Improving K-Ion Battery Performance By Using K-Metal-Pretreated Electrolyte Solutions

Abstract

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.