Here 𝐶𝐶 𝑝𝑝 , is the specific heat capacity, 𝜌𝜌 is the density, and 𝛼𝛼 is the thermal diffusivity. It was not possible to measure the thermal diffusivity of the separator as it is too thin to be characterised using laser flash analysis (LFA). The electrolyte volume fraction of traditionally used PP/PE/PP separator does significantly affect the thermal conductivity, therefore the thermal diffusivity is assumed to be similar across commercially used separator materials. The thermal conductivity of a wetted separator with similar properties is taken from literature, this is 0.3344 W m -1 K -1 . The density and the volumetric specific heat capacity of the wetted separator can be evaluated using Eq. [1]. At 25 °C, 𝜌𝜌, 𝐶𝐶 𝑝𝑝 , and 𝜀𝜀 𝑙𝑙 are 946 kg m -3 , 1700 J kg -1 K -1 and 0.47 for the dry separator. The volumetric heat capacity is 1745970 J m -3 K -1 . This corresponds to a density and gravimetric specific heat capacity of 1620 kg m -3 and 1080 J kg -1 K -1 for the wetted separator. Therefore the heat capacity for the wetted separator is represented by Equation [1], where 𝐶𝐶 𝑝𝑝,𝑠𝑠 is defined by Equation [S3], and the other values are summarised in Table .