Investigating the Reversibility of Structural Modifications of Li_xNi_yMn_zCo_1-y-zO₂ Cathode Materials during Initial Charge/Discharge, at Multiple Length Scales

In this work, we investigate the structural modifications occurring at the bulk, subsurface, and surface scales of Li_xNi_yMn_zCo_1-y-zO₂ (NMC; y, z = 0.8, 0.1 and 0.4, 0.3, respectively) cathode materials during the initial charge/discharge. Various analytical tools, such as X-ray diffraction, selected-area electron diffraction, electron energy-loss spectroscopy, and high-resolution electron microscopy, are used to examine the structural properties of the NMC cathode materials at the three different scales. Cutoff voltages of 4.3 and 4.8 V are applied during the electrochemical tests as the normal and extreme conditions, respectively. The high-Ni content NMC cathode materials exhibit unusual behaviors, which deviate from the general redox reactions during the charge or discharge. The transition metal (TM) ions in the high-Ni content NMC cathode materials, which are mostly Ni ions, are reduced at 4.8 V, even though TMs are usually oxidized to maintain charge neutrality upon the removal of Li. It was found that any changes in the crystallographic and electronic structures are mostly reversible down to the subsurface scale, despite the unexpected reduction of Ni ions. However, after the discharge, traces of the phase transitions remain at the edges of the NMC cathode materials at the scale of a few nanometers (i.e., surface scale). This study demonstrates that the structural modifications in NMC cathode materials are induced by charge as well as discharge, at multiple length scales. These changes are nearly reversible after the first cycle, except at the edges of the samples, which should be avoided because these highly localized changes can initiate battery degradation.