1.2V NiMH batteries

Key production processes and control factors of 1.2V NiMH batteries

1. Stirring

It can also be called mixing, homogenization, slurrying, batching, etc. The process of preparing key components such as positive and negative active materials, conductive agents, binders, solvents, etc. into non-Newtonian fluids with certain viscosity requirements and particle size requirements through a certain order of adding materials, stirring process, vacuum control, temperature control and other conditions. The slurry properties that need to be controlled are good fluidity, and the viscosity and particle size meet the coating requirements.

2. Coating

Some companies call it coating. Coating is the process of evenly coating the stirred slurry on the current collector and drying it to make a pole piece. The coating methods include continuous coating and intermittent coating. The main methods of coating lithium-ion battery slurry are direct coating with a scraper, roller coating with a scraper, and slit extrusion coating. The control parameters of the coated product are surface density, adhesion, moisture content, etc.

3. Pressing

Pressing is the process of passing the coated product through two steel rollers at a certain gap and pressure to compact the electrode to a specified thickness. The factors affecting the pressing include feed angle, gap value, pressure value, pressing speed, winding tension, electrode temperature, etc. The purpose of pressing is to further compact the loose and porous electrode, reduce the contact resistance between materials, and increase the battery capacity within a certain battery volume. At the same time, it cannot be over-pressed to ensure the electrolyte infiltration effect on the electrode.

4. Slitting

Slitting is also called stripping. The coated electrode is wide and needs to be cut into multiple strips. The slitting product is mainly affected by the quality of the cutter, the angle of the cutter and the tension.

5. Tab Forming

The process of shearing the electrode by controlling the meshing between the upper and lower cutting dies of the equipment to form the tab according to the design size requirements.

6. Electrode Defects

Electrode defects occur at various stages of electrode production. During coating, thick head, thin tail, thick edge, exposed foil, unstable surface density, horizontal and vertical stripes, dry material, pinhole shrinkage, white spot, pitting, foreign matter, etc. are easily produced. The specific reasons are analyzed in detail. The main defects of rolled electrode sheets are uneven winding, wrinkled edge, thickness rebound, material drop and sticking to roller, particles, unstable thickness, etc. The defects of electrode sheet slitting include wavy edges, burrs, curling, material drop, etc.

7. Winding

Winding is a way of composing battery cells, suitable for cylindrical batteries, square batteries and soft pack batteries. By controlling the speed, tension, size, deviation and other factors of the equipment, the negative electrode sheet, positive electrode sheet and diaphragm with matching size after slitting are rolled into a bare battery cell. The key point of winding is that the diaphragm wraps the positive and negative electrodes, and the negative electrode sheet wraps the positive electrode sheet. This requires that the negative electrode sheet cannot be exposed to the foil, which will cause local lithium deposition on the sheet to pierce the diaphragm and cause a short circuit.

8. Lamination

Another way of forming laminated cells, suitable for square and soft-pack lithium-ion batteries. The process of stacking the punched negative electrode sheet, positive electrode sheet and diaphragm into a bare cell through the control equipment. It is also required that the diaphragm wrap the positive and negative electrodes, and the negative electrode sheet wrap the positive electrode sheet.

9. Hot Pressing

Hot pressing is also called shaping. The cell after winding is in a bulging state, which is not conducive to shell entry and electrolyte infiltration. It needs to be hot pressed to promote the contact between the electrode sheet and the diaphragm, while reducing the volume and preventing the misalignment of the positive and negative electrode sheets. The factors affecting hot pressing include pressure, temperature, time, etc., and 60~80℃ hot pressing is generally selected.

10. Vacuum Baking

Vacuum baking is divided into electrode sheet baking and cell baking, both of which are to control the moisture content of the cell. Moisture can be fatal to 1.2V NiMH batteries. When moisture comes into contact with the electrolyte, the hydrofluoric acid formed will cause great damage to the battery, and the generated gas will also cause the battery to bulge. The baking efficiency is affected by the vacuum degree, drying temperature, and time. By adjusting the baking process, it is possible to dry efficiently with low energy consumption.

11. Tab Welding

Whether it is winding or lamination, the tabs and the current collector need to be welded. When the positive and negative tabs are welded to the current collector, the strength of the welding must be ensured to prevent the tabs from falling off. The tab welding method generally uses ultrasonic welding. Its principle is that under auxiliary pressure, high-frequency vibration waves are transmitted to the two objects to be welded through the welding head and welding seat. The two contact surfaces to be welded rub against each other, and the molecules diffuse with each other to form molecular fusion and weld together. The strength of ultrasonic welding is affected by welding pressure, amplitude, frequency, time, welding machine stability, welding head quality, tooling, material hardness, etc.

12. Packaging

The packaging forms mainly include metal shell packaging and aluminum-plastic film packaging. Metal shells include steel shells and aluminum shells, which are suitable for cylindrical batteries and square batteries, and aluminum-plastic film packaging is suitable for soft-pack batteries. For 18650 cylindrical batteries, the packaging steps are to put the winding core into the steel shell, then perform spot bottom welding, groove rolling, and liquid injection, and then perform laser welding capping and mechanical sealing. For square batteries, the packaging steps are to perform laser welding sealing after entering the shell, and then complete the final sealing after the liquid injection hole is filled.

The difference in soft-pack battery packaging is that it uses aluminum-plastic film packaging, which consists of three layers of material, PP layer, aluminum layer and nylon layer. The key to packaging is to melt the PP layer and bond it together. Heat sealing is mainly affected by temperature, heat sealing time, and heat sealing pressure. It is necessary to ensure that there is no poor heat sealing or over-sealing. Pay special attention to the top seal at the tabs, ensuring that the PP layer of the aluminum-plastic film is well bonded and sealed with the PP layer on the outer surface of the tabs, and that the PP layer of the aluminum-plastic film in the non-tab area is well bonded and sealed face to face. In addition, compared with metal shell batteries, aluminum-plastic film soft-pack batteries need to be covered with a protective film after heat sealing to prevent external substances from scratching the aluminum-plastic film.

13. Leakage Test

The process of detecting whether the battery has leakage through negative pressure detection method or helium detection method. Leakage detection is very necessary. Poorly sealed batteries will introduce moisture, impurities, etc., causing side reactions with the electrolyte in the battery, causing the battery to be scrapped.

14. Electrolyte Injection

The electrolyte is the channel for lithium ions to move inside the battery, mainly composed of solutes and solvents. The solute is lithium hexafluorophosphate, and the solvent is generally selected to be used in combination of three or more solvents, such as EC/DMC/DEC. The injection process is to inject the electrolyte into the battery cell. The parameters to be controlled mainly include the amount of electrolyte, injection pressure, time, etc.

15. Formation

Formation is the first charge and discharge of the battery after injection and sealing. There are two main purposes of formation: First, after the battery is made, the electrode material is not in the best applicable state, or the physical properties are not suitable (for example, the particles are too large, the contact is not tight, etc.), or the physical phase itself is not right (for example, some alloy mechanism metal oxide negative electrode), and the first charge and discharge is required to activate it. Second, when the lithium battery is charged and discharged for the first time, the electrons reach the surface of the graphite negative electrode through the external path, react with the electrolyte solvent and lithium ions to form a solid electrolyte membrane (SEI), which has an important influence on the performance of the lithium battery. The formation process has a great influence on the performance of the battery, because the charging and discharging current, time and other factors have a great influence on the key parameters such as the formation of high-quality SEI, gas production, and resistance in the battery.

16. Aging

Aging generally refers to the placement of the battery after the first charge and formation after the battery is assembled and filled. It can be either room temperature aging or high temperature aging. Both of them make the SEI film formed after the initial charge and formation more stable in nature and composition, ensuring the stability of the battery's electrochemical performance.

After aging is completed, the battery is finally divided into different capacities, and can be shipped after passing the resistance, voltage drop and other tests.

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