Characterization and Control of Irreversible Reaction in Li-Rich Cathode during the Initial Charge Process

Li-rich layered oxide has been known to possess high specific capacity beyond the theoretical value from both charge compensation in transition metal and oxygen in the redox reaction. Although it could achieve higher reversible capacity due to the oxygen anion participating in electrochemical reaction, however, its use in energy storage systems has been limited. The reason is the irreversible oxygen reaction that occurs during the initial charge cycle, resulting in structural instability due to oxygen evolution and phase transition. To suppress the initial irreversible oxygen reaction, we introduced the surface-modified Li­[Li_0.2Ni_0.16Mn_0.56Co_0.08]­O₂ prepared by carbon coating (carbonization process), which was verified to have reduced oxygen reaction during the initial charge cycle. The electrochemical performance is improved by the synergic effects of the oxygen-deficient layer and carbon coating layer formed on the surface of particles. The sample with suitable carbon coating exhibited the highest structural stability, resulting in reduced capacity fading and voltage decay, which are attributed to the mitigated layered-to-spinel-like phase transition during prolonged cycling. The control over the oxygen reaction of Li₂MnO₃ by surface modification affects the activation reaction above 4.4 V in the initial charge cycle and structure changes during prolonged cycling. X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy analyses as well as electrochemical performance measurement were used to identify the correlation between reduced oxygen activity and structural changes

<i>In Operando</i> Monitoring of the Pore Dynamics in Ordered Mesoporous Electrode Materials by Small Angle X‑ray Scattering

To monitor dynamic volume changes of electrode materials during electrochemical lithium storage and removal process is of utmost importance for developing high performance lithium storage materials. We herein report an <i>in operando</i> probing of mesoscopic structural changes in ordered mesoporous electrode materials during cycling with synchrotron-based small angel X-ray scattering (SAXS) technique. <i>In operando</i> SAXS studies combined with electrochemical and other physical characterizations straightforwardly show how porous electrode materials underwent volume changes during the whole process of charge and discharge, with respect to their own reaction mechanism with lithium. This comprehensive information on the pore dynamics as well as volume changes of the electrode materials will not only be critical in further understanding of lithium ion storage reaction mechanism of materials, but also enable the innovative design of high performance nanostructured materials for next generation batteries