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High-Resolution Battery Mapping Using Graphene-Based Sensors

GIPhotoStock X / Alamy Stock Photo Graphene.jpg
A new game-changing analysis method using graphene-based magnetic sensors makes the battery mapping process far more efficient and accurate.

Lithium-ion battery designs are constantly evolving, with efforts being made to accelerate charging times and fit more energy storage capacity into tighter spaces. But for battery manufacturers, recyclers, and resellers to take advantage of the business opportunities offered by this technology, they need access to detailed data on the batteries they are developing for their customer base. This will help them to differentiate their products, assure elevated levels of quality and reliability, ramp up their output and raise sales incomes.    

The technology currently available for carrying out battery analysis is far from ideal, and this is presenting suppliers with a major obstacle. What is needed is a new, more advanced method for carrying out detailed studies that is also quick, simple, and cost-effective.

What battery manufacturers and recyclers essentially want is to:

  • Make superior products that will give them a competitive edge.
  • Keep production output as high as possible (with minimal downtime) - so that customer demands can be met
  • Maintain elevated operational efficiency levels - curbing costs and raising profitability
  • Secure a reputation for supplying batteries with the best performance and quality

How better battery mapping will make a real difference

Detailed battery mapping will serve several purposes. By having data on the cells that manufacturers have integrated into their batteries, they will get better insight into what is happening inside those cells, rather than having to rely on theoretical modelling. 

During the initial development stages, battery manufacturers will often assess the cells from different vendors. They will look at current density data to see how well these cells are performing once they have been integrated into the battery. This will enable them to see how far they can push these cells while still ensuring safe operation is maintained (avoiding thermal runaway, etc.). If a new design is being experimented with, they will want to look at how the cells being used might be affected. Finally, once in full production, end-of-line tests will need to be conducted - to check that expected quality is maintained.

Alongside this, the recycling business is growing all the time - and companies located there need better battery analysis. When the charge capacities of primary use batteries (such as those in EVs) drop below around 75-80% of their original figure, they will be repurposed. They can go into domestic solar panel systems or peak shaving installations (for commercial sites, schools, hospitals, etc.). But before battery recyclers can resell these batteries, they must scrutinize the units they receive, so that their suitability for second life deployment can be gauged. This will allow them to locate any dead regions that could impact energy storage capacity, as well as identify any safety issues that need dealing with. 

Currently, the analysis of lithium-ion batteries will either be via the use of temperature sensors distributed around the cells to indirectly measure thcurrent densities or shunt resistors attached to the bus bars measuring the current going in and out of the cells. The data derived from these methods gives an incomplete picture, however. 

Getting meaningful data from temperature sensors takes longer because larger changes in the current density are needed for any temperature changes to be detected. Current measurement using shunt resistors also has its drawbacks, as these devices can only be placed at certain points on the batteries. This translates into very limited overall visibility of battery performance, slower operations, and increased downtime when testing equipment fails.

Higher-resolution real-time data       

Graphene-based magnetic sensors take a different approach to battery mapping. This overcomes the drawbacks of traditional techniques, resulting in a much more accurate indication of what is going on within the cells. The benefits of this can be seen throughout the entire supply chain - with cell vendors, battery manufacturers, and battery recyclers all able to gain from it. Paragraf is a company working on this different approach using to develop graphene-based magnetic sensors.

Paragraf’s Graphene Hall Sensor (GHS) devices, derived from a proprietary direct-deposition process that avoids contamination and structural integrity issues, have a sensing element that consists of a graphene monolayer which is only 0.34nm thick. The two-dimensional (2D) nature of these sensing elements eliminates the ‘Planar Hall Effect’ which is present in conventional three-dimensional (3D) silicon-based Hall sensors. Consequently, their performance is not impacted by stray in-plane electromagnetic fields. Thanks to the high magnetic field resolutions that can be achieved (down to sub-µTesla levels), even relatively small changes in current densities can quickly be determined. This means that magnetic fields generated at a granular level can be measured, to determine any current density fluctuations in real-time.

In contrast to shunt resistors and fluxgate devices, which have placement restrictions due to their size or function, the compactness of GHS sensors provides significant improvements in terms of spatial accuracy (with their sensing elements only covering an area of 1.3mm2). This makes these sensors much easier to place at numerous different points across the whole battery, thereby enabling a more comprehensive assessment of the battery. The GHS approach also offers greater built-in redundancy. If one sensor fails, then the rest of the system will keep working. It will only be necessary to replace the one faulty sensor node, not all of them, as would be the case with a shunt resistor arrangement. 

Image Courtesy of ParagrafPARA10 - Array kit copy.jpg

Battery cell testing and analysis can be incorporated into an existing test rig using the Paragraf GHS array starter kit.

Accelerating the test procedures

Data acquisition can be done faster using GHS so that testing can be completed within a shorter timeframe - which means quicker turnaround times and greater throughput. It also provides a quicker response to safety-critical issues (like current shorts) so the battery can be turned off sooner.

GHS sensors can cover all the measurement work needed for battery analysis, whereas others only deal with specific aspects. Consequently, it is no longer necessary for different sensor types to be used for different functions. This simplifies test set-ups significantly, making everything more standardized. Firstly, that means less capital investment on testing equipment. Secondly, the preparation time taken up by technicians between testing (changing set-ups) can be shortened - so more tests can be conducted each day and increase productivity. It also means that errors that will arise by swapping between different sensors can be eliminated.   

As GHS sensors can deliver constantly updated highly detailed information, battery designers will be much better positioned to adjust their design concepts. OEMs will be able to bring batteries to market that can store more energy, run for longer, be recharged faster, be smaller, and weigh less - all of which will make the products powered by the batteries more convenient for the end consumer to use.

Such enhancements will set these cells apart from those of the competition - allowing manufacturers to increase their market penetration. Since design optimization can be done within a shorter period, manufacturers will be able to exploit windows of opportunity quicker.  

Widespread potential

GHS is a solution that is applicable to all the key players in the battery supply chain. Cell vendors can use it for mapping during research phases so that they can make certain there is an even current distribution throughout their cells, and there are no hotspots or other anomalies. The battery manufacturers can then do their investigations once these cells have been integrated into battery units. Having compiled datasets, the manufacturers can then provide valuable information back to the cell vendors they are partnered with. This feedback loop enables manufacturers and suppliers to work together to innovate and develop products in line with what the market needs.

By having testing processes that can be completed markedly faster, recyclers will also gain. It will take less time to qualify the batteries they are looking to repurpose. Batteries could be deployed to customers quicker, and income generated sooner. In addition, with a better understanding of these batteries (their parameters, tolerances, etc.), there is the prospect of a greater proportion of them being reused. Recyclers can therefore have more confidence in the quality of their products they are selling, with potentially new markets opening up for them.

Dr Hugh Glass is responsible for the development of Paragraf’s graphene Hall sensors, ensuring future sensors meet market needs. He holds a Master’s degree in Chemistry from the University of Surrey, focusing on the synthesis of new materials and their use in green energy applications, and a PhD from Cambridge University in the synthesis and characterisation of novel electrode materials for batteries.

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