Long applied in metals and paper production processes, technologies for measuring flatness, tension and thickness are increasingly finding a new field of application in helping to meet the rapid growth in demand for lithium batteries.

The global shift to renewable energy sources is leading to a boom in electrification and energy storage worldwide. Lithium-ion batteries in particular — rechargeable, lightweight, energy-efficient and boasting a high energy density — will be part of a cleaner energy future.

The applications are important too. Demand for consumer electronics, like laptops, tablets and smartwatches, will continue to grow, making demands for battery power that lithium-ion is well placed to meet. The nascent electric vehicle (EV) market is possibly the most significant factor. The Economist suggests that, in the coming years, the proportion of vehicles powered by batteries will grow quickly, with China leading the way.

Thus, lithium-ion-based battery solutions will most likely dominate in the near future. With yearly battery demand growth forecasted to be more than 25%, one estimate suggests that more than 100 giga-size factories need to be built to keep up with demand until 2030.

Another important factor is cost. The combination of lower raw material prices for lithium and cobalt and economies of scale in battery production is driving down costs. This in turn fuels further growth, as more and more applications become economically feasible.

However, the challenge for manufacturers is to keep up with demand and, despite lower raw material prices, to manage costs; this is a highly competitive market.

Currently, battery supply is very tight, and in turn is driving interest in equipment for measuring and controlling flatness, tension and thickness, which play a crucial role in ensuring the final quality of the finished battery.

A force for quality

In a highly competitive and demanding manufacturing environment such as battery production, even small improvements in equipment performance can make a big difference. In terms of the battery manufacturing process, the primary applications demanding accurate measurement of flatness, tension and thickness are found on the downstream side, which compromises various stages including slurry mixing, electrode manufacturing and cell assembly, including winding and stacking.

The process starts with copper and aluminum foil of thicknesses of up to 0.2mm down to 10 µm or even thinner being coated with slurry used to create the anode (copper) and cathode (aluminum) electrodes.

The coated foils are then dried at temperatures from 20 to 150°C, before being passed to the calendering process, where they are compressed and compacted to achieve a consistent thickness and improved energy density of the battery.

Following these stages, the electrode coils are slitted into strips to be rolled up and stacked into packages and placed in a sleeve filled with electrolytes to allow electricity to flow.

Cell assembly is where the separated anode and cathode sheets are either wound or stacked, always alternating with a separator sheet between them. The assembled cell is packed into a pouch foil and filled with the electrolyte. The resulting pouch cells can be rolled or stacked in different configurations before they are finally placed into a housing, which is filled with electrolyte again.

Cell finishing involves processes such as pressing, high-quality formation, degassing, aging and testing. Throughout these processes, it is essential to control both the tension and flatness of the foils. With machines in the processes moving at typical speeds of 250ft per minute (76m) or more, there is risk of issues that could lead to foil strips breaking or that could cause potential imperfections or damage that could affect the final quality of the finished battery.

One problem is wrinkling of the foils that can occur if they are not correctly tensioned and flattened. If the foils are wrinkled, it can result in uneven distribution of the electrode slurries. As well as reducing battery performance, this could also present a safety risk, with the potential for batteries to explode. For this reason, and to avoid potentially costly product recalls, it is vital to ensure accurate application of tension and flatness throughout the production stages to minimize the risk of the foil becoming distorted.

Accurate web tensioning is also important during the cell assembly stage. Two alternative methods are used at this stage, specifically winding, or stacking. During the winding process, the anode foil is overlaid onto the cathode foil and a separator is inserted to prevent short circuits and enable the flow of ions during operation. Accurate tensioning is needed to ensure the foils and the separator are properly aligned. Failure to align them properly, or to apply the correct tensioning can both result in impaired performance and increase the risk of a short circuit that could affect the safety of the finished battery.

The other method, stacking, involves cutting the anode and cathode sheets and stacking them into alternate layers. As with the winding process, accurate tension control is key to ensuring that the sheets are positioned correctly to ensure optimum performance of the final assembled battery.

Avoiding production disruption

Satisfying the rapidly escalating demand for batteries calls for production lines that can be maintained at full operation with minimal downtime. As well as inspection checks and repairs and replacements for maintenance purposes, this downtime can also be incurred by the need to check and, if necessary, adjust the calibration of measurement equipment to ensure their continued accuracy.

In tension measurement applications, for example, typical load cells based on strain gauge and LVDT (Linear Variable Differential Transformer) technologies can be impacted by a variety of factors, including vibration, unexpected shock loads, electrical interference and incorrect specification and installation. This can increase the frequency of calibration checks, and/or recalibration, requiring the affected production line to be put out of service for several hours for the necessary work to be carried out.

This issue can be overcome by opting for devices using technologies that can function for long periods without drift or loss of calibration. Based on magnetoelastic transducer technology, load cells do not require a physical movement in the transducer, enabling it to withstand vibrations, shocks and overloads that can affect other sensor types. They are also impervious to the ingress of dirt and fluids that could impede performance. By eliminating the need for recalibration and enabling measurement accuracy to be maintained, the load cells can help to maximize yields by reducing disruption to production.

These same benefits apply to the flatness and thickness gauges, which both utilize highly stable measurement techniques that avoid the need for recalibration and are unaffected by external disturbances such as vibration and ambient temperatures.

Minimizing the risk of recalls

The importance of eliminating problems that could affect the quality of the assembled battery is highlighted by the potential implications of safety recalls in the event of a fault. In 2016, a major electronics producer recalled 2.5 million of its smartphones from across 10 countries after a flaw was discovered in the batteries used to power the devices that could cause overheating. To placate customers, the company issued replacements for each of the phones, resulting in its manufacturing lines being tied up for two weeks to produce the new units.

There have also been instances involving overheating and fire risks presented by faulty batteries used in everything from laptops to cars and even aircraft, highlighting the need for the highest levels of accuracy, reliability and consistency during the battery production process. Furthermore, with battery manufacturers held liable for the consequences of a failed battery even if the fault was due to improper handling or mistreatment by customers, there is the added impetus to ensure that every step is taken during production to achieve the required standards of quality.

Rolling out a new future for energy

The growing role of lithium-ion batteries in providing a renewable source of power to support everything from electronics to transportation and microgrids will see global demand continuing to increase, with projections from Statista estimating an elevenfold rise between 2020 and 2030¹.

As lithium-ion batteries are used to power an expanding range of products, manufacturers will need to ensure they are making the right choice when it comes to selecting the best measurement instruments for their processes. When it comes to selecting measurement and control technologies for flatness, tension and thickness applications, making the right choice will ensure both the quality and safety of one of the critical components in EVs and consumer electronics, potentially saving billions of dollars and minimizing the risk of injuries from defective products.

¹ Statista - Lithium-ion batteries - statistics & facts - https://www.statista.com/topics/2049/lithium-ion-battery-industry/