Jieqiong
Shan‡
a,
Yuxin
Liu‡
a,
Ping
Liu
b,
Yanshan
Huang
b,
Yuezeng
Su
*c,
Dongqing
Wu
*b and
Xinliang
Feng
bd
aSchool of Aeronautics and Astronautics, Shanghai Jiao Tong University, 800 Dongchuan RD, Shanghai, 200240, P. R. China
bSchool of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan RD, Shanghai, 200240, P. R. China. E-mail: wudongqing@sjtu.edu.cn
cSchool of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, 800 Dongchuan RD, Shanghai, 200240, P. R. China. E-mail: yzsu@sjtu.edu.cn
dCenter for Advancing Electronics Dresden (cfaed) & Department of Chemistry and Food Chemistry, Technische Universitaet Dresden, Dresden, 01062, Germany
First published on 28th October 2015
A dual-doping approach for nitrogen-doped carbon-coated SnO2–SnS/graphene nanosheets (N–C@SnO2–SnS/GN) has been developed, using hydrothermal carbonization of sucrose with SnO2-decorated graphene and ammonium thiocyanate and a subsequent thermal treatment. The resulting hybrid manifests a typical two-dimensional core–shell architecture, with a N-doped carbon coating over the SnS- and SnO2-decorated graphene nanosheets. Used as the anode material in lithium ion batteries (LIBs), N–C@SnO2–SnS/GN delivers a high specific capacity of 1236 mA h g−1 at a current density of 0.1 A g−1 after 110 cycles, which outperforms most state-of-the-art tin-based LIB anodes with core–shell structures.
Current state-of-the-art nanotechnology allows encapsulation of tin-based anodes in a carbon shell, which can provide both effective protection against pulverization of the electrode materials and rapid pathways for the transportation of charge carriers. Nevertheless, introduction of the carbon shell inevitably decreases the mass ratio of the tin components in the core, often leading to a reduction in performance concerning the weight or volume of the whole electrode. An ideal solution is to dope the carbon shell with heteroatoms such as nitrogen (N) and boron (B), which can significantly enhance conductivity and wettability in an electrolyte.6,26–28 Moreover, heteroatoms in carbon have been demonstrated to create extra active sites for accommodating lithium atoms, which is beneficial for further elevating the capacity of tin-based anodes.6,27–30 On the other hand, combining two different tin-containing anode materials can lead to a synergetic effect on the electrochemical properties of the whole electrode material. The varied phase behavior of different tin species such as SnO2 and SnS provides buffers against volumetric changes of electrodes during the charge/discharge process, thereby leading to stabilized cycling performance.20,21,31,32 Diffusion of lithium can also be improved along grain boundaries of different tin-based anode materials.33 Moreover, the rational design and construction of graphene-based carbon composites as highly efficient electrodes for various electrochemical applications has attracted great interest.34–37
Inspired by these results, an unprecedented dual-doping strategy for the fabrication of nitrogen-doped carbon encapsulated SnO2–SnS/graphene nanosheets (N–C@SnO2–SnS/GNs) is developed in this work, through the hydrothermal carbonization of sucrose in the presence of SnO2 nanoparticle-decorated graphene nanosheets (SnO2/GNs) and ammonium thiocyanate (NH4SCN), along with a subsequent thermal treatment of the resultant hybrid under a flow of nitrogen. During the hydrothermal treatment, decomposition of NH4SCN can release multiple gaseous fragments, including carbon disulfide (CS2), hydrogen sulfide (H2S) and ammonia (NH3), which can serve as dopants for SnO2/GNs.38 Due to different reaction energies, carbon and SnO2 are doped with nitrogen and sulfur atoms, respectively, leading to the formation of dual-doped N–C@SnO2–SnS/GN. As the anode material in LIB, N–C@SnO2–SnS/GN can keep a high capacity of 1236 mA h g−1 at a current density of 0.1 A g−1 for over 110 cycles. Even at a fast charge rate of 3.2 A g−1, a good capacity of 350 mA h g−1 can still be obtained by N–C@SnO2–SnS/GN. Such an outstanding electrochemical performance is superior to most state-of-the-art tin-based LIB anodes with core–shell structures.10,39–41
The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of N–C@SnO2–SnS/GN and C@SnO2/GN (Fig. 2a and b, S1a, S1c and S2a†) indicate that both samples have sheet-like architectures with uniform decoration of NPs, which are inherited from the 2D morphology of SnO2/GN (Fig. S1b and S2b†). The smooth strips without SnO2 NPs can be found on the edges of N–C@SnO2–SnS/GN and C@SnO2/GN (Fig. 2c and S2c†), suggesting that the SnO2 NPs are enwrapped in the carbon layers. High-resolution TEM (HRTEM) images (Fig. 2d and e) display the existence of two distinct particle morphologies in N–C@SnO2–SnS/GN. The lattice spacings of the two domains in Fig. 2f are 0.33 and 0.29 nm, corresponding to the (110) plane of SnO2 and the (101) plane of SnS, respectively.19,42 Strongly distinct from N–C@SnO2–SnS/GN, C@SnO2/GN only exhibits the lattice spacing for SnO2 (Fig. S2d†). The atomic force microscopy (AFM) images (Fig. 2f and S3†) suggest that the thickness of SnO2/GN and N–C@SnO2–SnS/GN are about 12 and 26 nm, respectively. These results unambiguously confirm that an additional carbon layer with a uniform thickness of about 7 nm has been successfully introduced on both sides of SnO2/GN, in agreement with the TEM results (Fig. 2c).
In the X-ray diffraction (XRD) patterns of C@SnO2/GN (Fig. 3a) and N–C@SnO2/GN (Fig. S4†), five diffraction peaks with 2θ value as 26.61°, 33.89°, 37.94°, 51.78°, and 65.94 °can be assigned to the (110), (101), (211), (200), and (301) faces of rutile SnO2 (JCPDS no. 41-1445).42 In contrast, the additional narrow peaks in the XRD patterns of N–C@SnO2–SnS/GN are attributed to the SnS phase (JCPDS no. 65-3766),19 which suggests the existence of both crystallized SnO2 and SnS NPs in N–C@SnO2–SnS/GN. According to the thermogravimetric analysis (TGA) results (Fig. 3b), the weight content of carbon shells in N–C@SnO2–SnS/GN is ∼34 wt%. As indicated by the Raman spectra of C@SnO2/GN and N–C@SnO2–SnS/GN (Fig. 3c), both samples exhibit two evident peaks at ∼1344 and ∼1584 cm−1, which can be attributed to D (disordered) and G (ordered) bands of carbon, respectively.43 The intensity ratios of D and G bands (ID/IG) of C@SnO2/GN and N–C@SnO2–SnS/GN are 0.70 and 0.83, respectively. The higher ID/IG value of N–C@SnO2–SnS/GN could be ascribed to defects introduced in the carbon shell by nitrogen doping.44 As indicated by their nitrogen adsorption and desorption isotherms (Fig. 3d), the Brunauer–Emmett–Teller (BET) specific surface area of N–C@SnO2–SnS/GN is 129 m2 g−1, which is much higher than that of C@SnO2/GN (52 m2 g−1), presumably owing to the pores and structural defects generated during the thermal decomposition of NH4SCN.38
The chemical compositions of C@SnO2/GN, N–C@SnO2/GN and N–C@SnO2–SnS/GN were further probed by X-ray photoelectron spectroscopy (XPS). The XPS survey spectra of N–C@SnO2–SnS/GN indicate the presence of C, N, O, S, and Sn elements (Fig. S5a†). The C 1s spectra of N–C@SnO2–SnS/GN (Fig. 4a) display four fitted peaks at 284.7, 285.5, 286.7 and 288.1 eV. The high intensity of the peak at 284.7 eV suggests that the carbon atoms of N–C@SnO2–SnS/GN predominantly exist in CC configuration. Additionally, the strong peaks at 285.5 and 286.7 eV reveal that nitrogen has been doped in the carbon shell and bonded together with carbon as sp2 C–N or sp3 C–N. The peak at 288.1 eV indicates the presence of C–O configuration, confirming the existence of oxygenated functional groups of GO; nevertheless, the low intensity of this peak testifies that most GO has been reduced into graphene.12,45 The N 1s spectra of N–C@SnO2–SnS/GN (Fig. 4b) can be assigned to pyridine N (398.3 eV, 43%), pyrrole N (400.1 eV, 45%), and quaternary N (401.1 eV, 12%), respectively.29,45–47 The S 2p spectra of N–C@SnO2–SnS/GN (Fig. 4c) only show S 2p3/2 peak at 162.1 eV, which can be ascribed to the presence of SnS.48–50 Based on the results from XPS analysis, the contents of N and S in the N–C@SnO2–SnS/GN are calculated to be 1.6 at% and 1.8 at%, respectively. Similarly, the amount of N in N–C@SnO2/GN is also determined as 1.6 at% by its XPS results (Fig. S5b†). In the XPS spectra of C@SnO2/GN, the binding energy (BE) peaks of Sn 3d3/2 (496.0 eV) and Sn 3d5/2 (487.6 eV) should be derived from SnO2 NPs.49 In contrast, an obvious shift of the BE peaks of Sn 3d3/2 and Sn 3d5/2 can be observed in the XPS spectra of N–C@SnO2–SnS/GN, confirming the existence of both Sn4+ and Sn2+ in the hybrid (Fig. 4d).47–51
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Fig. 4 (a) C 1s XPS spectra of N–C@SnO2–SnS/GN; (b) N 1s XPS spectra of N–C@SnO2–SnS/GN; (c) S 2p XPS spectra of N–C@SnO2–SnS/GN; (d) Sn 3d XPS spectra of C@SnO2/GN and N–C@SnO2–SnS/GN. |
As shown in the FTIR spectra of N–C@SnO2–SnS/GN (Fig. S6†), the evident peaks at 1220 and 1380 cm−1 can be assigned to the stretching vibrations of C–N and N–CH3,52 respectively. The bands around 1600 cm−1 indicates the existence of CC stretching vibration.53 The absorbance at ca. 650 cm−1 is attributed to the out-of-plane N–H deformation vibration.54 Additionally, the N–H symmetric stretching vibration appears at ca. 3418 cm−1.55 The FTIR analysis clearly demonstrates the existence of C–N, N–H, and C–O56 groups in the N-doped carbon, which agrees well with the XPS results.
The core–shell structure and high heteroatom content of N–C@SnO2–SnS/GN make it an appealing candidate for the anode material of LIBs. To determine the lithiation/delithiation mechanism of N–C@SnO2–SnS/GN, it was assembled as coin cells and tested by a cyclic voltammetry (CV) experiment at a scanning rate of 0.1 mV s−1 in the voltage range of 0.01–3 V vs. Li+/Li (Fig. 5a). During the initial cathodic scan, the first small peak at 1.1 V can be ascribed to the irreversible decomposition of SnS to Sn and Li2S [eqn (1)].17,19–21 It should be note that the inert phase of Li2S can effectively buffer the volume expansion of LixSn (0 < x ≤ 4.4) alloys during the lithiation/delithiation process, which thus is expected to endow N–C@SnO2–SnS/GN with good cycling stability.20,21,31,32 The other three cathodic peaks at 0.7, 0.12, and 0.01 V can be associated with the formation of Li2O [eqn (2)],49–51 LixSn alloys [eqn (3)] and lithium-inserted carbonaceous materials LixC [eqn (4)],57–59 respectively. Accordingly, in the anodic processes, oxidation peaks at 0.12, 0.52, and 1.3 V can be attributed to the extraction of lithium from carbonaceous materials, reversible de-alloying of LixSn, and partial transformation of Li2O to SnO2,57–59 respectively. Furthermore, the small anodic peak at 1.9 V in Fig. 5a is produced by transformation from Li2S to polysulfides.60,61 The shape of the CV profiles from N–C@SnO2–SnS/GN in the following cycles does not change significantly, which implies the completion of the irreversible reactions and the formation of stable SEI layers on the surface of the electrode.
SnS + 2Li+ + 2e− → Sn + Li2S | (1) |
SnO2 + 4Li+ + 4e− → Sn + 2Li2O | (2) |
Sn + xLi+ + xe− ↔ LixSn (0 < x ≤ 4.4) | (3) |
C + xLi+ + xe− ↔ LixC | (4) |
Correspondingly, the profile of N–C@SnO2/GN (Fig. S7†) strongly resembles that of N–C@SnO2–SnS/GN, except for the absence of a cathodic peak at 1.1 V and an anodic peak at 1.9 V, which indicates the lack of a SnS phase in N–C@SnO2/GN.
The electrochemical behavior of N–C@SnO2–SnS/GN was further investigated by galvanostatic discharge–charge measurements at a current density of 0.1 A g−1 within a voltage window of 0.01–3 V (Fig. 5b). Initial discharge and charge capacities of the N–C@SnO2–SnS/GN electrode are 1970 and 1305 mA h g−1, respectively, corresponding to a columbic efficiency (CE) of 66.3%. By contrast, N–C@SnO2/GN (Fig. S8†) and C@SnO2/GN (Fig. 5c) offer much lower discharge and charge capacities (1670 and 1029 mA h g−1 for N–C@SnO2/GN and 1543 and 903 mA h g−1 for C@SnO2/GN) in the first cycle, with CEs of 61.6% and 58.5%, respectively. As for SnO2/GN, it delivers initial discharge and charge capacities of 1823 and 1257 mA h g−1, with a CE of 68.9%. The capacity loss in the first cycle can be attributed to the formation of SEI film.17,19–21 It is worth noting that the capacities of the three electrodes are even higher than the theoretical capacities of SnO2 and SnS. In fact, the theoretical capacity of SnO2 (782 mA h g−1) is based on the conventional alloying mechanism, in which the reduction reaction of SnO2 [eqn (2)] is assumed to be irreversible.10,51 In this work, the peak located at 1.0–0.4 V in the CV curve of N–C@SnO2–SnS/GN (Fig. 5a) indicates that this reduction process is highly reversible. This implies that all the Sn in the alloy can be oxidized to Sn4+ during delithiation, which will effectively improve the capacity of the SnO2-based anodes over 1236 mA h g−1.27,62 In addition, the coexistence of two different tin-based phases could help to provide buffers against the volumetric changes of electrodes during cycling63 and improve the diffusion of lithium along grain boundaries of the two phases.33 It is also supposed that an organic polymeric/gel like film can be formed on the interface of the core/shell structured electrodes, which can also enhance their capacity via a “pseudo-capacitance”.64–67 Moreover, the 2D carbon shell of N–C@SnO2–SnS/GN could store lithium at the same time and serve as a mini current collector, facilitating electron transfer during lithiation and delithiation process.68 Besides, the nitrogen atoms doping in the carbon shell may provide accessible lithium active sites, as well as enhanced electrical conductivity.69 The combination of the above advantages thus enables N–C@SnO2–SnS/GN electrode to deliver excellent gravimetric capacity and good volumetric capacity (Fig. S9†).
During the cycling performance tests at a current density of 0.1 A g−1, the N–C@SnO2–SnS/GN electrode displayed an ever-rising capacity from the 15th cycle onwards. The increased capacity should be mainly ascribed to slow activation of electrode materials, promoted by gradual penetration of electrolyte through the nitrogen-doped porous carbon shells.65,70,71 After 110 cycles, a high capacity of 1236 mA h g−1 can still be delivered by N–C@SnO2–SnS/GN (Fig. 5c). Under the same conditions, the C@SnO2/GN-based electrode only gives a reversible capacity of 647 mA h g−1 after 110 cycles. In contrast, the capacity of N–C@SnO2/GN falls to 828 mA h g−1 after 12 cycles (Fig. S8†), while the capacity of SnO2/GN drops drastically to 250 mA h g−1 after 30 cycles, highlighting the crucial role of the heteroatom doping and carbon shell on the cycling stability of the electrode materials.
The rate capability of N–C@SnO2–SnS/GN, C@SnO2/GN and SnO2/GN were further evaluated at progressively increased current densities ranging from 0.1 to 3.2 A g−1 (Fig. 5d). Remarkably, the reversible capacities of N–C@SnO2–SnS/GN are about 800, 700, 650, 500, and 350 mA h g−1 at 0.2, 0.4, 0.8, 1.6, and 3.2 A g−1, respectively. Upon resetting the current density to 0.1 A g−1, a stable reversible capacity of 1000 mA h g−1 can be retained by the N–C@SnO2–SnS/GN electrode. In contrast, the capacities of C@SnO2/GN and SnO2/GN at 3.2 A g−1 are only maintained as 50 and 17 mA h g−1, respectively.
The excellent cycling stability and rate capability of N–C@SnO2–SnS/GN can be attributed to both 2D core–shell architecture and dual-doped heteroatoms. The effective combination of N-doped carbon shells and graphene-supported SnO2–SnS NPs in a 2D sheet-like manner greatly facilitates accommodation of the volume variation of the electrode and the rapid transport of electrons and lithium ions. The SEM with elemental mapping images of the N–C@SnO2–SnS/GN electrode after cycling (Fig. S10 and S11†) demonstrate that the nanosheet morphology was preserved.
Additionally, in their electrochemical impedance spectra (EIS), N–C@SnO2–SnS/GN manifests a more depressed semicircle at high frequencies than C@SnO2/GN (Fig. S12a†). According to the equivalent circuit (Fig. S12b†), the contact resistance (Rf) and charge transfer resistance (Rct) of N–C@SnO2–SnS/GN are fitted to be 17.69 and 70.34 Ω, which are significantly lower than those of C@SnO2/GN (44.6 and 229 Ω) (Table S1†). The small resistances explain the outstanding rate capability of N–C@SnO2–SnS/GN. Obviously, N-doped carbon shells in N–C@SnO2–SnS/GN significantly enhance electrical conductivity and structural stability of the composites. Moreover, the incorporation of nitrogen atoms in the carbon frameworks can facilitate the wetting process of an electrolyte and provide extra active sites for lithium storage.6,26–30,46,47,72
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ta06617d |
‡ These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2015 |