Electronic Origin of Different Discharge Curves in LixCoO2 and NaxCoO2 cathode Materials
In the case of NaxCoO2-y it was evidenced that the origin of the observed step-like character of the discharge/charge curve of NaxCoO2-y is due to the specific features of the electronic structure, arisen from the presence of the oxygen vacancies and sodium ordering. Performed comprehensive studies of the structural, transport and electronic specific heat properties of NaxCoO2-y cathode material in the characteristic points of the discharge curve i.e. on the pseudo-plateaus and on the potential jumps, show non-monotonous variations of its transport properties (i.e. a sequence of alternative, metallic-like or semiconducting-like, behaviours) during sodium intercalation, what suggest that the density of states near the Fermi level is spiky. This effect is coherently supported by KKR-CPA calculations of DOS in NaxCoO2-y, accounting for chemical disorder, i.e. oxygen vacancy defects as well as variable occupancy of two sodium sites. We concluded that these unusual electronic structure features lead to an abrupt changes in the position of EF, experiencing the oxygen-defect DOS peaks upon intercalation, and finally resulting in step-like character of the discharge curve of Na/Na+/NaxCoO2-y cell.
The project was funded by the National Science Centre Poland (NCN) on the basis of the decision number DEC-2011/02/A/ST5/00447.
This work is supported by the Polish-Swiss Research Programme under grant no. 080/2010 LiBeV (Positive Electrode Materials for Li-ion Batteries for Electric Vehicles).
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