Design of Nickel-rich Layered Oxides Using <i>d</i> Electronic Donor for Redox Reactions

Through first-principles calculations and experimental observations, we first present the correlation between the Ni and Mn ratio and the redox behaviors of the layered NCM cathodes. The equilibrium potentials based on redox reactions of Ni<sup>2+</sup>/Ni<sup>3+</sup> are highly dependent on the Mn ratio (NCM523 and NCM721: ∼3.7 and 3.5 V) because of a donor electron, in the e<sub>g</sub> band, transferred from Mn to Ni owing to their crystal field splitting (CFS) with different electronegativities, leading to oxidation states of Ni<sup>2+</sup>-like and Mn<sup>4+</sup>. Considering the electronic donor (Mn) based on CFS with electronegativity of transition metals (TMs), we finally expect V as a promising doping source to provide donor electrons for Ni redox reactions in Ni-rich layered oxides, leading to be higher delithiation potentials (NCV523: 3.8 V). From our theoretical calculations in the NCV oxide, the oxidation states of Ni and V are stable Ni<sup>2+</sup>-like and V<sup>5+</sup>, respectively, and the fractional d-band fillings of Ni are the highest value as compared with NCM523 and LiNiO<sub>2</sub> because of two donor electrons in the t<sub>2g</sub> band. Based on the underlying understanding on the CFS with electronegativity of TMs, it would be possible to design new Ni-rich layered cathodes with higher energy for use in Li-ion batteries.