Mapping internal temperatures during high-rate battery applications

Abstract Electric vehicles (EV) demand high charge/discharge rates creating potentially dangerous temperature rises. Lithium-ion cells are sealed during their manufacture, making internal temperatures challenging to probe1. Tracking current collector expansion using X-ray diffraction (XRD) permits non-destructive internal temperature measurements2; however, cylindrical cells are known to experience complex internal strain3,4. Here, we characterise the state-of-charge (SoC), mechanical strain, and temperature within Lithium-ion 18650 cells operated at high rates (>3C) via two advanced synchrotron XRD methods: firstly, as entire cross-sectional temperature maps during open-circuit cooling and secondly, single-point temperatures during charge/discharge cycling. We observed that a 20-minute discharge on an energy-optimised cell (3.5 Ah) resulted in internal temperatures >70 °C, whereas, a faster 12-minute discharge on a power-optimised cell (1.5 Ah) resulted in substantially lower temperatures (< 50 °C). However, when comparing the two cells under the same electrical current, the peak temperatures were similar, e.g., a 6A discharge resulted in 40°C peak temperatures for both cell types. We observe that the operando temperature rise is due to heat accumulation, strongly influenced by the charging protocol e.g., constant-current (CC) and/or constant-voltage (CV). Design mitigations for temperature-related battery issues should now be explored using this novel methodology to provide opportunities for improved thermal management during high-rate EV applications.