Sodium-ion batteries have attracted great deal of attention due to sodium’s abundance and lower price compared to lithium and cobalt. A major obstacle that prevents sodium-ion batteries going to a mass-scale is the negative electrode (anode during discharge). Graphite is currently the safest negative electrode material for lithium-ion batteries, but does not intercalate sodium reversibly, because sodium ions prefer prismatic or octahedral coordination sites [1].

Our previous work [2] focused on the synthesis and electrochemical characterization of glucose-derived hard carbon negative electrodes for sodium-ion batteries. Unfortunately, the tested electrochemical half-cell showed high capacity fade during cycling. At the time, we thought it was due to material’s spherical morphology (Fig. 1a). But when we substituted the previously used electrolyte (1M NaClO₄ in PC) with 1M NaPF₆ in PC:EC (9:1) capacity fade was significantly reduced. The discharge capacity for the 100^th discharge cycle was only 177 mAh g^-1 when using NaClO₄ in PC, but remained at 267 mAh g^-1 when using NaPF₆ in PC:EC (9:1) (Fig. 1d - e).

The results of full cell testing with NVP-type cathode [3] will also be discussed.

Acknowledgements

This research was supported by the EU through the European Regional Development Fund TK141 “Advanced materials and high-technology devices for energy recuperation systems”, Estonian target research project IUT20–13 and personal research grant PUT55. Mr. Väli thanks Estonian Students Fund in USA and University of Tartu Foundation for financial support.

References

1. J. Qian, X. Wu, Y. Cao, X. Ai, and H. Yang, Angew. Chem. Int. Ed., 52, 4633 (2013).

2. R. Väli, A. Jänes, T. Thomberg, and E. Lust, J. Electrochem. Soc., 163, A1619 (2016).

3. R. Väli, P. Möller, and A. Jänes, ECS Trans., 69, 27 (2015).