𝜀𝜀 𝑡𝑡𝑐𝑐𝑡𝑡 is the active material volume fraction, 𝜕𝜕 𝑒𝑒𝑙𝑙𝑒𝑒𝑐𝑐𝑡𝑡𝑟𝑟𝑡𝑡𝑒𝑒𝑒𝑒 is the electrode volume, 𝜕𝜕 𝑝𝑝𝑡𝑡𝑟𝑟𝑡𝑡𝑝𝑝𝑐𝑐𝑙𝑙𝑒𝑒 is particle volume, 𝐹𝐹 𝑝𝑝𝑡𝑡𝑟𝑟𝑡𝑡𝑝𝑝𝑐𝑐𝑙𝑙𝑒𝑒 is the particle surface area. For this the average particle sizes of 5.22 µm and 5.86 µm were used for the positive and negative electrode respectively, these parameters were measured previously using SEM. For the analysis of the positive electrode diffusivity it is assumed 𝐹𝐹 remains unchanged as NMC volume expansions is negligible, but for the negative electrode we account for the volume expansion of the graphite-SiOy. This involves applying a linear scaling factor across the diffusivity to capture the 4x volume expansion across the SOC range of the battery. The solid-phase diffusion coefficients were measured using GITT in a three-electrode cell comprising of the NMC811 positive electrode and the graphite-SiOy negative electrode (Figure ). The three-electrode set-up probed a limited stoichiometric range for each electrode, which was near equivalent to that observed under operation in the cylindrical cell. The diffusion coefficients vary significantly with lithium concentration, distinct regions can be identified in these profiles and attributed to distinct thermodynamic phases. For the NMC-based electrode these phases relate to the different crystal structures of LiNi0.8Mn0.2Co0.2O2 e.g. hexagonal and monoclinic. For graphite-SiOy, the Li insertion stages of graphite are responsible for the changes in solid-phase diffusivity e.g. I to IV. For the temperature range 5 °C to 45 °C, the diffusivity of the NMC811 was approximately 10 -10 cm 2 s -1 to 10 -11 cm 2 s -1 and for graphite-SiOy it was 10 -14 cm 2 s -1 to 10 -10 cm 2 s -1 , see Figure and S5 respectively for the raw data. The solid-phase diffusion coefficients for graphite-SiOy between 10 -14 cm 2 s -1 to 10 -12 cm 2 s -1 are underestimates due to difficultly observing the minor voltage changes that occur during the transient and relaxation periods of GITT for the plateau regions in graphite. In plateau regions the reaction overpotentials dominate and produce inaccuracies for the solid-phase diffusivity.