O3 to O1 Phase Transitions in Highly Delithiated NMC811 at Elevated Temperatures

Zachary Ruff; Chloe S Coates; Katharina Märker; Amoghavarsha Mahadevegowda; Chao Xu; Megan E Penrod; Caterina Ducati; Clare P Grey

doi:10.1021/acs.chemmater.3c00307

O3 to O1 Phase Transitions in Highly Delithiated NMC811 at Elevated Temperatures

Chem Mater. 2023 Jun 20;35(13):4979-4987. doi: 10.1021/acs.chemmater.3c00307. eCollection 2023 Jul 11.

Authors

Zachary Ruff^{1

2}, Chloe S Coates¹, Katharina Märker^{1

2}, Amoghavarsha Mahadevegowda^{1

3}, Chao Xu^{1

2}, Megan E Penrod^{1

2}, Caterina Ducati^{1

3}, Clare P Grey^{1

2}

Affiliations

¹ Yusuf Hamied Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, U.K.
² The Faraday Institution, Quad One, Harwell Science and Innovation Campus, Didcot OX11 0RA, U.K.
³ Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, U.K.

Abstract

Nickel-rich layered oxide cathodes such as NMC811 (Li_xNi_0.8Mn_0.1Co_0.1O₂) currently have the highest practical capacities of cathodes used commercially, approaching 200 mAh/g. Lithium is removed from NMC811 via a solid-solution behavior when delithiated to x_Li > 0.10, maintaining the same layered (O3 structure) throughout as observed via operando diffraction measurements. Although it is possible to further delithiate NMC811, it is kinetically challenging, and there are significant side reactions between the electrolyte and cathode surface. Here, small format, NMC811-graphite pouch cells were charged to high voltages at elevated temperatures and held for days to access high states of delithiation. Rietveld refinements on high-resolution diffraction data and indexing of selected area electron diffraction patterns, both acquired ex situ, show that NMC811 undergoes a partial and reversible transition from the O3 to the O1 phase under these conditions. The O1 phase fraction depends not only on the concentration of intercalated lithium but also on the hold temperature and hold time, indicating that the phase transition is kinetically controlled. ¹H NMR spectroscopy shows that the proton concentration decreases with O1 phase fraction and is not, therefore, likely to be driving the O3-O1 phase transition.