Phosphoric acid and thermal treatments reveal the peculiar role of surface oxygen anions in lithium and manganese-rich layered oxides

The interplay between cationic and anionic redox activity during electrochemical cycling makes layered Li-rich oxides appealing cathodes for state-of-the-art lithium-ion batteries. However, it remains challenging as the origin of lattice oxygen activity is not yet fully understood. Here we report on the effects of a lithium-deficient layer in the near-surface region of Co-free Li-rich Li[Li0.2Ni0.2Mn0.6]O-2 (LLNMO) achieved via a phosphoric acid surface treatment. Our results show that oxidized On- (0 < n < 2) species are formed on the surface of H3PO4-treated LLNMO resulting from Li ion deficiency and lattice distortion. The metastable On- could be easily released from the oxygen surface lattice forming O-2 via thermal treatment, accompanied by a surface reconstruction and a layered-to-rock-salt/spinel transition. The presented results demonstrate that the surface lattice structure plays a critical role in the electrochemical performance of LLNMO. This information provides new insights into the oxygen redox in LLNMO and opens up an opportunity for Li-rich cathodes to achieve long cycle life toward a broad range of applications in electrical energy storage devices.

Automated summary

The study shows that the use of phosphoric acid surface treatment can form oxidized species in LLNMO materials, improving their electrochemical performance. The surface lattice structure plays a critical role in the performance of LLNMO.