Introduction

Rechargeable lithium-ion batteries (LIBs) are the most effective power storage systems for portable electronic devices and considered as promising candidates for electric vehicles (EVs) and hybrid electric vehicles (HEVs) [1]. Compared with traditional fossil energy, LIBs are renewable and clean energy and friendly to the environment. Recently, with the rapid development of LIBs technology, demands for both energy and power density have continuously increased. One key challenge is developing high-performance electrode active materials, and the cathode material is a vital factor for improving the electrochemical properties of LIBs, including the specific capacity, cycling capability, rate capability, etc. [2, 3]. Commercialized cathode materials, such as LiCoO2 [4], LiMn2O4 [5], and LiFePO4 [6], suffer from low theoretical capacities due to the intercalation reaction involving only a single electron reaction, which cannot satisfy the demands of EVs. In the past several years, multi-electron materials have attracted substantial interest because they can realize the transfer of more than one electron through the conversion reaction [7]. Metal fluorides are ideal cathode materials with high theoretical capacities, energy densities, and operating voltages. Among them, FeF3 has been regarded as the most suitable cathode material due to its high theoretical specific capacity of 712 mAh g−1 (3e− transfer) and 237 mAh g−1 (1e− transfer), high discharge voltage plateau at approximately 2.7 V, and superb thermal stability [8,9,10].

Despite these remarkable merits, FeF3 as a cathode material still has several shortcomings, which have restricted its practical application. The main drawback of FeF3 is its electronic insulating behavior caused by a high ionicity, which induces a large band gap of the Fe-F bond and eventually leads to a low actual specific capacity, an inferior rate capability, and poor energy efficiency [11,12,13]. In order to resolve these issues, various strategies have been adopted to overcome the poor electronic and ionic conductivities. Generally, the methods to improve conductivity can be summarized in three aspects as follows: (1) element doping. Element doping can effectively decrease the band gap and actively effect microcrystal growth [14, 15]. Rahman et al. prepared Co-doped iron fluoride (Fe0.9Co0.1F3·0.5H2O) by a non-aqueous precipitation method, resulting in a high discharge capacity of 227 mAh g−1 at 0.1 C between 1.8 and 4.5 V [14]. (2) Surface coating. Modification by introducing a coating layer can significantly shorten the Li+ transport length and alleviate volume changes [16]. Ma et al. successfully fabricated FeF3 coated with poly (3,4-ethylenedioxythiophene) (PEDOT) via a novel in situ polymerization method, and the sample exhibited a high power capability of 120 mAh g−1 at 1 A g−1 at room temperature due to the improved ionic and electronic transport in the electrode [17]. (3) Fabricating composite with conductive additives. It can substantially enhance the cycling and rate performance of the FeF3 cathode material [18,19,20,21]. Jung et al. obtained FeF3/ordered mesoporous carbon (OMC) nanocomposite that showed a high reversible specific capacity (178 mAh g−1 at 0.1 C during the second cycle in the voltage range of 2.0–4.5 V) and better cycling stabilities (capacity fading of 8.8%) than bulk FeF3 (capacity fading of approximately 42%) at 30 cycles [22]. Noticeably, the fabrication of composite with conductive network is the most beneficial approach to improve both the ionic and electronic conductivities to eventually enhance the electrochemical performance of the cathode material.

Iron(III) fluoride cathode materials with different amounts of hydration water, for example, FeF3·0.33H2O [23], FeF3·0.5H2O [24], FeF3·3H2O [25], and FeF3 [26], have been extensively reported. Among them, hexagonal tungsten bronze-type FeF3·0.33H2O demonstrated that with the best electrochemical property, its characteristic one-dimensional hexagonal cavity is convenient for efficient Li+ transport and can facilitate electrolyte penetration [27]. In addition, the unique structure can effectively limit the movement of water and stabilize the crystal structure. Different functionalized carbon matrixes have been used as conductive networks, but overall, carbon nanotubes (CNTs) and graphene exhibit significant potential as conductive medium due to their distinguished electronic conductivities and excellent stabilities [28,29,30,31]. Graphene, with its large specific area, can promote sufficient contact at the electrode and electrolyte interface, and the graphene network plays an important role in electron transfer and ion migration. Furthermore, graphene provides excellent mechanical stability, which contributes to the bend and stretch of electrode [32, 33].

In this study, nanostructured FeF3·0.33H2O cathode material was synthesized via a liquid-phase method, and then, the precursor was milled with CNTs followed by sintering to obtain FeF3·0.33H2O/CNT composite that was further mixed with graphene conducting paste without a binder. Finally, the CNTs and graphene co-modified FeF3·0.33H2O nanocomposite was successfully prepared. The CNTs with intrinsic flexibility and large specific surface area can greatly facilitate the electron transport, and graphene with high mechanical strength and high chemical stability can effectively buffer the volume change and provides a support for electrochemical reaction [31, 34, 35]. Moreover, the interconnecting of CNTs and graphene sheets can construct an integrated three-dimensional conductive framework, which tremendously promotes Li+ diffusion and simultaneously increases the structure stability. Therefore, compared to the FeF3·0.33H2O composite with a single conductive network of CNTs and pure FeF3·0.33H2O, FeF3·0.33H2O nanocomposite with CNTs and graphene networks exhibits superior electrochemical properties. The morphologies, crystal structures, and electrochemical performances of all the samples were systematically investigated.

Results and Discussion

Structural and Morphology Analysis

Thermogravimetric-differential scanning calorimetry (TG-DSC) measurement was carried out to confirm the dehydration temperature of the FeF3·3H2O precursor and the result is shown in Fig. 1a. Four stages of the weight loss curve are found in the regions of 30–110 °C, 110–250 °C, 250–450 °C, and 450–700 °C. In the first stage of 30–110 °C, a slight weight loss of approximately 3% can be attributed to phase transformation of crystal. In the second stage of 110–250 °C, the TG curve has a rapid weight loss of about 15% and the DSC curve shows an evident endothermic peak around 170 °C; the corresponding reaction process is the removal of hydration water (2.67 H2O) from FeF3·3H2O. In the third stage of 250–450 °C, the weight loss is about 6% which may be due to the removal of hydration water for FeF3·0.33H2O transforming to FeF3 and a weak exothermic peak is observed from the DSC curve. In the last stage of 450–700 °C, a little weight loss of about 4% is probably due to the decomposition of FeF3. According to these results, the precursor was dried at 80 °C in a vacuum oven to remove the absorbed water and calcinated at 240 °C to obtain FeF3·0.33H2O.

Fig. 1
figure 1

a TG-DSC curve of the FeF3·3H2O precursor measured from 30 to 700 °C at a heating rate of 10 °C min−1 under an argon atmosphere. b XRD patterns of FeF3·0.33H2O, FeF3·0.33H2O/CNT, and FeF3·0.33H2O/C + G

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X-ray diffraction (XRD) measurements were conducted to investigate the crystal structure of the synthesized samples. The XRD patterns of FeF3·0.33H2O, FeF3·0.33H2O/CNT, and FeF3·0.33H2O/C + G are depicted in Fig. 1b. All samples reveal diffraction peaks positioned at 2θ = 13.79°, 23.62°, and 27.80° in accordance with the (110), (002), and (220) facets, which matched well with the standard spectrum of hexagonal tungsten bronze structure FeF3·0.33H2O (PDF No. 76-1262) [36]. No evident characteristic peak of CNTs and graphene are observed in the XRD pattern of the FeF3·0.33H2O/CNT and FeF3·0.33H2O/C + G samples, which is mainly due to the low contents of CNTs and graphene.

The SEM and EDS measurements were performed to analyze the microstructure of the composites. The morphologies and particle sizes of FeF3·0.33H2O, FeF3·0.33H2O/CNT and FeF3·0.33H2O/C + G nanocomposites are shown in Fig. 2. As distinctly seen from Fig. 2a, the particle size of pure FeF3·0.33H2O is around 100 nm, and the particles are uniform in size and well distributed, slight aggregation is observed, and the particle size of FeF3·0.33H2O can be further confirmed by particle size distribution diagram shown in Fig. 2e. Figure 2b presents the morphology of FeF3·0.33H2O/CNT nanocomposite. Clearly, the conductive network of CNTs is intimately intertwined on the surface of the FeF3·0.33H2O particles. For the FeF3·0.33H2O/C + G nanocomposite, the surface of the FeF3·0.33H2O particles is wrapped by CNTs and graphene sheets; as shown in Fig. 2c, the FeF3·0.33H2O particles and CNTs are well covered by graphene sheets. In addition, the graphene sheets are preserved well-layered structure in the FeF3·0.33H2O/C + G nanocomposite, which can provide a fast channel for Li+ transport. The conductive contact between the FeF3·0.33H2O material and current collector can be significantly improved by CNTs and graphene due to their outstanding electronic conductivity. Especially graphene with a large surface area can provide an additional transport channel for Li-ion diffusion, which makes the FeF3·0.33H2O/C + G nanocomposite with superior electrochemical performance. EDS test was carried out to further investigate the elemental composition of the FeF3·0.33H2O/C + G nanocomposite. The elements of Fe, F, O, and C can be observed from the EDS image in Fig. 2d.

Fig. 2
figure 2

SEM images, a FeF3·0.33H2O, b FeF3·0.33H2O/CNT, and c FeF3·0.33H2O/C + G. d EDS of the FeF3·0.33H2O/C + G. e Particle size distribution diagram of FeF3·0.33H2O

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The morphology and detailed microstructure of the FeF3·0.33H2O/C + G nanocomposite were further studied by TEM, and the TEM images are displayed in Fig. 3. As shown in Fig. 3a, b, the FeF3·0.33H2O particles and CNTs and graphene sheets are closely interconnected with each other, which is consistent with the result of the SEM images. The HRTEM image shown in Fig. 3c offers no evident delineation between the bulk and wrapping layer; the lattice fringe spacing of 0.64 nm coincides with the (110) facet of FeF3·0.33H2O. The SAED pattern of the FeF3·0.33H2O/C + G nanoparticle is shown in Fig. 3d; the planes of (110), (002), (220), (132), and (004) correspond to the XRD results, which indexed to the hexagonal tungsten bronze structure FeF3·0.33H2O. The FeF3·0.33H2O/C + G nanocomposite with small particle size and superb conductive network structure, favors sufficient contact between the electrode material and electrolyte and facilitates Li-ion transport; therefore, better electrochemical performance can be achieved.

Fig. 3
figure 3

a, b TEM images of FeF3·0.33H2O/C + G nanocomposite. c HRTEM image of FeF3·0.33H2O/C + G nanocomposite. d SAED image of FeF3·0.33H2O/C + G nanocomposite

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Electrochemical Characterization

To investigate the electrochemical properties of all samples, galvanostatic charge/discharge tests were implemented in the voltage range of 1.8–4.5 V (vs. Li+/Li), and this voltage range allows only one electron reaction to occur. The charge-discharge profiles of all samples are shown in Fig. 4. The initial charge-discharge curves of the three electrodes at 0.1 C (1 C = 237 mAh g−1) rate are shown in Fig. 4a; the pristine FeF3·0.33H2O electrode exhibits the lowest initial discharge capacity of 217.5 mAh g−1, which may be due to the poor electronic conductivity of FeF3·0.33H2O. While the FeF3·0.33H2O/CNT and FeF3·0.33H2O/C + G electrodes deliver higher initial discharge capacities of approximately 225.1 mAh g−1 and 234.2 mAh g−1, respectively. In our test, the initial discharge capacity of the FeF3·0.33H2O/C + G electrode is only 16.7 mAh g−1 higher than that of the pristine FeF3·0.33H2O electrode, illustrating CNTs and graphene almost deliver no capacity in the FeF3·0.33H2O/C + G sample. The slightly increased capacity can be attributed to the CNTs and graphene incorporation enhanced the electron transport and reduced electrochemical polarization. From the initial charge-discharge curves of all electrodes, all the curves have an evident discharge plateau at 2.7 V due to the insertion reaction (Li+ + e− + FeF3 ∙ 0.33H2O → LiFeF3 ∙ 0.33H2O). The charge-discharge profiles of different cycles at 0.2 C rate are presented in Fig. 4b–d. As shown in Fig. 4b, the FeF3·0.33H2O electrode only delivers a capacity of 146.2 mAh g−1 at 0.2 C rate after 50 cycles. The FeF3·0.33H2O/CNT electrode delivers a capacity of 170.3 mAh g−1 after 50 cycles shown in Fig. 4c. It is worth noting that the FeF3·0.33H2O/C + G electrode still retain a capacity of 193.1 mAh g−1 even after 50 cycles shown in Fig. 4d. In addition, the FeF3·0.33H2O/C + G electrode presents the lowest charge voltage plateau and the highest discharge voltage plateau, demonstrating that it has the smallest electrochemical polarization and excellent reversibility, thus mitigate the voltage hysteresis. The better performance of FeF3·0.33H2O/CNT and FeF3·0.33H2O/C + G electrodes demonstrate that adding CNTs and graphene can effectively improve the conductivity of the FeF3·0.33H2O cathode material. Particularly, the FeF3·0.33H2O/C + G electrode exhibits the best electrochemical performance due to the interlacing of CNTs and graphene forms a three-dimensional conductive structure, which tremendously facilitate the transport of Li-ion, and thus resulting in promoting the intercalation process of Li-ions [37, 38].

Fig. 4
figure 4

a The initial charge-discharge profiles of FeF3·0.33H2O, FeF3·0.33H2O/CNT, and FeF3·0.33H2O/C + G electrodes at 0.1 C rate. Charge and discharge profiles of different cycles (1st, 3rd, 30th, 50th) at 0.2 C rate, b FeF3·0.33H2O, c FeF3·0.33H2O/CNT, and d FeF3·0.33H2O/C + G

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To further demonstrate the excellent cycling stability of the FeF3·0.33H2O/C + G nanocomposite, the cycling capabilities of the FeF3·0.33H2O, FeF3·0.33H2O/CNT, and FeF3·0.33H2O/C + G electrodes up to the 50th cycle at 0.2 C rate in the voltage range of 1.8–4.5 V (vs. Li+/Li) are shown in Fig. 5a. The FeF3·0.33H2O electrode with a rapid capacity decay and a poor capacity retention of approximately 70.83% (capacity fading rate of 0.58% per cycle) after 50 cycles. The FeF3·0.33H2O/CNT electrode shows a capacity retention of about 79.65% (0.41% fading per cycle) after 50 cycles. Notably, the FeF3·0.33H2O/C + G electrode achieve the highest capacity retention of 85.48% (only 0.29% fading per cycle) after 50 cycles. Moreover, the coulombic efficiency of the FeF3·0.33H2O/C + G electrode can reach up to over 99% during the Li+ insertion and extraction processes. The above results demonstrate that CNTs and graphene can improve the electronic conductivity and enhance the discharge capacities of FeF3·0.33H2O. Particularly, the FeF3·0.33H2O/C + G electrode exhibits the best cycling performance, illustrating higher electrical conductivity, better reversibility, and lower polarization after adding of CNTs and graphene. CNTs with high surface area supply sufficient pathway for electron transfer and the graphene works as an excellent conductive network for enabling fast Li+ transport between the electrolyte and electrode [28, 35]. Moreover, the FeF3·0.33H2O particles and CNTs can work as spacers to impede the stacking of graphene sheets and thus provide high active surface area. Therefore, the interaction of FeF3·0.33H2O particles and CNTs and graphene can significantly improve the cycling performance.

Fig. 5
figure 5

a Cycling performances of FeF3·0.33H2O, FeF3·0.33H2O/CNT, and FeF3·0.33H2O/C + G electrodes at 0.2 C rate. b Rate performances of FeF3·0.33H2O, FeF3·0.33H2O/CNT, and FeF3·0.33H2O/C + G electrodes at different current density

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The rate capabilities of the FeF3·0.33H2O, FeF3·0.33H2O/CNT, and FeF3·0.33H2O/C + G electrodes were evaluated at 0.1 C, 0.5 C,1 C, 3 C, and 5 C rates and then again at 0.1 C rate and results are displayed in Fig. 5b. The discharge capacities of all samples are decreased with increased current density. As expected, the FeF3·0.33H2O/C + G electrode presents a superior rate performance among the three electrodes and delivers average discharge capacities of 228 mAh g−1, 210.7 mAh g−1, 194.4 mAh g−1, 170.5 mAh g−1, and 140.4 mAh g−1 at 0.1 C, 0.5 C, 1 C, 3 C, and 5 C rates. When the rate is returned to 0.1 C, the electrode can still deliver a discharge capacity of 226.7 mAh g−1. For comparison, FeF3·0.33H2O and FeF3·0.33H2O/CNT electrodes show inferior rate performance; they deliver poor discharge capacities of 81.7 mAh g−1 and 115.7 mAh g−1 at 5 C rate, which are remarkably lower than that of FeF3·0.33H2O/C + G electrode. As a result, the rate capability of the FeF3·0.33H2O/C + G electrode is significantly improved compared to those of FeF3·0.33H2O without or with a single CNT conductive network. Therefore, the good rate capability of the FeF3·0.33H2O/C + G electrode result from the CNTs and graphene conductive networks, which enhanced the electronic conductivity, and above all, the constructed three-dimensional conductive network is beneficial for Li-ion insertion and extraction between electrodes.

Cyclic voltammogram (CV) measurements were carried out to further examine the electrochemical properties of the FeF3·0.33H2O, FeF3·0.33H2O/CNT, and FeF3·0.33H2O/C + G electrodes at a scan rate of 1 mV s−1 between 1.8 V and 4.5 V (vs. Li+/Li) which are shown in Fig. 6. The three curves display similar shapes with apparent oxidation/reduction peaks corresponding to delithiation/lithiation processes. The oxidation and reduction peaks of the FeF3·0.33H2O/C + G electrode are detected at 3.32 V and 2.78 V, and the potential interval (ΔEp) is 0.54 V. While the ΔEp values of the FeF3·0.33H2O and FeF3·0.33H2O/CNT electrodes are 0.59 V and 0.62 V, respectively. Smaller ΔEp value indicates a smaller electrochemical polarization and a better reversibility of the electrode. In addition, the FeF3·0.33H2O/C + G electrode exhibits a higher current and a larger area than those of the FeF3·0.33H2O and FeF3·0.33H2O/CNT electrodes. The area surrounded by the CV curve represents the capacity of the material; the larger area is related to the higher capacity, and the change rate of area represents the decay rate of capacity. The results reveal that the FeF3·0.33H2O/C + G electrode has a higher capacity and better reversibility, which is well consistent with the galvanostatic charge/discharge tests.

Fig. 6
figure 6

CV curves of FeF3·0.33H2O, FeF3·0.33H2O/CNT, and FeF3·0.33H2O/C + G electrodes at a scan rate of 1 mV s−1

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Electrochemical impedance spectroscopy (EIS) measurements were performed to explore the electrochemical reaction kinetics behavior of the FeF3·0.33H2O, FeF3·0.33H2O/CNT, and FeF3·0.33H2O/C + G electrodes after the 3rd cycle and 50th cycle, and results are shown in Fig. 7a, b. All the Nyquist plots of the electrodes after activation are consisted of a semicircle and a sloping line. The semicircle in the high frequency is related to the charge transfer resistance (Rct), which represents the reaction kinetics of the electrode. The smaller radius of semicircle demonstrates the easier transport of Li+ and the electron transfer between the electrolyte and electrode interface, and the sloping line in the low frequency is associated with the Warburg resistance (Zw) of Li+ diffusion in the bulk of cathode material [39]. The corresponding equivalent circuit model was constructed to illustrate the impedance spectra shown in Fig. 7e; the uncompensated ohmic resistance (Rs) represents the resistance of the electrolyte and electrode material, and the constant phase-angle element (CPE) represents the double-layer capacitance and passive film capacitance [40]. The impedance values of Rs and Rct for the three electrodes after the 3rd and 50th cycle are listed in Table 1. No significant difference of Rs values for the three electrodes after the 3rd cycle is noted. However, the Rct value (50.9 Ω) of the FeF3·0.33H2O/C + G electrode is evidently lower than those of the FeF3·0.33H2O (115.7 Ω) and FeF3·0.33H2O/CNT (68.2 Ω) electrodes, which indicated less polarization of the FeF3·0.33H2O/C + G electrode. Moreover, the Rct value of the FeF3·0.33H2O/C + G electrode is 86.5 Ω after the 50th cycle, which is also the smallest among the three electrodes. The lower Rct value of the electrode after activation suggested better charge transfer kinetics behavior. The lithium ion diffusion coefficients (DLi+) of the FeF3·0.33H2O, FeF3·0.33H2O/CNT, and FeF3·0.33H2O/C + G electrodes are calculated from the following equation [41],

$$ {D}_{\mathrm{Li}+}=\frac{{\mathrm{R}}^2{\mathrm{T}}^2}{2{\mathrm{A}}^2{\mathrm{n}}^4{\mathrm{F}}^4{\mathrm{C}}^2{\upsigma}_{\upomega}^2} $$
(1)
Fig. 7
figure 7

The Nyquist plots of FeF3·0.33H2O, FeF3·0.33H2O/CNT, and FeF3·0.33H2O/C + G electrodes; a the 3rd cycle and b the 50th cycle. The relationship plots of FeF3·0.33H2O, FeF3·0.33H2O/CNT, and FeF3·0.33H2O/C + G electrodes between Z’ and ω−1/2 at low-frequency region; c the 3rd cycle and d the 50th cycle. e The corresponding equivalent circuit model of EIS

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Table 1 Rs, Rct, and DLi+ values of FeF3·0.33H2O, FeF3·0.33H2O/CNT, and FeF3·0.33H2O/C + G electrodes after the 3rd and 50th cycle
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In Eq. (1), R is the gas constant, T is the absolute temperature, A is the surface area of electrode, n is the number of electrons involved in the redox reaction, F is the Faraday constant, C is the molar concentration of Li+, and σω is the Warburg coefficient which can be obtained from the following relationship,

$$ {\mathrm{Z}}^{\hbox{'}}={R}_{\mathrm{s}}+{R}_{\mathrm{ct}}+{\upsigma}_{\upomega}{\upomega}^{-1/2} $$
(2)

where Z’ is the real part of impedance and ω is the angular frequency in the low-frequency region. The linearity of Z’ and ω− 1/2 after the 3rd cycle and 50th cycle are shown in Fig. 7c, d. The Li+ diffusion coefficients of the three electrodes are listed in Table 1. The DLi+ value (1.67 × 10−12 cm2 s−1) of the FeF3·0.33H2O/C + G electrode after the 3rd cycle is higher than those of the FeF3·0.33H2O/CNT (1.19 × 10−12 cm2 s−1) and FeF3·0.33H2O (7.63 × 10−13 cm2 s−1). In addition, the DLi+ values of the FeF3·0.33H2O, FeF3·0.33H2O/CNT, and FeF3·0.33H2O/C + G electrodes after the 50th cycle are 2.96 × 10−13 cm2 s−1, 7.10 × 10−13 cm2 s−1, and 1.21 × 10−12 cm2 s−1, respectively. Apparently, the DLi+ values of the FeF3·0.33H2O/C + G electrode are the highest among the three electrodes, indicating that the FeF3·0.33H2O/C + G shows better electrode reaction kinetics. The results confirm that the conductive network constructed by CNTs and graphene can effectively reduce the polarization of FeF3·0.33H2O/C + G electrode, which contribute to excellent electrochemical performance.

Conclusions

In summary, the FeF3·0.33H2O cathode material was successfully synthesized by a liquid-phase method, and the FeF3·3H2O precursor was milled with CNTs conductive network followed by sintering to obtain FeF3·0.33H2O/CNT nanocomposite, and then mixed with graphene conducting paste without a binder to obtain the FeF3·0.33H2O/C + G nanocomposite. The functional network consisted of CNTs and graphene provides an effective strategy to improve the electronic conductivity of FeF3·0.33H2O cathode material. The FeF3·0.33H2O/C + G nanocomposite exhibits better electrochemical performances with increased specific capacity, extended cyclic lifespan, and enhanced rate capability than that of pure FeF3·0.33H2O. The EIS results also indicate that the FeF3·0.33H2O/C + G electrode has the best electrochemical reaction kinetics behavior. The outstanding electrochemical performances of FeF3·0.33H2O/C + G can be attributed to the constructed three-dimensional conductive networks by CNTs and graphene, improving the electronic conductivity, facilitating the Li+ and electron transport, thus enhancing the cycling and rate capabilities. Therefore, the FeF3·0.33H2O cathode material modified with CNTs and graphene showed excellent electrochemical properties and exhibited great promise as a cathode material for LIBs application.

Methods

Synthesis of FeF3·0.33H2O Powder

FeF3·0.33H2O powder was synthesized via a liquid-phase method followed by an annealing treatment. For the synthesis of FeF3·0.33H2O powder, Fe(NO3)3·9H2O (Aladdin, 99.99%) and NH4F (Aladdin, 98%) were utilized as the iron and fluorine sources, respectively, and polyethylene glycol (PEG400, Aldrich, 20%) was used as a dispersant. First, 3.1 g Fe(NO3)3·9H2O was dissolved in 20 mL ethanol in a Teflon-lined stainless-steel autoclave, and then, three drops of PEG400 were added. Next, the solution was ultrasonicated for 10 min to obtain solution A. Then, 0.85 g NH4F was dissolved in 5 mL of deionized water and ultrasonicated to form solution B. Solution B was added dropwise into the constantly stirred solution A, and the yellow solution gradually became colorless, eventually gained conglobate precipitates. After stewing for 12 h at room temperature, the precipitates were alternately washed with deionized water and ethanol several times and then dried at 80 °C for 12 h in a vacuum oven. After cooling to room temperature naturally, the precipitates were ground into powder to obtain FeF3·3H2O precursor and then transferred into a tube furnace for calcination at 240 °C for 3 h under an argon atmosphere to remove the crystal water. Finally, the FeF3·0.33H2O powder was obtained.

Preparation of FeF3·0.33H2O/CNT + Graphene Combination Electrode

To prepare FeF3·0.33H2O with CNTs and graphene conductive networks, optimized amount of 5 wt% CNTs were added into the as-prepared precursor, uniformly ground and heated at tube furnace (240 °C for 3 h) under an argon atmosphere to obtain FeF3·0.33H2O/CNT powder. Then, 0.5 g FeF3·0.33H2O/CNT powder was added into 1.5 mL graphene N-methyl pyrrolidinone paste (Aladdin, graphene content: 1–1 .5wt%), stirred 4 h to form a homogeneous slurry. The slurry was pasted on an Al foil and dried at 85 °C overnight to obtain the FeF3·0.33H2O/CNT + graphene (denoted as FeF3·0.33H2O/C + G) combination electrode. Notably, the process of making combination electrode did not require the addition of a binder.

Characterization

Thermogravimetric-differential scanning calorimetry (TG-DSC) measurement of the precursor was carried out in the temperature range from 30 to 700 °C at a heating rate of 10 °C min−1 under an argon atmosphere. The crystal structures of all the samples were characterized by X-ray diffraction (XRD, Bruker AXS D8, Germany) with Cu Kα radiation in the 2θ range of 10°–80° at a scan rate of 8° min−1. The morphologies and particle sizes of the materials were observed by scanning electron microscopy (SEM, JEOL JSM-6610 LV) and energy-dispersive spectroscopy (EDS, JEOL JSM-6610 LV). Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were carried out to further investigate the microstructure of materials by using a transmission electron microscope (JEOL JSM-2100F).

Electrochemical Measurement

The electrochemical performances of the prepared cathode materials were characterized by CR2032 coin-type half-cells. The working electrodes were made by mixing the cathode materials (FeF3·0.33H2O or FeF3·0.33H2O/CNT), carbon black (Super P Li carbon), and polyvinylidene fluoride (PVDF) at a weight ratio of 90:5:5 in N-methyl pyrrolidinone (NMP). When the slurry was stirred uniform, it was pasted on an Al foil and dried at 85 °C overnight. The FeF3·0.33H2O/C + G combination electrode was fabricated as mentioned above. The cathode electrodes were pressed and cut into several disks and weighted, and then they were dried at 85 °C for 4 h in a vacuum oven. The coin-type cells were assembled in an argon-filled glove box, where the oxygen and water contents were controlled to less than 0.1 ppm, metal Li foils as anodes and Celgard 2400 membrane as separator; 1.0 M LiPF6 in ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) with a volume ratio of 1:1:1 were used as electrolyte. All the coin cells were aged for 4 h before testing. Galvanostatic charge/discharge tests were performed in the voltage range of 1.8–4.5 V (vs. Li+/Li) on a Land battery test system (LAND CT-2001A, Wuhan, China) at room temperature. The specific capacities of the working electrodes were calculated based on the mass of the active cathode materials. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were measured by an electrochemical workstation (CorrTest CS310). The scanning rate of the CV tests was 1 mV s−1 in the voltage range of 1.8–4.5 V (vs. Li+/Li). The frequency range of EIS was from 100 kHz to 0.01 Hz at potentiostatic signal amplitudes of 5 mV.