From "materials" to "battery packs" one article to understand the whole process of power battery production

　　Lithium-ion battery is a complex system, including positive electrode, negative electrode, separator, electrolyte, current collector and binder, conductive agent, etc. The reactions involved include electrochemical reaction of positive and negative electrodes, lithium ion conduction and electronic 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 batteries can be imagined. The entire design and production process from "material selection" to the final production of "battery pack".

　　Lithium-ion battery is a complex system, including positive electrode, negative electrode, separator, electrolyte, current collector and binder, conductive agent, etc. The reactions involved include electrochemical reaction of positive and negative electrodes, lithium ion conduction and electronic 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 batteries can be imagined. The entire design and production process from "material selection" to the final production of "battery pack".

　　Generally speaking, the development of lithium-ion batteries is divided into several cycles. The first is the basic research in the laboratory. This part is mainly applicable to button half-cells or simple soft-pack batteries. The main purpose of this step is to test materials and The performance of the formula, because the structure of the battery has not been optimized, so the results obtained here cannot be directly applied to production. After conducting preliminary tests and evaluations at the laboratory level, good materials and formulations will be transferred to the next stage—the pilot test stage. At this stage, the overall performance of the battery needs to be considered, such as battery energy density (positive and negative electrodes). The coating amount), fast charging, magnification and other characteristics, and find the process problems that may be faced in the large-scale production process, and make timely adjustments. Through the above process, after the battery formula and production process are improved, mature products can finally be put into formal production. Since there are many factors that affect the performance of lithium-ion batteries, each parameter of design and production or connection will have a significant impact on the final electrical performance and safety of the battery. Therefore, it is necessary for us to deeply understand the material, design and process parameters for the final product. Performance impact.

　　1. Battery material

　　The design of a battery must first start 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 indicators. In terms of cathode material selection, we can choose LiFePO4 with olivine structure. This material is more suitable for use in buses that require low energy density. In addition, there are high-capacity layered materials such as NCM and NCA. The cost is higher, and it is more suitable for use in pure electric vehicles, while the spinel structure of LiMn2O4 is more suitable for use in hybrid vehicles. In terms of anode materials, the current mainstream choice is artificial graphite, natural graphite and mesophase structure carbon microsphere materials. In the current situation of continuous improvement of power battery performance indicators, we will also add a small amount of Si to graphite materials. Material (generally <5%) in order to increase the specific capacity of the negative electrode. In order to improve the conductivity of the positive and negative electrodes, it is usually necessary to add a small amount of conductive agent. At present, the most common conductive agents are carbon black materials, carbon fiber materials, and carbon nanotubes and graphene materials that have emerged in recent years. .

　　In addition, in order to adhere the active material particles to the surface of the current collector, it is necessary to add 1-4% binder. The current binders are mainly divided into two categories. One is oil-based binders, mainly PVDF. Binder, PVDF has very good electrochemical stability and is currently one of the most widely used lithium-ion battery binders; the other major category is water-based binders, mainly CMC, and SBR and PAA binders. Knot agent.

　　In order to conduct the electrons in the lithium-ion battery, we also need the current collectors applied to the positive and negative electrodes, mainly Al foil and Cu foil. At present, the mainstream copper foil is 8um, and the Al foil is 15um. However, as the ratio of lithium-ion batteries With the continuous improvement of energy, manufacturers have begun to use thinner 6um copper foil and 12um Al foil, but their strength is poor, and they are prone to breakage and crease problems during use. Sometimes in order to reduce the internal resistance of lithium-ion batteries and improve adhesion, we will also coat a layer of carbon material (3-5um) on the surface of copper foil or aluminum foil. For example, carbon-coated Al foil can perform better in the LiFePO4 material system. Effect.

　　Diaphragm is also an important part of lithium-ion batteries. It plays the role of isolating electronic conduction ions. At present, the common preparation methods of diaphragm are mainly divided into dry stretching process and wet process. The dry stretching process has a cost Certain advantages, but the diaphragm prepared by the dry stretching process has obvious anisotropy. The strength of the wet diaphragm is basically the same in all directions, but the cost is higher. At present, in order to increase the specific energy of lithium-ion batteries, the thickness of the separator continues to become thinner. In order to ensure the safety of lithium-ion batteries, coated separators have become the mainstream trend in the development of current separators. Common coatings can be divided into two major categories. One is inorganic oxide coatings, such as Al2O3, MgO, etc. Organic coatings can significantly improve the thermal stability of the diaphragm; the other is organic polymer coatings, such as aramid coatings used by Japanese manufacturers. The layered diaphragm can effectively improve the oxidation resistance of the diaphragm.

　　Electrolyte is also an important part of lithium-ion batteries. It plays a role of conducting Li+ inside lithium-ion batteries. The current mainstream lithium-ion battery electrolytes are mainly carbonate electrolytes (generally containing at least two or more carbonates). Solvents, such as EC, DMC, EMC, etc.), Li salt generally uses 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 VC, etc. In the electrolyte developed for the silicon-carbon negative electrode, a considerable amount of FEC is generally added to produce an SEI film with a higher LiF content to improve the stability of the negative electrode 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 move on to the next link-electrode production. First of all, we have to start with homogenization. The homogenization of lithium-ion batteries is a key link in the production of lithium-ion batteries. The homogenization process is mainly to mix the active material, binder and conductive agent into a uniform suspension. Usually we will first disperse the binder into a homogeneous suspension. Glue, some processes will disperse the conductive agent and the glue into a conductive glue, and then mix it with the active material, and some processes will mix the conductive agent and the active material with the glue. The key to homogenization is how to mix the slurry In order to achieve this goal, the homogenization process needs to be optimized. At present, with the gradual popularity of nano-materials, in order to better disperse nano-level materials, lithium-ion battery manufacturers have also begun to use high-speed dispersion equipment, using high-speed shear to make the slurry dispersion more uniform. In addition, there are many materials. The manufacturer has developed a large number of additives to improve the dispersion of the slurry.

　　After finishing the dispersion of the slurry, the next step is the coating of lithium-ion batteries. The current common coating processes mainly include two types of roller coating and spraying. The roller coating equipment has been gradually eliminated, but the roller coating equipment is easy to clean. , The coating width is easy to adjust, and only a small amount of slurry is needed to complete the coating, so there are more applications in some Chinese lines and laboratories. Spraying equipment, by squeezing the slurry from the nozzle and transferring it to the current collector, the coating is completed. The spraying equipment can use slurry with higher viscosity and solid content, and the electrode surface condition is better, so it has been widely used . In actual production, the coating speed is generally controlled between 25-50m/min. To increase the drying speed is mainly by increasing the length of the oven. Although this will increase a part of the equipment investment, it can significantly speed up the production schedule and reduce the production cost. , But there is a limit to increasing the length of the oven. This is mainly because the increase in the length of the oven will increase the difficulty of controlling the tension of the current collector, especially when the ultra-thin current collector with lower strength is used. Becomes more prominent, so we cannot increase the length of the oven indefinitely. In addition, rapid drying at high temperature will aggravate the uneven distribution of PVDF binder in the electrode, resulting in a decrease in the bonding force of the active material. Therefore, it is difficult for us to increase the coating speed of the electrode by continuously increasing the oven temperature. There is a certain limit to the increase in cloth speed.

　　The electrode that has just been coated and dried will generally have a porosity between 60-70%, and then we will use a roller press to roll it down to reduce the porosity to about 40%, which can improve the battery on the one hand The specific energy can also significantly improve the conductivity and adhesion of the electrode. The diameter of the rollers of the roller press is generally 600-1000mm. A larger roller diameter can increase the length of the effective rolling area and slow down the pressure change rate during the rolling process. This is particularly important for thick electrodes (thick electrodes are very important). It is easy to cause failure due to pressure overload during the rolling process).

　　After finishing the electrode rolling, we need to divide the electrode into a certain width according to the structure of the battery, and then the electrode will be dried in a vacuum oven to remove the water involved in the electrode, usually the water in the battery needs to be removed The content is controlled below 500ppm in order to minimize the impact of moisture on the life of the lithium-ion battery and side reactions.

　　3. Single battery production

　　After the electrode drying process, we have entered the next link in the production of lithium-ion batteries-the production of single cells. In order to prevent the dried electrodes from absorbing moisture again, the entire cell production process needs to be carried out in a drying room, and the environmental dew point is generally controlled at -40°C to -60°C. There are three main types of production processes for prismatic power battery cells. One is the winding process. This process is generally used in the production of cylindrical batteries. It is also currently used in the production process of prismatic batteries. The main advantages of this process are It has high production efficiency and can achieve continuous production. The disadvantages are also obvious. Because the bending angle at the edge of the battery is relatively large, it is easy to break the electrode, produce wrinkles, and cause defects, especially in the case of thick electrodes. The problem will become more serious; the second is the lamination process. The lamination process is an ideal process. The positive and negative pole pieces will first be punched to obtain a specific shape of pole pieces, and then choose the positive electrode or the negative electrode. The sheet is made into a packaging bag with a diaphragm for protection, and then laminated by hand or by a laminator. The advantage of this process is that it will not cause the pole piece to deform, and thicker electrodes can be used, but because the lamination process is a non-continuous Therefore, the production efficiency of the lamination process is relatively low, and fewer manufacturers use this process; the third is the Z-type lamination process, which uses a continuous diaphragm and places the punched positive and negative pole pieces on the In the middle of the diaphragm, this process has retained the advantages of the lamination process, but has also accelerated the production process and improved production efficiency. At present, there are more applications.

　　A good battery cell must first be welded to the tab. The tab welding method is mainly ultrasonic welding. The cell produced by the winding process is limited by the structure of the cell. A single cell cannot be made very thick, so it is usually Two to four batteries are connected in parallel to weld the tabs to form a large cell. There are no restrictions on the structure of the battery produced by the lamination process, so it is generally a single cell to weld the tabs. The next step is the shelling process. After welding the tabs with the protective film on the outer surface of the battery cells, they are placed in the battery shell. After the shells are inserted, the positive and negative poles on the cover of the tabs and the battery shell need to be used. Ultrasonic welding, riveting and other processes are connected together, and then the upper cover of the battery and the outer shell are welded together by laser.

　　Here we need to talk about the upper cover of the prismatic battery separately. This is also the place with the highest technical content and the most complicated structure of the prismatic battery shell. This is because we not only need to ensure that the positive and negative poles, but also between the battery shell Insulation, it is necessary to ensure good sealing to prevent moisture in the environment from entering the inside of the battery case. At present, the most common sealing method is compression sealing, which is to use plastic parts for insulation between the electrode pole and the case, and pass The way of compressing the plastic part realizes the sealing of the battery structure. Although this method is simple and effective, the plastic parts will age during the long-term deformation process, resulting in a decrease in the reliability of the seal. Therefore, some manufacturers, such as BYD, have developed the Al2O3 ceramic sealing process to avoid the aging problem of plastic parts. It can ensure the battery's sealing reliability for more than 30 years, which is of great significance to the echelon utilization of power batteries.

　　After the welding is completed, it is usually necessary to carry out leak detection and remove the batteries with unqualified leak rates. Common leak detection methods include direct pressure, double pressure and differential pressure. Good sealing is to ensure the performance of lithium-ion batteries. The key to long-term stability and reliability, battery leak detection is also an indispensable link in the production of prismatic power batteries.

　　The battery that has been screened for leak detection then goes to the very important injection process. Since the electrolyte of lithium-ion batteries is very sensitive to moisture, the injection process must be carried out inside the drying room. In order to improve the infiltration effect of the electrolyte, it is usually necessary Vacuum liquid injection, that is, first exhaust the air inside the battery, then inject the electrolyte, and repeat it several times to make the electrolyte fully infiltrate the cell, then seal it, and place the battery in a high temperature environment to promote Infiltration of electrolyte.

　　The battery fully infiltrated by the electrolyte then enters the formation process. The formation process is mainly to activate the battery by charging and discharging the battery with a small current. During the first charge, the potential of the positive electrode will continue to rise, and the potential of the negative electrode will continue to rise. Generally speaking, when the negative electrode potential drops to about 1V, the EC components in the electrolyte and other film-forming additives, such as VC, FEC, etc. will decompose on the negative electrode surface to form SEI film, and accompanying gas generation, SEI film formation It can prevent the negative electrode from further reacting with the electrolyte. Therefore, a good SEI film is very important to improve the cycle performance of lithium-ion batteries. At present, special film-forming additives and high-temperature formation processes are usually used to improve the quality of the negative SEI film. In addition, due to the problem of gas produced during the decomposition process of the electrolyte, the gas produced may accumulate in the cell, resulting in insufficient electrolyte infiltration. Therefore, some manufacturers may also exhaust the gas produced during the formation process. Arrange the battery seal after formation.

　　The formed battery also needs to be aged. The so-called aging is to put the fully charged battery at a certain temperature. During the storage process, some side reactions inside the lithium-ion battery will cause the external voltage and internal resistance of the battery. Change, by monitoring the voltage, internal resistance and capacity of the battery pack, it is possible to eliminate those batteries with unqualified self-discharge and unqualified internal resistance to improve the consistency of single cells. At the same time, the aging result is also the follow-up battery An important reference basis for group matching. In order to accelerate battery aging and improve production efficiency, manufacturers usually perform aging at high temperatures (50-60°C) to shorten battery aging time.