Ultra-fine porous SnO₂ nanopowder prepared via a molten salt process: a highly efficient anode material for lithium-ion batteries

Abstract

Ultra-fine porous SnO₂nanoparticles for lithium ion batteries were prepared by a simple, easily scaled-up molten salt method at 300 °C. The structure and morphology were confirmed by X-ray diffraction and transmission electron microscopy. The as-prepared SnO₂ had a tetragonal rutile structure with crystal sizes around 5 nm. The electrochemical performance was tested compared with commercial nanopowder and previously reported nanowires. The as-prepared nanoparticles delivered a significantly higher discharge capacity and better cycle retention. The nanoparticle electrode delivered a reversible capacity of 410 mAh g⁻¹ after 100 cycles. Even at high rates, the electrode operated at a good fraction of its capacity. The excellent electrochemical performance of the ultra-fine porous SnO₂ can be attributed to the ultra-fine crystallites (which tend to decrease the absolute volume changes) and the porous structure (which promotes liquid electrolyte diffusion into the bulk materials and acts as a buffer zone to absorb the volume changes).

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Introduction

Large efforts are presently devoted to developing advanced materials to substitute for the currently used graphite anodes in lithium-ion batteries. Among them, tin-based oxides have attracted much attention, owing to their higher theoretical specific capacity (781 mAh g⁻¹ for SnO₂) compared with graphite electrode (372 mAh g⁻¹).^1–3 However, tin-based materials have a disadvantage, in that they undergo significant volume expansion and contraction, greater than 100%, during lithium intercalation and de-intercalation.^4,5 This causes cracking and crumbling, resulting in “dead volume”, which is electrically disconnected from the bulk material or the current collector. The mechanically inactive “dead volume” results in subsequent degradation of electrode performance during cycling. Special structures have been designed to stabilize the morphology of electrodes in order to alleviate the volume changes and mechanical stress, such as Sn-metal alloy^6–10 or tin-based composite.^11–13 In such cases, another inactive or less active composition served as a supporting and conducting matrix to buffer the volume change. Another approach was to make porous structures or nanostructures, such as nanowires,^14–16nanotubes,^17,18nanorods,¹⁹nanoparticles,^20,21 or hollow structures.^22–24 When the size was decreased, the absolute volume change and cracking would decrease. These reports showed improved electrochemical performance for the SnO₂-based materials, but they either used templates or involved tedious synthetic procedures, which handicapped them in terms of large-scale application. An alternative relatively simple synthetic technique was the hydrothermal method,^25,26 but it was not easy to control the real pressure and thus the batch to batch reproducibility. Furthermore, this method made it difficult to obtain large batch production.

In our previous work, we reported the effect of morphology on the electrochemical performance of SnO₂ anodes²⁷ and found that SnO₂nanowires exhibit better electrochemical performance than nanotubes and nanopowder with average crystal sizes of less than 30 nm, due to the higher electrical conductivity of the nanowires. However, the synthesis process for nanowires was expensive and complicated, which made it unsuitable for real industrial applications. Recently, it was reported that a porous framework was an ideal structure to buffer the volume changes of SnO₂ during charge/discharge.¹⁵ Most types of porous SnO₂ have been prepared through hard templating or soft templating synthesis, which were expensive and not easy to control. In this work, we have developed a simple, easily scaled-up method, the molten salt method, to synthesize ultra-fine porous SnO₂nanostructures. The as-prepared ultra-fine porous SnO₂ nanomaterials show excellent electrochemical performance, better than some reported to date in previous publications.^{14,20–22,28–32} Molten salts used as the reaction solvent have been reported for the preparation of cathode materials elsewhere.^33–35 The molten salt method can result in very fine morphology due to the large viscosity and dielectric behaviour of the eutectic system. Furthermore, this method is easy to achieve and scale up, the treatment temperature is low, and the raw molten salt materials are recyclable, so it is economical and promising for industrial application. The molten salt method could also be used to produce other metal oxides, such as nanosized magnetic or electrode materials.

Experimental

Material preparation

Porous SnO₂ nanopowder was synthesized by a molten salt method. 100 mmol of lithium nitrate (Aldrich), 100 mmol of lithium hydroxide monohydrate (Aldrich), and 10 mmol of tin(II) chloride dehydrate (Analar-BDH) were mixed in an agate mortar to obtain a homogeneous mixture. Then, 50 mmol hydrogen peroxide (Sigma-Aldrich) was added into the mixture, which was stirred for 24 h. The mixture was then given a heat-treatment at 120 °C for 4 h in a vacuum oven, followed by further heat-treatment in air at 300 °C for 3 h in a muffle furnace. After cooling down, the SnO₂ was separated from the eutectic mixture by washing with a large amount of de-ionized water and by centrifugation, and then the product was dried under vacuum at 60 °C for 24 h.

Material characterization

Thermogravimetric analysis (TGA) was carried out with Setaram 92 equipment at a heating rate of 10 °C min⁻¹ under air. X-Ray diffraction (XRD) patterns were obtained using a Philips PW1730 diffractometer with CuKα radiation and a graphite monochromator. Transmission electron microscope (TEM) investigations were performed using a JEOL 2011 200 keV analytical electron microscope. TEM samples were prepared by deposition of ground particles onto lacey carbon support films.

The electrochemical tests were carried out via CR2032 coin-type cells. The electrodes were prepared by mixing as-prepared nano-SnO₂ and commercial SnO₂ as active materials with 10 wt% carbon black (Super P, MMM, Belgium) and 10 wt% polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidinone (NMP) to form a homogeneous slurry, which was then spread onto copper foil. The coated electrodes were dried in a vacuum oven at 120 °C for 4 h. Coin cells were assembled in an argon-filled glove box (Mbraun, Unilab, Germany) by stacking a porous polypropylene separator containing liquid electrolyte between the composite electrode and a lithium foil counter electrode. The electrolyte used was 1 M LiPF₆ in a 50 : 50 (v/v) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC), provided by MERCK KgaA, Germany. The cells were galvanostatically charged and discharged in different voltage ranges at different current densities. All the electrochemical tests were carried out at room temperature (25 °C).

Results and discussion

The overall synthetic procedure is described in Scheme 1. The molten salt method shows an accelerated reaction rate and controllable particle morphology, because the salt melt acts as a strong solvent and exhibits a high ionic diffusion rate.³⁶ Furthermore, the melted salts with their low melting point are helpful in hindering the growth of SnO₂ particles. When cooled down, the SnO₂ particles were surrounded by the salts, which were then washed away by de-ionized water, leaving pores behind, and thus generating ultra-fine porous nanostructures.

Scheme 1 Schematic model of synthetic procedure: (a) mixed raw materials; (b) solid molten salts and SnO₂; (c) porous and nanosized SnO₂.

Temperature is a crucial parameter for crystal growth, so the temperature option is the first consideration. Fig. 1 shows the TGA result on the mixed raw materials. No weight losses above 250 °C can be observed in the curve. Since low temperatures are typically favourable for the synthesis of nanostructures and porous structures, 300 °C was chosen in the experiment to ensure that the reaction occurred in a liquid phase.

Fig. 1 TGA curve of raw material consisting of SnCl₂·2H₂O mixed with LiOH and LiNO₃ (molar ratio 1 : 1).

The structure of the as-prepared SnO₂ nanopowder was characterized using X-ray diffraction and compared with a commercial powder (with average crystal size of 61 nm), as shown in Fig. 2. All reflections are in good agreement with a tetragonal rutile structure (JCPDS 41-1445), belonging to the space groupP42/mnm (136). The highly broadened peaks indicate that the as-prepared powder is composed of SnO₂ crystallites with very small size. The calculated mean crystallite size of the SnO₂, estimated from Scherrer's formula, is 3.6 nm, based on the (110), (101), and (211) peaks.

Fig. 2 X-Ray diffraction patterns for commercial and as prepared SnO₂.

The morphology images are displayed in Fig. 3. The commercial powder was characterized by SEM (Fig. 3(a) and (b)), while the as-prepared SnO₂ was characterized by high resolution TEM (Fig. 3(c) and (d)). The TEM images show that the crystal sizes of the as-prepared SnO₂ are around 5 nm, and lattice fringes are present with d spacing values of 0.318 nm, corresponding to the (110) and (220) planes of SnO₂. There are also a great many pores among the nanoparticles, and the average pore width calculated from Brunauer–Emmett–Teller (BET) measurements is 2.015 nm. These pores provide void space to buffer volume expansion during lithium insertion.

Fig. 3 Scanning electron microscope (SEM) images (a, b) of commercial SnO₂ and transmission electron microscope (TEM) images (c, d) of the as-prepared nano-SnO₂. The scale bars are 1 µm, 0.5 µm, 20 nm and 5 nm, respectively.

The electrochemical performance of the as-prepared ultra-fine porous SnO₂ was characterized by charge/discharge cycling over different voltage ranges and compared with the performance of commercial SnO₂nanoparticles and of SnO₂nanowires prepared in our laboratory. Fig. 4 shows the first and the second charge/discharge curves of the SnO₂ anodes. The as-prepared SnO₂ nanopowder delivers a reversible capacity of 660 mAh g⁻¹ and 645 mAh g⁻¹ in the first and second cycles, respectively, while the commercial sample shows only 372 mAh g⁻¹ and 287 mAh g⁻¹, respectively. The initial Coulombic efficiency of the SnO₂ nanopowder is 44.2%, which is much higher than that of the commercial powder (37.8%).

Fig. 4 First and second cycles of charge and discharge curves for as-prepared (a) and commercial (b) SnO₂.

Fig. 5(a) and (b) compare the cycling performance of the as-prepared nanoparticles with that of SnO₂nanowires in the voltage range of 0.05–1.5 V. The as-prepared ultra-fine porous nanopowder shows a higher capacity and better cycling performance. The nanopowder electrode delivers a reversible capacity of 410 mAh g⁻¹ after 100 cycles, which is significantly better than that of the SnO₂nanowires. Curves (c) and (d), respectively, show the cycling performance of the as-prepared and the commercial SnO₂ in the voltage range of 0.01–3 V. Both the materials show a decrease in discharge capacity with time. However, obviously, the as-prepared ultra-fine porous nanopowder shows a much higher discharge capacity than the commercial powder.

Fig. 5 Cycling performance with different voltage profiles: (a, b) between 0.05 and 1.5 V; (c, d) between 0.01 and 3 V.

In order to fully estimate the electrochemical performance of the as-prepared sample, the cycling data at various charge/discharge rates (0.1, 0.5, 1, 2, 5 and 10 C) are shown in Fig. 6. The charge capacity at the 5 C rate is 57% of the capacity of 0.1 C; even as high as the 10 C rate, it still shows 43% of the capacity of 0.1 C. By returning to 1 C, the electrode delivers a capacity of about 500 mAh g⁻¹. We tried to compare with reported results, but only a few papers did high rate testing. The results are shown in the supporting information (Table S1).† Obviously, our results are comparable. Considering the simple synthetic process and template free, the molten salt method is very promising for large-scale application. The good cycling response is well supported by the TEM images of cycled SnO₂ (Fig. 7), which show that the crystal sizes are from 5–10 nm in size, very similar to the sizes in the as-prepared sample, and with pores among the particles. In further work, transition metal oxides or carbon will be incorporated to improve the conductivity and buffer volume change.

Fig. 6 Capacity delivered upon cycling at different rates between 0.05 and 1.5 V in coin-type half-cells.

Fig. 7 (a) TEM images of the SnO₂electrode after 20 cycles; (b) a high resolution image of (a).

The promising electrochemical performance of the as-prepared SnO₂nanoparticles could be attributed to: (1) the pores, which provides further buffering against the local volume change during the Li–Sn alloying/de-alloying reactions; (2) the ability of the pores to promote liquid electrolyte diffusion into the bulk of the anode and provide fast transport channels for the Li ions; (3) the lower absolute volume changes during the Li–Sn alloying/de-alloying reactions due to the nanosized oxide crystals (around 5 nm); and (4) the shorter diffusion length for Li insertion due to the ultra-fine nanosized particles, with benefits in retaining the structural stability, thereby leading to good cycling performance.

Conclusions

In this paper, a simple molten salt method was applied to synthesize ultra-fine porous SnO₂ nanopowder. The nanosized crystals (around 5 nm) delivered a higher capacity and more stable cyclability compared to SnO₂nanowires. The reversible capacity was 410 mAh g⁻¹ after 100 cycles in the voltage range of 0.05–1.5 V; even at high charge/discharge rates, the performance was comparable to the reported results. This template or surfactant free method is an efficient way to produce ultra-fine porous nanopowder and shows promise for industrial use.

Acknowledgements

Financial support provided by the Australian Research Council (ARC) through an ARC Discovery project (DP0878611) is gratefully acknowledged. Moreover, the authors would like to thank Dr Tania Silver at the University of Wollongong for critical reading of the manuscript.

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Footnote

† Electronic supplementary information (ESI) available: Comparison of the high rate performance of the SnO₂ in this work with those reported elsewhere. See DOI: 10.1039/b821519g

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