Estimation of the average oxidation number of nickel in a nickel oxide based on local structural information

We developed the methods to estimate the average oxidation number of Ni in a nickel oxide from the local structural information on the first nearest neighbor Ni–O shell obtained by extended X-ray absorption fine structure curve fitting analysis. The estimation method utilizes the Jahn-Teller distortion around Ni3+ sites, enabling the evaluation of local structural parameters and mole fractions for different Ni valence states. The average oxidation numbers of Ni in nickel oxides estimated by the methods proposed in this work exhibit good agreement with those reported or evaluated by a conventional method using instrumental and solution chemical analyses. It is demonstrated that the estimation methods also work well in an in situ X-ray absorption spectroscopy (XAS) study on the charge-compensating behavior of Ni in a Ni-containing battery electrode material. The estimation methods are based on Ni K-edge XAS using highly penetrating hard X-ray, and thus applicable to in situ XAS studies. The methods proposed in this work are expected to work as substitutes for the conventional empirical method using X-ray absorption near edge structure. The estimation methods would be important choices when the local environment around Ni could dramatically be changed during charge/discharge cycles, like in an overcharge condition. [Display omitted] •The average oxidation number of Ni is estimated from the local structure around Ni.•The proposed methods derive consistent Ni oxidation numbers with reported values.•The proposed methods are applicable to in situ/operando studies.