Uniform core–shell Cu₆Sn₅@C nanospheres with controllable synthesis and excellent lithium storage performances

Abstract

Metallic tin (Sn) is one of the most promising alternatives to graphite anodes for lithium ion batteries due to its higher theoretical capacity, higher packing density and safer thermodynamic potential, while the huge volume transformation during repeated cycling leads to rapid pulverization and consequently poor capacity retention. This work provides an easy-to-control method to prepare uniform core–shell Cu₆Sn₅@C nanospheres in which Cu@Sn cores (40–50 nm in diameter) are well encapsulated by PANI-derived carbon layers with a thickness of ∼5 nm. The obtained Cu₆Sn₅@C exhibits an excellent cycling ability and good rate capabilities. Both the reversible capacity (518 mA h g⁻¹) after 100 cycles and the initial coulombic efficiency (89.2%) are the highest values in Cu₆Sn₅-based materials. The impressive cycling performance is believed to result from the carbon coating that not only prevents particle agglomeration during the synthesis but also accommodates the vast structural transformation of the Cu₆Sn₅ nanocores during the electrochemical (de)lithiation process, so ensuring good ionic and electronic transport to the core. The effect of synthesis conditions on the composition are also investigated systematically.

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1. Introduction

Low volumetric capacity and safety issues limit the performances of graphite-based anodes in high energy/power applications, such as electric vehicles (EVs) and hybrid electric vehicles (HEVs).^1–4 Metallic tin (Sn) is one of the most promising alternatives to graphite due to its high theoretical capacity (990 mA h g⁻¹, 7200 mA h cm⁻³) (cf. graphite: 372 mA h g⁻¹, 800 mA h cm⁻³), high packing density, and safe thermodynamic potential.^5–8 However, the large specific volume change that occurs during charging and discharging leads to rapid pulverization of the Sn electrode and consequently a decrease of electrical conductivity, resulting in poor capacity retention upon extended cycling.⁹

In order to overcome this problem, an effective approach is to alloy Sn with inactive metals such as Fe,^10,11 Co,^12–17 Cu,^18–21 Ni,^22–25 and Zn^26,27 that do not react as readily with lithium and thus provide a buffer matrix that can absorb the extensive volume expansion and contraction of Sn. In this respect, many efforts have focused on Sn–Cu alloys, which react with lithium at a few hundred milli-volts above the potential of metallic lithium and lithiated graphite electrodes Li_xC₆ (x ≤ 1).^28–30 One of key issues in the synthesis of Cu–Sn alloys is to ensure intimate contact between Sn and Cu and at the same time inhibit the agglomeration of products due to the melting of Sn (and alloying with Cu) during heat treatment.

Ideally, the alloy should be synthesized in a nanostructured form to enhance electronic conduction and shorten the Li-ion migration paths.³¹ Various methods including high energy ball milling,³² direct melting,³³ electro-deposition,^34,35 and carbothermal reduction³⁶ have been used to prepare such Cu–Sn materials. However, it is difficult to prepare small/isolated particles with controlled size and morphology using these synthetic methods. Further, these relatively complicated synthesis routes are often not convenient for scaled-up production. Thus the efficiency of such approaches has been relatively limited. In recent years, electro-less deposition has been used to prepare Sn thin films on Cu substrates, with annealing leading to the formation of Cu–Sn thin film anodes.^37,38 The advantage of this method is that an intimate contact between Sn and Cu can be achieved, regardless of the shape of the Cu substrate.

It is believed that carbon surface modification presents significant advantages to enhance the cycleability of active materials, as it can prevent the aggregation of active particles, accommodate the strain of volume change, and enhance the surface electronic conductivity of such materials.^39,40 For example, Wang et al. found that polyamine (PANI) coating FePO₄ as opposed to the final LiFePO₄ product could effectively restrict the particle growth during the reaction of FePO₄ and CH₃COOLi to form LiFePO₄. PANI decomposed into carbon over the course of the reaction.⁴¹ The resultant LiFePO₄ nanoparticles are coated with conductive carbon nanolayers and exhibits close-to-theoretical capacity and much improved rate capabilities.⁴² This indicates that an intermediate coating can effectively control the size of final particles and in the process form a protective conducting shell around the particle. Similarly, pomegranate-like Sn@C,⁴³ Si@C,^44,45 Sb@C,⁴⁶ and Fe₃O₄@C⁴⁷ hierarchical nanocomposites are also reported recently and exhibited extraordinary performance for Li/Na storage.

Motivated by the above findings, herein we propose a new strategy to prepare uniform core–shell Cu₆Sn₅@C nanospheres. Our strategy involves the electro-less preparation of Cu–Sn powder and the formation of double-shelled Cu@Sn@PANI intermediate compounds by fabricating Sn and PANI double layers around Cu nanoparticles, followed by an annealing step to generate core–shell Cu₆Sn₅@C nanospheres. The role of PANI is thus not only to act as the carbon source, but also to suppress the particle aggregation by isolating the Cu@Sn@PANI intermediate particles, ensuring that there is no growth beyond the nanoscale at each step. Hence, we were able to control fully the synthesis of high quality core–shell Cu₆Sn₅@C nanospheres at the nanoscale level. The Cu₆Sn₅@C nanospheres were electrochemically tested as anodes for Li-ion batteries to verify its applicability. As shown below, the nanospheres demonstrated excellent Li⁺ storage properties over extended (de)lithiation cycling and exhibited high rate capabilities.

2. Experimental

2.1 Materials synthesis

All the reagents were of analytical grade and used without further purification. The Cu–Sn precursor powder was synthesized by electro-less deposition. The synthesis procedure of Cu–Sn@C nanospheres can be schematically illustrated in Scheme 1 and include three main steps: (i) the electro-less deposition of Sn on Cu nanoparticles, (ii) the polymerization of AN molecules on Cu@Sn surface to obtain Cu@Sn@PANI precursors, and (iii) the formation of Cu–Sn alloy cores and the carbonization of PANI shells via heating at 300 °C in an inert atmosphere.

Scheme 1 Schematic of the preparation of the core–shell Cu₆Sn₅@C nanospheres.

In a typical synthesis, the plating solution was prepared using reagent grade chemicals containing stannous sulfate for as a source of Sn²⁺ ions (SnSO₄ 0.32 g L⁻¹), citric acid for buffering the solution, sulphuric acid for pH adjustment to pH = 1.0, which was monitored by pH meter (Shanghai Jingke, PHS-3C). Thiourea (100 g L⁻¹) was added as a complexing agent to form soluble [Cu(NH₂CSNH₂)₄]²⁺ ions. First, Cu powder (20 nm, purity 99.99%, Suzhou Canfuo Nanotechnology Company) was cleaned using dilute hydrochloric acid (1%) and acetone before the coating process. Then, the pretreated Cu power was slowly added to the plating solution under vigorous stirring. Plating was performed for 10 min at 20 °C. After plating, 0.5 mL of aniline (AN) solution (2 mol L⁻¹) was added drop-wise to the Sn/Cu suspension. The solution immediately became grey/green in colour when slowly adding the a few drops of H₂O₂ (2 wt%), indicating the formation of PANI. The residual Cu²⁺ in the solution was analyzed by ICP-MS (PerkinElmer Elan DRC-e spectrometer). The powder product was dried under vacuum (10⁻⁵ Pa) at room temperature and heated under nitrogen (to avoid the oxidation of the Cu and Sn metals) to 300 °C at a heating rate of 5 °C min⁻¹ for 10 h to form the final Cu₆Sn₅@C product. Additional Cu–Sn@C composites with different Cu to Sn ratios were prepared for comparison. Expect for the (NH₂)₂SC and SnSO₄ concentrations, the synthesis procedures used for these additional samples were the same as those previously described.

2.2 Materials characterization

The structures and morphology of the samples were characterized by X-ray diffraction (XRD) (PANalytical X'Pert Pro with Cu Kα radiation, λ = 1.5418 Å, using a scan rate of 0.03° min⁻¹), scanning electronic microscope (SEM, Hitachi S-4700 operated at 15 kV) and transmission electronic microscopy (TEM, Tecnai G2 F30 S-Twin operated at 300 kV). The carbon contents of Cu₆Sn₅@C materials were measured using a ThermoFisher Flash EA 1112 analyzer. Fourier transform infrared spectroscopy (FTIR) analysis of Cu@Sn@PANI nanospheres was performed using a Nicolet 6700 spectrometer. Thermogravimetric analysis (TGA) of Cu@Sn@PANI and Cu₆Sn₅@C nanospheres was performed with a Perkin Elmer thermobalance at a heating rate of 5 °C min⁻¹ in a 50 mL min⁻¹ flow of argon and in air, respectively. Raman spectra were measured in a back scattering configuration using a micro-Raman spectrometer HR 800 (Jobin Yvon Horiba).

2.3 Electrochemical measurements

For electrochemical characterization, electrodes were prepared by mixing 80 wt% of active materials, 10 wt% of acetylene black (Changzhou dafu), and 10 wt% of polyvinylidene fluoride (PVdF) (Shanghai Sanaifu) in a 1-methyl-2-pyrrolidinone (NMP) solution (Shenzhen Huachang). The as-prepared slurry was coated onto a Cu foil and dried at 80 °C for 10 h under vacuum (10⁻⁵ Pa). The average loading mass of the electrodes was 1.0–1.1 mg cm⁻². Electrochemical cells used Li metal sheet as the counter electrode. The electrolyte was 1 mol dm⁻³ LiPF₆ in ethylene carbonate (EC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC) (1 : 1 : 1, v/v/v) (from Zhangjiagang GuotaiRonghua). Cycling tests were performed at 298 K with various charge–discharge current densities between 0.01–1.2 V and with a LAND CT2001A battery tester. Except noted, the presented capacities are calculated on the basis of the total weight of composites. Cyclic voltammetry (CV) was performed with a Solartron 1287A potentiostat/galvanostat and 1260A impedance/gain-phase analyzer at a scan rate of 0.1 mV s⁻¹. Electrochemical impedance spectra (EIS) were performed on a CHI 660B electrochemical workstation and recorded over a frequency range from 10⁻² to 10⁵ Hz, with voltage amplitude of 5 mV at room temperature.

3. Results and discussion

3.1 Synthesis mechanism, compositions, and performances of Cu–Sn@C nanocomposites

The formation mechanism of Sn shell on Cu nanoparticles by the electro-less deposition can be described as follows: (NH₂)₂SC forms stable complexes with Cu²⁺, e.g. [Cu(NH₂CSNH₂)₄]²⁺/Cu and so by its addition one would expect the electrochemical potential of [Cu(NH₂CSNH₂)₄]²⁺/Cu to move to a more negative value (e.g. −0.157 V when the concentration of (NH₂)₂SC is 1 mol L⁻¹).⁴⁸ Thus some Cu on the surface was replaced by Sn²⁺ with the addition of (NH₂)₂SC (Cu²⁺/Cu and Sn²⁺/Sn is 0.337 V and −0.136 V, respectively). The electro-less deposition process was initiated. The solvated Cu²⁺ would return to the Cu particle by combining the Sn²⁺ (to form Cu₆Sn₅), and the residual Cu²⁺ concentration was found to be as low as 0.0001 mol L⁻¹ after synthesis indicating that the previously replaced Cu²⁺ could be deposited to the Cu particles. Afterwards, Sn started to be deposited to form Sn layer around Cu particles.

The concentrations of (NH₂)₂SC and SnSO₄ were modified in an effort to exert a greater control over the final compositions of the Cu–Sn@C materials. Table 1 summarizes the effect of the preparation conditions on the phase properties for the obtained products (according to XRD analysis as shown in Fig. S1†). Diffraction data show that the concentrations of (NH₂)₂SC and Sn²⁺ greatly affect the final phase composition due to different Cu : Sn ratios resulting from the electro-less reduction. For instance, when the concentration of (NH₂)₂SC is lower than 100 g L⁻¹ and the concentration of Sn²⁺ fixes at 0.1 mol L⁻¹, metallic copper and other Cu-rich phases (e.g. Cu₃Sn) are found in the final product (samples a–d). The occurrence of these phases can be ascribed to the relatively small quantity of Sn plated in the Cu@Sn@PANI precursors. By increasing the concentration of Sn²⁺ ions, the Cu and Cu₃Sn phases are gradually removed and the reflections from Cu₆Sn₅ become dominant (samples d–f). The Cu : Sn mole ratio of sample f is 5.9 : 5.1 confirmed by EDX. Inevitably, a high concentration of Sn²⁺ ions resulted in a large amount of Sn in the product (e.g. sample g).

Table 1 Summary of phases and elemental compositions of Cu–Sn@C samples prepared under different electro-less reduction conditions

Samples	C(NH2)2SC^a	CSnSO4^b	Cu	Sn	Cu3Sn	Cu6Sn5	Mole ratio of Cu : Sn by EDX
a Concentrations of (NH2)2SC, g L^−1.b Concentrations of SnSO4, g L^−1.
Sample a	40	0.15	●		●		6.3 : 0.7
Sample b	80	0.15	●		●	●	3.3 : 1.1
Sample c	100	0.15	●	●	●	●	2.1 : 0.9
Sample d	120	0.15	●	●	●	●	2.9 : 1.1
Sample e	100	0.25			●	●	3.1 : 2.2
Sample f	100	0.32				●	5.9 : 5.1
Sample g	100	0.50		●		●	6.1 : 7.4

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Table 2 summarizes the electrochemical performances of Cu–Sn@C nanospheres (samples a–g). Cu-Rich samples (e.g. a–e) exhibit an extended reversible capacity of <370 mA h g⁻¹ after 100 cycles. For Sn-rich samples (e.g. sample g), however, the capacity fading over 100 cycles is significant although the initial capacity is high, presumably due to presence of unprotected Sn. Given that the sample f is with the best electrochemical properties, we therefore devoted our efforts to this sample to study its electrochemical properties in detail.

Table 2 Summary of electrochemical properties of Cu–Sn@C samples synthesized under different preparation conditions

Samples	Initial DC^a	100^th DC	Cycling retention	CC^b at 1C	CC at 2C	CC at 5C
a Discharge capacity.b Charge capacity.
Sample a	113	105	92.9%	87	73	69
Sample b	138	118	85.5%	104	81	75
Sample c	220	179	81.4%	137	94	82
Sample d	361	273	75.6%	217	138	91
Sample e	510	369	72.4%	251	143	91
Sample f	686	490	71.4%	334	275	206
Sample g	828	405	48.9%	175	118	101

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3.2 Materials characterization of core–shell Cu₆Sn₅@C

The electro-less deposition of Sn²⁺ ions and polymerization of AN were conducted in sequence on Cu nanoparticles to fabricate the Cu@Sn@PANI precursors. The phase composition of the starting and intermediate compounds for sample f was determined by XRD and the results are shown in Fig. 1. All the reflections of the Cu nanoparticles could be readily indexed to cubic Cu (JCPDS no. 01-1241). Reflections in the XRD pattern of Cu@Sn@PANI could also be attributed to Cu, with additional reflections that could be assigned to Sn (JCPDS no. 04-0673), implying that a Cu–Sn composite was formed. Note that, due to the high surface energy of the Cu nanoparticles, some of the deposited Sn at Cu/Sn interface would also readily diffuse into the Cu lattice to form Cu₆Sn₅, which appears in the Cu@Sn@PANI nanospheres. XRD patterns of the Cu₆Sn₅@C nanospheres obtained after heat treatment were indexed to monoclinic Cu₆Sn₅ (C2/c, JCPDS no. 45-1488) with an average crystalline size of ∼40 nm, (using the Scherrer equation: D = Kλ/β cos θ). The absence of a diffraction reflection at 2θ = 23° indicates that carbon resulting from the PANI decomposition may exist in an amorphous form.

Fig. 1 XRD patterns of Cu nanoparticles, Cu@Sn@PANI precursors, and core–shell Cu₆Sn₅@C nanospheres.

The formation of PANI in the Cu@Sn@PANI precursors can be observed via FTIR analysis as shown in Fig. 2a. The absorption bands at 1548 and 1497 cm⁻¹ are assigned to C=C stretching deformation of the quinoid and benzoid rings of PANI, respectively. The bands at 1300 cm⁻¹ belongs to the C–N stretching of the intermediate aromatic amine, while the bonds at 1136 and 814 cm⁻¹ correspond to the C–H deformations. After heat treatment, the characteristic peaks of PANI disappear as would be expected as PANI decomposes into carbon. TGA (Fig. 2b) in flow argon of Cu@Sn@PANI shows that the removal of adsorbed water and the carbonization process mainly occur before 300 °C and lead to a mass loss of 10.5 wt%. This observation can explain why we adopt 300 °C as the annealing temperature, as higher temperatures trend to result in agglomeration of alloy nanoparticles.

Fig. 2 (a) FTIR spectrum and (b) TGA curve in Ar of Cu@Sn@PANI precursors; (c) Raman spectrum and (d) TGA curve in air of Cu₆Sn₅@C nanospheres.

After heat treatment, the PANI converted into amorphous carbon confirmed by Raman spectroscopy, as shown in Fig. 2c, which is recognized as predictive for sp² bonded carbons and physical properties of carbon materials. The strong bands at 1340 and 1590 cm⁻¹ are respectively attributed to the D-band and G-band of carbon.⁴⁹ The I_D/I_G ratio is calculated to be 1.59, implying that the carbon obtained from the carbonization of PANI is almost amorphous. Fig. 2d shows TGA curve for Cu₆Sn₅@C nanospheres in air. The mass loss of ∼2 wt% before 200 °C can be mainly ascribed to the releasing of absorbed water, while the loss of ∼3 wt% at 200–380 °C belongs to the preliminary oxidation of carbon coating. However, the complete removal of carbon generally happens in the temperature range of 400–500 °C, where a significant mass increasing of over 10 wt% appears due to the surface oxidation of Cu₆Sn₅. Therefore, it is inaccurate to estimate the detail carbon content. Elemental analysis was conducted instead and showed the carbon content was 9.8 wt%.

TEM images of the Cu@Sn@PANI and Cu₆Sn₅@C nanospheres are shown in Fig. 3. The average size of the Cu₆Sn₅@C nanoparticles are in the range of 40–50 nm, in good agreement with the values calculated from XRD patterns and SEM image (Fig. S2†). The sizes of the Cu₆Sn₅@C nanoparticles are almost identical to those of the Cu@Sn@PANI intermediate (Fig. 3a and b). This observation indicates that the PANI layer can effectively prevent agglomeration during the melting and alloying of Sn with Cu to make Cu–Sn intermetallic phases while maintaining the nanoscale morphology of the composite. The transformation from Cu@Sn@PANI to Cu₆Sn₅@C is thus essentially pseudomorphic. A TEM image of a selected Cu₆Sn₅@C particle clearly reveals that a coarse carbon layer forms around the core particle (Fig. 3c), where the carbon layer originates from the decomposition of the previously deposited PANI shell. It also demonstrates that the thickness of the carbon layer is ca. 5 nm. Selected area electron diffraction (SAED) patterns further confirms the crystalline nature of the core material and could be indexed to the monoclinic Cu₆Sn₅. At high resolution, TEM image (Fig. 3d) shows that the distance between neighboring fringes is 2.1 Å, very close to the (132) d-spacing in the monoclinic structure of Cu₆Sn₅. Therefore, based on all the results discussed above, it can be deduced that core–shell Cu₆Sn₅@C nanospheres were successfully prepared from Cu@Sn@PANI precursors.

Fig. 3 TEM images of (a) Cu@Sn@PANI precursors and (b) resultant Cu₆Sn₅@C nanospheres. (c) HRTEM image and (d) SAED patterns of Cu₆Sn₅@C. Scale bars are 50, 20, 5, and 2 nm separately.

3.3 Electrochemical performances of core–shell Cu₆Sn₅@C nanospheres

The core–shell Cu₆Sn₅@C nanospheres were electrochemically characterized by CV, charge–discharge profiles, and cycling performances at different rates in CR-2032 coin cells (shown in Fig. 4). CV curves for the Cu₆Sn₅@C sample are shown in Fig. 4a. Over the course of the lithiation process the CV profiles show two apparent reduction peaks at ca. 0.35 and 0.55 V, corresponding to the formation of Li₂CuSn and Li_4.4Sn as shown in eqn (1) and (2), respectively. In contrast to previous studies, irreversible peaks at potentials above 1.0 V due to catalytic decomposition of the electrolyte on the exposed active Sn surface were not observed in our data.⁵⁰ The absence of these features at higher potential could be ascribed to the protection of the Cu₆Sn₅ core by the carbon shell, which inhibits such side reactions.

Fig. 4 Electrochemical performances of Li/Cu₆Sn₅@C cells: (a) CV curves at a scan rate of 0.1 mV s⁻¹, (b) charge/discharge profiles at 0.1C (1C = 605 mA g⁻¹), (c) the cycleability and coulombic efficiency at 0.1C, and (d) high-rate performances.

Typical charge–discharge voltage curves of the Cu₆Sn₅@C nanospheres cycled between 0.01 and 1.20 V were given in Fig. 4b. Several different potential regions can be identified in the discharge profiles between 0.4 and 0.1 V. In the first cycle, the discharging and charging capacity were 914.2 and 815 mA h g⁻¹, respectively, resulting in a coulombic efficiency of 89.2%. The irreversible capacity observed in the first cycle can be attributed to the formation of a solid electrolyte interface (SEI) film. The level of SEI formation is proportional to the surface area and the quality of coating, and a homogeneous carbon coating would effectively suppress the formation of coarse SEI layer. The initial coulombic efficiency (ICE) of the Cu₆Sn₅@C nanospheres is 89.2% higher than the uncoated nanosized Cu–Sn anode materials (usually less than 75%, see details in Table 3). In the following cycles, Cu₆Sn₅@C demonstrates a coulombic efficiency of more than 99%. The charge–discharge profiles are in accord with the CV results and the close overlap of the curves offer further evidence for the observed high coulombic efficiency.

Table 3 Summary of lithium storage performances between core–shell Cu₆Sn₅@C nanospheres herein and representative Cu₆Sn₅-based materials reported previously

Materials	Carbon (%)	RC (mA h g^−1)	ICE^c (%)	Current (mA g^−1)	Cycles	Ref.
a Not given in references.b Calculated according to the given data.c ICE: initial coulombic efficiency.
Porous Cu6Sn5	0	404	73	100	100	38
Porous Cu6Sn5	0	420	83^b	0.5 mA cm^−2	20	21
Microporous Cu6Sn5–Sn	0	400	58	666	54	51
Cu6Sn5/carbon fiber	—^a	250	68	131	30	37
Cu6Sn5@C on carbon	53.7	366	56.5	1000	200	18
Cu6Sn5/C	8.86	400	—	140^b	100	52
Cu6Sn5/GNS	52.6^b	411	70	500	1600	20
Sn rich Cu6Sn5	0	440	98	200	50	53
Cu6Sn5@SnO2–C	—^a	619	65	200	500	54
Cu6Sn5 nanoparticles	0	490	71.4	60	100	This work
Core–shell Cu6Sn5@C	9.8	518	89.2	60	100	This work

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The cycling performance of the Cu₆Sn₅@C nanospheres is shown in Fig. 4c. It shows a ability of maintaining a reversible capacity of 518 mA h g⁻¹ without significant fading after an initial decrease in the first few cycles. If excluding the contribution of 9.8 wt% carbon, the exact capacity of Cu₆Sn₅ cores is ca. 570 mA h g⁻¹, that is very close to its theoretical value of 605 mA h g⁻¹ calculated according to eqn (1) and (2). Rate capability is an important factor in evaluating the potential application of anode materials. This motivated us to investigate the rate performance of the Cu₆Sn₅@C nanospheres further. Fig. 4d displays the rate capability of the Cu₆Sn₅@C nanospheres at rates of 0.1–5C. The Cu₆Sn₅@C nanospheres maintain a steady capacity output at high current densities, e.g. 488 mA h g⁻¹ at 0.2C, 331 mA h g⁻¹ at 1C, 275 mA h g⁻¹ at 2C, and 206 mA h g⁻¹ at 5C. Fig. S3† demonstrates that the Cu₆Sn₅@C nanospheres still maintain a structural integrity and a reversible crystalline phase of Cu₆Sn₅ after 30 cycles at 0.2C.

Table 3 compares the lithium storage performance of the core–shell Cu₆Sn₅@C nanospheres with representative Cu₆Sn₅-based materials reported previously. Totally speaking, the core–shell Cu₆Sn₅@C nanospheres in this work exhibit comparable or even better performance especially at a relatively low rate. To the best of our knowledge, both the reversible capacity of 518 mA h g⁻¹ and the initial coulombic efficiency of 89.2% are the highest value in pure Cu₆Sn₅ and Cu₆Sn₅/C materials. Note that, with the help of high-capacity materials such as Sn⁵³ and SnO₂,⁵⁴ the counterpart performance can be further enhanced. The impressive performance can be ascribed to the reduced Li⁺ ion diffusion pathways in the nanoscale material and the enhanced conductivity imparted by the carbon shell.

4. Conclusions

In conclusion, a controllable electro-less reduction strategy was reported to prepare uniform core–shell Cu₆Sn₅@C nanospheres from Cu@Sn@PANI precursors. The composition and lithium storage performance of the Cu–Sn@C nanospheres were heavily influenced by the synthesis parameters. Isolated, approximately isotropic Cu₆Sn₅/C nanospheres (40–50 nm in diameter) exhibited a demonstrably superior electrochemical performance to those composites prepared with alternative Sn : Cu ratios and offers promise as a candidate high power anode material.

Acknowledgements

This work was supported by the NSFC (21403195), the Key Research and Development Program (2016C01SA500898, 2015C01001, and 2017C01023) of Science and Technology Department of Zhejiang Province, the Shenzhen Peacock Plan (KQTD2015071616442225), and the Chinese Government “Thousand Talent” Program (Y62HB31601).

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Footnote

† Electronic supplementary information (ESI) available: XRD patterns, components, and cycle performance of Cu–Sn@C composites synthesized under different conditions; SEM image of Cu₆Sn₅@C nanospheres. See DOI: 10.1039/c7ra02214j

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