Three-dimensional porous carbon nanosheet networks anchored with Cu₆Sn₅@carbon as a high-performance anode material for lithium ion batteries

Abstract

The poor cycling stability resulting from large volume change is the major obstacle to the application of tin-based anode materials. In this paper, three-dimensional porous carbon nanosheet networks anchored with Cu₆Sn₅@carbon nanoparticles (10–35 nm) as a high-performance anode for lithium ion batteries are synthesized via a self-assembly NaCl template-assisted in situ chemical vapor deposition strategy. The composite exhibits superior rate capability (523, 443, 395, 327, 281, and 203 mA h g⁻¹ at 0.2, 0.5, 1, 2, 5, and 10 A g⁻¹, respectively) and excellent cycling stability (396.8 mA h g⁻¹ at 1 A g⁻¹ for the first cycle and maintains 92.3% after 200 cycles). The superior performance is attributed to the unique architecture: inactive metal copper serves as a “buffer matrix” and relaxes the large volume change of the tin; a uniform distribution of nano-sized Cu₆Sn₅ makes the inevitable stress/strain small, meanwhile it provides a short path for lithium ion diffusion; onion-like carbon shells not only prevent the Cu₆Sn₅ nanoparticles from agglomerating and growing but also offer mechanical support to accommodate the stress associated with the volume change of tin upon cycling, thus alleviating pulverization; 3D porous carbon nanosheet networks ensure the mechanical integrity and facilitate lithium ion diffusion as well as electron transportation.

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Introduction

Tin is considered to be a suitable candidate for anode materials of next generation Li-ion batteries (LIBs) due to its high theoretical gravimetric capacity (994 mA h g⁻¹) and volumetric capacity (7246 mA h cm⁻³), compared with the commercial graphite anode¹ (372 mA h g⁻¹ and 837 mA h cm⁻³). However, its large volume change (up to 260%) associated with the insertion and extraction of lithium leads to electrode pulverization, resultant loss of contact with the current collector, severe particle aggregation, and continual formation of a very thick solid electrolyte interphase (SEI) on the Sn surfaces upon cycling, thereby resulting in rapid capacity fading and poor cycling stability.^2,3

Recently, extensive efforts have been made to address the issues posed by the volume change of Sn and to improve its electrochemical performance. The three most effective approaches are summarized as follows. First, reducing Sn particle size to nanoscale can smaller stress/strain over the entire electrode during lithiation/delithiation and prevent local cracking. In this context, Sn-based nanomaterials with different nanostructures, including nanoparticles,^4,5 nanowires/nanotubes,^6,7 nanosheets/nanoplates,^8,9 and 3D nanoarchitectures^10,11 have been explored and their electrochemical performances are apparently superior to that of the bulk materials. Second, dispersing nano-sized Sn into a conductive matrix (such as carbon materials) can accommodate volume change, maintain mechanical integrity, and enhance conductivity of the composite electrode. So nanostructured Sn–C composites with Sn nanoparticles uniformly dispersed in a supporting matrix have been synthesized to enhance the electrochemical performance.^12–17 For instance, Bruno Scrosati reported that nanostructured Sn–C composite delivered a specific capacity of 500 g⁻¹ and remained stable over more than 200 cycles.¹² Recently, we reported a graphene network anchored with Sn@graphene synthesized by in situ chemical vapor deposition (CVD) technique, and it demonstrated excellent electrochemical performance.¹⁴ Third, using intermetallic alloy containing both active and inactive elements can relieve volume change.¹⁸ In such alloy anodes, active metal reacts with lithium to increase capacity, and inactive metal serves as “buffer matrix” and relieves the volume change of the active phase to some extent. Han synthesized monodisperse nanospheres of intermetallic M–Sn (M = Fe, Co, Cu, and Ni) phases with improved electrochemical performance.³

In summary, significant improvements in electrochemical performances of Sn anode have been achieved by above modifications. However, any single strategy cannot overcome the volume change problem completely. It is expected a method that integrate those three approaches could construct a stable uniform composite structure which formed by nano-sized Sn-based alloy uniformly embedded in a conductive carbon matrix will alleviate volume change effectively and achieve an excellent cycling stability. Among all the alternative metals that can form intermetallic alloy with Sn, such as nickel,^19–21 stibium,^22–24 iron,^25–27 cobalt,^28–30 and copper,^31–35 copper is remarkable because of its high conductivity and elasticity. Xia reported a reduced graphene oxide (RGO)/Cu₆Sn₅ composite in which nano-sized Cu₆Sn₅ was sandwiched between multilayer graphene sheets. The composite with graphene network displayed better electrochemical performance than the Cu₆Sn₅ nanoparticles anode.³⁶ Yang also fabricated graphene/Cu₆Sn₅ nanocomposite anode with enhanced electrochemical performance.³⁷ Unfortunately, the Cu₆Sn₅ in the as-prepared samples exhibit a disordered morphology, size, and distribution, which is largely impeded by the synthesis process and very difficult to be tailored. Moreover, both of two methods are very costly, time and energy consuming due to the tedious preparation of reduced graphene oxide in advance.

Obviously, intermetallic alloy, nanoparticle size, uniform distribution, and porous conductive network structure are the key factors for tin-based materials to achieve high electrochemical performance. However, it is a challenge to fabricate such tin-based composite with all the above features and the related report is rare. Herein, we report a facile and scalable strategy to synthesize such composite of 3D porous carbon nanosheet networks anchored with Cu₆Sn₅@carbon (3D PCNNWs-Cu₆Sn₅@C) as an advanced anode material for high performance LIBs. 3D porous carbon nanosheet networks are synthesized via a simple freeze-drying process using the self-assembly cubic NaCl salt as a sacrificial template and Cu₆Sn₅@carbon with 10–35 nm in diameter are homogeneously embedded in the 3D porous carbon networks. It is expected that the unique structure will display an outstanding electrochemical performance including large reversible capacity, high coulombic efficiency, excellent cyclic performance, and good rate capability.

Experimental section

Synthesis of 3D PCNNWs-Cu₆Sn₅@C

The synthesis of 3D PCNNWs-Cu₆Sn₅@C consisted of solution preparation, freeze drying, and CVD process. All the reagents were purchased from Sinopharm Chemical Reagent Co.,Ltd and used without further purification. Citric acid (2.5 g), SnCl₂·2H₂O (0.185 g), Cu(NO₃)₃·9H₂O (0.234 g) and NaCl (20.7 g) were dissolved in deionized water (75 mL) by magnetic stirring. The obtained solution was frozen in a refrigerator at −20 °C for 24 h. After that the resulting gel was put in a freeze drier to eliminate the water completely and then was ground to a fine composite powder. Then 20 g composite powder was put in a quartz boat, annealed at 750 °C for 2 h under mixed gas H₂ and Ar (1 : 3), and cooled to room temperature under the protection gas. Finally, the as-synthesized product was washed with deionized water several times to remove NaCl, and then pure 3D PCNNWs-Cu₆Sn₅@C was obtained. For comparison, Cu₆Sn₅ particles embedded in amorphous carbon (Cu₆Sn₅/carbon composite) were also prepared at the same conditions by carbonizing the mixture of Cu(NO₃)₃·9H₂O, SnCl₂, and citric acid without NaCl as template.

Physical characterization

XRD pattern was obtained from an X-ray diffractometer (Rigaku Smartlab) with Cu Kα radiation (λ = 0.15406 nm) in the 2θ range from 10° to 80°. The morphology was examined by a field emission scanning electron microscope (SEM, ZEISS SUPRA55) and a high-resolution transmission electron microscope (HRTEM, JEM 2100F). Nitrogen adsorption and desorption isotherms of the as-prepared material were measured at 77 K using an autosorb iQ instrument (Quantachrome U.S.). The total surface area was calculated by the Brunauer–Emmett–Teller (BET) method, and the pore size distribution was calculated by the Density Functional Theory (DFT) method based on the adsorption and desorption data. The content of Cu₆Sn₅ in the composite was measured by the thermogravimetry and differential thermal analysis (TG/DTA) (Hengjiu Instrument Co., Ltd., HCT-2, Beijing, China) from room temperature to 1000 °C at a rate of 10 °C min⁻¹ in air. A micro-Raman spectrometer (Renishaw, InVia microscope) with a 532 nm laser was used in the Raman study to analyze the characteristics of carbon in the composite.

Electrochemical measurements

To evaluate the electrochemical performance of 3D PCNNWs-Cu₆Sn₅@C and Cu₆Sn₅/carbon composite, the composites were assembled in a CR 2032-type coin cell. First, the synthesized materials were mixed with carbon black and polyvinylidene fluoride in N-methyl-2-pyrrolidinone solvent with a weight ratio of 80 : 10 : 10. After stirring for 4 h, the slurry was coated on a copper foil, dried at 120 °C overnight in a vacuum oven and cut into a wafer with area of 0.785 cm², the loading amount is about 1.5–2 mg cm⁻² for the electrode, coin cell was assembled in an argon-filled glove box where the lithium plays as the counter electrode and reference electrode, Celgard 2300 polypropylene film as separator and 1 M LiPF₆ dissolving in ethylene carbonate/ethyl methyl carbonate/dimethyl carbonate (1 : 1 : 1 in volume) as electrolyte. Galvanostatic charge/discharge tests were performed using a battery test system (CT2001A, Wuhan LAND Electronic Co., Ltd., China) in a voltage range of 0.005–3 V with different current densities. The cyclic voltammetry (CV) test was conducted by an electrochemical workstation (Solartron 1260 + 1287) between 0.005 and 3 V versus Li⁺/Li at a scan rate of 0.1 mV s⁻¹ at room temperature. The electrochemical impedance spectroscopy (EIS) measurement was performed through the same electrochemical workstation with an amplitude voltage of 5 mV and frequency range of 10 mHz–100 kHz.

Results and discussion

The fabrication process of 3D PCNNWs-Cu₆Sn₅@C is schematically depicted in Fig. 1. The entire fabrication process consists of three main steps. Firstly, the raw materials citric acid, SnCl₂·2H₂O, Cu(NO₃)₃·9H₂O and NaCl in a certain proportion were dissolved in deionized water to form solution. Herein, SnCl₂·2H₂O and Cu(NO₃)₃·9H₂O were used as the precursor to form Cu₆Sn₅ alloy. Citric acid, a weak organic acid containing three carboxyl groups, formed complexation with metal ions and inhibited the hydrolysis of Sn²⁺ and Cu³⁺; meanwhile, it is used as precursors to synthesize three-dimensional porous carbon nanosheet networks through dehydration and carbonization. Secondly, the solution was freeze dried. During this process, the NaCl grew into cubes and further self-assembled into three-dimensional structure, the mixture (citric acid, SnCl₂·2H₂O, Cu(NO₃)·9H₂O) covered on the surface of the self-assembled 3D NaCl. Finally, the precursor reacted under chemical vapor deposition (CVD) conditions and 3D macroporous graphene networks anchored with Cu₆Sn₅ nanoparticles directly grown on the scaffold of self-assembled NaCl template and then the NaCl skeleton was removed by washing with water, which left the free-standing 3D PCNNWs-Cu₆Sn₅@C, as displayed in Fig. 1 and 2d–i.

Fig. 1 Schematic representation of the fabrication process of 3D PCNNWs-Cu₆Sn₅@C.

Fig. 2 (a) XRD pattern, (b) nitrogen adsorption–desorption isotherm, (c) pore size distribution profile, (d–f) SEM images of 3D PCNNWs-Cu₆Sn₅@C, and (g–i) SEM images of the CVD synthesized products before eliminating NaCl.

X-ray diffraction (XRD) measurement was performed to determine the crystal structure of the as-prepared product (3D PCNNWs-Cu₆Sn₅@C). As shown in Fig. 2a, all the peaks in the pattern can be well aligned with a monoclinic Cu₆Sn₅ crystal structure (JCPDS no. 45-1488), and no other characteristic peaks of Sn, Cu or SnO₂ is observed in XRD pattern, which implies complete formation of the targeted alloy (Cu₆Sn₅) from SnCl₂ and Cu(NO₃)₂ by annealing under the protection gas according to the Cu–Sn phase diagram (Fig. S1†). And the sharpness of the diffraction peaks indicates that the Cu₆Sn₅ phase in the composite is well crystallized. The average particle size of Cu₆Sn₅ calculated from the largest low-angle diffraction peak (221̄) according to the Scherrer equation is about 25.9 nm, which is well consistent with the average diameter (24 nm) revealed by the TEM analysis. To determine the content of Cu₆Sn₅ in the 3D PCNNWs-Cu₆Sn₅@C composite, TG–DTA was carried out in air at a heating rate of 10 °C min⁻¹ (Fig. S2†). The sample was heated to 1000 °C and kept for 30 min so that Cu₆Sn₅ alloy could be oxidized to CuO and SnO₂, and carbon could be oxidized to CO₂ completely. According to the eqn (S1) (ESI†), the Cu₆Sn₅ is calculated to be 46.4% based on the remaining weight (CuO and SnO₂) and the TG–DTA curves.

The morphology of the 3D PCNNWs-Cu₆Sn₅@C was investigated by scanning electron microscopy (SEM). As shown in Fig. 2d–f, the composite exhibits a unique interconnected 3D porous network appearing as a foam-like structure and many Cu₆Sn₅ nanoparticles with uniform particle size (10–35 nm) were homogenously anchored on the walls of porous carbon framework. The unique 3D porous carbon networks were obtained by using NaCl as a sacrificial template, which was demonstrated in our previous work.¹⁴ In Fig. 2d–i, we compared the SEM images of the CVD-synthesized product before and after eliminating NaCl, and found that the self-assembled 3D NaCl structure acts as a template for directing synthesis of the 3D porous carbon networks as well as plays a role of preventing the growth and aggregation of Cu₆Sn₅ nanoparticles during the CVD process, resulting in the formation of small and uniform Cu₆Sn₅ nanoparticles with high catalytic activity. The Cu₆Sn₅ nanoparticles can catalyze the growth of onion-like carbon shells around them, as demonstrated in the TEM characterization. After washing with water to remove the NaCl, the as-prepared sample is supposed to be a close-packed uniform cubic empty “cells” of 1–2 μm in side length, which consists of carbon nanosheets anchored with Cu₆Sn₅ nanoparticles. However, it is obvious that the close-packed uniform the cubic empty “cells” transformed to irregular interconnected 3D porous networks after drying because the ultrathin carbon walls turned into curved caused by van der Waals forces during drying (see Fig. 2d–f). Furthermore, the size of the interconnected macropores was approximately the same as the size of cubic NaCl particle, which confirms the role of NaCl as the template for growing 3D porous structure. Such an interconnected 3D porous network is beneficial for penetration of electrolyte and mechanical stability of the electrode.

Nitrogen adsorption–desorption measurement was carried out to further investigate the porous structure of 3D PCNNWs-Cu₆Sn₅@C, as illustrated in Fig. 2b. The as-prepared sample exhibits a type-IV isotherm with a large hysteresis loop, which can be ascribed to a mesoporous structure with a large amount of uniform pores, as reported previously.³⁸ The Brunauer–Emmett–Teller (BET) surface area of the 3D PCNNWs-Cu₆Sn₅@C is measured to be 396 m² g⁻¹. Pore size distribution was calculated by the Density Functional Theory (DFT) method based on the adsorption and desorption data, as displayed in Fig. 2c. The total pore volume is 0.576 cm³ g⁻¹ and a uniform micropores distribution with an average size of 1.2 nm is determined by DFT analysis curve (inset of Fig. 2c). The micropores maybe come from the carbon nanoshells outside Cu₆Sn₅ nanoparticles. Furthermore, a broad peak at around 3.8 nm is found for the 3D PCNNWs-Cu₆Sn₅@C, corresponding to the mesopores among the clusters (Fig. 2c). The above results demonstrate the 3D carbon networks have a large number of pores and a high surface area. Porous structure is favourable to electrolyte ions diffusion and large surface area provides more active sites for Li⁺ storage. Meanwhile, 3D carbon networks can offer more freedom space to accommodate the large volume change of Cu₆Sn₅ nanoparticles during insertion and extraction of lithium ion.

To better investigate the microstructure and morphology of 3D PCNNWs-Cu₆Sn₅@C, the as-obtained sample was further observed using TEM and HRTEM. Fig. 3a and b presents the typical TEM images of 3D PCNNWs-Cu₆Sn₅@C, it is obvious that the sample exhibits a 3D porous ultrathin architecture with many black Cu₆Sn₅ nanoparticles homogeneously embedded in the carbon nanosheets. The nanosheets exhibit a curved characteristic with a low contrast, implying ultrathin carbon nanosheets (less than 30 nm). Particle size analysis (Fig. 3c) displays a narrow distribution and an average diameter of 24 nm. The lattice fringe in the HRTEM image (Fig. 3f) reveals a clear lattice spacing of 0.29 nm, in good agreement with that of the (221̄) plane of Cu₆Sn₅ (JCPDS 45-1488). The scanning TEM elemental mapping (Fig. 3g) further remarkably reveals that the alloy elements (Cu and Sn) are uniformly distributed in the carbon nanosheets. What's more, these Cu₆Sn₅ nanoparticles are entirely encapsulated in thin onion-like carbon shells on the graphitic carbon nanosheets (Fig. 3d). The interplanar spacing for the shells is 0.34 nm, corresponding to (002) planes of graphite. The ordered graphitized carbon nanoshells outside the Cu₆Sn₅ seems to be induced by the Cu₆Sn₅ nanoparticles during the heating process. As a result, carbon nanoshells can prevent Cu₆Sn₅ nanoparticles from agglomerating and growing, as well as provide mechanical support to accommodate the stress associated with the volume change of nano-Sn upon prolonged cycling, thus it can alleviates electrode pulverization. Interestingly, when the sample was prepared for TEM observation and suffered from long-time intense ultrasonication, we found only a few of the embedded Cu₆Sn₅ particles were removed by the ultrasonication and some hollow carbon nanocages were remained (Fig. 3e). Most of the Cu₆Sn₅ nanoparticles are still firmly immobilized on the carbon nanosheets (Fig. 3a and b), indicating the strong connection between Cu₆Sn₅@C nanoparticles and carbon matrix. Moreover, when the Cu₆Sn₅ nanoparticles were further observed in detail by HRTEM (Fig. 3d and e), we found that some Cu₆Sn₅ nanoparticles were not a uniform crystallized structure but exhibited a core–shell structure, in which the central part is highly crystallized, while the edge of the particle is amorphous. We inferred that the rapid cooling process during CVD might be the main reason for formation of this crystallized-amorphous core–shell structure of Cu₆Sn₅ nanoparticles.

Fig. 3 (a and b) TEM images of 3D PCNNWs-Cu₆Sn₅@C. (c) Size distribution of Cu₆Sn₅ particles, (d–f) HRTEM of an individual Cu₆Sn₅ nanoparticle, (g) STEM-EDS mapping images, (h) Raman spectrum of 3D PCNNWs-Cu₆Sn₅@C.

Raman spectroscopy was performed to further analyze the carbon material in the composite, as illustrated in Fig. 3h. The spectrum displays a D band at 1355 cm⁻¹ and a G band at 1586 cm⁻¹. It is reported that the D band is ascribed to sp³ carbon and defects such as topological defects, dangling bonds, and vacancies, while the G band is attributed to ordered sp² carbon.^14,39 Therefore, the I_D/I_G ratio represents the disorder degree of carbon material. In the Raman spectrum (Fig. 3h), the I_D/I_G ratio of 3D PCNNWs-Cu₆Sn₅@C is calculated to 0.85, which indicating that the carbon materials in the 3D networks are mainly composed of well-crystallized nanocrystalline graphite, in good agreement with HRTEM investigation. And the graphitic carbon not only enhances the conductivity of the composite but also offers more reversible active sites for lithium ions storage.

In the structure of 3D PCNNWs-Cu₆Sn₅@C, the uniform Cu₆Sn₅ nanoparticles make the inevitable stress/strain become small and uniform, and the copper in the Cu₆Sn₅ alloy can serve as “buffer matrix” to relax the volume change of Sn phase to some extent. The carbon shells confine the active Cu₆Sn₅ alloy within a closed volume so that the capacity fading due to pulverization and agglomeration can be minimized, the electric contact of active Cu₆Sn₅ with carbon nanosheets can be enhanced, and the side reaction for continuous formation of the solid electrolyte interphase (SEI) can be reduced. Furthermore, the assembled 3D carbon networks can facilitate electron transport and improve lithium ion diffusion within the composite.

As mentioned above, the unique structure may endow them with superior lithium ion storage performance. To confirm this supposition, the Li-ion insertion/extraction property of the composite was investigated by CR2032 coin cell. Fig. 4a shows cyclic voltammograms (CV) of 3D PCNNWs-Cu₆Sn₅@C for the initial three cycles at a sweep rate of 0.1 mV s⁻¹ in the voltage range 0.005–3.0 V. Similar to the CV curves of Cu₆Sn₅ alloy previous reported,^31,35 the cathodic part below 0.5 V is mainly ascribed to the stepwise formation process of Li_3.5Sn alloy from Cu₆Sn₅ (corresponding to theoretical capacity 480 mA h g⁻¹), agreeing well with the following discharge curves (Fig. 4b). During discharging at 0.5 V, Li⁺ insert into the crystal structure of Cu₆Sn₅ to form partially lithiated phase Li_xCu₆Sn₅ (eqn (1-1)). Further insertion of Li⁺ into Li_xCu₆Sn₅ leads to the formation of Li₂CuSn at 0.3 V (eqn (1-2)), when the potential (<0.01 V) close to the metal lithium's, the lithiated compound Li₂CuSn transforms into Li_3.5Sn alloy (eqn (1-3)) that is surrounded by Cu matrix. The Cu in the Cu–Sn alloy seems to have no effect on lithiation/delithiation process since it is an inactive metal with Li. As a consequence, the Cu is beneficial to the electrochemical performance of the Cu₆Sn₅ alloy anode for its excellent electrical conductivity and buffering effect, which is demonstrated by the good rate capability and long cycle life observed in the electrochemical tests (Fig. 4c and e). Upon charging, multiple anodic humps located at around 0.56 V and 0.86 V are attributed to the de-alloying process, corresponding to the recovery of Li₂CuSn from Li₇Sn₂ and Cu₆Sn₅ from Li₂CuSn, respectively. The anodic peak at 1.26 V corresponds to the deintercalation of lithium ion from ultrathin carbon nanosheet networks.¹⁶ The overlapping of the curves for the second and third cycles indicates good reversibility of Li⁺ insertion/extraction during charging/discharging process.

xLi + Cu₆Sn₅ → Li_xCu₆Sn₅ (1-1)

(10 − x)Li + Li_xCu₆Sn₅ → 5Li₂CuSn + Cu (1-2)

3Li + 2Li₂CuSn → 2Li_3.5Sn + 2Cu (1-3)

Fig. 4 (a) CV curves of 3D PCNNWs-Cu₆Sn₅@C at a scan rate of 0.1 mV s⁻¹ with voltage range from 0.005 to 3.0 V. (b) Voltage profiles of 3D PCNNWs-Cu₆Sn₅@C at 0.2 A g⁻¹ for the initial three times. (c and d) Rate capability of 3D PCNNWs-Cu₆Sn₅@C and Cu₆Sn₅/carbon composite at various rates from 0.2 A g⁻¹ to 10 A g⁻¹ for 60 cycles. (e) Cycle performance of 3D PCNNWs-Cu₆Sn₅@C and Cu₆Sn₅/carbon composite at a current density of 1 A g⁻¹. (f) Nyquist plots of 3D PCNNWs-Cu₆Sn₅@C and Cu₆Sn₅/carbon composite electrodes.

Fig. 4b presents the charge and discharge curves of 3D PCNNWs-Cu₆Sn₅@C for the initial three cycles at a current rate of 0.2 Ag⁻¹. It exhibits a relatively smooth voltage profile due to the multi-step lithium ion intercalation reactions of Cu₆Sn₅, and the charge and discharge curves are nearly overlapping except for the first discharge curve, which also demonstrates a relatively excellent reversibility of Li⁺ insertion/extraction, in consistent with the CV results (Fig. 4a). As shown in Fig. 4b, the prepared sample delivers a discharge capacity of 893.8 mA h g⁻¹ and a reversible capacity of 504.9 mA h g⁻¹ in the first cycle. The reversible capacity is slightly higher than the maximum theoretical capacity of Cu₆Sn₅ alloy (480 mA h g⁻¹). The exceed capacity may derive from three aspects: (i) Cu₆Sn₅ nanoparticles generate larger surface and interface for lithium storage; (ii) the 3D porous carbon nanosheet networks with high surface area provide more active sites for Li⁺, contributing to higher initial capacity; (iii) the unique structure of 3D PCNNWs-Cu₆Sn₅@C offer high structural stability during the first alloying process.

The cycle performance of 3D PCNNWs-Cu₆Sn₅@C and Cu₆Sn₅/carbon composite were evaluated by galvanostatic charging–discharging in a potential range of 0.005–3.0 V (vs. Li⁺/Li) at a current density of 1 A g⁻¹ (Fig. 4e). The 3D PCNNWs-Cu₆Sn₅@C delivers a reversible capacity of 396.8 mA h g⁻¹ for the first cycle and remains 366.4 mA h g⁻¹ after 200 cycles with an outstanding capacity retention of 92.3%, which demonstrates that its structure is very stable during cycling. In contrast, the Cu₆Sn₅/carbon composite delivers a much lower reversible capacity (∼224 mA h g⁻¹) and exhibits very fast capacity fading during cycling. Moreover, the average capacity value of the 3D PCNNWs-Cu₆Sn₅@C is higher than that of the previous reported Cu–Sn alloy and Cu₆Sn₅–carbon nanostructures tested at the same conditions (Table S1†),^33,36,37 which is closely related to structure features: nano-sized Cu₆Sn₅, homogenous distribution, carbon nanoshells coating, and 3D carbon networks. However, the initial Coulomb efficiency of the prepared sample is only 56.5%, which is undesirable for practical application. Fortunately, the charge–discharge efficiency increases to 96.8% in the third cycle and remains over 98% till 200 cycles, which suggests a highly reversible lithium insertion/extraction associated with fast electron transport in the electrodes. The low initial coulomb efficiency is mainly due to the irreversible processes that the ‘rearrangement’ of the electrode structure and some active sites (disorder structure) become inactive, resulting in dead lithium. To identify if the unique architecture is stable after the repeated cycling, we investigated the morphology and structure of 3D PCNNWs-Cu₆Sn₅@C electrode after 100 electrochemical cycles by TEM, as shown in Fig. S3.† It is clear that the Cu₆Sn₅ nanoparticles do not aggregate after cycling and they are still uniformly and firmly anchored on the surface of carbon nanosheets, which is almost the same as the morphology of the pristine product (Fig. 3b) except for the covered SEI layer. This demonstrates a strong interaction between Cu₆Sn₅ nanoparticles and carbon nanosheets as well as the superior mechanical flexibility of 3D porous carbon nanosheet networks, which can restrain the aggregation of Cu₆Sn₅ nanoparticles and the cracking of the electrode during charge/discharge cycling. As a result, the 3D PCNNWs-Cu₆Sn₅@C electrode displays an excellent cycling stability.

The 3D PCNNWs-Cu₆Sn₅@C anode not only presents a stable cycle performance, but also exhibits an excellent rate capability, as illustrated in Fig. 4c and d and Table 1S.† The reversible capacities at 0.2, 0.5, 1, 2, 5, and 10 A g⁻¹ are 523, 443, 395, 327, 281, and 203 mA h g⁻¹, respectively. When compared with the as-prepared Cu₆Sn₅/carbon composite (Fig. 4c) and some available representative results of the Cu–Sn alloy anodes reported by other researchers (Table S1†), notably, for the 3D PCNNWs-Cu₆Sn₅@C, the achieved rate capability is superior to the Cu₆Sn₅/carbon composite prepared without NaCl template and recent reported Cu–Sn alloy synthesized by complex methods.^{32–37,40,41} This good rate capability may benefit from the combination of the nano-Cu₆Sn₅@carbon with short path for Li⁺ diffusion and 3D carbon nanosheet networks with high electric conductivity.

In order to gain further understanding of the excellent rate performance for the 3D PCNNWs-Cu₆Sn₅@C, the coin cells of 3D PCNNWs-Cu₆Sn₅@C and Cu₆Sn₅/carbon composite after the first cycle were measured by electrochemical impedance spectroscopy (EIS). Fig. 4f shows the Nyquist plots, which are composed of a depressed semicircle in the high-medium frequency region and an oblique straight line in the low frequency region. According to the previous reports,^13,14 the depressed semicircle relates to the charge transfer resistances (R_ct) and the oblique straight line corresponds to Warburg impedance (Z_W) associated with the Li⁺ diffusion in the bulk of anode material. The 3D PCNNWs-Cu₆Sn₅@C electrode presents a smaller high-medium frequency semicircle compared with Cu₆Sn₅/carbon composite, indicating that a good electronic conductivity can be achieved owing to the 3D porous carbon nanosheet networks anchored with nano-sized Cu₆Sn₅@carbon, which results in a superior rate capability, as discussed above (Fig. 4c and d). To sum up, the superior electrochemical performance can be ascribed to the unique structure of 3D PCNNWs-Cu₆Sn₅@C that offers porous structure for electrolyte infiltration, higher surface area for more active sites, copper in Cu₆Sn₅ for relaxing the large volume, nano-sized alloy for shorter path of Li⁺ diffusion, carbon shells outside Cu₆Sn₅ for minimizing the agglomeration, 3D conducting carbon matrix for fast electron transport and more freedom space for volume change, which can keep a stable connection between active material and the current collector and enhance the reaction kinetics.

Conclusions

In summary, we have developed a practical synthetic method to prepare 3D porous carbon nanosheet networks anchored with Cu₆Sn₅@carbon nanoparticles (10–35 nm) that exhibiting an outstanding electrochemical performance for lithium ion battery anodes. Importantly, compared to the previous work, this strategy is easier to tailor and control the component of Cu–Sn alloy, nanoparticle size, uniform distribution, and porous conductive network structure. As a result, such a nanostructured composite exhibits a high reversible capacity (523 mA h g⁻¹ at 0.2 A g⁻¹), excellent rate capability (443, 395, 327, 281, and 203 mA h g⁻¹ at 0.5, 1, 2, 5, and 10 A g⁻¹, respectively) and superior cycling stability (capacity retention attained 92.3% after cycled at 1 A g⁻¹ for 200 times), which could maximum of electrochemical activity of Cu₆Sn₅ and carbon nanosheet networks and highlight the preeminence of the anchoring of Cu₆Sn₅ nanoparticles on 3D porous carbon nanosheet networks. Thus it is a promising anode material for the next generation LIBs with high energy and power density.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grants No. 51404055; No. 51374056; No. 51571054), the Special Fund for Basic Scientific Research of Central Colleges, Northeastern University (No. N142304002; N100123003; N120523001), the Natural Science Foundation of Hebei (No. B2015501020; No. E2013501135), the Youth Foundation of Hebei Educational Committee (No. QN2014325), the program for New Century Excellent Talents in University (No. NCET-10-0304), the Key Technologies R & D Program of Qinhuangdao of Hebei Province (No. 201402B005), and the Doctoral Scientific Research Foundation of Northeastern University at Qinhuangdao, China (No. XNB201511).

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Footnote

† Electronic supplementary information (ESI) available: The Cu–Sn phase diagram, TG–DTA curves of the 3D PCNNWs-Cu₆Sn₅@C composite in air, TEM images of 3D PCNNWs-Cu₆Sn₅@C electrode after 100 cycles with different magnification, and electrochemical performances of 3D PCNNWs-Cu₆Sn₅@C and some representative Cu–Sn alloy anodes in literature. See DOI: 10.1039/c6ra04778e

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