Oxygen Activity Promoting the Surface Transformation of High Voltage Layered Oxide Cathodes for Lithium Ion Batteries

The demand for high-energy dense materials that are also capable of extended charging and discharging cycling has brought the lithium-rich layered oxide compounds as leading contenders for the next generation of lithium ion battery cathode materials for consumer use.¹ While promising, these materials suffer from thermodynamic instabilities and surface transformations when charged to higher voltages, which are unique and deviate from the degradation mechanisms of the classical layered oxides². One considerable difference between the classical and lithium rich layered oxides is the oxygen activity or oxygen loss and the impact it has on the surrounding environment. Figure 1 is the electrochemical charging and discharging profile for the stoichiometric LiNi_0.4Mn_0.4Co_0.2O₂ (442) and the lithium rich Li_1.2Ni_0.133Mn_0.533Co_0.133O₂ (Li-rich) where a plateau can be observed for the Li-rich material at 4.5 V that corresponds to a simultaneous extraction of lithium and oxygen.  To obtain a better understanding of differences in the oxygen activity, x-ray absorption spectroscopy (XAS) was used to probe the O K-edge and the different transition metal L-edges on the surface and sub-surface using three penetrations depths (Auger, Electron-Yield, and Fluorescence) at different states of charge for 442 and Li-rich. Scanning transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) were also employed in an attempt to elucidate key differences within the surfaces between the classical and lithium rich layered oxide materials when subjected to similar high voltage cycling. Differences in the d-p hybridization can be observed between 442 and Li-rich materials, where 442 shows a more dramatic decrease within the d-p hybridization compared to the Li-rich after charging to 4.8 V and discharged to 2.0 V. This is evident from  the drop of intensity of the low energy band within the O K-edge XAS spectra (Figure 2). Within the Co-free analogous material, the drop in hybridization showed higher dependence on Ni (Figure 3).³With the involvement of Co within the charge compensation mechanism, the oxygen activity is more complex where, unlike the Co-free materials, the increase of the spectral weight of the pre-edge of the lower energy band is more pronounced. In addition, this change is highly dependent on the current rate that is applied where slower rates give rise to larger changes within the oxygen environment while high current rates after extended cycling show minimal decrease of the hybridization.

Acknowledgements

The authors acknowledge the financial support from the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. DOE under Contract No. DE-AC02- 05CH11231, under the Batteries for Advanced Transportation Technologies (BATT) Program. The synchrotron X-ray portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the US Department of Energy Office of Science by Stanford University. S/TEM and EELS experiments were performed at the Center for Functional Nanomaterials, Brookhaven National Laboratory.

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