KPF6-KFSA Binary Salt Electrolytes for 4 V-Class Potassium Batteries
Authors: Hosaka Tomooki , Matsuyama Tatsuo , Kubota Kei , Komaba Shinichi 
Published: February 2020

Potassium-ion battery (KIB) has attracted much attention as a potential high-voltage and high-power secondary battery due to a lower standard electrode potential of potassium in carbonate ester-based electrolytes and weaker Lewis acidity of potassium ion than those of lithium (1). Recently, our group has demonstrated a high voltage KIB with graphite//K1.75Mn[Fe(CN)6]0.93·0.16H2O configuration by using 0.7 mol dm-3 KPF6/EC:DEC electrolyte (2). Although the Coulombic efficiency increase up to 99% after several cycles, the initial one is ca. 60% and needs to be improved. Instead of the KPF6 based electrolytes, KN(SO2F)2 (KFSA) based electrolytes are reported to exhibit higher Coulombic efficiencies of negative electrodes such as K-metal (3) and graphite electrodes (4) while the KFSA ester-based electrolytes corrode an Al current collector at high potential above approximately 3.5 V in the conventional concentration. In this study, KPF6-KFSA binary-salt electrolytes are prepared in the conventional concentration to realize high Coulombic efficiency of graphite electrodes and Al passivation at the high potential, and the electrolyte properties are examined.
The KPF6-KFSA binary-salt electrolytes of 0.75 mol kg-1 (m) K[PF6:FSA (= 90:10)] (KPF6-90), 1 m K[PF6:FSA (= 75:25)] (KPF6-75), and 1 m K[PF6:FSA (= 50:50)] (KPF6-50) were prepared by dissolving them in battery-grade solvents of ethylene carbonate (EC) and diethyl carbonate (DEC) at 1:1 v/v, and were compared with single-salt electrolytes of 0.75 m KPF6/EC:DEC and 1 m KFSA/EC:DEC. Al corrosion behavior was evaluated by cyclic voltammetry (CV) of an Al-foil working electrode. Charge/discharge tests were conducted using K//graphite half-cells filled with the electrolytes.
Figure 1 (a) shows CV curves of Al working electrodes in single KFSA salt electrolyte and binary ones of KPF6-90, KPF6-75, and KPF6-50. Significantly large and irreversible anodic current appears above 3.5 V in the KFSA and KPF6-50 electrolytes and Al corrosion is confirmed as many pits of 10–50 μm in diameter on the Al electrode after 3 cycles as shown in Fig. 1(b). On the other hand, much smaller oxidation current is observed in KPF6-75 and KPF6-90 electrolytes in the CV curves. As a result, a few tiny pits are observed on the Al electrodes in KPF6-75 electrolyte, and no significant change of the Al electrode is achieved in KPF6-90 electrolyte. Therefore, the KPF6-90 electrolyte successfully prevents Al corrosion at high potential above 3.5 V. Figure 1c shows charge/discharge curves of K//graphite cells filled with KPF6, KPF6-90 and KPF6-75, and KFSA electrolytes. All cells deliver reversible capacities of ca. 260 mAh g-1 based on KC8 formation. Whereas the KPF6 cell shows a low Coulombic efficiency of ca. 72%, the KPF6-90 and KPF6-75 electrolyte cells exhibit much higher Coulombic efficiency of ca. 86% similar to the KFSA cell at the initial cycle. Furthermore, the binary-salt electrolyte cells demonstrate higher Coulombic efficiencies at subsequent cycles than that of KPF6 one (Fig. 1d), which indicates stable SEI formation on the graphite electrode in the KPF6-KFSA binary salt electrolytes. Electrochemical performance and surface layer on the graphite electrode analyzed using hard X-ray photoemission spectroscopy will be discussed in the presentation.

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Figure 1