Green Synthesis of Dual Carbon Conductive Network-Encapsulated

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Green synthesis of dual carbon conductive networks encapsulated hollow SiO spheres for superior lithium-ion batteries x

Tao Xu, Qian Wang, Jian Zhang, Xiaohua Xie, and Baojia Xia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03070 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 15, 2019

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ACS Applied Materials & Interfaces

Green Synthesis of Dual Carbon Conductive Networks Encapsulated Hollow SiOx Spheres for Superior Lithium-Ion Batteries Tao Xu, †,‡ Qian Wang, § Jian Zhang, *,† Xiaohua Xie *,† and Baojia Xia † †

Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai, 200050, China

‡

University of Chinese Academy of Sciences, Beijing, 100049, China

§

School of Physical and Mathematical Sciences, Nanjing Tech University, Nanjing 211800, China

* Corresponding author: E-mail address: [email protected] (Xiaohua Xie) [email protected] (Jian Zhang)

KEYWORDS: lithium-ion batteries, dual carbon layers, hollow SiOx, soft-template synthesis, electrochemistry

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ABSTRACT Designing hollow/porous structure is regarded as an effective approach to address the dramatic volumetric variation issue for Si-based anode materials in Li-ion batteries (LIBs). Pioneer works mainly focused on acid/alkali etching to create hollow/porous structures, which are, however, highly corrosive and may lead to complicated synthetic process. In this paper, a dual carbon conductive networks encapsulated hollow SiOx (DC-HSiOx) is fabricated through a green route where polyacrylic acid is adopted as an eco-friendly soft template. Low electrical resistance and integrated electrode structure can be maintained during cycles due to the dual carbon conductive networks distributed both on the surface of single particle formed by amorphous carbon and among particles constructed by reduced graphene oxide. Importantly, hollow space is reserved within SiOx spheres to accommodate the huge volumetric variation and shorten the transport pathway of Li+ ions. As a result, the DC-HSiOx composite delivers a large reversible capacity of 1113 mAh g-1 at 0.1 A g-1, an excellent cycling performance up to 300 cycles with capacity retention of 92.5% at 0.5 A g-1, and a good rate capability. Furthermore, the DC-HSiOx//LiNi0.8Co0.1Mn0.1O2 full cell exhibits high energy density (419 Wh kg1)

and superior cycling performance. These results render an opportunity for the unique

DC-HSiOx composite as a potential anode material for use in high-performance LIBs.

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1. INTRODUCTION Lithium-ion batteries (LIBs), as one of the main energy storage devices, play an important role in a wide range of fields due to their outstanding characteristics such as high gravimetric and volumetric energy densities, long cycling life and superior power performance.1-3 However, the burgeoning energy demands of emerging applications such as electric and hybrid electric vehicles have raised higher requirements for current LIBs technology. This has spurred intensive investigations into new electrode materials with higher specific capacity, aimed at improving the energy density of current LIBs.4-8 With regard to the anode materials for current LIBs, considerable efforts have been made to replace the carbonaceous materials that are commercially applied to LIBs, with an outstanding cycling performance but limited reversible capacity. Among them, silicon (Si) is the most attractive candidate owing to its high theoretical specific capacity (3579 mAh g-1 for the lithiated stoichiometry of Li15Si4), which is an order of magnitude higher than that of graphite (372 mAh g-1).9-11 Nevertheless, the massive volumetric variation (~300%) upon alloying/dealloying of Si particles leads to pulverization of electrode and subsequent loss of electrical contact between active materials and the current collector, resulting in rapid capacity degeneration, which thereby hampers its practical applications.12, 13 In this context, nonstoichiometric SiOx (099%, size is 0.5~3 μm, Aladdin) was firstly ultrasonicated with 200 ml deionized water for 1 h, followed by the addition of 1 g HSiOx/C composite under magnetic stirring for 2 h. The homogeneous suspension was dried by a spray dryer with an inlet temperature of 200 oC

and outlet temperature of 100 oC. Subsequently, the collected powder was heated

under an Ar/H2 (95/5 vol%) atmosphere at 900 oC for 3 h to yield the DC-HSiOx composite. 2.3. Physical characterization X-ray diffraction (XRD, Rigaku D/max 2200/PC, Japan) was used to characterize the crystal structures of materials by using Cu Kα (λ = 0.15406 nm) radiation scanning from 10 o to 80 o at a rate of 4 o min-1. Microstructures, morphologies and elemental

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distributions of the prepared materials were characterized by scanning electron microscopy (SEM, Hitachi S4700, Japan), transmission electron microscopy (TEM, JEM-2100F, Japan) as well as energy dispersive X-ray spectrometer (EDX). Raman spectrum was acquired on a DXR Raman system with 532 nm diode laser excitation. Thermogravimetric analysis (TGA) was performed on a STA409PC TG-DSC/DTA instrument (Netzsch, Germany) with a heating rate of 10 °C min-1 under air. The chemical valence state of Si in the composites was analyzed by X-ray photoelectron spectroscopy (XPS) with Al Kα radiation (hv = 1486.71 eV) on a PHI-5000C ESCA system (Perkin-Elmer). 2.4. Electrochemical characterization The slurry for working electrodes consisted of active material (80 wt%), Super P (10 wt%) and PAA (10 wt%), which was cast onto a copper foil and subsequently dried at 120 oC for 12 h under vacuum. The mass loading of the active material was ca. 1.0 mg cm-2 for each cell. The coin-type (CR2025) cells were assembled in an argon-filled glove box (Mikrouna, China) with less than 0.1 ppm of oxygen and moisture. 1 M LiPF6 dissolved in diethyl carbonate/ethylene carbonate (1:1 vol%) containing 10 vol% fluoroethylene carbonate was used as the electrolyte (Smoothway Electronic Materials Co., Ltd., China), Celgard 2320 PP/PE/PP film (Celgard Inc., USA) was used as the separator and Li foil was used as both reference and counter electrode. Galvanostatic charge/discharge tests were used to measure the cycling performances of all electrodes at a current density of 0.1 A g-1 for the first cycle and 0.2 A g-1 for the following cycles over the voltage range of 0.01~2.5 V with a Neware battery testing system (Shenzhen,

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China). Cyclic voltammetry (CV) tests were carried out over the voltage range of 0.01~2.5 V at a scan rate of 0.1 mV s-1. Electrochemical impedance spectroscopy (EIS) measurements were conducted over the frequency range of 100 kHz~0.01 Hz with an amplitude of 10 mV on an Autolab electrochemical workstation (Metrohm, Switzerland). Unless stated otherwise, all electrochemical tests were performed at 25 oC.

In this research, the charge process refers to delithiation and the discharge process

refers to lithiation, and the mentioned specific capacity was calculated based on the total mass of the composite. The DC-HSiOx composite is used as anode material and LiNi0.8Co0.1Mn0.1O2 (NCM811) (Ronbay Lithium Battery Material Co., Ltd., China) as cathode material to assemble 2025-type coin full cells. The slurry for cathode consisted of NCM811, Super P and poly (vinyl difluoride) (PVDF) with a weight ratio of 85:9:6, which was cast onto an aluminium foil and dried at 120 °C for 12 h. The DC-HSiOx electrode was firstly prelithiated and then paired up with the NCM811 electrode with the capacity ratio of 1.3:1 to assemble the full cell. The mass loading of the cathode for full cell is ca. 5 mg cm2. The DC-HSiOx electrode was prelithiated by direct contacting with lithium foil wetted in electrolyte before full cell assembly to compensate for the initial irreversible capacity loss. Galvanostatic charge/discharge tests were carried out for the DCHSiOx//NCM811 full cell between 2.0 to 4.2 V at the current rate of 1 C (1 C = 180 mA g−1). 3. RESULTS AND DISCUSSION Figure 1a illustrates the typical synthesis route of the DC-HSiOx composite. PAA

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spontaneously formed small aggregates with time when immersed in ethanol and TEOS with stirring.43,44 During the hydrolysis process of TEOS, PAA aggregates acted as soft templates. The SiO2 precursors would combine onto the surfaces of the aggregates, gradually generating the SiO2 layer. When soaked in deionized water, PAA templates were dissolved in water and removed by subsequent centrifugation, resulting in the formation of HSiO2 spheres. The preparation of HSiO2 spheres is highly reproducible, facile and eco-friendly, and no etching reagent or toxic chemical is required to remove the template. The obtained HSiO2 spheres were then mixed with phenolic resin, followed by a carbothermic reduction process.45-48 During this process, phenolic resin was carbonized, which then reduced HSiO2 spheres to HSiOx matrix, resulting in the formation of the HSiOx/C composite. Finally, the GO sheets were distributed uniformly and wrapped HSiOx/C spheres through a simple spray-drying method, which immobilized the distribution of GO and HSiOx/C spheres immediately during fast solvent evaporation, preventing the agglomeration or delamination of the mixture induced by the large specific surface area of GO sheets or varied densities of different components. GO sheets were then transformed to RGO under high temperature. The amorphous carbon on the surface of HSiOx spheres could improve the electronic conductivity of SiOx matrix and help facilitate the electron transportation within the hollow SiOx clusters. RGO assisted the amorphous carbon to improve the electronic conductivity of composite. Besides, it also played a vital role in promoting stable SEI layer formation, encapsulating the HSiOx/C spheres within the conductive network and maintaining their electrical contact during long-lasting cycles.

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The structure and morphology of the HSiO2 spheres, HSiOx/C and DC-HSiOx composites were characterized by SEM and TEM. As shown in Figure 1b, the primary HSiO2 particles display spherical shape with an average diameter of ca. 350 nm. After carbon coating and subsequent carbothermic reduction process, the resultant HSiOx/C particles inherit the spherical morphology of the HSiO2 spheres yet show rougher surfaces (Figure 1c), and the color of the HSiO2 spheres changes from white to black (Figure S1), which both indicate the successful coating of carbon layer. TEM images of the HSiO2 spheres and HSiOx/C composite both display a distinct contrast between the well-defined shell and interior cavity (Figure 1e and f), demonstrating their hollow structures. High-resolution TEM images of the HSiO2 sphere and HSiOx/C composite (Figure S2) show that the shell of the HSiOx/C composite consists of a 40 nm thick SiOx layer and an amorphous carbon layer with an average thickness of 10 nm. Moreover, EDX was employed to detect the homogenous distribution of elements Si, O and C (Figure 1i-k), which further confirms that the HSiOx spheres are conformally coated by carbon layer. After spray drying and subsequent heat treatment, the obtained DC-HSiOx composite exhibits sphere-like morphology with diameters ranging from 1 to 3 μm (Figure 1d and g). Combining the magnified images shown in Figure S3 and the inset in Figure 1d, it can be concluded that numerous spheres are packed within thin conductive sheets, displaying an ideal dual carbon conductive networks-encapsulated hollow HSiOx structure. XRD was used to characterize the phase components and crystal structures of the HSiO2 spheres, HSiOx/C and DC-HSiOx composites. As shown in Figure 2a, a broad peak at

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around 22

o

can be observed for all the samples, suggesting the amorphous states of

HSiO2 and SiOx matrix.45,46 A broad peak located at around 510 cm-1 is observed from Raman spectra (Figure 2b) of the HSiOx/C and DC-HSiOx composites, further indicating the formation of SiOx through carbothermal reduction.49 In addition, two characteristic peaks corresponding to the disordered D band and graphitic G band of carbon at around 1348 and 1594 cm-1 can be observed. The ratio of ID/IG indicates the degree of graphitization and also implies the electronic conductivity of material.37 The lower ratio (ID/IG = 0.72) for the DC-HSiOx composite compared to the HSiOx/C composite (ID/IG = 1.18) suggests that a more efficient conductive network is constructed through dual carbon encapsulation, which can be expected to play a positive role in addressing the inferior electronic conductivity of SiOx. During the calcining process, SiO2 is regarded to be reduced by carbon component. The chemical valence states of Si in the HSiOx/C and DC-HSiOx samples were detected by XPS. As shown in Figure S4, XPS spectra of the HSiOx/C and DC-HSiOx composites present the peaks for Si 2p, Si 2s, C 1s, O 1s and O KLL, confirming the presence of Si, C and O elements. The high-resolution Si 2p spectra of the HSiOx/C and DC-HSiOx composites are presented in Figure 2c and d, respectively. On the basis of previous report, the broad Si 2p peak can be divided into three small peaks at 101.6, 102.5, and 103.5 eV, respectively, indicating different valence states of Si in SiOx.50 The average valence states of Si in the HSiOx/C and DC-HSiOx composites are determined to be 2.58 and 2.44 (corresponding to x values of 1.29 and 1.22), respectively. XPS results further confirm the carbothermal reduction of SiO2, in accordance with previous reports.45,46

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Moreover, TGA was carried out to determine the carbon content in the HSiOx/C and DC-HSiOx composites (Figure S5a). The weight losses of 8.0 and 19.8% correspond to carbon contents in the HSiOx/C and DC-HSiOx composites, respectively. The mass contents of amorphous carbon and RGO in the DC-HSiOx composite are ca. 6.5% and 13.3%, respectively, which are reflected by two stages with different slopes in the TG curve of DC-HSiOx composite due to the different degrees of graphitization. For comparison, the HSiOx/C composite was prepared with similar carbon content (21.5 wt%) to that of the DC-HSiOx composite (Figure S5b). The electrochemical performances of the DC-HSiOx composite were evaluated by employing it as the anode material for 2025-type coin half battery. Figure 3a shows typical CV curves of the DC-HSiOx electrode at a scanning rate of 0.1 mV s-1 over the potential range of 0.01~2.5 V. In the cathodic polarization process of the first cycle, a distinct cathodic peak at 0.75 V (vs. Li/Li+) that disappears in the following cycles can be attributed to the formation of SEI layer. Moreover, the distinct cathodic peak at below 0.25 V is ascribed to the lithiation of SiOx, which evolves to two new peaks at 0.35 and 0.01 V that can be assigned to the formation of different LixSi alloys.51 The anodic peak at 0.48 V is attributed to the dealloying processes of the LixSi alloys.46 In addition, the intensities of cathodic and anodic peaks both increase during cycles, indicating an activation process of the active materials. The representative galvanostatic charge/discharge curves of the DC-HSiOx electrode during different cycles is displayed in Figure 3b. The oxidative and reductive peaks appeared in CV curves well correspond to the voltage plateaus in the charge and discharge curves. Besides, the shape of the

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charge/discharge profiles keep similar with a slight increase in specific capacity during the early cycles and are basically overlapped after long cycles, indicating a good cycling performance. Figure 3d displays the cycling performance of the DC-HSiOx composite. For comparison, the HSiOx/C composite with similar carbon content was also tested as coin-type battery anode. The DC-HSiOx electrode delivers an initial reversible capacity of 1113 mAh g-1 with an ICE of 59.9%, whereas the HSiOx/C electrode shows a reversible capacity of 909 mAh g-1 with a relatively low ICE of 55.2%. The low ICE is mainly attributed to the irreversible reaction occurring during the first cycle and the formation of SEI layer, and the higher ICE for the DC-HSiOx electrode may profit from the obtained secondary particles that effectively decrease the interfacial areas in contact with electrolyte.52 The CEs for both electrodes rise to 99% after 5 cycles and maintain above 99.2% during the following cycles. The average CE for DC-HSiOx electrode is 99.6%, larger than that of HSiOx/C electrode (99.3%), indicating an improved reversibility and stability for DC-HSiOx electrode. The high CE during cycling for DCHSiOx electrode is beneficial for full cell cycling, and its average CE is comparable to previously reported SiOx-based materials.15,39 Furthermore, the DC-HSiOx electrode delivers a high reversible capacity of 982 mAh g-1 over 200 cycles with a capacity retention of 95.4% at a current density of 0.2 A g-1, whereas the HSiOx/C electrode shows inferior cycling performance, with a reversible capacity of 691 mAh g-1 (capacity retention of 77.1%). The performance difference is mainly attributed to the more robust structure of the DC-HSiOx composite, as will be discussed in later section. Additionally, the DC-HSiOx electrode shows outstanding rate capability (Figure 3e), delivering

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reversible capacities of 885, 828, 714 and 533 mAh g-1 at 0.5, 1, 2 and 5 A g-1, respectively. On the contrary, the HSiOx/C electrode only delivers reversible capacities of 788, 621, 410 and 161 mAh g-1 under the same current densities, respectively. This suggests that the dual carbon conductive networks effectively reduce the interparticle resistance of SiOx and significantly boost the capacity at high current densities. The DC-HSiOx electrode exhibits a high specific capacity of 1019 mAh g-1 while the current density is reversed back to 0.2 A g-1, indicating its superior cycling reversibility at different current densities. However, a reversible capacity of only 669 mAh g-1 is delivered by the HSiOx/C electrode while the current density shifts back to 0.2 A g-1, indicating its degenerated structure and inferior conductive network. In addition, it can be observed that the CEs for DC-HSiOx electrode maintain high values under different current densities, demonstrating the stable reversible electrochemical reaction and a high structural stability. On the contrary, the HSiOx/C electrode shows inferior CEs, which fluctuate and decline especially under high current densities, indicating the unstable structure of HSiOx/C composite during cycling. Figure 3c presents the typical charge/discharge profiles of the DC-HSiOx electrode at different rates, illustrating its highly reversible electrochemical behavior and slightly increased overpotentials at higher rates. The cycling performances of the DC-HSiOx electrode for long term are shown in Figure 3f. The electrodes exhibit high capacity retentions of 92.5% and 85.1% over 300 cycles, with reversible capacities of 823 and 682 mAh g-1 under the current densities of 0.5 and 1 A g-1, respectively. The excellent electrochemical behaviors of the DC-HSiOx composite are ascribed to the hollow structure of SiOx providing short

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diffusion length for Li+ ions, the dual carbon conductive networks facilitating fast charge transfer, as well as a robust RGO mechanical backbone confining HSiOx/C in the 3D conductive network during cycles. Table S1 provides the electrochemical information of the anodes prepared in this work and other SiOx-based materials reported in recent literature.26,

33, 45-47, 49

It is worth noting that the DC-HSiOx sample are

comparable to or better than previously reported SiOx-based anodes in term of electrochemical performances. To further disclose the reason that the DC-HSiOx composite exhibits superior electrochemical properties, EIS measurements of the HSiOx/C and DC-HSiOx electrodes were conducted after 1 and 200 cycles and the results are shown in Figure 3g. It is observed that Nyquist plots show two compressed semicircles in highfrequency region and an inclined line in low-frequency region, corresponding to SEI layer resistance (RSEI), charge transfer resistance (Rct), and Warburg resistance (Wo), respectively.37, 53 The equivalent circuit model is shown in Figure S6 and the fitting resistance values for electrodes during different cycles are shown in Table S2. After the first cycle, the Rct value of the DC-HSiOx electrode (49.1 Ω) is lower than that of the HSiOx/C electrode (54.0 Ω). This can be attributed to the synergistic effect of dual carbon conductive networks that leads to a higher electronic conductivity for the DCHSiOx composite. After 200 cycles, the values of Rct and RSEI for both DC-HSiOx and HSiOx/C electrodes increase. However, Rct and RSEI values of the DC-HSiOx electrode (68.1 and 14.5 Ω) are much smaller than those of the HSiOx/C electrode (96.9 and 20.5 Ω). The result indicates the more stable structure and SEI layer for the DC-HSiOx

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electrode. TEM and SEM were utilized to further unveil the role of the dual carbon layers played in the DC-SiOx composite. Schematic illustrations along with corresponding TEM and SEM images show structural evolutions of the HSiOx/C and DC-HSiOx composites before and after 200 cycles. The HSiOx/C samples experience severe structural damage, with abundant cracks and isolated fragments lacking electrical contact observed in TEM and SEM images after 200 cycles (Figure 4a and c), resulting in inferior cycling performance. On the contrary, although cracks and fragments can be also observed in some junior units (HSiOx/C spheres) of DC-HSiOx sample (Figure S7), its original microstructure with HSiOx/C spheres encapsulated in 3D RGO conductive network can be well maintained after 200 cycles, attributed to the outstanding mechanical strength of RGO sheets (Figure 4b and d). This guarantees the integrity of the entire electrode and electrical connectivity for most active particles, hence contributing to the excellent electrochemical performances. The result indicates that the amorphous carbon and the RGO network both play a role in lowering the electric resistance, while RGO can further improve the stability of the whole structure. The cross-sectional SEM images of the HSiOx/C and DC-HSiOx electrodes were also measured before and after 200 cycles (Figure S8a-d), and the results are summarized in Figure 4e. Both DC-HSiOx and HSiOx/C electrodes display dramatically suppressed volume expansion (17.7% and 18.2%, respectively), indicating that hollow structure can effectively address the issue of huge volumetric variation for SiOx material. It should be noted that the synthesis of hollow structure inevitably comes with low tap density, which sacrifices the volumetric

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energy density of full cell. In our case, the DC-HSiOx composite shows an increased tap density of 0.48 g cm-3 compared to that of HSiOx/C composite (0.41 g cm-3), which can be attributed to the denser stacking of HSiOx/C spheres in the DC-HSiOx composite. For practical application, the tap density can be further improved by mixing with commercial graphite and the full cell of the obtained mixture is expected to show increased energy density compared to that of graphite.42 In practical applications, it is of great importance to obtain high ICE for full cells because the active Li+ ions are provided by the cathode materials. The compromised ICE of anode material usually necessitates an additional loading of cathode active material, which inevitably leads to a lessened total energy density. Hence, anodes with low ICE should be prelithiated before use to compensate for the initial irreversible capacity loss. Here, a common method was adopted to prelithiate the DC-HSiOx electrode and test its cycling performance in half cell.54 The prelithiated DC-HSiOx electrode shows a significantly declined open-circuit voltage (OCV) of 0.28 V compared to that of the DC-HSiOx electrode, indicating successful prelithiation of SiOx matrix (Figure S9a). Accordingly, the ICE increases from 59.9% to 95.2%, confirming that the large irreversible capacity loss in the irreversible reaction of SiOx and SEI formation is effectively compensated. Importantly, the prelithiated electrode exhibits comparable cycling performance as that of the DC-HSiOx electrode (Figure S9b). Coin-type DC-HSiOx//NCM811 full cells were assembled and tested to further assess the practical viability of the DC-HSiOx composite. NCM811 is regarded as a promising cathode material for use in LIBs, with a high theoretical specific capacity of ca. 200

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mAh g-1. In our case, it delivers a discharge capacity of 209 mAh g-1 at 0.1 C and maintains a high capacity retention of 92.9% with reversible capacity of 177 mAh g-1 at 1 C (1 C = 180 mA g-1) after 100 cycles, along with a good rate capability (Figure 5a-c). The DC-HSiOx anode was first prelithiated to diminish the initial irreversible capacity loss and then assembled into a coin-type full cell with NCM811 cathode. As shown in Figure 5e, the DC-HSiOx//NCM811 full cell delivers an initial discharge capacity of 198 mAh g-1 at 0.1 C with an ICE of 83.9%. Besides, a high reversible capacity of 172 mAh g-1 is retained after 100 cycles at 1 C based on the weight of cathode material, with a capacity decay ratio of 0.04% per cycle (Figure 5e). With an average discharge voltage of ~3.4 V (Figure 5d), a high energy density of ~419 Wh kg-1 is achieved by the DC-HSiOx//NCM811 full cell based on the total weight of the cathode and anode materials. It is worth noting that the value is comparable to those of reported Si-based full cells.42,55 In addition, the rate capability of the DCHSiOx//NCM811 full cell was also evaluated. The full cell delivers reversible specific capacities of 197, 183, 175 and 165 mAh g-1 at rates of 0.2, 0.5, 1 and 2 C, respectively (Figure 5f). The specific capacity recovers to 200 mAh g-1 when the current rate is reversed back to 0.1 C, indicating that the DC-HSiOx//NCM811 full cell exhibits good cycling reversibility. The high energy density and good cycling performance of the DCHSiOx//NCM811 full cell make it a potential candidate for use in high-energy electric devices. 4. CONCLUSIONS In conclusion, this study has demonstrated the preparation of a novel DC-HSiOx

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composite with dual carbon conductive networks encapsulating hollow HSiOx spheres for stable lithium-ion battery anode through a green synthetic route. The hierarchical structure was prepared through soft-template and carbothermic reduction processes, followed by a facile spray-drying method. The synergistic effect of hollow structure of SiOx and dual carbon conductive networks effectively overcame the shortcomings of SiOx-based anode used in high-energy LIBs, improving electrical conductivity of SiOx matrix, maintaining structural integrity of electrode materials and promoting stable SEI layer formation. As a result, the DC-HSiOx electrode delivered a high reversible specific capacity of 1113 mAh g-1 at the current density of 0.1 A g-1, a good cycling performance with capacity retention of 92.5% over 300 cycles at 0.5 A g-1, and a superior rate capability. In addition, the DC-HSiOx//NCM811 full cell exhibited good rate capacity, stable cycling performance and a high energy density of 419 Wh kg-1, demonstrating that the DC-HSiOx composite is a highly competitive candidate for potential applications in LIBs for powering sustainable vehicles.

ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.

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Digital photographs and TEM images of the composites. XPS spectra and TGA curves of the HSiOx/C and DC-HSiOx composites. Equivalent circuit and cross-sectional SEM images of the HSiOx/C and DC-HSiOx electrodes. Galvanostatic discharge/charge profiles of the prelithiated DC-HSiOx anode. Performance comparison of present work with reported SiOx-based materials. Impedance parameters of the HSiOx/C and DCHSiOx electrodes after 1 and 200 cycles. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Xiaohua Xie) *E-mail: [email protected] (Jian Zhang) ORCID Xiaohua Xie: 0000-0002-6585-2592 Notes The authors declare no competing financial interest ACKNOWLEDGEMENT This work was supported by the National Natural Science Foundation of China (No. 21603260, 51777208).

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Rice

Husk-Derived

Hierarchical

Silicon/Nitrogen-Doped

Carbon/Carbon Nanotube Spheres as Low-Cost and High-Capacity Anodes for Lithium-Ion Batteries. Nano Energy 2016, 25, 120-127. (7) Xu, R.; Wang, G.; Zhou, T.; Zhang, Q.; Cong, H.-P.; Sen, X.; Rao, J.; Zhang, C.; Liu, Y.; Guo, Z.; Yu, S.-H. Rational Design of Si@Carbon with Robust Hierarchically Porous Custard-Apple-Like Structure to Boost Lithium Storage. Nano Energy 2017, 39, 253-261. (8) Yang, Y.; Ni, C.; Gao, M.; Wang, J.; Liu, Y.; Pan, H. Dispersion-Strengthened Microparticle Silicon Composite with High Anti-Pulverization Capability for Li-Ion Batteries. Energy Storage Mater. 2018, 14, 279-288. (9) Zhang, W.-J. A Review of The Electrochemical Performance of Alloy Anodes for

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Lithium-Ion Batteries. J. Power Sources 2011, 196 (1), 13-24. (10)Wu, H.; Cui, Y. Designing Nanostructured Si Anodes for High Energy Lithium Ion Batteries. Nano Today 2012, 7 (5), 414-429. (11)Yao, Y.; McDowell, M. T.; Ryu, I.; Wu, H.; Liu, N. A.; Hu, L. B.; Nix, W. D.; Cui, Y. Interconnected Silicon Hollow Nanospheres for Lithium-Ion Battery Anodes with Long Cycle Life. Nano Lett. 2011, 11 (7), 2949-2954. (12)Liu, X. H.; Zheng, H.; Zhong, L.; Huang, S.; Karki, K.; Zhang, L. Q.; Liu, Y.; Kushima, A.; Liang, W. T.; Wang, J. W.; Cho, J. H.; Epstein, E.; Dayeh, S. A.; Picraux, S. T.; Zhu, T.; Li, J.; Sullivan, J. P.; Cumings, J.; Wang, C.; Mao, S. X.; Ye, Z. Z.; Zhang, S.; Huang, J. Y. Anisotropic Swelling and Fracture of Silicon Nanowires during Lithiation. Nano Lett. 2011, 11 (8), 3312-3318. (13)Kim, H.; Seo, M.; Park, M.-H.; Cho, J. A Critical Size of Silicon Nano-Anodes for Lithium Rechargeable Batteries. Angew. Chem., Int. Ed. 2010, 49 (12), 2146-2149. (14) Liu, Z.; Yu, Q.; Zhao, Y.; He, R.; Xu, M.; Feng, S.; Li, S.; Zhou, L.; Mai, L. Silicon Oxides: A Promising Family of Anode Materials for Lithium-Ion Batteries. Chem. Soc. Rev. 2019, 48 (1), 285-309. (15) Yu, Q.; Ge, P.; Liu, Z.; Xu, M.; Yang, W.; Zhou, L.; Zhao, D.; Mai, L. Ultrafine SiOx/C Nanospheres and Their Pomegranate-Like Assemblies for High-Performance Lithium Storage. J. Mater. Chem. A 2018, 6 (30), 14903-14909. (16) Yang, W.; Liu, H.; Ren, Z.; Jian, N.; Gao, M.; Wu, Y.; Liu, Y.; Pan, H. A Novel Multielement, Multiphase, and B-Containing SiOx Composite as A Stable Anode Material for Li-Ion Batteries. Adv. Mater. Interfaces 2019, 6 (5), 1801631.

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(17) Huang, T.; Yang, Y.; Pu, K.; Zhang, J.; Gao, M.; Pan, H.; Liu, Y. Linking Particle Size to Improved Electrochemical Performance of SiO Anodes for Li-Ion Batteries. RSC Adv. 2017, 7 (4), 2273-2280. (18)Jung, S. C.; Kim, H.-J.; Kim, J.-H.; Han, Y.-K. Atomic-Level Understanding toward a High-Capacity and High Power Silicon Oxide (SiO) Material. J. Phys. Chem. C 2016, 120 (2), 886-892. (19)Chen, T.; Wu, J.; Zhang, Q.; Su, X. Recent Advancement of SiOx Based Anodes for Lithium-Ion Batteries. J. Power Sources 2017, 363, 126-144. (20)Hwa, Y.; Park, C.-M.; Sohn, H.-J. Modified SiO as A High Performance Anode for Li-Ion Batteries. J. Power Sources 2013, 222, 129-134. (21)Yamamura, H.; Nobuhara, K.; Nakanishi, S.; Iba, H.; Okada, S. Investigation of The Irreversible Reaction Mechanism and The Reactive Trigger on SiO Anode Material for Lithium-Ion Battery. J. Ceram. Soc. Jpn. 2011, 119 (1395), 855-860. (22)Kim, J.-H.; Park, C.-M.; Kim, H.; Kim, Y.-J.; Sohn, H.-J. Electrochemical behavior of SiO anode for Li secondary batteries. J. Electroanal. Chem. 2011, 661 (1), 245-249. (23)Zhang, L.; Deng, J.; Liu, L.; Si, W.; Oswald, S.; Xi, L.; Kundu, M.; Ma, G.; Gemming, T.; Baunack, S.; Ding, F.; Yan, C.; Schmidt, O. G. Hierarchically Designed SiOx/SiOy Bilayer Nanomembranes as Stable Anodes for Lithium Ion Batteries. Adv. Mater. 2014, 26 (26), 4527-4532. (24)Kim, H. J.; Choi, S.; Lee, S. J.; Seo, M. W.; Lee, J. G.; Deniz, E.; Lee, Y. J.; Kim, E. K.; Choi, J. W. Controlled Prelithiation of Silicon Monoxide for High Performance Lithium-Ion Rechargeable Full Cells. Nano Lett. 2016, 16 (1), 282-288.

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(25)Zhao, J.; Lu, Z.; Liu, N.; Lee, H.-W.; McDowell, M. T.; Cui, Y. Dry-Air-Stable Lithium Silicide-Lithium Oxide Core-Shell Nanoparticles as High-Capacity Prelithiation Reagents. Nat. Commun. 2014, 5. (26)Pan, Q.; Zuo, P.; Mu, T.; Du, C.; Cheng, X.; Ma, Y.; Gao, Y.; Yin, G. Improved Electrochemical Performance of Micro-Sized SiO-based Composite Anode by Prelithiation of Stabilized Lithium Metal Powder. J. Power Sources 2017, 347, 170-177. (27)Zhao, J.; Lee, H.-W.; Sun, J.; Yan, K.; Liu, Y.; Liu, W.; Lu, Z.; Lin, D.; Zhou, G.; Cui, Y. Metallurgically Lithiated SiOx Anode with High Capacity and Ambient Air Compatibility. Proc. Natl. Acad. Sci. USA 2016, 113 (27), 7408-7413. (28) Ashuri, M.; He, Q.; Liu, Y.; Zhang, K.; Emani, S.; Sawicki, M. S.; Shamie, J. S.; Shaw, L. L. Hollow Silicon Nanospheres Encapsulated with A Thin Carbon Shell: An Electrochemical Study. Electrochim. Acta 2016, 215, 126-141. (29) Chen, D.; Mei, X.; Ji, G.; Lu, M.; Xie, J.; Lu, J.; Lee, J. Y. Reversible Lithium-Ion Storage in Silver-Treated Nanoscale Hollow Porous Silicon Particles. Angew. Chem., Int. Ed. 2012, 51 (10), 2409-2413. (30) Teng, Z.; Su, X.; Zheng, Y.; Sun, J.; Chen, G.; Tian, C.; Wang, J.; Li, H.; Zhao, Y.; Lu, G. Mesoporous Silica Hollow Spheres with Ordered Radial Mesochannels by A Spontaneous Self-Transformation Approach. Chem. Mater. 2012, 25 (1), 98-105. (31) An, W.; Fu, J.; Su, J.; Wang, L.; Peng, X.; Wu, K.; Chen, Q.; Bi, Y.; Gao, B.; Zhang, X. Mesoporous Hollow Nanospheres Consisting of Carbon Coated Silica Nanoparticles for Robust Lithium-Ion Battery Anodes. J. Power Sources 2017, 345, 227-236.

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(32)Wang, J.; Zhou, M. J.; Tan, G. Q.; Chen, S.; Wu, F.; Lu, J.; Amine, K. Encapsulating Micro-Nano Si/SiOx into Conjugated Nitrogen-Doped Carbon as Binder-Free Monolithic Anodes for Advanced Lithium Ion Batteries. Nanoscale 2015, 7 (17), 8023-8034. (33)Wu, W.; Shi, J.; Liang, Y.; Liu, F.; Peng, Y.; Yang, H. A Low-Cost and Advanced SiOx-C Composite with Hierarchical Structure as An Anode Material for Lithium-Ion Batteries. Phys. Chem. Chem. Phys. 2015, 17 (20), 13451-13456. (34)Hwang, S. W.; Lee, J. K.; Yoon, W. Y. Electrochemical Behavior of CarbonCoated Silicon Monoxide Electrode with Chromium Coating in Rechargeable Lithium Cell. J. Power Sources 2013, 244, 620-624. (35)Yuan, X. Q.; Xin, H. X.; Qin, X. Y.; Li, X. J.; Liu, Y. F.; Guo, H. F. Self-Assembly of SiO/Reduced Graphene Oxide Composite as High-Performance Anode Materials for Li-Ion Batteries. Electrochim. Acta 2015, 155, 251-256. (36)Zhang, W.; Zuo, P.; Chen, C.; Ma, Y.; Cheng, X.; Du, C.; Gao, Y.; Yin, G. Facile Synthesis of Binder-Free Reduced Graphene Oxide/Silicon Anode for HighPerformance Lithium Ion Batteries. J. Power Sources 2016, 312, 216-222. (37)Zhang, J.; Zhang, L.; Xue, P.; Zhang, L.; Zhang, X.; Hao, W.; Tian, J.; Shen, M.; Zheng, H. Silicon-Nanoparticles Isolated by In Situ Grown Polycrystalline Graphene Hollow Spheres for Enhanced Lithium-Ion Storage. J. Mater. Chem. A 2015, 3 (15), 7810-7821. (38) Cai, X.; Liu, W.; Zhao, Z.; Li, S.; Yang, S.; Zhang, S.; Gao, Q.; Yu, X.; Wang, H.; Fang, Y. Simultaneous Encapsulation of Nano-Si in Redox Assembled rGO Film as

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Binder-Free Anode for Flexible/Bendable Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2019, 11 (4), 3897-3908. (39) Xu, Q.; Sun, J. K.; Yu, Z. L.; Yin, Y. X.; Xin, S.; Yu, S. H.; Guo, Y. G. SiOx Encapsulated in Graphene Bubble Film: An Ultrastable Li-Ion Battery Anode. Adv. Mater. 2018, 30 (25), 1707430. (40)Gao, X.; Li, J.; Xie, Y.; Guan, D.; Yuan, C. A Multilayered Silicon-Reduced Graphene Oxide Electrode for High Performance Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2015, 7 (15), 7855-7862. (41)Park, J. H.; Moon, J.; Han, S.; Park, S.; Lim, J. W.; Yun, D.-J.; Kim, D. Y.; Park, K.; Son, I. H. Formation of Stable Solid–Electrolyte Interphase Layer on Few-Layer Graphene-Coated Silicon Nanoparticles for High-Capacity Li-Ion Battery Anodes. J. Phys. Chem. C 2017, 121 (47), 26155-26162. (42)Shi, L.; Pang, C.; Chen, S.; Wang, M.; Wang, K.; Tan, Z.; Gao, P.; Ren, J.; Huang, Y.; Peng, H.; Liu, Z. Vertical Graphene Growth on SiO Microparticles for Stable Lithium Ion Battery Anodes. Nano Lett. 2017, 17 (6), 3681-3687. (43) Yu, Y. W. a. S.-H. Polyelectrolyte Controlled Large-Scale Synthesis of Hollow Silica Spheres with Tunable Sizes and Wall Thicknesses. J. Phys. Chem. C 2008, 113, 3641-3647. (44) Nakashima, Y.; Takai, C.; Razavi-Khosroshahi, H.; Fuji, M. Influence of the PAA Concentration on PAA/NH3 Emulsion Template Method for Synthesizing Hollow Silica Nanoparticles. Colloids Surf. A 2018, 546, 301–306. (45)Lv, P.; Zhao, H.; Gao, C.; Du, Z.; Wang, J.; Liu, X. SiOx-C Dual-Phase Glass for

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Lithium Ion Battery Anode with High Capacity and Stable Cycling Performance. J. Power Sources 2015, 274, 542-550. (46)Lv, P.; Zhao, H.; Gao, C.; Zhang, T.; Liu, X. Highly Efficient and Scalable Synthesis of SiOx/C Composite with Core-Shell Nanostructure as High-Performance Anode Material for Lithium Ion Batteries. Electrochim. Acta 2015, 152, 345-351. (47)Wang, J.; Zhao, H.; He, J.; Wang, C.; Wang, J. Nano-Sized SiOx/C Composite Anode for Lithium Ion Batteries. J. Power Sources 2011, 196 (10), 4811-4815. (48) Liu, Z.; Guan, D.; Yu, Q.; Xu, L.; Zhuang, Z.; Zhu, T.; Zhao, D.; Zhou, L.; Mai, L. Monodisperse and homogeneous SiO/C microspheres: A Promising High-Capacity and Durable Anode Material for Lithium-Ion Batteries. Energy Storage Mater. 2018, 13, 112-118. (49)Xu, Q.; Sun, J.-K.; Yin, Y.-X.; Guo, Y.-G. Facile Synthesis of Blocky SiOx/C with Graphite-Like Structure for High-Performance Lithium-Ion Battery Anodes. Adv. Funct. Mater. 2018, 28 (8), 1705235. (50)Takezawa, H.; Iwamoto, K.; Ito, S.; Yoshizawa, H. Electrochemical Behaviors of Nonstoichiometric Silicon Suboxides (SiOx) Film Prepared by Reactive Evaporation for Lithium Rechargeable Batteries. J. Power Sources 2013, 244, 149-157. (51)Gao, C.; Zhao, H.; Lv, P.; Wang, C.; Wang, J.; Zhang, T.; Xia, Q., Superior Cycling Performance of SiOx/C Composite with Arrayed Mesoporous Architecture as Anode Material for Lithium-Ion Batteries. J. Electrochem. Soc. 2014, 161 (14), A2216-A2221. (52)Liu, N.; Lu, Z.; Zhao, J.; McDowell, M. T.; Lee, H.-W.; Zhao, W.; Cui, Y. A Pomegranate-Inspired Nanoscale Design for Large-Volume-Change Lithium Battery

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Anodes. Nat. Nanotech. 2014, 9 (3), 187-192. (53)Xu, T.; Wang, D.; Qiu, P.; Zhang, J.; Wang, Q.; Xia, B.; Xie, X. In Situ Synthesis of Porous Si Dispersed in Carbon Nanotube Intertwined Expanded Graphite for HighEnergy Lithium-Ion Batteries. Nanoscale 2018, 10 (35), 16638-16644. (54)Chae, C.; Noh, H.-J.; Lee, J. K.; Scrosati, B.; Sun, Y.-K. A High-Energy Li-Ion Battery Using a Silicon-Based Anode and a Nano-Structured Layered Composite Cathode. Adv. Funct. Mater. 2014, 24 (20), 3036-3042. (55)Chen, S.; Shen, L.; van Aken, P. A.; Maier, J.; Yu, Y. Dual-Functionalized Double Carbon Shells Coated Silicon Nanoparticles for High Performance Lithium-Ion Batteries. Adv. Mater. 2017, 29 (21), 1605650.

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Figure 1. (a) Schematic illustration of the preparation procedure for the DC-HSiOx composite. SEM and TEM images of (b, e) the hollow SiO2 spheres, (c, f) the HSiOx/C composite, (d, g) and the DC-HSiOx composite with an inset of magnified SEM image. (h) TEM image of the HSiOx/C spheres and (i-k) corresponding EDX mapping images of elements Si, O and C.

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Figure 2. (a) XRD patterns of the HSiO2 spheres, the HSiOx/C and DC-HSiOx composites. (b) Raman spectra of the HSiOx/C and DC-HSiOx composites. Highresolution Si 2p spectra of (c) the HSiOx/C and (d) DC-HSiOx composites.

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Figure 3. (a) CV curves of the DC-HSiOx electrode at a scanning rate of 0.1 mV s-1. (b) Galvanostatic charge/discharge profiles of the DC-HSiOx electrode in different cycles. (c) Galvanostatic charge/discharge profiles of the DC-HSiOx electrode at different current densities. (d) Cycling performances of the DC-HSiOx and HSiOx/C electrodes at a current density of 0.1 A g-1 for the first cycle and 0.2 A g-1 for the following cycles. (e) Rate capabilities of the DC-HSiOx and HSiOx/C electrodes. (f) Cycling performances of the DC-HSiOx electrodes at current densities of 0.5 and 1 A g-1, respectively. (g) Electrochemical impedance spectra of the DC-HSiOx and HSiOx/C electrodes after 1 and 200 cycles.

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Figure 4. Schematic illustrations and corresponding TEM images of (a) the HSiOx/C and (b) DC-HSiOx composites before and after 200 cycles. SEM images of (c) the HSiOx/C and (d) DC-HSiOx electrodes after 200 cycles. (e) Histogram showing the thickness changes of the HSiOx/C and DC-HSiOx electrodes before and after 200 cycles.

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Figure 5. (a) Charge/discharge profiles of the NCM811 electrode at different rates. (b) Cycling performance of the NCM811 electrode at 0.1 C for the first cycle and 1 C for the following 100 cycles. (c) Rate capability of the NCM811 electrode. (d) Charge/discharge profiles of the DC-HSiOx//NCM811 full cell at different rates. (e) Cycling performance of the DC-HSiOx//NCM811 full cell at 0.1 C for the first 3 cycles and 1 C for the following 100 cycles. (f) Rate capability of the DC-HSiOx//NCM811 full cell.

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Table of Contents

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Figure 1. (a) Schematic illustration of the preparation procedure for the DC-HSiOx composite. SEM and TEM images of (b, e) the hollow SiO2 spheres, (c, f) the HSiOx/C composite, (d, g) and the DC-HSiOx composite with an inset of magnified SEM image. (h) TEM image of the HSiOx/C spheres and (i-k) corresponding EDX mapping images of elements Si, O and C. 82x98mm (300 x 300 DPI)


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Figure 2. (a) XRD patterns of the HSiO2 spheres, the HSiOx/C and DC-HSiOx composites. (b) Raman spectra of the HSiOx/C and DC-HSiOx composites. High-resolution Si 2p spectra of (c) the HSiOx/C and (d) DCHSiOx composites. 82x67mm (300 x 300 DPI)



Figure 3. (a) CV curves of the DC-HSiOx electrode at a scanning rate of 0.1 mV s-1. (b) Galvanostatic charge/discharge profiles of the DC-HSiOx electrode in different cycles. (c) Galvanostatic charge/discharge profiles of the DC-HSiOx electrode at different current densities. (d) Cycling performances of the DC-HSiOx and HSiOx/C electrodes at a current density of 0.1 A g-1 for the first cycle and 0.2 A g-1 for the following cycles. (e) Rate capabilities of the DC-HSiOx and HSiOx/C electrodes. (f) Cycling performances of the DCHSiOx electrodes at current densities of 0.5 and 1 A g-1, respectively. (g) Electrochemical impedance spectra of the DC-HSiOx and HSiOx/C electrodes after 1 and 200 cycles. 80x59mm (300 x 300 DPI)


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Figure 4. Schematic illustrations and corresponding TEM images of (a) the HSiOx/C and (b) DC-HSiOx composites before and after 200 cycles. SEM images of (c) the HSiOx/C and (d) DC-HSiOx electrodes after 200 cycles. (e) Histogram showing the thickness changes of the HSiOx/C and DC-HSiOx electrodes before and after 200 cycles. 82x69mm (300 x 300 DPI)



Figure 5. (a) Charge/discharge profiles of the NCM811 electrode at different rates. (b) Cycling performance of the NCM811 electrode at 0.1 C for the first cycle and 1 C for the following 100 cycles. (c) Rate capability of the NCM811 electrode. (d) Charge/discharge profiles of the DC-HSiOx//NCM811 full cell at different rates. (e) Cycling performance of the DC-HSiOx//NCM811 full cell at 0.1 C for the first 3 cycles and 1 C for the following 100 cycles. (f) Rate capability of the DC-HSiOx//NCM811 full cell. 82x79mm (300 x 300 DPI)


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Green Synthesis of Dual Carbon Conductive Network-Encapsulated

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