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Amorphous-silicon nanoshell on artificial graphite composite as the anode for lithium-ion battery
Solid State Sciences 93 (2019) 24–30
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Amorphous-silicon nanoshell on artificial graphite composite as the anode for lithium-ion battery
T
Seh-Yoon Lim School of Advanced Materials Science and Engineering, SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Amorphous Si MCMB CVD Anode Lithium-ion battery
The fabrication of uniform amorphous silicon (a-Si) nanoshell on graphite is recognized as a high energy technique for building block into macroscopic materials. Here in, various thick containing nanoscaled A-Si nanoshells could easily be adjusted by controlling the preparation conditions. In this structure, A-Si nanoshell thickness with up to 24 nm are uniformly coated and supported on anode composite. The resulting AG@a-Si90min composites (a-Si shell thickness ≈ 15 nm) exhibited better cycle reversibility and stability than among samples. The G@a-Si-90 sample delivered a relatively stable specific capacity of ca. 384 mAh.g-1 at 1.3 A g−1 (1C: 650 mAh.g-1) for 200 cycles. Furthermore, the designed A-Si shell by CVD can also be applied to the study of the synergetic properties of great importance for the large-scale production for batteries and other devices.
1. Introduction Recently, the increasing demands with lithium ion batteries (LIBs) have stimulated intense study on the fabrication of various energy systems for portable IT devices, electric vehicles (EVs) and renewable energy [1–5]. Among the current graphite anode options for LIBs, the performance is limited by the low capacity (372 mAh.g-1). The property of graphite anode cannot meet increasing satisfaction about needs due to their low reversible capacity and poor rate capability because the slow solid-state diffusion of Li+ within the structure [4,6,7]. Therefore considerable researchers are searching for novel anode materials for high-performance with LIBs. Among these anode materials, elemental silicon (Si) has attracted remarkable attention as the most promising anode to replace commercial graphite because the alloy mechanism with Li ions with the high theoretical capacity near 4000 mAh.g-1, low discharge potential (∼0.4 V versus Li+/Li) and rich resources, which is higher advantages than that of other anode [3,6,8–12]. Despite of these promise, Si experiences a large volume expansion (> 300%) during continuous cycling, which can cause heavy strain in electrode, leading to pulverization, excessive growth of the solid electrolyte interphase (SEI) and increased internal resistance by loss of electrical conductivity [11,13,14]. As a result, several challenges exist for Si that hindered their commercial applications. Over the past few decades, many efforts have been devoted to compensate to improve the cycle life of silicon by fabricating nanosized silicon-based materials (including nanowires [15–18], nanotubes [19–21] and nanoparticles [19,22], hollow Si materials [23], porous Si [24–29] and active/-inactive composites [30,31].
Of these, one promising the design of amorphous based silicon has become an important research direction on cycle performance because of the good electronic conductivity and fast Li+ diffusion in improving the stability of silicon-based anodes [11,13]. The amorphous phase is dominated to Li diffusion due to the propensity of disordered like grain boundaries, which leads to an isotropic volume expansion by lithium diffusion [11,13], while the crystalline is relevant to the difference in reaction with crystallographic direction. Various methods previously have been worked for preparing amorphous a-Si-based composite anodes, such as a high-energy mechanical milling (HEMM) [32], hydrothermal method [33], spray pyrolysis [34] and a chemical vapor deposition (CVD). Among them CVD process can affect facile process without multi steps and a more uniform quality with a controllable thickness, which proceed to the less electrical conductivity loss between electrolyte and electrode materials during cycling. Recently, Ko et al. [35] reported carbon coated amorphous Si around natural graphite in large amounts by repeated CVD treatment to improve the high reversible capacity uniformity. The obtained material delivered a high reversible capacity of over (> 500 mAh.g-1) after 100 cycles. Although all these methods exhibits better cycling performance and solve the defects to some extents for Si anodes, it is still remained that electrode performance with various amorphous Si shell thicknesses with artificial graphite such as mesocarbon microbeads (MCMB) on the electrochemical behavior during cycling has not been reported. In this work, commercially available MCMB was employed. Microsized MCMB with lamella structure has many benefits than graphite from perspective structure such as high tap density (1.2–1.3 g.cm-3), small surface area,
E-mail address: [email protected]. https://doi.org/10.1016/j.solidstatesciences.2019.05.002 Received 9 April 2019; Received in revised form 30 April 2019; Accepted 1 May 2019 Available online 02 May 2019 1293-2558/ © 2019 Elsevier Masson SAS. All rights reserved.
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Fig. 1. Schematic illustration of the synthesis of amorphous Si nanoshell on artificial graphite composite.
SEM, JEOL 6500) and high-resolution transmission electron microscopy (HRTEM, JEOL 2100). Raman spectra were collected using a 514 nm laser with RM100 under ambient condition with a laser spot size of about 1 mm. Surface areas were measured at 77 K with a Quantachrome Autosorb-3B analyzer (USA). The amorphous Si weight fractions in AG@a-Si samples were determined by evaluating the mass loss of AG@ a-Si composite after calcination by thermogravimetric analysis (TGA; TGA-50 Shimadzu instrument), which is carried out from temperature to 1100 °C at a ramp of 10 °C min−1 in air atmosphere.
and low irreversible capacity corresponding to side reaction. Compare with natural graphite, surfaces of MCMB are made up of exposed edgeplane surfaces, thus enhanced Li+ intercalation and rate capability during cycling [36]. From an electrochemical point of view, micro sized materials experience advantage such as high tap density and generate high energy and volumetric capacity. However, it may have heavy stress and the reach to the center long ion/electron transport pathways during charge/discharge process. On the other hand, nanosized materials have light stress for volume expansion and short the Li+ transport path lengths, however, it exhibits the lower tap density, which cause to the low energy and volumetric density. Taking into account these merits and drawbacks, building blocks coated nanoscale on macrosized graphite template can provide harness these advantages while avoiding the above-mentioned shortcomings. Herein, we report on the effects of the CVD approach processing time reaction on the amorphous Si content, the thickness of shell, and the homogeneity of the coating on artificial graphite (MCMB). The AG@a-Si composites synthesized using one-step, where amorphous Si shells were deposited by CVD approach using a silane gas (SiH4) as shown Fig. 1. Uniform coated nanoscaled aSi nanoshell has beneficial to electron conduction as well as the prevention of stress for contact between the Si-shell and electrolyte (upon prolonged cycling) and exposed inner lamella graphite has high Li+ intercalation. During the synthetic process, it could successfully control the thickness of the a-Si nanoshell thickness by adjusting the time reaction. The AG@a-Si composites with well controlled a-Si thickness (aSi shell thickness ≈ 15 nm and mass ≈ 4.7%) provides high capacity (384 mAh.g-1 at 2C (1C: 650 mA g-1)), volumetric capacity (288 mAh.cm3), long cycle life (200 cycles with 83% capacity retention), and high initial Coulombic efficiency (93.8%). The a-silicon-shell coated MCMB composite in a one-step approach method can merit industrially scalable.
2.3. Electrochemical measurements The working electrode was fabricated as follows. For the electrochemical half-cell testing, all of the active material (AG@a-Si-20 min to 120 min) at a weight ratio of 96.5 w.t% and poly-(vinylidene fluoride) (PVDF) 3.5 w.t% were mixed in N-methyl-2-pyrrolidone (NMP) to form a slurry. The slurry was then covered onto the aluminum (Cu) sheet. The coated electrode was dried at 200 °C for 2 h under vacuum. A lithium foil acted as both the counter electrode and reference electrode, and microporous polypropylene membrane (Celgard 2500) was used as the separator. CR2032 coin cells were assembled in an argon-filled glove box moisture and oxygen contents below 0.1 ppm. The electrolyte used was commercially available 1.3 M LiPF6 in a 1:1:1 vol mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC), to which 5 wt % fluoroethylene carbonate (FEC-Panax Starlyte) was added. The galvanostatic charge and discharge experiment was performed with a battery tester WBCS 3000, WonATech in the voltage rage of 0.001–1.5 V at room temperature at constant current rate of 0.2C. For the electrochemical measurements, an active materials loading were about 3.0 mg cm-2 and thickness of the electrode was 40 μm. Specific capacities of all materials were calculated on the basis of the total mass of the vanadium dioxide. To investigate the electrochemical kinetics, electrochemical impedance spectroscopy (EIS) measurements were performed suing fresh cells in a frequency range of 100 kHz to 0.01 Hz and 10 mV. Cyclic voltammetry (CV) was conducted on electrochemical workstation at a scanning rate of 0.1 mV s-1 in the voltage range of 0.001–1.5 V (vs. Li+/Li). The electrodes were washed with dimethyl carbonate after 5th lithiation and 30th charge/discharge cycles and observed using TEM and SEM.
2. Experimental 2.1. Preparation of amorphous Si-shell on artificial graphite (AG@a-Si) All chemicals were purchased from Sigma Aldrich and used direct without purification. In a typical synthesis, 3 g mesocarbon microbeads (MCMB) were placed in the center of a small open tube, at which both front and rear sides were blocked with wool to prevent the powders from blowing away pumping in CVD chamber. To grow amorphous silicon on MCMB, 10 sccm of silane (SiH4, 10% dilute in H2 gas) was introduced into the CVD chamber which was maintained at a total pressure of 10 Torr and 500 °C for 20 min to 120 min. The mass of the resulting AG@a-Si composite was increased about 3.53 g.
3. Results and discussion To analyze the amorphous Si deposition of artificial graphite after CVD process, SEM examination of samples artificial graphite (MCMB) and amorphous silicon nanoshell on MCMB (AG@a-Si) before (Fig.2a,c) and after (Fig. 2b,d) was performed. Generally, the artificial graphite has smooth surface and an average particle size of 15 μm. After CVD process, the AG@a-Si sample (Fig. 2d) confirmed the coating on the surface compared with pristine morphology (Fig. 2c). Synthesized samples were well retained without crack and the loss of the pristine morphology. Further, amorphous Si deposition was confirmed by TEM analysis as shown in Fig. 3.
2.2. Characterization Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku Dmax-2200 Ultima II powder X-ray diffractometer with Cu Kα at a scanning rate of 100 min-1. The structure of the sample was characterized by using Field emission scanning electron microscopy (FE25
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diffraction (SAED) images. The nitrogen sorption measurements were investigated to determine the surface area of the artificial graphite and AG@a-Si sample. The AG@a-Si sample exhibited a specific area of 0.83 m2 g-1, an average pore diameter of 0.8–1.1 nm, and a pore volume of 0.007 cm3 g-1 while the artificial graphite sample exhibited a specific area of 1.15 m2 g-1, an averages pore diameter of 0.8–1.4 nm, and a pore volume of 0.0079 cm3 g-1 (Fig. 5a and b). The decrease in specific area and pore volume for artificial AG@a-Si sample compared to the artificial graphite is related to the decease of graphite porosity by silicon deposition. The crystalline artificial graphite are further confirmed by Raman spectra (Fig. 5c), the presence of a peak of ∼1320 cm-1 and 1590 cm-1, respectively, recognized as the characteristic disorder-induced D band and graphitic G band [37]. As the synthesis time continues, the ID/IG intensity is gradually low, which ratio has a decreased 0.48 to 0.11. The cause of the effect of a-Si coating deposited on the graphite surface is seen as covering the defect. On the other hand, the intensity of the Gband appears to be gradually reduced with respect to the graphite concentration. The a-Si intensity of Raman band around 480 cm-1 is increased, revealing the well-sealed Si nanoshell structure [38,39]. As the reaction increase from 20 min to 120 min, the intensities of the Raman bands due to a-Si increase relative to the intensity of the carbon Raman band, which indicates that increase of Si amount of graphite in the AG@a-Si composite. Further, the gradual increase of Si amount in the AG@a-Si composites was also measured by TGA analysis of the sample at a heating rate of 10 °C min-1to 1000 °C in air. The weight loss 950 °C can be attributed to the combustion of graphite in the AG@a-Si composite [40,41], and the slight increase in weight after ∼975 °C is due to the formation of SiOx [38,42]. From these results, the weight fractions of a-Si in AG@a-Si-20 min to 120 min samples were determined to be about 1.03%, 2.08%, 3.13%, 4.71% and 6.22%, respectively. This is reasonable that the a-Si content in the increase in the composites increased with the prolonged reaction time. In order to investigate the effect of the a-Si nanoshell thicknesses, the electrochemical performances of electrodes fabricated with AG@aSi composites with five different shell thickness from ∼2 nm to ∼24 nm were tested as shown in Fig. 6a. a-Si film-15 nm thickness on SUS film and artificial graphite samples were also used as a reference. The cycling performance of the six samples at 0.2C between 1.5 and 0.001 V shows that the 100th initial capacities are 408 mAh.g-1 (20 min), 445 mAh.g-1 (40 min), 487 mAh.g-1 (50 min), 544 mAh.g-1 (90 min), 608 mAh.g-1 (120 min), 2410 mAh.g-1 (a-Si film-15nm thickness) and 328 mAh.g-1 (graphite). With the increasing the a-Si nanoshell thickness, in case of AG@a-Si composites, the first discharge capacities are slightly increased from 408 mAh.g-1 to 611 mAh.g-1, which is probably due to the increase of a-Si ratio in the AG@a-Si composite materials. The second initial discharge reversible capacities range from 369 mAh.g-1 to 492 mAh.g-1, respectively. The initial irreversible capacity loss could be associated with the inevitable formation of side reaction on the surface of the electrodes due to the decomposition of electrolytes and the volume expansion of a-Si, which is also caused to the capacity retention with 98 to 72% as the shell thickness increased from ∼2 nm to ∼24 nm. The AG@a-Si composite with 120 min electrode demonstrates a high initial capacity in the first cycles and decrease quickly in the following cycles. After 100 cycles, the capacity shows 371 mAh.g-1. The maximum reversible capacity after 100 cycles was 468 mAh.g-1, which was obtained from the AG@a-Si composite (aSi shell thickness ≈ 15 nm) synthesized 90 min process. The content of thicker than ∼20 nm and mass of a-Si is more than 5 wt%, the a-Si shell is probably too thick to withstand the mechanical stress during cycling and also lead to a continuous reaction between lithiated Si and electrolyte. As a result, this reaction may lead to continuous increase of cell impedance. This result is consistent with previously reported literature [35,41]. On the other hand, reference with a-Si film (15 nm thickness) exhibits 2280 mAh.g-1 (2nd) and 1756 mAh.g-1 (after 100th cycle), which is leads to the capacity retention with 77%. Artificial graphite
Fig. 2. FE-SEM image of (a) commercial artificial graphite, (b) prepared amorphous Si nanoshell on artificial graphite composite after CVD process, (c and d) each image shows high magnification before and after, respectively.
Fig. 3. HR-TEM images of the amorphous Si shell on artificial graphite composite obtained with CVD time reaction of (a) 20 min (b) 40 min (c) 60 min (d) 90 min (e) 120 min. Each images shows amorphous Si-shell thickness.
As shown by transmission electron microscopy (TEM) images in Fig. 3. After coating with different thickness of a-Si nanoshell by time reaction, the products obviously show gradually increasing of the shell thickness and silicon contents to form the AG@a-Si composite. Notably it can be seen from TEM images that all artificial graphite surface are homogeneously sealed by a smooth a-Si nanoshell, leading to the formation of AG@a-Si composite, which suggesting good electrical contact with the artificial graphite. These resultant a-Si nanoshell layers have tailored thickness of ∼2, ∼6, ∼12, ∼18, and ∼24 nm from 20 min to 120 min during the synthetic process, respectively. This increase in thickness is due to the fact that as the relative content of a-Si mass increase. According to our TEM analysis, as the deposition time increased, it was uniformly deposited as amorphous silicon particles to layer form on the surface. Energy dispersive spectrometry (EDS) elemental mapping analysis indicate that carbon (green), silicon (brown) and oxygen (yellow) are uniformly deposited on the artificial graphite surface (Fig. 4a). Fig. 4b shows the section between both the artificial graphite and the amorphous Si in the AG@a-Si composite exhibits interfaces, which clearly crystalline and amorphous phase in the selected area electron 26
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Fig. 4. High-resolution images of (a) an individual amorphous Si shell on artificial graphite composite and (b) the corresponding HR-TEM images showing the interfacial region.
Fig. 5. (a) N2 adsorption-deposition isotherm curves and (b) pore size distribution plot and (c) Raman spectra and (d) TG curves of the amorphous Si shell on artificial graphite composite with CVD reaction time.
electrolyte and electrode, which helps loss of Li+ during reaction. (2) the synthesized nanoscaled coated microstructure composite is favorable for electrolyte penetration and all nanoscaled a-Si nanoshell could reactivity with the electrochemical reactions Finally optimized the a-Si nanoshell layers thickness on graphite composite can provide a surface for formation of stable side reaction by preventing direct contact between the electrolyte and electrode of the anode, while an internal carbon matrix allows for free expansion of the anode material without increasing the macroscopic volume. The electrochemical performances of the AG@a-Si composite composites are summarized in Table 1. Cyclic voltammetry within a potential window 0.001–2.0 V is initially investigated and showed in Fig. 6c. A cathode peak at around 0.8 V is obviously observed in the first discharge process and disappeared in the following cycles, which indicates the formation of SEI layer in the first cycle. Simultaneously, the intense cathodic peak at around 0.32, 0.24 and 0.07 V is related to the lithiation of a-Si and around 0.24 and 0.07 V is the formation of LixSi
shows stable capacity retention (99%). As a result, the stable cycle performances of the AG@a-Si composite as anode materials for LIBs are attributed to the synergistic effect of optimized amorphous layer thickness, compensating effective volume expansion. Carbon matrix, which helps the contact between a-Si nanoshell can compensate continuous Li+ conductivity and suppress peel off of a-Si and increase the electronic conductivity. In addition to the cycling performance, Coulombic efficiency (C.E) of AG@a-Si composites with an initial C.E were maintained the 95.2, 94.8, 94.3, 93.8 and 93.01%, respectively. The cause of the initial capacity decrease is seen as the side reaction of the electrolyte between electrolyte and electrode and activation reaction by lithiation process. The Coulombic efficiency of all of the samples reached over 99% after 5 cycles (Fig. 6b). The AG@a-Si composite90 min (a-Si shell thickness ≈ 15 nm and mass of a-Si ≈ 4.7%) with nanoscaled coated micro structure anode merited an initial high CE up to 93.8%, which could be attributed to the following aspects: (1) the lower specific surface area provides stable side reaction between 27
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Fig. 6. (a) Cycle performance and (b) Coulombic efficiency of the amorphous Si nanoshell on artificial graphite composite obtained with CVD reaction time. (c) Cyclic voltammetry profiles for the amorphous Si nanoshell on artificial graphite composite obtained with CVD reaction time for 90 min at a scan rate 0.1 mV s-1 between 0.001 and 2.0 V. (d) 1st, 2nd, 3rd discharge-charge profile for the amorphous Si nanoshell of artificial graphite composite obtained with CVD reaction time for 90 min and 120 min.
morphological changes were studied by STEM image (Fig. 7). It shows that the AG@a-Si-90 min composite sample expand during full lithiation process but the a-Si shell clearly preserved and a-Si nanoshell coating on surface could still be observed (Fig. 7a–c). The STEM image also shows that the AG@a-Si-90 min with the uniform distribution of Si and C atoms are well preserved, revealing the well-sealed nanoshell structure than 120 min sample. In contrast, the AG@a-Si-120 min composite sample eventually cracks and loses the form of the pristine morphology (Fig. 7b–d). The AG@a-Si-90 min composite electrode showed the average capacity of 470 mAh.g-1, 456 mAh.g-1, 433 mAh.g-1, 403 mAh.g-1, 368 mAh.g-1, 327 mAh.g-1, 287 mAh.g-1 at the current rate of 0.2C, 0.5C, 1C, 2C, 3C, 4C and 5C, respectively (Fig. 8a). Interestingly, the capacity
from a-Si and intercalation of graphite, respectively [43–45]. In the charge process, there are two anodic peaks appeared at around 0.28 and 0.46 V, attributing to the delithiation from LixSi and de-intercalated graphite [43–45]. In addition, the intensity of the two cathodic and anodic peaks increase in latter cycles, indicating the improvement of reversibility and thus resulting in the better cycles stability of electrode. Fig. 6d shows the charge/discharge profiles of the two samples with 90 min (black) and 120 min (red) during 1st, 2nd and 3rd cycles at a current rate of 0.2C (130 mA g-1). As the a-Si content increased, the voltage polarization became higher, which can be ascribed to the larger resistance to Li+ ion transport due to the thicker a-Si nanoshells. Further to observe the morphological preservation of the AG@a-Si-90 min and 120 min composite surface samples after 5th full lithiation, the
Table 1 Comparison of performance characteristics of amorphous Si nanoshell on artificial graphite composite samples. Sample Name AG@a-Si composite
20 min 40 min 60 min 90 min 120 min
Weight content (%)
Reversible Capacity (mAh g-1)
a-Si
C
2nd
100th
1.03% 2.08% 3.13% 4.71% 6.22%
99,04% 98.02% 96.87% 95.29% 83.68%
369 403 441 492 513
362 390 424 468 371
Tap density (g cm)
a-Si Shell Thickness (nm)
Capacity Retention (nm)
Initial Coulombic Efficiency (%)
3
1–2 nm 3–6 nm 8–12 nm 15–18 nm 21–24 nm
98.1% 97.0% 96.2% 95.1% 72.2%
95.2% 94.8% 94.3% 93.8% 93.0%
1.29 1.26 1.23 1.18 1.13
28
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which shows ∼82% retention compared to the second cycles. To further inderstand effect of a-Si nanoshell thickness, electrochemical impedance spectrums (EIS) for the AG@a-Si-90 min and 120 min are conducted characterize the electrochemical kinetic performance of electrode before and after 100 cycles, which are well fit to the equivalent circuit model shown in the inset (Fig. 8c and d). According the model, the charge transfer resistance (Rct) for the 90 min and 120 min are calculated as 78 Ω and 89 Ω, respectively, indicating more efficient a charge-transfer reaction in the AG@a-Si composite with 90 min. A thick a-Si nanoshell of > 20 nm may have the disadvantage of acting as a barrier to the transport of Li+ through the layer, despite high reactivity for Li reaction. Also, as the shell thickness increases, the mass ratio of carbon decreases, leading to a decrease in electric conductivity. To further understand the kinetics of the AG@a-Si composite90 min and 120 min electrode, the EIS of ∼90 sample and ∼120 min sample after 100 cycles are measured again as shown in Fig. 8d. The observed impedance spectra may correspond to the resistance (Rsei) of the SEI film between the electrode and electrolyte and the Rct related to the AG@a-Si composite. The diameters of the semicircles frequency areas become noticeably smaller evidently after cycle, indicating that the several cycles are the electrode activation process and the decrease in the impedance value of AG@a-Si composite electrode. The Rsei values of ∼90 min and ∼120 min sample are 1.33 Ω and 6.42 Ω and the Rct values increase to 7.36 Ω from 15.64 Ω, respectively. This fact confirms that the thicker then ∼20 nm a-Si nanoshell cause to impedance of kinetics of lithium into the electrode due to the pulverization and interfacial large side reaction, which gradually inhibit the growth of the passive layer to get a stable SEI film and penetration of electron with lithium storage. Fig. 9 shows the scanning electron microscopy (SEM) images of the electrode paste on the AG@a-Si-90 min and 120 min composite electrode, after 2nd cycles (Fig. 9a and b) and after 10th (Fig. 9c and d) and 30th (Fig. 9e and f) charging/discharging cycles. As shown in Fig. 9a
Fig. 7. HR-TEM images of the amorphous Si nanoshell on artificial graphite composite pristine with (a) 90 min, (b) 12min (c,d) after 5th lithiation and the corresponding elemental mapping images showing carbon silicon sealed shell structure, respectively.
recovers to 473 mAh.g-1 when the current rates return to 0.2C. Compared to AG@a-Si-90 min composite, the artificial graphite has a poor rate performance and decrease to 62 mAh.g-1 when the current density up to 5C. As a result, because the high diffusivity and reactivity of a-Si nanoshell for Li reaction during high c-rate process, the AG@a-Si composite exhibited the power capability and the highest current density in comparison with graphite anodes. To investigate the stability in repetitive fast lithiation, de-lithiation processs, a cycle test was carried out at a current rate of 2C for 200 cycles (Fig. 8b). The first and second reversible capacities are 470 mAh.g-1 and 463 mAh.g-1, respectively. After 200 cycles, the reversible capacity shows 379 mAh.g-1,
Fig. 8. (a) Rate performance, (b) cycling performance at a C-rate of 2C over 200 cycles of the amorphous Si nanoshell on artificial graphite composite with ∼15 nm shell thickness, which was prepared CVD reaction of 90 min. (c–d) EIS curves of the 90 min and 120 min sample before and after 100th cycles. 29
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Fig. 9. FE-SEM images of the amorphous Si nanoshell on artificial graphite composite with (a) 90 min and (b) 120 min after 2nd cycles, (c,d) after 10th cycles and (e,f) after 30th cycles.
and b, no distinct surface morphology difference and no fracture or voids were observed. After 10th cycles (Fig. 9c and d), we observed distinct differences in the surface morphologies of the two electrodes. From Fig. 9c, the AG@a-Si-90 min composite was observed a little void on the surface but the electrode surface has even smoother compared with 120 min electrode. Fig. 9d shows distinct differences in the surface after 10th cycles, which was observed about cracks and swelling. The size of these cracks varies from a few hundred nanometers to several micrometers. Finally, after 30 cycles, it could observe that the difference between the two electrodes was more pronounced. As shown in Fig. 9e, no obvious large crack and void was observed, and the electrode surface even still smoother than 120 min electrode (Fig. 9f). Concerning Fig. 9f, this pulverization is mainly by the a-Si volume expansion; it reads to higher inner, interfacial resistance and decreased cyclic performance, which means a-Si nanoshell thick ∼20 nm cannot fully constrain the volume change of the electrode than ∼15 nm. (Fig. 9e). As a result, these electrochemical performances of the AG@aSi -90 min composite could be attributed to the following aspects: (1) Optimized a-Si nanoshell on artificial graphite design for enhanced cycle performance, and enabling effective accommodation of volume change and facilitation of electron transport during charge/discharge process. In particular, the outer a-Si nanoshell can provide high reactivity with Li+ and a surface for formation of stable side reaction by preventing direct contact between the electrolyte and electrode, which helps shortened the Li+ diffusion length (2) The lower specific surface area and higher tap density are beneficial to increase the high energy density for practical applications in LIBs with anode materials. 4. Conclusion In summary, we have prepared a simple dynamic CVD process to coat a uniform a-Si nanoshell on artificial graphite. When silan (SiH4) gas is used as the amorphous Si source, the a-Si shell thickness can be effectively controlled by the CVD reaction time. The resulting AG@a-Si composites were examined as the anode material for LIBs. It was found that the a-Si nanoshell thickness greatly influences the specific capacity, cycle stability, and high-rate capability. The optimal AG@a-Si-90 composite with a-Si nanoshell of ca. 15 nm (Si mass: 4.7%) showed a fairly stable capacity of about 468 mAh.g-1 during 100 cycles at 130 mA g−1. The significant improvement in the electrochemical performance obtained by applying optimum a-Si nanoshell can be attributed to the suppressed volume effect, improved Li+ reactivity, and stable composite structure upon long-term cycling. Acknowledgment This work was supported by the National Research Foundation of
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Amorphous-silicon nanoshell on artificial graphite composite as the anode for lithium-ion battery
T
Seh-Yoon Lim School of Advanced Materials Science and Engineering, SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Amorphous Si MCMB CVD Anode Lithium-ion battery
The fabrication of uniform amorphous silicon (a-Si) nanoshell on graphite is recognized as a high energy technique for building block into macroscopic materials. Here in, various thick containing nanoscaled A-Si nanoshells could easily be adjusted by controlling the preparation conditions. In this structure, A-Si nanoshell thickness with up to 24 nm are uniformly coated and supported on anode composite. The resulting AG@a-Si90min composites (a-Si shell thickness ≈ 15 nm) exhibited better cycle reversibility and stability than among samples. The G@a-Si-90 sample delivered a relatively stable specific capacity of ca. 384 mAh.g-1 at 1.3 A g−1 (1C: 650 mAh.g-1) for 200 cycles. Furthermore, the designed A-Si shell by CVD can also be applied to the study of the synergetic properties of great importance for the large-scale production for batteries and other devices.
1. Introduction Recently, the increasing demands with lithium ion batteries (LIBs) have stimulated intense study on the fabrication of various energy systems for portable IT devices, electric vehicles (EVs) and renewable energy [1–5]. Among the current graphite anode options for LIBs, the performance is limited by the low capacity (372 mAh.g-1). The property of graphite anode cannot meet increasing satisfaction about needs due to their low reversible capacity and poor rate capability because the slow solid-state diffusion of Li+ within the structure [4,6,7]. Therefore considerable researchers are searching for novel anode materials for high-performance with LIBs. Among these anode materials, elemental silicon (Si) has attracted remarkable attention as the most promising anode to replace commercial graphite because the alloy mechanism with Li ions with the high theoretical capacity near 4000 mAh.g-1, low discharge potential (∼0.4 V versus Li+/Li) and rich resources, which is higher advantages than that of other anode [3,6,8–12]. Despite of these promise, Si experiences a large volume expansion (> 300%) during continuous cycling, which can cause heavy strain in electrode, leading to pulverization, excessive growth of the solid electrolyte interphase (SEI) and increased internal resistance by loss of electrical conductivity [11,13,14]. As a result, several challenges exist for Si that hindered their commercial applications. Over the past few decades, many efforts have been devoted to compensate to improve the cycle life of silicon by fabricating nanosized silicon-based materials (including nanowires [15–18], nanotubes [19–21] and nanoparticles [19,22], hollow Si materials [23], porous Si [24–29] and active/-inactive composites [30,31].
Of these, one promising the design of amorphous based silicon has become an important research direction on cycle performance because of the good electronic conductivity and fast Li+ diffusion in improving the stability of silicon-based anodes [11,13]. The amorphous phase is dominated to Li diffusion due to the propensity of disordered like grain boundaries, which leads to an isotropic volume expansion by lithium diffusion [11,13], while the crystalline is relevant to the difference in reaction with crystallographic direction. Various methods previously have been worked for preparing amorphous a-Si-based composite anodes, such as a high-energy mechanical milling (HEMM) [32], hydrothermal method [33], spray pyrolysis [34] and a chemical vapor deposition (CVD). Among them CVD process can affect facile process without multi steps and a more uniform quality with a controllable thickness, which proceed to the less electrical conductivity loss between electrolyte and electrode materials during cycling. Recently, Ko et al. [35] reported carbon coated amorphous Si around natural graphite in large amounts by repeated CVD treatment to improve the high reversible capacity uniformity. The obtained material delivered a high reversible capacity of over (> 500 mAh.g-1) after 100 cycles. Although all these methods exhibits better cycling performance and solve the defects to some extents for Si anodes, it is still remained that electrode performance with various amorphous Si shell thicknesses with artificial graphite such as mesocarbon microbeads (MCMB) on the electrochemical behavior during cycling has not been reported. In this work, commercially available MCMB was employed. Microsized MCMB with lamella structure has many benefits than graphite from perspective structure such as high tap density (1.2–1.3 g.cm-3), small surface area,
E-mail address: [email protected]. https://doi.org/10.1016/j.solidstatesciences.2019.05.002 Received 9 April 2019; Received in revised form 30 April 2019; Accepted 1 May 2019 Available online 02 May 2019 1293-2558/ © 2019 Elsevier Masson SAS. All rights reserved.
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Fig. 1. Schematic illustration of the synthesis of amorphous Si nanoshell on artificial graphite composite.
SEM, JEOL 6500) and high-resolution transmission electron microscopy (HRTEM, JEOL 2100). Raman spectra were collected using a 514 nm laser with RM100 under ambient condition with a laser spot size of about 1 mm. Surface areas were measured at 77 K with a Quantachrome Autosorb-3B analyzer (USA). The amorphous Si weight fractions in AG@a-Si samples were determined by evaluating the mass loss of AG@ a-Si composite after calcination by thermogravimetric analysis (TGA; TGA-50 Shimadzu instrument), which is carried out from temperature to 1100 °C at a ramp of 10 °C min−1 in air atmosphere.
and low irreversible capacity corresponding to side reaction. Compare with natural graphite, surfaces of MCMB are made up of exposed edgeplane surfaces, thus enhanced Li+ intercalation and rate capability during cycling [36]. From an electrochemical point of view, micro sized materials experience advantage such as high tap density and generate high energy and volumetric capacity. However, it may have heavy stress and the reach to the center long ion/electron transport pathways during charge/discharge process. On the other hand, nanosized materials have light stress for volume expansion and short the Li+ transport path lengths, however, it exhibits the lower tap density, which cause to the low energy and volumetric density. Taking into account these merits and drawbacks, building blocks coated nanoscale on macrosized graphite template can provide harness these advantages while avoiding the above-mentioned shortcomings. Herein, we report on the effects of the CVD approach processing time reaction on the amorphous Si content, the thickness of shell, and the homogeneity of the coating on artificial graphite (MCMB). The AG@a-Si composites synthesized using one-step, where amorphous Si shells were deposited by CVD approach using a silane gas (SiH4) as shown Fig. 1. Uniform coated nanoscaled aSi nanoshell has beneficial to electron conduction as well as the prevention of stress for contact between the Si-shell and electrolyte (upon prolonged cycling) and exposed inner lamella graphite has high Li+ intercalation. During the synthetic process, it could successfully control the thickness of the a-Si nanoshell thickness by adjusting the time reaction. The AG@a-Si composites with well controlled a-Si thickness (aSi shell thickness ≈ 15 nm and mass ≈ 4.7%) provides high capacity (384 mAh.g-1 at 2C (1C: 650 mA g-1)), volumetric capacity (288 mAh.cm3), long cycle life (200 cycles with 83% capacity retention), and high initial Coulombic efficiency (93.8%). The a-silicon-shell coated MCMB composite in a one-step approach method can merit industrially scalable.
2.3. Electrochemical measurements The working electrode was fabricated as follows. For the electrochemical half-cell testing, all of the active material (AG@a-Si-20 min to 120 min) at a weight ratio of 96.5 w.t% and poly-(vinylidene fluoride) (PVDF) 3.5 w.t% were mixed in N-methyl-2-pyrrolidone (NMP) to form a slurry. The slurry was then covered onto the aluminum (Cu) sheet. The coated electrode was dried at 200 °C for 2 h under vacuum. A lithium foil acted as both the counter electrode and reference electrode, and microporous polypropylene membrane (Celgard 2500) was used as the separator. CR2032 coin cells were assembled in an argon-filled glove box moisture and oxygen contents below 0.1 ppm. The electrolyte used was commercially available 1.3 M LiPF6 in a 1:1:1 vol mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC), to which 5 wt % fluoroethylene carbonate (FEC-Panax Starlyte) was added. The galvanostatic charge and discharge experiment was performed with a battery tester WBCS 3000, WonATech in the voltage rage of 0.001–1.5 V at room temperature at constant current rate of 0.2C. For the electrochemical measurements, an active materials loading were about 3.0 mg cm-2 and thickness of the electrode was 40 μm. Specific capacities of all materials were calculated on the basis of the total mass of the vanadium dioxide. To investigate the electrochemical kinetics, electrochemical impedance spectroscopy (EIS) measurements were performed suing fresh cells in a frequency range of 100 kHz to 0.01 Hz and 10 mV. Cyclic voltammetry (CV) was conducted on electrochemical workstation at a scanning rate of 0.1 mV s-1 in the voltage range of 0.001–1.5 V (vs. Li+/Li). The electrodes were washed with dimethyl carbonate after 5th lithiation and 30th charge/discharge cycles and observed using TEM and SEM.
2. Experimental 2.1. Preparation of amorphous Si-shell on artificial graphite (AG@a-Si) All chemicals were purchased from Sigma Aldrich and used direct without purification. In a typical synthesis, 3 g mesocarbon microbeads (MCMB) were placed in the center of a small open tube, at which both front and rear sides were blocked with wool to prevent the powders from blowing away pumping in CVD chamber. To grow amorphous silicon on MCMB, 10 sccm of silane (SiH4, 10% dilute in H2 gas) was introduced into the CVD chamber which was maintained at a total pressure of 10 Torr and 500 °C for 20 min to 120 min. The mass of the resulting AG@a-Si composite was increased about 3.53 g.
3. Results and discussion To analyze the amorphous Si deposition of artificial graphite after CVD process, SEM examination of samples artificial graphite (MCMB) and amorphous silicon nanoshell on MCMB (AG@a-Si) before (Fig.2a,c) and after (Fig. 2b,d) was performed. Generally, the artificial graphite has smooth surface and an average particle size of 15 μm. After CVD process, the AG@a-Si sample (Fig. 2d) confirmed the coating on the surface compared with pristine morphology (Fig. 2c). Synthesized samples were well retained without crack and the loss of the pristine morphology. Further, amorphous Si deposition was confirmed by TEM analysis as shown in Fig. 3.
2.2. Characterization Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku Dmax-2200 Ultima II powder X-ray diffractometer with Cu Kα at a scanning rate of 100 min-1. The structure of the sample was characterized by using Field emission scanning electron microscopy (FE25
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diffraction (SAED) images. The nitrogen sorption measurements were investigated to determine the surface area of the artificial graphite and AG@a-Si sample. The AG@a-Si sample exhibited a specific area of 0.83 m2 g-1, an average pore diameter of 0.8–1.1 nm, and a pore volume of 0.007 cm3 g-1 while the artificial graphite sample exhibited a specific area of 1.15 m2 g-1, an averages pore diameter of 0.8–1.4 nm, and a pore volume of 0.0079 cm3 g-1 (Fig. 5a and b). The decrease in specific area and pore volume for artificial AG@a-Si sample compared to the artificial graphite is related to the decease of graphite porosity by silicon deposition. The crystalline artificial graphite are further confirmed by Raman spectra (Fig. 5c), the presence of a peak of ∼1320 cm-1 and 1590 cm-1, respectively, recognized as the characteristic disorder-induced D band and graphitic G band [37]. As the synthesis time continues, the ID/IG intensity is gradually low, which ratio has a decreased 0.48 to 0.11. The cause of the effect of a-Si coating deposited on the graphite surface is seen as covering the defect. On the other hand, the intensity of the Gband appears to be gradually reduced with respect to the graphite concentration. The a-Si intensity of Raman band around 480 cm-1 is increased, revealing the well-sealed Si nanoshell structure [38,39]. As the reaction increase from 20 min to 120 min, the intensities of the Raman bands due to a-Si increase relative to the intensity of the carbon Raman band, which indicates that increase of Si amount of graphite in the AG@a-Si composite. Further, the gradual increase of Si amount in the AG@a-Si composites was also measured by TGA analysis of the sample at a heating rate of 10 °C min-1to 1000 °C in air. The weight loss 950 °C can be attributed to the combustion of graphite in the AG@a-Si composite [40,41], and the slight increase in weight after ∼975 °C is due to the formation of SiOx [38,42]. From these results, the weight fractions of a-Si in AG@a-Si-20 min to 120 min samples were determined to be about 1.03%, 2.08%, 3.13%, 4.71% and 6.22%, respectively. This is reasonable that the a-Si content in the increase in the composites increased with the prolonged reaction time. In order to investigate the effect of the a-Si nanoshell thicknesses, the electrochemical performances of electrodes fabricated with AG@aSi composites with five different shell thickness from ∼2 nm to ∼24 nm were tested as shown in Fig. 6a. a-Si film-15 nm thickness on SUS film and artificial graphite samples were also used as a reference. The cycling performance of the six samples at 0.2C between 1.5 and 0.001 V shows that the 100th initial capacities are 408 mAh.g-1 (20 min), 445 mAh.g-1 (40 min), 487 mAh.g-1 (50 min), 544 mAh.g-1 (90 min), 608 mAh.g-1 (120 min), 2410 mAh.g-1 (a-Si film-15nm thickness) and 328 mAh.g-1 (graphite). With the increasing the a-Si nanoshell thickness, in case of AG@a-Si composites, the first discharge capacities are slightly increased from 408 mAh.g-1 to 611 mAh.g-1, which is probably due to the increase of a-Si ratio in the AG@a-Si composite materials. The second initial discharge reversible capacities range from 369 mAh.g-1 to 492 mAh.g-1, respectively. The initial irreversible capacity loss could be associated with the inevitable formation of side reaction on the surface of the electrodes due to the decomposition of electrolytes and the volume expansion of a-Si, which is also caused to the capacity retention with 98 to 72% as the shell thickness increased from ∼2 nm to ∼24 nm. The AG@a-Si composite with 120 min electrode demonstrates a high initial capacity in the first cycles and decrease quickly in the following cycles. After 100 cycles, the capacity shows 371 mAh.g-1. The maximum reversible capacity after 100 cycles was 468 mAh.g-1, which was obtained from the AG@a-Si composite (aSi shell thickness ≈ 15 nm) synthesized 90 min process. The content of thicker than ∼20 nm and mass of a-Si is more than 5 wt%, the a-Si shell is probably too thick to withstand the mechanical stress during cycling and also lead to a continuous reaction between lithiated Si and electrolyte. As a result, this reaction may lead to continuous increase of cell impedance. This result is consistent with previously reported literature [35,41]. On the other hand, reference with a-Si film (15 nm thickness) exhibits 2280 mAh.g-1 (2nd) and 1756 mAh.g-1 (after 100th cycle), which is leads to the capacity retention with 77%. Artificial graphite
Fig. 2. FE-SEM image of (a) commercial artificial graphite, (b) prepared amorphous Si nanoshell on artificial graphite composite after CVD process, (c and d) each image shows high magnification before and after, respectively.
Fig. 3. HR-TEM images of the amorphous Si shell on artificial graphite composite obtained with CVD time reaction of (a) 20 min (b) 40 min (c) 60 min (d) 90 min (e) 120 min. Each images shows amorphous Si-shell thickness.
As shown by transmission electron microscopy (TEM) images in Fig. 3. After coating with different thickness of a-Si nanoshell by time reaction, the products obviously show gradually increasing of the shell thickness and silicon contents to form the AG@a-Si composite. Notably it can be seen from TEM images that all artificial graphite surface are homogeneously sealed by a smooth a-Si nanoshell, leading to the formation of AG@a-Si composite, which suggesting good electrical contact with the artificial graphite. These resultant a-Si nanoshell layers have tailored thickness of ∼2, ∼6, ∼12, ∼18, and ∼24 nm from 20 min to 120 min during the synthetic process, respectively. This increase in thickness is due to the fact that as the relative content of a-Si mass increase. According to our TEM analysis, as the deposition time increased, it was uniformly deposited as amorphous silicon particles to layer form on the surface. Energy dispersive spectrometry (EDS) elemental mapping analysis indicate that carbon (green), silicon (brown) and oxygen (yellow) are uniformly deposited on the artificial graphite surface (Fig. 4a). Fig. 4b shows the section between both the artificial graphite and the amorphous Si in the AG@a-Si composite exhibits interfaces, which clearly crystalline and amorphous phase in the selected area electron 26
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Fig. 4. High-resolution images of (a) an individual amorphous Si shell on artificial graphite composite and (b) the corresponding HR-TEM images showing the interfacial region.
Fig. 5. (a) N2 adsorption-deposition isotherm curves and (b) pore size distribution plot and (c) Raman spectra and (d) TG curves of the amorphous Si shell on artificial graphite composite with CVD reaction time.
electrolyte and electrode, which helps loss of Li+ during reaction. (2) the synthesized nanoscaled coated microstructure composite is favorable for electrolyte penetration and all nanoscaled a-Si nanoshell could reactivity with the electrochemical reactions Finally optimized the a-Si nanoshell layers thickness on graphite composite can provide a surface for formation of stable side reaction by preventing direct contact between the electrolyte and electrode of the anode, while an internal carbon matrix allows for free expansion of the anode material without increasing the macroscopic volume. The electrochemical performances of the AG@a-Si composite composites are summarized in Table 1. Cyclic voltammetry within a potential window 0.001–2.0 V is initially investigated and showed in Fig. 6c. A cathode peak at around 0.8 V is obviously observed in the first discharge process and disappeared in the following cycles, which indicates the formation of SEI layer in the first cycle. Simultaneously, the intense cathodic peak at around 0.32, 0.24 and 0.07 V is related to the lithiation of a-Si and around 0.24 and 0.07 V is the formation of LixSi
shows stable capacity retention (99%). As a result, the stable cycle performances of the AG@a-Si composite as anode materials for LIBs are attributed to the synergistic effect of optimized amorphous layer thickness, compensating effective volume expansion. Carbon matrix, which helps the contact between a-Si nanoshell can compensate continuous Li+ conductivity and suppress peel off of a-Si and increase the electronic conductivity. In addition to the cycling performance, Coulombic efficiency (C.E) of AG@a-Si composites with an initial C.E were maintained the 95.2, 94.8, 94.3, 93.8 and 93.01%, respectively. The cause of the initial capacity decrease is seen as the side reaction of the electrolyte between electrolyte and electrode and activation reaction by lithiation process. The Coulombic efficiency of all of the samples reached over 99% after 5 cycles (Fig. 6b). The AG@a-Si composite90 min (a-Si shell thickness ≈ 15 nm and mass of a-Si ≈ 4.7%) with nanoscaled coated micro structure anode merited an initial high CE up to 93.8%, which could be attributed to the following aspects: (1) the lower specific surface area provides stable side reaction between 27
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Fig. 6. (a) Cycle performance and (b) Coulombic efficiency of the amorphous Si nanoshell on artificial graphite composite obtained with CVD reaction time. (c) Cyclic voltammetry profiles for the amorphous Si nanoshell on artificial graphite composite obtained with CVD reaction time for 90 min at a scan rate 0.1 mV s-1 between 0.001 and 2.0 V. (d) 1st, 2nd, 3rd discharge-charge profile for the amorphous Si nanoshell of artificial graphite composite obtained with CVD reaction time for 90 min and 120 min.
morphological changes were studied by STEM image (Fig. 7). It shows that the AG@a-Si-90 min composite sample expand during full lithiation process but the a-Si shell clearly preserved and a-Si nanoshell coating on surface could still be observed (Fig. 7a–c). The STEM image also shows that the AG@a-Si-90 min with the uniform distribution of Si and C atoms are well preserved, revealing the well-sealed nanoshell structure than 120 min sample. In contrast, the AG@a-Si-120 min composite sample eventually cracks and loses the form of the pristine morphology (Fig. 7b–d). The AG@a-Si-90 min composite electrode showed the average capacity of 470 mAh.g-1, 456 mAh.g-1, 433 mAh.g-1, 403 mAh.g-1, 368 mAh.g-1, 327 mAh.g-1, 287 mAh.g-1 at the current rate of 0.2C, 0.5C, 1C, 2C, 3C, 4C and 5C, respectively (Fig. 8a). Interestingly, the capacity
from a-Si and intercalation of graphite, respectively [43–45]. In the charge process, there are two anodic peaks appeared at around 0.28 and 0.46 V, attributing to the delithiation from LixSi and de-intercalated graphite [43–45]. In addition, the intensity of the two cathodic and anodic peaks increase in latter cycles, indicating the improvement of reversibility and thus resulting in the better cycles stability of electrode. Fig. 6d shows the charge/discharge profiles of the two samples with 90 min (black) and 120 min (red) during 1st, 2nd and 3rd cycles at a current rate of 0.2C (130 mA g-1). As the a-Si content increased, the voltage polarization became higher, which can be ascribed to the larger resistance to Li+ ion transport due to the thicker a-Si nanoshells. Further to observe the morphological preservation of the AG@a-Si-90 min and 120 min composite surface samples after 5th full lithiation, the
Table 1 Comparison of performance characteristics of amorphous Si nanoshell on artificial graphite composite samples. Sample Name AG@a-Si composite
20 min 40 min 60 min 90 min 120 min
Weight content (%)
Reversible Capacity (mAh g-1)
a-Si
C
2nd
100th
1.03% 2.08% 3.13% 4.71% 6.22%
99,04% 98.02% 96.87% 95.29% 83.68%
369 403 441 492 513
362 390 424 468 371
Tap density (g cm)
a-Si Shell Thickness (nm)
Capacity Retention (nm)
Initial Coulombic Efficiency (%)
3
1–2 nm 3–6 nm 8–12 nm 15–18 nm 21–24 nm
98.1% 97.0% 96.2% 95.1% 72.2%
95.2% 94.8% 94.3% 93.8% 93.0%
1.29 1.26 1.23 1.18 1.13
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which shows ∼82% retention compared to the second cycles. To further inderstand effect of a-Si nanoshell thickness, electrochemical impedance spectrums (EIS) for the AG@a-Si-90 min and 120 min are conducted characterize the electrochemical kinetic performance of electrode before and after 100 cycles, which are well fit to the equivalent circuit model shown in the inset (Fig. 8c and d). According the model, the charge transfer resistance (Rct) for the 90 min and 120 min are calculated as 78 Ω and 89 Ω, respectively, indicating more efficient a charge-transfer reaction in the AG@a-Si composite with 90 min. A thick a-Si nanoshell of > 20 nm may have the disadvantage of acting as a barrier to the transport of Li+ through the layer, despite high reactivity for Li reaction. Also, as the shell thickness increases, the mass ratio of carbon decreases, leading to a decrease in electric conductivity. To further understand the kinetics of the AG@a-Si composite90 min and 120 min electrode, the EIS of ∼90 sample and ∼120 min sample after 100 cycles are measured again as shown in Fig. 8d. The observed impedance spectra may correspond to the resistance (Rsei) of the SEI film between the electrode and electrolyte and the Rct related to the AG@a-Si composite. The diameters of the semicircles frequency areas become noticeably smaller evidently after cycle, indicating that the several cycles are the electrode activation process and the decrease in the impedance value of AG@a-Si composite electrode. The Rsei values of ∼90 min and ∼120 min sample are 1.33 Ω and 6.42 Ω and the Rct values increase to 7.36 Ω from 15.64 Ω, respectively. This fact confirms that the thicker then ∼20 nm a-Si nanoshell cause to impedance of kinetics of lithium into the electrode due to the pulverization and interfacial large side reaction, which gradually inhibit the growth of the passive layer to get a stable SEI film and penetration of electron with lithium storage. Fig. 9 shows the scanning electron microscopy (SEM) images of the electrode paste on the AG@a-Si-90 min and 120 min composite electrode, after 2nd cycles (Fig. 9a and b) and after 10th (Fig. 9c and d) and 30th (Fig. 9e and f) charging/discharging cycles. As shown in Fig. 9a
Fig. 7. HR-TEM images of the amorphous Si nanoshell on artificial graphite composite pristine with (a) 90 min, (b) 12min (c,d) after 5th lithiation and the corresponding elemental mapping images showing carbon silicon sealed shell structure, respectively.
recovers to 473 mAh.g-1 when the current rates return to 0.2C. Compared to AG@a-Si-90 min composite, the artificial graphite has a poor rate performance and decrease to 62 mAh.g-1 when the current density up to 5C. As a result, because the high diffusivity and reactivity of a-Si nanoshell for Li reaction during high c-rate process, the AG@a-Si composite exhibited the power capability and the highest current density in comparison with graphite anodes. To investigate the stability in repetitive fast lithiation, de-lithiation processs, a cycle test was carried out at a current rate of 2C for 200 cycles (Fig. 8b). The first and second reversible capacities are 470 mAh.g-1 and 463 mAh.g-1, respectively. After 200 cycles, the reversible capacity shows 379 mAh.g-1,
Fig. 8. (a) Rate performance, (b) cycling performance at a C-rate of 2C over 200 cycles of the amorphous Si nanoshell on artificial graphite composite with ∼15 nm shell thickness, which was prepared CVD reaction of 90 min. (c–d) EIS curves of the 90 min and 120 min sample before and after 100th cycles. 29
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Fig. 9. FE-SEM images of the amorphous Si nanoshell on artificial graphite composite with (a) 90 min and (b) 120 min after 2nd cycles, (c,d) after 10th cycles and (e,f) after 30th cycles.
and b, no distinct surface morphology difference and no fracture or voids were observed. After 10th cycles (Fig. 9c and d), we observed distinct differences in the surface morphologies of the two electrodes. From Fig. 9c, the AG@a-Si-90 min composite was observed a little void on the surface but the electrode surface has even smoother compared with 120 min electrode. Fig. 9d shows distinct differences in the surface after 10th cycles, which was observed about cracks and swelling. The size of these cracks varies from a few hundred nanometers to several micrometers. Finally, after 30 cycles, it could observe that the difference between the two electrodes was more pronounced. As shown in Fig. 9e, no obvious large crack and void was observed, and the electrode surface even still smoother than 120 min electrode (Fig. 9f). Concerning Fig. 9f, this pulverization is mainly by the a-Si volume expansion; it reads to higher inner, interfacial resistance and decreased cyclic performance, which means a-Si nanoshell thick ∼20 nm cannot fully constrain the volume change of the electrode than ∼15 nm. (Fig. 9e). As a result, these electrochemical performances of the AG@aSi -90 min composite could be attributed to the following aspects: (1) Optimized a-Si nanoshell on artificial graphite design for enhanced cycle performance, and enabling effective accommodation of volume change and facilitation of electron transport during charge/discharge process. In particular, the outer a-Si nanoshell can provide high reactivity with Li+ and a surface for formation of stable side reaction by preventing direct contact between the electrolyte and electrode, which helps shortened the Li+ diffusion length (2) The lower specific surface area and higher tap density are beneficial to increase the high energy density for practical applications in LIBs with anode materials. 4. Conclusion In summary, we have prepared a simple dynamic CVD process to coat a uniform a-Si nanoshell on artificial graphite. When silan (SiH4) gas is used as the amorphous Si source, the a-Si shell thickness can be effectively controlled by the CVD reaction time. The resulting AG@a-Si composites were examined as the anode material for LIBs. It was found that the a-Si nanoshell thickness greatly influences the specific capacity, cycle stability, and high-rate capability. The optimal AG@a-Si-90 composite with a-Si nanoshell of ca. 15 nm (Si mass: 4.7%) showed a fairly stable capacity of about 468 mAh.g-1 during 100 cycles at 130 mA g−1. The significant improvement in the electrochemical performance obtained by applying optimum a-Si nanoshell can be attributed to the suppressed volume effect, improved Li+ reactivity, and stable composite structure upon long-term cycling. Acknowledgment This work was supported by the National Research Foundation of
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