Theoretical Investigation on the Structural Stability of Na2+XC6O6 As a Sodium-Ion Battery Cathode

Lithium-ion batteries play a vital role in our daily life due to their broad applicability as power sources for portable electronic devices and electric vehicles. However, the use of lithium-ion batteries in large scale application is currently under debate due to the high cost of lithium and its possible supply risk. Thus, minor-metal free or low-cost materials that can be derived from more abundant resources have become increasingly desirable along with the world's growing ecological concerns. Sodium-ion batteries have been explored as a promising alternative to lithium-ion batteries because there is no doubt that the sodium resources are inexhaustible and unlimited everywhere around the world. However, its ionic volume is almost double and its atomic mass is triple than that of lithium. Hence, it has been difficult to find an insertion host that can readily accept repeated insertion and extraction of the large sodium-ion into the matrix. Recently, it has been reported that disodium rhodizonate, Na₂C₆O₆, exhibit good electrochemical properties and cycle performance as a rare organic cathode for sodium-ion batteries [1]. The minor-metal free Na₂C₆O₆ can be considered as a promising cathode candidate for high cost performance sodium-ion batteries. However, its crystal structures during discharge/charge cycle, which correspond to Na_2+xC₆O₆ (0 < x < 2) in composition, still remain unclear.

   In this work, we theoretically propose feasible crystal structures of Na_2+xC₆O₆ during discharge/charge processes and show their electronic structures. Density functional theory calculations have been performed using the full-potential linearized augmented plane wave method [2, 3]. As a result of total energy calculations for Na_2+xC₆O₆ with several space groups, we have found a structural phase transition: The most stable structure for Na₄C₆O₆ has a different C₆O₆ packing arrangement from that in Na₂C₆O₆. Our calculations have also revealed that the calculated electronic structure of Na₂C₆O₆ crystal is quite analogous to that of a C₆O₆ molecule. Na₂C₆O₆ has a stable structure where bonding states around Fermi level are filled and antibonding states are not. If Na insertion occurs without the structural phase transition, additional electrons would occupy antibonding states, resulting in unstable structure. On the other hand, the calculated electronic structure of Na₄C₆O₆ is similar to that of two stacked C₆O₆ molecules. In this C₆O₆ frame, additional electrons can occupy new bonding states which arise from two stacked C₆O₆. Consequently, this structural phase transition, i.e. two stacked C₆O₆, allows for Na-ion insertion. Moreover, electrode potentials vs. Na/Na⁺ of Na_2+xC₆O₆ have been evaluated. The calculated electrode potentials are consistent well with experimental results. Our predictions from first-principles calculations could be the key to understanding the mechanism of the discharge/charge process for Na_2+xC₆O₆ cathode and be of benefit to the application of the minor-metal free sodium-ion batteries.

Reference

[1] K. Chihara, N. Chujo, A. Kitajou, and S. Okada, Electrochim. Acta 110, 240 (2013).

[2] E. Wimmer, H. Krakauer, M. Weinert, and A. J. Freeman, Phys. Rev. B 24, 864 (1981).

[3] J. M. Soler and A. R. Williams, Phys. Rev. B 40, 1560 (1989).