The performance of EV battery cells decrease with use over time, meaning that manufacturers typically only warranty the battery for around 8 years. This is less than the average lifetime of a car. The reasons for this loss of performance is from the “degradation of battery materials,” that is, the complex mix of chemical and mechanical processes that weaken the battery’s ability to store energy.
Scientists have an incomplete understanding of degradation processes and measuring these changes as they happen within operating batteries is problematical. The traditional method has been to open up a cell after degradation has taken place to analyse internal components, but this is destructive to the cell. Batteries with high nickel cathodes (that cell manufacturers are moving towards to increase the range of EVs) are highly air- and moisture-sensitive, so destructively opening these cells for analysis is particularly problematic.
The challenge was to develop a measurement technique that can detect chemical and mechanical changes as they take place in a battery as it charges and discharges. The ideal sensor would be non-destructive, not interfere with the normal operation of the cell, yet be sensitive enough to detect the slow and subtle chemical changes taking place.
This is a great way to peer inside the black box, finding out why batteries fail and how to treat them right. Rather than just probing a single material, real-time techniques provide an understanding of how the various materials function cooperatively, crucially within working batteries.”
Dr Ermanno Miele, University of Cambridge.
A multidisciplinary research team at the University of Cambridge, working as part of the Faraday Institution’s Battery Degradation project, has developed a technique to embed hollow optical fibres within a battery cell that allows a chemical analysis technique called Raman spectroscopy to be performed on a small sample of electrolyte using an external laser source.
A protype probe has been built that has proven capable of measuring small variations in the electrolyte composition in commercial pouch cells as it is charged and discharged. The liquid electrolyte is critical to the performance of batteries, and its chemical composition can give an insight to the reactions underway within the cell and the health of the system overall.
Use of the Raman spectroscopy while the battery is in operation is allowing researchers to improve their understanding of the effect on the electrolyte of lithium-ion transport, investigate chemical reactions at the electrode interfaces, and can be used to monitor the formation of solid electrolyte interphase layers on the electrodes, which are critical to long term battery performance.
Critically, the probe can be embedded in battery designs relevant to industrial uses with minimal disturbance to the system.
A UK patent has been filed to protect the invention.
Commercialisation partner Cambridge Enterprise is currently looking to work with an instrumentation company to jointly develop and commercialise the technology.