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LiNi0.8Co0.15Al0.05O2 Cathode Material: New Insights via 7Li and 27Al Magic-Angle Spinning NMR Spectroscopy

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journal contribution
posted on 13.09.2016, 00:00 by Nicole Leifer, Onit Srur-Lavi, Irina Matlahov, Boris Markovsky, Doron Aurbach, Gil Goobes
Aluminum doped mixed metal oxides are popular positive electrode materials for Li-ion batteries. They are used extensively in many applications, yet their operation and limitations are not entirely understood. This work shows the advantage of using solid-state 7Li and 27Al NMR for monitoring the electrochemical properties of the doped nickel–cobalt oxide cathode material, LiNi0.8Co0.15Al0.05O2 (NCA), particularly during the first few charge/discharge cycles. The changes in the state of the material as lithium ions are intercalated and deintercalated during discharge and charge, respectively, are highlighted via the Li nuclei as a dynamic reporter and the Al nuclei as a static, material-embedded reporter. In particular, the NMR view of the cyclic change of Ni ions between paramagnetic and diamagnetic oxidation states is enhanced by monitoring both nuclei. Two protocols of cycling the NCA electrode are compared: one employing a smaller voltage window, cycled against graphite as anode, and one using a wider voltage window, cycled against a lithium metal anode. The NMR analysis unveils notable differences in the reversibility of the changes in the Ni oxidation states as charge carriers are shuttled in and out of the cathode material. The 27Al NMR data of the pristine material shows the existence of at least two distinct configurations of Ni ions around the Al dopant ions, suggesting coexistence of two disparate phases, which remain intact upon cycling. The protocol employing slower cycling versus Li anode delivers better cathode performance in the sense that more extensive relithiation occurs, and here, it is shown that the return of the local environments to their pristine electronic configurations is more complete. The 27Al and 7Li NMR results are integrated into a simple scheme exemplifying how better understanding of the local electronic changes in paramagnetic electrode materials can be captured in simple progressive plots.