Free-Standing Selenium Impregnated Carbonized Leaf Cathodes for High-Performance Sodium-Selenium Batteries
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A novel approach of carbonizing leaves by thermal pyrolysis with melt diffusion followed by selenium vapor deposition is developed to prepare the carbon-selenium composite cathodes for sodium-selenium batteries. The carbonized leaf possesses internal hierarchical porosity and high mass loading; therefore, the composite is applied as a binder- and current collector-free cathode, exhibiting an excellent rate capability and a high reversible specific capacity of 520 mA h g−1 at 100 mA g−1 after 120 cycles and 300 mA h g−1 even at 2 A g−1 after 500 cycles without any capacity loss. Moreover, the unique natural three-dimensional structure and moderate graphitization degree of leaf-based carbon facilitate Na+/e− transport to activate selenium which can guarantee a high utilization of the selenium during discharge/charge process, demonstrating a promising strategy to fabricate advanced electrodes toward the sodium-selenium batteries.
KeywordsCarbonized leaf Free-standing Binder-free Sodium-selenium battery
Electrochemical impedance spectroscopy
Field emission scanning electron microscopy
Galvanostatic intermittent titration technique
Lithium ion batteries
Solid electrolyte interface
Scanning electron microscopy
Sodium ion batteries
Transmission electron microscopy
X-ray photoelectron spectroscopy
With the rapid growth of electronic devices, sustainably rechargeable batteries are urgently needed, giving the urgent rise to exploit energy storage devices with high capacity and satisfactory rate performance [1, 2, 3, 4, 5]. Lithium ion batteries (LIBs) are the dominant power for electronic devices because of the advantages of high energy/power density and long-term stability [4, 6]. While the commercial LIBs cannot meet the future energy requirement of electric vehicles, lithium-sulfur (Li-S) batteries were greatly developed by the reasons of the low cost and high theoretical energy density of S [7, 8, 9, 10]. However, the insulated nature of S and the dissolution of polysulfides are major challenges, leading to sluggish electrochemical reaction and low utilization of S, which hinders their practical applications [11, 12, 13, 14, 15].
Sodium ion batteries (SIBs) are considered to be a promising alternative for LIBs due to the low-cost and large-scale electrical energy storage applications [2, 16, 17, 18, 19]. Especially, sodium-selenium (Na-Se) batteries have drawn increasing interest in these years [20, 21, 22]. The Se element is in the same group with S and has similar electrochemistry versus Na while the energy density of Na2Se (3254 mAh cm−3) is comparable to Li2Se (3467 mAh cm−3) [23, 24, 25, 26]. Moreover, the electric conductivity of Se (10−3 S cm−1) is much higher than that of S (10−30 S cm−1 at 25 °C) . The shuttle effect of polyselenides (which is similar to the polysulfides, Na2Sen, 3 < n < 8) can also deteriorate the cycle life of Na-Se batteries; therefore, it is a key challenge to overcome the hurdle of polyselenides shuttle [28, 29, 30]. Carbon matrixes with appropriate porosity and high electric conductivity, which are always used to load Se, have been regarded as an effective way to address the above issues in recent years [20, 21, 31, 32]. Much endeavor has been made to trap the soluble polyselenides within various carbons including carbon nanofibers [33, 34], carbon spheres [35, 36], and carbon nanosheets , which have been proved to effectively improve the electrochemical performance of Na-Se batteries. Nevertheless, the reported materials involve complex multistep processes and additional components (carbon black and binders); moreover, they are usually environmental harmful and economical costly.
Fortunately, renewable materials with remarkable properties provided by nature can meet our needs [5, 37]. For example, natural leaves are diversified with heteroatom-doping and exceptional porous structure and these natural hard carbons, which possess the impressive ability to store sodium ions, can act as alternative substitutes of traditional materials as electrode materials for SIB devices [32, 37]. The leaves of Ficus can be carbonized by thermal pyrolysis, and it is extremely satisfying that obtained leaves possess a hierarchical porous structure and moderate surface area. In brief, the porous voids can endow the pyrolysis products with high loading capacity and serve as ion-buffering reservoirs during electrochemical process, improving rate capability and power density [5, 38].
Herein, we prepared a new type of the free-standing Se impregnated electrode by melting diffusion followed Se vapor deposition into carbonized leaf which is obtained by thermal pyrolysis of natural leaves in a facile way. The highly reversible specific capacity (84% of the theoretical capacity of Se) is achieved for the first time when the biochar-selenium composite is applied as binder- and current collector-free cathodes for Na-Se batteries. In addition, the as-prepared composite electrode exhibits satisfactory rate capability and cycling stability. With the superiority properties, the carbonized leaf electrode demonstrated desirable electrochemical performance, which is a potential anode material for the Na-Se batteries.
Preparation of Carbonized Leaf
Preparation of Se-R800A
Se powder was put on bottom of the porcelain boat, and the free-standing R800A films were suspended by an irony support in midair right above the Se powder, and the weight ratio of Se:C is not less than 2:1 in order to ensure excess Se powder as shown in Fig. 1a. Then the Se was melted at 260 °C under N2 atmosphere and maintained for 10 h to ensure a good penetration of Se. The weight of Se in the final Se-R800A electrode was measured by thermogravimetric analysis.
The morphology and microstructure were observed by the scanning electron microscopy (SEM, Hitachi SU-70), the field emission scanning electron microscopy (FESEM, JSM-7800F, and TEAM Octane Plus), and transmission electron microscopy (TEM, JEM-2100, and X-Max80). The structure and Raman spectra were collected on the X-ray diffraction (XRD, PANalytical Empyrean with Cu-Kα radiation) and Raman microscope (Renishaw, inVia), respectively. Thermogravimetric analysis (TGA, STA409PC) was tested from room temperature to 700 °C with a heating rate of 10 °C min−1 under N2 atmosphere. BELSORP-max Surface Area and Porosimetry instrument was used to measure the N2 adsorption/desorption isotherms of electrodes. X-ray photoelectron spectroscopy (XPS) tests were carried out using a Thermo K-Alpha+ system.
The electrochemical tests were carried out using CR2032 coin cells, which were assembled with manual Na foil prepared by tableting press as the counter electrode inside an argon-filled glove box (MBRAUN, UNILab2000) with moisture and oxygen levels lower than 1 ppm. Glass fiber (Whatman) was used as the separator. The electrolyte was 1 M of NaClO4 in a mixture of ethylene carbonate /diethyl carbonate (EC/DEC, 1: 1 in volume). The free-standing Se-R800A was directly used as the working electrode without any binder and carbon conductor. The cyclic voltammogram (CV) measurements were performed on an electrochemical workstation (CHI660D). The galvanostatic charge-discharge tests were carried out over a voltage range of 0.005–3.0 V (vs. Na+/Na) on a battery test system (Land, CT-2001A). Electrochemical impedance spectroscopy (EIS) measurements were tested using the electrochemical workstation (CHI 760D) by applying a voltage of 5 mV over a frequency of 10−2–105 Hz. The galvanostatic intermittent titration technique (GITT) test was performed by the discharge/charge process of the cells for 10 min at 10 mA g−1 and followed by a 40-min relaxation at most 50 cycles. All the cells were held at room temperature for at least 12 h before tests. All the specific capacity in this work was calculated on the basis of the loading Se weight. For the ex situ SEM tests, tested electrodes were carefully washed with DEC solvent for three times and dried overnight in vacuum oven.
Results and Discussion
The Se-R800A free-standing electrode was fabricated by carbonization, KOH activation, and Se impregnation processes, which is presented in Fig. 1.
After carbonization process at 800 °C, the size of R800 (Fig. 1b) barely shrank (17 mm to 12 mm in diameter) and the thickness changed hugely (800 μm to 240 μm) with the weight loss of 74%. Figure 1c shows the R800 turned into black indicating that R was successfully transformed into carbon. After activation process, the weight of R800 continued to decrease ~ 9%. However, after Se impregnation process, the weight of R800A (Fig. 1d) increased 90% to transform into the Se-R800A as shown in Fig. 1e. It is noteworthy that the R800A films suspended in midair right above the Se powder were surrounded by Se vapor. This is an original idea of melt diffusion and vapor deposition due to avoiding isolated stray of Se in carbon matrixes . Finally, the Se-R800A maintains well mechanical strength as a free-standing electrode for Na-Se batteries.
In order to evaluate the electrochemical performance of the Na-Se batteries, the Se-R800A was directly used as the cathode in CR2032 coin cell. It is worthy to mention that the back surface of Se-R800A faces the metal Na and upper surface is as the current collector.
The comparison of cycling performance for the C-Se cathodes for Na-Se batteries reported in literature
Current density (A g−1)
Reversible capacity (mA h g−1)
520 at 80th cycle
258 at 50th cycle
340 at 380th cycle
441 at 100th cycle
430 at 300th cycle
410 at 240th cycle
478 at 200th cycle
535 at 150th cycle
410 at 300th cycle
400 at 150th cycle
520 at 120th cycle
It is noteworthy that the cycling stability at high current density, even at 2 A g−1, is better than that at 0.1 A g−1. This may be due to the following reasons: (i) the inartificially hierarchical biochar and moderate graphitization degree of the Se-R800A tremendously accelerate the Na+ and e− transport to activate amorphous Se, therefore ensuring facile electrochemical kinetics even at high current density; (ii) the intermediates (Na2Sen, 3 < n < 8) at low current density have more chances to dissolve into the carbonate electrolyte, but the polyselenides are firmly confined in the micropores and retained by overlapping carbon sheets, which is effective to alleviate the shuttle effect, resulting in a high efficient utilization of Se during the long-term cycling .
The resistance values were obtained by modeling the equivalent circuit for experimental impedance
When the loaded Se occupies the most of micropores, the Se-R800A electrode shows obviously smaller Rct and Rid confirmed by the excellent electrochemical performance. The pores of the carbonized leaf are most in the range of 0.1–2 nm, and these abundant micropores are more suitable to load and confine Se, finally bringing a moderate surface area for higher coulombic efficiency [31, 37]. The Na+ diffusion coefficients of the three samples are calculated by the GITT method during discharge/charge process in Fig. 6c, d . It can be observed that the Na+ diffusion coefficients of R800, R800A, and Se-R800A are the same order of magnitude (10−16 cm2/s) but the Se-R800A is higher than the others, which reveals that the Na+ diffusion in the carbon matrixes is notably improved due to the presence of Se [49, 50]. Together with these properties, both the electronic conductivity and the ionic diffusion efficiency in the carbon-selenium composite were effectively enhanced, resulting in an excellent electrochemical performance of Se-R800A electrode for Na-Se batteries.
In conclusion, it was demonstrated that a novel fabrication of the Se-R8000A can be finished by a tube furnace. It was successful to confine Se into the microporous carbonized leaf by common melt-infusion methods, which can effectively reduce the shuttle effect of polyselenides, resulting in excellent electrochemical performance for Na-Se batteries. The Se-R8000A shows a reversible capacity as high as 520 mA h g−1 at 100 mA g−1 after 120 cycles, which supports the superior cycling stability and rate capability. The inartificially hierarchical leaf structure and moderate graphitization degree of the Se-R800A were proved to significantly promote the efficient utilization of Se. Generally, the Se-R800A, owing to the free-standing, high-performance, and cost-effective characteristics, was demonstrated to be a promising alternative to conventional and substantial electrode materials in Na-Se batteries.
This work was financially supported by the National Natural Science Foundation of China (Nos. 21878189 and 51774203), Shenzhen Science and Technology Project Program (Nos. KQJSCX20170327151152722 and JCYJ20160422112012739), the Natural Science Foundation of SZU (No. 827–000039).
Availability of Data and Materials
The datasets generated during and/or analyzed during the current study are available from the corresponding authors on reasonable request.
YL conceived and designed the experiments. BG performed the experiments and analyzed the data. HM, XR and PZ contributed the analysis tools. BG and YL wrote the paper. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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