YolkâShell-Structured Bismuth@N-Doped Carbon Anode for Lithium

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Functional Nanostructured Materials (including low-D carbon)

Yolk-Shell Structured Bismuth@N-Doped Carbon Anode for Lithium-Ion Battery with High Volumetric Capacity Wanwan Hong, Peng Ge, Yunling Jiang, Li Yang, Ye Tian, Guoqiang Zou, Xiao-Yu Cao, Hongshuai Hou, and Xiaobo Ji ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20477 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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ACS Applied Materials & Interfaces

Yolk-Shell Structured Bismuth@N-Doped Carbon Anode for Lithium-Ion Battery with High Volumetric Capacity Wanwan Hong, † Peng Ge, † Yunling Jiang, † Li Yang, † Ye Tian, † Guoqiang Zou, † Xiaoyu Cao, ‡ Hongshuai Hou, *,† Xiaobo Ji†

† State

Key Laboratory of Powder Metallurgy, College of Chemistry and

Chemical Engineering, Central South University, Changsha 410083, Hunan, China

‡ College of Chemistry Chemical and Environmental Engineering, Henan University of Technology, Zhengzhou 450000, Henan, China

KEYWORDS yolk-shell structure, Bi nanorod, N-doped carbon nanotube, lithium-ion battery, high volumetric capacity

1

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ABSTRACT As anode for lithium-ion batteries (LIBs), metallic bismuth (Bi) can provide a superb volumetric capacity of 3800 mAh cm-3, showing perspective value for application. It is a pity that the severe volume swelling during the lithiation process would lead to the dramatic deterioration of the cycling performances. To overcome this issue, Bi nanorods encapsulated in N-doped carbon nanotubes (yolk-shell Bi@C-N) are elaborately designed through in-situ thermal reduction of Bi2S3@polypyrrole nanorods. In comparison with the commercial Bi, the lithium storage capacities of Bi@C-N are significantly enhanced, and it presents a stable volumetric capacity of 1700 mAh cm-3 over 500 cycles at a high current density of 1.0 A g-1, nearly 2.2 times that of graphite. The N-doped carbon nanotube and the cavity between carbon wall and Bi jointly contribute to this superior performance. Especially, the failure mechanism of Bi nanorod and the protective effect of carbon shell is revealed by ex-situ transmission electron microscopy (TEM), which illuminates the decreasing tendency in the initial 10-20 cycles and the subsequent stable 2


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trend of cyclic performance. INTRODUCTION Bismuth (Bi) is well-known as an environmental friendly element which has

been

widely

utilized

in

different

fields,

such

as

metallurgy,

semiconductors, pharmaceutics and medicine treatment, due to its low melting point, low toxicity and other special properties.1-12 Meanwhile, at the same group of phosphorus (P) and antimony (Sb) in the periodic table, Bi has been certified to be a perspective anode for Li storage.13-15 With a typical layer crystal structure, Bi presents a large interlayer spacing of 3.95 Å along the

c-axis, providing enough space to accommodate lithium-ions. Theoretically, it can deliver a specific capacity of 385 mAh g-1 or a superb volumetric capacity of 3800 mAh cm-3, which is nearly five-times that of graphite (756 mAh cm-3) and much higher than other metal anodes.16-18 The comparison of graphite with different metals in lithium storage performance is listed in Table 1.19 The excellent volumetric capacity and low potential hysteresis (0.11 V) make Bi a potential anode for lithium ion batteries. Furthermore, the formation of lithium dendrite could be effectively prevented by the slightly high operating voltage platform ( ~0.8 V vs. Li/Li+).17,20 Nonetheless, it suffers from the extreme volume expansion on Li+ insertion (~215%), giving rise to the dramatic pulverization and the loss of electronic connection, accompanying with dreadful capacity fading.21 Restricted by the afore-mentioned drawbacks, several

available

methods

are

prompted,

including

3


the

design

of


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nano-structure materials and the introduction of carbonaceous materials.22-31 Table 1. The comparison of graphite with different metals in lithium storage performance

Gravimetric capacity (mAh g-1)

Metal

Volumetric capacity (mAh cm-3)

Potential hysteresis (V)

graphite

372

756

0.11

Al

994

1383

0.26

Sb

660

1889

0.19

Sn

993

1991

0.14

Ge

1600

2180

0.21

Si

4200

2190

0.25

Bi

384

3800

0.11

Based on the previous reports, some controlled strategies have been employed to induce various nano-scale materials, such as nanosphere,32 nanorods,16

nanotubes,33

core-shell

nanomaterials,34,35

yolk-shell

nanomaterials,36-38 etc. Considering the merits of neoteric structure with distinguished physicochemical properties, yolk-shell-structured nanomaterials have triggered plenty of exploring activities.39-42 Interestingly, different from close-connected core-shell structures, yolk-shell structures show a void cavity like frogspawn, efficiently providing adequate space for yolk.43 Utilized as LIBs anodes, the existing void cavity can alleviate the volume swelling and further prevent the aggregation of the inner yolks, facilitating the prompting of the electrochemical

performance.41,44-46

Other

advantages

like

enhanced

lithium-ion diffusion and accelerated electrolyte infiltration also contribute to the better Li-storage ability.40 Currently, yolk-shell structure has shown its 4


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merits in improving the performance of electrodes. A Sb@C yolk-shell nanosphere displayed a reversible capacity of 405 mAh g-1 at 1.0 A g-1 over 300 cycles as LIBs anode.41 Yolk-shell ZnO-C microspheres fabricated through a chemical solution reaction delivered a rate capability of 764, 465, 339, and 212 mAh g-1 at stepwise current densities of 0.1, 0.2, 0.5 and 1.0 A g-1, respectively.47 Alternatively, introducing carbon matrix is regarded as the classical way to optimized the properties of electrodes. As known, carbon substrate can relieve

the

stress

derived

from

the

volume

change

during

the

insertion/extraction of Li+, which can reliably inhibit the pulverization of the electrode materials.48-50 Furthermore, it is demonstrated that hetero-atoms (N, B, S, and P) doping could improve the electrical conductivity and modify the surface characteristic of carbon substrate.51-53 In this regard, N-doped carbon materials are widely explored to prompt the performance with two distinct merits: (i) enhancing the electronic conductivity with the faster reaction kinetics; (ii) creating numerous defects and active sites with the facile redox process.54-56 For example, a homogeneous rose-like Bi@N-doped carbon composite obtained by polydopamine coating delivered a good rate capacity of 250 mAh g-1 at 1.0 A g-1.20 Also a Bi@NC composite prepared by a replacement process showed great cyclic stability with a charge capacity of 285 mAh g-1 after 100 cycles 0.08 A g-1.19 According to the above-mentioned factors, in this work, a special Bi@C 5



yolk-shell nanotube with nitrogen-doping (denoted as Bi@C-N) is designed through in-situ thermal-reduction and carbonization of Bi2S3@PPy nanorods. Bi@C-N exhibits a satisfactory Li-storage performance stemmed from its stable yolk-shell structure, accompanied with excellent electronic conductivity from N-doping and shorter Li-ion diffusion pathways in equally distributed 1D Bi nanorods. When utilized as LIBs anode, it shows an excellent initial charge capacity of 3715 mAh cm-3 at 0.1 A g-1. Surprisingly, the long cycle capacity can be maintained at 1700 mAh cm-3 after 500 cycles at the current density of 1.0 A g-1. RESULT AND DISCUSSION The morphology of the designed samples are presented in Fig.1. It is obvious that all the samples display uniform one-dimensional structure. As shown in Fig. 1A1-A3, Bi2S3 rods are dispersed irregularly and exhibited well-defined homogeneous nanorod-like structure with an average diameter of ~150 nm, indicating a narrow size distribution. After the coating of polypyrrole (Fig. 1B1-B3), the obtained core-shell Bi2S3@PPy composite inherit the nanorod-like morphology with uniform radial dimension, and no aggregation is observed. Moreover, it is distinct that Bi2S3@PPy nanorods are distributed more loosely than pure Bi2S3 nanorods. At the terminal of the nanorod (Fig. 1B3), it is obvious that Bi2S3 nanorod is wrapped by polypyrrole layer with distinct interface. Interestingly, nanorod structure with contractive size (Fig. 1C) could be reserved after annealing with H2 thermal reduction. Notably, 6


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some void cavities are arisen between Bi rods and carbon nanotube walls, suggested that the rod-like structure is reconstituted through directed self-assembly of Bi atoms after the reduction of trivalent Bi with the release of S, and the void cavities are formed simultaneously.28 Theoretically, compared with the bulk Bi, the disordered one-dimensional nanorod structure would shorten the paths of ions diffusion and enlarge the contact area of electrode/electrolyte, which are in favor of the better kinetic characteristics. In addition, the generated void cavities and outer carbon nanotubes are able to accommodate the volume swelling and limit the pulverization of Bi in the repeated lithiation-delithiation process, which is beneficial for the better electrochemical stability.41,45 Therefore, the formed yolk-shell Bi@C-N is feasible to provide favorable Li-storage performance. However, the solid Bi2S3 nanorods are transformed into chaotic structurer accompanied with the appearance of bulk product after the direct H2 reduction, as shown in Fig. S1, which may be caused by the low melt point of Bi (273.1 oC) and structural rearrangement after the release of S. The energy-dispersive spectrum manifests the final product of Bi. The structure difference compared with Bi@C-N indicates that the outer PPy layer could restrict the inner Bi in a tiny space, which lead to the inheriting of one-dimensional structure.

7



Figure 1. SEM images at different magnifications. (A1-A3) Bi2S3 nanorods; (B1-B3) Bi2S3@PPy nanorods; (C1-C3) yolk-shell Bi@N-C. The element mapping images and energy-dispersive spectra (EDS) of the three obtained products are shown in Fig. 2. In Fig. 2A1-A3, element S and Bi are distributed uniformly, and the atomic ratio of Bi and S is about 2:3, further confirming the formation of Bi2S3. In Fig. 2B1-B3, C and N elements are observed with homogeneous distribution, indicating the successful introduction of PPy, which is further verified by EDS spectrum in Fig. 2B4. After calcining and reduction of Bi2S3@PPy, only C, N and Bi are detected (Fig. 2C1-C4), and no elemental S signal is found, suggesting that Bi2S3 was 8


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completely transformed into Bi.

Figure 2. Mapping images and energy-dispersive spectra. (A1-A4) Bi2S3 nanorods; (B1-B4) Bi2S3@PPy nanorods; (C1-C4) yolk-shell Bi@N-C. (The color for C, N, S, Bi are red, yellow, purple, green, respectively) In Fig. 3A1-A2, TEM images display solid rod-like structure of Bi2S3 with an average diameter of 150 nm, agreeing well with the results of SEM. The lattice fringe with the spacing of ~ 0.285 nm is related to the (211) crystal plane of orthorhombic Bi2S3 (JCPDS:17-0320).38,57 In the TEM images of Bi2S3@PPy, it is revealed that the sample displays a typical core-shell structure, with a uniform thickness of about 50 nm PPy layer. In considering the existence of the outer PPy layer, no distinct crystal face can be observed 9



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in the HRTEM image (Fig. 3B3). It is notable that the length and diameter of nanorods are decreased after annealing as shown in Fig. 3C1-C2. There is an apparent void cavity between the carbon shell and Bi nanorods, in good accordance with SEM results, which is conductive to the infiltration of electrolyte and the alleviation of volume swelling.41 In Fig. 3C3, obvious lattice space of ~0.34 nm and ~0.324 nm can be observed, which are assigned to the

graphite

(002)

and

Bi

(012)

crystal

face

(JCPDS:44-1246),

respectively.19,57

Figure 3. TEM and HRTEM images. (A1-A3) Bi2S3 nanorods; (B1-B3) Bi2S3@PPy nanorods; (C1-C3) yolk-shell Bi@N-C. The crystal structures of the as-prepared samples are analyzed by XRD in Fig. 4a. As displayed, all the peaks of pure Bi2S3 nanorods and Bi2S3@PPy are

indexed

well

to

the

standard

card

10


of

orthorhombic

Bi2S3

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(JCPDS:17-0320), indicating the high purity of the obtained samples. The absence of the broad peak at 2 = 10° ~ 30° for PPy in Bi2S3@PPy pattern could be attributed to the strong intensity of Bi2S3 and the low content of PPy.58 After annealing of Bi2S3@PPy in H2/Ar, the XRD pattern displays typical diffraction peaks of rhombohedral Bi (JCPDS:44-1246), manifesting the transformation from Bi2S3 to Bi. Owing to the small amount of carbon stemmed from PPy, only weak wide peak at 2 = ~24° is observed. The Raman spectra are presented in Fig. 4b, where the peaks located at ~250 cm-1 and ~310 cm-1 in the spectra of pure Bi2S3 and Bi2S3@PPy are well consistent with Bi2S3 lattice vibrations.57,59 Furthermore, the peaks at ~310 cm-1 and ~450 cm-1 of Bi@C-N are in good accordance with purchased commercial Bi (marked as P-Bi, in Fig. S2), further indicating the existence of Bi in Bi@C-N. Notably, there are two distinct scattering bands at ~1340 cm-1 and ~1580 cm-1 in the spectra of Bi2S3@PPy and Bi@C-N. The first band is indexed to D-bands (disorder-induced phonon mode), while another is assigned to G-bands (graphite band) of carbon.48 According to previous study, the two broad peaks of D-bands and G-bands can be separated into four peaks based on Gaussian numerical simulation.60,61 As shown in Fig. 4c-d, the two peaks at ~1200 cm-1 (peak 1) and ~1540 cm-1 (peak 3) are corresponding to sp3-type carbon (amorphous carbon and the defects), while the other two peaks at ~1350 cm-1 (peak 2) and ~1600 cm-1 (peak 4) are well consistent with sp2-type carbon (graphitized carbon). The integrated area ratio 11



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of sp3 to sp2 (Asp3/Asp2) offers significant information to analyze the existential state of carbon matrix.62 The Asp3/Asp2 value of Bi@C-N (0.617) is lower than the value of Bi2S3@PPy (0.816), indicating that the content of graphitized carbon is increased after calcining. The specific Bi content in Bi@C-N is further investigated by TG as shown in Fig. 4e. The combustion of C and N, and the oxidation of Bi, synergistically contribute to the weight loss of Bi@C-N. The XRD pattern of residual product is presented in Fig. 4f, all diffraction peaks of which are indexed to Bi2O3 (JCPDS: 65-2366), confirming that the final product is Bi2O3. The specific Bi content could be calculated by following Eqs. 1. Bi(wt%) = 100 ×

2 × 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝐵𝑖 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝐵𝑖2𝑂3

𝑓𝑖𝑛𝑎𝑙 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝐵𝑖2𝑂3

× 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝐵𝑖@𝐶_𝑁 𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒

Eqs. 1

The Bi content of Bi@C-N is calculated to be 66.9% according to Eqs.1. Moreover, the surface area and pore size distribution of Bi@C-N are analyzed by N2 adsorption-desorption isotherms. As shown in Fig. 4g, the Brunauer-Emmett-Teller (BET) surface area is calculated to be 205.42 m2 g-1. In the inset, pores that are smaller than 5 nm may originate from the mass loss of PPy shell during the carbonization, and those larger than 20 nm are attributed to the interspace between the outer carbon shell and the inner Bi rods, which effectively reveals the mesoporous structure of Bi@C-N.

12


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Figure 4. Characterization of Bi2S3, Bi2S3@PPy and Bi@N-C. (a) and (b) are XRD pattern and Raman spectra of Bi2S3, Bi2S3@PPy and Bi@N-C, respectively; (c) and (d) are peak-differentiating of D-band and G-band for Bi2S3@PPy and Bi@N-C, respectively; (e) TG and DCS curves of Bi@N-C; (f) XRD pattern of residual product after TG; (g) N2 adsorption-desorption isotherm and pore size distribution of Bi@N-C (inset). The chemical composition of Bi@C-N is further explored by XPS, presented in Fig. 5. Clearly, the signals of Bi, C, N and O are exhibited in the full survey scan, certifying the existence of above-mentioned elements. The additional signal of O may be derived from the adsorbed O2 on the surface of sample. In order to analyze the element existence state, high resolution spectra are collected. As shown by the previous work, the peaks at ~284.8 and ~285.6 eV in C 1s spectrum are associated with C=C and C-C bonds, 13



respectively.63 Besides, the small peak at ~288.5 eV is assigned to C-N bonds, suggesting the successful doping of N into the carbon matrix.56 The N 1s spectrum (Fig. 5c) presents three types of N: pyridinic N at ~397.2 eV, pyrrolic N at ~399.5 eV and graphite N at ~402.8 eV.64 The ratios of various N species are 27.8%, 56.1% and 16.1%, respectively. Benefitting from the PPy precursor of carbon matrix, pyrrolic N is prominent among the three types. It is verified that the large amount of pyridinic N and pyrrolic N could induce abundant defects and active sites, which contribute to the better electrochemical performances.63,64 The atomic ratio of introduced N is confirmed to be 9.79%. Bi 4f spectrum is displayed in Fig. 5d, which exhibits two peaks at 163.6 eV and 158.3 eV, agreeing well with previous works.26

14


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Figure 5. XPS spectra of Bi@C-N. (a) XPS full spectrum, (b) C 1s, (c) N 1s and (d) Bi 4f. Based on the above discussion, the formation process of Bi@C-N is proposed in Scheme 1. First in the hydrothermal process, Bi3+ and S2- were combined into crystal nucleus, which were constructed into Bi2S3 nanorod structure

through

an

oriental

attachment

mechanism.

Then,

a

uniform-thickness polypyrrole layer was generated on the surface of Bi2S3 through a room-temperature oxidative polymerization of pyrrole monomer. Subsequently, in H2/Ar (volume ratio: 5:95) atmosphere at 600 oC, the outer PPy layer was carbonized into N-doped carbon shell. At the same time the Bi2S3 was pyrolyzed into Bi nanorods and confined into the carbon shell, leading to the introduction of void cavity.

15



Scheme 1. The reaction mechanism diagram of Bi@C-N The detailed Li storage performance of Bi@C-N and P-Bi are evaluated. The volumetric capacity is the product of specific capacity and true density (5.66 g cm-3 for Bi@C-N, 9.78 g cm-3 for P-Bi). As shown in Fig. 6a, Bi@C-N exhibits an initial charge and discharge capacities of 3715 mAh cm-3 and 5078 mAh cm-3 at 0.1 A g-1, with a coulombic efficiency of 73%. The irreversible capacity at the first cycle is commonly due to the formation of solid electrolyte interface (SEI), the decomposition of electrolyte and other irreversible side reactions.19 The reversible capacity could be remained at 2150 mAh cm-3 after 50 cycles, with a capacity retention of 63% (based on the charge capacity of 2nd cycle). And it is nearly 2.8 times that of graphite (756 mAh cm-3). By 16


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contrast, P-Bi displays a poor cycle stability with a sharp decay of capacity, and the residual capacity is only 1300 mAh cm-3 after 50 cycles, corresponding to an inferior capacity retention of 32% (Fig. S3). The better cyclic ability of Bi@C-N could be deduced from its well-designed structure. During the repetitive lithiation/delithiation process, the bare Bi particles will be broken by huge volume expansion, leading to dramatic capacity fading. But for Bi@C-N, Bi is encapsulated in the carbon matrix, which is acted as a buffer to accommodate the volume change and maintain the integrity of structure, along with superior performance. In Fig. 6b, two voltage plateaus could be observed at 0.79 V and 0.72 V in the discharge curves and only one plateau appears at 0.86 V in the charge curves. The difference between the first discharge voltage plateaus and the succedent cycles may be attributed to the irreversible SEI formation. The rate performances of Bi@C-N and P-Bi were also evaluated at different current densities (Fig. 6c and Fig. S4). When cycled at 0.05, 0.1, 0.2, 0.5, 1.0 and 2.0 A g-1, the average capacities of Bi@C-N are 3910, 3460, 2900, 2300, 1940 and 1635 mAh cm-3, respectively. When the current density is decreased to 0.1 A g-1 again, the capacity surprisingly recovers back to 2830 mAh cm-3 (equal to 500 mAh g-1 in Fig. S5). It is amazing that at the highest current density of 2.0 A g-1, the volumetric capacity can still maintain at a high value of 1635 mAh cm-3, much higher than that of graphite. However, P-Bi nearly loses its capacity at high current density, with a capacity of only 107 mAh cm-3 at 2.0 A g-1 (Fig. S4). The evident gap 17



between Bi@C-N and P-Bi reveals the high performance of Bi@C-N contributed by the designed tolerant structure. As shown in Fig. 6d, the voltage plateaus and the shape of curves maintain well at various current densities, which also proves the stable electrochemical performances. The long-term cycle performance is further examined at the current density of 1.0 A g-1. A sharp-drop tendency of capacity in the initial 20 cycles is observed, from 3850 to 2430 mAh cm-3, which is attributed to the activation and adjustment of structure. When testing up to 500 cycles, the capacity could still maintain at 1700 mAh cm-3 (nearly 2.2 times that of graphite), corresponding to a capacity decay of 0.063% per cycle based on 20th cycle, demonstrating the superior cycling stability of Bi@C-N. The specific capacity of Bi@C-N and P-Bi is displayed in Fig. S5. And the performance of designed Bi@C-N is compared with other previous Bi-based materials, as displayed in Table S1. The N-doped carbon nanotubes could confine the Bi nanorods in it and provide space for volume change in the repeating lithiation/delithiation process, and also increase the electrical conductivity, hence yielding superior performance.

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Figure 6. Electrochemical performance of Bi@C-N. (a) cycle stability of Bi@C-N at the current density of 0.1 A g-1; (b) first five charge-discharge profiles of Bi@C-N; (c) rate performance of Bi@C-N at different current densities; (d) charge-discharge curves of Bi@C-N at different current densities; (e) long cycling performance of Bi@C-N at 1.0 A g-1. To further clarify the lithium-storage mechanism of Bi@C-N and P-Bi, the cyclic voltammetry (CV) tests are executed and the initial three cycles of the fresh Li-cells at scan rate of 0.1 mV s-1 are displayed in Fig. 7a-b. As shown in Fig. 7a, the broad peaks between 1.0 V and 2.0 V in the first cathodic scan 19



are related to the formation of SEI, which disappeared in the following cycles.19,23 The peak located at 0.64 V is shifted to high voltage and divided into two peaks (peak 3 at 0.79 V and peak 2 at 0.68 V), which are assigned to the stepwise transformation from Bi to LiBi3: Bi + Li+ + e- →LiBi (peak 3) and LiBi + 2Li+ + 2e- →Li3Bi (peak 2).19,20,24 Only one distinct peak at 0.93 V in the anodic scan is observed, which is associated with the reversible extraction of Li+ from Li3Bi: Li3Bi → Bi + 3Li+ + 3e- (peak 1).20,24 The peaks of P-Bi are located at the similar voltage position (Fig. 7b), but much sharper and with a weaker peak 3, deducing to the inadequate lithiation at the first step. Compared the morphology of as-prepared Bi@C-N and P-Bi (Fig. S2), the much smaller Bi@C-N could offer more reactive sites leading to more complete conversion at the first step from Bi to LiBi. The comparison of first CV curves for Bi@C-N and P-Bi is depicted in Fig. 7c. It is obvious that Bi@C-N processed a much broader peak 2 (0.6 V) than P-Bi (0.39 V). Theoretically, smaller particles of Bi@C-N would enlarge the exposed surface and create more active sites with wide energy distribution, benefiting to the redox reaction and resulting in the broader peak.65 The CV curves of Bi@C-N at different scan rates is displayed in Fig. 7d. As the scan rates are increased from 0.1 mV s-1 to 0.5 mV s-1, the characteristic peaks and shape of Bi@C-N CV curves could still preserve well. The diffusion coefficient of Li-ion could be obtained through the following Randles-Sevick equation19,66 (Eqs 2),

Ip=2.69×105n3/2AD1/2v1/2CLi+

Eqs 2 20


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in which Ip is the value of peak current in CV curves, n is the value of transfer electrons, A is the effective connecting area of electrode, v is the value of scan rate and CLi+ is the concentration of Li+. The value of D is calculated through the slope of the fitting linear between v1/2 and Ip, shown in Fig. 7e, with the slope values 2.22 for peak 1 and -1.56 for peak 2. The corresponding D for oxidation and reduction are 1.06×10-12 and 5.26×10-13 cm2 s-1, respectively. And the average value is 7.93×10-13 cm2 s-1. The detailed lithium storage mechanism could be qualitatively evaluated by the correlation between the value of peak current and scan rate66 (Eqs 3):

i=avb

Eqs 3

in which a and b are constants calculated from the log (|i|) - log (v) fitted linear. According to the previous research, when b-value is approximate to 0.5, the storage mechanism is mainly controlled by ion-diffusion, whereas the b-value approaching to 1.0 discloses capacitance-controlled mechanism. The b-values in this work are estimated to be 0.64 for peak 1 and 0.72 for peak 2 based on the plots in Fig. 7f, suggesting a diffusion-controlled mechanism of Bi@C-N.

21



Figure 7. The cyclic voltammetry analysis of Bi@C-N and P-Bi. (a) and (b) are the first three CV curves of Bi@C-N and P-Bi at 0.1 mV s-1, respectively; (c) the first CV curves comparison of Bi@C-N and P-Bi; (d) the CV curves at stepwise scan rates (0.1, 0.3 and 0.5 mV s-1) for Bi@C-N; (e) the fitting linear between Ip and v1/2; (f) the fitting linear between log(i) and log(v). Electrochemical impedance spectroscopy (EIS) method is applied to further analyze the electrochemical performance of Bi@C-N and P-Bi. As 22


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shown in Fig. 8B1-B2, the EIS is measured before and after cycling and the equivalent circuit model is displayed in Fig. 8B3. Both of the spectra have the similar shape: a flattened semicircle in the high and medium frequency range, which corresponds to the sum resistance of SEI film (Rf) and charge transfer (Rct) , and an approximate straight line in the low frequency range, representing the resistance associated with the ions-diffusion.16,19 It is distinct that the initial R at high frequency of Bi@C-N (about 140 ) is much smaller than that of P-Bi (about 280 ). After cycling, the R of Bi@C-N still maintains at a lower value. The above results are closely related to the structure of electrode materials. The bare P-Bi is easily pulverized into small particles and exposed to electrolyte, accompanied with continuous growth and destroy of SEI film, which causes the increase of resistance. Whereas, Bi is covered by carbon matrix and therefore the integrity is reserved, with stable SEI film and low resistance. According to the former reports, the phase evolution during the discharge and charge process could be explored by in-situ EIS method. The discharge-charge curve of 10th loop is shown in Fig. 8A. Based on that different voltage platforms correspond to different phase change, a series of voltage points, including 3.0, 1.4, 0.76, 0.3 and 0.01 V at discharge process and 0.76, 1.4 and 3.0 V at charge process are selected and the corresponding

in-situ EIS spectra are displayed in Fig. 8C1-C2. The diffusion coefficient of Li+ (DLi+) could be decided by using the following Eqs. 4-6.66

=2πf

Eqs 4 23



Zre=R+-1/2

Eqs 5

DLi+=0.5R2T2/A2n4F4C22

Eqs 6

R, T, A, n, F and C are constants, corresponding to gas constant (8.314 J mol-1 K-1), Kelvin temperature (298.15 K), effective contact area of electrode (1.54 cm2), electronic transfer number (3), Faraday constant (96485 C mol-1) and the molar concentration of Li+, respectively.  is Warburg coefficient, determined by the slope of the fitting linear of Z'--1/2. In Table 2, clearly, the DLi+ is decreased from 1.11×10-14 cm2 s-1 (D 3.0 V) to 2.16×10-15 cm2 s-1 (D 1.4 V) before Li-ion insertion, which may be induced by the ions adsorption on the surface of electrode. At the voltage point of 0.76 V, the DLi+ increased to 7.29×10-14 cm2 s-1, which could be explained that the primary lithiation process may create loose structure and more moving channel for fast ion diffusion. As more ions embedded into the electrode to form Li3Bi, the volume was expanded and compressed into the carbon layer, with DLi+ lessening to 6.67×10-15 cm2 s-1 at 0.01 V. The reversible change for DLi+ could be found during the delithiation process. The results of DLi+ are different from the above-mentioned results analyzed by CV curves because of the different selected models. The value of R at the high frequency range in Nyquist plots is also associated with the inner variation of materials. As shown in Fig. 8E and Table 2, R increased to 234 Ω during the discharge process and went

24


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nearly back to the initial value 78 Ω at the end point of charge, which exhibited excellent reversibility of the electrode material.

Figure 8. Electrochemical impedance spectroscopy (EIS) analysis of Bi@C-N and P-Bi. (A) phase evolution; (B1-B2) EIS of Bi@C-N and P-Bi measured before cycling and after cycling; (B3) equivalent circuit model for EIS; (C1-C2) 25



Page 26 of 40

Nyquist plots, (D1-D2) fitting linear of Z'--1/2 and (E) the value of R at different voltage points.

Table 2 The diffusion coefficient and R of Bi@C-N at different voltage Discharged Voltage [V]

3.0

1.4

0.76

0.3

0.01

DLi+ [10-14 cm2 s-1]

1.11

0.216

7.29

2.49

0.667

Charged Voltage [V]

0.01

0.76

1.4

3.0

DLi+ [10-14 cm2 s-1]

0.667

2.19

8.06

0.0514

Discharged Voltage [V]

3.0

1.4

0.76

0.3

0.01

R []

73

89

95

138

234

Charged Voltage [V]

0.01

0.76

1.4

3.0

R []

234

140

107

78

Summarizing the previous research works so far, most of the Bi-based anodes

suffer

from

dreadful

capacity

decay

in

the

initial

10~20

cycles,19,20,23,28,29 including this work. In order to deeply illuminate this phenomenon, ex-situ TEM is employed to study the morphology evolution. First, the images of Bi@C-N anode at 2nd cycle are exhibited in Fig. 9a. At the beginning of discharge process (3.0 V), the Bi anode maintains the initial complete structure similar to the images in Fig. 3. After the primary lithiation at 0.76 V, the inner Bi rod expands and fills up the carbon shell. With the insertion of more Li+ forming Li3Bi at 0.3 V, the nanorod is broken into 26


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nanoparticles which are wrapped in carbon matrix and distribute loosely. When the discharge process is fully completed (0.01 V), nanoparticles are still encapsulated in carbon shell with a fraction of them attempting to overflow. And Fig. 9b displays the schematic illustration of lithiation reaction. The structure maintains relatively well at 0.01 V of the end of 2nd discharge process. On the contrary, a certain amount of Bi particles overflow from carbon layer and are exposed to the electrolyte after 20 cycles (Fig. 9c). Losing contact with carbon matrix, SEI film will continually grow and break on the surface of the isolated Bi particles, which may lead to the increasing of resistance and the fading of capacity. Fig. 9d vividly presents the expansion and crushing of the nanorod from 2nd cycle to 20th cycle, giving information on the structure evolution. Based on the ex-situ TEM results, it can be deduced that the broken of nanorod and the reintegration of structure lead to the capacity fading at the first 10~20 cycles. The remained Bi particles are embedded and protected by carbon shell, contributing to the stable capacity later. Visibly, the introduction of carbonaceous material could effectively alleviate the structure destruction and improve the cyclic stability.

27



Figure 9. Ex-situ TEM images of Bi@C-N. (a) ex-situ TEM images of Bi@C-N in 2nd cycle at different voltage points; (b) schematic diagram for (a); (c) TEM image of Bi@C-N at 20th cycle; (d) structure change diagram of Bi@C-N from 2nd cycle to 20th cycle. CONCLUSIONS In summary, a neoteric yolk-shell structured 1D Bi@C-N nanocomposite

28


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has been fabricated in this work. The interspace between Bi and carbon shell could accommodate the volume change and inhibit the structure destroy, facilitating the enhancement of performance. Interacting with the advantages of (i) shorter diffusion paths caused by one-dimension stacking nanorods, (ii) numerous reactivity sites rooting from N-doping and (iii) the tolerant structure with

carbon

layer,

the

Bi@C-N

composite

could

present

excellent

lithium-storage performance. When used as anode for LIBs, Bi@C-N delivered a high reversible capacity, a considerable high-rate capability and excellent long-cycle stability with a capacity of 1700 mAh cm-3 at the high current density of 1.0 A g-1 after 500 cycles. The in-situ EIS results further proved the excellent reversibility of the material. And the ex-situ TEM was employed to observe the structure evolution, which shed light on the capacity decay mechanism of the initial cycles. The considerable electrochemical performance makes Bi@C-N a viable anode candidate for LIBs. Moreover, the synthetic method introduced cavity structure and heteroatom doping at the same time in this work could be referential for fabricating other materials for advanced applications. EXPERIMENTAL SECTION The reagents used in the work were purchased in analytical purity and used without further purification. Preparation of Bi2S3 nanorods. The Bi2S3 nanorods were prepared by hydrothermal method. Briefly, 1.5 g Na2S were dissolved in 15 mL deionized 29



water under magnetically stirring to form uniform solution. The obtained solution was dropwise added into Bi(NO3)3 solution (5 mL ethylene glycol (EG) contained 0.73 g Bi(NO3)3·5H2O ) with continuous stirring. Next, 0.76 g urea and 15 mL water were added into the mixture solution. After stirring for a short time, the homogeneous solution was transferred into a 100 mL stainless steel Teflon-lined autoclave and kept at 120 oC for 12 h. When the autoclave was cooling down, the Bi2S3 nanorods were collected through centrifugation and washed by alcohol and deionized water for later use. Preparation of Bi2S3@PPy nanorods. The Bi2S3@PPy nanorods were obtained by a room-temperature polymerization of pyrrole monomer outside the Bi2S3 nanorods. First, 80 mg as-prepared Bi2S3 and 4 mg sodium dodecyl sulfate (SDS) were dispersed into 40 mL deionized water under ultrasound for 0.5 h and then stirred for 1 h. Subsequently, 42 µL pyrrole monomer was added and stirred for another 1 h. After that, the oxidant, 8 mL 0.1 M (NH4)2S2O8 solution, was introduced into the suspension liquid dropwise and reacted for 4 h. The black product was obtained by centrifugation and washed with deionized water and alcohol for several times, dried at 60 oC overnight. Preparation of yolk-shell Bi@C-N. The Bi@C-N yolk-shell nanotubes were obtained by calcining Bi2S3@PPy nanorods at 600 oC for 1 h under Ar/H2 atmosphere (volume ratio: 95:5) with a heating rate of 3 oC min-1. Materials Characterization. The morphology features and internal structure of the samples were characterized by field-emission scanning electron 30


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microscopy (FESEM, FEI Quanta 200) and transmission electron microscopy (TEM, JEM-2100F). The compositions and structures of the samples were investigated by power X-ray diffraction (XRD, RIGAKU Ultima IV), Raman spectroscopy (Jobin-Yvon Lab RAM HR-800), and X-ray photoelectron spectroscopy (XPS, K-Alpha 1063, UK). The carbon content of the products was determined by thermogravimetric analysis (TGA, NETZSCH STA449F3). The sample density is tested by automatic true density analyzer (AccuPyc 1330, American). Electrochemical measurements. In order to investigate the lithium storage behavior of the as-obtained Bi@C-N composite, CR2016-type cells were assembled in a Mbraun glove box full filled with argon. The prepared sample, super P and carboxymethyl cellulose sodium (CMC) were mixed completely in mortar at the mass ratio of 70:15:15 and dispersed in deionized water. Then the uniform slurry was painted on the Cu foil and further dried at 60 oC overnight in vacuum oven. The foil was cut, pressed and then used as work electrode. The mass loading of active material is 0.5-0.7 mg per slice (effective electrode area is 1.50 cm2). Metallic lithium was employed as the counter electrode, and Celgard 2400 was used as separator. 1.0 M LiPF6 in a mixture of ethylene carbonate, diethyl carbonate and dimethyl carbonate (volume ratio, EC:DEC:DMC=1:1:1) with additive of 5% (volume ratio) fluoroethylene carbonate (FEC) was used as electrolyte. Taking advantage of CHI660d, the cyclic voltammetry (CV) measurements were performed 31



between 0.01 and 3 V (vs. Li/Li+), and the electrochemical impedance spectroscopy (EIS) results were obtained at an AC voltage of 5 mV amplitude in the range of 100 kHz to 0.01 Hz. Galvanostatic charge-discharge stability and rate capability were carried out by Land battery systems (CT 2001A) at different current densities in the voltage range of 0.01-3 V (vs. Li/Li+).

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (H.H.) ACKNOWLEDGMENT This work was financially supported by Young Elite Scientists Sponsorship Program by CAST (2017QNRC001), National Natural Science Foundation of China (51622406, 21673298), China Postdoctoral Science Foundation (2017M6203552), National Key Research and Development Program of China (2017YFB0102000, 2018YFB0104200), Innovation Mover Program of Central South University (2017CX004, 2018CX005), Hunan Provincial Science and Technology Plan (2017TP1001, 2016TP1009). ASSOCIATED CONTENT Supporting Information

32


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This material is available free of charge via the Internet at http://pubs.acs.org. SEM image and EDS spectrum of the product after the H2 reduction of solid Bi2S3, Raman spectrum and SEM image of commercial Bi, cyclic and rate performance of P-Bi, electrochemical properties of Bi@C-N and P-Bi with the dates in the form of specific capacity; the table of property comparison.

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Bismuth (Bi), which has a superb theoretical volumetric capacity of 3800 mAh cm-3, is restrained by the volume expansion during the lithiation process as anode for lithium-ion batteries (LIBs). Here, 1D yolk-shell structured Bi@C-N nanocomposite is designed and it delivers a stable reversible capacity of 1700 mAh cm-3 over 500 loops at high current density of 1.0 A g-1, nearly 2.2 times that of graphite.

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YolkâShell-Structured Bismuth@N-Doped Carbon Anode for Lithium

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YolkâShell-Structured Bismuth@N-Doped Carbon Anode for Lithium