Hydrothermal synthesis of K3FeF6 and its electrochemical characterization as cathode material for lithium-ion batteries
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K3FeF6 was synthesized through a simple hydrothermal reaction for a novel cathode material of lithium-ion batteries. From the SEM and TEM images, the synthesized K3FeF6 particle are about 30–50 nm after high energy ball milling. K3FeF6 electrode delivers a high reversible capacity of 212.6 mAh g−1 and it maintained 131 mAh g−1 after 30 cycles. Additionally, in rate performance test, when the current density returns back to the 10 mA g−1 again, the full recovery of the capacity exhibits its superior rate performance. The electrochemical redox mechanism, herein studied through Ex-situ XRD of K3FeF6 electrodes at different polarization voltages, shows the satisfactory reversibility of structure.
KeywordsPotassium iron fluoride Cathode materials Lithium-ion batteries Electrochemical impedance spectroscopy
Lithium ion batteries (LIBs) possess overwhelming advantages over other cell counterparts in developing reliable power sources. However, the limited capacity of conventional cathode materials, such as LiCoO2 (~ 140 mAh g−1) [1, 2, 3, 4], can hardly meet the ever-growing demands. Moreover, sources of lithium and cobalt are relatively expensive. Thus, it is extremely urgent to explore cost-effective cathodes with readily available materials. For example, metal fluorides [5, 6, 7, 8], oxides [9, 10], sulfides  and nitrides  have been explored for LIBs.
Fluorides as an intriguing candidate recently have attracted significant attention due to their high theoretical capacities, economical merits, low toxicity and good thermodynamic stability when used as cathodes in LIBs. Notably, fluorine shows high electronegativity. Therefore, the fluoride cathodes can deliver a high redox potential and operating voltage of as Li cells [13, 14, 15, 16]. In fact, several binary metal fluorides have been already reported for their high specific energies [8, 17, 18, 19, 20, 21], such as iron fluoride (712 mAh g−1, 1950 Wh kg−1) . On the other hand, researches on ternary fluorides are intensified to facilitate the development of lithium-ions batteries. The Li-rich fluorides Li2MnF5 and Li3MF6 (M = V, Cr, Fe) concerning theoretical investigations have been published recently [13, 23, 24, 25, 26]. And Li3VF6 , LiMnF4  and LiFe2F6  relating to electrochemical properties for Li cells have been reported as well.
Herein, we have studied a concise solvothermal reaction to fabricate K3FeF6 as a promising cathode material for LIBs. Thermogravimetric analysis (TGA) of K3FeF6 exhibits only 0.3% weight loss when heated up to 500 °C. Meanwhile, the electrochemical properties of K3FeF6 electrode are also characterized and the electrochemical redox mechanism is checked by Ex-situ XRD.
2.1 Preparation of the materials
The crystal structures of K3FeF6 were investigated by Cu Kɑ radiation on a D/MAX-3B X-ray diffractometer. While, S-4800 and JEOL JEM-2010SEM were used to get scanning electron microscopy (SEM) images. The thermogravimetric analysis (TGA) data was recorded from room temperature up to 500 °C under nitrogen atmosphere (10 °C min−1). X-ray spectroscopy (XPS) spectra were obtained to analyze the chemical bonds and elements of K3FeF6.
2.3 Electrochemical measurements
K3FeF6 electrode was prepared by spreading a mixture onto an aluminum foil, which were composed of 70 wt% K3FeF6, 20 wt% carbon black and 10 wt% PVDF binder. The electrolyte consisted of 1 mol·L−1 LiPF6 in a mixture of EC, DMC and DEC (wt%,1:1:1). Charge–discharge test was carried out with CR2025-type coin cells. Cyclic voltammetry (CV, 0.2 mV s−1, 1.0–4.5 V) and electrochemical impedance spectroscopy (EIS) was measured using an electrochemical workstation (CHI660D).
3 Results and discussion
3.1 XRD analysis of synthesized material
3.2 XPS and TGA tests
3.3 SEM and TEM images of K3FeF6 powders
3.4 CV and charge–discharge test
Meanwhile, there are two consecutive oxidation peaks at around 2.5 V(O1) and 4.0 V(O2) in the delithiation process, O1 is ascribed to the reversible oxidation of Fe0 to Fe3+, while O2 is corresponded to the decomposition of the electrolyte . In addition, after 2 cycles, the well overlapped CV curves displays an excellent stability and superior reversibility of Li+ insertion/extraction in the subsequent cycles.
Figure 5b shows the large initial irreversible capacity which due to the formation of the SEI film on the active material surface and the decomposition of electrolyte [47, 48]. The decent reversible capacity (212.6 mAh g−1) K3FeF6 electrode is superior to some of the reported fluorides in Li cells, showing in Tab.S1. Though the coulombic efficiency (CE) of the initial cycle is only 65.3%, it increases to 86.3% at the third cycle. Figure 5c shows the cycling data of K3FeF6 electrode. After 30 cycles, K3FeF6 electrode maintains 131 mA h g−1 (CE, 97.9%) with a capacity retention of 78.4% compared with the second cycle (167 mA h g−1). In rate performance test (Fig. 5d), K3FeF6 electrode delivers the average reversible capacities of 145 (10 mA g−1), 92.3 (20 mA g−1), 53.7 (50 mA g−1) as well as 36.5 mA h g−1 (100 mA g−1), respectively. Notably, the discharge capacity of K3FeF6 electrode can return to about 150 mA h g−1, when the current density changes from 100 to 10 mA g−1, implying the stable structure and superior reversibility of K3FeF6 electrode.
3.5 XRD analysis of K3FeF6 electrodes at different voltages
3.6 EIS analysis of K3FeF6 electrodes
Figure 8b shows the R2 changes of K3FeF6 electrode in middle-frequency region in the initial discharge–charge process. From the open circuit potential (3.4 V) to 1.6 V, R2 almost keeps stable. However, after 1.6 V, R2 begins to increase, originating from the first step conversion reaction (2K3FeF6 + Li↔LiF + 6KF + Fe2F5). Then R2 shows swift growth after 1.3 V, which obviously is ascribed to the Fe, LiF and KF generating in the discharge process. In the charge process, R2 firstly keeps growth (1.1–1.5 V) and then decreases (1.5–2.7 V), which shows a superior reversibility.
K3FeF6 was synthesized by a simple solvothermal method and applied to cathode materials for Li cells. TGA data exhibits only 0.3% weight loss (~ 500 °C), implying the good thermal stability of K3FeF6. After high energy ball milling, K3FeF6 particles decrease to 30-50 nm. The 1st and 30th discharge capacities (10 mA g−1) are 212.6 mA h g−1 and 131 mA h g−1, respectively. In rate performance test, the average reversible capacities of first 10 cycles (10 mA g−1) is 145, when the current density changes back to 10 mA g−1 again, the discharge capacities can return to about 150 mA h g−1, showing a superior rate performance. The electrochemical redox mechanism, investigated by Ex-situ XRD of K3FeF6 electrodes at different discharge–charge voltages, shows the reversibility of the reaction and the stable structure. EIS results revealed that, the EIS of K3FeF6 electrode consists of HFS, MFS and LFC/LFL, respectively, which coincides with the insertion/extraction reaction mechanism model.
This work was supported by the Fundamental Research Funds for the China University of Mining and Technology (2017XKQY063).
Compliance with ethical standards
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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