Bi Nanoparticles Anchored in N-Doped Porous Carbon as Anode of High Energy Density Lithium Ion Battery
KeywordsPorous N-doped carbon Bi nanoparticles Anode Lithium-ion battery High energy density
The Bi nanoparticles anchored in N-doped porous carbon (Bi@NC) composite was prepared by a facile replacement reaction method, in which ultrasmall Bi nanoparticles were homogeneously encapsulated in the carbon matrix
The N-doped carbon matrix enhanced the electric conductivity and alleviated the mechanical strain of Bi nanoparticles on Li insertion/extraction due to the larger void space, and Bi@NC exhibits excellent cyclic stability and rate capability for LIBs
The strategy developed in this work solves the cyclic instability issue of bismuth as anode for LIBs and provides a new approach to improve high volumetric energy density for electrochemical energy storage devices.
Power sources with high volumetric and gravimetric energy densities are urgently needed to meet the small size and long service life requirements of various applications from information technology to transportation [1, 2, 3, 4, 5, 6]. Lithium-ion batteries (LIBs) are the dominant power sources for these applications owing to their superior energy densities and cycle lives compared to other secondary batteries, but their energy densities are still unsatisfactory for quickly developing society [7, 8, 9, 10, 11].
Graphite is the most commonly used anode in commercial LIBs because of its superior cycling stability and high coulombic efficiency. However, the low theoretical capacity of the graphitic anode (372 mAh g−1) limits the development of graphite-based LIBs. Therefore, it is necessary to look for high energy density LIBs anodes.
Like other metal anodes, however, bismuth exhibits poor cycling stability due to its large volume change during lithiation/delithiation . Some efforts have been made to solve this problem. For example, Park et al.  prepared a nanostructured Bi@C composite that delivered a relatively high capacity of 300 mAh g−1 after 100 cycles at current density 100 mA g−1 by varying the voltage from 0.0 to 2.0 V. Yang et al.  revealed that Bi@C microspheres as anode materials for LIBs retained capacity of 280 mAh g−1 after 100 cycles at current density 100 mA g−1. The improved cycling stability of bismuth in these efforts can be attributed to the controlled coating of carbon layer on bismuth, which enhances electronic conductivity and alleviates the mechanical strain of bismuth during lithiation/delithiation [23, 24]. Moreover, the controlled coating of carbon layer acts as host to stabilize the solid electrolyte interphase (SEI) on the bismuth surface . However, the above-mentioned achievements are unsatisfactory for the practical application of bismuth as anode in LIBs.
Various carbon materials have been extensively studied for performance improvement of anode or cathode materials in LIBs [26, 27, 28, 29, 30]. Metal organic frameworks (MOFs) characterized by diverse skeletal structures, high surface areas, tunable pore sizes, and open metal sites in the skeleton have been demonstrated as promising templates or precursors for fabricating nanostructured carbon for various applications [31, 32, 33, 34, 35, 36, 37]. Except for the advantages mentioned above, MOFs can also be designed and synthesized in a straightforward and cost-effective manner by assembling varied metal ions/clusters and organic ligands under mild conditions . Therefore, without any processing equipment, it can be simply mass-produced just by increasing the amounts of raw materials. In addition, it has been noted that nitrogen-containing MOFs yield nitrogen-doped carbon that exhibits enhanced electronic conductivity and activity toward reactions on carbon [39, 40]. Zeolitic imidazolate framework (ZIF-8), a kind of nitrogen-containing MOFs, combines high stability of inorganic zeolite with high surface area and porosity, and is a good precursor for preparing carbon matrices to enhance cycling stability of some electrode materials for LIBs [41, 42, 43]. For example, Si@ZIF8 composites were prepared by Han et al.  via in situ mechanochemical synthesis, which shows superior electrochemical properties with lithium storage capacity up to 1050 mAh g−1 and excellent cycle stability (> 99% capacity retention after 500 cycles).
In this work, a novel carbon/bismuth composite is introduced through a novel synthetic strategy wherein ZIF-8 was used as precursor for N-doped porous carbon to improve the cycling stability of the bismuth anode. ZIF-8 was obtained by a simple hydrothermal method at low temperature and underwent pyrolysis in H2/Ar atmosphere to form N-doped porous carbon with dispersed zinc nanoparticles. Based on the potential difference between redox couples of Zn2+/Zn (− 0.76 V vs. SHE) and Bi3+/Bi (0.31 V) , bismuth nanoparticles were anchored on the carbon matrix through a replacement reaction. The carbon matrix afforded an electronically conductive network and served as support to restrain the aggregation of bismuth nanoparticles . Most importantly, the pores in the carbon matrix provided space to alleviate the mechanical strain of bismuth during lithiation/delithiation. With these features, the resultant carbon/bismuth composite exhibited excellent performance as anode for LIBs when compared to other bismuth anodes that have been reported in other literatures.
3 Experimental Section
3.1 Sample Syntheses
ZIF-8 was synthesized hydrothermally . Typically, 3 mmol zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99%) and 8 mmol 2-methylimidazole (MeIm, 98%) were separately dispersed in 40 mL methanol (99.5%) with moderate magnetic stirring for 10 min and then mixed under stirring for another 30 min at room temperature. The mixture was sealed in a Teflon−lined autoclave and maintained at 100 °C. After a certain period of time, a white precipitate was harvested by centrifugation at 8000 rpm for 3 min, thoroughly washed with methanol, followed by drying in a vacuum oven overnight.
To obtain N-doped porous carbon with dispersed zinc nanoparticles (Zn@NC), carbonization process was carried out. The as-obtained ZIF-8 was heated at 500, 600, 700, and 800 °C for 3 h at the rate 2 °C min−1 under H2/Ar atmosphere with slow flow. Finally, a tan product was produced after high temperature calcination.
Bismuth nanoparticles were anchored in N-doped porous carbon matrices by galvanic replacement reaction. Typically, 1 mmol as-obtained Zn@NC and 1 mmol BiCl3 were homogeneously dispersed in 75 mL mixed solvent of glycerin and methanol (2:1 in volume) under ultrasonic treatment at room temperature for 30 min. The mixture was sealed in a 100 mL Teflon−lined autoclave, maintained at 120 °C for a certain period of time and then cooled naturally. To obtain the product (Bi@NC), the precipitation was thoroughly washed with methanol via centrifugation–redispersion cycles at 9000 rpm for 5 min and finally dried in a vacuum oven overnight.
The NC sample was obtained by washing Zn@NC with dilute HCl and then deionized water several times to remove the residual Zn component.
For performance comparison, Bi nanospheres (bare Bi, Beijing Dekedao, 99.95%, OD 100 nm) were used and a bismuth/carbon composite (Bi@C) was prepared hydrothermally by coating Bi nanospheres with carbon. Typically, 0.63 g Bi nanospheres were dispersed in 15 mL deionized water, which was mixed with 48 mL aqueous solution containing 1.8 g glucose. Methanol (15 mL) was added under stirring at room temperature for 15 min. The mixture was then sealed in a 100 mL Teflon-lined autoclave and heated at 190 °C for 15 h. After cooling naturally, the precipitate harvested as Bi@NC was prepared. Finally, the product Bi@C was obtained by heating the precipitate at 550 °C for 3 h under N2 at the rate 2 °C min−1.
3.2 Physical Characterizations and Electrochemical Measurements
The crystal configurations and crystallographic planes of the synthetic materials were identified by X-ray diffractometry (XRD, Ultima IV Germany). The specific surface area and pore diameter distribution were tested at liquid nitrogen temperature (77 K) with a surface area and porosimetry analyzer (V-Sorb 2800P). Scanning electron microscopy (SEM, JEOL JSM-6380LA) and transmission electron microscopy (TEM, JEOL JEM-2100HR) were carried out to observe the morphologies, structures, and particle sizes of the samples. During SEM observation, energy dispersion spectrum (EDS) and EDS mapping were also obtained. Fourier transition infrared (FTIR) spectrum of ZIF-8 was determined using infrared spectroscopy (Bruker Tensor 27) within 500–4000 cm−1. X-ray photoelectron spectrometer (XPS, Thermo Fisher Scientific, UK) was used with monochromatic Al-Kα X-ray source (excitation energy = 1468.6 eV) under ultra-high vacuum (lower than 5 × 10−8 mbar). Spectra were collected from 0 to 1350 eV using an X-ray spot size of 400 μm with pass energy 100 eV for wide scan and 30 eV for individual elements. Binding energies were corrected based on the carbon 1s signal at 284.8 eV. Raman spectra were examined on an Alpha 300R Raman instrument at room temperature. The apparent densities of the samples were obtained by keeping the samples in a volumetric cylinder and then vibrating the cylinder until the volumes of the samples remained unchanged.
The Bi electrodes were composed of active materials, bare Bi, Bi@C or Bi@NC, acetylene black, and PVDF in the ratio 7:1.5:1.5 by mass, which were mixed in N-methyl pyrrolidone and coated on Cu foil (S = 1.13 cm2) with the weight of active materials being about 0.5 mg. CR2025 type coin cells were assembled with Bi electrode, lithium foil electrode, electrolyte of 1.0 M LiPF6 in ethyl methyl carbonate (EMC)/ethylene carbonate (EC)/diethyl carbonate (DEC) (EMC/EC/DEC = 5:3:2, by weight), and a microporous membrane (Celgard 2400), in an Ar-filled glove box (Vigor-CH) where water and oxygen contents were controlled to less than 0.1 ppm.
The assembled coin cells were patiently tested on a multi-channel battery tester (LAND CT2001A, Wuhan, China) at 25 °C by discharging to 0.01 V and charging to 2.5 V at various current rates. Under certain operation conditions, cyclic voltammetry (CV) was collected from multichannel potentiostats (Bio-Logic SAS VMP-3) at scan rate 0.1 mV s−1. The electrochemical impedance spectroscopy of coin cells was carried out on an Autolab (PGSTAT302N) with AC signal 10 mVrms from 0.1 MHz to 0.01 Hz.
4 Results and Discussion
According to previous reports, nitrogen doping in carbon can enhance the electronic conductivity of carbon matrices and create abundant defects (for instance, nano-pores) on carbon [60, 61]. XPS was performed to determine the nitrogen species in Bi@NC. As shown in Fig. S3c, the N atomic ratio in Bi@NC is about 27.48%, in agreement with EDS analysis. The nitrogen species consisted of pyridinic-N (N1, 398.50 eV), pyrrolic-N (N2, 399.70 eV), graphitic-N (N3, 400.56 eV), and oxidized-N (N4, 401.8 eV) . Both pyridinic-N and pyrrolic-N in NC provide more active sites for lithium ion storage, benefiting mass transport and electron transfer .
Figure S3d presents N2 adsorption–desorption and corresponding pore diameter distribution curves of Bi@NC. The N2 adsorption–desorption isotherm of Bi@NC could be classified as a typical IV (H3) isotherm with a distinct hysteresis loop, indicative of the presence of distinct mesoporous microstructures . From the BET result, the specific surface area is 492.08 m2 g−1 while the single point adsorption total pore volume is 0.2749 cm g−1 (P/P0 = 0.9889). According to the narrow pore size distribution in the range 2.1–5 nm, Bi@NC had an average pore size of about 2.23 nm, which was calculated via desorption data using the Barrett–Joyner–Halenda (BJH) model. Obviously, the as-prepared Bi@NC exhibits a porous structure, which is related to its precursor ZIF-8. This porous structure was helpful for volume buffering during lithium insertion/extraction in bismuth. The nanoparticles of bismuth in Bi@NC reduce the distance for lithium transportation in bismuth while the NC increases the electronic conductivity and activity of lithium insertion/extraction. Therefore, the hierarchical configuration of Bi@NC contributes to its excellent electrochemical performances as anode in the lithium ion battery in terms of cyclic stability and rate capability.
Figure 6c presents the cyclic stability of Bi@NC at 80 mA g−1 after the initial three cycles at 40 mA g−1, with comparisons to bare Bi and Bi@C. A drop-off trend of capacity in the initial cycles was distinctly observed in all samples. This is because Bi particles pulverized upon cycling, resulting in the loss of electrical integrity leading to rapid capacity fading . Besides, the sizes of Bi particles in bare Bi and Bi@C (OD = 100 nm) are larger than that in Bi@NC (OD = 5 nm), leading to easier pulverization and faster fading of capacity . The initial first cycle coulombic efficiencies of these three samples (Fig. 6d) are only about 65%, which was ascribed to the formation of SEI on the fresh sample surface during the first cycle. It can be found from Fig. 6c that bare Bi exhibits poor cyclic stability. Its charge capacity decayed quickly before the initial 20 cycles and retained only 60 mAh g−1 after 100 cycles. This poor cyclic stability resulted from pulverization of Bi nanoparticles due to their volume change during lithium insertion/extraction and electronic insulation of pulverized particles due to surface SEI [1, 17]. The pulverization of Bi nanoparticles can be clearly indicated by SEM and TEM images of cycled bare Bi, as shown in Fig. S4a. The poor cyclic stability of bare Bi is improved to some extent by coating carbon on Bi nanoparticles. As shown in Fig. 6c, the charge capacity of Bi@C is retained at 100 mAh g−1 after 100 cycles. However, this capacity is lower than the theoretical specific capacity of bismuth. Obviously, simple carbon coating did not improve the cyclic stability of bismuth. The large volume change of the bismuth could destroy the carbon-coating layer and expose bismuth to the electrolyte, resulting in pulverization and continuous growth of SEI layers on Bi particle surfaces . The TEM and SEM images in Fig. S4b confirm the destruction of Bi@C particles. Bi@NC, in contrast, showed excellent cyclic stability, with charge capacity (285 mAh g−1) that is higher than those of bare Bi or Bi@C. Bi@NC showed excellent cyclic stability with charge capacity (285 mAh g−1) that is significantly higher than those of bare Bi or Bi@C. The electrochemical performances of Bi@NC were compared with previous reports in the literature, as displayed in Table S1. The corresponding volumetric capacity is about 430 mAh cm−3 (specific capacity × apparent density = 285 mAh g−1 × 1.51 g cm−3) at current density 80 mA g−1, which is also 1.5 times that of graphite (275 mAh cm−3, specific capacity × apparent density = 372 mAh g−1 × 0.74 g cm−3). This excellent performance is attributed to the porous structure of the carbon matrix, which provides space to alleviate the mechanical strain of Bi nanoparticles during lithium insertion/extraction and maintains the structural integrity of Bi (Fig.S5b) . The NC matrix is just like a huge conductive network where the smaller and higher active Bi nanoparticles are anchored, resulting in preferable electrochemical performance compared to bare Bi and Bi@C. Even after 100 cycles, Bi@NC maintains its pristine morphology, as indicated by the TEM and SEM of cycled Bi@NC (Fig. S4c).
Bi@NC exhibited excellent rate capability. Figure 6e presents the rate capability of Bi@NC compared to bare Bi and Bi@C. Obviously, bare Bi and Bi@C almost lost their charge capacities, but Bi@NC delivered a capacity as high as 100 mAh g−1 under a high rate current of 3840 mA g−1. This excellent rate capability is related to the smaller bismuth nanoparticles uniformly anchored in the NC than those in bare Bi and Bi@C. The smaller nanoparticles shortened the path for lithium transport in the particles and the nitrogen-doped carbon enhanced the electronic conductivity of Bi. It should be noted from Fig. 6c that at low rate current, Bi@NC delivered a charge capacity (over 400 mAh g−1) higher than the theoretical specific capacities of bismuth and carbon. This could be ascribed to the capacitive contribution of the high specific surface of the carbon matrix in Bi@NC.
To understand the electrochemical behavior of NC during cycling, its cycle and rate performance were investigated, as shown in Fig. S6a, b. According to previous literature, we propose that Li ions were stored in NC because the Li ions had strong interactions with N atoms [69, 70]. Figure S6a presents the cyclic stability of NC at 80 mA g−1 after the initial three cycles at 40 mA g−1. The first cycle coulombic efficiency of NC is also low, about 58%. The low coulombic efficiency is attributed to the formation of SEI and storage of Li ions in nanoporous voids, which are difficult to extract . As the cycling at 80 mA g−1 proceeded further, the capacity of NC quickly stabilized to exhibited good electrochemical performance with high reversible capacity of about 215 mAh g−1 up to 100 charge/discharge cycles. The rate performance of NC was evaluated at various current densities from 80 to 3840 mA g−1, as shown in Fig. S6b. As can be seen, the reversible capacities remain stable and decreased regularly with increase in rate. Therefore, there is reason to believe that NC is an excellent carbon matrix that could improve the electrochemical performance of Bi particles in Bi@NC relative to bare Bi or its simple composite with carbon.
The electrochemical impedance test was also measured to examine the kinetic process. In Fig. S7a–c, the electrochemical impendence spectra of Bi@NC, bare Bi, and Bi@C half-cells are presented. The semicircle’s diameter stands for charge-transfer resistance. Although the Bi@NC half-cells had larger internal resistance than the other two in the initial stage, its rate of increase in resistance is slower, which can be clearly observed in Fig. S7d. This is attributed to the differences in the structures of Bi@NC, bare Bi, and Bi@C. In Bi@NC, the Bi nanoparticles are uniformly dispersed in the carbon matrix and most maintain their structural integrities with few SEI layers on the surface after cycling (Fig. S5a, b). In bare Bi and Bi@C, the pulverization of Bi nanoparticles and continuous growth of SEI layers on Bi particle surfaces result in fast growth of resistance.
A novel bismuth–carbon composite, in which bismuth nanoparticles were anchored in nitrogen-doped porous carbon matrices (Bi@NC), was successfully fabricated by galvanic replacement reaction in an MOF (ZIF-8). In this composite, the carbon matrices maintain the morphology of ZIF-8 and exhibit a porous structure, providing space to alleviate the mechanical strain of Bi nanoparticles during Li insertion/extraction. Nitrogen-doped carbon increased the electronic conductivity of the matrix and the reaction activity of bismuth for lithium insertion/extraction. Bismuth nanoparticles uniformly distributed in the carbon matrix reduced the path for lithium transport in the particles. With these features, the as-prepared Bi@NC exhibits excellent cyclic stability and rate capability. The strategy developed in this work solves the cyclic instability issue of bismuth as anode for the lithium ion battery and provides a new approach to high volumetric energy density for electrochemical energy storage devices.
This work is supported by the Natural Science Foundation of Guangdong Province (Grant No. 2017B030306013) and the key project of Science and Technology in Guangdong Province (Grant No. 2017A010106006).
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