【Background】
Potassium-ion batteries (PIBs) have the advantages of low redox potential and high natural abundance of elements, making them promising to become electrochemical energy storage technologies in the energy market segment. In order to realize the application of potassium-ion batteries, cathode materials (such as layered metal oxides, polyanionic oxides, etc.) and anode materials (carbon-based materials, alloys, etc.) have been widely studied. At present, graphite is the most promising anode material for PIB commercialization, which not only enables reversible intercalation of potassium ions (K+) (KC8) in a specific electrolyte, but also has a high theoretical capacity (279 mA h g-1). However, most of the carbonate-based electrolytes applied to PIB, especially propylene carbonate (PC)-based electrolytes, are not compatible with graphite electrodes and are prone to solvent co-embedding of graphite, resulting in graphite stripping or electrolyte decomposition. Therefore, it is of scientific significance to study how to effectively improve the compatibility between the electrolyte and the graphite electrode in PIB.
In recent years, in order to improve the compatibility between PC-based electrolytes and graphite electrodes, researchers have proposed many effective strategies, including the introduction of additives, increasing the concentration of electrolytes, and designing local high-concentration electrolytes. At the same time, especially in the field of lithium-ion batteries, the reasons for the incompatibility between PC and graphite electrodes are also the difficulty of PC to form a strong SEI film without inhibiting solvent co-embedding, graphite stripping and electrolyte decomposition, and the strength of the interaction between metal ions and solvents in the solvation structure of the electrolyte is the fundamental reason affecting the electrode performance (ACS Energy Lett. 2018, 3, 335-340; ACS Energy Lett. 2019, 4, 1584-1593), and then to fine-tune how the interaction between electrolyte components (e.g., solvent-solvent interaction) affects the interaction between metal ions and solvents, and then how it affects the compatibility of PC-based electrolyte with graphite electrode (Adv. Sci. 2022, 9, 2202405; ACS Nano 2023, 17, 18062-18073; Adv. Funct. Mater. 2024, 2401118）。 Since the beginning of this series of studies, the focus of electrolyte research has gradually shifted from the SEI membrane to the regulation of the solvation structure of the electrolyte, as well as further deep peeling and understanding of the true role of the SEI membrane (Adv. Mater. 2021, 2005993; ACS Energy Lett. 2022, 7, 490-513；ACS Energy Lett. 2022, 7, 3545-3556; Adv. Funct. Mater. 2023, 2210292), rather than just SEI films, especially considering the differences and effects of the desolvation behavior of the solvation structure on the positive/negative electrode surfaces (Adv. Mater. 2021, 2102964; Angew. Chem. Int. Ed. 2023, 135, e202216189; ACS Energy Lett. 2024, 9, 1604; Adv. Energy Mater. 2024, 202304321）。 However, for PC-based electrolytes, how to understand the bulk phase and interface behavior of PC solvents, as well as the relationship between PC solvent and graphite compatibility, so as to inhibit K+-PC co-embedding, graphite stripping and electrolyte decomposition, are still scientific problems worth exploring. For example, how to characterize and resolve the interaction differences between PC solvent and other solvents in the solvation structure of electrolyte at the molecular scale, and how to structure the microstructure of electrolyte and its relationship with electrode properties need to be further studied.
Figure 1. Schematic diagram of the detection of intermolecular interactions and the behavior of different electrolytes in graphite.
【Research Introduction】
Recently, Ming Jun, a researcher at Changchun Institute of Applied Chemistry of the Chinese Academy of Sciences, has realized the reversible intercalation of K+ in the graphite electrode system of PC-based electrolyte by introducing different types of fluoroethers. In particular, the effect of the interaction between fluoroethers (e.g., 1,1,2,2-tetrafluoroethyl-2,2,2,3,3-tetrafluoropropyl ether (HFE), 1,1,2,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (TFTFE)) and PC was determined by two-dimensional nuclear magnetic spectroscopy (1H-19F HOESY) on the electrolyte and electrode performance. Studies have shown that the strong interaction between HFE and PC can effectively attenuate the interaction between K+-PC and promote the reversible intercalation of K+ in graphite electrodes (Fig. 1). Based on this, the authors proposed a corresponding interface model, analyzed the differences in the kinetic and thermodynamic properties of K+-solvent-anionic complexes in different electrolyte systems, and clarified the stability of the electrolyte at the electrode interface. The electrolyte designed in this study not only has flame retardant properties, but also can make potassium-sulfur batteries exhibit excellent performance, which provides a new perspective for the application and design of potassium-ion batteries. The work was published in the internationally renowned academic journal ACS Energy Letters, titled "Electrolyte Intermolecular Interaction Mediated Nonflammable Potassium-Ion Sulfur Batteries".
Figure 2. Electrochemical performance of PC-based electrolytes.
【Content Description】
1. Electrolyte design and electrochemical performance
Conventional PC-based electrolytes (e.g., 1.0 M KFSI in PC) are often incompatible with graphite and will react at 0.8 V (vs. K/K+) appeared on the left and right of K+-PC co-embedding, and the PC continued to reduce and decompose, and finally destroyed the graphite structure. In this study, fluoroethers (e.g., HFE/TFTFE) were introduced into PC-based electrolytes, and the reversible intercalation of ultra-low concentration PC-based electrolytes (0.3 M KFSI in PC/HFE and 0.3 M KFSI in PC/TFTFE) in graphite electrodes was realized, and the initial coulombic efficiency was as high as 88.65% and 86.73%, and the electrolyte had flame retardant characteristics, which greatly improved the safety performance of the battery. Cyclic voltammetry and in-situ XRD further proved that the designed low-concentration electrolyte has good compatibility with graphite. In addition, the authors confirmed that the SEI film was insufficient to stabilize the electrode and inhibit K+-PC co-embedding through "exchange experiments", and then proved that the main reason for the improvement of electrolyte compatibility may be the change of electrolyte microstructure. In subsequent tests, the authors compared the cycling stability of PC/HFE and PC/TFTFE-based electrolytes, and the PC/HFE-based electrolytes exhibited excellent performance, the root cause of which was the difference in the interaction between the two diluents and PC. Therefore, how the diluent and the PC molecule interact and how the difference in action affects battery performance deserves further exploration at the molecular scale (Figure 2).
2. Characterization of solvent-solvent interactions
The authors characterized solvent-solvent interactions by NMR, Raman, and infrared spectroscopy in pure solvent systems. The NMR results showed that the charge distribution of atoms in PC was altered with the addition of diluent, which was mainly due to the dipole-dipole interaction between PC and diluent, and HFE had a greater effect on PC than TFTFE. Raman and infrared spectroscopy results showed that the number of C=O bonds in PC increased more significantly after the addition of HFE, confirming the strong interaction between PC and HFE. In addition, the binding energy (ΔE) of the formation of co-solvent complexes (e.g., PC+HFE → PC-HFE) between solvents further demonstrates the strong interaction between PC and HFE. Specifically, the binding energy ΔE2 of the PC-HFE mixture was -91.1 kJ/mol, which was much lower than that of the PC-PC solvent (i.e., ΔE1 = -68.3 kJ/mol) and the PC-TFTFE solvent (i.e., ΔE3 = -74.3 kJ/mol), demonstrating a stronger interaction between PC-HFE compared to PC-TFTFE (Figure 3).
Figure 3. Solvent-solvent interaction characterization.
3. Characterization of the solvation structure of the electrolyte
The authors investigated the K+ solvation structure of the electrolyte to elucidate the compatibility of the electrolyte with graphite. The results show that the density of the electron cloud around C=O in the PC/HFE and PC/TFTFE-based electrolytes decreases, and the comparison of the effects of TFTFE and HFE on PC shows that HFE has a greater effect on the charge distribution in PC, weakens the binding between K+ and PC, and effectively inhibits the co-embedding of PC in graphite electrodes (Fig. 4).
Figure 4. Characterization of solvation structures of different electrolytes.
4. Solvation structure and interface characterization
Based on the above analysis, the authors constructed the K+ solvation structure by the formula of K+ [solvent] x [anion] (where x is the molar ratio of solvent to K+), and proposed an interface model of the K+ desolvation process to understand the effect of solvent-solvent interaction on the performance of graphite electrodes. In a 1.0 M KFSI in PC electrolyte (i.e., K+[PC]11.8[FSI-]), due to the large dielectric constant of PC, K+ and PC are tightly bound (d1), and the FSI- anion is far away from K+ (i.e., f1), so PC cannot be effectively desolvated (d1'), resulting in K+-PC co-embedding in the graphite structure and ultimately graphite stripping. However, in PC/HFE-based (i.e., K+[PC]11.8[HFE]13.2[FSI-]) and PC/TFTFE-based (i.e., K+[PC]11.8[TFTFE]14.9[FSI-]) electrolytes, the binding between K+ and PC is weakened (i.e., d2>d1, d3>d1) and enhanced between K+ and FSI- (i.e., f2< f1, f3, f3) due to the interaction between the diluent (i.e., HFE and TFTFE) and PC<f1）。 At the same time, the interaction between HFE and PC is stronger (U1 < U2) than TFTFE, so the binding of K+ and PC in the PC/HFE-based electrolyte is weakened during desolvation (i.e., d2'>d3'), promoting a more efficient desolvation process and thus obtaining more stable cycling performance.
Figure 5. Interface models and theoretical simulations.
In addition, the authors also evaluated the electrochemical stability (i.e., electrolyte stability) of the desolvated clusters at the graphite interface by the energy difference between the orbitals (ΔE = HOMO'-LUMO). In the PC/HFE-based electrolyte, there is a strong interaction between molecules, which increases the energy difference between the orbitals, and it is difficult for electrons to transfer electrons (Fig. 5).
5. Electrode interface characterization
In order to verify the rationality of the interface model, SEM and XPS characterization were performed on the electrodes after cycling in different electrolytes. The results show that compared with the original graphite, the electrode surface after cycling in 1.0 M KFSI in PC has serious spalling, while the graphite surface of PC/HFE and PC/TFTFE-based electrolytes is intact, and only a small number of by-products exist, indicating that the electrolyte has good compatibility with graphite. Further analysis of the graphite surface products after cycling in PC/HFE and PC/TFTFE-based electrolytes by deep XPS showed that the SEI formed in PC/HFE-based electrolytes was rich in inorganic components (mainly KF and sulfides) compared to PC/TFTFE-based electrolytes, which contributed to lower desolvation energy and thus stable cycling performance (Fig. 6).
Figure 6. Graphite electrode interface characterization.
6. K-S battery application
The authors assemble potassium-embedded graphite (KC8)||Vulcanized polyacrylonitrile (SPAN) further validated the compatibility of the designed PC-based electrolyte (i.e., 0.3 M KFSI in PC/HFE) with graphite electrodes. After 200 cycles at a current density of 100 mA g-1, the capacity retention rate was 73.3%, and the electrolyte showed good rate performance at different current densities. In addition, when the temperature drops to -10°C, the battery can still cycle stably. These results show that the designed electrolyte exhibits good compatibility with graphite (Figure 7).
Figure 7. K-S full battery performance.
【Conclusion】
The authors developed a non-flammable, low-concentration, graphite-compatible PC-based electrolyte in potassium-ion batteries by introducing fluoroether as a diluent to modulate solvent-solvent interactions. The difference in intermolecular interaction between fluoroethers (i.e., HFE, TFTFE) and PC solvents was determined by 1H-19F HOESY, and it was found that fluoroethers could interact with PC solvents through dipole-dipole interactions to promote reversible K+ (de)embedding, so that the designed K-S cells exhibited wide temperature adaptability. This work mainly highlights the importance of solvent-solvent interactions in electrolytes, contributing to a deeper understanding of electrolytes at the molecular level.
【Paper Link】
Honghong Liang, Pushpendra Kumar, Zheng Ma, Fei Zhao, Haoran Cheng, Hongliang Xie, Zhen Cao, Luigi Cavallo, Qian Li, Jun Ming*, Electrolyte Intermolecular Interaction Mediated Nonflammable Potassium-Ion Sulfur Batteries, ACS Energy letters, 2024.
https://pubs.acs.org/doi/full/10.1021/acsenergylett.4c00591
Article source: Energy Scholar
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