Lithium-ion batteries could be charged five times faster
UK researchers have developed a precise test of lithium-ion batteries’ internal temperatures and their electrodes’ potentials, and found that the batteries can be safely charged much faster than the current recommended charging limits.
If a battery becomes overheated, it risks severe damage — particularly to its electrolyte — and can even lead to dangerous situations where the electrolyte breaks down to form gases that are both flammable and cause significant pressure build-up. Overcharging of the anode can lead to so much lithium electroplating that it forms metallic dendrites and eventually pierces the separator, causing an internal short circuit with the cathode and, subsequently, catastrophic failure.
In order to avoid this, manufacturers stipulate a maximum charging rate or intensity for batteries based on what they think are the crucial temperature and potential levels to avoid. However, internal temperature testing (and gaining data on each electrode’s potential) in a battery has previously proved either impossible or impractical without significantly affecting the batteries’ performance.
As a result of this, manufacturers have had to rely on a limited, external instrumentation which is unable to provide precise readings. This has led them to assign very conservative limits on maximum charging speed or intensity to ensure the battery isn’t damaged and doesn’t suffer catastrophic failure.
Now, researchers at the University of Warwick have developed a range of methods that allow direct, highly precise internal temperature and ‘per-electrode’ status monitoring of lithium-ion batteries of various formats and destination. The technology works in situ during a battery’s normal operation without impeding its performance, provides more precise data than external sensing methods and has been tested on commercially available automotive-class batteries.
The technology employs miniature reference electrodes and Fibre Bragg gratings (FBG) threaded through bespoke strain protection layer. An outer skin of fluorinated ethylene propylene (FEP) was applied over the fibre, adding chemical protection from the corrosive electrolyte. The result is a device that can have direct contact with all the key parts of the battery and withstand electrical, chemical and mechanical stress inflicted during the batteries’ operation while still enabling precise temperature and potential readings.
“This method gave us a novel instrumentation design for use on commercial 18650 cells that minimises the adverse and previously unavoidable alterations to the cell geometry,” said Associate Professor Rohit Bhagat, a co-author on the study. “The device included an in situ reference electrode coupled with an optical fibre temperature sensor. We are confident that similar techniques can also be developed for use in pouch cells.”
Writing in the journal Electrochimica Acta, the team from the Warwick Manufacturing Group (WMG) revealed that current commercially available lithium batteries could be charged at least five times faster than the recommended maximum rates of charge. Their technology is expected to enable advances in battery materials science, flexible battery charging rates, and thermal and electrical engineering of new battery materials/technology, and it has the potential to help the design of energy storage systems for high-performance applications such as motor racing and grid balancing.
“This could bring huge benefits to areas such as motor racing, which would gain obvious benefits from being able to push the performance limits, but it also creates massive opportunities for consumers and energy storage providers,” said Dr Tazdin Amietszajew, who led the research.
“Faster charging as always comes at the expense of overall battery life, but many consumers would welcome the ability to charge a vehicle battery quickly when short journey times are required and then to switch to standard charge periods at other times. Having that flexibility in charging strategies might even, further down the line, help consumers benefit from financial incentives from power companies seeking to balance grid supplies using vehicles connected to the grid.”
Dr Amietszajew stated that the technology is ready to apply now to commercial batteries, though the researchers would need to ensure that these vehicles’ battery management systems are able to accommodate variable charging rates. The WMG team have also conducted further research on the subject of battery sensing, according to Associate Professor Bhagat, who says they hope to publish their work on other innovative approaches within the next year.
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