performance. This is due to the agglomeration and huge volume change of Fe2O3 during Li ion insertion/deintercalation. An effective method is to apply carbonaceous material on Fe2O3 to buffer its volume expansion, thereby improving the electrochemical performance of Fe2O3. According to the literature, many graphene-based metal oxide materials have been reported as anode materials for lithium-ion batteries, such as Fe2O3, iron tetroxide (Fe3O4), titanium dioxide (TiO2), tin oxide (SnO2), cobalt tetroxide (Co3O4), Three manganese (Mn3O4).
The reported Fe2O3/graphene composites prepared by hydrothermal method exhibited higher reversible capacity (660mAh/g after 100 cycles of charge-discharge at a current density of 160mA/g) and higher rate capability, The cycle performance is superior to Fe2O3 and graphene electrodes. The exfoliated graphene oxide/Fe3O4 composite prepared under ultrasonic irradiation had very uniform Fe3O4 particles attached to the graphene oxide. As a negative electrode material for lithium-ion batteries, it has very good cyclability. The Co3O4/graphene nanocomposite prepared by hydrothermal method exhibits very high cycling performance and capacity, its reversible capacity reaches 906.6 mAh/g and maintains 93.1% capacity after 50 cycles.
4. Graphene battery product and technology patent analysis
Table 3 summarizes existing graphene battery products. The highlights of these products are fast charging and high capacity.
The high electrical conductivity, thermal conductivity, low resistivity, high strength and hardness of graphene, as well as its open-sided structure that can be easily synthesized with other materials, will hopefully greatly improve the performance of existing lithium-ion batteries. According to the research progress of many graphene lithium-ion batteries, the potential of graphene in improving the performance of lithium-ion batteries can be found. The composite of graphene and the cathode material of lithium ion battery can increase the specific surface area of the electrode material, improve the electrical conductivity, and thus increase the effective capacity of the material. Compounding with metal oxides can increase the conductivity of the material. Due to the structural characteristics of graphene itself, it can prevent the volume expansion of the metal oxide during the charge and discharge process, thereby increasing the stability of the material and improving the charge and discharge life of the material.
At this stage, the quality of graphene itself is difficult to achieve defect-free and 100% single-layer rate, resulting in the performance of graphene lithium-ion batteries, which cannot reach the expected performance. However, with the continuous development of graphene preparation technology, the quality of graphene has been greatly improved compared with before, and we can expect that the performance of graphene batteries in the future will have a greater performance improvement.