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

PRINCIPAL PUBLICATION AND AUTHORS

Mapping internal temperatures during high-rate battery applications, T.M.M. Heenan (a,b), I. Mombrini (a,c), A. Llewellyn (a), S. Checchia (c), C. Tan (a,b), M.J. Johnson (a), A. Jnawali (a), G. Garbarino (c), R. Jervis (a,b), D.J.L Brett (a,b), M. Di Michiel (c), P.R. Shearing (a,b), Nature 617 (2023); https:/doi.org/10.1038/s41586-023-05913-z (a) Electrochemical Innovation Laboratory, Department of Chemical Engineering, University College of London (UK) (b) The Faraday Institution, Harwell Science and Innovation Campus, Didcot (UK) (c) ESRF

In this work, a new technique was developed to accurately map the temperature distribution in a Li-ion cell without modification. Utilising non-destructive X-ray diffraction (XRD) measurements, the technique can monitor the thermal behaviour of batteries during operando cycling to establish an improved understanding of temperature gradients across the cell diameter.

The technique relies on the ability to accurately determine changes to the crystallography of constituent cell materials. In Li-ion batteries, the negative electrode (generally graphite) is supported by a metallic copper current collector. Tracking the changes to the copper lattice parameter during battery cycling can be linked to thermal expansion behaviour of the electrode, which can be used as a proxy for direct temperature measurement. However, this is not straightforward since the expansion of the graphite negative electrode during the charging process (as it lithiates) imposes a mechanical strain on the copper substrate, which needs to be decoupled from the thermal strain resulting from the cell heating during electrochemical cycling.

At beamline ID15A, XRD computed tomography (XRD-CT) and highly localised XRD measurements (using a multi- channel collimator, or MCC) were carried out to decouple the thermal and mechanical strains experienced by the cell materials during charging and discharging. Figure 76 illustrates the setup and procedure. The temperature distribution was compared in two cell chemistries, a high- energy-density cell based on a nickel-rich transition metal oxide (NMC) cathode (Figure 77), and a high-power cell using a lithium iron phosphate (LFP) cathode. The effects of long-term cycling on cell degradation and the resulting changes in temperature distribution were also explored. This revealed an increase in internal temperature for degraded cells, providing valuable information regarding performance loss as the cell approaches end-of-life.

With the increasing trend towards larger-format cells for automotive applications, and a demand for higher- rate charging, temperature distributions in cells become increasingly important. The methodology developed here will provide a new tool to evaluate the thermal behaviour of emerging battery designs in real-world operational conditions, accelerating the safe deployment of high- performance batteries and maximising their lifetime.

Fig. 77: a) The high-rate discharge currents for a commercial 18650 Li-ion battery with measurement of the peak temperature at the point of open circuit (stars). b-c) Internal temperatures for eight zones divided as radial (b) and azimuthal zones (c). All error bars are statistical, precision error is roughly ±3 °C throughout. Note that ambient temperature was assumed to be a consistent 20°C.