Improving K-Ion Battery Performance By Using K-Metal-Pretreated Electrolyte Solutions
Authors: Hosaka Tomooki , Matsuyama Tatsuo , Tatara Ryoichi , Gossage Zachary Tyson , Komaba Shinichi 
Published: February 2024

Nonaqueous K-ion batteries (KIBs) have attracted much attention as a potential high-voltage and high-power secondary battery due to the low standard electrode potential of K+/K in carbonate ester solutions and small Stokes radius of K+ ions compared with Li cases.1 In studies of new electrode materials and electrolytes for KIBs, half-cell measurements using highly reactive K metal counter electrodes are a standard practice.1 The presence of K metal has been reported to alter both the electrochemical performance and interface chemistry of the working electrode via migration of electrolyte decomposition products across the cell.2-4 Herein, we systematically evaluated the electrolyte decomposition products formed between K metal and electrolytes prepared with KPF6, KN(SO2F)2 (KFSA) or the combined salts (K(PF6)0.75(FSA)0.25) dissolved in ethylene carbonate/diethyl carbonate (EC/DEC).5, 6

To investigate the effect of degradation products on KIB performance, we prepared electrolytes containing KPF6, KFSA or K(PF6)0.75(FSA)0.25 dissolved in EC/DEC. We stored 2 mL of each electrolyte in contact with a freshly cut K metal disk with a diameter of 15 mm for 7 days (Figure 1a-d). Then, the K metal was removed, and all the concentration and composition of electrolytes were adjusted to 1 mol kg-1 K(PF6)0.75(FSA)0.25/EC/DEC by adding either KFSA or KPF6, as shown in Figure 1b-d. We denote the obtained electrolytes based on their treatment by K metal (K-KPF6, K-KFSA, and K-KPF6:KFSA).
Using gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS), we identify significant formation of decomposition products derived from carbonate esters, such as oligocarbonates for K-KPF6 (Table 1), while K-KFSA predominantly generates anions combining FSA- with the solvent structures (Table 1). Using three-electrode cells, we delineated positive effects of the K-KFSA and K-KPF6:KFSA electrolytes on graphite negative electrode performance and the negative impact of oligocarbonates in K-KPF6 on K2Mn[Fe(CN)6] positive electrodes.
We further evaluated the K-KPF6:KFSA electrolyte in a graphite||K2Mn[Fe(CN)6] full cell. As shown in Figure 2a, the initial charge/discharge curves for the K-KPF6:KFSA electrolyte (purple) cell exhibited a larger reversible capacity (119 mAh g-1 positive) and higher Coulombic efficiency (CE, 74.5 %) than the untreated electrolyte cell shown in red (110 mAh g-1 and 69.6 %). The improved initial CE for the K-KPF6:KFSA electrolyte led to a higher energy density of 277 Wh (kg-active materials)-1 compared with that of 257 Wh kg-1 in the untreated electrolyte. Figure 2b,c displays the CE and capacity retention over 500 cycles. For more than 100 cycles, the K-KPF6:KFSA cell continued to demonstrate a higher CE than the untreated electrolyte (Figure 2b), though both cells eventually showed CEs of approximately 99.9%. Furthermore, the use of K-KPF6:KFSA improved the full cell’s capacity retention (78%) compared with the untreated cell (72%), as shown in Figure 2c. These results suggest that the electrolyte decomposition products formed by K metal treatment can minimize battery capacity loss by promoting initial SEI formation. Indeed, density functional theory calculation and hard X-ray photoelectron spectroscopy indicated the incorporation of the FSA-derived structures into the solid electrolyte interphase at graphite, which is not observed in K metal-free cells. In the presentation, the impact of electrolyte decomposition products on the electrochemical properties of KIBs’ electrodes and their mechanisms will be further discussed.

References:

T. Hosaka, K. Kubota, A. S. Hameed and S. Komaba, Chem. Rev., 2020, 120, 6358–6466.

T. Hosaka, S. Muratsubaki, K. Kubota, H. Onuma and S. Komaba, J Phys Chem Lett, 2019, 10, 3296-3300.

L. Madec, V. Gabaudan, G. Gachot, L. Stievano, L. Monconduit and H. Martinez, ACS Appl. Mater. Interfaces, 2018, 10, 34116-34122.

H. Wang, D. Zhai and F. Kang, Energy Environ. Sci., 2020, 13, 4583-4608.

T. Hosaka, T. Matsuyama, K. Kubota, S. Yasuno and S. Komaba, ACS Appl. Mater. Interfaces, 2020, 12, 34873-34881.

T. Hosaka and S. Komaba, Bull. Chem. Soc. Jpn., 2022, 95, 569-581.

Figure 1