Composition of lithium-ion batteries

Li-ion batteries are developed from lithium batteries. So before introducing Li-ion, introduce lithium batteries. For example, the button batteries used in the previous camera belonged to lithium batteries. The positive electrode material of the lithium battery is manganese dioxide or thionyl chloride, and the negative electrode is lithium. After the battery is assembled, the battery has a voltage and does not need to be recharged. This battery may also be charged, but the cycle performance is not good. During the recharging and discharging cycle, lithium dendrites are easily formed, resulting in a short circuit in the battery. Therefore, this type of battery is generally prohibited. Charge. Later, Sony Corporation of Japan invented the negative electrode of carbon materials and the positive electrode of lithium-containing compounds. During the charging and discharging process, there was no metallic lithium, only lithium ions, which was a lithium-ion battery. When the battery is charged, lithium ions are generated on the positive electrode of the battery, and the resulting lithium ions move through the electrolyte to the negative electrode. The carbon as a negative electrode has a layered structure. It has many micro-pores. Lithium ions that reach the negative electrode are embedded in the micro-pores of the carbon layer. The more lithium ions embedded, the higher the charging capacity. Similarly, when the battery is discharged(that is, when we use the battery), the lithium ions embedded in the negative carbon layer are removed and moved back to the positive pole. The more lithium ions that return to the positive pole, the higher the discharge capacity. What we usually refer to as battery capacity refers to discharge capacity. In the process of Li-ion's charging and discharging, lithium ions are in the state of motion from the positive pole → negative pole → positive pole. Li-ion Gates is like a rocking chair. The ends of the rocking chair are the poles of the battery, and lithium ions run back and forth in the rocking chair like athletes. So Li-ion Batters are also called rocking chair batteries.

1,Performances and general preparation methods of 1 cathode materials

The important parameter to characterize the transport properties of ions in positive poles is the chemical diffusion coefficient. In general, the diffusion coefficient of lithium ions in positive polar active substances is relatively low. Lithium is embedded in or removed from a positive material, accompanied by a change in the phase of the crystal. Therefore, the electrode membranes of lithium-ion batteries are very thin, generally in the order of several tens of micrometers. Lithium embedded compounds of cathode materials are temporary storage containers for lithium ions in lithium ion batteries. In order to obtain a higher battery voltage, lithium-embedded compounds with high potential are preferred. Positive materials should satisfy:

1) Electrochemical compatibility with electrolyte solution within the required charge and discharge potential range;

2) Moderate electrode process dynamics;

3) High reversibility;

4) Stability in air in 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 de-intercalation 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 have 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) Solid phase method generally uses lithium salts such as lithium carbonate and cobalt compounds or nickel compounds after grinding and mixing to perform sintering reactions[ 10] And ... The advantage of this method is that the process is simple and the raw materials are easy to obtain. It belongs to the method that has been widely studied and developed in the early development of lithium ion batteries. The foreign technology is more mature. The disadvantages are the limited capacitance of the positive material, the poor mixing uniformity of the raw materials, the poor performance stability of the prepared material, and the poor quality consistency between the batch and the batch.

2) The complex precursor containing lithium ions and cobalt or vanadium ions is first prepared by the complex method and then sintered. The advantage of this method is that the molecular scale is mixed, the material uniformity and performance stability are good, the electrical capacity of the cathode material is higher than that of the solid phase method, and the industrialization method used as a lithium ion battery has not been tested abroad. The technology is not mature, and there are few reports in China.

3) The Sol gel method developed in the 1970s

The method for the preparation of ultrafine particles and the preparation of cathode materials has the advantages of the complex method, and the electrical capacity of the prepared electrode materials has been greatly improved, which is a method that is rapidly developing at home and abroad. The disadvantage is that the cost is high and the technology is still in the development stage[ 11] .

4) LiMnO2 prepared by the ion exchange method Armstrong et al. by ion exchange method has obtained a high value of reversible discharge capacity of 270 mA · h/g. This method has become a new research hotspot. It has stable electrode performance and high capacitance. Features. However, the process involves the time consuming steps such as solution crystallization evaporation, and there is still a considerable distance from practical application.

The study of positive materials can be seen from foreign literature that its capacitance is increasing at a rate of 30 to 50 mA · h/g per year, and its development tends to be lithium embedded compounds with smaller and smaller microstructures and larger capacitance., The raw material scale advances toward the nanometer level, and the theoretical research on the structure of lithium embedded compounds has made some progress, but its development theory is still changing. The problem of increasing the capacity of lithium batteries and decreasing the cyclic capacity that troubles this field has been proposed by researchers to add other components to overcome the problem[ 12] [ 13] [ 14] [ 15] [ 16] [ 17] And ... However, at present, the theoretical mechanism of these methods has not been clearly studied, leading Japanese scholar Yoshio. Nishi to believe that there has been little substantial progress in this area over the past decade and it is urgent to study it further.

2,Properties and General Preparation Methods of 2-negative Polarized Materials

The conductivity of the negative electrode material is generally higher, and a lithium-embedded compound with a potential as close as possible to the lithium potential, such as various carbon materials and metal oxides, is selected. The negative electrode material requirements for reversible embedding and de-embedding lithium ions have:

1) Small change of free energy in the embedding reaction of lithium ions;

2) Lithium-ion has a high diffusion rate in the solid structure of the negative electrode;

3) Highly reversible embedding reaction;

4) Good conductivity;

5) Thermodynamically stable and does not react with electrolytes.

Research work mainly focuses 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 nano-tubes, and bucky ball C60 are being studied [18][19][20][21] [22] [23]. Japan Honda Research and Development Co. K. of Ltd. Sato et al. used the pyrolysis product PPP-700 of polyparaphenylene (PPP) (heating PPP to 700 ° C at a certain heating rate and heat-dissolving the product obtained for a certain period of time) as the negative electrode, and the reversible capacity was as high as possible. 680 mA·h/g. MJ Matthews of MIT, USA, reported that the PPP-700's lithium storage capacity (Storage capacity) can reach 1170 mA·h/g. If the lithium storage capacity is 1170 mA·h/g, and as the amount of lithium insertion increases, and thus the performance of the lithium ion battery is improved, the author believes that future research will focus on the smaller nano-scale 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 lag, that is, the embedding reaction of lithium is performed between 0 and 0.25 V(relative to Li + / Li) and the de-embedding reaction occurs around 1 V;

2) The recycling capacity gradually decreases. After 12 to 20 cycles, the capacity decreases to 400 to 500 mA · h/g.

The further deepening of the theory depends on the preparation of various high-purity, structured raw materials and carbon materials and the establishment of more effective structural characterization methods. Fuji Corporation of Japan has developed a new type of tin composite oxide base negative electrode material for lithium-ion batteries. In addition, the existing research mainly focuses on some metal oxides, and its quality is greatly improved compared with that of carbon negative electrode materials. Such as SnO2, WO2, MoO2, VO2, TiO2, LixFe2O3, Li4 Ti5O12, Li4 Mn5O12, etc.[ 24] , but not as mature as carbon electrodes. The reversible high storage mechanism of lithium in carbon materials mainly includes lithium molecule Li2 formation mechanism, multilayer lithium mechanism, lattice matrix mechanism, elastic global-elastic network model, laminar-side-surface lithium storage mechanism, nanometer graphite storage Lithium mechanism, carbon-lithium-hydrogen mechanism and micro-pores Lithium storage mechanism. Graphite, as one of the carbon materials, has long been found to form graphite-embedded compounds(Graphite International Compounds) LiC6 with lithium, but these theories are still in the development stage. The difficulty to overcome negative electrode materials is also a problem of capacity cyclic decay, but it is known from the literature that the preparation of high-purity and well-structured micro-structured carbon negative materials is a direction of development.

The general methods for preparing negative electrode materials can be summarized as follows.

1) Heating soft carbon at a certain high temperature to obtain highly graphed carbon; The molecular formula of lithium graphite Ionic compound is LiC6, in which the dynamic changes of lithium ion embedding and de-embedding process in graphite, the relationship between graphite structure and electrochemical properties, the reason for irreversible capacitance loss and the improvement method have been studied by many researchers. 2) The structure of hard carbon obtained from the decomposition of cross-linked resins with special structures at high temperatures has a higher reversible capacitance than graphite carbon, and its structure is greatly affected by raw materials. However, it is generally believed that the nano-pores in these carbon structures have a greater influence on the content of lithium inlays. The research mainly focuses on the use of polymer with special molecular structure to prepare hard carbon containing more nanometer micro-pores[ 25] [ 26] [ 27] .

3) Hydrocarbon prepared by high-temperature thermal decomposition of organic matter and polymers[ 28] [ 29] . This type of material has a reversible capacity of 600 to 900 mA · h/g and is therefore concerned, but its voltage lag and reduced cyclic capacity are the biggest obstacles to its application. The improvement of the 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 been noticed by researchers that the main research direction is to obtain metal oxides of new structures or composite structures.

5) As a lithium-incorporated material, carbon nano-tubes and bucky-ball C60 are also a new hot spot in current research, and become a branch of nano-material research. The special structure of carbon nano-tubes and bucky-ball 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

To sum up, the research and development of lithium-ion battery negative electrode active material has been very active in the world and has made great progress. The crystal structure of the material is regular, and the irreversible change of the structure during charging and discharging is the key to obtain lithium ion batteries with high capacity and long cycle life. However, the structure and properties of lithium-embedded materials are still the weakest link in this field. The study of lithium-ion batteries is a type of battery system that is constantly updated. 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., This will greatly increase the application range of lithium ion batteries. In short, the study of lithium-ion batteries is a cross-cutting field involving many disciplines such as chemistry, physics, materials, energy, and electronics. Progress in this area has attracted considerable interest from the chemical power industry and industry. It can be expected that with the deepening of the study of the relationship between the structure and performance of electrode materials, various normalizing structures or positive and negative materials with doped composite structures designed from the molecular level will vigorously promote the research and application of lithium ion batteries. Lithium-ion batteries will be the second type of batteries with the best market prospects and the fastest development in the long term after nickel-cadmium and nickel-metal hydride batteries.

There are different methods for classifying batteries. The classification methods can be roughly divided into three categories.

Type I: Divided by type of electrolyte, including: alkaline batteries, electrolytes are mainly batteries based on potassium hydroxide aqueous solutions, such as alkaline zinc manganese batteries(commonly known as alkaline manganese batteries or alkaline batteries), cadmium nickel batteries, hydrogen nickel batteries, etc.; Acid batteries, mainly aqueous sulfuric acid as a medium, such as lead-acid batteries; Neutral batteries, using salt solution as a medium, such as zinc manganese dry batteries(some consumers also call acid batteries), seawater activated batteries, etc.; Organic electrolyte batteries, mainly batteries with organic solution as a medium, such as lithium batteries, lithium ion batteries.

The second type: According to the nature of work and storage methods, it includes: primary batteries, also known as primary batteries, IE batteries that can not be recharged, such as zinc manganese dry batteries, lithium primary batteries, etc.; Secondary batteries, rechargeable batteries, such as hydrogen nickel batteries, lithium ion batteries, cadmium nickel batteries, etc.; Batteries are customarily referred to as lead-acid batteries and are also secondary batteries; Fuel cells, that is, active materials are continuously added from the outside to the battery when the battery is working, such as hydrogen and oxygen fuel cells; Storage batteries, that is, batteries are not directly exposed to electrolytes when they are stored, and electrolytes are not added until the battery is used, such as magnesium-silver chloride batteries, also known as seawater activated batteries.

The third category: According to the positive and positive materials used in the battery, it includes: zinc series batteries, such as zinc manganese batteries, zinc silver batteries, etc.; Nickel series batteries, such as cadmium nickel batteries, hydrogen nickel batteries, etc.; Lead series batteries, such as lead-acid batteries; Lithium series batteries, lithium magnesium batteries; Manganese dioxide series batteries, such as zinc manganese batteries, alkaline manganese batteries, etc.; Air(oxygen) series batteries, such as zinc empty batteries, etc.

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