Formation and Impact of Nanoscopic Oriented Phase Domains in Electrochemical Crystalline Electrodes Revealed by Collocated 4D-STEM and EELS Mapping

We study the structural and compositional changes in primary cathode nanoparticles in the electrochemical phase transformation. We map the formation of oriented phase domains and the associated development of strain gradients quantitatively during ion insertion. A collocated four-dimensional scanning transmission electron microscopy and electron energy loss spectroscopy approach, coupled with data mining, enables the research. Results show that in the spinel cathode nanoparticles their phase transformation upon Mg2+ insertion leads to the formation of domains of similar chemical identity but different orientations at the nanoscale. The domain formation process follows the nucleation, growth and coalescence process, different from the conventional diffusion- or reaction-limited mechanisms. Furthermore, electrolytes have significant impacts on the transformation microstructure (‘island’ versus ‘archipelago’). Large strain gradients built up from the development of phase domains across their boundaries change the chemical diffusion coefficient in the cathode nanoparticles by a factor of ten or more. Our findings provide critical insights into the microstructure formation mechanism and its impact on ion insertion, suggesting another dimension of transformation structure control for energy storage materials.