Lithium-ion batteries are a complex system comprising a positive electrode, a negative electrode, a separator, an electrolyte, a current collector and a binder, a conductive agent, and the like, and the reactions involved include electrochemical reactions of positive and negative electrodes, lithium ion conduction and electron conduction, As well as the diffusion of heat, the electrical performance and safety of lithium-ion batteries are affected by many factors. Therefore, the complexity of the design and production of lithium-ion batteries can be imagined. Today, Xiao Bian brings readers and friends to learn about the power. The entire design and production process of the battery from the 'material selection' to the final production of the 'battery pack'.
In general, the development of lithium-ion batteries is divided into several cycles. The first is basic research in the laboratory. This part mainly applies to button-type half-cells, or simple soft-pack batteries. The main purpose of this step is to test materials and The performance of the formulation, because the structure of the battery is not optimized, so the results obtained here can not be directly applied to production. After conducting preliminary tests and assessments at the laboratory level, good materials and formulations will be transferred to the next stage. - During the pilot test phase, it is necessary to consider the overall performance of the battery, such as the energy density of the battery (the amount of positive and negative electrodes applied) and the characteristics such as fast charge and rate, and find out the process problems that may be faced in large-scale production. , Make timely adjustments. Through the above process, after perfecting the battery formulation and production process, mature products can finally be put into formal production. As there are many factors affecting the performance of lithium ion batteries, each parameter of design and production or connection will be It has a major impact on the ultimate electrical performance and safety of the battery, so we need to understand the material, design and process The effect of parameters on the final performance of the product.
Battery material
The design of a battery starts with the selection of materials. It is necessary to select suitable materials according to the target requirements, such as energy density, rate characteristics, cycle life, and safety. For the cathode material selection, we can choose the olivine structure of LiFePO4. This type of material is more suitable for use on buses that do not require high energy density. In addition, there are high-capacity layered materials such as NCM and NCA. These materials are more suitable for use in pure electric vehicles because of their higher cost. Spinel structure of LiMn2O4 is more suitable for hybrid vehicles. The negative electrode material, the current mainstream choice is artificial graphite, natural graphite and mesophase structure carbon microsphere materials, in the current power battery capacity indicators continue to increase In the case of, we also add a small amount of Si material to the graphite material (generally<5%) , 以便提高负极的比容量. 为了改善正负极的导电性, 通常还需要在其中添加少量的导电剂, 目前最常见的导电剂为炭黑类材料, 碳纤维类材料, 以及近几年兴起的碳纳米管和石墨烯类材料.
In addition, in order to adhere the active material particles to the surface of the current collector, 1-4% of binder needs to be added. The current binder is mainly divided into two major categories: one is oil-based binder, and the other is mainly PVDF. Binder, PVDF has very good electrochemical stability. It is one of the most widely used lithium-ion battery adhesives at present. The other major category is water-based adhesives, mainly CMC, and SBR, PAA-type adhesives. Consolidation.
In order to conduct the electrons out of the lithium-ion battery, we also need to apply the positive and negative current collectors, the main Al foil and Cu foil, the current mainstream copper foil is 8um, Al foil is 15um, but with the lithium-ion battery ratio As energy continues to increase, manufacturers have begun to use thinner 6um copper foil and 12um Al foil, but its strength is poor. In use, problems such as breakage and wrinkling are more likely to occur. Sometimes, in order to reduce the internal resistance of lithium ion batteries, To improve the adhesion, we will also coat a layer of carbon material (3-5um) on the surface of copper foil or aluminum foil. For example, the Al foil coated with carbon can play a better role in LiFePO4 material system.
The separator is also an important part of the lithium-ion battery. It bears the role of isolating the electron-conducting ions. Currently, common diaphragm preparation methods are mainly divided into a dry drawing process and a wet process. The dry drawing process has a cost advantage. Certain advantages, but there is obvious anisotropy in the separators prepared by the dry-drawing process. The strength of the wet-laid separators in all directions is basically the same, but the cost is higher. At present, in order to increase the specific energy of lithium-ion batteries, the thickness of the separators Continuous thinning, in order to ensure the safety of lithium-ion batteries, the coating separator has become the mainstream of the current development of the diaphragm, the common coating can be divided into two major categories, one is inorganic oxide coating, such as Al2O3, MgO, etc. , organic coating can significantly improve the thermal stability of the diaphragm; the other is the organic polymer-based coating separator, such as the Japanese manufacturers use more aramid coating separator, can effectively improve the separator's anti-oxidation.
The electrolyte is also an important part of the lithium-ion battery. It plays the role of conducting Li+ inside the lithium-ion battery. At present, the main lithium-ion battery electrolyte is mainly carbonate electrolyte (generally containing at least two types of carbonates Solvents, such as EC, DMC, EMC, etc., Li Li6 are generally LiPF6. In order to improve the quality of electrolyte film formation on the negative electrode surface, we usually add some film-forming additives to the electrolyte, such as common VCs. Electrolytes developed for silicon carbon anodes generally also incorporate a considerable amount of FEC to produce a higher LiF content SEI film to improve the stability of the negative SEI. In addition, in order to improve the reliability and safety of lithium ion batteries , We will also add a small amount of anti-overcharge additives, flame retardant additives and other ingredients in the electrolyte.
2. Electrode production
After completing the selection of materials, we have entered the next step - electrode production. First of all we have to start from the homogenate. The homogenization of lithium-ion batteries is the key link in the production of lithium-ion batteries. The substances, binders and conductive agents are mixed to form a uniform suspension. Usually, we first disperse the binder into a glue solution. In some processes, the conductive agent and glue are dispersed into a conductive adhesive, and then the active material is mixed. Mixing, some of the process will be conductive agent and active material together with the glue solution, the key to homogenization is how to uniformly disperse the components in the slurry, in order to achieve this goal requires the optimization of the homogenization process. With the increasing popularity of nanomaterials, in order to better disperse nanoscale materials, lithium-ion battery manufacturers have also begun to use high-speed dispersion equipment, the use of high-speed shear, making the slurry more uniform dispersion, in addition there are also many material manufacturers to develop A large number of additives to improve the dispersion of the slurry.
After the slurry is dispersed, the next step is the coating of lithium-ion batteries. Currently, the common coating processes are mainly two types: roll coating and spray coating. The roll coating equipment has gradually been eliminated, but the roller coating equipment is well cleaned. The coating width is easy to adjust and requires only a small amount of paste to complete the coating. Therefore, there are many applications in some Chinese lines and laboratories. Spraying equipment, transfer by squeezing the slurry from the nozzle On the collector, the coating is completed, the spray equipment can use a slurry with a higher viscosity and solid content, and the surface state of the electrode is also better, so it has been widely used. In the actual production, the coating speed is generally controlled at 25-50m Between /min, to increase the drying speed is mainly to increase the length of the oven. Although this will increase the investment in some equipment, it can significantly speed up the production schedule and reduce the production cost. However, there are certain limits to increasing the oven length. The reason is that as the length of the oven increases, the tension control of the current collector will increase, especially when ultra-thin collectors with lower intensity are used. This problem will also become more serious. Therefore, we cannot increase the length of the oven indefinitely. In addition, rapid drying at high temperatures can also increase the phenomenon of uneven distribution of PVDF binders in the electrodes, resulting in a decrease in the adhesion of active substances. Therefore, it is difficult for us to improve continuously. The oven temperature raises the coating speed of the electrode, so there is a limit to the increase in coating speed.
Immediately after coating, the porosity of the electrode after drying will be between 60-70%. Then we will use a roller press to roll it to reduce its porosity to about 40%. This can on the one hand improve the battery The specific energy can also significantly improve the electrical conductivity and adhesion of the electrode. The roller diameter of the roller press is generally 600-1000mm. The larger roller diameter can increase the length of the effective roller zone, which can slow down the roller compaction process. This is especially important for thick electrodes (thick electrodes can easily fail due to pressure overload during rolling).
After finishing the electrode rolling, we need to divide the electrode into a certain width according to the structure of the battery. Then the electrode will be dried in a vacuum oven so as to remove the participating moisture in the electrode. It is usually necessary to put the water in the battery. The content is controlled below 500ppm in order to minimize the effect of moisture on the life and side reactions of Li-ion batteries.
Due to limited space, today we mainly introduced the two processes of 'material selection' and 'electrode coating' for power battery. In the next article, we will continue to introduce 'single cell production' and 'battery combination'. Process, stay tuned.
