Figure 2. Phase evolution probed by in situ electron diffraction.

(a) Electron diffraction intensity profile (the colour map in the middle) as a function of reaction time during an in situ lithiation of Fe₃O₄ nanoparticles (see Supplementary Movie 1). The SAED patterns and corresponding radially integrated intensity profiles obtained at pristine (0 s) and fully lithiated (3400, s) states are shown below and above the colour map, respectively. Arrows indicate diffraction peak shift towards a lower value in the reciprocal space. The sample was lithiated at a rate of ∼980 mA g⁻¹. (b) Intensity profiles of Fe₃O₄ (311) and Fe (110) Bragg reflections during the lithiation, indicating the phase evolution of Fe₃O₄ and Fe. (c) Lattice parameter measured from Fe₃O₄ (311) and (511) reflections showing an incremental shift as a function of lithiation time, which quantitatively agrees with the SXRD results. Error bars are given by 1 pixel precision of SAED measurements. Blue lines indicate lattice parameters of Fe₃O₄ and LiFe₃O₄ measured by SXRD in Fig. 1g.