Introduction
There are many factors affecting the high-power charging and amplifier performance of lithium-ion batteries. This paper starts with the selection of the material system of high-power lithium-ion batteries; the factors affecting the performance of high-power batteries: the structure of the positive material, the particle size of the positive material and the thickness of the electrode film; the conductivity of the electrode; and the factors brought about by the negative electrode. Material structure, material size, electrode surface resistance, electrode conductivity; electrolyte influencing factors: thermal stability and chemical stability and other factors that affect the performance of high-power batteries; battery design includes the design of pole surface density, formula optimization and special assembly process of batteries. Summary. Keywords: high-power lithium-ion batteries; influencing factors; positive electrode; negative electrode.
With the rapid development of social economy and science and technology, energy use has become an indispensable and key part of the human development process. Over time, some non-renewable energy sources are not only exploited, but also create environmental problems that cannot be ignored in the process of consumption. This Article starts from the material system of high-power lithium-ion batteries; the positive electrode affecting the performance of high-power batteries: the structure of the positive material, the particle size of the positive material and the thickness of the electrode film; the conductivity of the electrode; the factors brought about by the negative electrode: material structure, material size, electrode surface resistance, electrode conductivity; electrolysis Factors influencing quality: Factors affecting the performance of high-power batteries in several aspects, such as thermal stability and chemical stability; the design of batteries, including the design of polar one-sided density, the optimization of battery formulas and the special assembly process of batteries, expounds the effects on high-power batteries, and deeply analyzes the causes of these effects.
1. Factors affecting high-power batteries:
The high power performance of lithium-ion batteries is affected by the selection of material systems, battery structure design, battery process design, electrode selection and temperature.
1.1 Material system of lithium-ion batteries
(1) Positive electrode: Small particle lithium cobalt ate (LiCo02), nanometer lithium iron phosphate (LiFePO4), lithium nickel cobalt manganese acid (LiNiCoMnO2), lithium vanadium phosphate (Li3V2 (PO4)3) (the positive electrode is generally small particles, large BET substances, with high reaction activity) Adhesive (PVDF), conductive agent (CNT), solvent (NMP), collector-substrate (aluminum foil-Al).
(2) Negative electrode: The negative electrode is coated with hard carbon-coated graphite or lithium titanate or silica anode. Graphite system: hard carbon-coated graphite with high magnification charging characteristics. Lithium titanate system: three-dimensional structure, which facilitates the embedding and elapse of lithium ions. Adhesive (SBR), thickener (CMC), conductive agent (SP), solvent (water), collector-substrate (copper foil-Cu).
(3) The conductive agent adopts conductive carbon tubes or carbon tubes + graphene conductive agents.
(4) Electrolyte: 1 In terms of electrolyte solvent, in order to obtain a high conductivity, more chain esters, even EA carboxylates, are selected. The general antioxidant ability is poor, and secondary reactions are prone to occur at high temperatures. 2 In terms of electrolyte additives, in order to reduce SEI impedance, there are generally fewer additives or thinner films, so the positive and negative electrodes and electrolyte reactions are relatively violent at high temperatures. 3 In terms of electrolyte salts, consider using lithium salts with higher molar concentrations.
(5) Isolation membrane: High porosity, high breathability and good thermal shrinkage temperature.
(6) Polar ear: The cross-sectional area of the polar ear is enough to withstand the impact of high current.
(7) Aluminum plastic film or square aluminum shell (outside packaging).
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1.2 Design of high-power cells:
The performance of high-power batteries is inseparable from the comprehensive design of cells. There are many factors to be considered for high-power cells. When selecting positive and negative electrode materials, materials with high diffusion coefficient should be selected to shorten the path of ion transmission. In terms of process design, the pole thinning, that is, reducing the density of coating surface, and widening the polar ear area or more should be considered. The method of increasing the polar ear increases the overflow area and other technical means to meet the requirements of high-power output of the battery.
The specific design rules are as follows:
(1) Positive and negative electrode materials will choose materials with high diffusion coefficient and large surface area;
(2) The whole conductive network will choose multiple conductors or conductive agents with excellent conductivity to improve electrical conductivity;
(3) The design of the collector coating is relatively thin;
(4) Generally, the laminated structure is used to increase the collector capacity of the cell;
(5) Use carbon-coated aluminum foil to reduce contact internal resistance and improve electrical conductivity;
(6) Use wide cross-sectional polar ears and collectors to undertake high currents.
1.3 Model process design of high-power batteries
In addition to the selection of key materials, the model process design considered from the overall basis of the battery, such as the determination of the positive and negative pole capacity N/P (Negative/Positive) ratio, the thickness selection of the collector substrate and the treatment of the end of the plate coating are of great significance to battery safety.
(1) N/P ratio
The well-designed N/P ratio is usually controlled between 1.2 and 1.25 in order to improve the high power performance of the battery.
(2) Thickness selection of collector substrate
The collector selects copper foil (15um) of moderate thickness to increase heat dissipation, while thin aluminum foil (12um) is selected to minimize the short-circuit impact of aluminum foil.
(3) Pole plate coating end treatment
Lamination tape is attached to the beginning and end of the cathode plate to avoid direct contact between aluminum foil and negative active substances and prevent short circuit.
1.4 Effect of electrode on high-power charging and discharging performance:
1.4.1. Factors influencing the positive electrode on the high-power charging and discharging performance of the battery:
(1) Influence of the thickness of the electrode of the cathode material: the thinner the battery electrode, the better its circulation performance under the condition of high current discharge; the thicker the battery electrode, the worse the cycle will become in the case of high current discharge.
(2) Influence of the structure of cathode materials: An important factor affecting the high-power charging and discharging performance of lithium-ion batteries is the diffusion of lithium ions inside the cathode material. Another important factor affecting the high magnification performance is the diffusion rate of lithium ions on the surface of the material. All factors affecting lithium ion diffusion, such as the size and structure of cathode materials, have a great impact on the high magnification performance, and the surface area and the electrode thickness, porosity and conductivity of the electrode also have a great impact on the high magnification performance of lithium-ion batteries.
(3) Conductivity of electrodes: Positive materials such as LiCoO2, LiMnxFe1-xPO4, Li3V2 (PO4)3, LiMn2O4, Life-PO4, etc. have low conductivity. When making battery electrodes, conductive agents (CNT, acetylene black) should be added. OR SUPER-P) TO IMPROVE ITS CONDUCTIVITY. When charging and discharging at a high magnification rate, the influence of conductive agents on the properties of cathode materials is particularly obvious, and the addition of conductive agents on the performance of high magnification also plays a great role.
1.4.2 Influence of negative electrode on high-power charging and discharging performance:
(1) Influence of the size of the anode material: The length of the lithium-ion diffusion path depends on the size of the anode material of the lithium-ion battery, which has a great impact on the high-power performance of the battery. When the size of the electrode material is large, it is generally smaller than the surface area; when the size of the electrode material is large, it is generally smaller than the surface area. Reducing the current density of the battery electrode can reduce the polarization of the electrode. In order to improve the high power performance of the battery, it can also be achieved by shortening the migration path and reducing diffusion impedance. Materials with smaller particle size and nanostructure materials, such as nanowires, Nano spheres, nanotubes and Nano rods, will have good high power performance.
(2) Influence of negative material structure : The spherical sheet structure of graphite intermediate as phase asphalt carbon microspheres is conducive to the embedding and effuse of lithium ions from all directions of the ball, reducing the diffusion resistance of lithium ions in the solid phase, thus improving the high-power performance of the battery. Carbon fiber with a radial structure is also conducive to lithium ion diffusion. The negative material.
(3) Pole conductivity: Lithium ions are embedded in the negative electrode and electrons are transferred at the same time. In this process, the conductivity of the electrode has a great impact on the electrochemical performance of the electrode. When investigating the influence of different electrode density on the high-power charging and discharging capacity in the experiment, with the increase of electrode density, the capacity of the battery increases first and then decreases. With the increase of electrode density, the surface area and porosity of the electrode will decrease, which is especially not conducive to the diffusion of lithium ions, thus increasing the polarization internal resistance and the ohm internal resistance decrease. The conductivity will increase. When MCMB is used as an anode material in the experiment, under the charging and discharge system of 0.2C charging and 3C magnification discharge, the influence of the conductor on the high power of the electrode was observed. It was found that the electrode with the electric conductor could discharge a capacity of
160mAh/g, while the electrode without a conductor only discharged 105mAh/g. . . After electroless plating Cu on the surface of artificial graphite, its Coulomb efficiency, reversible capacity and high current performance have been greatly improved. It can be seen that the conductivity plays a great role in its high power performance.
(4) Influence of pole surface resistance: Pole surface resistance is a threshold in the diffusion process of lithium ions, which affects the embedding and effuse of lithium ions, and is more prominent when charging and discharging high power.
The factors affecting the performance of high-power charging and discharging of negative materials can be summarized as follows:
(1) Observe the length of the diffusion path of lithium ions in the material or electrode from the influence of the size of the material and the thickness of the electrode on the high magnification performance, so the reason for the high magnification performance of the electrode is that the concentration difference between lithium ions in the electrode is the internal resistance of the concentration difference polarization.
(2) From the perspective of the influence of pole conductivity and pole surface resistance on the negative high magnification performance, the size of ohm internal resistance is another aspect that affects the performance of negative high magnification.
The size of these two internal resistance is the main reason affecting the high power performance of the negative electrode. The size of the internal resistance (polarization internal resistance + ohm internal resistance) directly affects the degree of polarization when the negative electrode is charged and discharged at high. There is still a certain relationship between polarization internal resistance and ohm internal resistance. The size of concentration difference polarization internal resistance is the root cause affecting the negative high magnification charging and discharge performance, while the increase of ohm internal resistance is the direct cause of the poor charging and discharging performance of negative high magnification.
1.5 Factors influencing electrolyte:
The research of electrolyte is the key to improving the high-power performance of lithium-ion batteries. Compared with electrode materials, the composition and properties of electrolytes are easier to operate and modify in practical applications. Therefore, this research is a breakthrough to improve the high-power performance of lithium-ion batteries. For the reasons for the poor multiplier performance of lithium-ion batteries, it is mainly for low ion conductivity, high charge transfer impedance and SEI membrane impedance, the research on lithium salts in lithium-ion battery electrolytes focuses on the selection of a new system of lithium salt with low charge transfer impedance. In the future, the research of ionic liquids will focus more attention to its preparation process to reduce costs. Some electrolytes will be oxidized and decomposed on the positive surface. Because lithium-ion batteries are more likely to overcharge and overcharge when charged and discharge at a high magnification rate, choosing electrolytes with high chemical stability in a wider electrochemical window has become a basic requirement for high-power lithium-ion batteries.
The high-power electrolyte of lithium-ion batteries mainly studies two aspects: first, the charge migration impedance of the electrode boundary mask (SEI) under high magnification charging increases, which increases the polarization of the charging process; second, under the condition of high magnification charging, lithium-ion batteries are prone to lithium analysis in the later stage of constant current charging, leading to SE The I-membrane has deteriorated, and the battery performance has deteriorated. Therefore, first, by optimizing lithium salt, adding lithium salt conducive to high magnification and discharging can improve the high power performance of the battery to a certain extent. Second, by adding high-power performance additives, by adding film-forming additives with better than EC, the charge transfer impedance at the interface under high magnification charging and discharging, or adding lithium salt deposition. Improve the agent to prevent lithium-supported crystal growth during high-power charging and improve the high-power performance of the battery.
1.6 Effects of diaphragm
The diaphragm mainly is to isolate the positive and negative electrodes to prevent short circuit and self-discharge of the two electrodes, and to provide a good ion channel between the two electrodes. Diaphragms mainly include PP-PE-PP multi-layer diaphragms, polymer ceramic coated diaphragms (MFS) and non-woven diaphragms. High-power batteries usually choose a diaphragm with high porosity and large porosity to increase the diffusion rate of lithium ions and reduce the polarization of batteries.
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1.7 Effect of temperature on high-power batteries
The high-power batteries on the market are analyzed now. The test shows that the power of all batteries can be significantly affected by temperature, while the power performance of LTO batteries is significantly affected by SOC, and the power performance of LFP batteries is less affected by the SOC state. Three batteries are analyzed. Among them, the positive pole of battery A is NMC, the negative pole is LTO, the positive pole of battery B is NMC, the negative pole is graphite, the battery C is positive pole LFP, and the negative pole is graphite. These three batteries are optimized for magnification performance, with high peak current and a wide operating temperature range. The influence of pulse current on battery charging and discharging at different temperatures, and testing the impact of different temperatures and charging and discharging current on the discharge capacity of the battery. The capacity of lithium titanate batteries is significantly affected by temperature, but lithium titanate batteries can also withstand a charging magnification of 15C at -25°C, so it also makes lithium titanate batteries. Become the only battery that can charge and discharge at high current at all temperatures.
Battery B has poor low temperature performance and cannot be charged or discharged at -25°C. For lithium iron phosphate batteries, the discharge magnification has little significant impact on the discharge capacity at more than 25°C, but below 25°C, the capacity of the battery will be significantly affected by the magnification.
Summary:
In summary, the factors restricting the charging and discharge performance of high magnification are mainly the influence of material system selection, battery structure design, battery process design, electrode selection and temperature.
The positive factors affecting the performance of high magnification batteries include the structure of the material, the particle size of the material and the thickness of the electrode film; the conductivity of the electrode; the factors brought by the negative electrode are also the material structure, material size, electrode surface resistance, and electrode conductivity, because they directly or indirectly affect the resistance of the electrode. The degree of charging and discharging electrodes; the factors affecting electrolytes are thermal stability and transmission ability, because they affect the lithium inlaying degree, cycle and safety performance of lithium-ion batteries; battery design includes the design of pole surface density, the optimization of formulas and the special assembly process of the battery.
Therefore, the key to improving the high magnification performance of lithium-ion batteries in the future is, on the one hand, to develop new electrode materials and electrolytes with high conductivity and stability conducive to the rapid diffusion of lithium ions, and on the other hand, to further optimize the process design scheme and structural design scheme of high-power batteries.
In order to implement the relevant deployment arrangements for national scientific and technological innovation during the 14th Five-Year Plan, the national research and development plan has launched the implementation of the key special project of “new energy vehicles”. According to the deployment of this key special implementation plan, the project declaration guide for 2022 is hereby issued.
The overall goals of this key special project are: to adhere to the pure electricity-driven development strategy, consolidate the basic research and development capabilities of the industry, solve the key technical problems of the new energy vehicle industry, break through the core bottleneck technology of the industrial chain, and achieve independent controllability of key links, form a number of internationally forward-looking and leading scientific and technological achievements, and consolidate me. China’s new energy vehicles have a leading advantage in scale, and gradually established technological advantages. The special implementation cycle is 5 years.
The 2022 guide deployment adheres to the principles of problem orientation, step-by-step implementation and focus.
Focusing on the six technical directions of energy power, electric drive system, intelligent driving, vehicle network integration, support technology, vehicle platform, and in accordance with basic research and common key technologies, 14 guidance tasks are planned to be deployed. The national allocation is 508 million yuan. Among them, focusing on the direction of power battery technology in the new system, two projects of young scientists are proposed to be deployed, and the state allocation will not exceed 8 million yuan, and each project will not exceed 4 million yuan. Focusing on the key technologies of self-evolutional learning self-driving system, real-time protection of expected functions of intelligent vehicles and test verification technology, it is proposed to deploy two young scientists, each of which does not exceed 3 million yuan. In principle, basic research projects and young scientist’s projects do not require supporting funds, and common key technology projects require that the ratio of supporting funds to state-funded funds should not be less than 2:1.
Projects are uniformly declared in accordance with the research direction of the second-level title of the guide (such as 1.1).
Except for special instructions, the number of support to be carried out from 1 to 2 per project, and the implementation cycle shall not exceed 3 years. The research content of the declared project must cover all the research contents and assessment indicators listed in the guide under the second-level heading. The number of topics under basic research projects shall not exceed 4, the total number of project participants shall not exceed 6, the number of common key technical projects shall not exceed 5, and the total number of project participants shall not exceed 10. The project has 1 person in charge, and each project has 1 person in charge.
The Young Scientist Project no longer has a project, and the total number of project participants does not exceed 3. The Young Scientist Project has a project leader. The age of the project leader of the Young Scientist Project requires that men should be born after January 1, 1984 and women should be born after January 1, 1982. In principle, the age requirements of other participants on the team are the same as above. The age requirements of young scientists and participants are in line with the young scientist project.
In the guide, “the number of proposed support is 1 to 2” means that in the same research direction, when the first two evaluation results of the declared project are similar and the technical route are obviously different, these two projects can be supported at the same time. Two projects will be supported in two stages. After the completion of the first phase, the implementation of the two projects will be evaluated, and the follow-up support modalities will be determined based on the results of the evaluation.
Energy and electricity
1.1 New system power battery technology (basic research, including young scientist’s project)
Research content: Develop the key materials and technologies of the next generation of lithium-ion batteries, including the design and low-cost preparation of new high-capacity lithium storage electrode materials, charge compensation, coupling mechanism and dynamic lifting technology of electrode reactions, structural evolution and stabilization strategies of materials and electrodes, non-flammable electrolytes, Design and application technology of high-temperature and high-voltage diaphragm, design and preparation method of high-face capacity electrode; carry out forward-looking research on batteries in new systems, including new principles and mechanisms of battery reaction, new electrode materials and battery structures, electrode reaction dynamics regulation mechanism and improvement strategies, battery performance decline mechanism and stability.
Strategy
Assessment indicators: lithium storage positive polar ratio capacity >350 mAh/g; lithium storage negative polar ratio capacity >1200 mAh/g; new material system lithium-ion battery capacity >2 ampere, specific energy ≥ 500 watt-hour/kg, cycle life ≥ 600 times; new system battery specific energy ≥600 Watt-hours/kg, cycle life ≥200 times.
Related instructions: 1. Support two conventional projects with different technical routes. In addition, parallel support for two young scientists whose technical routes are different and different from those of two conventional projects; 2. The implementation cycle of all projects shall not exceed 4 years; 3. The research content of the young scientist’s project is the same as that of the conventional project, but the assessment indicators are slightly different. The specific assessment indicators are as follows: the specific energy of lithium-ion batteries in the new material system is ≥ 500 watt-hours/kg, and the cycle life is ≥ 100 times; the specific energy of the battery of the new system is ≥ 600 watt-hours/kg, and the cycle life is ≥ 100 times.
1.2 Solid-liquid hybrid high specific energy lithium-ion battery technology (common key technology)
Research content: study high-performance hybrid electrolyte system and high-capacity electrode materials, new principles and new technologies of positive and negative electrode efficiency regulation; develop model-based pole/battery design technology, new pole/battery manufacturing process and new equipment, and study built-in sensor integration technology and new methods for high-precision state estimation. ; develop new technologies for in-situ/real-time characterization, study failure mechanisms and performance improvement strategies, thermal runaway mechanisms and prevention mechanisms, establish a safety risk assessment system; carry out supporting applications and assessment and verification.
Assessment indicators: Composite semi-solid electrolyte film thickness <15 microns, room temperature ion conductivity > 5 millimeter Siemens/cm; battery monomer specific energy ≥ 400 watt-hours/kg, cycle life ≥ 1,500 times, through acupuncture and 150oC hot box test, other safety meets national standards. Please, the loading application should not be less than 500 vehicles.
1.3 Cobalt-free power batteries and ladder application technologies (common key technologies)
Research content: design and preparation of cobalt-free low-cost materials, high-strength diaphragm and functional electrolyte development; construction of ion transmission model of porous electrode structure and table interface; design and manufacture of new structural power batteries and systems suitable for gradient utilization; study of dynamic and rapid power batteries in multi-scenes of complex working conditions Non-destructive testing technology and the evolution law of battery electrical and safety performance, establish a battery life cycle performance evaluation method and a residual evaluation index system for retired batteries; study the health thresholds and safety thresholds of the cascade utilization of power batteries, and establish technical specifications for the application of retired batteries. Assessment indicators: new structural power batteries and systems to meet the needs of gradient utilization. The monomer battery specific energy is ≥240Wh/kg, and the energy density is ≥ 500Wh/L; it meets the requirements of the whole vehicle for 10 years/500,000 kilometers (passenger vehicles) or 8 years/800,000 kilometers (commercial vehicles), the safety meets the requirements of national standards, and the loading application is not less than 1000. Vehicles; formulate a power battery health evaluation system and retired battery residual value evaluation system, and the error of fast evaluation of decommissioned battery capacity is ≤3%; the ladder utilization scenarios of retired batteries are ≥ 3, and 1 technical standard for the gradient application of retired batteries is formulated.
Conclusion
With the charging and discharging process of lithium-ion batteries, the internal resistance of the electrode is constantly changing, and the internal resistance increases more during discharge. Because some fragments from the SEI film on the negative surface fall off during the cycle process enter the electrolyte and electrophoresis occurs under the action of voltage, the internal resistance is increased and discharged at high current. When the debris is deposited on the surface of the electrode, the resistance increases, which in turn affects the elapse of lithium ions. When the lithium-ion battery is charged and discharged at a high magnification rate, the internal resistance of the battery increases a lot in the process of charging and discharging. The increase in internal resistance mainly comes from the negative electrode, and the increase in negative resistance is caused by the thickening of the SEI film. At the same time, the process of thickening of the negative electrode at high magnification is also simulated to verify this conclusion.
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