Electronic Origin of Different Discharge Curves in LixCoO2 and NaxCoO2 cathode Materials

This paper presents a novel approach to an explanation of apparently different character of the discharge/charge curve in Li_xCoO₂ (monotonous curve) and Na_xCoO₂ systems (step-like curve). Comprehensive experimental and theoretical studies on crystallographic, electronic  and electrical transport properties of the Li_xCoO₂ battery cathode material are reported. During electrochemical deintercalation of lithium we observe an increase of the electrical conductivity by 6 orders of magnitude. The thermal activated character of electrical conductivity for Li_xCoO₂ is detected in the region 0.95 ≤ x ≤ 1. However, only for stoichiometric Li₁CoO₂ the activated character of electrical conductivity indicates clearly semiconducting behavior. Based on the calculated DOS from the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) method allowing to take into account Li- and O-deficiency in self-consistent way, it is found that for 0.99 ≤ x ≤ 0.95 the Fermi level is situated inside the tails of the valence states and its activated character corresponds to activated character of electron hole mobility [1]. The electrical conductivity of Li_xCoO₂ samples with x ≤ 0.94 does not depend on temperature and the values tend to increase with lowering of the lithium concentration. On the whole, we postulate that the observed insulator-metal transition in Li_xCoO₂ can be interpreted on the basis on the Anderson type transition. Drastic evolution of the top valence band features (the relative spectral weight of the Co atom orbitals e_g/t_2g)  well corroborates crystallographic data as well as the particular effect of the oxygen octahedron distortion on computed DOS shape [2]. We suppose that irregular behavior of the positional parameter z (O sites) with Li concentration is a reason of significant discrepancies in observed crystallographic parameters, transport properties and phase diagram of Li_xCoO₂ system as reported in different works.

In the case of Na_xCoO_2-y  it was evidenced  that the origin of the observed step-like character of the discharge/charge curve of Na_xCoO_2-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 Na_xCoO_2-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 Na_xCoO_2-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 E_F, experiencing the oxygen-defect DOS peaks upon intercalation, and finally resulting in step-like character of the discharge curve of Na/Na⁺/Na_xCoO_2-y cell.

 

Acknowledgements

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).

References:

[1] A. Milewska, K. Świerczek, J. Toboła, F. Boudoire, Y. Hu, D.K. Bora, B.S. Mun, A. Braun, J. Molenda, The nature of the nonmetal-metal transition in Li_xCoO₂ oxide, Solid State Ionics (2014)

[2] J. Molenda, D. Baster, K. Świerczek, J. Toboła, Anomaly in electronic structure of Na_xCoO_2-y cathode as a source of its step-like discharge curve, Physical Chemistry Chemical Physics (2014)