C L E A N E N E R G Y T R A N S I T I O N A N D S U S T A I N A B L E T E C H N O L O G I E S

S C I E N T I F I C H I G H L I G H T S

9 8 H I G H L I G H T S 2 0 2 3 I

Mapping internal temperatures with X-ray diffraction during high-rate battery applications A new technique uses X-ray diffraction to map the internal temperatures of lithium-ion batteries under real working conditions, decoupling the thermal and mechanical strains experienced by the cell during charging and discharging. This tool will help evaluate the thermal behaviour of new battery designs, leading to safer, higher-performing devices.

Lithium (Li)-ion batteries are the cornerstone of the drive towards electrified vehicles. The opportunity for the use of these batteries in an increasingly diverse and demanding range of applications, including grid-scale storage and aerospace, is compelling, but requires an improved understanding of battery performance in real-world, operational conditions. Temperature plays a pivotal role in the performance, safety and lifespan of Li-ion batteries. Maintaining an appropriate temperature range is essential to maximise the performance, safety and longevity of these devices.

During charge and discharge, the passage of current within a battery leads to temperature changes within the device, but it is difficult to accurately measure the internal temperatures in a cell. Inside the cylindrical cell casing, positive and negative electrodes form a spiral structure. At the outside of the spiral, the electrodes adjacent to the cell wall can more effectively reject heat compared with the inner windings of the electrode architecture; this is particularly important in commercial devices, where there may be a significant temperature gradient from the surface to the cell interior. High-current operation (e.g., fast charging) may be limited by the battery management system to ensure cells do not overheat and cause safety hazards. Moreover, hot spots inside the cell may accelerate degradation, leading to premature cell death. To understand these complex phenomena requires new techniques to monitor cell temperature.

Until now, accurately determining the temperature distribution in operating cells has been challenging; thermography can accurately monitor surface temperatures but does not reveal internal thermal behaviour. Embedded sensors and thermocouples have been used to provide internal point measurements of temperature, but require invasive modifications to the cell architecture.

Fig. 76: a) Photograph of the cell holder with electrical connections as mounted on the diffractometer at ID15A

and (inset) an example of the X-ray diffraction rings collected from the cells. b) Example diagram of the

sample-detector geometry used for all experiments: The X-ray beam is directed along x and normal to

the detector plane; sample translation perpendicular to the beam goes along y; ω is the angle about the

rotation axis. c) A schematic image of the cell holder with electrical terminals (red/black) and 18650 cell.

d) Electrochemical data obtained using the cell holder during characterisation at various C-rates.