Improved Na+/K+ Storage Properties of ReSe2–Carbon Nanofibers Based on Graphene Modifications
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Graphene modifications effectively improved conductivity but also resulted in a regulatory effect on the decrease in its diameter.
The synergistic action of graphene and carbon fibers protected the structure of the electrode material and shortened the ion diffusion path.
ReSe2@G@CNFs exerted high capacity and long cyclic stability in Na+/K+ half cells. When this compound was assembled in Na+ full cells, the cells displayed excellent performances
KeywordsRhenium diselenide Carbon nanofiber Graphene Sodium-/potassium-ion batteries Full cell
With the rapid development of electronic equipment and the emergence of electric and hybrid electric vehicles in recent years, it has become necessary to investigate energy storage materials with high efficiency, the existence of alternative and abundant resources, and the environment-friendliness [1, 2, 3]. Owing to the characteristics of the high energy density and broad voltage range, lithium-ion batteries (LIBs) have gained considerable attention since their development [4, 5, 6, 7]. However, the limited distribution of lithium resources in the earth has hindered their widespread applications in grid energy storage system [8, 9, 10]. As promising alternatives to LIBs, sodium-ion batteries (NIBs) and potassium-ion batteries (KIBs) have attracted considerable interest owing to the abundance of sodium/potassium sources and their low prices [7, 11, 12, 13, 14]. In addition, NIBs and KIBs have redox voltages which are closer to Na+/Na (− 2.71 V vs. normal hydrogen electrode) and K+/K (− 2.93 V) to that of Li+/Li (− 3.04 V) [15, 16, 17]. Nevertheless, Na+ (1.02 Å) and K+ (1.38 Å) have larger radii than Li+ (0.76 Å), which means that they are more likely to induce sluggish reaction kinetics and massive volume expansions, thus resulting in poor rate performances and unstable circulation [13, 18, 19]. Moreover, graphite has a capacity of 372 mAh g−1 in LIBs, exhibits a low capacity in KIBs (279 mAh g−1), but cannot be used in NIBs. Therefore, the need to identify electrode materials with larger physical spaces to adapt Na+/K+ fuel cells is imminent.
Transition metal dichalcogenides (TMDs) [20, 21, 22] are known for their excellent electrochemical properties as negative electrode materials. Rhenium diselenide (ReSe2) is a representative TMD and consists of a plane of Re atoms sandwiched between two planes of Se atoms. Every layer is coupled based on weak van der Waals interactions (18 MeV) owing to the extra valence electrons in Re atom . Additionally, this particular compound has a larger interlayer spacing (6.37 Å) compared to other TMDs, such as ReS2 (6.14 Å) and MoS2 (6.15 Å). The larger distance between layers allows the insertion of Na+/K+ without significant destruction . Secondly, based on acquired knowledge from most types of dichalcogenides arranged in hexagonal phases, ReSe2 is a distorted 1 − T phase with triclinic symmetry , thus possessing superior potential for strain engineering. And also, ReSe2 is an anisotropic semiconductor . Results published by de Groot et al.  and Tiong et al.  showed that in terms of electrical conductivity, the orientation of the atomic link of Re–Re was better than those associated with other crystalline directions. However, the volume change during circulation will result in rapid capacity decline, and the conductivity of ReSe2 needs further improvements to better match the dynamics of the electrochemical reaction. Therefore, some efforts should be dedicated to engineer the composition and morphology of ReSe2.
As it is known, graphene exhibits tremendous potential and promise for the improvement of conductivity, cyclic stability, and control of the morphology of materials [29, 30]. Additionally, various porous carbon nanocomposites, including carbon nanofibers , carbon spheres , and carbon nanobelts , possess desirable characteristics given their capabilities for accommodating volume expansion, their large specific surface areas, and shortened ion or electron diffusion paths. To solve the aforementioned problems, in this study, we synthesized ReSe2–carbon nanofibers (termed as ReSe2@G@CNFs) with the use of electrospinning and solid-phase heat treatments. When these nanofibers are used as negative electrodes, they exhibit superb Na+/K+ storage performances. To research the effects of graphene and different rhenium contents on the properties of these materials, we also prepared ReSe2 composite nanofibers without graphene (termed as ReSe2@CNFs) and different concentrations of Re composite nanofibers. (The concentrations of rhenium are 1 and 0.4 mmol, and are thus denoted as 1 mM ReSe2@G@CNFs and 0.4 mM ReSe2@G@CNFs.)
2.1 Material Synthesis
Ammonium perrhenate (NH4ReO4), poly(methyl methacrylate) (PMMA, Mw = 35,000), polyacrylonitrile (PAN, Mw = 150,000), and N,N-dimethylformamide (DMF) were used without further processing. Graphene oxide (GO) was obtained in accordance with a previously published document . The ReSe2 composite nanofibers were synthesized by electrospinning and heat treatment as follows. Firstly, 0.7 mmol NH4ReO4 was added into 6 mL DMF and stirred for several minutes to completely dissolve. Small amounts of PAN and PMMA were then dissolved in the aforementioned solution with vigorous stirring at 50 °C in a water bath. Subsequently, 0.5 mL GO was added to form a light black solution and stirring was continued for another several hours. To prepare comparative carbon fibers, a precursor without GO and precursors containing 1 mmol/0.4 mmol NH4ReO4 were also prepared in the same way. All precursor solutions were poured into a 6-mL noncorrosive steel needle injector that was electrically connected to a high-voltage power supply (11–12 kV). The flow rate of the solution was controlled to an approximate value of 0.3 mL h−1. The distance between the needle and collector was set at to a value of 12 cm. After it was peroxided at 235 °C in air for 2 h, the as-spun reddish brown mats were annealed at 620 °C in argon for 2 h. Finally, the black membrane which was obtained was ground and mixed with selenium powder at the ratio of 1:1.5. The mixture was then placed into an ark and calcined in Ar/H2 at 270 °C for 3 h. The temperature was then increased to 600 °C and was maintained for 2 h. The ReSe2 composite fibers were obtained after this selenizing process.
2.2 Material Characterization
The crystal structure of the as-prepared nanofibers was characterized using powder X-ray diffraction [XRD, Siemens D-5000 diffractometer with Cu–Kα irradiation (λ = 1.5406 Å)]. The microstructure of the samples was characterized using a HITACHI S4800 scanning electron microscope (SEM), a transmission electron microscope (TEM), and a high-resolution TEM operating at 200 kV. The samples were also analyzed by X-ray photoelectron spectroscopy (XPS, Surface Science Instruments S-probe spectrometer). The Raman spectrum was acquired at room temperature with excitation laser lines of 514 nm (Renishaw). Specific surface areas were measured using a Tristar II 3020 instrument by adsorption of nitrogen at 77 K. The pore diameter distribution of the mesopores was tested by nitrogen adsorption/desorption analysis (MicroActive ASAP 2460). The thermal gravimetric analysis was recorded on a thermogravimetric analyzer (TGA, PerkinElmer, Diamond TG/DTA) with a heating rate of 10 °C min−1 in air from 30 to 800 °C.
2.3 Electrochemical Measurements
In a glove box with argon, the half cells were assembled using CR2025 coin cells. The anodes consisted of active materials, carbon black, and carboxymethyl cellulose (8:1:1). The mixed suspension was then coated on a copper foil and dried at 70 °C in a vacuum oven. The electrodes were cut into small round pieces with a diameter of 12 mm. The loading mass of active material was in the range of 1.1–1.5 mg cm−2 for the anode. Na or K metal was used as a counter electrode, and the glass fiber (Whatman, CF/F) was used as a separator to assemble NIBs and KIBs. The electrolyte was consisted of 1 M NaClO4 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1) with 5% fluoroethylene carbonate (FEC) in the case of NIBs, and 3 M potassium bis(fluorosulfonyl)imide (KFSI) in methoxymethane (DME) in the case of KIBs, respectively. A Neware test system was used to conduct the galvanostatic charge–discharge measurements, within a voltage range of 0.01–3 V for anode materials, 2.0–3.9 V for the Na3V2(PO4)3 cathode, and 1–3.5 V for full cells. The calculation of the specific capacity was based upon the quality of the active substance of the entire electrode. The cyclic voltammetry (CV) curves at a scan rate of 0.1 mV s−1 and electrochemical impedance spectrum (EIS) were recorded using a CHI660E electrochemical workstation.
3 Results and Discussion
The galvanostatic discharge–charge profiles of ReSe2@G@CNFs at the current density of 200 mAh g−1 are presented in Fig. 4b. The first discharge/charge capacities are 384/253 mAh g−1 and correspond to the relatively low CE of 66%. The main cause of the irreversible capacity loss is the formation of the SEI film . In the subsequent cycles, the coulombic efficiency is nearly 100%. All curves are going to be highly consistent, thus indicating a good reversible capacity. In addition, the charge and discharge voltage platforms are consistent with the CV curves. Figure 4c displays the cyclic performance of ReSe2@G@CNFs at 200 mA g−1. This shows a stable long-term cycle life yielding 227 mAh g−1 after 500 cycle and a capacity at 89% compared to the second discharge capacity. Specifically, the coulombic efficiency is close to 100% during most of the cycles. In contrast, ReSe2@CNFs yield a lower specific capacity of 175 mAh g−1. This phenomenon results from the regulation of graphene which modifies the structural characteristic of carbon nanofibers. Pure ReSe2 was also synthesized (Fig. S8) and exhibited a high initial capacity, but significantly decayed as the cyclic number increased. The measurement of electrical conductivity was completed with the use of the two-probe method. At first, ReSe2 carbon fibers mats were cut into rectangular sheets with sizes of 25 × 20 mm2. Subsequently, the two ends of the mats were clamped through two metal probes which were connected to the semiconductor parameter analyzer (Agilent 4156). As depicted in Fig. S9, the ReSe2@G@CNFs exhibit a linear response with an increased slope and correspond to a smaller resistance compared to ReSe2@CNFs. The results show that graphene can effectively improve the conductivity, consequently leading to a higher capacity. Additionally, the cyclic performances of ReSe2@G@CNFs (1 mM) were evaluated, which delivered a capacity of 199 mAh g−1, while the capacity retention was 81% after 500 cycles (Fig. S10a). Moreover, ReSe2@G@CNFs (0.4 mM, Fig. S10c) yielded the minimum specific capacity which was lower than 200 mAh g−1 owing to the relatively low content of ReSe2. Therefore, the appropriate concentration also had a great impact on the electrochemical performance. The rate capability of the composites was also investigated (Fig. 4d). The outcomes showed that reversible capacities of 283, 241, 197, 170, and 140 mAh g−1 were maintained when the current densities, respectively, increased from 100 to 2000 mA g−1. Meanwhile, the capacity can return to 223 mAh g−1 and continue to cycle without a significant decay as the current density returns to 200 mA g−1. Obviously, the rate performances of the ReSe2@CNFs were secondary, and they delivered an average capacity of approximately 125 mAh g−1 at large current values. In Fig. S10b, d, both ReSe2@G@CNFs (1 mM) and ReSe2@G@CNFs (0.4 mM) exhibit much lower capacities. Furthermore, at low current density values, the responses exhibit attenuation trends. In general, the outstanding electrochemical property of ReSe2@G@CNFs is owing to its specific composition and morphology. Graphene modification enhances its electrical conductivity, while the moderate amount of ReSe2 will relieve the agglomeration of nanoparticles. Meanwhile, the structure of nanofibers can effectively alleviate the volume change of the electrode during the sodiation/desodiation process and provide a fast electronic/ionic transmission path.
For better understanding of the kinetics of ReSe2@G@CNFs, the electrochemical impedance spectra of the composite before/after 100 cycles were acquired. The Nyquist plot shows a depressed semicircle at high frequencies. This is attributed to the SEI resistance and charge transfer. In addition, the slope of the straight line obtained at low frequencies is attributed to ionic diffusion [52, 53]. As shown in Fig. S11, the size of the semicircle becomes larger after circulation in comparison with previously published results [54, 55, 56]. The enlarged resistance may be attributed to the stable SEI films and the phase transition during the conversion reaction process. As the reaction progresses, a large amount of electrolyte is consumed and Na ions continually deposit on the surface of SEI films, thus resulting in the increase in the interface impedance [57, 58, 59]. From another perspective, the diffusion rates of ions before and after the completion of the multiple cycles are close in value. This is verified again by the superior electrochemical performance of ReSe2@G@CNFs. The EIS of ReSe2@CNFs was also conducted and yielded a much larger circle in the medium–high-frequency area compared to those for ReSe2@G@CNFs. The result is also in accordance with the I–V curve. More importantly, as it can be observed from the SEM diagrams of the ReSe2@G@CNFs electrodes in Fig. S12a, b, the original smooth fiber surface becomes coarse and thick after 100 cycles, but there is no obvious pulverization in the structure. However, as shown in Fig. S12c, d, the structure of the ReSe2@CNF electrode material changed dramatically, as the carbon fibers melted and accumulated into larger particles. This discovery can also explain the outstanding stability of ReSe2@G@CNFs.
In summary, we have successfully synthesized ReSe2–carbon nanofibers through electrospinning and heat treatment. Its advantages lie in the extremely weak van der Waals coupling interaction and large interlayer spacing of ReSe2, and shortened diffusion channel for the ion and electron because of carbon nanofibers. Based on reasonable control schemes of the morphology and composition, the ReSe2@G@CNFs led to excellent electrochemical performances when used in NIBs and KIBs, even in full cells. These compounds delivered reversible capacities of 227 mAh g−1 after 500 cycles in NIBs, 230 mAh g−1 at 200 mA g−1 after 200 cycles, and 212 mAh g−1 at 500 mA g−1 after 150 cycles in KIBs. Additionally, they also led to a capacity retention of 82% after 200 cycles in full cells. Most importantly, this was the first time we investigated the battery applications of ReSe2 and obtained good results. Based on the study’s findings, we envisage that ReSe2 will draw more attention for energy storage applications.
The present work has been supported by the National Natural Science Foundation of China (Grants 51772082, 51574117, and 51804106), the Research Projects of Degree and Graduate Education Teaching Reformation in Hunan Province (JG2018B031, JG2018A007), the Natural Science Foundation of Hunan Province (2019JJ30002, 2019JJ50061), and project funded by the China Postdoctoral Science Foundation (2017M610495, 2018T110822).
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