High-Performance Cathode Material of FeF3·0.33H2O Modified with Carbon Nanotubes and Graphene for Lithium-Ion Batteries
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The FeF3·0.33H2O cathode material can exhibit a high capacity and high energy density through transfer of multiple electrons in the conversion reaction and has attracted great attention from researchers. However, the low conductivity of FeF3·0.33H2O greatly restricts its application. Generally, carbon nanotubes (CNTs) and graphene can be used as conductive networks to improve the conductivities of active materials. In this work, the FeF3·0.33H2O cathode material was synthesized via a liquid-phase method, and the FeF3·0.33H2O/CNT + graphene nanocomposite was successfully fabricated by introduction of CNTs and graphene conductive networks. The electrochemical results illustrate that FeF3·0.33H2O/CNT + graphene nanocomposite delivers a high discharge capacity of 234.2 mAh g−1 in the voltage range of 1.8–4.5 V (vs. Li+/Li) at 0.1 C rate, exhibits a prominent cycling performance (193.1 mAh g−1 after 50 cycles at 0.2 C rate), and rate capability (140.4 mAh g−1 at 5 C rate). Therefore, the electronic conductivity and electrochemical performance of the FeF3·0.33H2O cathode material modified with CNTs and graphene composite conductive network can be effectively improved.
KeywordsFeF3·0.33H2O cathode material Conductive network Electrochemical performance Lithium-ion batteries
Constant phase-angle element
Lithium ion diffusion coefficients
Electrochemical impedance spectroscopy
- FeF3·0.33H2O/C + G
FeF3·0.33H2O/CNT + graphene
Hybrid electric vehicles
Ordered mesoporous carbon
Charge transfer resistance
Selected area electron diffraction
Scanning electron microscopy
Transmission electron microscopy
Thermogravimetric-differential scanning calorimetry
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) . 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 , LiMn2O4 , and LiFePO4 , 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 . 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 . (2) Surface coating. Modification by introducing a coating layer can significantly shorten the Li+ transport length and alleviate volume changes . 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 . (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 . 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 , FeF3·0.5H2O , FeF3·3H2O , and FeF3 , 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 . 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
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) . 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 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.
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
DLi+ (cm2 s−1)
DLi+ (cm2 s−1)
7.63 × 10−13
2.96 × 10−13
1.19 × 10−12
7.10 × 10−13
FeF3·0.33H2O/C + G
1.67 × 10−12
1.21 × 10−12
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.
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.
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.
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).
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.
This work was supported by Science and Technology Plan Foundation of Guangdong (2017B010119002), Science and Technology Plan Foundation of Guangzhou (201803030015), and Science and Technology Plan Foundation of Guangzhou (201704030031).
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The data supporting the conclusions of this article are included within the article and its additional files.
JL and LL designed the experiments. LL and SX performed the experiments. LL, SL, and LL analyzed the data. LL and SL wrote the manuscript. LL, YL, and SX drawn the figures. SH, CP, and FZ reviewed and edited the manuscript. All authors read and approved the final manuscript.
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