Branched Graphene Nanocapsules for Anode Material of Lithium-Ion

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Chemistry of Materials

Branched Graphene Nanocapsules for Anode Material of Lithium-Ion Batteries Chuangang Hu, Lingxiao Lv, Jiangli Xue, Minghui Ye, Lixia Wang, and Liangti Qu*

Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Key Laboratory of Cluster Science, Ministry of Education of China, School of Chemistry, Beijing Institute of Technology, Beijing 100081, P. R. China E-mail: [email protected] Abstract: The promising complex structures of graphene nanocapsules with in-situ formed graphene sheets (GC-Gs) have been generated by partially peeling the multiwalled graphene capsules (MWGCs) with a small size of ca. 15 nm. The abundant edges and defects on the in-situ induced graphene sheets and capsule walls largely favored the lithiation/de-lithiation reaction and resulted in a high Li-ion storage level. Since the surface area loss of GC-Gs during stacking and aggregation is generally avoided due to the branched structures and the active doping atoms (N, S) can be intercalated into the carbon lattices during sample preparation, the unique GC-Gs possess an excellent reversible capacity of 1373 mAh g−1 at 0.5 A g−1 as anode material in lithium-ion batteries. This value is more than three times that of the theoretical capacity of state-of-the-art graphite counterpart, and higher than those of most carbon materials reported to date and even the composites of metal, alloys with carbon materials.

1. Introduction In recent years, the ever-increasing demand for electric or hybrid electric vehicles has largely promoted the development of high-performance electrode materials for lithium-ion batteries (LIBs) 1

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that have high reversible capacity, excellent rate capability and cycling stability.1−3 A considerable amount of research has been focused on exploiting the attractive anode materials including Ge, Sn, Si based composites,4−7 transition metals and their oxides8−12 with theoretical capacities far exceeding that of commercial graphitic anodes (372 mAh g−1).1 However, these materials usually suffer from the low Coulombic efficiency and the decreasing performance during practical cycling operation, resulting from the instability of the solid-electrolyte interphase (SEI) film and a large specific volume change commonly occurred in the host matrix during the discharge/charge process.13 It is one of the key challenges to achieve the high-capacity anode materials with stable response over extended cyclings. Nowadays, attention has been back to the carbon based materials with rationally defined structures and compositions that could combine both good stability and high capacity. Among them, zero dimensional (0D) carbon nanocapsules, 1D carbon nanotubes (CNTs), 2D graphene sheets and their nanocomposites have presented advantageous characteristic as material platform for high performance anodes. Meanwhile, their carbon skeleton could be doped with heteroatoms (e.g., O, N, B, P or S) to further tune the electronic structure14,15 for enhanced lithiation capabilities. It was found that the presence of heteroatoms in the basal plane of graphene can enhance the reactivity and electric conductivity, and hence favours the lithium ions diffusion and increases active sites.16–18 Furthermore, Wu et al. demonstrated electrochemical lithiation possibly occurred at heteroatom centers based on theoretical calculation.19 Although some progresses have been achieved so far, the capacity of such developed anode materials are still far from satisfying.20 The graphitic carbon materials with good electrical conductivity commonly favor the high performance. However, Li-ion storage capacity of crystallized carbon materials is relatively low associated with the few available active sites. The latest theoretical research shows that Li-ion can 2


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hardly stabilize and diffuse in the highly crystallized graphene without defects.20 Introduction of defects while retaining the electrical conductivity of the anode material could be an effective method to afford abundant active sites for electrochemical process, which has been demonstrated by partially unzipping CNTs for efficient oxygen reduction reaction.21 Very recently, it was also revealed that defects in the graphene lattice could act as capture sites for entrapment of lithium ions for high specific capacities and energy densities.22 In this contribution, we develop a promising complex structure of branched graphene nanocapsules with in-situ formed graphene sheets (GC-Gs), which were generated by partially peeling the multiwalled graphene capsules (MWGCs) with a small size of ca. 15 nm through chemical oxidation. The initial graphene capsule (GC) consists of 7~9 layers of graphenes. Its outer walls are peeled into branch like graphene sheets in nanoscale surrounding the whole capsule. During this process, abundant edges and defects on the in-situ induced graphene sheets and capsule walls were yielded. Combining experimental results and density functional theory calculations, Yao et al. proved that defects and edges of graphene nanosheets enhance not only the reactivity of the anode material toward the adsorption of Li ions, but also their diffusion properties as the existence of vacancies and grain boundaries.16,23,24 As the defect density was increased, the maximum capacity was also found to increase.22 The unique structure of GC-Gs has the great potential in favouring of the lithiation/de-lithiation reaction and resulting in a high Li-ion storage level.25 Since the surface area loss of GC-Gs during stacking and aggregation is generally avoided due to the branched structures and the active doping atoms (N, S) can be intercalated into the carbon lattices during sample preparation, plus intact capsule frameworks with ultrathin inner walls of ca. three graphene layers, providing abundant active sites, efficient mass transport as well as large and accessible 3



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electroactive surface, and boosting the capacity and long-time cycling stability of GC-Gs as an advanced anode material in LIBs. The unique branched GC-Gs possess an excellent reversible capacity of 1334 mAh g−1 at 0.5 A g−1 as an anode material in LIBs. This value is more than three times that of the theoretical capacity of state-of-the-art graphite counterpart, and higher than those of most carbon materials reported to date and even the composites of metal, alloys with carbon materials. 2. Experiments Preparation of N,S co-doped graphitic layers coated on Ni seeds (Ni@GCs): In a typical procedure, 20 g of melamine and 3 g of nickel sulfate were added into 20 mL of deionized water to form a homogeneous emulsion with ultrasound and violently stirring for 3 h at room temperature, respectively. The as-formed nickel sulfate adsorbed melamine sample was collected by lyophilization. Subsequently, the power was grinded, and then annealed at 800 °C for 3 h under Ar atmospheres with a ramp rate of 6 °C min−1; finally, the N,S co-doped graphitic capsules grown from Ni seeds were obtained, and were labelled as Ni@GCs. Purification of multiwalled graphitic capsules (MWGCs): The above resulting sample was treated with HCl solution (8 M) to remove redundant Ni contained impurities. Finally, the sample was obtained by repeatedly washing with deionized water and lyophilization, and labeled as MWGCs. Preparation of few-walled N,S co-doped graphitic capsule-graphene complexes (GC-Gs): A moderate oxidation condition was identified to afford oxidized carbon nanocapsules with partially unzipped outer walls. The annealed product was exploited through acid oxidation according to the modified Hummers’ method.26,27 Briefly, Ni@GCs (0.3 g) was added to concentrated sulfuric acid (20 mL) under stirring at room temperature, then sodium nitrate (0.3 g) was added, and the mixture 4


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was cooled to 0 °C. Potassium permanganate (0.9 g) was added slowly under vigorous agitation. Successively, the reaction system was maintained at 30 °C for ca. 40 min. Then, 40 mL of water was added, and the solution was stirred for another 15 min at 70 °C. An additional 150 mL of water was added followed by a slow addition of 8 mL of H2O2 (30 % in wt %). The mixture was filtered and washed with 1:10 HCl aqueous solution (100 mL) for the removal of metal ions followed by repeated washing with deionized water to remove the acid. Finally, the obtained sample was lyophilized and then subjected to Ar atmosphere annealing at 900 °C for 1 h with a ramp rate of 3 °C min−1 to afford the final product of few-walled N,S co-doped graphitic capsule-graphene complexes, which was denoted as GC-Gs. Preparation of graphene nanoplates (GNPs): To optimize the experimental conditions and make sure the carbon based material with high electrochemical properties, controlled experiments carried out at exploited times of 60 min at 30 °C were carried out for comparison. The corresponding samples were labeled as GNPs, respectively. Preparation of GC-Gs sample for TEM measurement after long-term electrochemical test: The cells were disassembled in the glove box after 5000 cycles and the composite electrodes containing the active material was recovered and washed thoroughly with the solvent, diethyl carbonate (DEC) to remove the electrolyte. Then the sample was further washed with ethanol and methanol over 3 times to remove residual Li2O. 3. Results and discussion Our strategy for controllable fabrication of the branched GC-Gs is based on an outer wall peeling approach. Firstly, structure-well-defined capsules of a few layers of graphenes were generated at a high temperature of 800 °C by Ni nucleation induced growth (Figure 1a). Ni2SO4 and melamine 5



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were specifically selected as the Ni precursor and carbon source, respectively. By doing so, S and N heteroatoms could be simultaneously doped into the carbon lattice of the as-prepared graphene capsules with Ni cores (Ni@GCs, Figure 1b). Thereafter, a well-controlled moderate acid oxidation process (Figure S1) was applied to peel the outer walls of nanocapsules into graphene nanosheets, accompanied with the removal of Ni species. The final N and S containing branched GC-Gs complexes were obtained after thermally annealing under Ar atmosphere (Figure 1c). The experimental details were included in the Supporting Information. Figure S2 depicted the morphology and structure of Ni@GCs, revealing the formation of Ni core with graphene shells. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images display the GC-Gs maintained well the capsule structures with a good uniformity of ca. 15 nm in diameter (Figure 2a&b). Each of branched GC-Gs has walls of ca. 3 graphene layers (Figure 2c), much less than that of the initial GCs with ca. 7~9 layers (Figure S3), indicating the moderate oxidation process applied in this study only selectively peeled 4~6 graphene layers. The high-resolution TEM (HR-TEM) image (Figure 2d, Figure S4) verified a number of graphene nanosheets arranging along the outer walls of capsules. FFT image (Figure 2e) derived from the corresponding insert HR-TEM image of GC-Gs reveals its microcosmic structure. The lattice spacing from the two symmetrical diffraction spots was calculated as 0.34 nm, corresponding to the (002) plane of graphite. Additionally, the pore-rich structure was formed on capsule walls during the oxidation process as pointed by arrows in Figure 2d. High-angle annular dark-ﬁeld scanning transmission electron microscope (HAADF-STEM) images (Figure 2f, Figure S5) further confirmed the hollow cavities of GC-Gs, the existence of nanopores in the wall and the attached graphene nanosheets. It was also verified that the C, N and S elements were well dispersed along the whole 6


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capsules as shown by HAADF-STEM mappings (Figure 2g–i). Figure 3a shows the Raman spectrum of GC-Gs. The G-band at 1595 cm−1 is associated with E2g mode, while the D-band located at 1350 cm−1 corresponds to defect induced mode, and heteroatom doping of N, S, O and Ni.28 The defect-rich texture of GC-Gs is evident from the D-band/G-band ratio of 1.27, which is slightly lower than that of GC-Gs before annealing (Figure S6a), indicating the structure of GC-Gs is repaired partly. X-ray diffraction (XRD) pattern (Figure 3b) shows that the representative peak of GO (2θ ≈ 11°) (Figure S6b) has completely disappeared in GC-Gs, and only a broad peak at around 24° assigned to the (002) diffraction plane of hexagonal graphite (JCPDS card no. 41-1487) was left.29 The energy dispersive spectroscopy (EDS) (Figure S7) reveals the sample is mainly composed of C, N, O, S elements with trace of Ni. X-Ray photoelectron spectroscopy (XPS) measurements were performed to further determine the composition and state of the elements (Figure 3c). Apart from the C, N elements from the melamine, the existence of O was mainly derived from the oxidation process (Figure S8). S element was associated with the used NiSO4 and Ni was also detected with a negligible amount of ca. 0.8 at %. Accordingly, the contents of N, S elements were ca. 2.8 at% and 1 at%, respectively. Consistently, the spectrum of C 1s (Figure 3d) can be deconvoluted into four peaks corresponding to the typical “C-C” (284.7 eV), “C-O” (286.8 eV),29 “C-S-C” (283.9 eV)30 and “C-N” (285.8 eV)31 bonds. The successful incorporation of N, S heteatoms into the C backbone was also confirmed by the HR-XPS in Figure 3e&f.28,30,31 As shown in Figure 3e, two peaks positioned at 398.7 and 401.0 eV can be identified for pyridinic N and quaternary N, respectively,31,32 which are formed through substituting a carbon atom by N on edges or defect sites in the plane.20 Similarly, two predominately peaks at 163.8 and 164.9 eV are identified in the S 2p XPS spectrum, which correspond to the covalent “C-S-C” and “C=S” bonds, respectively30,33 (Figure 7



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3f). Upon peeling, the Brunauer-Emmett-Teller (BET) specific surface area increased from 131 m2 g−1 (MWGCs) to 733 m2 g−1 for GC-Gs (Figure S9). Notably, the surface area of GC-Gs is higher than that of typical N doped CNTs (100 m2 g−1) and graphenes (300 m2 g−1),34,35 implying the presence of defect-rich structures in branched GC-Gs. Branched GC-Gs were expected to act as an advanced anode material in LIBs. To ensure the repeatability of the experiment, we prepared more than 10 batches of materials under the same experimental conditions, and randomly selected 3 batches of the samples for the study on their battery performance. At least 12 fabricated batteries were measured for every batch of the sample, and the modest testing data was selected for the performance evaluation. Figure 4a shows the discharge/charge profiles at 0.1 A g-1 (0.27 C). An initial discharge capacity of 2196 mAh g−1 is achieved, and the reversible capacity maintains a high level of ca. 1454 mAh g−1. Although a capacity loss of ca. 30 % is observed because of common irreversible processes such as the trapping of some lithium in the lattice, electrolyte decomposition and the formation of SEI,20,28 the reversible capacity is still ca. 3.9 times that of the theoretical value of graphite (372 mAh g−1, corresponding to LiC6 intercalation recation) and higher than many values reported (

Branched Graphene Nanocapsules for Anode Material of Lithium-Ion

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