Some possible reasons and corresponding explanations for the preparation and process of button cells/soft packs

Button battery

1.Basic Introduction

Lithium-ion button batteries are mainly composed of the following parts: positive electrode shell, negative electrode shell, (positive/negative) electrode sheet, diaphragm, gasket, spring, and electrolyte. The C in the commonly used regular commercial button battery CR2032 indicates that the positive electrode is MnO2, and there is also a BR series, and B indicates that the positive electrode is carbon fluoride. C or B represents the button battery system, and R represents that the battery shape is round.

A half-battery refers to a battery with an electrode sheet as the positive electrode and a lithium sheet as the negative electrode (due to the lower potential of lithium, it serves as the negative electrode in the half-battery); a half-battery is a device used only to study the electrochemical properties of the positive or negative electrode materials of the battery, with a focus on the study of the electrochemical properties of battery materials.

A full battery refers to a battery that uses electrodes for both the positive and negative electrodes (need to consider the N/P ratio of the positive and negative electrodes, the overhang of the positive and negative electrodes, etc.); a full battery is a complete battery, including the positive and negative electrodes, separators, electrolytes, and shells. It is usually used to test the matching degree between the positive or negative electrode materials and the rest of the battery, focusing on the research of the electrochemical performance of the battery system and the battery manufacturing process.

2.Assembly (glove box, vacuum mixer, oven, roller machine, punching machine, sealing machine)

Assembly: negative electrode shell - lithium sheet (placed in the center of the negative electrode shell, the tablet pressing mold is flattened)  - diaphragm (placed flat on the upper layer of the lithium sheet ) -  electrolyte (dropped on the surface of the diaphragm ) -  pole piece ( the active material side needs to be close to the diaphragm )  -  gasket  -  spring  -  positive electrode shell is placed on the upper layer of the diaphragm in sequence

Note:

(1)Place the negative electrode side upward in the button battery sealing machine mold (absorb the overflowed electrolyte with a paper towel, the residual electrolyte on the battery surface will cause a low open circuit voltage);

(2) For button-type battery cases, the positive electrode case is larger, while the negative electrode case has a mesh structure on the surface and is smaller, so the assembly process generally starts with the negative electrode case (there is no right or wrong answer to this question, it all depends on personal habits, for reference only).

(3) The positive and negative current collectors of lithium-ion battery electrodes are aluminum foil and copper foil respectively. If a single-sided smooth foil is used, it is often coated on the rough surface to increase the bonding force between the current collector and the material. The thickness of the foil is not strictly required, but there are relatively high requirements for the uniformity of the surface density of the foil. Silicon-based negative electrode materials generally use carbon-coated copper foil to improve adhesion, reduce contact resistance, increase the reproducibility of test results, and improve charge and discharge cycle performance.

(4) Lithium sheets are relatively soft and easily deformed, so you can use the positive electrode shell (because it is relatively large) to flatten the deformed lithium sheets before installing the battery. In addition, metallic lithium is very easy to oxidize and deteriorate in the air, and it is easy to explode when it comes into contact with water, so the purchased metallic lithium sheets need to be opened in the glove box . Be careful not to damage the gloves when opening.

3.Testing suggestions

(1)The pole piece needs to be rolled before assembly and baked before assembly. The baking condition is 105℃ for 4h. After baking, the pole piece needs to be weighed before assembly. The amount of active material in the pole piece needs to be calculated based on the actual weight of the pole piece.

(2) Before placing the battery assembly parts in the glove box, they must be vacuum dried for about 4 hours. The temperature should not be too high and can be set at 60-80°C. After the battery is assembled, it needs to be left to stand for more than 8 hours.

(3) A glove box with a larger capacity can store battery components in advance, which is cleaner. After the raw materials enter the hatch, strictly follow the operating procedures to perform exhaust-air intake operations at least three times. It is recommended to place a small operating table in the glove box to prevent the reagents from corroding the glove box.

(4)The recommended amount of electrolyte to be added during assembly is about 130ul. The electrolyte can cause severe corrosion to gloves and the inner wall of the glove box, so operating errors should be avoided as much as possible.

Soft pack battery

1.Basic Introduction
Soft-pack batteries are actually batteries that use aluminum-plastic packaging film as packaging materials. Relatively speaking, lithium-ion battery packaging is divided into two categories, one is soft-pack batteries and the other is metal-shell batteries. Metal-shell batteries include steel shells and aluminum shells, etc. In recent years, due to special needs, some batteries use plastic shells, which can also be classified as this category.　　

The difference between the two is not only the shell material, but also the packaging method. Soft-pack batteries use thermal packaging, while metal shell batteries generally use welding (laser welding). The reason why soft-pack batteries can use thermal packaging is that they use aluminum-plastic packaging film.

2. Assembly (glove box, vacuum mixer, oven, coating machine, spot welding machine, die cutting machine, laminating machine, aluminum plastic film forming machine, packaging machine)

Taking soft-package lithium-ion batteries as an example, the battery manufacturing process includes the following main steps:
material pretreatment - weighing - homogenization - coating - rolling - slitting - drying - winding or lamination - assembly - cleaning - primary aging - formation - secondary aging - sorting - appearance inspection and shipment

 

 

Electrode-electrode Calendering-rolling      Slitting-slitting      Notching-cross-cutting      SRS (safety reinforced separator)-diaphragm Assembly-assembly      Cathode-negative electrode      Anode-positive      electrode Lamination-lamination      Folding-folding (combining the stacked positive and negative electrodes)  Welding-welding Forming-shell      Sealing-shelling      Electrolyte-electrolyte (liquid injection) Package-encapsulation      Formation-forming      Degas-vacuum degassing      EOL-"EOL" in the battery industry usually refers to "End of Life", that is, the end of the battery's life. The end of a battery's life may mean that the battery's performance has deteriorated to the point where it no longer meets the requirements of use, or that the battery has completely failed and can no longer be charged or release energy.

3.Possible causes and explanations ( poor punching depth, poor top seal, battery corner damage, poor second seal, poor edge voltage, etc. )

3.1 Poor drawing depth

3.1.1 Mould

(1) Corner R ≥ vertical R ≥ 1mm ---- -- Cracks are likely to occur if the angle is less than 1mm  

(2) Die gap = 0.25-0.35mm: about 2-3 times the film thickness

(3) Mold surface roughness = 3.2s (Ra = 0.8um); R zone surface roughness = 1.6s (Ra = 0.4um) ---- --If the surface roughness is too rough, the molding depth will be poor.   

3.1.2 Molding conditions(1) Surface control pressure = 0.3-0.5MPa - lower pressure will cause wrinkles, higher pressure will cause cracks  (2) Punching speed 5 mm/sec -- Faster speed will cause cracks  (3) Stamping maintenance time 2 seconds ---- -- Short time affects the rebound rate   

3.1.3 Countermeasures for defects(1) When cracks or holes occur - reduce the pressure of surface control(2) When wrinkles are formed ------ Increase surface control pressure(3) When warping occurs --- reduce the surface control pressure and adjust the stamping speed(4) Corner depression ------ Extend the holding time and increase the punching speed        Note : Under normal circumstances, the mold gap is controlled at 0.25mm (twice the thickness of ALF). The R angle is related to the punching depth, forming size and ALF. Usually, the R angle is set to 1.5mm for a punching depth below 4mm, and 2mm for a punching depth above 5mm. The R angle is reduced for smaller models to ensure the appearance; the smaller the size, the smaller the effective compensation area at the bottom, and the shallower the punching depth. The angle and R size of the punching die core of the flexible packaging shell depend on the forming area/depth. Generally, the single-side gap between the upper and lower molds is 0.15-0.3mm, and R is 1.5-3.5mm. To ensure the safety of the four corners without damage, R is as large as possible . 

3.2 Poor top seal

3.2.1 Solutions to wrinkle defects in top and side sealing processesPoor film punching : The punching depth does not match the thickness of the battery core, which makes it difficult to seal and cause wrinkles;The pit and the shallow pit cannot overlap effectively.Large and thick battery models require a long time to seal the top and side after shell punching, which causes the membrane shell to rebound and deform;The bottom surface of the top seal fixture is not on the same plane as the head;There is PP glue on the head that is not wiped off in time, which causes the seal to wrinkle easily;Poor operation will cause wrinkling of the seal. 

3.2.2 Top seal leakage problemTo prevent leakage from top seal, first eliminate the problem of the tabs. The reasons for leakage caused by packaging are:The actual temperature of the head is too low, resulting in insufficient heating temperature of PP and ear glue;The tab glue is exposed too long, causing the head to be pressed on the tab metal strip;The fixture is not positioned accurately, and the alignment between the fixture and the head is not adjusted properly, resulting in the side edge sealing and the top edge sealing not overlapping;Auxiliary heating module failure 

3.3 Battery corner damage

Battery corner damage usually occurs at the bottom of the folded edge of the battery secondary package (some battery cell processes have the second seal on the positive ear side, and some have the second seal on the negative ear side);Cause: During the production process from battery filling to secondary sealing, one side of the air bag is easily bent repeatedly, making the aluminum-plastic film at this position easy to be damaged, especially at the top sealing corner and the bottom corner;Solution: The battery cannot hold the air bag and the number of bends should be reduced. For the aluminum-plastic film, it is recommended to be bent 180 degrees at least 5 times without pinholes. In addition, this position should be pre-sealed before injection. (Pre-sealing process: general temperature 150-160℃, time 1s, pressure 0.1-0.2MPa) 

3.4 Second edge sealing defect

3.4.1 Possible causes of secondary seal leakage or poor soldering(1) The temperature fluctuation of the head is too large: confirm the temperature stability and replace the heating tube(2) After injection, the electrolyte is not resealed for a long time, resulting in the PP layer being soaked in electrolyte for a long time: Control the time from injection to resealing(3) The flatness of the head is not enough, resulting in insufficient actual pressure in the virtual sealing position: head flatness inspection(4) Too much electrolyte: This situation is obvious in rate batteries. It is recommended to pre-seal the battery fixture at the corners and bake it first, so that the electrolyte is concentrated on the second sealing edge and the air pocket faces upward when the fixture is baked.

3.4.2 Detailed Analysis(1) The main reason for insufficient heat sealing of the second sealing edge (i.e., the liquid injection edge or the vacuum extraction edge) is that the electrolyte sticks to or adheres to the CPP surface during liquid injection or vacuum extraction, resulting in a decrease in the heat sealing strength of the CPP.(2) If the CPP surface is adhered to the electrolyte, its strength will not be sufficient even if the heat seal is done well, because the electrolyte on the CPP surface can be evaporated by increasing the heat sealing temperature or taking a secondary heat sealing method, but there will still be residual electrolyte film between the CPP and CPP, leaving a safety hazard.(3) The best solution is to pay attention when injecting and vacuuming, and do not let too much electrolyte adhere to the surface of CPP 

3.4.3 Solution(1) Make a larger round opening on the packaging film near the injection edge.(2) Place the injection needle as close to the battery cell as possible for injection.(3) Placing the battery cell vertically or on an inclined surface during aging is conducive to the electrode fully absorbing the electrolyte(4) Vacuum the battery cell at an angle 

3.5 Bad side voltage

After the soft-pack battery is produced, the voltage level is measured to evaluate the packaging effect and determine the risk of battery swelling and leakage. 

3.5.1 Bad side voltageTheoretically: the positive and negative electrodes, the aluminum layer between them and the aluminum-plastic film is insulated, that is to say, their voltage should be 0.In fact: Considering the battery materials and processing, there will be local damage, which will lead to local conduction between them, forming a micro short circuit, and thus a potential difference. The higher the voltage, the higher the risk of the battery. The voltage standard is different for each battery cell factory. The industry is set at less than 1V. 

3.5.2 Measurement standards

Connect the red pen of the multimeter to the positive/negative pole of the battery, and slide the black pen on the aluminum layer of the aluminum-plastic film. The maximum voltage measured initially is the edge voltage. Generally, ≥1V is a defective product (Guangyu ≥0.5V) 

3.5.3 Causes of edge voltage

It is usually caused by poor top and second seal packaging of the battery, resulting in poor insulation. 

3.5.4 Solution(1) Control the thickness of the aluminum-plastic film after sealing and welding, and control the PP residual rate to 70-90%(2) Control the reserved position for sealing welding 

3.6 Internal corrosion ( electrical corrosion, also called black spot or internal corrosion)

3.6.1 Causes

The aluminum layer of the aluminum-plastic film forms a circuit with the negative electrode ear, an electron channel is generated through the electrode ear, and the primary charge is generated through the contact between the electrolyte and the aluminum layer.pool, causing corrosion. 

3.6.2 Solution(1) The hard seal process requires precise control. The skew and offset of the tabs can easily cause internal short circuits between the aluminum layer and the tabs. Generally, this is detected in a short circuit test. If not screened, the probability of black spot corrosion is high.(2) Failure of the reserved control of each edge seal is also one of the causes of black spot corrosion. If there is no reserved edge seal, the PP layer at the stretching position of the aluminum-plastic film is easily affected by the heat seal and ruptured, resulting in the risk of battery bulging and leakage.(3) After the top seal, the exposure of the tab glue must be guaranteed to prevent the tab metal from short-circuiting from the external aluminum layer. 

3.7 Nylon layer nylon layer

3.7.1 Nylon layer delamination phenomenonWhen the aluminum-plastic film is formed, the nylon layer and the Al layer will be delaminated. Once heat-sealed, bubbles or delamination will be found at a certain corner or folded edge. This is the delamination of the nylon layer and the aluminum layer. There is no problem with other edges and corners (if there is a problem with the aluminum-plastic film itself, all edges will have problems). 

3.7.2 Reasons for stratification

The nylon layer is over-extended during the molding process, and shrinks after heat sealing. When the shrinkage force is greater than the bonding strength between the nylon layer and the aluminum layer, delamination occurs. 

3.7.3 Solution(1) Reserve more aluminum-plastic film for the layered sides(2) Polish the R angle of the mold on the layered side (if possible, the R angle can be appropriately increased) and try to achieve mirror polishing(3) When stamping, adjust the clamping force of the mold on the layer side to be smaller, which is beneficial for the aluminum-plastic film to be pulled down during stamping to reduce the tensile burden of the nylon layer.(4) When heat sealing, push as flatly as possible and do not use too much force when pressing the top angle. The smaller the top angle, the greater the force, the greater the burden on the nylon layer.(5) Storage conditions of aluminum-plastic film: avoid direct sunlight, humidity RH ≤ 70%, temperature ≤ 40°C, otherwise the effective substances formed on the nylon surface will escape. 

3.8 Analysis of abnormal leakage

(1) Forming: Forming breaks through 4 corners(2) Battery assembly: The internal force of the battery body is T1, and the heat-sealing adhesion of the aluminum-plastic film is T2. When T1 ≥ T2, the four corners of the upper part of the battery cell are prone to damage and may leak after heat sealing.(3) During heat sealing: a) Improper mold design during heat sealing causes damage to the Al layer, resulting in leakage; b) Insufficient space between the battery and the mold during heat sealing causes stratification or even leakage; c) Insufficient heat sealing conditions (time, pressure, temperature) may cause leakage(4) The electrolyte injection remains in the seal, resulting in insufficient thermal strength: When overmelting (CPP layer forms crystals on CPP layer, and the adhesion is very tight), CPP/CPP>CPP/Al, a tearing surface will be generated, one side is white CPP layer, and the other side is bright Al layer. Normally, CPP/CPP<CPP/AL(5) The tabs are corroded by the electrolyte for a long time and leak: Al surface treatment, if not treated HF is corrosive to Al; CPP is too thin to compensate for the gap between the metal strip and the tape.(6) Excessive edge removal causes damage to the heat seal, resulting in leakage

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