13 Main Steps of Manufacturing Lithium Battery
As the “heart” of an electric vehicle, the performance of the power battery determines the key performance of the vehicle such as range, charging time, power and speed. With the continuous development of technology, various new manufacturing processes and technologies for batteries have emerged in an endless stream. Today we will take a look at the detailed production process of lithium batteries.
First of all, the production of lithium batteries can be divided into 13 steps: positive electrode batching, negative electrode batching, coating, positive electrode production, negative electrode production, positive electrode sheet preparation, negative electrode sheet preparation, winding, shedding, rolling groove, battery baking, liquid injection, and super welding cap.
1.CONFIGURING CATHODE MATERIAL
The cathode material of a lithium battery is composed of an active substance, a conductive agent, and a binder. The specific production process is as follows：
Confirm incoming material & baking
Generally, the conductive agent needs to be baked at about 120℃ for 8 hours, and the binder PVDF needs to be baked at about 80℃ for 8 hours. The active substance (LFP, NCM, etc. ) depends on the state of the incoming material and the process. Whether it needs to be baked and dried. The current workshop requires temperature ≤40℃ and humidity ≤25%RH.
Configure PVDF glue
If the wet process is used, the PVDF glue (solute PVDF, solution NMP) needs to be prepared in advance. The quality of PVDF glue is very important to the internal resistance and electrical performance of the battery. The factors that affect the gluing are temperature and stirring speed. The higher the temperature, the easier the glue is to yellow out, which affects the adhesion; the stirring speed is too high, and the glue is easy to break. The specific speed depends on the size of the dispersion disc. Under normal circumstances, the linear speed of the dispersion disc is 10-15m/s (high dependence on equipment). At this time, the mixing tank is required to turn on the circulating water, and the temperature is ≤30℃.
Positive electrode slurry
At this time, it is necessary to pay attention to the order of feeding (first add the active substance and the conductive agent to mix slowly, and then add the glue), the feeding time, and the feeding ratio, which should be strictly implemented in accordance with the process.
Secondly, it is necessary to strictly control the revolution and rotation speed of the equipment (generally, the linear speed of dispersion should be above 17m/s, depending on the performance of the equipment, which varies greatly from manufacturer to manufacturer), the vacuum degree of stirring, and the temperature.
At this stage, the particle size and viscosity of the slurry need to be tested regularly, and the particle size and viscosity are closely related to the solid content, material properties, feeding sequence and process technology. At this time, the conventional process requires a temperature ≤30℃, humidity≤25%RH, and vacuum ≤-0. 085MPa.
After the slurry is mixed, the slurry must be transferred out to the transfer tank or coating workshop. When the slurry is transferred out, it needs to be sieved. The purpose is to filter large particles, precipitate and remove ferromagnetic substances.
Large particles affect the coating to the end, which may cause the battery to self-discharge or the risk of short circuit; the ferromagnetic material of the slurry is too high, which will cause the battery to self-discharge too much and other defects. The process requirements at this time are temperature ≤40℃, humidity≤25%RH, screen ≤100 mesh, particle size ≤15um (the parameters are for reference only).
CONFIGURING ANODE MATERIAL
The negative electrode is similar to the positive electrode. In addition to active substances, conductive agents, and adhesives, dispersants are also required. The negative electrode is similar to the positive electrode.
Confirm incoming material
The conventional negative electrode system is an aqueous mixing process (the solvent is deionized water), so there is no need for drying requirements for incoming materials. This process requires the conductivity of deionized water to be ≤1us/cm. The workshop requires temperature ≤40℃ and humidity ≤25%RH.
Preparation of glue
After the material confirmation is completed, the glue is first prepared. At this time, graphite C and conductive agent are poured into a mixer for dry mixing. It is recommended not to vacuum and turn on the circulating water (the particle extrusion friction is serious during dry mixing), at a low speed of 15-20rpm, and the scraping cycle is 2-3 times at intervals of 15 minutes.
Next, pour the glue into the mixer and turn on the vacuum (≤-0. 09mpa), scrape the material twice at a low speed of 15-20rpm, then adjust the speed (low speed 35rpm, high speed 1200-1500rpm), and run for 15-60 minutes (depending on each manufacturer’s own wet process).
Finally, pour SBR into a mixer. It is recommended to stir quickly and at low speed at this time (SBR is a long-chain polymer, and the molecular chain is easily interrupted and loses activity if the speed is too high and the time is too long). It is recommended to low speed 35-40rpm, high speed 1200-1800rpm, 10-20mins.
The reference values are as follows: viscosity 2000-4000m Pa. s, particle size ≤35µm, solid content 40-70%, vacuumed and sieved ≤100 mesh. The specific process value needs to vary according to the physical properties of the material, the mixing process, etc. The workshop requires temperature ≤30℃ and humidity ≤25%RH.
Squeeze or spray the positive electrode slurry on the AB surface of the aluminum collector fluid, with a single-sided density of 20-40mg/cm2 (NCM power type), the coating oven temperature is conventional 4-8 knots or more, and the baking temperature of each section is 95-120℃. Adjust according to actual needs to avoid transverse cracks and solvent drops in baking cracking.
The transfer coating roller speed ratio is 1. 1-1. 2, the gap position is thinned by 20-30um (to avoid trailing and cause excessive compaction in the pole ear position, lithium is analyzed during the battery cycle), and the coating moisture is ≤2000-3000ppm (depending on the material and process). Workshop positive electrode temperature≤30℃, humidity≤25%
Squeeze or spray the negative electrode slurry on the AB surface of the copper collector fluid, with a single-sided density of about 10-15mg/cm2, and the coating oven temperature is generally 4-8 knots or more, and the baking temperature of each section is 80-105℃. Adjust according to actual needs to avoid transverse cracks in baking cracking.
The transfer roller speed ratio is 1. 2-1. 3, the gap position is thinned by 10-15um, the coating moisture is ≤3000ppm, the workshop negative electrode temperature is ≤30℃, and the humidity is≤25%.
PRESS CATHODE MATERIAL
After the positive electrode coating is dried, it needs to be rolled within the process time. The roller compacts the pole piece, and there are currently two processes: hot pressing and cold pressing.
Hot pressing compaction is relatively high compared to cold pressing, and the rebound rate is low; the cold pressing process is relatively simple and easy to operate and control.
The main equipment for the roller has the following process values: compaction density, rebound rate, and elongation. At the same time, pay attention to the fact that the surface of the pole piece is free of chips, lumps, dropped materials, wavy edges, etc. , and no fracture is allowed at the gap. At this time, the ambient temperature of the workshop is ≤23℃ and the humidity is ≤25%.
PREPARATION OF CATHODE ELECTRODE SHEET
After the slitting is completed, the positive electrode sheet needs to be dried (120℃), and then the welding aluminum pole ear and pole ear gluing process. At this time, the length of the pole ear and the width of the shaping need to be considered. Taking the design of a lithium battery as an example, the design of the exposed pole ear mainly takes into account the reasonable cooperation of the positive electrode ear when welding the cap and rolling groove.
The exposure of the pole ear is too long, and it is easy to short circuit the pole ear and the steel shell when rolling the groove; the pole ear is too short to weld the cap.At present, ultrasonic welding heads are wired and dot-shaped, and domestic processes are more linear (considering overcurrent and welding strength).
In addition, high-temperature glue is used to coat the pole ears, mainly taking into account the risk of short circuit caused by metal burrs and metal debris.The ambient temperature of this workshop is ≤23℃, the dew point is ≤-30℃, and the positive electrode moisture content is ≤500-1000ppm.
PREPARATION OF ANODE ELECTRODE SHEET
The negative electrode sheet needs to be dried (105-110℃), and then the nickel electrode ear and electrode ear gluing process are welded.It is also necessary to consider the length of the pole ear and the width of the shaping. The ambient temperature of this workshop is ≤23℃, the dew point is ≤-30℃, and the negative electrode moisture content is ≤500-1000ppm.
Winding is to pass the diaphragm, positive electrode sheet, and negative electrode sheet through the winding machine into a single core. The principle is to use the negative electrode to wrap the positive electrode, and then isolate the positive and negative electrode pieces through the diaphragm. Because the negative electrode of the conventional system is used as the control electrode of the battery design, the capacity design is higher than that of the positive electrode, so that the Li+ of the positive electrode can be stored in the “empty space“ of the negative electrode when it is turned into charging.Winding requires special attention to winding tension and pole piece alignment.
The winding tension is small, which will affect the internal resistance and shell entry rate; excessive tension can easily cause short circuit or fragmentation risk.Alignment refers to the relative position of the negative electrode, the positive electrode and the diaphragm. The width of the negative electrode is 59.5mm, the positive electrode is 58mm, and the diaphragm is 61mm. The three are centered and aligned to avoid the risk of short circuit.
The winding tension is generally 0.08-0.15Mpa for positive tension and 0.08-0.15Mpa for negative tension; the tension of the upper diaphragm is 0.08-0.15Mpa, and the tension of the lower diaphragm is 0.08-0.15Mpa, which should be adjusted according to the equipment and process.The ambient temperature of this workshop is ≤23℃, the dew point is ≤-30℃, and the moisture content is ≤500-1000ppm.
ROLL THE CORE INTO THE SHELL
Before the core is put into the shell, the Hi-Pot test voltage is 200-500V (to test whether there is a high-voltage short circuit), and the dust is vacuumed (to further control the dust before entering the shell). Here we need to emphasize the three major control points of lithium batteries: moisture, burrs, and dust.
After the previous process is completed, put the bottom pad into the bottom of the core and bend the negative electrode ear so that the polar ear is facing the pinhole of the core, and finally insert the steel shell or aluminum shell vertically (take a certain model as an example, the outer diameter is about 18mm, and the height is about 71.5mm).
Of course, the cross-sectional area of the core should be smaller than the cross-sectional area of the steel shell, and the shell entry rate is approximately 97-98.5%, because the rebound value of the polar piece and the degree of liquid injection in the later stage should be taken into account.In the same process as the surface pad, the upper pad is also assembled.The ambient temperature of this workshop is ≤23℃ and the dew point is ≤-40℃.
Insert the welding needle (usually copper or alloy material) into the middle hole of the core.The commonly used welding needle specifications are Φ2.5*1.6mm, and the welding strength of the negative electrode ear ≥12N is qualified. If it is too low, it is easy to virtual welding and the internal resistance is too large; if it is too high, it is easy to weld off the nickel layer on the surface of the steel shell, resulting in hidden dangers such as rust and liquid at the solder joints.
The simple understanding of the rolling groove is to fix the core in the shell without shaking.In this process, special attention needs to be paid to the matching of the transverse extrusion speed and the longitudinal downward pressure speed to avoid the transverse speed is too fast to cut the shell, and the longitudinal speed is too fast. The nickel layer of the notch falls off or affects the groove height to affect the sealing.
It is necessary to detect whether the process values of groove depth, flaring, and groove height are up to standard (through actual and theoretical calculations).Common hob specifications are 1.0, 1.2, and 1.5mm.After the rolling groove is completed, the overall vacuum treatment needs to be carried out again to avoid metal debris, the vacuum degree is ≤-0.065Mpa, and the vacuum time is 1-2s.The ambient temperature of this workshop is ≤23℃ and the dew point is ≤-40℃.
After the cylindrical battery cell passes through the rolling groove, the next step is a very important step: baking.During the production process of the battery, a certain amount of moisture will be brought in. If the moisture is not controlled within the standard in time, it will seriously affect the performance and safety of the battery.
Generally, an automatic vacuum oven is used for baking, and the batteries to be baked are neatly placed in the oven. The desiccant is placed in the oven, the parameters are set, and the heating is heated to 85℃ (take lithium iron phosphate batteries as an example). It takes several vacuum drying cycles to reach the standard.
The baked battery cell is tested for moisture, and after it meets the previous baking standards, the next step can be carried out: the electrolyte is injected.
Quickly put the baked battery cells into the vacuum glove box, weigh them, record the weight, put on the injection sleeve cup, and add the designed weight of the electrolyte to the sleeve cup (generally, a liquid soaking experiment will be carried out: put the battery into the electrolyte, soak for a period of time, test the maximum liquid absorption of the battery, and generally inject the liquid according to the experimental amount), put it in a vacuum box and vacuum it (vacuum degree ≤-0.09Mpa) to accelerate the electrolyte invasion of the polar piece.
After several cycles, take out the battery cell and weigh it to calculate whether the injection volume meets the design value. If it is less, rehydration is required, and the excess part needs to be drained until it meets the design requirements. Glove box environment: temperature ≤23℃, dew point ≤-45℃.
WIELDING BATTERY CAP
Put the cap in the glove box in advance, fasten the cap tightly to the mold under the super welding machine in one hand, and hold the battery in the other. The positive electrode ear of the battery is aligned with the pole ear of the cap. After confirming that the positive electrode ear is aligned with the pole ear of the cap, press the pedal switch of the super welding machine.
After that, you need to fully check the battery cell: self-check the welding effect of the pole ear, the first is to observe whether the pole ear is aligned; the second is to gently pull the pole ear to see if the pole ear is loosened.The batteries that are over-soldered and the caps are over-soldered need to be over-soldered again.