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Hollow-in-Hollow Carbon Spheres for Lithium-ion Batteries with Superior Capacity and Cyclic Performance
Electrochimica Acta 186 (2015) 436–441
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Hollow-in-Hollow Carbon Spheres for Lithium-ion Batteries with Superior Capacity and Cyclic Performance Jun Zanga,c, Jianchuan Yea , Xiaoliang Fangb,** , Xiangwu Zhangc, Mingsen Zhenga , Quanfeng Donga,* a State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Collaborative Innovation Centre of Chemistry for Energy Materials (iChEM), Xiamen 361005, China b Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen 361005, China c Fiber and Polymer Science Program, Department of Textiles Engineering, Chemistry and Science, North Carolina State University, Raleigh, NC 27695-8301, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 September 2015 Received in revised form 29 October 2015 Accepted 1 November 2015 Available online 4 November 2015
Hollow spheres structured materials have been intensively pursued due to their unique properties for energy storage. In this paper, hollow-in-hollow carbon spheres (HIHCS) with a multi-shelled structure were successfully synthesized using a facile hard-templating procedure. When evaluated as anode material for lithium-ion batteries, the resultant HIHCS anode exhibited superior capacity and cycling stability than HCS. It could deliver reversible capacities of 937, 481, 401, 304 and 236 mAh g 1 at current densities of 0.1 A g 1, 1 A g 1, 2 A g 1, 5 A g 1 and 10 A g 1, respectively. And capacity fading is not apparent in 500 cycles at 5 A g 1. The excellent performance of the HIHCS anode is ascribed to its unique hollow-inhollow structure and high specific surface area. ã 2015 Published by Elsevier Ltd.
Keywords: Hollow-in-hollow Carbon spheres Anode Lithium-ion batteries
1. Introduction Currently, rechargeable lithium ion batteries (LIBs) have been widely used as the power source in portable electronic devices and electric vehicles [1,2]. Electrode materials are the determining factor for the battery performance. Although graphite performs well as the anode material for commercial LIBs, its theoretical capacity (372 mAh g 1 for the nominal composition LiC6) and relatively low lithium diffusion coefficient (10 710 10 cm2 s 1) are insufficient to satisfy the increasing demand for batteries with higher capacity and rate capability [3–5]. Therefore, continued efforts have been devoted to exploring new anode materials during the past decade. Based on the improved electrochemical performance and structure stability during discharge/charge processes, carbonaceous nanomaterials are still the preferred choices. It is believed that morphology and structure are important factors for the electrochemical performance of the carbonaceous nanomaterials. Many carbon nanoarchitectures, such as nanotubes, nanosheets, and nanospheres, have been used to fabricate high-preformance
* Corresponding author. Tel.: +86 0592 2185905; fax: +86 0592 2183905. ** Corresponding author. Email-address: Tel.: +86 0592 2183063; fax: +86 0592 2183063. E-mail addresses: [email protected] (X. Fang), [email protected] (Q. Dong). http://dx.doi.org/10.1016/j.electacta.2015.11.002 0013-4686/ ã 2015 Published by Elsevier Ltd.
anodes [6–9]. Among these materials, hollow carbon spheres (HCS) are thought to be profitable for achieving improved performance owing to their high surface area and short path length for Li transport [10–12]. It is well-known that high surface area can accommodate extra lithium ions at the electrode surface and in the inner cavities and a reduced lithium transport path length can allow better reaction kinetics, resulting in improved capacity and rate performance [13,14]. For instance, Tang et al. synthesized the monodisperse hollow carbon nanospheres via a combined polystyrene latex/hydrothermal carbonization templating approach. A high rate performance of ca. 100 mAh g 1 at 50 C (18.6 A g 1) was achieved [12]. However, compared with solid structures, the low packing density of HCS originated from its large empty space usually leads to the low volumetric energy density, which is undesirable for practical applications. As a special class of hollow structures, multi-shelled hollow structures have recently attracted increasing attention since they can significantly reduce lithium ions and electrons diffusion paths, meanwhile increase the volumetric capacity [15]. When used as the anode materials for Liion batteries, hollow metal oxide spheres with a multi-shelled structure exhibited excellent rate capability, good cycling performance, and ultrahigh specific capacity [16–20]. From the viewpoint of fundamental research and practical application, the multi-shelled hollow structures provide new opportunities to upgrade the electrochemical performance of LIB anodes.
J. Zang et al. / Electrochimica Acta 186 (2015) 436–441
Herein, we utilized a facile hard-templating route to synthesize a hollow-in-hollow structured carbon spheres (HIHCS). The assythesized HIHCS has a uniform multi-shelled structure and high specific surface area. These characteristics make HIHCS a potential anode material for lithium ion batteries. Compared with HCS, the multi-shelled structure endowed HIHCS with much higher capacity and rate capability 2. Experimental 2.1. Synthesis of HCS and HIHCS HCS and HIHCS were prepared by a facile hard-templating method [21]. The starting material SiO2 spheres were obtained by the hydrolysis of tetraethyl orthosilicate (TEOS) in a mixture containing ethanol and ammonium aqueous solution. After treated with the resorcinol formaldehyde resin (RF) coating process and silica coating process in sequence, the SiO2 spheres were converted into SiO2@RF@SiO2 spheres or SiO2@RF@SiO2@RF@SiO2 spheres. The procedures for RF coating and silica coating were carried out according to Ref. [22] and [23]. Subsequently, the as-synthesized SiO2@RF@SiO2 spheres and SiO2@RF@SiO2@RF@SiO2 spheres were further sintered to achieve the intermediates SiO2@C@SiO2 spheres and SiO2@C@SiO2@C@SiO2 spheres under nitrogen atmosphere at 800 C for 2 h with a heating rate of 1.5 C/min. After washing with a 10% HF aqueous solution for one day, the intermediates were converted into HCS and HIHCS, respectively. 2.2. Characterization Product morphologies were observed on a Hitachi S-4800 scanning electron microscopy (SEM) with a field-emission electron gun. Transmission electron microscopy (TEM) images were taken on a TECNAI F-30 high-resolution transmission electron microscope operated at 300 kV. The structure was characterized by X-ray powder diffraction (XRD, Philips Panalytical X-pert) using Cu Ka1 radiation l = 1.5405 Å. Nitrogen adsorption and desorption isotherms were measured at 77 K with a Micrometrics ASAP 3020 system. The specific surface area and pore size distributions of HCS and HIHCS were obtained by the Brunauer-Emmett-Teller (BET) method and the BarrettJoyner-Halenda (BJH) method, respectively.
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2.3. Electrochemical Measurements Two-electrode CR2016 type coin cells were used to evaluate the electrochemical performance of the products. The working electrode was made up by 80wt% active materials, 10wt% Super P Li and 10wt% poly(vinylidenedifluoride) (PVDF). The current collector was copper foil and the counter electrode was Li foil. Celgard 2400 polypropylene membrane was utilized as the separator. The electrolyte was 1 M LiPF6 in a 1:1:1 v/v/v mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) (Zhuhai, China). The cells was assembled in an Ar-filled glovebox with the concentrations of moisture and oxygen below 1.0 ppm. The cells were galvanostatically charged and discharged between 0.01 V and 3.0 V vs. Li/Li+ using a BTS Battery test system (Neware, Shenzhen, China) under various current densities (100 mA g 1, 500 mA g 1, 1 A g 1, 2 A g 1, 5 A g 1, and 10 A g 1). Electrochemical impedance spectrometry (EIS) tests were performed on IM6 (Zahner elektrik) in the frequency range from 100 kHz to 100 mHz. 3. Results and discussion Fig. 1 presents the typical hard-templating procedure for the synthesis of HCS and HIHCS. Firstly, the SiO2 spheres were coated with RF resin and silica to form a sandwich-like SiO2@RF@SiO2 spheres [22]. Secondly, according to the same procedure, SiO2@RF@SiO2@RF@SiO2 spheres were obtained by using the SiO2@RF@SiO2 spheres as the templates. The weight ratio of inner layer RF precursors and outer layer RF precursors in SiO2@RF@SiO2@RF@SiO2 spheres was 1:1. Finally, after the carbonization under N2 atmosphere and the subsequent removal of SiO2, thesynthesized SiO2@RF@SiO2 spheres and SiO2@RF@SiO2@RF@SiO2 spheres were converted into HCS and hollow-in-hollow structured HIHCS, respectively. Generally, SEM and TEM were used to characterize the morphology and structure of HCS and HIHCS. As shown in Fig. 2a, uniform HCS has a well-defined spherical morphology with an average diameter of 300 nm. The TEM image of HCS (Fig. 2b) further indicates that the HCS possesses a uniform hollow structure. The void volume of HCS conforms with the original SiO2 template size (see Supplementary Materials, Fig. S1). Whilst the average thickness of the porous carbon shell in HCS is ca. 30 nm. Additionally, the external surface of HCS is clean and smooth, indicating no impurities left over the carbonization process. High-
Fig. 1. Schematic illustration: the hard-templating procedures for the synthesis of HCS and HIHCS.
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J. Zang et al. / Electrochimica Acta 186 (2015) 436–441
Fig. 2. (a) SEM image and (b, c) TEM images of HCS, (d) SEM image and (e, f) TEM images of HIHCS.
magnification TEM image of HCS (Fig. 2c) further shows that the carbon shell of HCS exhibits a porous network structure. Pore sizes are in the range of several to tens of nanometers, which is beneficial for the transfer and storage of lithium ion (0.60 Å) [11,24]. In contrast, HIHCS shows the similar morphology to HCS with enlarged diameters up to 350 nm (Fig. 2d). According to the Fig. 2e and f, the porous HCS core encapsulated in a hollow carbon form the unique hollow-in-hollow structure of HIHCS. The inner porous HCS core, the outer hollow carbon shell, and the void space between the core and the shell can be clearly distinguished in TEM images of HIHCS. The average thickness of the outer carbon shell in HIHCS is about 15 nm. Since the diameter of the SiO2@RF@SiO2 template is bigger than the SiO2 template and the amount of inter layer RF precursors is the same as that of outer layer, the thickness of the outer carbon shell in HIHCS is thus smaller than that of the inner HCS core. The morphology of intermediates was also
characterized by SEM and TEM (see Supplementary Materials, Fig. S2, 3). The porosities of HCS and HIHCS were measured by Nitrogen adsorption-desorption analysis. As illustrated in Fig. 3, the N2 sorption isotherms with high nitrogen uptake have clearly demonstrated the high porosity of HCS and HIHCS. The Brunauer-Emmett-Teller (BET) surface area of HCS is 1144.8 m2 g 1. The type IV isotherm with a hysteresis loop indicates the existence of mesopores [25,26]. Based on the Barrett-Joyner-Halenda (BJH) method, HCS has a rather narrow pore size distribution centered 12 nm. The N2 sorption isotherm of HIHCS exhibits an analogous shape to that of HCS, indicating that the pore structure of HIHCS is similar as HCS. Compared with HCS, HIHCS has a little higher surface area (1190.1 m2 g 1), total pore volume (1.36 cm3 g 1), and a pore size distribution at 6 nm. The pore structures of HCS and HIHCS determined from N2 adsorption-desorption measurements
Fig. 3. (a) N2 sorption isotherms and (b) pore size distributions of HCS and HIHCS.
J. Zang et al. / Electrochimica Acta 186 (2015) 436–441
agree well with the TEM observation. With the high specific surface areas, large pore volume, and pore structure, the assynthesized HCS and HIHCS are expected as the potential anode materials for lithium-ion batteries. Galvanostatic discharge (lithium insertion) - charge (lithium extraction) method between 0.01 and 3.0 V was carried out to investigate the lithium storage performance of the HCS and HIHCS anode. The first and second charge/discharge curves of the HCS and HIHCS anode at a current density of 500 mA g 1 are shown in Fig. 4a and b, respectively. Interestingly, the capacity of the HIHCS anode is much higher than that of traditional graphite anode (shown in Fig. 4b). The reversible capacity of HIHCS anode can reach values up to 800 mAh g 1, which suggests the existence of different mechanism from traditional graphite anode. During the initial lithium insertion process, no obvious plateau was observed, implying a low degree of graphitization which agrees with the XRD result (see Supplementary Materials, Fig. S4) [27]. The steep charge/discharge slope is similar to that of typical disordered carbon materials [28,29]. Lithium ions can be stored in not only the disordered graphite layers but also some defect sites and micropores at the carbon surfaces, which increases the reversible capacity of the HIHCS anode. A large irreversible capacity of 1000 mAh g 1 measured in first cycle was also observed, which is common for porous carbonaceous anodes. The reason can be attributed to irreversible lithium insertion into special positions, such as cavities or sites in the carbonaceous materials and the
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formation of solid/electrolyte interface (SEI film) [11,28]. In the following cycles, the Coulombic efficiency (CE) increases and reaches nearly 100%. In contrast, as shown in Fig. 4a, the HCS anode only delivers the reversible capacity of 388 mAh g 1 in the first cycle. The initial coulombic efficiency of HCS (39%) is also lower than that of HIHCS (44%). Fig. 4c shows the cyclic performance of the HCS anode and HIHCS anode at a current density of 500 mA g 1. The HIHCS anode exhibits much higher reversible capacity than the HCS anode. The decrease in capacity during initial cycles is mainly due to the SEI film formation and irreversible lithium insertion. At 100th cycle, a high reversible capacity of 555 mAh g 1 is retained for the HIHCS anode. More importantly, from 10th cycle to 100th cycle, the discharge capacity of HIHCS maintains nearly constant, indicating the excellent cyclic stability. After the initial cycles, the Coulombic efficiency was almost equal to 100% during the measurement. In comparison, the discharge capacity of HCS was only 271 mAh g 1 (half capacity of HIHCS anode) after 100 cycles in the same measure condition. The rate performance of the HCS and HIHCS anode is shown in Fig. 4d. The HIHCS anode can deliver the discharge capacities of 481 and 304 mAh g 1 upon increasing the current densities to 1 A g 1 and 5 A g 1, respectively. Even the current density increases to a very high rate of 10 A g 1, a capacity of 236 mAh g 1 is still maintained. To our knowledge, this result is much better than that of traditional graphite electrode and many other similar carbon
Fig. 4. 1st and 2nd cycle discharge/charge curves of (a) HCS anode and (b) HIHCS anode at a current density of 500 mA g 1. (c) Cycling performances of HCS anode and HIHCS anode at a current density of 500 mA g 1. (d) Rate capabilities of HCS anode and HIHCS anode.
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in-hollow structure of HIHCS. With large surface area and total pore volume, HIHCS is a potential electrode material for high capacity and high rate lithium-ion batteries. As a result, HIHCS exhibits favorable capacity, superior rate and cyclic performance. A high reversible capacity (800 mAh g 1) and excellent rate capability (236 mAh g 1 at the current density of 10 A g 1) was achieved. The excellent electrochemical performance of the HIHCS anode could be derived from the unique hollow-in-hollow structure and high specific surface area. We believe that the hollow-in-hollow carbon structure presented here can also provide scope for further optimization of low-conductivity electrode materials such as Sn, Si, transition metal oxide, and sulfur. Acknowledgements
Fig. 5. Long cyclic performance of the HIHCS anode at a current density of 5 A g 1.
anodes [6–9]. In addition, after 40 cycles discharging/charging at different current densities, the reversible capacity can return to 800 mAh g 1 when the current density goes back to 100 mA g 1. These results indicate the excellent rate performance at high current density. Compared with the HIHCS anode, the discharge capacities of the as-synthesized HCS anode are only 301, 258, 200, and 157 mAh g 1 at the current densities of 1 A g 1, 2 A g 1, 5 A g 1 and 10 A g 1, respectively. The reason for excellent rate performance can be investigated by electrochemical impedance spectra (EIS) measurements (see Supplementary Materials, Fig. S3). It is well-known that the semicircle at high frequency correspondes to the charge transfer reaction at the electrolyte/electrode interface while the inclined line at low frequency was attributed to the lithium-diffusion process [30,31]. Apparently, the diameter of semicircle of the HIHCS anode was smaller than that of HCS electrode, indicating that the HIHCS anode possesses a favorable transport kinetict for both lithium ions and electrons [32]. To further investigate the long-term cycling stability, the HIHCS anode was tested for 500 cycles at a high rate of 5 A g 1. As shown in Fig. 5, after activated for initial 10 cycles, the HIHCS anode can be stabilized at a specific reversible capacity of 240 mAh g 1. At the same time, the Coulombic efficiency of the HIHCS anode is maintained at around 99%. All these results clearly indicate that the hollow-in-hollow structure can provide a favorable route to achieve high-performance carbon anode material for lithium ion batteries. Compared with the single-shelled hollow structure, the hollow-in-hollow structure of HIHCS guarantees: i) much special cages created by the hollow-in-hollow structure, in which the Li+ can be stored reversibly, resulting in more capacity capability; ii) a short lithium diffusion distance and large electrode/electrolyte contact area, resulting in superior rate capability. More importantly, such hollow-in-hollow structure could not only provide high gravimetric energy densities and excellent cycling performance but also exhibit higher volumetric energy densities compared to traditional hollow structure counterpart [19,33]. In addition, the large specific surface area and pore volume of the hollow-in-hollow structure can also accommodate extra lithium ions at the surface upon discharge/charge cycling. 4. Conclusions In summary, hollow-in-hollow structured carbon spheres (HIHCS) were successfully synthesized by a facile hard-templating method. The inner HCS core, the outer hollow carbon shell, and the void space between the core and the shell constitute the hollow-
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Hollow-in-Hollow Carbon Spheres for Lithium-ion Batteries with Superior Capacity and Cyclic Performance Jun Zanga,c, Jianchuan Yea , Xiaoliang Fangb,** , Xiangwu Zhangc, Mingsen Zhenga , Quanfeng Donga,* a State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Collaborative Innovation Centre of Chemistry for Energy Materials (iChEM), Xiamen 361005, China b Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen 361005, China c Fiber and Polymer Science Program, Department of Textiles Engineering, Chemistry and Science, North Carolina State University, Raleigh, NC 27695-8301, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 September 2015 Received in revised form 29 October 2015 Accepted 1 November 2015 Available online 4 November 2015
Hollow spheres structured materials have been intensively pursued due to their unique properties for energy storage. In this paper, hollow-in-hollow carbon spheres (HIHCS) with a multi-shelled structure were successfully synthesized using a facile hard-templating procedure. When evaluated as anode material for lithium-ion batteries, the resultant HIHCS anode exhibited superior capacity and cycling stability than HCS. It could deliver reversible capacities of 937, 481, 401, 304 and 236 mAh g 1 at current densities of 0.1 A g 1, 1 A g 1, 2 A g 1, 5 A g 1 and 10 A g 1, respectively. And capacity fading is not apparent in 500 cycles at 5 A g 1. The excellent performance of the HIHCS anode is ascribed to its unique hollow-inhollow structure and high specific surface area. ã 2015 Published by Elsevier Ltd.
Keywords: Hollow-in-hollow Carbon spheres Anode Lithium-ion batteries
1. Introduction Currently, rechargeable lithium ion batteries (LIBs) have been widely used as the power source in portable electronic devices and electric vehicles [1,2]. Electrode materials are the determining factor for the battery performance. Although graphite performs well as the anode material for commercial LIBs, its theoretical capacity (372 mAh g 1 for the nominal composition LiC6) and relatively low lithium diffusion coefficient (10 710 10 cm2 s 1) are insufficient to satisfy the increasing demand for batteries with higher capacity and rate capability [3–5]. Therefore, continued efforts have been devoted to exploring new anode materials during the past decade. Based on the improved electrochemical performance and structure stability during discharge/charge processes, carbonaceous nanomaterials are still the preferred choices. It is believed that morphology and structure are important factors for the electrochemical performance of the carbonaceous nanomaterials. Many carbon nanoarchitectures, such as nanotubes, nanosheets, and nanospheres, have been used to fabricate high-preformance
* Corresponding author. Tel.: +86 0592 2185905; fax: +86 0592 2183905. ** Corresponding author. Email-address: Tel.: +86 0592 2183063; fax: +86 0592 2183063. E-mail addresses: [email protected] (X. Fang), [email protected] (Q. Dong). http://dx.doi.org/10.1016/j.electacta.2015.11.002 0013-4686/ ã 2015 Published by Elsevier Ltd.
anodes [6–9]. Among these materials, hollow carbon spheres (HCS) are thought to be profitable for achieving improved performance owing to their high surface area and short path length for Li transport [10–12]. It is well-known that high surface area can accommodate extra lithium ions at the electrode surface and in the inner cavities and a reduced lithium transport path length can allow better reaction kinetics, resulting in improved capacity and rate performance [13,14]. For instance, Tang et al. synthesized the monodisperse hollow carbon nanospheres via a combined polystyrene latex/hydrothermal carbonization templating approach. A high rate performance of ca. 100 mAh g 1 at 50 C (18.6 A g 1) was achieved [12]. However, compared with solid structures, the low packing density of HCS originated from its large empty space usually leads to the low volumetric energy density, which is undesirable for practical applications. As a special class of hollow structures, multi-shelled hollow structures have recently attracted increasing attention since they can significantly reduce lithium ions and electrons diffusion paths, meanwhile increase the volumetric capacity [15]. When used as the anode materials for Liion batteries, hollow metal oxide spheres with a multi-shelled structure exhibited excellent rate capability, good cycling performance, and ultrahigh specific capacity [16–20]. From the viewpoint of fundamental research and practical application, the multi-shelled hollow structures provide new opportunities to upgrade the electrochemical performance of LIB anodes.
J. Zang et al. / Electrochimica Acta 186 (2015) 436–441
Herein, we utilized a facile hard-templating route to synthesize a hollow-in-hollow structured carbon spheres (HIHCS). The assythesized HIHCS has a uniform multi-shelled structure and high specific surface area. These characteristics make HIHCS a potential anode material for lithium ion batteries. Compared with HCS, the multi-shelled structure endowed HIHCS with much higher capacity and rate capability 2. Experimental 2.1. Synthesis of HCS and HIHCS HCS and HIHCS were prepared by a facile hard-templating method [21]. The starting material SiO2 spheres were obtained by the hydrolysis of tetraethyl orthosilicate (TEOS) in a mixture containing ethanol and ammonium aqueous solution. After treated with the resorcinol formaldehyde resin (RF) coating process and silica coating process in sequence, the SiO2 spheres were converted into SiO2@RF@SiO2 spheres or SiO2@RF@SiO2@RF@SiO2 spheres. The procedures for RF coating and silica coating were carried out according to Ref. [22] and [23]. Subsequently, the as-synthesized SiO2@RF@SiO2 spheres and SiO2@RF@SiO2@RF@SiO2 spheres were further sintered to achieve the intermediates SiO2@C@SiO2 spheres and SiO2@C@SiO2@C@SiO2 spheres under nitrogen atmosphere at 800 C for 2 h with a heating rate of 1.5 C/min. After washing with a 10% HF aqueous solution for one day, the intermediates were converted into HCS and HIHCS, respectively. 2.2. Characterization Product morphologies were observed on a Hitachi S-4800 scanning electron microscopy (SEM) with a field-emission electron gun. Transmission electron microscopy (TEM) images were taken on a TECNAI F-30 high-resolution transmission electron microscope operated at 300 kV. The structure was characterized by X-ray powder diffraction (XRD, Philips Panalytical X-pert) using Cu Ka1 radiation l = 1.5405 Å. Nitrogen adsorption and desorption isotherms were measured at 77 K with a Micrometrics ASAP 3020 system. The specific surface area and pore size distributions of HCS and HIHCS were obtained by the Brunauer-Emmett-Teller (BET) method and the BarrettJoyner-Halenda (BJH) method, respectively.
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2.3. Electrochemical Measurements Two-electrode CR2016 type coin cells were used to evaluate the electrochemical performance of the products. The working electrode was made up by 80wt% active materials, 10wt% Super P Li and 10wt% poly(vinylidenedifluoride) (PVDF). The current collector was copper foil and the counter electrode was Li foil. Celgard 2400 polypropylene membrane was utilized as the separator. The electrolyte was 1 M LiPF6 in a 1:1:1 v/v/v mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) (Zhuhai, China). The cells was assembled in an Ar-filled glovebox with the concentrations of moisture and oxygen below 1.0 ppm. The cells were galvanostatically charged and discharged between 0.01 V and 3.0 V vs. Li/Li+ using a BTS Battery test system (Neware, Shenzhen, China) under various current densities (100 mA g 1, 500 mA g 1, 1 A g 1, 2 A g 1, 5 A g 1, and 10 A g 1). Electrochemical impedance spectrometry (EIS) tests were performed on IM6 (Zahner elektrik) in the frequency range from 100 kHz to 100 mHz. 3. Results and discussion Fig. 1 presents the typical hard-templating procedure for the synthesis of HCS and HIHCS. Firstly, the SiO2 spheres were coated with RF resin and silica to form a sandwich-like SiO2@RF@SiO2 spheres [22]. Secondly, according to the same procedure, SiO2@RF@SiO2@RF@SiO2 spheres were obtained by using the SiO2@RF@SiO2 spheres as the templates. The weight ratio of inner layer RF precursors and outer layer RF precursors in SiO2@RF@SiO2@RF@SiO2 spheres was 1:1. Finally, after the carbonization under N2 atmosphere and the subsequent removal of SiO2, thesynthesized SiO2@RF@SiO2 spheres and SiO2@RF@SiO2@RF@SiO2 spheres were converted into HCS and hollow-in-hollow structured HIHCS, respectively. Generally, SEM and TEM were used to characterize the morphology and structure of HCS and HIHCS. As shown in Fig. 2a, uniform HCS has a well-defined spherical morphology with an average diameter of 300 nm. The TEM image of HCS (Fig. 2b) further indicates that the HCS possesses a uniform hollow structure. The void volume of HCS conforms with the original SiO2 template size (see Supplementary Materials, Fig. S1). Whilst the average thickness of the porous carbon shell in HCS is ca. 30 nm. Additionally, the external surface of HCS is clean and smooth, indicating no impurities left over the carbonization process. High-
Fig. 1. Schematic illustration: the hard-templating procedures for the synthesis of HCS and HIHCS.
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Fig. 2. (a) SEM image and (b, c) TEM images of HCS, (d) SEM image and (e, f) TEM images of HIHCS.
magnification TEM image of HCS (Fig. 2c) further shows that the carbon shell of HCS exhibits a porous network structure. Pore sizes are in the range of several to tens of nanometers, which is beneficial for the transfer and storage of lithium ion (0.60 Å) [11,24]. In contrast, HIHCS shows the similar morphology to HCS with enlarged diameters up to 350 nm (Fig. 2d). According to the Fig. 2e and f, the porous HCS core encapsulated in a hollow carbon form the unique hollow-in-hollow structure of HIHCS. The inner porous HCS core, the outer hollow carbon shell, and the void space between the core and the shell can be clearly distinguished in TEM images of HIHCS. The average thickness of the outer carbon shell in HIHCS is about 15 nm. Since the diameter of the SiO2@RF@SiO2 template is bigger than the SiO2 template and the amount of inter layer RF precursors is the same as that of outer layer, the thickness of the outer carbon shell in HIHCS is thus smaller than that of the inner HCS core. The morphology of intermediates was also
characterized by SEM and TEM (see Supplementary Materials, Fig. S2, 3). The porosities of HCS and HIHCS were measured by Nitrogen adsorption-desorption analysis. As illustrated in Fig. 3, the N2 sorption isotherms with high nitrogen uptake have clearly demonstrated the high porosity of HCS and HIHCS. The Brunauer-Emmett-Teller (BET) surface area of HCS is 1144.8 m2 g 1. The type IV isotherm with a hysteresis loop indicates the existence of mesopores [25,26]. Based on the Barrett-Joyner-Halenda (BJH) method, HCS has a rather narrow pore size distribution centered 12 nm. The N2 sorption isotherm of HIHCS exhibits an analogous shape to that of HCS, indicating that the pore structure of HIHCS is similar as HCS. Compared with HCS, HIHCS has a little higher surface area (1190.1 m2 g 1), total pore volume (1.36 cm3 g 1), and a pore size distribution at 6 nm. The pore structures of HCS and HIHCS determined from N2 adsorption-desorption measurements
Fig. 3. (a) N2 sorption isotherms and (b) pore size distributions of HCS and HIHCS.
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agree well with the TEM observation. With the high specific surface areas, large pore volume, and pore structure, the assynthesized HCS and HIHCS are expected as the potential anode materials for lithium-ion batteries. Galvanostatic discharge (lithium insertion) - charge (lithium extraction) method between 0.01 and 3.0 V was carried out to investigate the lithium storage performance of the HCS and HIHCS anode. The first and second charge/discharge curves of the HCS and HIHCS anode at a current density of 500 mA g 1 are shown in Fig. 4a and b, respectively. Interestingly, the capacity of the HIHCS anode is much higher than that of traditional graphite anode (shown in Fig. 4b). The reversible capacity of HIHCS anode can reach values up to 800 mAh g 1, which suggests the existence of different mechanism from traditional graphite anode. During the initial lithium insertion process, no obvious plateau was observed, implying a low degree of graphitization which agrees with the XRD result (see Supplementary Materials, Fig. S4) [27]. The steep charge/discharge slope is similar to that of typical disordered carbon materials [28,29]. Lithium ions can be stored in not only the disordered graphite layers but also some defect sites and micropores at the carbon surfaces, which increases the reversible capacity of the HIHCS anode. A large irreversible capacity of 1000 mAh g 1 measured in first cycle was also observed, which is common for porous carbonaceous anodes. The reason can be attributed to irreversible lithium insertion into special positions, such as cavities or sites in the carbonaceous materials and the
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formation of solid/electrolyte interface (SEI film) [11,28]. In the following cycles, the Coulombic efficiency (CE) increases and reaches nearly 100%. In contrast, as shown in Fig. 4a, the HCS anode only delivers the reversible capacity of 388 mAh g 1 in the first cycle. The initial coulombic efficiency of HCS (39%) is also lower than that of HIHCS (44%). Fig. 4c shows the cyclic performance of the HCS anode and HIHCS anode at a current density of 500 mA g 1. The HIHCS anode exhibits much higher reversible capacity than the HCS anode. The decrease in capacity during initial cycles is mainly due to the SEI film formation and irreversible lithium insertion. At 100th cycle, a high reversible capacity of 555 mAh g 1 is retained for the HIHCS anode. More importantly, from 10th cycle to 100th cycle, the discharge capacity of HIHCS maintains nearly constant, indicating the excellent cyclic stability. After the initial cycles, the Coulombic efficiency was almost equal to 100% during the measurement. In comparison, the discharge capacity of HCS was only 271 mAh g 1 (half capacity of HIHCS anode) after 100 cycles in the same measure condition. The rate performance of the HCS and HIHCS anode is shown in Fig. 4d. The HIHCS anode can deliver the discharge capacities of 481 and 304 mAh g 1 upon increasing the current densities to 1 A g 1 and 5 A g 1, respectively. Even the current density increases to a very high rate of 10 A g 1, a capacity of 236 mAh g 1 is still maintained. To our knowledge, this result is much better than that of traditional graphite electrode and many other similar carbon
Fig. 4. 1st and 2nd cycle discharge/charge curves of (a) HCS anode and (b) HIHCS anode at a current density of 500 mA g 1. (c) Cycling performances of HCS anode and HIHCS anode at a current density of 500 mA g 1. (d) Rate capabilities of HCS anode and HIHCS anode.
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in-hollow structure of HIHCS. With large surface area and total pore volume, HIHCS is a potential electrode material for high capacity and high rate lithium-ion batteries. As a result, HIHCS exhibits favorable capacity, superior rate and cyclic performance. A high reversible capacity (800 mAh g 1) and excellent rate capability (236 mAh g 1 at the current density of 10 A g 1) was achieved. The excellent electrochemical performance of the HIHCS anode could be derived from the unique hollow-in-hollow structure and high specific surface area. We believe that the hollow-in-hollow carbon structure presented here can also provide scope for further optimization of low-conductivity electrode materials such as Sn, Si, transition metal oxide, and sulfur. Acknowledgements
Fig. 5. Long cyclic performance of the HIHCS anode at a current density of 5 A g 1.
anodes [6–9]. In addition, after 40 cycles discharging/charging at different current densities, the reversible capacity can return to 800 mAh g 1 when the current density goes back to 100 mA g 1. These results indicate the excellent rate performance at high current density. Compared with the HIHCS anode, the discharge capacities of the as-synthesized HCS anode are only 301, 258, 200, and 157 mAh g 1 at the current densities of 1 A g 1, 2 A g 1, 5 A g 1 and 10 A g 1, respectively. The reason for excellent rate performance can be investigated by electrochemical impedance spectra (EIS) measurements (see Supplementary Materials, Fig. S3). It is well-known that the semicircle at high frequency correspondes to the charge transfer reaction at the electrolyte/electrode interface while the inclined line at low frequency was attributed to the lithium-diffusion process [30,31]. Apparently, the diameter of semicircle of the HIHCS anode was smaller than that of HCS electrode, indicating that the HIHCS anode possesses a favorable transport kinetict for both lithium ions and electrons [32]. To further investigate the long-term cycling stability, the HIHCS anode was tested for 500 cycles at a high rate of 5 A g 1. As shown in Fig. 5, after activated for initial 10 cycles, the HIHCS anode can be stabilized at a specific reversible capacity of 240 mAh g 1. At the same time, the Coulombic efficiency of the HIHCS anode is maintained at around 99%. All these results clearly indicate that the hollow-in-hollow structure can provide a favorable route to achieve high-performance carbon anode material for lithium ion batteries. Compared with the single-shelled hollow structure, the hollow-in-hollow structure of HIHCS guarantees: i) much special cages created by the hollow-in-hollow structure, in which the Li+ can be stored reversibly, resulting in more capacity capability; ii) a short lithium diffusion distance and large electrode/electrolyte contact area, resulting in superior rate capability. More importantly, such hollow-in-hollow structure could not only provide high gravimetric energy densities and excellent cycling performance but also exhibit higher volumetric energy densities compared to traditional hollow structure counterpart [19,33]. In addition, the large specific surface area and pore volume of the hollow-in-hollow structure can also accommodate extra lithium ions at the surface upon discharge/charge cycling. 4. Conclusions In summary, hollow-in-hollow structured carbon spheres (HIHCS) were successfully synthesized by a facile hard-templating method. The inner HCS core, the outer hollow carbon shell, and the void space between the core and the shell constitute the hollow-
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