Read the power battery production process in one article

  Industry News     |      2019-01-17
Lithium batteries are mainly used in digital products in the traditional field. Today, with the rapid development of new energy sources, lithium batteries are widely used in power batteries and energy storage.
 
Battery technology itself is not so inscrutable, the basic principle is the redox reaction. But the specific chemical processes involved in energy carriers are ever-changing. Specific to practical applications involve many problems interwoven with materials science, inorganic chemistry, organic chemistry, physics, surface, interface, thermodynamics, kinetics, engineering machinery processing, and electronic circuit technology.
 
Redox reactions
 
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 electron conduction. And the spread of heat, etc.
 
In general, the development of lithium-ion batteries is divided into several cycles. The first is the basic research in the laboratory. This part is mainly applied 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 a preliminary laboratory-level test and evaluation, good materials and formulations will be transferred to the next stage, the pilot phase, where the overall performance of the battery, such as battery energy density (positive and negative), needs to be considered. The coating amount) and the characteristics of fast charge, rate, etc., and found that the process problems that may be encountered in the mass production process, timely adjustments. Through the above process, after perfecting the battery formulation and production process, mature products can be finally put into formal production.
 
Due to the many factors affecting the performance of lithium-ion batteries, each parameter designed and produced or connected will have a significant impact on the final electrical performance and safety of the battery, so it is necessary to have an in-depth understanding of the materials, design and process parameters for the final product. The impact of performance.
 
Battery material selection
 
Lithium battery raw material resources are mined and processed, mainly lithium resources, cobalt resources and graphite.
 
Energy density, cost, safety, thermal stability, and cycle life are the five key indicators of power lithium batteries. The ternary materials, potassium manganate and lithium iron phosphate do not have absolute advantages in these five aspects, leading to power. Lithium battery pole material route differences.
 
A battery design begins with the choice of materials and requires the selection of the right material based on target requirements such as energy density, rate characteristics, cycle life and safety. For the selection of positive electrode materials, we can choose LiFePO4 with olivine structure, which is more suitable for use on buses with low energy density requirements, in addition to high-capacity layered materials such as NCM and NCM. Suitable for use in pure electric vehicles, the spinel-structured LiMN2O4 is more suitable for use in hybrid vehicles.
 
In terms of anode materials, graphite has always been the choice of anode materials for lithium batteries. In fact, metal tin is more suitable as a cathode material if only energy density is considered.
 
In order to improve the conductivity of the positive and negative electrodes, it is usually necessary to add a small amount of conductive agent thereto. Currently, the most common conductive agents are carbon black materials, carbon fiber materials, and carbon nanotubes and graphene materials which have emerged in recent years. .
 
In addition, in order to adhere the electrode to the surface of the current collector, it is necessary to add 1-4% of the binder. The current binders are mainly divided into two categories: oil-based binders, and the other is water systems. Binder.
 
The four components of the battery are critical: positive (discharge to cathode), negative (discharge to anode), electrolyte, diaphragm. The positive and negative electrodes are the places where chemical reactions take place, and the important position can be understood. But what is the use of electrolytes? Doing work is still very weighty.
 
Inside the battery, metallic lithium loses electrons in the negative electrode and is oxidized to become lithium ions, which is transferred to the positive electrode through the electrolyte; the positive electrode material is reduced in electrons and neutralized by lithium ions coming from the positive electrode. The ideal function of the electrolyte is to transport and transport only lithium ions. Outside the battery, electrons are transferred from the negative electrode to the positive electrode through the external circuit, and work is performed in the middle. Ideally, the electrolyte should be a good carrier for lithium ions, but it must not be a good electron carrier. Therefore, in the absence of external circuits, electrons cannot be transferred from the negative electrode to the positive electrode inside the battery; electron transfer can only be performed when there is an external circuit.
 
Single cell production
 
The first step, the production of monomer electrodes
 
Mixing: The electrode active material, the binder, the solvent, and the like are mixed together, and the mixture is sufficiently stirred and dispersed to form a slurry.
 
Coating: The prepared slurry is uniformly applied to a current collector (aluminum foil or copper foil, etc.) at a specified thickness.
 
Baking: high temperature baking and drying treatment.
 
“Mixed” and “baked” are related. “Bake” is to better fix the mixed slurry on aluminum foil or copper foil, and the “baking” process is a high energy-consuming part, such as improving the link. , can reduce the production cost of the positive and negative electrodes of the lithium battery.
 
The homogenization of lithium-ion batteries is the key link in the production of lithium-ion batteries. The homogenization process mainly combines the active substances, binders and conductive agents into a uniform suspension. Usually we will first disperse the binders into The glue, and then some processes will first disperse the conductive agent and the glue into a conductive paste, and then mix with the active material.
 
Some processes will mix the conductive agent and the binder together with the glue. The key to the homogenization is how to disperse the components in the slurry evenly. In order to achieve this goal, the homogenization process needs to be optimized. The homogenization process is mainly divided into dry homogenization and wet homogenization. At present, with the gradual popularization of nanomaterials, the current lithium ion battery manufacturers have begun to adopt high-speed dispersing equipment, which utilizes high-speed shearing to make the slurry more dispersed. Uniform, in addition, many material manufacturers have developed a large number of additives to improve the dispersion of the slurry.
 
The second step, the production of single cells
 
After completing the above electrode drying process, we entered the next step in the production of lithium-ion batteries - the production of single cells.
 
Pressing: Rolling is a method of pressing the coated positive and negative materials to make it compact and attach it to aluminum foil or copper foil.
 
Cutting: slitting is to cut the strips that have been rolled into strips according to the process standard.
 
In order to prevent the electrode after drying from absorbing moisture again, the entire cell production process needs to be carried out in a dry room.
 
There are three main types of production processes for square power battery cells. One is the winding process. This process is generally applied to the production of cylindrical batteries. It is also applied to the production process of square batteries. The main advantages of this process. It is high in production efficiency and can achieve continuous production. The disadvantages are also obvious. Because the bending angle at the edge of the cell is relatively large, electrode breakage is likely to occur and defects are generated. Especially in the case of thick electrodes, this problem will become more complicated. serious;
 
The second method is the lamination process. The lamination process is an ideal process. The positive and negative pole pieces are first punched to obtain a pole piece of a specific shape, and then the positive or negative pole piece is selected as a package bag. For protection, and then lamination by hand or lamination machine, the advantage of this process is that it does not cause pole piece deformation, thicker electrodes can be used, but since the lamination process is a discontinuous process, the lamination process The production efficiency is relatively low, and there are fewer manufacturers adopting this process;
 
The third is the Z-type lamination process, which uses a continuous diaphragm to place the punched positive and negative pole pieces in the middle of the diaphragm. This process accelerates the advantages of the lamination process. The production process has improved production efficiency and there are many applications at present.
 
The production of good batteries must first be welded to the ear. The welding method of the ear is mainly by ultrasonic welding. The battery produced by the winding process is limited by the structure of the battery. A single battery cannot be made very thick, so it will usually 2-4 batteries are connected in parallel with the tabs. There is no limit to the structure of the battery produced by the lamination process, so it is generally a single cell welding tab. The next step is to enter the shell process. The outer surface of the welded cell is wrapped with a protective film and then placed in the battery case. After the case is placed, the positive and negative poles on the cover of the electrode and the battery case are used. The processes of ultrasonic welding, riveting, etc. are joined together, and then the upper cover and the outer casing of the battery are welded together by laser welding.
 
After the welding is completed, it is usually necessary to perform leak detection and remove the batteries whose leakage rate is unqualified. Common leak detection methods include direct pressure, double voltage and differential pressure. Good sealing performance is to ensure the performance of lithium ion battery. The key to long-term stability and reliability, battery leak detection is also an indispensable part of the production of square power batteries.
 
The battery after the leak detection screen is followed by a very important liquid injection process. Since the electrolyte of the lithium ion battery is very sensitive to moisture, the liquid injection process must be carried out inside the drying chamber. In order to improve the electrolyte infiltration effect, it is usually required. Vacuum injection.
 
The battery in which the electrolyte is sufficiently wetted is then subjected to a chemical conversion step, and the battery is activated mainly by charging and discharging a small current to the battery.
 
In addition, due to the problem that the production gas is usually generated during the decomposition process of the electrolyte, the generated gas may accumulate in the battery core, resulting in insufficient electrolyte infiltration, so some manufacturers will discharge the gas in the chemical conversion process. Arrange the battery seal after the formation.
 
After the formation of the battery, it needs to be aging. The so-called aging is to leave the fully charged battery at a certain temperature. During the process of suspension, some external reactions of the lithium ion battery will cause the external voltage and internal resistance of the battery. By monitoring the voltage, internal resistance and capacity of the battery pack, it is possible to eliminate those batteries that fail to pass the self-discharge and fail the internal resistance, so as to improve the consistency of the single battery, and the aging result is also the subsequent battery. An important reference for group matching. In order to accelerate the aging of the battery and improve the production efficiency, the manufacturer usually aging at a high temperature (50-60 ° C) to shorten the battery aging time.
 
Assembly of battery modules and battery packs
 
After the aging battery completes the aging, it enters the stage of module combination. Before the combination, it must first be screened, that is, test the capacity, dynamic internal resistance and voltage of the single battery, and try to select the battery with the same parameters to match.
 
A large battery pack usually consists of a plurality of battery modules, each of which is composed of a plurality of single cells connected in series and in parallel. The series connection can increase the voltage of the battery module, and the parallel connection can increase the capacity of the battery module. The principle to be followed when performing cell matching for a battery module is generally to prioritize capacity in series to reduce overcharging or overdischarging of a module having a lower capacity during charging and discharging of the battery pack. In parallel, the internal resistance is prioritized to avoid overcharging or overdischarging of the battery with less internal resistance caused by uneven current distribution during high current charging and discharging.
 
After the matching of the single cells is completed, the combination process of the battery modules is entered. This process usually fixes the matched single cells to the module structure of the battery pack, and then uses the bus bar to connect the single cells. The electrode poles are connected together.
 
Although the cells in the battery pack have been carefully matched, the uniformity of the capacity and internal resistance of the single cell is very good, but the battery pack can also be caused by the inconsistency in the decay rate of the single cell during the cycle. In the internal cell, there is a voltage deviation. In order to reduce the inconsistency of the single cell in the battery pack, we usually add an equalizer to the battery pack. When the voltage deviation of some cells in the battery pack reaches a certain level, we will Start the equalizer to restore the unit cells in the battery pack.
 
According to the working principle, the equalizer can be generally divided into dissipative type equalization and non-dissipative type equalization. The dissipative type equalization structure is the simplest, that is, the battery with higher voltage in the battery pack is directly discharged, and the electric energy is converted into heat and dissipated to the environment. In the middle, the non-dissipative type equalization is more complicated, and the battery with a higher voltage will charge the battery with a lower voltage through the equalizer, thereby achieving 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 lithium-ion batteries. Especially in the case of a large number of batteries in the battery pack, it is easy to cause the temperature inside the battery pack under the influence of charge and discharge heat. The distribution is uneven, affecting the electrical performance and reliability of the battery pack.
 
Experiment to verify battery pack inconsistency
 
According to the user's needs, a power battery pack usually consists of several battery modules, which are connected in series to provide external power supply to meet the needs of different usage scenarios.
 
In addition, we also need to install a management system for the battery pack, which is what we usually call BMS. The main function of BMS is to control the charging and discharging of the battery pack, to prevent overcharging or over-discharging of the battery, and to manage the battery pack. The equalization system and thermal management system enhance 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 to the battery pack to reduce the damage caused by the thermal runaway of the battery pack.