Phase interface structures in Li_1+xRh₂O₄ zero strain cathode material analyzed by scanning transmission electron microscopy

For reliable use of lithium ion batteries, their cyclic property must be further improved. Several phase separating materials called zero strain electrodes are very promising because they possess negligible lattice mismatches at the phase interface and significantly reduce the mechanically induced deterioration. However, the structures and chemistry of actual phase interfaces are still not well understood in this class of materials. In this study, the phase interfaces of Li_1+xRh₂O₄ cubic spinel, one of the stable zero strain cathodes, are analyzed in an atomic scale using high-angle annular dark-field (HAADF)/annular bright-field (ABF) scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). Electrochemically induced phase interfaces can be clearly visualized in the ABF STEM images, where they tend to lie on {111} crystallographic lattice planes. The zero strain nature near the observed interface can be confirmed by real space strain analysis, which is in good agreement with the previous X-ray diffraction experiments. By performing the canonical Monte Carlo simulations with effective cluster interaction energies, it is concluded that the stability of {111} phase interfaces will be attributed to their small number density of close lithium ionic pairs formed near the interface plane.