Positive and negative materials of lithium battery and preparation method thereof

The electronic information age has led to a rapid increase in demand for mobile power. Because lithium-ion battery has the important advantages of high voltage and high capacity, and has long cycle life and good safety performance, it has broad application prospects in portable electronic equipment, electric vehicles, space technology,special industry, etc, a research hotspot that has been widely concerned in the year. Lithium-ion battery mechanism According to the general analysis, lithium-ion battery as a chemical power source refers to a secondary battery composed of two compounds capable of reversibly intercalating and deintercalating lithium ions as a positive and negative electrode. When the battery is charged, lithium ions are deintercalated from the positive electrode and embedded in the negative electrode, and vice versa when discharging. Lithium-ion batteries are the result of research in the fields of physics, materials science and chemistry. The physical mechanism involved in lithium-ion batteries is currently explained by the embedded physics in solid physics. Intercalation refers to the reversible embedding of movable guest particles (molecules, atoms, ions) into a host lattice of suitable size. In the network space point in the point, the positive and negative materials of the electron transport lithium ion battery are mixed conductor embedded compounds of ions and electrons. Electrons can only move in the positive and negative materials [4][5][6]. There are many types of embedded compounds known, and the guest particles can be molecules, atoms or ions. At the same time as the ions are embedded, charge compensation is required by the main structure to maintain electrical neutrality. The charge compensation can be achieved by a change in the energy band structure of the host material, and the conductivity changes before and after embedding. Lithium-ion battery electrode materials can be stably present in the air and are closely related to this property. The intercalation compound can be used as a lithium ion battery electrode material only if it satisfies the structural change and is reversible and can compensate for the charge change by structure.

The key material for controlling the performance of lithium-ion batteries--positive and negative active materials in batteries is the key to this technology, which is the consensus of researchers at home and abroad.

1 cathode material properties and general preparation methods

An important parameter characterizing the transport properties of ions in the positive electrode is the chemical diffusion coefficient. In general, the diffusion coefficient of lithium ions in the positive active material is relatively low. Lithium is embedded in or deintercalated from the positive material, accompanied by a change in crystal phase. Therefore, the electrode film of a lithium ion battery is required to be very thin, generally on the order of several tens of micrometers. The lithium intercalation compound of the positive electrode material is a temporary storage container for lithium ions in a lithium ion battery. In order to obtain a higher cell voltage, a high potential lithium intercalation compound is preferred. The cathode material should meet:

1) Having electrochemical compatibility with the electrolyte solution within the required range of charge and discharge potentials;

2) Mild electrode process kinetics;

3) Highly reversible;

4) Stability in air in a fully lithiated state.

The research focus is mainly on the compound of layered LiMO2 and spinel LiM2O4 structure and the similar electrode materials of two kinds of M (M is a transition metal ion such as Co, Ni, Mn, V). As a positive electrode material for lithium ion batteries, the degree of structural change and reversibility of Li+ ions during deintercalation and embedding determine the stable repeated charge and discharge properties of the battery. In the preparation of the positive electrode material, the raw material properties and the synthesis process conditions will have an impact on the final structure. A variety of promising cathode materials had been used to reduce the capacitance during cycling, which is the primary problem in research. Commercially available positive electrode materials are Li1-xCoO2 (0<x<0.8), Li1-xNiO2 (0<x<0.8), LiMnO2[7][8]. They have advantages and disadvantages as positive electrode materials for lithium ion batteries. Lithium-ion battery with lithium cobalt oxide as positive electrode has the advantages of high open circuit voltage, large specific energy, long cycle life, fast charge and discharge, etc., but the safety is poor; lithium nickel oxide is cheaper than lithium cobalt oxide, and its performance is equivalent to lithium cobalt oxide. It has excellent lithium intercalation performance, but it is difficult to prepare; while lithium manganese oxide is cheaper, it is relatively easy to prepare, and its overcharge and safety performance is good, but its lithium insertion capacity is low, and the spinel structure is not charged and discharged stable. From the perspective of application prospects, the pursuit of abundant resources, low cost, pollution-free, and low requirements for voltage control and circuit protection during overcharging, high-performance cathode materials will be the research of lithium-ion battery cathode materials. Focus. It has been reported in foreign countries that LiVO2 can also form a layered compound, which can be used as a positive electrode material [9]. It can be seen from these reports that although the chemical composition of the electrode materials is the same, the performance changes more after the preparation process changes. Successful commercial electrode materials have their own unique features in the preparation process, which is the gap in the current research in China. The advantages and disadvantages of various preparation methods are listed below.

1) The solid phase method generally uses a lithium salt such as lithium carbonate and a cobalt compound or a nickel compound to grind and mix, and then performs a sintering reaction [10]. The advantage of this method is that the process flow is simple and the raw materials are easy to obtain. It belongs to the method of extensive research and development in the early stage of development of lithium ion batteries, and the foreign technology is relatively mature; the disadvantage is that the prepared positive electrode material has limited capacitance, poor uniformity of raw material mixing, and preparation of materials, poor performance and poor quality consistency between batch and batch.

2) Complex method A complex precursor containing lithium ions and cobalt or vanadium ions is first prepared with an organic complex, and then sintered. The method has the advantages of molecular scale mixing, good material uniformity and performance stability, and the positive electrode material has higher capacitance than the solid phase method. It has been tested abroad as an industrial method for lithium ion batteries, and the technology is not mature report.

3) The sol-gel method uses the method for preparing ultrafine particles developed in the 1970s to prepare a positive electrode material. The method has the advantages of the complex method, and the prepared electrode material has a large increase in capacitance, a method that is rapidly developing at home and abroad. The disadvantage is that the cost is higher and the technology is still in the development stage [11].

4) Ion exchange method, such as LiMnO2 prepared by ion exchange method, obtained a reversible discharge capacity of 270 mAh/g. This method has become a new hotspot of research. It has the characteristics of stable electrode performance and high capacitance. . However, the process involves time-consuming steps such as recrystallization of the solution, and there is still a considerable distance from the practical use.

The study of the cathode material can be seen from the foreign literature, its capacitance is increasing at a rate of 30-50 mAh/g per year, and the development tends to be smaller and smaller, and the lithium-filled compound with larger and larger capacitance the scale of raw materials advances to the nanometer level. Theoretical research on the structure of lithium intercalation compounds has made some progress, but its development theory is still changing. The problem of lithium battery capacity increase and cycle capacity attenuation that has plagued this field has been proposed by researchers to add other components to overcome [12][13][14][15][16][17]. But for now, the theoretical mechanism of these methods has not been clearly studied, leading Japanese scholar Yoshio. Nishi to believe that there has been little progress in this field in the past decade [1], and further research is urgently needed.

2 anode material properties and general preparation methods

The conductivity of the negative electrode material is generally high, and a lithium-intercalable compound having a potential as close as possible to the lithium potential, such as various carbon materials and metal oxides, is selected. A negative electrode material that reversibly intercalates and deintercalates lithium ions is required to have:

1) The free energy changes little in the intercalation reaction of lithium ions;

2) Lithium ions have a high diffusivity in the solid state structure of the negative electrode;

3) A highly reversible embedding reaction;

4) Have good conductivity;

5) Thermodynamically stable, while not reacting with the electrolyte.

Research work has focused on carbon materials and other metal oxides with special structures. Graphite, soft carbon and medium-phase carbon microspheres have been developed and studied in China. Hard carbon, carbon nanotubes, and buckyball C60 are being studied [18][19][20][21] [22] [23]. Japan HondaResearchandDevelopmentCo. K. of Ltd. Sato et al. used polypyridyl ethylene (Polyparaphenylene-PPP) pyrolysis product PPP-700 (heating PPP to 700 ° C at a certain heating rate and heat-dissolving the product for a certain period of time) as the negative electrode, and the reversible capacity is as high as 680mAh/g. MJ Matthews of MIT, USA, reported that the PPP-700's storage capacity can reach 1170 mA?h/g. If the lithium storage capacity is 1170 mA?h/g, and as the lithium insertion amount increases, and then the performance of the lithium ion battery is improved, the author believes that future research will focus on the smaller nanoscale lithium intercalation microstructure. At the same time as the study of carbon negative electrodes, the search for other negative electrode materials with potentials similar to Li+/Li potential has been paid attention to. There are two problems with the carbon materials used in lithium-ion batteries:

1) Voltage hysteresis, that is, the lithium intercalation reaction is carried out between 0 and 0.25 V (relative to Li+/Li) and the deintercalation reaction occurs at around 1 V;

2) The cycle capacity gradually decreases. After 12 to 20 cycles, the capacity is reduced to 400-500 mAh/g.

Further deepening in theory depends on the preparation of various high-purity, structurally-regulated raw materials and carbon materials and the establishment of more effective structural characterization methods. Fujifilm Corporation of Japan has developed a new tin composite oxide based anode material for lithium ion batteries. In addition, the existing research has focused on some metal oxides, and its mass ratio energy is much higher than that of carbon anode materials, such as SnO2, WO2, MoO2, VO2, TiO2, LixFe2O3, Li4Ti5O12, Li4Mn5O12, etc. [24], but not as mature as carbon electrodes. The reversible high storage mechanism of lithium in carbon materials mainly includes lithium Li2 formation mechanism, multi-layer lithium mechanism, lattice lattice mechanism, elastic sphere-elastic network model, layer-edge-surface lithium storage mechanism, and nano-scale graphite storage, Lithium mechanism, carbon-lithium-hydrogen mechanism and microporous lithium storage mechanism. Graphite, as one of the carbon materials, has long been found to form graphite intercalation compounds (Liph6) with lithium, but these theories are still in the development stage. The difficulty to overcome the negative electrode material is also a problem of capacity cycle attenuation. However, it is known from the literature that the preparation of high purity and regular microstructured carbon negative electrode materials is a development direction.

A general method for preparing a negative electrode material can be summarized as follows.

1) heating soft carbon at a certain high temperature to obtain highly graphitized carbon; the molecular formula of lithium intercalated graphite ionic compound is LiC6, wherein the lithium ion is dynamically changed in the process of intercalation and deintercalation in graphite, and the relationship between graphite structure and electrochemical performance, Problems such as the cause of irreversible capacity loss and improvement methods have been discussed by many researchers. 2) The hard carbon obtained by decomposing the crosslinked resin with special structure at high temperature has higher reversible capacity than graphite carbon, and its structure is greatly affected by raw materials, but the general literature considers that the nanopores in these carbon structures are embedded. Lithium capacity has a great influence, and its research mainly focuses on the use of high molecular weight polymer to prepare hard carbon with more nanometer-scale micropores [25][26][27].

3) Hydrogen-containing carbon prepared by high temperature thermal decomposition of organic matter and high polymer [28] [29]. Such materials have a reversible capacity of 600-900 mA?h/g, which has attracted attention, but its voltage lag and cycle capacity are the biggest application obstacles. The improvement of its preparation method and the explanation of the theoretical mechanism will be the focus of the research.

4) The mechanism of various metal oxides is similar to that of cathode materials [24],

It has also attracted the attention of researchers, and the research direction is mainly to obtain metal oxides of new structures or composite structures.

5) As a lithium-incorporated material, carbon nanotubes and buckyball C60 are also a new hot spot in current research, and become a branch of nanomaterial research. The special structure of carbon nanotubes and buckyball C60 makes it the best choice for high capacity lithium intercalation materials [22][23][30]. In theory, nanostructures can provide lithium-insertion capacity higher than currently available materials, and their microstructure has been extensively studied and made great progress, but how to prepare appropriate stacking methods to obtain excellent performance. Electrode materials, this should be an important direction of research [31] [32] [33].

3. Conclusion

In summary, the research and development and application of positive and negative active materials in lithium ion batteries have been quite active internationally in recent years, and great progress has been made. The crystal structure of the material is regular, and the irreversible change of the structure during charging and discharging is the key to obtaining a lithium ion battery with high specific capacity and long cycle life. However, the study of the structure and properties of lithium-incorporated materials is still the weakest link in the field. Lithium-ion battery research is a constantly updated battery system. Many new research results in physics and chemistry will have a major impact on lithium-ion batteries, such as nano-solid electrodes, which may make lithium-ion batteries have higher energy density. And power density, which greatly increases the range of applications of lithium-ion batteries. In short, the research of lithium-ion batteries is a cross-cutting field involving many disciplines such as chemistry, physics, materials, energy, and electronics. Advances in the field have generated great interest in the chemical power industry and industry. It is expected that with the deepening of the research on the relationship between the structure and properties of electrode materials, various regular structures or doped composite structures of positive and negative materials designed at the molecular level will strongly promote the research and application of lithium ion batteries. Lithium-ion batteries will be the second-battery with the best market prospects and the fastest development in the future for a long time after nickel-cadmium and nickel-hydrogen batteries.

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