Rechargeable lithium batteries have been recognized as the most successful and sophisticated energy storage devices since their first commercialization in 1991 by Sony.  Furthermore, they are used now as an alternative power source for electric motors instead of combustion engines equipped with a fuel tank.¹  Electric vehicles equipped with large-scale lithium batteries as power sources have been introduced to the automotive market, promising to reduce the dependence of transportation on fossil fuels in the future.  The element lithium is widely distributed in the Earth’s crust and sea, but is not regarded as an abundant element.  In contrast, potassium is the seventh most-abundant element in the continental crust and a major element in oceanic waters.²  Furthermore, the standard potential of potassium is lower than that of lithium in propylene carbonate.³ In this study, the electrochemical potassium intercalation into graphite negative electrode is investigated to realize rechargeable potassium-ion batteries.

The working electrodes consisted of 90 wt% graphite and 10 wt% binder: poly(vinilydene fluoride) (PVdF), carboxymethyl cellulose (CMC), or sodium polyacrylate (PANa). Metallic potassium foil was used as a counter electrode.  The galvanostatic charge/discharge measurements were carried out at C/10 (25 mA g^-1) in the voltage range of 0-2 V vs. K/K⁺.  In situ X-ray diffraction (XRD) was conducted for electrodes in K cell.  The surface of the electrodes after cycles was characterized by hard X-ray photoelectron spectroscopy (HAXPES), soft X-ray photoelectron spectroscopy (SOXPES), time-of-flight secondary ion mass spectrometry (TOF-SIMS) and scanning electron microscopy (SEM).

In Fig. 1a, graphite electrode with PANa binder shows better cyclability than those of electrodes with PVDF and CMC binder due to the interfacial modification, with delivering ca. 250 mAh g^-1 of reversible capacity and excellent capacity retention in 1 mol l^-1 KFSI EC:DEC solution.  In addition, graphite electrode with PANa demonstrates ultrahigh oxidation rate capability of depotassiation up to 40C (11160 mA g^-1) rate in a non-aqueous potassium cell as shown Fig. 1b.  Furthermore, in-situ XRD confirms reversible phase transition from graphite to different stage intercalation compounds, such as KC₈ and KC₂₄.  In addition, HAXPES results demonstrate that surface of a graphite electrode is covered with a passive layer after 1^st cycle, and no obvious difference between 1^st and 10^thcycle is observed in the surface layer.  From these results, we will further discuss graphite negative electrode for potassium-ion batteries as next-generation battery beyond lithium-ion and sodium-ion.

References

1.  M. Dahbi, N. Yabuuchi, K. Kubota, K. Tokiwa, S. Komaba, Phys. Chem. Chem. Phys.,16, 15007 (2014)

2.  T. Hasegawa, M. Dahbi, S. Komaba, et al., Proceeding of Renewable and Sustainable Energy Conference, IEEE 642 (2014)

3.  S. Komaba, T. Hasegawa, M. Dahbi, K. Kubota, Electrochem. Commun., 60, 172 (2015)