Full process of power lithium battery production -Lithium - Ion Battery Equipment
Lithium ion battery is a complex system, including positive pole, negative pole, diaphragm, electrolyte, collector, binder, conductive agent, etc. The reactions involved include the electrochemical reaction of positive and negative poles, lithium ion conduction and electronic conduction, as well as the diffusion of heat. Therefore, the electrical performance and safety of lithium ion battery are affected by many factors. Therefore, the design and manufacture of lithium ion battery not only requires rich electrochemical knowledge, More importantly, we need a lot of practical experience in production. Today, Xiao Bian will take you to review the status quo and challenges of power battery technology.(Lithium - Ion Battery Equipment)
In general, the development of lithium-ion batteries can be divided into several cycles. First, basic research in the laboratory. This part is mainly applicable to button type half batteries or simple flexible batteries. This step is mainly aimed at testing the performance of materials and formulas. Because the battery structure has not been optimized, the results obtained here cannot be directly applied to production. After the preliminary test and evaluation at the laboratory level, good materials and formulas will be transferred to the next stage - the pilot stage. In this stage, it is necessary to consider the comprehensive performance of the battery, such as the battery energy density (coating amount of positive and negative electrodes), fast charging, magnification and other characteristics, and find out the process problems that may be faced in the mass production process, and make timely adjustments. Through the above process, mature products can be finally put into formal production after the battery formula and production process are improved. As there are many factors that affect the performance of lithium ion batteries, each parameter of design, production or connection will have a significant impact on the final electrical performance and safety of batteries. Therefore, it is necessary for us to deeply understand the impact of materials, design and process parameters on the final performance of products. Recently, ArnoKwade of Brunswick University of Technology combed for us the influence of design and production process parameters on the final product performance in the whole production process from electrode to single battery to battery pack.
1. Battery material
The design of a battery should start from the selection of materials, and the appropriate materials should be selected according to the target requirements, such as energy density, magnification characteristics, cycle life, safety and other indicators. In terms of cathode material selection, we can choose LiFePO4 with olivine structure, which is more suitable for buses with low demand for energy density. In addition, there are layered materials with high capacity, such as NCM and NCM, which are more suitable for pure electric vehicles, while LiMN2O4 with spinel structure is more suitable for hybrid vehicles. In terms of anode materials, the current mainstream choice is artificial graphite, natural graphite and mesophase carbon microsphere materials. With the continuous improvement of the specific capacity index of power batteries, we will also add a small amount of Si materials to the graphite materials to improve the specific capacity of the anode. In order to improve the conductivity of the positive and negative electrodes, it is usually necessary to add a small amount of conductive agents. At present, the most common conductive agents are carbon black materials, carbon fiber materials, and carbon nanotubes and graphene materials emerging in recent years. In addition, in order to adhere the electrode to the surface of the collector fluid, 1-4% of the adhesives need to be added. At present, the adhesives are mainly divided into two categories: oil based adhesives, mainly PVDF adhesives, and water based adhesives, mainly CMC, SBR and PAA adhesives.
In order to transmit the electrons in the lithium ion battery, we also need to apply the collector to the positive and negative electrodes, mainly aluminum foil and copper foil. At present, the mainstream copper foil is 8um and aluminum foil is 15um. However, with the continuous improvement of the specific energy of the lithium ion battery, manufacturers have begun to use thinner 6um copper foil and 12um aluminum foil. Sometimes, in order to reduce the internal resistance of the lithium ion battery and improve the adhesion, We will also coat a layer of carbon material (3-5 um) on the surface of copper foil or aluminum foil. Diaphragm is also an important part of lithium ion battery. It plays a role in isolating electrons and conducting ions. At present, the common preparation methods of diaphragm are mainly divided into dry stretching process and wet stretching process. The dry stretching process has certain advantages in cost, but the diaphragm prepared by dry stretching process has obvious anisotropy. The wet stretching process has basically the same strength in all directions, but the cost is high. At present, in order to improve the specific energy of the lithium ion battery, the thickness of the diaphragm continues to thin. In order to ensure the safety of the lithium ion battery, the coated diaphragm has become the mainstream trend in the development of the diaphragm. The common coatings can be divided into two categories: inorganic oxide coatings, such as Al2O3, MgO, etc. Organic coatings can significantly improve the thermal stability of the diaphragm, and organic polymer membranes, For example, the aramid coated diaphragm used by Japanese manufacturers can effectively improve the oxidation resistance of the diaphragm.
Electrolyte is also an important part of lithium-ion battery, which plays a role in conducting Li+inside the lithium-ion battery. At present, the mainstream electrolyte of lithium-ion battery is mainly carbonate electrolyte (generally including at least two kinds of carbonate solvents, such as EC, DMC, EMC, etc.). Li salt generally uses LiPF6. In order to improve the quality of electrolyte film on the negative electrode surface, We usually add some film forming additives, such as common VC, to the electrolyte, and generally add a considerable amount of FEC to the electrolyte developed for silicon carbon negative electrodes to improve the stability of 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 components in the electrolyte.
2. Electrode production
The homogenization of lithium-ion battery is a key link in the production of lithium-ion battery. The homogenization link is mainly to mix active substances, adhesives, conductive agents and other components into a uniform suspension. Usually, we will first disperse the adhesive into glue, and then some processes will first disperse the conductive agent and glue into conductive glue, and then mix with the active substances. Some processes will mix the conductive agent and adhesive together with the glue, The key to homogenization is how to disperse all the components in the slurry evenly. In order to achieve this goal, the homogenization process needs to be optimized. At present, the main homogenization processes are mainly divided into dry homogenization and wet homogenization. At present, with the gradual popularization of nano materials, lithium ion battery manufacturers have begun to use high-speed dispersion equipment to make the slurry more evenly dispersed by using high-speed shearing, In addition, many material manufacturers have developed a large number of additives to improve the size dispersion.
After the dispersion of the slurry is completed, the next step is the coating process of the lithium ion battery. At present, the common coating processes mainly include 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. Therefore, there are many applications in some Chinese lines and laboratories. Spraying equipment is used to complete coating by extruding the slurry from the nozzle and transferring it to the collector. Spraying equipment can use slurry with higher viscosity and solid content, and the electrode surface is in better condition, so it is widely used. In actual production, the coating speed is generally controlled between 25-50 m/min, and the drying speed is mainly increased by increasing the length of the oven. Although this will increase part of the equipment investment, it can significantly accelerate the production progress and reduce the production cost, but there is a certain limit to increase the length of the oven, which is mainly because the increase in the length of the oven will increase the difficulty in controlling the collector tension, Especially when the ultra-thin collector with lower strength is used, this problem will become more prominent, so we can not infinitely increase the length of the oven. In addition, too fast drying speed will also aggravate the uneven distribution of PVDF binder in the electrode, resulting in a decrease in the adhesion of active substances. Therefore, it is difficult for us to improve the coating speed of the electrode by continuously increasing the oven temperature, so there is a certain limit to the improvement of the coating speed.
Generally, the porosity of the electrode just coated and dried will be 60-70%, and then we will use the roller press to roll it to reduce the porosity to about 40%. On the one hand, this can improve the specific energy of the battery, and also can significantly improve the conductivity and cohesiveness of the electrode. The diameter of the roller 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 speed of pressure change during rolling, which is particularly important for thick electrodes (thick electrodes are easy to lose efficacy due to pressure overload during rolling).
After the electrode rolling is completed, we need to cut the electrode into a certain width according to the structure of the battery, and then the electrode will be dried in the vacuum oven to remove the moisture involved in the electrode. Generally, the moisture content in the battery needs to be controlled below 500ppm, so as to minimize the impact of moisture on the life and side reaction of the lithium ion battery.
3. Production of single battery
After the above electrode drying process is completed, we will enter the next link in the production of lithium ion batteries - the production of single batteries. In order to prevent the dried electrode from absorbing water again, the whole production process of single battery needs to be carried out in the drying room, and the environmental dew point is generally controlled at - 40 ℃ to - 60 ℃. There are three main types of production processes for square power battery cells. One is the winding process. This process is generally used in the production of cylindrical batteries, and it is also used in the production process of square batteries at present. The main advantage of this process is high production efficiency, which can achieve continuous production. The disadvantages are also obvious. Because the bending angle at the edge of the cell is relatively large, it is easy to break the electrode and produce defects, Especially in the case of thick electrodes, this problem will become more serious; The second is the lamination process. The lamination process is an ideal process. First, the positive and negative pole pieces will be punched to obtain the pole pieces with a specific shape. Then, the positive or negative pole pieces will be made into packaging bags with diaphragms for protection, and then laminated manually or with a laminating machine. The advantage of this process is that it will not cause deformation of the pole pieces. Thicker electrodes can be used, but because the lamination process is a discontinuous process, Therefore, the production efficiency of lamination process is relatively low, and few manufacturers adopt this process; The third is the Z-type lamination process. This process uses a continuous diaphragm and places the punched positive and negative pole plates in the middle of the diaphragm. On the basis of retaining the advantages of the lamination process, this process also speeds up the production process and improves the production efficiency. At present, it is also widely used.
The produced electric core should first weld the lug. The lug welding method is mainly ultrasonic welding process. For the electric core produced by winding process, a single electric core cannot be made very thick due to the restriction of the electric core structure. Therefore, 2-4 electric cores are usually welded in parallel with the lug. There is no restriction on the battery structure produced by lamination process. Therefore, the lug is generally welded by a single electric core. The next step is to enter the shell. After the outer surface of the cell welded with the lug is wrapped with a protective film, it is put into the battery shell. After entering the shell, the lug needs to be connected with the positive and negative pole posts on the cover of the battery shell by ultrasonic welding, riveting and other processes, and then the upper cover and the shell of the battery are welded together by laser welding.
Here we need to talk about the upper cover of the square battery separately. This is also the place where the square battery case has the highest technical content and the most complex structure. This is because we should not only ensure the insulation between the positive and negative pole, as well as between the battery case, but also ensure good sealing to prevent water in the environment from entering the battery case. At present, the most common sealing method is compression sealing, That is, plastic parts are used between the electrode pole and the shell for insulation, and the battery structure is sealed by compressing the plastic parts. Although this method is simple and effective, plastic parts will age in the process of long-term deformation, leading to a decline in sealing reliability. Therefore, some manufacturers, such as BYD, have developed the Al2O3 ceramic sealing process to avoid the aging problem of plastic parts. It is said that it can ensure the sealing reliability of batteries for more than 30 years, which is of great significance for the cascade utilization of power batteries.
After welding, it is usually necessary to conduct leakage detection, and remove the batteries with unqualified leakage rate. Common leakage detection methods include direct pressure, double pressure and differential pressure. Good sealing is the key to ensure the long-term stability and reliability of lithium ion battery performance. Therefore, battery leakage detection is also an indispensable link in the production of square power battery.
After leak detection and screening, the battery goes to a very important liquid injection process. Because the electrolyte of the lithium ion battery is very sensitive to water, the liquid injection process must be carried out in the drying room. In order to improve the wetting effect of the electrolyte, vacuum liquid injection is usually required, that is, the air inside the battery is exhausted first, then the electrolyte is injected, and repeated several times, so that the electrolyte is fully soaked in the cell, The battery is then sealed and placed in a high temperature environment to promote electrolyte infiltration.
The battery fully saturated with electrolyte then enters into the formation process. The formation is mainly to activate the battery by charging and discharging the battery with a small current. During the first charging process, the positive electrode potential will continue to rise, while the negative electrode potential will continue to decline. Generally, when the negative electrode potential drops to about 1V, EC components in the electrolyte and other film forming additives, such as VC, FEC, will decompose on the negative electrode surface to form a SEI film, With gas production, the formation of SEI film can prevent the negative electrode from further reacting with the electrolyte. Therefore, a good SEI film is important for improving the cycle performance of lithium ion batteries. At present, the quality of negative electrode SEI film is usually improved through special film forming additives and high-temperature formation processes. In addition, the problem of gas generation usually occurs during the decomposition of electrolyte, and the gas generated may accumulate in the cell, leading to insufficient electrolyte infiltration. Therefore, some manufacturers also arrange the battery seal after the formation in order to discharge the gas generated during the formation process.
The formed battery also needs to be aged. The so-called aging is to put the fully charged battery aside at a certain temperature. During the shelving process, some side reactions inside the lithium ion battery will lead to changes in the external voltage and internal resistance of the battery. By monitoring the electrical voltage, internal resistance, capacity and other indicators of the battery pack, those batteries with unqualified self discharge and unqualified internal resistance can be eliminated, In order to improve the consistency of the single battery, and the aging results are also an important reference for the subsequent battery pack matching. In order to accelerate the battery aging speed and improve the production efficiency, manufacturers usually age at high temperatures (50-60 ℃) to shorten the battery aging time.
4. Assembly of battery module and battery pack
After the aging of single battery, it will enter the stage of module combination. Before combination, it is necessary to screen, that is, test the capacity, dynamic internal resistance, voltage and other data of single battery, and try to select the battery with the same parameters for matching. A large battery pack is usually composed of multiple battery modules. Each battery module is composed of multiple single batteries in series and parallel. Series connection can improve the voltage of the battery modules, and parallel connection can improve the capacity of the battery modules. The principle followed in matching single batteries for the battery modules is generally that the capacity is given priority to in series connection, To reduce overcharge or overdischarge of modules with low capacity during charging and discharging of the battery pack. In parallel connection, the internal resistance is given priority to avoid overcharging or discharging of batteries with small internal resistance due to uneven current distribution during high current charging and discharging.
After the matching of the single battery is completed, the combination process of the battery module is started. This process is usually to fix the matched single battery into the module structure of the battery pack, and then use the busbar to connect the electrode poles of the single battery together. Although the single cells in the battery pack have been carefully matched, and the consistency of the capacity and internal resistance of the single cells is very good, the voltage deviation of the single cells in the battery pack will also occur due to the inconsistent decay speed of the single cells during the cycle. In order to reduce the inconsistency of the single cells in the battery pack, we usually add an equalizer in the battery pack, When the voltage deviation of some cells in the battery pack reaches a certain level, we will start the equalizer to make the cells in the battery pack consistent. According to the working principle, the equalizer can generally be divided into dissipative equalization and non dissipative equalization. The dissipative equalization has the simplest structure, which is to directly discharge the batteries with higher voltage in the battery pack, convert the electric energy into heat and dissipate it to the environment. The non dissipative equalization is more complex. The battery with higher voltage will charge the battery with lower voltage through the equalizer, so as to achieve the voltage balance between the cells.
The temperature management of the battery pack is also a part that cannot be ignored. Temperature is a key factor affecting the performance of the lithium-ion battery. Especially when there are many batteries in the battery pack, under the influence of charging and discharging heat, it is easy to lead to uneven temperature distribution in the battery pack, which affects the electrical performance and reliability of the battery pack. Some studies have shown that the maximum temperature difference in the battery pack is 4.62 ℃, The reliability of the battery pack can be improved from 0.0635 to 0.9328 when the temperature drops to 2.5 ℃ (200 cycles without BMS and balance system management). Therefore, the battery pack needs a good supporting heating and cooling device. The heating is relatively simple, usually by means of heating strips. In recent years, some scholars have proposed some methods to quickly increase the battery temperature by means of internal heating of the battery. The heat dissipation mainly includes air cooling (forced and non forced), and water cooling with better heat dissipation effect.
According to the needs of users, a power battery pack is usually composed of several battery modules, which are connected in series to provide external power to meet the needs of different use scenarios.
In addition, we also need to install a management system for the battery pack, which is commonly referred to as the BMS. The main function of the BMS is to control the charging and discharging of the battery pack and prevent overcharge or overdischarge of the battery. In addition, we also need to manage the battery pack balancing system and thermal management system to improve the performance and life of the battery pack. In order to improve the safety of the power battery pack, we will also add some thermal runaway warning and blocking devices in the battery pack to reduce the harm caused by thermal runaway of the battery pack.
From "material" to "battery pack", ordinary materials have been transformed into a power source to drive a better new life through the smart hands of lithium battery users. It can be said that every electric vehicle and battery cell is the result of the efforts of lithium battery users.
In general, the development of lithium-ion batteries can be divided into several cycles. First, basic research in the laboratory. This part is mainly applicable to button type half batteries or simple flexible batteries. This step is mainly aimed at testing the performance of materials and formulas. Because the battery structure has not been optimized, the results obtained here cannot be directly applied to production. After the preliminary test and evaluation at the laboratory level, good materials and formulas will be transferred to the next stage - the pilot stage. In this stage, it is necessary to consider the comprehensive performance of the battery, such as the battery energy density (coating amount of positive and negative electrodes), fast charging, magnification and other characteristics, and find out the process problems that may be faced in the mass production process, and make timely adjustments. Through the above process, mature products can be finally put into formal production after the battery formula and production process are improved. As there are many factors that affect the performance of lithium ion batteries, each parameter of design, production or connection will have a significant impact on the final electrical performance and safety of batteries. Therefore, it is necessary for us to deeply understand the impact of materials, design and process parameters on the final performance of products. Recently, ArnoKwade of Brunswick University of Technology combed for us the influence of design and production process parameters on the final product performance in the whole production process from electrode to single battery to battery pack.
1. Battery material
The design of a battery should start from the selection of materials, and the appropriate materials should be selected according to the target requirements, such as energy density, magnification characteristics, cycle life, safety and other indicators. In terms of cathode material selection, we can choose LiFePO4 with olivine structure, which is more suitable for buses with low demand for energy density. In addition, there are layered materials with high capacity, such as NCM and NCM, which are more suitable for pure electric vehicles, while LiMN2O4 with spinel structure is more suitable for hybrid vehicles. In terms of anode materials, the current mainstream choice is artificial graphite, natural graphite and mesophase carbon microsphere materials. With the continuous improvement of the specific capacity index of power batteries, we will also add a small amount of Si materials to the graphite materials to improve the specific capacity of the anode. In order to improve the conductivity of the positive and negative electrodes, it is usually necessary to add a small amount of conductive agents. At present, the most common conductive agents are carbon black materials, carbon fiber materials, and carbon nanotubes and graphene materials emerging in recent years. In addition, in order to adhere the electrode to the surface of the collector fluid, 1-4% of the adhesives need to be added. At present, the adhesives are mainly divided into two categories: oil based adhesives, mainly PVDF adhesives, and water based adhesives, mainly CMC, SBR and PAA adhesives.
In order to transmit the electrons in the lithium ion battery, we also need to apply the collector to the positive and negative electrodes, mainly aluminum foil and copper foil. At present, the mainstream copper foil is 8um and aluminum foil is 15um. However, with the continuous improvement of the specific energy of the lithium ion battery, manufacturers have begun to use thinner 6um copper foil and 12um aluminum foil. Sometimes, in order to reduce the internal resistance of the lithium ion battery and improve the adhesion, We will also coat a layer of carbon material (3-5 um) on the surface of copper foil or aluminum foil. Diaphragm is also an important part of lithium ion battery. It plays a role in isolating electrons and conducting ions. At present, the common preparation methods of diaphragm are mainly divided into dry stretching process and wet stretching process. The dry stretching process has certain advantages in cost, but the diaphragm prepared by dry stretching process has obvious anisotropy. The wet stretching process has basically the same strength in all directions, but the cost is high. At present, in order to improve the specific energy of the lithium ion battery, the thickness of the diaphragm continues to thin. In order to ensure the safety of the lithium ion battery, the coated diaphragm has become the mainstream trend in the development of the diaphragm. The common coatings can be divided into two categories: inorganic oxide coatings, such as Al2O3, MgO, etc. Organic coatings can significantly improve the thermal stability of the diaphragm, and organic polymer membranes, For example, the aramid coated diaphragm used by Japanese manufacturers can effectively improve the oxidation resistance of the diaphragm.
Electrolyte is also an important part of lithium-ion battery, which plays a role in conducting Li+inside the lithium-ion battery. At present, the mainstream electrolyte of lithium-ion battery is mainly carbonate electrolyte (generally including at least two kinds of carbonate solvents, such as EC, DMC, EMC, etc.). Li salt generally uses LiPF6. In order to improve the quality of electrolyte film on the negative electrode surface, We usually add some film forming additives, such as common VC, to the electrolyte, and generally add a considerable amount of FEC to the electrolyte developed for silicon carbon negative electrodes to improve the stability of 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 components in the electrolyte.
2. Electrode production
The homogenization of lithium-ion battery is a key link in the production of lithium-ion battery. The homogenization link is mainly to mix active substances, adhesives, conductive agents and other components into a uniform suspension. Usually, we will first disperse the adhesive into glue, and then some processes will first disperse the conductive agent and glue into conductive glue, and then mix with the active substances. Some processes will mix the conductive agent and adhesive together with the glue, The key to homogenization is how to disperse all the components in the slurry evenly. In order to achieve this goal, the homogenization process needs to be optimized. At present, the main homogenization processes are mainly divided into dry homogenization and wet homogenization. At present, with the gradual popularization of nano materials, lithium ion battery manufacturers have begun to use high-speed dispersion equipment to make the slurry more evenly dispersed by using high-speed shearing, In addition, many material manufacturers have developed a large number of additives to improve the size dispersion.
After the dispersion of the slurry is completed, the next step is the coating process of the lithium ion battery. At present, the common coating processes mainly include 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. Therefore, there are many applications in some Chinese lines and laboratories. Spraying equipment is used to complete coating by extruding the slurry from the nozzle and transferring it to the collector. Spraying equipment can use slurry with higher viscosity and solid content, and the electrode surface is in better condition, so it is widely used. In actual production, the coating speed is generally controlled between 25-50 m/min, and the drying speed is mainly increased by increasing the length of the oven. Although this will increase part of the equipment investment, it can significantly accelerate the production progress and reduce the production cost, but there is a certain limit to increase the length of the oven, which is mainly because the increase in the length of the oven will increase the difficulty in controlling the collector tension, Especially when the ultra-thin collector with lower strength is used, this problem will become more prominent, so we can not infinitely increase the length of the oven. In addition, too fast drying speed will also aggravate the uneven distribution of PVDF binder in the electrode, resulting in a decrease in the adhesion of active substances. Therefore, it is difficult for us to improve the coating speed of the electrode by continuously increasing the oven temperature, so there is a certain limit to the improvement of the coating speed.
Generally, the porosity of the electrode just coated and dried will be 60-70%, and then we will use the roller press to roll it to reduce the porosity to about 40%. On the one hand, this can improve the specific energy of the battery, and also can significantly improve the conductivity and cohesiveness of the electrode. The diameter of the roller 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 speed of pressure change during rolling, which is particularly important for thick electrodes (thick electrodes are easy to lose efficacy due to pressure overload during rolling).
After the electrode rolling is completed, we need to cut the electrode into a certain width according to the structure of the battery, and then the electrode will be dried in the vacuum oven to remove the moisture involved in the electrode. Generally, the moisture content in the battery needs to be controlled below 500ppm, so as to minimize the impact of moisture on the life and side reaction of the lithium ion battery.
3. Production of single battery
After the above electrode drying process is completed, we will enter the next link in the production of lithium ion batteries - the production of single batteries. In order to prevent the dried electrode from absorbing water again, the whole production process of single battery needs to be carried out in the drying room, and the environmental dew point is generally controlled at - 40 ℃ to - 60 ℃. There are three main types of production processes for square power battery cells. One is the winding process. This process is generally used in the production of cylindrical batteries, and it is also used in the production process of square batteries at present. The main advantage of this process is high production efficiency, which can achieve continuous production. The disadvantages are also obvious. Because the bending angle at the edge of the cell is relatively large, it is easy to break the electrode and produce defects, Especially in the case of thick electrodes, this problem will become more serious; The second is the lamination process. The lamination process is an ideal process. First, the positive and negative pole pieces will be punched to obtain the pole pieces with a specific shape. Then, the positive or negative pole pieces will be made into packaging bags with diaphragms for protection, and then laminated manually or with a laminating machine. The advantage of this process is that it will not cause deformation of the pole pieces. Thicker electrodes can be used, but because the lamination process is a discontinuous process, Therefore, the production efficiency of lamination process is relatively low, and few manufacturers adopt this process; The third is the Z-type lamination process. This process uses a continuous diaphragm and places the punched positive and negative pole plates in the middle of the diaphragm. On the basis of retaining the advantages of the lamination process, this process also speeds up the production process and improves the production efficiency. At present, it is also widely used.
The produced electric core should first weld the lug. The lug welding method is mainly ultrasonic welding process. For the electric core produced by winding process, a single electric core cannot be made very thick due to the restriction of the electric core structure. Therefore, 2-4 electric cores are usually welded in parallel with the lug. There is no restriction on the battery structure produced by lamination process. Therefore, the lug is generally welded by a single electric core. The next step is to enter the shell. After the outer surface of the cell welded with the lug is wrapped with a protective film, it is put into the battery shell. After entering the shell, the lug needs to be connected with the positive and negative pole posts on the cover of the battery shell by ultrasonic welding, riveting and other processes, and then the upper cover and the shell of the battery are welded together by laser welding.
Here we need to talk about the upper cover of the square battery separately. This is also the place where the square battery case has the highest technical content and the most complex structure. This is because we should not only ensure the insulation between the positive and negative pole, as well as between the battery case, but also ensure good sealing to prevent water in the environment from entering the battery case. At present, the most common sealing method is compression sealing, That is, plastic parts are used between the electrode pole and the shell for insulation, and the battery structure is sealed by compressing the plastic parts. Although this method is simple and effective, plastic parts will age in the process of long-term deformation, leading to a decline in sealing reliability. Therefore, some manufacturers, such as BYD, have developed the Al2O3 ceramic sealing process to avoid the aging problem of plastic parts. It is said that it can ensure the sealing reliability of batteries for more than 30 years, which is of great significance for the cascade utilization of power batteries.
After welding, it is usually necessary to conduct leakage detection, and remove the batteries with unqualified leakage rate. Common leakage detection methods include direct pressure, double pressure and differential pressure. Good sealing is the key to ensure the long-term stability and reliability of lithium ion battery performance. Therefore, battery leakage detection is also an indispensable link in the production of square power battery.
After leak detection and screening, the battery goes to a very important liquid injection process. Because the electrolyte of the lithium ion battery is very sensitive to water, the liquid injection process must be carried out in the drying room. In order to improve the wetting effect of the electrolyte, vacuum liquid injection is usually required, that is, the air inside the battery is exhausted first, then the electrolyte is injected, and repeated several times, so that the electrolyte is fully soaked in the cell, The battery is then sealed and placed in a high temperature environment to promote electrolyte infiltration.
The battery fully saturated with electrolyte then enters into the formation process. The formation is mainly to activate the battery by charging and discharging the battery with a small current. During the first charging process, the positive electrode potential will continue to rise, while the negative electrode potential will continue to decline. Generally, when the negative electrode potential drops to about 1V, EC components in the electrolyte and other film forming additives, such as VC, FEC, will decompose on the negative electrode surface to form a SEI film, With gas production, the formation of SEI film can prevent the negative electrode from further reacting with the electrolyte. Therefore, a good SEI film is important for improving the cycle performance of lithium ion batteries. At present, the quality of negative electrode SEI film is usually improved through special film forming additives and high-temperature formation processes. In addition, the problem of gas generation usually occurs during the decomposition of electrolyte, and the gas generated may accumulate in the cell, leading to insufficient electrolyte infiltration. Therefore, some manufacturers also arrange the battery seal after the formation in order to discharge the gas generated during the formation process.
The formed battery also needs to be aged. The so-called aging is to put the fully charged battery aside at a certain temperature. During the shelving process, some side reactions inside the lithium ion battery will lead to changes in the external voltage and internal resistance of the battery. By monitoring the electrical voltage, internal resistance, capacity and other indicators of the battery pack, those batteries with unqualified self discharge and unqualified internal resistance can be eliminated, In order to improve the consistency of the single battery, and the aging results are also an important reference for the subsequent battery pack matching. In order to accelerate the battery aging speed and improve the production efficiency, manufacturers usually age at high temperatures (50-60 ℃) to shorten the battery aging time.
4. Assembly of battery module and battery pack
After the aging of single battery, it will enter the stage of module combination. Before combination, it is necessary to screen, that is, test the capacity, dynamic internal resistance, voltage and other data of single battery, and try to select the battery with the same parameters for matching. A large battery pack is usually composed of multiple battery modules. Each battery module is composed of multiple single batteries in series and parallel. Series connection can improve the voltage of the battery modules, and parallel connection can improve the capacity of the battery modules. The principle followed in matching single batteries for the battery modules is generally that the capacity is given priority to in series connection, To reduce overcharge or overdischarge of modules with low capacity during charging and discharging of the battery pack. In parallel connection, the internal resistance is given priority to avoid overcharging or discharging of batteries with small internal resistance due to uneven current distribution during high current charging and discharging.
After the matching of the single battery is completed, the combination process of the battery module is started. This process is usually to fix the matched single battery into the module structure of the battery pack, and then use the busbar to connect the electrode poles of the single battery together. Although the single cells in the battery pack have been carefully matched, and the consistency of the capacity and internal resistance of the single cells is very good, the voltage deviation of the single cells in the battery pack will also occur due to the inconsistent decay speed of the single cells during the cycle. In order to reduce the inconsistency of the single cells in the battery pack, we usually add an equalizer in the battery pack, When the voltage deviation of some cells in the battery pack reaches a certain level, we will start the equalizer to make the cells in the battery pack consistent. According to the working principle, the equalizer can generally be divided into dissipative equalization and non dissipative equalization. The dissipative equalization has the simplest structure, which is to directly discharge the batteries with higher voltage in the battery pack, convert the electric energy into heat and dissipate it to the environment. The non dissipative equalization is more complex. The battery with higher voltage will charge the battery with lower voltage through the equalizer, so as to achieve the voltage balance between the cells.
The temperature management of the battery pack is also a part that cannot be ignored. Temperature is a key factor affecting the performance of the lithium-ion battery. Especially when there are many batteries in the battery pack, under the influence of charging and discharging heat, it is easy to lead to uneven temperature distribution in the battery pack, which affects the electrical performance and reliability of the battery pack. Some studies have shown that the maximum temperature difference in the battery pack is 4.62 ℃, The reliability of the battery pack can be improved from 0.0635 to 0.9328 when the temperature drops to 2.5 ℃ (200 cycles without BMS and balance system management). Therefore, the battery pack needs a good supporting heating and cooling device. The heating is relatively simple, usually by means of heating strips. In recent years, some scholars have proposed some methods to quickly increase the battery temperature by means of internal heating of the battery. The heat dissipation mainly includes air cooling (forced and non forced), and water cooling with better heat dissipation effect.
According to the needs of users, a power battery pack is usually composed of several battery modules, which are connected in series to provide external power to meet the needs of different use scenarios.
In addition, we also need to install a management system for the battery pack, which is commonly referred to as the BMS. The main function of the BMS is to control the charging and discharging of the battery pack and prevent overcharge or overdischarge of the battery. In addition, we also need to manage the battery pack balancing system and thermal management system to improve the performance and life of the battery pack. In order to improve the safety of the power battery pack, we will also add some thermal runaway warning and blocking devices in the battery pack to reduce the harm caused by thermal runaway of the battery pack.
From "material" to "battery pack", ordinary materials have been transformed into a power source to drive a better new life through the smart hands of lithium battery users. It can be said that every electric vehicle and battery cell is the result of the efforts of lithium battery users.
