Anion Redox Chemistry in the Cobalt Free 3d Transition Metal Oxide Intercalation Electrode Li[Li_0.2Ni_0.2Mn_0.6]O₂

Conventional intercalation cathodes for lithium batteries store charge in redox reactions associated with the transition metal cations, e.g., Mn^3+/4+ in LiMn₂O₄, and this limits the energy storage of Li-ion batteries. Compounds such as Li­[Li_0.2Ni_0.2­Mn_0.6]­O₂ exhibit a capacity to store charge in excess of the transition metal redox reactions. The additional capacity occurs at and above 4.5 V versus 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 (Mn⁴⁺/Li⁺). The results have been obtained by combining operando electrochemical mass spec on ¹⁸O labeled Li­[Li_0.2Ni_0.2­Mn_0.6]­O₂ 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