Anion Redox Chemistry in the Cobalt Free 3d Transition Metal Oxide Intercalation Electrode Li[Li 0.2 Ni 0.2 Mn 0.6 ]O 2

Luo, Kun and Roberts, Matthew R. and Guerrini, Niccoló and Tapia-Ruiz, Nuria and Hao, Rong and Massel, Felix and Pickup, David M and Ramos, Silvia and Liu, Yi-Sheng and Guo, Jinghua and Chadwick, Alan V. and Duda, Laurent C. and Bruce, Peter G. (2016) Anion Redox Chemistry in the Cobalt Free 3d Transition Metal Oxide Intercalation Electrode Li[Li 0.2 Ni 0.2 Mn 0.6 ]O 2. Journal of the American Chemical Society, 138 (35). pp. 11211-11218. ISSN 0002-7863

Abstract

Conventional intercalation cathodes for lithium batteries store charge in redox reactions associated with the transition metal cations, e.g. Mn 3+/4+ in LiMn 2 O 4 , and this limits the energy storage of Li-ion batteries. Compounds such as Li[Li 0.2 Ni 0.2 Mn 0.6 ]O 2 exhibit a capacity to store charge in excess of the transition metal redox reactions. The additional capacity occurs at and above 4.5 V vs. Li + /Li. The capacity at 4.5 V is dominated by oxidation of the O 2-anions accounting for ~0.43 e -/formula unit, with an additional 0.06 e -/formula unit being associated with O loss from the lattice. In contrast, the capacity above 4.5 V, is mainly O loss, ~ 0.08 e -/formula. The O redox reaction involves the formation of localized hole states on O during charge, which are located on O coordinated by (Mn 4+ /Li +). The results have been obtained by combining operando electrochemical mass spec on 18 O labelled Li[Li 0.2 Ni 0.2 Mn 0.6 ]O 2 with XANES, soft X-ray spectroscopy, Resonant Inelastic X-ray spectroscopy and Raman spectroscopy. Finally the general features of O-redox are described with discussion about the role of comparatively ionic (less covalent) 3d metal-oxygen interaction on anion redox in lithium rich cathode materials.