Bonding for batteries
In the continuing fight against climate change, national targets are being set to reduce carbon emissions and increase the production of electric vehicles. In China, the government intends to mobilise five million ‘new energy’ vehicles on its roads by 2020, while Germany’s federal council, Bundesrat, looks to completely ban the production of internal combustion engines by 2030. As a result, the volume of electric vehicles being manufactured has grown dramatically.
The jury is still out, however, on which of type of battery cell will be powering these vehicles of the future. While this is still being determined, manufacturers are seeking the fastest, most efficient way to produce these cells.
While techniques such as ultrasonic welding have been used to produce battery cells in the past, they have proven to be limited in functionality – and are now being replaced by superior laser welding methods that grant higher flexibility, faster throughput and reduced production costs. One of the world’s largest e-mobility battery manufacturers, BYD, has recently acquired 120 fibre lasers for the purpose of battery production, while Fraunhofer ILT is currently devising new and improved laser welding methods that have the capacity to replace standard ultrasonic techniques.
According to Dr Alexander Olowinsky, group manager of microjoining at Fraunhofer ILT, fibre lasers have been a key tool in reducing the energy input, weld seam widths, and weld depths used in battery production, thanks to their precision and flexibility. ‘Fibre lasers with high beam quality provide a fine focal spot with a diameter less than 40µm or even smaller,’ he explained. ‘A fast modulation technique, combining a linear feed with a circular oscillation, enables a stable control of the weld depth.’ Lasers are therefore a ‘perfect tool in terms of energy input, reliability and productivity,’ Olowinsky said.
Scaling up production
In line with national emission targets, London Transport has pledged to incorporate 300 electric buses into its fleet of vehicles by 2020 – one of which is the world’s first electric double-decker bus, which is currently in operation. The vehicle was designed and built by BYD, a Chinese technology firm, and is powered by the company’s own iron phosphate battery – a safer and recyclable alternative to the lithium-ion batteries that feature heavily in e-mobility. In order to mass-produce these batteries, BYD has installed 120 Trumpf fibre lasers at its battery plant in China – 70 TruDisk lasers and 50 TruPulse lasers.
TruDisk fibre lasers are suitable for the welding of highly-reflective materials such as copper and aluminium. They are used by BYD to weld the connections between the cells in the iron phosphate battery. The TruPulse lasers are then used to seal the battery enclosures to make them completely gas-tight.
Marc Kirchhoff, head of industry and key account management automotive at Trumpf, said these systems allow the user to synchronise power and movement of the laser spot very precisely, allowing very small focal diameters of 80 to 100µm. This level of precision is required in battery production in order to protect the internal components of cells. ‘When you want to seal an aluminium can for a prismatic battery cell, if too much energy is brought in, then the chemical contents of the cell could be destroyed,’ Kirchhoff explained.
‘The welding process must have the least possible impact on the surrounding material,’ said Liu Huaping, process department manager at BYD. ‘The majority of our welding operations involve part thicknesses of no more than two millimetres or even less… fine weld seams, low heat diffusion, low intrinsic stress and minimal distortion are our key requirements.’ Trumpf’s lasers are able to achieve this with a high degree of precision due to the closed loop power control they offer, which enables them to provide a highly stable power output, and therefore a low variation in weld depth to a workpiece.
Two TruDisk lasers in a solid-state system, together with a programmable focussing optic, can form a typical set up for battery welding. ‘By adjusting the focus of the laser beam, we can rapidly move from one weld point to the next,’ said Huaping. ‘This process must not generate too much heat, because this could damage the internal components… the chosen method [therefore] fits the bill perfectly, because it allows time for the material to cool down between successive laser pulses.’
E-mobility is a very fast growing market for Trumpf, according to Kirchhoff. ‘Roughly 10 years from now, we think more than 30 per cent of the cars will be electric,’ he commented. ‘With the carbon emission targets existing around the world in countries and cities, there will be an increasing prohibition of combustion engines in particular areas, so there will be a big change.’
BYD has produced the world's first double-decker electric bus for London Transport (Image: Trumpf)
Researchers at the Fraunhofer Institute of Laser Technology ILT have been developing an alternative method for connecting battery cells using lasers, with the intention of replacing the ultrasonic wire and ribbon bonding techniques sometimes used. The work1, which has been conducted as part of the publicly funded RoBE (Robust Bonds in Electric Vehicles) project, was presented at ICALEO, in San Diego, last October.
According to Olowinsky, ultrasonic bonding has certain limitations. ‘The substrates have to be clean and non-vibrating because the energy input is done using an ultrasonic tool… if the substrate vibrates in the same frequency as the ribbon, then there’s no friction and therefore no connection.’ Ultrasonic welding also only works with ribbons up to widths of approximately three millimetres, he said, which limits the current capacity of each bond due to the small cross section. Using widths greater than this requires more force to create a joint than can be applied with ultrasonic welding.
Tesla has previously used ultrasonic wire and ribbon bonding to connect the approximately 7,800 cells in each of its car battery packs, according to Olowinsky. The technique required at least two strips per battery cell to provide sufficient current capacity, meaning approximately 15,600 wires per battery pack needed to be welded. The small thickness of the wires and ribbons therefore resulted in longer processing times.
Olowinsky and his colleagues are instead using lasers to form ribbon bonds, which removes the limitations of ultrasonic bonding and enables larger widths of ribbon to be used between battery cells. ‘We are considering ribbons as wide as 10mm, which are three times the standard of today,’ Olowinsky confirmed. ‘When we replace the wire by a thicker ribbon, we create a much larger cross section to conduct the current from cell to cell. This enables us to get a better usage of energy from the cells.’ The increased thickness also leads to only a single piece of material being needed to provide sufficient current capacity, leading to reduced production times, as less welds need to be done.
Using a fibre laser of less than 1kW in power, the group has been able to achieve times of 100ms per weld, corresponding to one loop of ribbon welding per second. Applying this to both the positive and negative ends of the cell results in needing only two seconds to connect each cell.
Three major types of battery cell are currently in the running to become the e-mobility standard: 18,650 cells, which are small, cylindrical and offer relatively low energy content, meaning around 10,000 are required to power a standard electric vehicle; pouch bag cells, which offer a much higher energy content and therefore only require 120 cells to provide the same power output; and prismatic cells, which offer a similar power output to pouch bag cells.
‘It is still open for which type of cells will become the standard,’ said Olowinsky. ‘The big question is, which type of battery will win? There will definitely be changes and new battery systems coming in the future, but one thing for sure … from a production technology point of view, even for new types of battery cells… the need for reliable joining technologies remains unaffected.’
Olowinsky also believes lasers will also feature more heavily in other applications, ‘like the drying and functionalisation of slurry and layers, ablation of material, cutting of electrodes and sealing of hermetic packaging.’
He concluded by explaining that, in order to produce the high number of vehicles in the future, a lot more laser installations have to be made within the battery industry, and that this will further stimulate the laser market. ‘More and more logistics as well as [shorter] cycle times will drive the development of the production technology. As we do see lasers as an important tool in this area, electromobility will boost the laser industry.’
Coherent-Rofin's FL series are well equipped to deal with back-reflection in laser processing
Right tool for the job
According to Mathias Schlett, responsible for the key account for the battery industry for high power fibre lasers at Coherent-Rofin, there is currently a trend towards higher production volumes and higher production speeds of batteries, which is in turn demanding higher laser powers. After merging with Rofin-Sinar Technologies late last year, Coherent (now Coherent-Rofin) sees itself in a better position to address these demands.
‘We are prepared to react to the market, to scale up our production of lasers to meet a larger demand,’ said Schlett. ‘We are a larger firm now and can dedicate our service activities to growing markets.’
The company offers its combined Rofin FL and SPS (scanner processing solution) package for the manufacturing of battery cells which, according to Schlett, is gaining more and more traction in the e-mobility industry. ‘The SPS helps to increase productivity,’ explained Wolfram Rath, product line manager of high power fibre lasers at Coherent-Rofin. ‘The scanning device… [offers] lightweight manipulation of the beam, granting high flexibility in the field.’
The SPS can be used alongside single mode lasers with spot sizes between 40 and 80µm, and enables users to perform ‘smart welds’ by manipulating welding results using oscillation techniques to modulate a laser beam spatially.
‘It enables fast jumping from one welding position to the next, so you can have several battery housings in one scanning field and weld one after the other,’ Rath said. ‘Connectors of different shapes can also be welded one after the other.’
The Rofin FL laser, according to Rath, is suitable for processing stainless steel, aluminium or copper connections in battery production, for example the aluminium-aluminium welds of the safety components and electrodes in prismatic cells. The high reflectiveness of the metals often presents a challenge in battery production. However, due to the effects of back reflection and the damage it can inflict on laser equipment. Coherent-Rofin therefore accounts for the issue in the design of its FL series. ‘We have technologies to make the laser robust and heatproof, because temperature rise is the main problem with fibre laser components,’ Rath explained.
‘We then have special techniques to dissipate the back-reflected power and cool the components with water. We also have sensors to detect the back-reflected core power, and a fast switch-off function to protect the laser from damage.’
Asia is regarded by both Trumpf and Coherent-Rofin, as the dominant region for the manufacturing of batteries for e-mobility, which, according to Kirchhoff, mostly takes place in South Korea, Japan and China. ‘This where our main customers for these types of battery cells are located today.’
According to an article in the Financial Times, Chinese battery manufacturers are beginning to gain dominance in the market, which, for the past three decades, has been led by South Korean and Japanese manufacturers such as Panasonic, which manufactures battery cells for Tesla. It is estimated by Goldman Sachs this dominance will be achieved by 2025, by which time the e-mobility battery market will be worth $40 billion.
BYD, while being in the top three companies for e-mobility battery production globally, also produces entire electric vehicles – having sold 11,000 electric-buses last year. The company is currently increasing its production capabilities by building more and more production lines in China, according to Kirchhoff.
While Asia is the dominant region at the moment, a diffusion to other regions, such as the US and Europe, can also be seen, according to Rath. This is because local production will eventually take place once e-mobility becomes more commonplace around the world.
 Connecting battery cells by aluminium ribbon bonding using laser micro welding: Helm et al.