Oleic-acid-assisted carbon coating on Sn nanoparticles for Li ion battery electrodes with long-term cycling stability

Abstract

We report the one-pot synthesis of high-performance Sn@C nanocomposite anode materials obtained by uniform carbon coating onto Sn nanoparticles induced by electrical Sn-wire explosion in oleic acid liquid media at room temperature. This Sn@C nanocomposite exhibits highly reversible Li-storage performance with a specific capacity of ∼730 mA h g⁻¹, even after 200 cycles.

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Great effort has been devoted to developing high-capacity anode materials with a variety of nanostructures that enable high Li-storage performance because of the capacity limitations of conventional carbonaceous materials based on Li-intercalation/de-intercalation reactions.¹ Recently, group IV elements such as Si, Ge, and Sn, which are capable of forming alloys with lithium (M + 4.4Li⁺ + 4.4e⁻ ↔ Li_4.4M, M = Si, Ge, and Sn), have received tremendous interest for use in Li-electroactive anode materials because of their high theoretical capacity and volumetric energy density.² Nanostructured metallic Sn is a promising option for high-capacity anode materials upon undergoing this Li-alloying/de-alloying reaction as it has theoretical gravimetric and volumetric capacities of ∼993 mA h g⁻¹ and 7262 A h cm⁻³, respectively, according to the Li_4.4Sn stoichiometry in the fully lithiated state.³ Furthermore, it not only has the advantages of being naturally abundant and inexpensive, but also has a relatively low operating potential that can lead to enhanced battery safety. Nevertheless, the practical application of Sn anode materials in Li-ion batteries (LIBs) is still restricted because of the following chronic problems: (i) the low intrinsic electrical conductivity of Sn, and (ii) the mechanical stress induced by the severe volume variation (>300%) that occurs upon full lithiation and de-lithiation of Sn; this ultimately leads to a loss of electrical connection with the current collector as well as pulverization of the Li-electroactive Sn materials, resulting in rapid capacity decay and poor electrochemical cycle life.⁴

Numerous approaches have been suggested to overcome these drawbacks; the use of Sn-based nanocomposites, e.g., Sn-amorphous carbon,⁵ Sn-CNT,⁶ and Sn-graphene,⁷ has been primarily investigated as an effective strategy. In most of these studies, the carbonaceous materials are used not only as effective buffer zones to minimize the volume changes but also as conductive phases to alleviate the low electrical conductivity and poor electrochemical stabilities of Sn anode materials, resulting in enhanced cycle stability and rate capability. However, preparation of these Sn-based composites involves multistep syntheses and substantial retooling of the current fabrication process, making these materials relatively costly and not amenable to mass production. Furthermore, obtaining long-term cycle stability with high specific capacities and rate performance remain challenging for the application of Sn anode materials in high-performance LIBs.

In this communication, we report a simple approach to the one-pot synthesis of a high-performance Li-anode material with a uniform carbon nanopainted coating on Sn nanoparticles (denoted as Sn@C nanocomposites); in these Sn@C nanocomposites, the carbon-coated layers play important roles not only as buffering layers against large volume variations of Sn but also as conducting networks and mechanical barriers to prevent aggregation of the Sn nanoparticles.

The Sn@C nanocomposites were easily prepared through a facile electrical pulse technique in oleic acid (OA) liquid media at room temperature: commercial Sn wire was continuously fed using an automatic synthetic system and the electrical Sn-wire explosion process was repeatedly conducted to fabricate the Sn@C nanocomposites (Scheme 1). This electrical explosion process proved to be an efficient method that is cost-effective, eco-friendly, and feasible for mass production of Sn@C nanocomposites. Characterization of the structural and electrochemical properties of the Sn@C nanocomposites as an anode material for LIBs were systematically investigated; their excellent Li-storage performance compared with those of nano- and micro-sized Sn nanoparticles was investigated. In particular, the Sn@C nanocomposite electrodes were superior in several key ways for high-performance LIBs; they featured a high specific capacity, long-term cycling life, and enhanced rate capability. The detailed experimental procedures, structural characterizations, and electrochemical evaluation methods are described in the ESI.†

Scheme 1 Schematic diagram of the one-pot synthesis of Sn@C nanocomposites induced by the electrical Sn-wire explosion process at room temperature.

Fig. 1 displays the representative characteristics of the as-synthesized Sn@C nanocomposites. The crystallographic structure of the Sn@C nanocomposites was determined using X-ray powder diffraction (XRD) analysis (Fig. 1a). Despite the presence of the carbon matrix, the well-defined diffraction peaks matched well with the cubic Sn structure with an Fd3m space group (JCPDS card #04-0673), and no secondary phases corresponding to impurities such as SnO_x (SnO and/or SnO₂), which can be caused by oxidation of Sn nanoparticles, were detected. Fig. 1b shows a typical FE-SEM image of the Sn@C nanocomposites. Relatively uniform sphere-like Sn nanoparticles (NPs) were observed with sizes ranging from a few nanometers to several tens of nanometers, and the Sn NPs appeared to be embedded in a carbon matrix (Fig. S1, ESI†). In particular, the low-magnification TEM image (Fig. 1b) clearly exhibited that the sphere-like Sn NPs were well-dispersed within the carbon matrix without major agglomeration of the Sn NPs. Further insights into the morphology and microstructure of the Sn@C nanocomposites were obtained using TEM and HR-TEM (Fig. S2, ESI†). Most of the Sn@C nanocomposites exhibited the morphologies of Sn NPs in a carbon matrix and had relatively uniform diameters of approximately 10–25 nm. Fig. 1d shows a representative TEM micrograph of an individual Sn@C nanocomposite. As expected, the carbon was uniformly nanopainted around the Sn NP, indicating that the Sn@C nanocomposites had core–shell structures. The HR-TEM image (Fig. 1e) taken from an individual Sn@C nanocomposite clearly featured two distinct zones: (i) a thin, uniform carbon nanopainted coating (∼2.5 nm thick), and (ii) a single crystalline Sn particle showing highly ordered lattice fringes. These well-resolved lattice planes indicated an interplanar distance of 0.21 nm, which corresponds to the d-spacing of the (220) plane of the cubic Sn structure. The indexed SAED pattern image of Fig. 1f also revealed the lattice planes of both the Sn and carbon elements without any reaction between them, demonstrating the structure of Sn@C nanocomposite. Furthermore, the high-angle annular dark-field (HAADF) image and EDS elemental mapping also revealed that the Sn NPs were homogeneously and densely dispersed in the carbon matrix (Fig. 1g).

Fig. 1 Representative characterization of Sn@C nanocomposites: (a) XRD pattern indexed to Sn (JCPDS card #04-0673), (b and c) typical FE-SEM and TEM micrographs, (d) TEM image of an individual Sn@C nanocomposite showing the uniform carbon coating on the Sn NP, (e) HR-TEM image, (f) SAED pattern and (g) HAADF STEM image of the Sn@C nanocomposite and corresponding EDS elemental mapping of tin (Sn, red) and carbon (C, yellow). The EDS elemental mapping was taken from the open-square zone (red line) of the HAADF STEM image.

As mentioned above, this uniform carbon nanopainted coating on Sn NPs was facilely achieved using the pulsed electrical-wire explosion method. According to Sedoi et al.,⁸ application of the electrical pulse resulted in a high-temperature because of the highly accelerated voltage; concurrently, the wires underwent consecutive melting, evaporation (and plasma), and condensation processes, resulting in the formation of nanoparticles. Notably, applying an electrical pulse also induced the structural collapse of OA, which generated carboxylic groups (–COOH) and some alkene chains, which serve as both a barrier to prevent agglomeration of the Sn NPs and a carbon source to ensure uniform carbon nanopainting onto the Sn NPs.⁹ Consequently, the Sn NPs in OA were converted into Sn@C nanocomposites.

In addition, the carbon content (by mass) of the Sn@C nanocomposites was determined by TG analysis (Fig. 2a): after heat-treatment up to 1000 °C in air, the weight loss was about 45%. This result corresponded with the quantitative analysis of the weight ratio of carbon performed using an elemental analyzer (EA) (see ESI, Table S1†). Fig. 2b shows the N₂ adsorption–desorption isotherm of the Sn@C nanocomposites.

Fig. 2 (a) TG analysis of Sn@C nanocomposites in air, (b) N₂ adsorption–desorption isotherm of the Sn@C nanocomposites.

The hysteresis loop corresponded with a II- and IV-type isotherm,¹⁰ indicating that the surface was porous; the BET specific surface area was calculated to be 55 m² g⁻¹. In combination with the carbon nanopainted coating, this large BET surface area with some porosity could create desirable synergy for high Li-storage.

Fig. 3 shows the electrochemical evaluation of Sn@C nanocomposite anodes measured using half-cells. The Li-electroactivity was investigated for the first 10 cycles at a scan rate of 0.3 mV s⁻¹ (Fig. 3a). The first sweep commenced cathodically from the open-circuit-voltage. The main peaks were assigned to Li-alloying and de-alloying of Sn with the corresponding structural changes of Li_xSn_y, which was in good agreement with previous literatures.^5,11 Fig. 3b displays the typical voltage profiles versus the specific capacity of Sn@C nanocomposite electrodes cycled at a constant C-rate of C/10 (1C, based on the theoretical capacity of 993 mA h g⁻¹ from the Li-alloying reaction: Sn + 4.4Li⁺ + 4.4e⁻ ↔ Li_4.4Sn) in the potential range of 0.01–3.0 V (vs. Li⁺/Li). In the first discharge reaction, a rapid potential drop and large sloping plateau were observed in the voltage range of 3.0–0.5 V, which indicated that irreversible reactions between the electrode and electrolyte as well as electrolyte decomposition occurred, i.e., a solid-electrolyte interphase (SEI) formed on the electrode surface¹² and the FEC additive in the electrolyte decomposed.¹³ Thereafter, a long slope with a short voltage plateau at around 0.5 V was evident until the cut-off voltage, which coincided well with the CV results. The specific capacities stabilized after the first cycle and even slightly increased with increasing number of charge–discharge cycles. More importantly, we confirmed a highly reversible specific capacity of ∼790 mA h g⁻¹ even after 50 cycles. In particular, the gravimetric specific capacity of Sn@C nanocomposite electrode here is slightly higher than that of the theoretical gravimetric capacity, which can be calculated to be ∼714 mA h g⁻¹ (based on the theoretical capacity of both Sn (∼993 mA h g⁻¹) and carbon (∼372 mA h g⁻¹)) because of the Sn (∼55%) and carbon (∼45%) contents of Sn@C nanocomposite resulted in TG analysis. This extra capacity above the theoretical capacity of Sn@C nanocomposite can be attributed to the formation of a Li-bearing solid/electrolyte interphase (SEI) layer, similar to the one forming on carbon electrodes. Similarly, this extra capacity above the theoretical capacity is also observed in case of transition metal oxide having Li-conversion reaction. In particular, the formation/dissolution of this polymeric/gel-like SEI layer around transition metal oxide structures has been proposed as the possible reason for the extra capacity over the theoretical capacity during the Li-conversion reaction.¹⁴

Fig. 3 (a) Representative CV behaviors (scanning rate: 0.3 mV s⁻¹) and (b) voltage-capacity profiles of the Sn@C nanocomposite electrodes at a constant C-rate of C/10, (c) cycling performance (at a constant C-rate of C/10) and (d) rate capability of the Sn@C nanocomposite electrode compared with those of the nano- and micro-sized Sn electrodes, (e) long-term cycling stability of the Sn@C nanocomposite electrode at a constant C-rate of C/5, and (f) Nyquist plots of the Sn@C nanocomposite and nano-Sn electrodes after a 50-cycle galvanostatic charge–discharge test.

To highlight the superiority of the Li-storage performance of the Sn@C nanocomposite electrodes, we compared their electrochemical performance with those of commercial nano- and micro-sized Sn powder electrodes (denoted as nano-Sn and micro-Sn, respectively). Fig. 3c presents the specific capacity as a function of the number of cycles for all electrodes at a constant C-rate of C/10. It was found that the specific capacity of both electrodes fabricated from the commercial powders rapidly faded with increasing number of cycles. In contrast, the Sn@C nanocomposite electrode manifested excellent cycling performance with a high specific capacity of ∼800 mA h g⁻¹, even after 100 cycles. Besides, after the first cycle, the coulombic efficiency was greater than 96%. The rate performances were also evaluated from C/20 to C/10 in discrete steps. As shown in Fig. 3d, the Sn@C nanocomposite electrode exhibited a better rate capability with higher specific capacities than the nano-Sn electrode. The Sn@C nanocomposite electrode delivered reversible capacities of approximately 780, 740, 706, 600, and 420 mA h g⁻¹ at the end of C-rate steps of C/20, C/10, C/5, C/2, and 1 C, respectively. Furthermore, the specific capacities almost recovered to 100% when the reverse C-rates were applied, revealing the excellent rate stabilities and efficient reversible reactions of the Sn@C nanocomposite. Fig. 3e demonstrates the outstanding long-term cycling stability of the Sn@C nanocomposite electrode at a constant C-rate of C/5: the specific capacity was almost ∼731 mA h g⁻¹, even after 200 cycles, and its capacity at the 200^th cycle was almost 75% of that at the first cycle. Furthermore, the specific capacity of ∼731 mA h g⁻¹ after 200 cycles indicated a high-capacity value of as high as approximately 74% versus the theoretical capacity of Sn (993 mA h g⁻¹). In addition, the microstructural stability of Sn@C nanocomposite after galvanostatic charge–discharge test over 200 cycles was investigated using FE-SEM and TEM observation. As shown in Fig. S3 (ESI†), the most Sn@C nanocomposite maintained the original sphere-like structure and morphology without significant transformation or collapse. It is confirmed that the pulverization of Sn induced by volume change during Li-alloying/de-alloying reactions could be effectively prevented even after 200 cycles.

The remarkably outstanding electrochemical performance of the Sn@C nanocomposite electrodes was mainly attributed to the following effects of the uniform carbon nanopainted coating: (i) enhancement of the electrical conductivity, which resulted in a highly efficient conducting network of the electrode through an electron percolation pathway from the current collector to the entire surface area of each individual active Sn NP, and (ii) formation of a mechanical barrier and buffer zone against the reaggregation and volume variation of Sn NPs during repeated cycling.

To elucidate the origin of the highly reversible Li-storage performance of the Sn@C nanocomposite electrodes, EIS analyses were carried out after the 50^th cycle and compared with those of a nano-Sn electrode (Fig. 3f). The Nyquist plots obtained for both electrodes showed similar characteristics: a partial semicircle and an inclined line along the imaginary axis (Z′′) in the high- and low-frequency regions, respectively. Each feature was attributed to various resistance phenomena.¹⁵ Importantly, the size of the semicircle portion of the plot for the Sn@C nanocomposite electrode was clearly smaller than that of the nano-electrode; this indicated a lower charge-transfer resistance, which means that the Sn@C nanocomposite electrode had better electrical conductivity, as attributed to the uniform carbon nanopainted coating formed on the Sn NPs. Therefore, we believe that this combination of structural features led to improved electrochemical performance.

In summary, we developed Sn@C nanocomposites with core–shell structures using a facile electrical Sn-wire explosion process in OA liquid media. The uniform carbon nanopainted coating on the Sn NPs was easily formed using a one-pot synthetic route at room temperature. As a result of the effect of the uniform carbon-coating layer on the Sn NPs, the Sn@C nanocomposite electrodes realized a highly reversible Li-storage performance with a high-capacity value of approximately 74% versus the theoretical capacity of Sn, even after 200 cycles. Both the outstanding long-term cycling stability with a high specific capacity and enhanced rate performance of the Sn@C nanocomposite electrodes were proven to originate from buffering of the volume variation as well as the facile electronic delivery provided by the uniform distribution of the carbon-coating on the Sn NPs.

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning (2012R1A2A2A01045382 and 2010-0029027).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental detail and FE-SEM, TEM, and HR-TEM images of Sn@C nanocomposite and Table S1. See DOI: 10.1039/c4ra07928k

‡ These authors contributed equally to this work.

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