Smoothing out NaxCo0.7Mn0.3O2 ocv Curve – Role of the Oxygen Nonstoichiometry

Abstract

It is well known that NaxCoO2-y exhibits complicated character of the OCV (Open Circuit Voltage) curve, with several potential plateaus and jumps in 2.0-3.8 V range [1,2]. Origin of such the behavior is commonly attributed to the Na cations ordering [3]. However, often neglected influence of the electronic structure of the cathode material, which manifest itself as an additional energy states present in the main energy gap, seems to play a major role in this case. These states can be directly related to the presence of the oxygen vacancies [4].In this work we show a novel approach, based on the consideration of the influence of cation dopants replacing partially cobalt, which aims to smooth out the OCV characteristic of Na/Na+/NaxCoO2-y battery. On a basis of work [4], one can state that in order to remove most of the potential jumps, the material has to be oxygen stoichiometric (NaxCo0.7Mn0.3O2) and no sodium ordering should be present. Since +4 oxidation state for cobalt cations is not the most favourable one, it gives rise to a presence of the oxygen vacancies, which decrease the average oxidation state of Co. If an appropriate dopant with stable +4 state would be introduced in cobalt positions, the concentration of the oxygen vacancies shall decrease considerable. Furthermore, random distribution of such dopant in Co sublattice will diminish the tendency of Na cations to form ordered structure.As predicted from the considerations shown above, in the case of Mn substitution, OCV curve of Na/Na+/NaxCo0.7Mn0.3O2 is almost completely smoothed out, with only one major potential jump, which is associated with x = 0.5 mol·mol-1. Apart from basic scientific aspect, the obtained result is interesting from the point of view of application, as in commercial batteries voltage cannot show any non-monotonous behaviour.