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Hydrothermal synthesis of K3FeF6 and its electrochemical characterization as cathode material for lithium-ion batteries

  • Minqing Liu
  • Yueli ShiEmail author
  • Quanchao ZhuangEmail author
Research Article
Part of the following topical collections:
  1. Engineering: Energy, Power and Industrial Applications


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.


Potassium iron fluoride Cathode materials Lithium-ion batteries Electrochemical impedance spectroscopy 

1 Introduction

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 [11] and nitrides [12] 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) [22]. 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 [27], LiMnF4 [28] and LiFe2F6 [29] relating to electrochemical properties for Li cells have been reported as well.

Recently, it has been reported that Li3FeF6 [26, 30, 31, 32, 33] and Na3FeF6 [34] electrodes for Li cells can deliver the reversible capacity up to ~ 100 mAh g−1 and ~ 150 mAh g−1, respectively, after 20 cycles. Thus, it is particularly urgent to explore new metal fluorides so as to take advantage of their electrochemical characterization. Among the various ternary fluorides, the cubic K3FeF6 in Fm-3m space group has a more symmetrical crystal structure, which means a more stable structure [35]. Meanwhile, K3FeF6 electrode possesses a theoretical capacity of 280 mAh g−1, which is much higher than the conventional cathodes (LiCoO2, LiFePO4, 140–170 mAh g−1).
$$C = \frac{N \times F}{3.6 \times M}$$
where C (mAh g−1) is specific capacity of a material, N represents the number of electrons transferred (N = 3), F is Faraday constant (96,485 C mol−1), M (g mol−1) is the molecular weight of material, and the unit of factor 3.6 is C mAh−1.

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 Experimental

2.1 Preparation of the materials

K3FeF6 was fabricated via a solvothermal reaction as illustrated in Fig. 1. Briefly, NH4HF2, Fe(NO3)3 9H2O and KF were dissolved in 8 mL distilled water to form a homogeneous solution, then adding 1 mL HF solution (40%) to the above solution and stirring for 0.5 h at room temperature. Simultaneously, the mixed solution was poured into a sealed PTFE bottle, heating at 180 °C for 72 h. Finally, the precipitation was collected by centrifugation and drying at 60 °C for 12 h.
Fig. 1

Schematic of the synthetic process for K3FeF6

2.2 Characterization

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

The X-ray powder diffraction pattern of K3FeF6 (Fig. 2a) match well with the standard data of K3FeF6 (PDF#22-1223). According to XRD analysis, the synthesized K3FeF6 possesses a cubic structure with space group Fm-3m. The Fe in K3FeF6 crystal structure (Fig. 2b) is surrounded by six fluorine atoms in a regular octahedron. There are two potassium sites (K1, K2),which are located at the octahedral site of (KFe6) and the tetrahedral site of KFe4) [35, 36, 37], respectively.
Fig. 2

The XRD patterns of K3FeF6 (a) and schematic illustration of the K3FeF6 structures (b)

3.2 XPS and TGA tests

XPS measurements are carried out to analyse the K3FeF6 chemical bonds, and F 1s, Fe 2p and K 2p spectra are shown in Fig. 3. The spectrum of F 1s has one peak (Fig. 3a), which is located at 684.01 eV, belonging to F–Fe bond [38, 39, 40]. The Fe2p core level spectrum (Fig. 3b) shows two main peaks at 714.7 eV and 728.01 eV, indicating Fe3+ in K3FeF6 [41, 42, 43], which are corresponded to Fe2p1/2 and Fe2p3/2, respectively. Meanwhile, there are three diminutive peaks at 723.32 eV, 713.24 eV and 710.25 eV. The 723.32 eV peak can be attributed to the Fe–F bonds [44]. For the K2p spectrum (Fig. 3c), the peaks at 292.66 eV and 295.36 eV are corresponded to K2p3 and K2p1, respectively. Through the above analysis, it is known that K3FeF6 has been synthesized. TGA is conducted to investigate the thermal stability of K3FeF6 (Fig. 3d), which exhibits only 0.3% weight loss when heated up to 500 °C. The K3FeF6 weight loss only appears at about 200 °C, which is corresponding to the moisture evaporation from the material. Based on TGA data, the synthesized material is very stable in term of high temperature (~ 500 °C).
Fig. 3

XPS spectra of K3FeF6 range from 0 to 1400 eV F 1s (a), Fe 2p (b), K 2p (c) and TGA curve (d) of K3FeF6 in nitrogen when heated up to 500 °C

3.3 SEM and TEM images of K3FeF6 powders

The different magnifications morphology of K3FeF6 powders are studied by SEM and TEM. K3FeF6 particles shows an irregular spherical shape with the size of about 200–300 nm (Fig. 4a–d). After high-energy ball milling (Fig. 4e, f), they are about 30-50 nm.
Fig. 4

SEM images of K3FeF6 (a, b); TEM images of K3FeF6 before (c, d) and after (e, f) high energy ball milling

3.4 CV and charge–discharge test

CV curves (Fig. 5a) are given to study the K3FeF6 electrode for Li cells (1.0–4.5 V, 0.2 mV s−1). In the initial lithiation process, there are two reversible peaks at 1.6 V (R1), and 1.4 V (R2). R1 and R2 are related to the conversion reaction step in Eq. (2) and (3) [34, 45], respectively.
Fig. 5

a CV curves (0.2 mV s−1, 1.0–4.5 V), b galvanostatic charge–discharge curves (10 mA g−1), c cycling performance (10 mA g−1) and d rate performance of K3FeF6 electrodes

$$2{\text{K}}_{ 3} {\text{FeF}}_{6} + {\text{Li}} \leftrightarrow {\text{LiF}} + 6{\text{KF}} + {\text{Fe}}_{ 2} {\text{F}}_{ 5}$$
$${\text{Fe}}_{ 2} {\text{F}}_{ 5} + 5{\text{Li}} \leftrightarrow 5{\text{LiF}} + 2{\text{Fe}}$$

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 [46]. 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

Ex-situ XRD analysis was performed to explored the electrochemical redox mechanism of K3FeF6 electrode (Fig. 6). In details, the typical peaks of K3FeF6 can still be observed when discharging to 1.6 V and meanwhile new peaks at 38.7°, 45.0°, 65.5° appears, which is related to LiF (PDF#78-1217). When the potential decreases to 1.4 V, the typical peaks of K3FeF6 are not very obvious and new peaks at 44.6° and 65°associated with Fe0 (PDF#06-0696) appears, suggesting that Fe3+ reduction to Fe0. The peaks of LiF and Fe0 become sharper when discharged to 1.0 V, illustrating that more LiF and Fe0 generated. The results above are corresponding to the CV observations. In charging process, the typical peaks of K3FeF6 become strong again when potential increases to 4.5 V. This phenomenon suggests the reversible reaction of K3FeF6 with lithium-ions, indicating a superior reversibility and a stable structure of K3FeF6 electrode.
Fig. 6

Ex-situ XRD analysis of K3FeF6 in Li cells at different voltages of the first cycle

3.6 EIS analysis of K3FeF6 electrodes

In Fig. 7, EIS is used to further investigate the electrochemical performances. At the open circuit voltage (3.42 V, Fig. 7a), the EIS curve of K3FeF6 electrodes consisted of two parts: high-frequency semicircle (HFS) and low-frequency line (LFL), which is ascribed to the SEI film and solid-state diffusion, respectively [49, 50]. With the potential decrease, the small HFS increases and elongate at 1.6 V (Fig. 7b), then it turned into two semicircles and LFL converts into a large arc at 1.4 V (Fig. 7c). Therefore, the Nyquist graphs at 1.4 V are composed of three parts: HFS, middle-frequency semicircle (MFS) and low-frequency arc (LFA). With the electrode potential decreasing to 1.0 V (Fig. 7d), both HFS and MFS increase rapidly. When the potential further increase to 2.5 V (Fig. 7e), an inclined line replaces the arc in the low frequency. Moreover, when charging to 4.0 V (Fig. 7f), both HFS and MFS become small. As shown in Figs. 1, S2, the Nyquist plots can consist of three parts: HFS, MFS and LFA/LFL, corresponding to lithium ion migration through the SEI film, the charge transfer resistance and lithium ions solid-state diffusion within the active electrode, respectively [51].
Fig. 7

EIS experimental data of K3FeF6 electrode at various potential 3.42 V (a), 1.6 V (b), 1.4 V (c), 1.0 V (d) at the first discharge process and 2.5 V (e), 4.0 V (f) at the first charge process and equivalent circuit proposed for analysis of the K3FeF6 electrodes (g)

An equivalent circuit (Fig. 7g) is given to fit the EIS of K3FeF6 electrode. In details, Rs, R1 and R2 represent Ohmic resistance, resistance of SEI film and charge transfer resistance, respectively. Additionally, Q1 and Q2 are the constant phase elements (CPE). Figs. S3, S4 show the simulated EIS compared with experimental EIS curves (discharge: 1.4 V, charge: 2.8 V), and Table S2, S3 show the corresponding parameters of K3FeF6 electrodes. In details, the uncertainties of all parameters are below 10%, suggesting that the equivalent circuit model can fit well with experimental data. Figure 8a provides the changes of R1 of K3FeF6 electrode received from the fitting EIS data in the initial discharge–charge process. In the discharge process, R1 almost have no changes from 3.4 to 1.4 V. However, from 1.4 to 1.0 V, it grows swiftly. From the above experimental results (Figs. 5a, 6), LiF appears due to the conversion reactions at 1.4 V. According to the literature, LiF is one of the main components of SEI film. Thus, the growing up of HFS radius in the vicinity of reaction potential is ascribed to SEI film formation. In the initial charge process (Fig. 8a), R1 rises firstly and reaches the maximum at around 2.6 V, then it decreases rapidly after 2.6 V. This phenomenon mainly results from the reversible generating and decomposition of SEI film. Based on previous research, LiF generated in the conversion reactions is the composition of the SEI film, so insertion/extraction lithium-ion reaction should also affect the change of the SEI film.
Fig. 8

Variations of R1(a) and R2(b) of K3FeF6 electrode with the electrode potential in the initial discharge–charge process

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.

4 Conclusion

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.

Supplementary material

42452_2019_904_MOESM1_ESM.docx (865 kb)
Supplementary material 1 (DOCX 864 kb)


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© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Lithium-ion Batteries Laboratory, School of Materials Science and EngineeringChina University of Mining and TechnologyXuzhouChina

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