DEVELOPMENT TRENDS OF LITHIUM ION POWER BATTERIES AND THEIR KEY MATERIALS

1. Introduction

The development of new energy vehicles is widely regarded as an important strategic measure to effectively meet the challenges of energy and environment. In addition, for China, the development of new energy vehicles is the only way for China to move from a "big automobile country" to a "strong automobile country". In recent years, the production and sales of new energy vehicles have shown blowout growth, and the global ownership has exceeded 130. Ten thousand vehicles have entered the stage of large-scale industrialization. China has also surpassed the United States as the world's largest producer and seller of new energy vehicles in 2015. Electric vehicles powered partly or wholly by power batteries are the main development direction of new energy vehicles because of their remarkable advantages of high efficiency, energy saving and off-site emission. Electric vehicles need to run farther, faster, safer and more convenient. Further improving specific energy and specific power, prolonging service life and shortening charging time, improving safety and reliability, and reducing cost are the themes and trends of power battery technology development.

Recently, the roadmap of energy-saving and new energy automobile technology published by China Automobile Engineering Society has drawn a blueprint for the development of power battery technology in China. The roadmap proposes that the specific energy of pure electric vehicle power battery will reach 350 Wh/kg by 2020, 400 Wh/kg by 2025 and 500 W/kg by 2030. While the existing system of lithium-ion power battery technology meets the needs of large-scale development of new energy vehicles, it focuses on the development of new lithium-ion power batteries, improves their safety, consistency and life, and carries out prospective research and development of new system of power batteries at the same time. In the medium and long term, it will continue to optimize and upgrade the new lithium-ion power. At the same time, we focus on the research and development of new system power batteries, which can significantly improve energy density, significantly reduce costs, and realize the practical and large-scale application of new system power batteries.

It can be seen that lithium-ion batteries will remain the mainstream products of power batteries for a long time to come. Lithium-ion batteries have the advantages of high specific energy, long cycle life, friendly environment, good energy density and power density. They are the best power batteries with comprehensive performance and have been widely used in various electric vehicles.

This paper briefly introduces the industrial and technological development of lithium-ion power batteries, and reviews the development trend of key materials for lithium-ion power batteries from the aspects of positive and negative materials, diaphragms and electrolytes. The selection and matching technology of positive and negative materials for lithium-ion power batteries are also discussed in this paper. Key technologies such as battery safety and battery manufacturing technology are briefly analyzed, and basic scientific issues that should be paid attention to in the research of lithium-ion power battery are put forward.

2. Technical Development of Lithium Ion Power Battery Industry

From the perspective of industry development, the world-renowned manufacturers of electric vehicle power batteries, including Panasonic, AESC, LG Chemistry and Samsung SDI, are actively promoting the research and development of high specific energy lithium ion batteries. In a word, the technical route of Japanese lithium battery industry is lithium manganate. (LMO) to lithium nickel cobalt manganate ternary (NCM) materials. For example, Matsushita's power battery technology route used lithium manganate in the early stage, and now developed lithium nickel cobalt manganate ternary and lithium nickel cobalt aluminate (NCA) as cathode materials. Its power batteries are mainly mounted on Tesla and other vehicles. Korean enterprises are based on lithium manganate materials, such as LG chemical early mining. In recent years, Samsung's SDI and LG chemistry has turned to lithium nickel cobalt manganate ternary materials in the application of lithium manganate as cathode material in Chevrolet Volt models.

At present, the mainstream power lithium battery manufacturers in China, such as BYD, are still dominated by lithium iron phosphate. While lithium iron phosphate batteries have been widely used, their energy density has increased from 90Wh/kg in 2007 to 140 W/kg at present. However, due to the limited space for improving the energy density of lithium iron phosphate batteries, with the increasing of power battery energy. With the increase of density requirement, there is an obvious trend for domestic power battery manufacturers to convert their technical routes to nickel-cobalt-manganese ternary, nickel-cobalt-aluminium or their mixtures.

3. Development Trend of Key Materials for Lithium Ion Power Batteries

Lithium-ion batteries use lithium compounds containing lithium ions as cathode, materials containing lithium ions embedded and removed reversibly at low potential as cathode, electronic insulating layer conducting lithium ions as separator, and lithium salts dissolved in organic solvents as electrolyte. Positive materials, negative materials, separators and electrolysis Liquid is the four key materials for lithium ion batteries.

3.1 Cathode material

Lithium manganate (LMO) has the advantages of low cost of raw materials, simple synthesis process, good thermal stability, excellent rate performance and low temperature performance. However, due to the Jahn-Teller effect and the formation of passivation layer, the dissolution of Mn and the decomposition of electrolyte at high potential, its high temperature cycling and storage performance is poor. Chemical electrolyte, control of material specific surface and surface modification to improve the high temperature and storage properties of LMO materials are common and effective modification methods in current research.

Lithium iron phosphate (LFP) cathode material has good thermal stability and cycling performance, which is due to the stable role of phosphate polyanions in the structure of the whole material framework. At the same time, lithium iron phosphate material is relatively low cost and environmentally friendly, which makes LFP become the mainstream material in the power battery of electric vehicles. Because lithium ions migrate through one-dimensional channels in olivine structure, LFP materials have some shortcomings, such as poor conductivity and low diffusion coefficient of lithium ions.

From the point of view of material preparation, the synthesis of LFP involves complex multiphase reactions, so it is difficult to ensure the consistency of the reactions, which is determined by the fundamental thermodynamic reasons of the chemical reactions. The improvement of lithium iron phosphate mainly focuses on three aspects: surface coating, ion doping and nano-materials. Automation of production process is the basic solution to improve the batch stability of LFP. However, due to the low voltage platform of lithium iron phosphate material (about 3.4V), the energy density of lithium iron phosphate battery is low, which limits its application in the field of long-term small passenger cars.

The advantages of nickel-cobalt-manganese ternary (NCM) or multicomponent materials lie in moderate cost, high specific capacity, adjustable nickel-cobalt-manganese ratio in a certain range, and different properties. At present, the power lithium cathode materials used abroad are mainly concentrated in the nickel-cobalt-manganese ternary or multicomponent materials, but there are still some urgent problems to be solved. Problems include low electronic conductivity, poor stability at large rate, poor cyclic characterization at high voltage, cationic mixing (especially nickel-rich ternary), poor high and low temperature performance and poor safety performance. In addition, due to the poor safety performance of ternary cathode materials, the adoption of appropriate safety mechanisms, such as ceramic diaphragm materials, has become a consensus in the industry.

Considering the safety issues, there is limited space for improving the energy density of power lithium-ion batteries by improving the process (such as reducing the weight of the electrode shell). In order to further improve the energy density of power lithium-ion batteries, the development of high-voltage and high-capacity cathode materials has become the main way to greatly increase the specific energy of power lithium-ion batteries.

3.1.1 High Voltage Cathode Material

The development of cathode materials that can output higher voltage is one of the important ways to improve the energy density of materials. In addition, another remarkable advantage of high voltage is that when the batteries are assembled into groups, the rated output voltage can be achieved by using fewer single batteries in series, which can simplify the control unit of the batteries. Voltage cathode material is spinel transition metal doped LiM x Mn 2?X O 4 (M=Co, Cr, Ni, Fe, Cu, etc.). The most typical material is LiNi 0.5Mn 1.5O 4. Although its specific capacity is only 146 mAh/g, the energy density can reach 686 W h/kg due to the working voltage of 4.7V. Spinel lithium nickel manganese oxide (LNMO) materials with spherical shape were synthesized by impregnation method from nano-polyhedral aggregates. The structure is very beneficial to the immersion of electrolyte and the insertion and removal of lithium ions. It can adapt to the volume change of materials during charging and discharging process and reduce the tension between materials particles. The electrochemical performance of micro Mn 3+ LMMO is better. After 80 cycles of charging and discharging, the discharge specific capacity can be maintained at 107mAh/g and the capacity retention rate is close to 100%. The specific capacity attenuation of LiNi 0.5Mn 1.5O 4 restricts its commercialization process. The reasons are mostly related to active materials and the interaction between collector and electrolyte. Because of the electrolyte, the specific capacity attenuation of LiNi 0.5Mn 1.5O 4 restricts its commercialization process. Instability at high potential, such as oxidation and decomposition of traditional carbonate electrolytes above 4.5V voltage, makes lithium-ion batteries swell and cycle performance worse under high voltage charging and discharging.

Therefore, high voltage cathode materials need to solve the problem of electrolyte matching. The methods to solve the above problems include the following three aspects. (1) Surface coating and doping of materials. For example, recently LiNi 0.5 Mn1.2 Ti 0.3 O 4 materials were obtained by surface 4-valent Ti substitution by Kim et al. Transmission electron microscopy (TEM) showed that a solid passivation layer was formed on the surface of materials. Therefore, the interface side effects are reduced. Full cell experiments at 30℃ show that the capacity retention rate increases by about 75% after 200 cycles at 4.85V cut-off voltage. However, a single surface coating/doping does not seem to provide long-term cyclic stability (e.g., more than 500 cycles). Combination with other strategies must be considered in application. (2) Use Electrolyte additives or other new electrolyte combinations.

The Yamada team achieved a 90% capacity of LiNi 0.5 Mn 1.5 O 4/graphite battery by using a simple LiFSA/DMC (1:1.1, molar ratio) electrolyte system after 100 cycles at 40℃, although the ionic conductivity of the highly concentrated system decreased by an order of magnitude (about 1.1 mS/cm at 30℃), it still maintained and enabled. It has been proved that the cycle life of LiNi 0.5 Mn 1.5 O 4 can be greatly improved by using electrochemically active Li4+x Ti 5 O 12 membranes and composite membranes of lithium Nafion membranes and commercial PP membranes.

In addition, some new spinel high voltage materials derived from LiNi 0.5Mn 1.5O 4, such as LiTiMnO 4, LiCoMnO 4 and olivine phosphate/fluorophosphate, have been extensively studied, such as LiCoPO 4, LiNiPO 4 and LiVPO 4F.

3.1.2 High capacity cathode material

Because the specific capacity of cathode material is much higher than that of cathode material, the effect of cathode material on the energy density of lithium-ion battery is greater. Simple calculation shows that at the present level, if the specific capacity of cathode material is doubled, the energy density of battery can be increased by 57%. However, the specific capacity of cathode material is even higher than that of cathode material. The energy density of the battery can only be increased by 47% when it is 10 times as high as the current one.

Among nickel-cobalt-manganese ternary materials, Ni is the main active element. Generally speaking, the higher the content of active metal, the larger the material capacity. Low-nickel multicomponent materials such as NCM111 and NCM523 have lower energy density. The energy density of power battery can reach 120-180 Wh/kg, which can not meet the higher energy density requirement. One of the development directions of quantitative cathode materials is to develop ternary or multicomponent systems with high nickel content.

In the high nickel multicomponent system, the energy density of the multicomponent materials (NCA or NCM811) with more than 80% nickel content has obvious advantages. The energy density of the batteries made of these materials can reach more than 300 Wh/kg after matching the suitable high capacity negative electrodes and electrolytes. However, the poor cycle stability, thermal stability and storage performance of the high nickel multicomponent materials are very great. It is generally believed that when the content of nickel is too high, it will cause Ni2+ to occupy the Li+ position, resulting in cationic mixing, hindering the embedding and removal of Li+ and resulting in capacity reduction. In addition, the material surface is prone to side reactions with air and electrolyte, the material structure stability is poor and the surface catalytic activity is poor at high temperature. Larger size is also considered to be an important cause of capacity degradation.

There are three ways to solve the above problems.

(1) Effective surface coating or bulk doping of materials. For example, recently Chae et al. coated NCM811 with N, N-dimethyl pyrrole sulfonate by wet chemical method, effectively blocked the interface between materials and electrolyte, inhibited the catalytic decomposition of electrolyte on the surface of high nickel ternary materials, and the average Coulomb efficiency of the first 50 cycles at 1C rate. The rate was 99.8% and the capacity retention rate was 97.1%.

(2) Developing a high-nickel ternary system with concentration gradient. Sun's research team prepared a dual-slope concentration gradient ternary material by coprecipitation method. The material has a higher content of nickel in its interior, which is conducive to the acquisition and maintenance of high capacity, and a higher content of manganese in its outer layer, which is conducive to cycle stability and thermal stability. Through Al doping, the capacity retention of LiNi 0.61 Co 0.12 Mn 0.27 O 2 with concentration gradient increased from 65% to 84% after 3000 cycles.

(3) Development of electrolyte additives or new electrolyte systems suitable for high capacity cathode materials.

At present, the mass production technology of high-nickel multi-material is mainly in the hands of a few Japanese and Korean enterprises, such as Sumitomo, Honda in Japan, Samsung SDI, LG, GS in Korea, etc. According to different application fields, the nickel content of the material is 78-90 mol, and the capacity of the material is concentrated in 190-210 mA h/g. Companies are trying to apply it in the field of electric vehicles, especially in the field of electric vehicles. Nickel-cobalt-aluminium (NCA) used by Tesla has attracted wide attention. It should be pointed out that there are many similarities between NCA and NCM811 in terms of capacity and production process. Panasonic 18650 battery cathode uses NCA cathode, and the energy density of the battery is about 250Wh/kg. However, NCA materials are difficult to grow due to the uneven distribution of aluminium elements. It is mainly used in the field of cylindrical batteries. Cylindrical batteries need high technology and cost in battery management system.

In addition, Li-rich cathode material zLi-2 MnO-3 (1?Z) LiMO-2 (0) with high specific capacity (200-300 mAh/g) based on Li-2 MnO-3

3.2 Negative electrode material

Lithium ion battery anode materials are classified into carbon materials and non-carbon materials. Carbon materials are classified into graphite and amorphous carbon, such as natural graphite, artificial graphite, mesophase carbon microspheres, soft carbon (such as coke) and some hard carbon. Other non-carbon anode materials are nitride, silicon-based materials, tin-based materials, titanium-based materials and alloy materials.

Anode materials will continue to develop in the direction of low cost, high specific energy and high safety. Graphite materials (including artificial graphite, natural graphite and mesophase carbon microspheres) are still the mainstream choice of lithium ion power battery at present. In the near to medium term, new large capacity anode materials such as silicon-based materials will gradually mature, with lithium titanate as the representative of high-capacity anode materials. Power density and high safety anode materials will be widely used in hybrid electric vehicles and other fields. In the medium and long term, silicon-based anode materials will replace other anode materials in an all-round way.

Silicon-based anode material is considered to be one of the best choices to improve the energy density of lithium batteries. Its theoretical specific capacity can reach more than 4000 mAh/g. After matching with high-capacity cathode material, the theoretical specific energy of single battery can reach 843 Wh/kg. However, there is a huge volume expansion and shrinkage effect in the charging and discharging process of silicon anode material. It will lead to powdering of the electrodes, decrease the first coulomb efficiency and cause capacity decay.

Researchers have tried many ways to solve this problem.

(1) Nanostructured materials have relatively small volume change, smaller ion diffusion paths and higher intercalation/delithium properties, including nano-silicon particles, nanowires/tubes, nano-films/sheets, etc.

(2) Introducing other metals or non-metals into silicon materials to form composite materials, which can buffer the volume change of silicon. Common composite materials include silicon-carbon composite materials, silicon-metal composite materials and so on. The silicon-carbon composites with egg yolk shell structure were obtained. The effect of the voids between the carbon shell and the silicon core on the stability and electrochemical properties of the materials was studied by in situ transmission electron microscopy. Because the structure of egg yolk shell reserved sufficient space between the silicon and the carbon layer, silicon did not destroy the outer layer when lithium was inserted and expanded. On this basis, through secondary granulation of carbon-coated nanoparticles, carbon film is coated on the surface of large particles. Finally, pomegranate-like structure is prepared by etching. The increase of the size of composite materials reduces the specific surface area of materials and improves the stability of materials. Qualitatively, the material's 1000-cycle capacity retention rate increased from 74% to 97%.

(3) Selecting binders with different flexibility and interfacial properties to improve the bonding effect; recently, Choi et al. obtained a two-component PR-PAA binder with special structure by crosslinking polyacrylic acid PAA with polyrotaxane ring component PR by forming ester bonds. The charging and discharging process stability of silicon negative electrode was greatly improved.

(4) Amorphous silicon materials, such as porous silicon materials, with relatively mild volume change are used. In terms of applications, Hitachi Maxell announced that it had successfully applied silicon-based anode materials to small batteries with high energy density; Japan GS Tangshao Company had introduced silicon-based anode materials lithium batteries and successfully applied them to Mitsubishi automobiles; Tesla claimed that it had added 10% silicon-based materials to artificial graphite and had been installed in its latest model 3. Silicon-carbon composite material is used as negative material of power battery.

3.3 Electrolyte

High safety and environmental adaptability are the basic requirements of lithium-ion power batteries for electrolyte. With the continuous improvement and updating of electrode materials, the requirements for matching electrolyte are becoming higher and higher. Due to the great difficulty in developing new electrolyte systems, carbonate organic solvents are compatible with conventional electrolyte of lithium hexafluorophosphate. The system will remain the mainstream choice for power batteries for quite a long time to come.

In this case, it is particularly important to optimize the solvent ratio and develop functional electrolyte additives for different power batteries and electrode materials with different characteristics. For example, the high and low temperature performance of power batteries can be improved by adjusting the solvent ratio content and adding special lithium salts; the over-charging additives and flame retardant additives can be added. Additives can greatly improve the safety of batteries under overcharging, short circuit, high temperature, needling and thermal shock conditions; by purifying solvents and adding positive film-forming additives, the charging and discharging requirements of high-voltage materials can be met to a certain extent; by adding SEI film-forming additives, the composition and structure of SEI films can be regulated. In recent years, with the first successful use of butadiene nitrile (SN) as an electrolyte additive by Kim et al. to improve the thermal stability of graphite/LiCoO_2 batteries, the nitrile additives represented by butadiene nitrile (SN) and adiponitrile (ADN) can be well inhibited because of their strong complexing force with metal atoms on the cathode surface. The advantages of oxidative decomposition of electrolyte and leaching of transition metals have become a kind of high voltage additives widely recognized by academia and industry. Another kind of high voltage additives, i.e. positive film-forming additives, represented by 1,3-propane sulfonate lactone (PS) and 1,3-propylene sulfonate lactone (PES), preferentially generate oxygen on the surface of positive electrode. A dense passivation film is formed on the surface of the cathode, which can prevent the contact between the electrolyte and the cathode active substance and inhibit the oxidation decomposition of the electrolyte at high voltage.

At present, the development of high and low temperature functional electrolyte is relatively mature, and the environmental adaptability of power batteries is basically solved. Further improvement of energy density and safety of batteries is the primary issue in the development of electrolyte. In the medium and long term, the development trend of electrolyte materials for lithium-ion power batteries will mainly focus on new solvents and new lithium salts. In terms of ionic liquids and additives, gel electrolytes and solid electrolytes will also be the direction of future development. Solid state batteries, which are one of the key features of solid state electrolytes, have potential excellent characteristics in terms of safety, life, energy density and system integration technology, and will also be the important direction of exhibition in future development of power batteries and energy storage batteries.

3.4 The diaphragm

At present, the main diaphragm materials used in commercial lithium-ion power batteries are microporous polyolefin films, such as polyethylene (PE) and polypropylene (PP) single or multi-layer composite membranes. Polyolefin diaphragm materials have the advantages of mature manufacturing process, high chemical stability and high processability for a period of time. Interior is still the mainstream of commercial diaphragm materials, especially the thermal closure temperature of PE is of great significance to restrain some side reactions and prevent thermal runaway in batteries. Further development of high performance modified diaphragm materials based on polyolefin (especially polyethylene) diaphragm (such as inorganic ceramic modified diaphragm, polymer modified diaphragm, etc.) Improving the safety and electrochemical properties of diaphragms will remain the focus of research and development of diaphragm materials.

Recently, the thermal stability of the diaphragm was improved to 160℃ by coating nano-Al_2O_3 on the monolayer surface of commercial PE diaphragm with high-temperature resistant polyimide as binder. On the basis of the SiO_2 ceramic diaphragm developed earlier, the group also coated the diaphragm with a high-temperature-resistant polymer by in-situ polymerization between its surface and pore size. Batamine protective layer, not only does the diaphragm not shrink but also maintains good mechanical properties after being treated at 230℃ for 30 minutes, which can effectively guarantee the safety of the battery. The polyetherimide diaphragm obtained by using heat-resistant polyetherimide resin as the base material, which is dissolved by heating with NMP, is re-cast into a film. With the application of lithium ion batteries in electric vehicles and other fields, it will be an important direction for the development of polyolefin diaphragms to establish effective control methods for diaphragm structure, diaphragm aperture size and distribution, and to introduce electrochemical active groups to make polyolefin diaphragms multifunctional. And industrialization will also be vigorously promoted.

In summary, the cathode materials will develop towards high voltage and high capacity; the negative materials will mainly develop silicon-carbon composites, which will make silicon-carbon composite cathode materials truly practical application through the development of new binders and SEI film control technology; the electrolyte will be in the near future. The development of high voltage electrolyte and high environmental adaptability electrolyte materials will be the main goal in the future, while solid electrolyte materials will be the development goal in the medium and long term. Composite and controllable separator materials with multiple materials will be the key development direction of lithium-ion power battery separators.

4. Key Technologies and Basic Scientific Problems of Lithium Ion Power Batteries

4.1 Key Technologies of Lithium Ion Power Batteries

Lithium-ion power battery is a complex system, the optimization of single component, material or component may not have a prominent effect on the improvement of the overall performance of the battery. To develop high specific energy, low cost, long life and high safety power batteries for electric vehicles, the key technologies of lithium-ion power battery system need to be focused on. Note: Solve the performance constraints in the final application process.

4.1.1 Selection and Matching Technology of Positive and Negative Materials

The basic performance of lithium-ion power battery, such as life, safety and cost, largely depends on the selection and matching of its electrode material system. Therefore, how to select a material system with high specific energy, long life, high safety and low cost is an important technology of lithium-ion power battery.

4.1.2 Power Battery Safety

Safety is a prerequisite for the application of power batteries in vehicles. With the gradual improvement of energy density of lithium-ion batteries, the safety problems of batteries will undoubtedly become more prominent. The fundamental cause of safety accidents of lithium-ion batteries is thermal runaway, heat release side reaction releases a large number of heat and organic small molecular gases, causing electricity. The sharp rise of temperature and pressure inside the cell, in turn, will accelerate the side reaction exponentially, and produce more heat, which will lead to uncontrollable thermal runaway state of the battery, and eventually lead to explosion or combustion of the battery. The high specific energy NCM and NCA ternary cathode, manganese-based solid melt cathode is more stable than LFP material. Poor performance makes people pay more attention to the safety problems while developing high energy density power batteries. Solving the safety problems of batteries requires at least two aspects: (1) preventing short circuit and overcharge to reduce the probability of thermal runaway of batteries; (2) developing highly sensitive thermal control technology to prevent thermal runaway of batteries.

4.1.3 Battery manufacturing process

With the application of power batteries deepening, single batteries are developing towards large-scale and easy-to-group. In this process, the manufacturing technology of single batteries is particularly important. Improving product consistency, so that the safety and life of batteries after grouping are higher and the manufacturing cost is lower will be the future lithium-ion battery manufacturer. The development direction of technology is: (1) developing efficient automation technology of production equipment, developing high-speed continuous slurry mixing, coating, roll slicing, winding/laminating and other technologies, which can reduce production costs; (2) developing automatic measurement and closed-loop control technology, improving the measurement technology level of battery production process, and realizing real-time dynamic quality detection in the whole process. To achieve closed-loop quality control in the process and on the whole line to ensure product consistency and reliability; (3) to establish automatic logistics technology development to realize automatic transfer of materials between processes and reduce manual intervention; (4) to develop intelligent production control technology, using information control, communication, multimedia and other technologies to develop effective production. Process automation control and manufacturing execution system can maximize production efficiency and reduce labor costs.

4.2 Basic Scientific Problems of Lithium Ion Power Batteries

4.2.1 Basic scientific issues such as electrode reaction process, reaction kinetics and interface control are studied.

At present, element doping and coating methods are widely used in material modification, but the reason is often "know it or not". For example, LFP can significantly improve electronic conductivity through hetero-valent lithium doping, but whether it is lattice doping or surface penetration is still controversial. In addition, it is generally believed that LFP has lower electronic conductivity. Electricity and ion diffusion characteristics are the main reasons for the poor rate characteristics, but studies have shown that the transport of lithium ions at the electrode/electrolyte interface is also an important factor affecting the rate characteristics of LFP. By improving the ion transport characteristics at the interface, better rate characteristics can be obtained. Therefore, in-depth study of surface electrochemical reactions on the electrodes can be carried out. The corresponding mechanism, especially the formation and properties of SEI film and the interaction between electrodes and electrolytes, can clarify the structure evolution mechanism and performance improvement strategies of materials, and provide theoretical guidance for improving the performance of materials and batteries.

4.2.2 Development of in-situ characterization of electrode surface interface

The performance of electrode materials for lithium-ion batteries depends mainly on their composition and structure. It is very important to study the composition-structure-performance structure-activity relationship of materials by in-situ characterization technology for understanding the reaction mechanism of electrode materials, optimizing the composition and structure of materials to improve their performance and guiding the development and application of high performance new materials. Significance. For example, in-situ Raman spectroscopy can detect the structural changes of materials in real time by lattice vibration (e.g. metal-oxygen coordination structure), which can help to find out the causes of material structural deterioration. Synchrotron radiation technology can not only obtain the oxidized states and localities of the constituent elements in electrode materials by studying the chemical environment around atoms in electrode materials. The information of domain structure and near-neighbor coordination atoms can also be obtained in situ, such as the evolution of electrode material structure, the oxidation state of transition metal ions and the change of local structure during battery charging and discharging, which can reveal accurately the reaction mechanism of the battery. Solid state nuclear magnetic resonance spectroscopy (NMR) can provide the local structure information of solid state materials and obtain dynamics information of the ion diffusion phase.

5. Conclusion

Lithium-ion power batteries are the most practical power batteries at present. In recent years, the rapid development of industrialization has strongly supported the development of electric vehicle industry. However, there are still many application problems to be solved, especially the endurance, safety, environmental adaptability and cost of lithium-ion power batteries. It can be expected that related technologies will make great progress and achieve large-scale application in recent years. With the rapid development of electric vehicles, lithium-ion power batteries will usher in a golden period of explosive growth.