Laser Welding in EV battery market - PhotonWeld

Growing EV battery market demands better joining solutions: Wobble laser Welding is the best

Laser Beam Welded Joints for Lithium-Ion Batteries

The growing electrification of vehicles and tools increases the demand for low resistance contacts. Today’s batteries for electric vehicles consist of large quantities of single battery cells to reach the desired nominal voltage and energy. Each single cell needs a contacting of its cell terminals, which raises the necessity of an automated contacting process with low joint resistances to reduce the energy loss in the cell transitions. A capable joining process suitable for highly electrically conductive materials like copper or aluminium is the laser beam welding.

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Over the past years, the demand for large battery packs for electric vehicles (EV) has steadily increased with the ongoing electrification of the transportation sector and a growing demand for greater ranges. State of the art EV battery packs consist of a large quantity of cells connected in series to achieve the desired voltage level and in parallel in order to enable higher charge- and discharge-currents.
For example, the EV Tesla Model S comprises of total count of over type 18,650 battery cells inside its battery pack. A single defective connection can lead to failure or a reduction in performance.
The quality of the joint has a decisive influence on the sustainability and safety of electric vehicles:  Increased resistance at a welded joint causes more heat loss at this spot and leads to an increased electrical and thermal load on the individual cells, which in turn can lead to failure or accelerated aging.

Laser beam welding is a promising technology to contact battery cells enabling automated, fast and precise production of conductive joints. In comparison to other conventional welding techniques, such as resistance spot welding, the laser beam welding has a reduced thermal energy input. Compared to ultrasonic welding, the laser beam welding technique does not induce a mechanical force. The resulting transition resistances are in the range of the basic material resistances. The overall performance of the battery pack is therefore improved by the reduction of the ohmic resistance of the joints and heat loss inside the battery cell.

High currents must flow through the welds between battery cells in order to deliver the electricity needed to power a battery electric vehicle. These welds are the bottleneck of the electric circuit. Electrical resistance causes the temperature in the welds to raise when a current is conducted. This temperature increase may be harmful to lithium-ion battery cells. Therefore, larger weld areas which are created with our wobble laser system, and thus lower resistance.  Thewelds made by wobble welding system increase the mechanical strength of the welds drastic, and reduce the temperature and thermal stress at the joints. Considering this, Wobble Laser Welding is much more suitable for battery tab joining than other types of welding.

Furthermore, laser beam welding produces a small heat-affected zone. Hence, it is crucial to understand how much heat is generated in the weld and whether the heat can damage the battery. Lithium-ion batteries must operate within a safe and reliable operating area, which is restricted by temperature and voltage windows. Exceeding the restrictions of these windows will lead to rapid attenuation of battery performance and even result in safety problems.

In the context of production, laser beam welding is well suited to be integrated into almost fully automated production lines in the manufacturing process of battery packs and EVs. 

Using Lasers for Battery Tab Welding Applications

Battery Tabs Welding with laser

From a welding perspective, the most important aspects of tab welding are the thickness and material of both the tab and the terminal. Conductivity is the name of the game, so battery tabs are generally made of aluminum or copper, sometimes plated with nickel or tin. Terminals may be cold rolled steel, aluminum, or copper, depending upon the physical size of the finished battery.

The most common battery types are cylindrical lithium ion cells around the size (18 mm x 65 mm), large prismatic cells, and lithium polymer pouch cells. Each cell type has a different set of welding requirements.

Cylindrical batteries

The key to welding the cylindrical cell type lies in the negative terminal weld, where the battery tab is welded directly to the can as opposed to the separate platform on the positive side. The weld on the negative terminal must not penetrate the can thickness which is typically around 0.3mm. The thickness of the can dictates how thick the tab can be – a rule of thumb is that the tab should be 50-60 % that of the can. Cylindrical battery can material is usually nickel-plated steel, and the tab material nickel or tin-coated copper. Nickel plating is preferred over tin because it is more stable; tin’s very low boiling point can lead to weld porosity and excessive spatter.

Large prismatic batteries

These high capacity cells need thick tabs to ensure a sufficient current carrying cross-section to deliver the pack output. However, the tab connection needs only to deal with the capacity of a single cell. Therefore, thinning or “coining” of the thick tab material to enable a lap weld or creating a through hole for a fillet weld greatly reduces the size of the weld needed. This in turn reduces heat input to the can, which is always a concern when welding thicker tabs.

For a lap weld geometry, reducing the tab thickness to a 0.25-0.5 mm thickness enables sufficient weld area for strength and capacity while keeping the temperature during the weld low enough to avoid battery damage. Material selection is generally aluminum for both terminal and tab – recommended tab materials are and .  Avoid aluminum alloy , which cracks when welded. If this material is already specified and cannot be changed, use a pre-form as a third material which will introduce a large amount of silicon into the weld, which prevents weld cracking.

Lithium polymer batteries

These pouch type cells, which are thin with a rectangular footprint, are really gaining traction for consumer electronics. The terminals on these batteries are made up of thin layers of copper and aluminum foil which are laser welded to tab of copper and aluminum respectively. This weld is traditionally made using ultrasonic technology due to the need to weld through a stack of foil, however, fiber laser welders are now being used for increased weld quality and strength.

The key to success in welding polymer batteries with a fiber laser is making sure that the foils are in close contact and you’re using a pulsed laser or even better a wobbling laser to avoid overheating.

Welding battery Tabs to battery Terminals

From a welding perspective, the important aspects of tab welding are the thickness and material of both the tab and the terminal.

Resistance welding is suited to welding nickel tab material up to 0.4 mm thickness, and nickel or steel clad copper tab material to around 0.3 mm thickness to a wide variety of terminal materials.

Laser welding is able to weld both thin and thick tab materials, with a capability of welding copper based or bi-metal tab material above and beyond 1.5 mm thickness

Although able to weld both thin and thick tab materials, laser welding is particularly well suited to addressing the needs of high power battery welding. The tab material used in the development of high power cells must be able to accommodate the associated higher capacities and power levels. In order to provide effcient energy transfer, a tab thickness of minimum 0.3 mm or greater is required, as is the use of more conductive materials. For high power lithium ion cells, the terminal material for certain battery manufacturers is different. Therefore the need for bi-metal and smart terminal design solutions is required. Defining the optimal tab material may require some development work both on the welding and material costing. In these cases, the laser is an invaluable tool that offers outstanding welding performance and flexibility.

Battery Pack Manufacturing Solutions

When planning an automated or semi automated solution based on our Wobble cube, the primary factors to consider are loading/unloading, motion and tooling that fit the planned production flow and production rate.
Loading and unloading can range from manual to conveyer or pick-and-place, motion options center around whether the laser head or the part will be moved, with options including XYZ tables and gantry’s or robotic manipulators. For tooling, the laser is non contact, so tooling of the parts can be achieved either by using a fixture that the batteries and tabs are loaded into, or using actuated tooling that is deployed prior to the welding process.
The most suitable technology and process for battery pack manufacture relates to a number of factors including the pack size, thickness and material of the tab itself, and the necessary production rate.  Laser welding processes enable high quality volume production, and, of the two joining technologies today used, spot welding and laser welding, the selection is usually made based on the specific requirements in each situation, but laser welding is taking over very fast from the spot welding, especially with the excelent wobble laser welding technology.

High efficiency lithium battery tabs laser welding machine

1. Welding of battery explosion-proof valve

The explosion-proof valve of the battery is a thin-walled valve body on the battery sealing plate. When the internal pressure of the battery exceeds the specified value, the valve body of the explosion-proof valve ruptures to prevent the battery from bursting. The safety valve has an ingenious structure, and this process requires extremely strict laser welding technology. Continuous laser welding can achieve high-speed and high-quality welding, and welding stability, welding efficiency and yield can be guaranteed.

2. Welding of battery tabs

The tabs are usually divided into three materials. The positive electrode of the battery uses aluminum material, and the negative electrode uses nickel material or copper nickel-plated material. In the manufacturing process of power batteries, one of the steps is to weld the battery tabs and poles together. In the production of the secondary battery, it needs to be welded with another aluminum safety valve. Welding must not only ensure the reliable connection between the tab and the pole, but also requires a smooth and beautiful weld.

3. Spot welding of battery poles

The materials used for the battery poles include pure aluminum tape, nickel tape, aluminum-nickel composite tape, and a small amount of copper tape. The welding of battery electrode strips generally uses pulse welding machines. Due to its good beam quality and small welding spot, Modulated CW lasers or QCW quasi-continuous lasers are suitable for high reflectivity aluminum strips, copper strips and narrow-band battery strips (polar strip width (Under 1.5mm) welding has unique advantages.

4. The power battery shell and the cover plate are sealed and welded

The shell materials of the power battery are aluminum alloy and stainless steel (stainless and acid-resistant steel). Among them, aluminum alloy is mostly used, generally aluminum alloy, and a few use pure aluminum. Stainless steel is a laser weldable material, especially 304 stainless steel, whether it is pulsed or continuous laser, it can obtain welds with good appearance and performance.

5. Power battery module and pack welding

The series and parallel connections between power batteries are generally completed by welding the connecting piece and the single battery. The positive and negative electrodes are made of different materials. Generally, there are two kinds of materials: copper and aluminum. Ultrasonic welding was usually used before, but its being replaced by laserwelding due to the regulary mechanical damage in the battery resulting from the ultrasonic vibrations. Copper and copper, aluminum and aluminum are generally used. Using laser welding. Both copper and aluminum conduct heat very quickly and have a very high reflectivity to the laser. The relatively large thickness of the connecting piece requires a higher power laser welding.

As a high-precision and high-efficiency welding technology, battery laser welding plays an important role in the manufacture of lithium battery packs. This article will introduce the application of battery laser welding technology in lithium battery packs, and discuss its advantages and future development prospects.

1. Advantages of battery laser welding technology

The application of battery laser welding technology in lithium battery pack including ternary lithium battery and lifepo4 battery has the following advantages:

High-precision welding: Battery laser welding can achieve micron-level weld seam control, making the welding connection more uniform and reliable.

Small heat-affected zone: Battery laser welding has the characteristics of high energy density and rapid heating, which makes the heat-affected zone extremely small and reduces thermal damage to surrounding materials.

High efficiency and automation: Laser welding has the ability of high-speed welding and automatic operation, which can greatly improve production efficiency and quality stability.

No need for welding materials: Battery laser welding is a non-contact welding technology that does not require additional welding materials, reducing material costs and pollution.

2. Key technology of battery laser welding in lithium battery pack production line

Lithium battery laser welding machine battery module automatic production line, generally including battery loading, scanning code, testing, cleaning, sorting, module stacking, stacking inspection, module welding, welding inspection, module unloading and other processes .

Material transfer system, self-adaptive system, visual positioning system, MES manufacturing execution management, etc. are the key technologies in battery laser welding, and also important technical support for adapting to small-batch and multi-variety production forms.

Material transfer system

From cell loading to final module unloading, the entire material transfer is completed through the material transfer system. The material transfer system can also flexibly expand the station according to the adjustment requirements of the process. The transfer between different stations does not require manual operation.

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The module positioning plate has a product size adjustment mechanism, which can adapt to the clamping of modules of different sizes, and is very suitable for the production needs of small batches and multiple varieties.

Self-adaptive system

In the production process of battery packs, pouch, prismatic, and cylinders are the most common types of incoming battery packs. After batteries of different types and sizes are stacked into battery packs of different sizes, they need to be adapted after each process. Adaptive system to ensure the linkage of the whole line beat. Especially in the welding process, the packing process can only be completed if modules of different sizes are adapted.

The self-adaptive system adopts multi-axis combined linkage to implement position positioning in the product processing area, and is not restricted by any form of incoming materials, completes the welding work and transmits it to the next process.

Visual positioning system

Cell welding surface cleaning, module marking, and busbar welding are usually done by laser processing. After the battery module is assembled, the dimensional tolerance is often large, and it is difficult to meet the gap position size requirements for laser processing, resulting in a rapid decline in processing quality.

The introduction of the visual positioning system can meet the needs of precise positioning, and the accuracy can reach ± 0.05mm. Through visual camera data collection, and feed back the deviation of incoming materials to the control system, high-precision positioning of the processing position is realized.

MES manufacturing execution management system

From cell loading to final module unloading, the parameters, data, and other incoming material information of each process can be quickly queried and analyzed in a timely manner through the MES system, truly achieving process controllability and high productivity.

The process data package in the battery laser welding process is directly integrated in the MES system. In order to facilitate users to call and switch, the whole set of MES system can directly build the production line into a quasi-unmanned production workshop, and manual labor only needs to replenish materials at the periphery, which improves safety.

3. Application of battery laser welding in lithium battery pack

The application of battery laser welding technology in lithium battery pack mainly includes the following aspects:

Battery cell connection: Battery laser welding can realize the connection between battery cells to ensure the reliability and stability of the connection. Through laser welding, high-strength welds can be achieved, improving the overall performance of the battery pack.

Battery pack connection: Battery laser welding can be used to different battery hookup, ensuring electrical and mechanical connection between modules. The high precision and high efficiency of laser welding make the module connection more firm and reliable.

Conductive sheet connection: Laser welding can be used to connect conductive sheets and battery cells to realize current transmission. The high energy density of battery laser welding can quickly complete the welding process and ensure the stability and conductivity of the connection.

4. Application and welding advantages of laser welding equipment in energy storage batteries

The energy storage battery is a whole composed of battery energy storage equipment, PCS and filtering links. In the field of laser welding of energy storage batteries, pulsed lasers, continuous lasers, and quasi-continuous lasers are currently the most used.

Pulse laser: YAG laser, MOPA laser;

Continuous laser: continuous semiconductor laser, continuous fiber laser;

Quasi-continuous laser: QCW laser series.

The welding advantages of laser welding equipment in terms of energy storage batteries include the following aspects:

The welding process is non-contact welding, and the internal stress of the welding rib is reduced to the minimum during the welding process;

The welding process does not produce other flashes and other released substances to prevent secondary pollution;

The strength and airtightness of the welding are high, which can meet the functional requirements;

Battery laser welding can meet the welding between different substances, and can also realize the connection technology between membrane materials and heterogeneous substances;

Battery laser welding is convenient for automatic integration, and can also achieve synchronous laser welding process schemes according to production capacity needs, with high efficiency and small welding internal stress;

The structure involved in battery laser welding is simple and convenient, and the difficulty factor of the mold structure is reduced;

The welding process can realize digital intelligent monitoring, which meets the needs of data visualization in the welding process;

This type of welding process scheme can be effectively integrated with automated production lines, meeting the needs of mass production schemes, achieving high-efficiency production, and low consumption.

5. The future development prospect of battery laser welding

The application of laser welding technology in lithium battery packs is still developing and has broad prospects:

Further improve efficiency: With the continuous innovation and improvement of laser welding technology, its welding speed and quality will be further improved, further improving production efficiency and reducing costs.

Adapt to a variety of materials: Battery laser welding technology can be applied to different types of materials, including different battery cells and conductive sheet materials, which improves applicability and flexibility.

Develop intelligent production: The combination of laser welding technology and intelligent production can realize automatic operation and real-time monitoring, and improve production efficiency and quality control.

6. Conclusion

The application of battery laser welding technology in lithium battery packs has a wide range of advantages, including high-precision welding, small heat-affected zone, high efficiency and automation.

With the continuous development and innovation of technology, the application prospect of battery laser welding in the manufacture of lithium battery energy storage pack is very broad.

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