The overall phase transformation behaviour of Li-ion battery electrode materials is well known. However, even for well-studied electrode materials such as LiFePO4, it is unknown how the transformation proceeds within individual grains and how it is distributed over the many grains present in an electrode, mainly due to the difficulty to probe the phase nucleation and growth within individual grains. This is crucial information for future battery improvement because the local conditions, such as the local current density, govern the Li-ion battery cycle life and rate performance. Using the microbeam at ID11, operando X-ray diffraction was employed to monitor the transformation of many individual LiFePO4 particles inside an Li-ion battery electrode from slow to fast battery (dis)charging rates, as shown schematically in Figure 26a. The diffraction results revealed the cycling-rate dependent phase transition mechanism, shown in Figure 27, within individual electrode grains. Measuring the transformation process of individual LiFePO4 particles allowed the first quantification of local current densities, i.e. the current per surface area of individual grains.

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