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Large-scale fabrication of egg-cartons-inspired Bi/C composite towards high volumetric capacity and long-life lithium-ion batteries Haocheng Yuan, Yuqiang Jin, Xiaona Chen, Jinle Lan, Yunhua Yu, and Xiaoping Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06149 • Publication Date (Web): 18 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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ACS Sustainable Chemistry & Engineering

Large-scale fabrication of egg-cartons-inspired Bi/C composite towards high volumetric capacity and longlife lithium-ion batteries Haocheng Yuan, Yuqiang Jin, Xiaona Chen, Jinle Lan,* Yunhua Yu* and Xiaoping Yang

State Key Laboratory of Organic−Inorganic Composites, College of Materials Science and Engineering, Beijing University of Chemical Technology, North Third Ring Road 15, Chaoyang District, Beijing, China.

*Corresponding author: [email protected]; [email protected]

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ABSTRACT: Nanostructuring strategies have immensely improved energy storage performance for Li-ion batteries (LIBs). However, the consequent low tap density and the severe side reaction caused by excessive specific surface area make it difficult for nanomaterials to exhibit satisfactory performance in their practical applications. Herein, inspired by the structural characteristics of egg cartons, a rational nano-micro structure was designed to address these problems. Specifically, compact micron-size 2D dimpled carbon frameworks (cartons) with densely-distributed Bismuth nanoparticles (eggs) were prepared via a large-scale pyrolysis methods. The composite electrode exhibits a high volumetric capacity of 1461 mA h cm-3 at 100 mA g-1, which can outperform all reported Bi-based anodes so far and even be comparable to prevailing Si-based anodes. Furthermore, a high capacity retention of 89 % can be maintained after 1000 cycle at 1 A g-1. The excellent electrochemical lithium-storage performances are attributed to the unique egg-cartons-like nano-micro hierarchical structure with robust mechanical integrity and compact sheet morphology.

KEYWORDS: Bi/C anode, nano-micro structure, volumetric capacity, long-life cycling, Li-ion battery


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

INTRODUCTION Nowadays, energy storage devices have become an indispensable part for the use of

clean energy such as providing power sources for electric vehicles, or as large-scale energy storage system for intermittent energy sources like solar or wind energy. In order to better meet these applications, high energy and power density Li-ion batteries (LIBs) have become research hotspots among global scientists.1-6 In the field of anode materials, many alloy-type high-capacity anodes like Si, Sn, Sb and Bi have been developed to replace the traditional graphite anode which has a low theoretical capacity of 372 mA h g-1.7-9 The major problem for hindering the application of these alloy-type anodes is the large volume change during cycling, which results in poor cycling performance. Sophisticated nanostructure engineering, such as dimension reducing, carbon coating, controlled etching and pore-forming, have been used to solve aforementioned problem. With the design of elaborate nanostructures, the great improvement of cycling stability has been achieved.10-16 However, consequent low tap density of nanomaterials results in low volumetric capacity of the electrodes, which is a key parameter in practical application, and severe side reactions caused by excessive specific surface area make the electrodes suffer from the low Coulombic efficiency. In addition, high surface energy of nanomaterials induces aggregation, which will require for more cost to preserve separations. These disadvantages get in the way of the widespread use of nanomaterials in LIBs industry. Building nano-micro structure is an effective strategy to overcome these disadvantages, which can not only inherit the high electrochemical activity from



nanomaterials, but also synergistically gain additional advantages from unique micronsized architectures, such as high tap density and moderate specific surface area.17, 18 A variety of methods have been proposed to build nano-micro structure such as chemical grafting and mechanical pressing approaches.19-22 Liu et al. reported a hierarchical pomegranate-like structure of the Si/C nanocomposites to improve the Coulombic efﬁciency and volumetric capacity of Si-based anodes.23 Xu et al. further developed watermelon-like nano-sized Si/carbon composite by adjusting the size distribution of Si/C microspheres to achieve a high pressing density.24 Nevertheless, the problems of the complex preparation process, the high cost and the low yield should not be neglected for the practical application. More recently, Metal-organic frameworks (MOFs) or metal–organic coordination compounds (MOCCs) have been used as the precursors to fabricate complicated nano-micro structures with controllable structure, specific composition and uniform pore distribution. Various nano-micro structures such as 3D carbon framework from sodium citrate, triphenylantimony

26

25

freestanding Sb/C framework from

and rGO-Wrapped MoO3 porous structure by calcination of

GO/Mo-MOF hybrids 27 have been fabricated to improve the cycling stability and rate capability of electrodes. However, the relative small scale as well as the expensive organic ligands are the huge problems of these elaborated MOF-derived materials for the further application. Therefore, it is still challenging to construct the nano-micro structures with satisfactory performance and mass production for electrode materials. In this work, we propose an “egg-cartons-inspired” nano-micro hierarchically structured Bi/C composite as a high-volumetric capacity and long-life cycling anodes


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for LIBs. This nano-micro structure is constructed with compact micron-size 2D dimpled carbon frameworks (cartons) integrating densely-distributed ~20 nm Bi nanoparticles (eggs), which was prepared by directly carbonizing the bismuth citrate. The design has three advantages: (1) Bismuth nanoparticles (eggs) can be uniformly distributed with high mass fraction and still possess nanostructure’s features —high rate and high capacity; (2) compact hierarchical carbon sheets (cartons) protect active materials against stresses exerted during cycling and limit the incidents of fracture to the fragile nanoparticles; (3) the overall egg-cartons-like structure can make full use of volumetric capacity of electrode material by close and dense packing. In addition, facile preparation process of this material make it easily to fabricate in large-scale. Consequently, the as-prepared hierarchical Bi/C anodes present a high volumetric capacity of 1461 mA h cm-3, excellent rate performance and cycling stability with capacity retention of 89 % over 1000 cycles at 1 A g-1. Furthermore, high pseudocapacitive contribution of 69 % in hierarchical Bi/C anodes can also be attributed

to

the

egg-cartons-inspired

nano-micro

structure.

The

excellent

electrochemical performance and the ability of mass production make hierarchical Bi/C composite attractive in large-scale energy storage systems. 

EXPERIMENTAL SECTION

Preparation

of

hierarchical

bismuth/carbon

composites:

Hierarchical

bismuth/carbon composites were prepared by the calcination of bismuth citrate (99 %, Macklin) at 500, 700 and 1000 oC for 2 h in nitrogen atmosphere with a heating rate of 5 oC min-1. Then, the obtained black powders were directly used as active materials



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without any post processing. The electrodes were denoted as Bi/C-500, Bi/C-700 and Bi/C-1000 according to the carbonized temperature. Characterization: The microstructure and morphologies of as-prepared samples were characterized by a wide angle X-ray diffractometer (XRD) (WAXD, D8 Advance, Bruker, Cu Kα, λ=0.154 nm), a Raman spectroscope (JY-HR800, the excitation wavelength of 532 nm), a field emission scanning electron microscope (FE-SEM, Supra55, Zeiss) and a high resolution transmission electron microscope (HRTEM, JEM-3013, JEOL). The surface chemical states were characterized by the X-ray photoelectron spectroscopy (XPS, Escalab 250, Thermo Fisher Scientific Inc). The specific surface areas and pore-size distributions of the products were estimated using a

Brunauer–Emmett–Teller

(BET)

analyzer

(Micromeritics,

ASAP

2020).

Thermogravimetric analysis (TGA) was carried out on a TGA instrument (TA-Q50, America) at a heating rate of 10 oC min-1 from 25 to 800 oC in air. Electrochemical

measurements:

For

electrochemical

performance

testing,

hierarchical Bi/C composites were mixed with polyvinylidene fluoride (PVDF) and super P in a ratio of 8:1:1 with aid of N-methyl pyrrolidone (NMP). The obtained slurry was coated on a copper foil, and then dried under vacuum at 120 oC for 12 h. Lithium metal foil, Celgard 2400 membrane and 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 v/v) were used as counter electrode, separator and electrolyte, respectively. The half cells were assembled in an argon-filled glove-box (OMNI-LAB). The cycle and the rate performance of the half cells were evaluated though a LANDCT2001A battery tester. Cyclic voltammetry (CV) measurements were


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performed between 0 to 3 V using an Autolab PGSTAT 302 N (Metrohm) workstation. 

RESULTS AND DISCUSSION In this work, bismuth citrate was used as single precursor to directly realize the

egg-cartons-like structure because of four considerations: (1) the natural micron-sized sheet morphology of raw bismuth citrate (Figure S1); (2) high theoretical volumetric capacity of Bi (3765 mAh cm-3) and low carbothermal reduction temperature for the Bismuth-based materials; (3) Bismuth has a moderate operating voltage (0.5~0.9 V vs Li+/Li), which make it not have to take risks for the growth of Lithium dendrites at high rate; (4) environmental friendliness and toxic-free property of Bismuth.28,

29

The

preparation processes for the formation of egg-cartons-like Bi/C composites is shown in Scheme 1. During the carbonization, the organic ligands of bismuth citrate can be converted into carbon in N2 atmosphere. Meanwhile, Bi3+ species will be in-situ transformed into bismuth oxide at 350 oC and subsequently reduced into metallic Bi above 400 oC. The detail of the formation was studied by analyzing the XRD results of calcination products of bismuth citrate at 300 oC, 350 oC and 400 oC, respectively (Figure S2). Specifically, Bi is a typical low melting point metal (melting point: ~ 271 oC).

When the calcination temperature is higher than the melting point of Bi, the citrate

ligands of bismuth citrate sheets will be converted into carbon sheets to encapsulate liquid Bi nano-droplets, which in turn act as a template for formation of dimpled carbon. As a result, the egg-cartons-like Bi/C composite can be obtained after cooling. It is worth noting that no any catalysts, templates or additives are added in the synthesis process of hierarchical Bi/C composites. Moreover, comparing the weight of the



product before and after carbonization, a high yield of 68.7 % can be obtained in the preparation of Bi/C composite from bismuth citrate even the calcination temperature is as high as 1000 oC. We can easily obtain > 10 g Bi/C composites from one batch (Figure S3). The facile and high yield synthesis of Bi/C composite will make it possible for large-scale production.

Scheme 1. Schematic presentation of egg-cartons-inspired design and the synthesis procedure of Bi/C composite sheets.

The morphologies of Bi/C-500, Bi/C-700 and Bi/C-1000 were analyzed by FE-SEM as shown in Figure 1a-c. All samples show typically micron-sized sheet structure with the size of 2-5 μm. As the carbonization temperature increases, the sheet structure becomes more regular and compact. The SEM images with high magnification (Figure 1d and e) of Bi/C-1000 clearly show the egg-cartons-like structure, that is, numerous Bi nanoparticles with the size of ~20 nm were accommodated in the micron-sized carbon sheets. BET tests (figure S4) revealed that the specific surface area and the total pore volume of three samples as shown in Table 1. The specific surface area and the total pore volume decreased with the increasing of carbonization temperature, indicating that Bi nanoparticles become more tightly packed under higher temperature.


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The tap density of Bi/C-500, Bi/C-700 and Bi/C-1000 are 0.87, 0.99 and 1.07 g/cm3, respectively. As shown in figure S5, Bi/C-1000 shows the lowest volume at the same mass among three samples, which is good agree with the SEM images and BET results. Fig. 1f shows the X-ray diffraction (XRD) patterns of Bi/C-500, Bi/C-700 and Bi/C1000. All sharp diffraction peaks of samples are corresponding to the hexagonal Bi phase (PDF#44-1246). The sharp peaks located at 27.2 o, 37.9 o, 39.6 o, 48.7 o and 55.6 o

are attribute to (012), (104), (110), (202) and (024) planes of Bi, respectively.

Moreover, a wide peak around 28° can be assigned to amorphous carbon. 30

Figure 1. SEM images of (a) Bi/C-500, (b) Bi/C-700 and (c) Bi/C-1000. (d, e) SEM images of Bi/C-1000 with high magnification. (f) The XRD patterns of hierarchical Bi/C composites. Table 1 Specific surface area and total pore volume of Bi/C-500, Bi/C-700 and Bi/C1000. Sample Specific surface area total pore volume 2 -1 (m g ) (cm3 g-1) Bi/C-500 Bi/C-700 Bi/C-1000

57.6 36.1 29.3


0.0927 0.0698 0.0596


The egg-cartons-like structure can be unambiguously seen in the HRTEM images of the as-prepared samples as shown in Figure 2a, S6a and S6b. Densely-distributed Bi nanoparticles are embedded in the two-dimensional (2D) micron-sized carbon sheets without aggregation, which helps to fully exhibit the electrochemical activity of Bi nanoparticles. Higher magnification HRTEM images of Bi/C-1000 (Figure 2b and 2c) further confirm the egg-cartons-like structure in the Bi/C-1000 sample. It can be clearly seen that Bi nanoparticles were individually encapsulated by each of the carbon dimples. The interconnected carbon dimples can greatly improve the electrical conductivity of composites and lead to a high-rate performance. Furthermore, mesoporous between Bi/C nanoparticles can be determined as shown in Figure 2c, which is good agree with BET results. Both thin carbon layers (2-3 nm) and relative thick carbon layers (8-10 nm) can be observed in the carbon dimples. It can be confirmed in the Figure 2d and 2e by etching Bi nanoparticles using FeCl3/HCl solution. The ultra-thin layers of carbon dimples provide a shorter Li+ diffusion pathway from electrolyte to Bi nanoparticles, which helps the rate performance. While the thicker carbon layer at the joint of dimples can act as a mechanical buffer to effectively absorb the phase transformation-induced stresses, which can prevent fracture and pulverization of the fragile Bi nanoparticles during cycling. The particle size of the three samples was measured by TEM images (Figure 2b, S6c and S6d) as shown in Figure 2f. It's worth noting that the size of Bi nanoparticles decreased with the increasing of carbonization temperature. The average particle size of Bi/C-500, Bi/C-700 and Bi/C-1000 are 30, 27 and 19 nm, respectively.


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In general, smaller nanoparticles have a shorter diffusion pathway, which can help the improvement of rate performance. The energy dispersive spectrometer (EDS) mappings of C-K, O-K, Bi-L and Bi-M (Figure 2g-k) show a uniform spatial distribution of Bi nanoparticles and the feature of carbon sheets. The diffraction rings in selected area electron diffraction (SAED, Figure 2l) demonstrate that the structure of Bi nanoparticles are polycrystalline. The presented diffraction rings can be well indexed to the hexagonal phase of Bi, which are in good agreement with XRD results. The weight percentages of C, O and Bi in all samples were detected by the EDS as shown in Figure S7 and Table S1. The weight percentages of Bi in Bi/C-500, Bi/C-700 and Bi/C-1000 were 72 %, 75 % and 80 %, respectively. The increase in Bi content with temperature is attributed to the more complete carbonthermal reduction occurred in higher temperatures. TG tests in Air atmosphere were also conducted in order to know the content of Bi more accurate as shown in Figure S8. The weight loss process involved the oxidation of bismuth and the consumption of carbon. The content of Bi was calculated by follow equation:

Content of Bi=Residual weight(%) 

M(Bi) M(Bi 2 O3 )

(1)

M (Bi) and M (Bi2O3) mean the relative molecular mass of Bi and Bi2O3, respectively. The calculated content of Bi of Bi/C-500, Bi/C-700 and Bi/C-1000 are 79 %, 83 % and 85 %, respectively. To the best of our knowledge, such a high content as 85 % of Bi for Bi/C-1000 has not been reported in other Bi-based anode materials as shown in Table S2.



Figure 2. HRTEM images of (a) Bi/C-500, (b) Bi/C-700 and (c) Bi/C-1000. (d, e) HRTEM images of Bi/C-1000. (f) Particles size distribution of Bi/C composite sheets. (g-k) EDS element mapping of (e) C-K, (f) O-K, (g) Bi-L and (h) Bi-M of Bi/C-1000. (l) SAED pattern corresponding to (e).

X-ray photoelectron spectroscopy (XPS) was performed on the three samples to reveal their surface chemical states. Figure 3a shows the XPS general spectra in the range of 0-1000 eV. It can be seen that C, O and Bi elements clearly exist in all three samples. Figure 3b shows the curve-fitted XPS spectra of Bi 4f for Bi/C-1000. Bi0


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exhibits very low peak intensity at 161.7 eV and156.5 eV and two other clear peaks at 163.6 eV and 158.0 eV could be observed in Bi 4f spectra, indicating a transition layer, such as Bi-O (163.8 eV, 158.5 eV) or Bi-O-C (163.1 eV, 157.8 eV), might exist between metallic Bi and ultra-thin carbon layer. The transition layer also can be observed in C 1s spectra as shown in Figure 3c. Besides C-O and C-C bonds, a peak with lower binding energy at 283.4 eV could be assigned to C-O-Bi due to the lower electron density at the O site. The similar phenomenon can also be found in reported C-O-Sb system.31 In addition, we compared the Bi 4f and C1s spectra among the three samples as shown in Figure S9. The binding energy of Bi 4f and C1s decreased with the increasing of carbonization temperature. The decreased binding energy can be attributed the decreased number of O atoms which have higher electronegativity compared with Bi atom and C atom (O atom, 3.44; C atom, 2.55; Bi atom, 2.02), further indicating the more complete reduction of Bi2O3 and more graphitized carbon. The increasing degree of graphitization was also confirmed by Raman spectra as shown in Figure 3d. The relative intensity ratio of the D-band (1320 cm-1) to G-band (1590 cm-1) could be used to estimate the disordered degree and defects of carbon materials. The ratio of Id/Ig decreased from 1.00 to 0.88 with the increasing of carbonization temperature, indicating that the gradually reduced defect sites and increased degree of graphitization. High degree of graphitization can increase the electrical conductivity of carbon layer, and improving the rate performance of the electrode.



Figure 3. (a) XPS general spectra of Bi/C composite sheets. High-resolution XPS spectra of (b) Bi 4f and (c) C 1s regions for Bi/C-1000. (d) Raman spectra of Bi/C composite sheets.

The lithium ion storage properties of Bi/C were evaluated in Li-ion half-cells. Figure 4a shows the first three CV curves of Bi/C-1000 at a scan rate of 0.1 mV/s within a potential range of 0.005-3 V. Two sharp reduction peaks at 0.58 V and 0.70 V during the first cathodic scan can be assigned to the alloying reaction of Li3Bi and the formation of solid-electrolyte interface (SEI) film. In the subsequent two cycles, these two sharp reduction peaks transfer to 0.61 V and 0.76 V, corresponding to the formation of LiBi and Li3Bi, respectively. The sharp oxidation peak at 0.94 V attributes to the reverse process of the Li-Bi alloying reaction. Two pairs of small redox peaks at


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1.35/1.73 V and 1.85/2.41 V may be assigned to the conversion of small amounts of Bi3+ into metallic Bi. The small redox peak close to 0 V can be attributed to the intercalation of Li-ion into carbon layers. It is worth noting that after first scan, the CV curves of the second and third scans are basically coincided, suggesting the high reversibility of lithiation/ delithiation. Figure 4b illustrates the galvanostatic charge– discharge curves of Bi/C-1000 at a current density of 100 mA g−1 within a potential range of 0.005-3 V. The initial discharge capacity and charge capacity were 1057 and 688 mAh g-1, corresponding to the initial Coulombic efficiency of 68 %. The initial irreversible capacity loss is mainly attributed to the formation of SEI layer and the electrolyte decomposition. Compared with other Bi-based anode materials, our Bi/C1000 electrode with the highest reported Bi content as shown in Table S2 shows a relatively high initial Coulombic efficiency. The high initial Coulombic efficiency can be attributed the relatively low specific surface area and structural stability during the charge/discharge process, which will be further analyzed in Figure 6 and Figure S12.



Figure 4. Electrochemical characterization of hierarchical Bi/C sheets. (a) First three CV curves and (b) charge-discharge curves of Bi/C-1000. (c) Cycle and (d) rate performances of Bi/C composite sheets. (e) The volumetric capacity of Bi/C-1000 and the comparison with some representative reported anodes.

Figure 4c presents the cycle stability of egg-cartons-like Bi/C sheets at 100 mA g-1. Commercial Bi was also tested for comparison. The capacity of commercial Bi


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decayed very quickly, only 11.4 % of the initial capacity (57 mAh g-1) can be maintained after 100 cycles. Compared with the commercial Bi, Bi/C electrodes with egg-cartons-like hierarchical sheet structure exhibit excellent capacity retention. For Bi/C-1000, a high specific capacity of 523 mAh g-1 can be obtained after 100 cycles, which is about 9 times higher than that of pure Bi and 1.40 times higher than the theoretical capacity of graphite. Bi/C electrodes obtained at different carbonization temperatures show basically the same lithium-ion storage performance at 100 mA g-1 after 100 cycles, which confirms the outstanding stability of the egg-cartons-like structure. Figure 4d shows the rate performance of Bi/C electrode. Among three samples, Bi/C-1000 electrode exhibits the highest specific capacity of 499, 456, 400, 342, 286, 200, 99 mAh g-1 at 100, 200, 500, 1000, 2000, 5000 and 10000 mA g-1, respectively, and when the current density turns back to 100 mA g-1, the specific capacity returned to 481 mAh g-1. The capacity retention is as high as 96 % after the rate performance testing, indicating outstanding structural stability of egg-cartons-like Bi/C composites. Compared with Bi/C-500 and Bi/C-700, Bi/C-1000 anode shows the higher rate performance due to the higher degree of graphitization of carbon layer and smaller particle size of Bi nanoparticles as mentioned above, which improve the electrical conductivity and shorten the Li-ion diffusion distance for the electrode. Table S2 lists the Li-ion storage properties of other reported Bi-based anode materials including Bi@C core-shell nanowires,32 Bi@C microshperes,33 Bi/C nanofiber,34 Bi@C nanocomposite,35 Bi/Al2O3 36 and Bi@NC 37. Obviously, egg-cartons-like Bi/C composites performed the best in terms of the reversible capacity and cycling stability.



The volumetric capacity of Bi/C-1000 electrode was calculated as being 1461 mAh cm3 at 100 mA g-1 according to mass loading and the thickness of active materials as shown

in Figure S10. The high volumetric capacity of Bi/C-1000 electrode surpasses previously reported Bi-based anodes 37 and most of previously reported anodes. 38 And even compared with Si-based anode materials, 23, 39-43 Bi/C-1000 electrode is still at the forefront in terms of volumetric capacity. There are two reasons that Bi/C-1000 has a high volumetric capacity. First, Metallic Bi itself has a high theoretical volumetric capacity of 3765 mAh cm-3. Second, it is the egg-cartons-like structure that can fully exhibit the volumetric capacity as high as possible by compacting micron-sized carbon sheets with densely-packed Bi nanoparticles.


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Figure 5. (a) Long cycling test for Bi/C-1000 at 1000 mA g-1. (b, c) SEM images and (d, e) TEM images of Bi/C-1000 electrode after 1000 cycles.

In order to investigate the long cycle stability of Bi/C-1000 electrode, the long cycle testing was conducted at a high current density of 1000 mA g-1 for 1000 cycles as shown in Figure 5a. After a slow capacity fading in the initial 50 cycles, Bi/C-1000 electrode presented superior performance and long-term stability with a high specific



capacity of 315 mAh g-1, as well as nearly 100 % Coulombic efficiency. The capacity retention is as high as 89 % even after 1000 cycles, and the stable cycling is still going without obviously decay. The morphology changes of Bi/C-1000 after 1000 cycles were characterized using SEM and TEM techniques. Compared with the fresh Bi/C1000 electrode (Figure S11), the Bi/C-1000 sheets after cycles (Figure 5d and 5e) showed a nearly original hierarchical structure with uniform and thin SEI film coated on the surface. TEM images further reveal that the densely-distributed Bi nanoparticles were still individually confined in carbon framework without obvious aggregation or growth. The size of Bi nanoparticles remains at 10-20 nm after 1000 cycles, indicating that the carbon layers as the mechanical protection are strong enough to accommodate the volume change of Bi nanoparticles.

Figure 6. The cross-sectional SEM images of Bi/C-1000 electrode; (a) Fresh electrode; (b) Fully lithiation; (c) After delithiation. (d) Schematic illustration of the thickness change of egg-carton-inspired Bi/C composite.


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To further explore the robust cycling stability of egg-cartons-inspired Bi/C composite, the thickness change of Bi/C-1000 electrode after lithition and delitihition was observed via cross-sectional SEM images as shown in Figure 6. It is worth noting that the change in thickness before and after lithition is only 20 %, 10 times lower than the theoretical value (~215 %) of pure Bi. And after delithition, the electrode recovered to the pristine thickness, indicating that two-dimensional dimpled carbon and a small amount of mesopores can greatly restrict the volume change of Bi nanoparticles during the cycles as shown in the scheme of Figure 6d. Another major challenge of metallic Bi as anode materials is low Coulombic efficiency during the cycling. When the bismuth swells and shrinks, the SEI layer will deform and break. The formation of new SEI on the freshly exposed bismuth surface will cause the low Coulombic efficiency. As shown in Figure S12, Coulombic efficiency of Bi/C-1000 quickly climbed to near 100 % after few cycles rather than fluctuated as bulk Bi did. The fast-rising and stable Coulombic efficiency in the first few cycles indicates that the SEI film can be statically grown on the surface of Bi/C-1000 electrode due to the small volume change about 20 % during the cycling. In addition, we conducted the XRD pattern of Bi/C-1000 electrode before and after the rate and 100th cycle performance test as shown in Figure S13. It's worth noting that the peaks of Bi are still maintain sharp without obvious widened, indicating that Bi nanoparticles are still maintain a good crystallinity without further pulverization, which further confirmed the structural stability of Bi/C-1000 electrode. Therefore, The synergy of outstanding structural stability and appropriate specific



surface area (29.3 m2 g-1) make egg-carton-inspired Bi/C possess high Coulombic efficiency and robust cycle stability.

Figure 7. (a) CV curves of Bi/C-1000 at various scan rates of 0.2 to 1.0 mV s-1. (b) Log (peak current, A) versus log (sweep rate, mV s-1) plots and the corresponding fitting line. (c) Capacitive and diffusion-controlled contribution to charge storage of Bi/C-1000 at 1.0 mV s-1. (d) Normalized contribution ratios of capacitive and diffusion-controlled capacities at different scan rates.

The kinetics analyses were conducted to investigate the mechanism of Li-ion storage in Bi/C-1000. Figure 7a shows the CV curves at various scan rates from 0.2 to 1.0 mV s-1. Based on above phase and element analysis, we mainly consider the below two types of charge-storage mechanisms: one is faradaic contribution arising from alloying reaction between Bi and Li-ion (diffusion-control), another is the charge


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transfer with surface/subsurface atoms (The essence of pseudocapacitance). Electrical double-layer effect is not considered due to the specific surface area is only 29.3 m2 g1.

It is known that the relationship between the current (i) and sweep rate (v) obey the

following formula:

i = av b

(2)

The b-value is close to 0.5, suggesting the diffusion-dominated process, and the value of b is near 1.0, indicating a capacitive-controlled process. For Bi/C-1000 electrode, the b value calculated by the slope of log (v)-log (i) plots of the anodic peak at 0.37 V is 0.855 (Figure 7b), indicating the fast kinetics of alloying reaction between Bi and Li. The ratio of Li-ion capacitive contribution can be further quantified by separating the current response i at a fixed potential V into capacitive effects (k1v) and diffusion-controlled reactions (k2v1/2):

i (V) = k1v + k2 v1/2

(3)

i /v1/2 =k1v1/2 +k2

(4)

As shown in Figure 7c and 7d, the capacitive contribution ratios in Bi/C-1000 electrode are 50 %, 57 %, 64 %, 66 % and 69 % corresponding to the scan rates of 0.2, 0.4, 0.6, 0.8 and 1.0 mV/s, respectively. The large ratio of capacitive contribution leads to an excellent rate capability for the electrode. It should be noted that the egg-cartonslike Bi/C-1000 can possess a capacitive-dominated fast dynamic process even the specific surface area is only 29.3 m2 g-1. The high capacitive contribution can be attributed two parts of Bi/C composite. The first part is from the ultra-thin carbon layer with the thickness of 2-3 nm; the second part can be attributed to the uniform Bi



nanoparticles with the size of sub-20 nm. 44 The structure of ultrathin carbon framework encapsulated uniformly-distributed Bi nanoparticle give the electrode fast kinetics of Li-ion lithition/delithition. High capacitive contribution of Bi/C-1000 electrode demonstrating that the electrochemical performance was optimized well by nano-micro structure design, which ensure the electrochemical performance and electrode density simultaneously, and greatly help the improvement of volumetric capacity and cycle stability.


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

CONCLUSIONS In summary, we have proposed a unique egg-cartons-inspired nano-micro design

for fabricating high volumetric capacity and long-life lithium-ion battery anodes. As a proof-of-concept, compact Bi/C hierarchical composite sheets were successfully synthesized through a facile and large-scale pyrolysis method using bismuth citrate as a precursor. As-prepared Bi/C hierarchical composites can not only inherit the high electrochemical activity of the Bi nanoparticles, but also synergistically gain additional advantages from the compact micron-sized 2D dimpled carbon frameworks, such as high pressing density, moderate specific surface area and robust mechanical protection. As a result, the typical Bi/C-1000 electrode shows a volumetric capacity as high as 1461 mAh cm-3 and excellent cycling stability with a capacity retention of 89 % after 1000 cycle at 1 A g-1. Kinetics analyses demonstrates that high capacitive contribution exists in Bi/C-1000 electrode, leading to a high rate performance. The nano-micro design developed in this paper can also be used in other low-melting-point alloy-type anode materials (e.g. Sn and Sb) for Li-ion batteries with high volumetric energy density and long-life cycling performance.



ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website at DOI: SEM, TEM images; photograph; BET data; EDS data; TGA curves; High-resolution XPS spectra; Coulombic efficiency in the first 15th cycle; Table of electrochemical properties of Bi-based electrodes for lithium-ion batteries.

Author Contributions This manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by National Natural Science Foundation of China (51772016, 51272021 and 51142004) and Fundamental Research Funds for the Central Universities (XK1802-2)


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For Table of Contents Use Only

Compact micron-size 2D dimpled carbon frameworks (cartons) with denselydistributed Bismuth nanoparticles (eggs) were prepared for high-performance LIBs anodes.


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C composite

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