SnO₂ decorated graphene nanocomposite anode materials prepared via an up-scalable wet-mechanochemical process for sodium ion batteries

Abstract

A facile and up-scalable wet-mechanochemical process is designed for fabricating ultra-fine SnO₂ nanoparticles anchored on graphene networks for use as anode materials for sodium ion batteries. A hierarchical structure of the SnO₂@graphene composite is obtained from the process. The resultant rechargeable SIBs achieved high rate capability and good cycling stability.

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Introduction

Rechargeable batteries are not only important to satisfy our energy requirements in daily life, but are also vital to address the emerging environmental issues and the world energy crisis. In the last decade, lithium ion batteries (LIBs) have achieved remarkable progress in electronic appliance applications and electrical vehicles.^1–5 However, large-scale applications of LIBs have been heavily restricted by their high cost and limited supply of lithium on the earth. In this regard, sodium ion batteries (SIBs) are considered as one of the most promising alternatives to LIBs, due to their significantly lower cost and the abundance of sodium. This is especially true in the smart grid applications where large scale, long term stability and low cost are the dominant factors.^6–8 Though LIBs and SIBs share similar chemistry in the charge/discharge processes, the essential components of LIBs cannot be simply adopted for SIBs owing to the fact that the Na ion is substantially larger (55% in radius) than the Li ion. For instance, graphite, the most commonly used LIB anode material, is not suitable for SIBs due to its poor Na ion insertion property.^9–11 The electrode materials for SIBs should contain large and abundant transport channels for Na ion transportation.

Nevertheless, several anode materials such as carbon, alloys, metal sulfates and metal oxides have been investigated for SIBs.^7,12–21 Among them, SnO₂ is one of the most promising candidates due to its high theoretical specific capacity of 667 mA h g⁻¹.^18,22 In order to achieve the full capacity, the inherent deficiencies of SnO₂, such as poor electronic conductivity, and severe volume change during the charge/discharge processes which causes pulverization of the electrode and needs to be addressed.^8,23 Mounting small SnO₂ particles on a resilient conductive substrate could be an effective strategy to tackle the above limitations. In this regard, graphene could benefit the entire battery process as it possesses superior conductivity, good mechanical strength, large surface area and unique sheet structure.^21,24

In this work, a wet-mechanochemical method is designed for fabricating uniform SnO₂@graphene nanocomposites from SnCl₂ and graphene oxide (GO) which are ball-milled in aqueous media. Compared with other multi-step sophisticated methods,^8,22,25,26 this strategy is simple, rapid, facile, economical and most importantly up-scalable. The precursors (SnCl₂ and GO) are firstly dispersed homogeneously in water, before the constant and powerful mechanical striking of the milling media constrains Sn²⁺ at the surface of GO sheets where redox reactions are initiated by the high impact energy. As a result, GO is reduced to graphene while Sn²⁺ is oxidized to SnO₂, as shown in eqn (1):

Due to the necessity of both an oxidant and a reductant, only the impacts at the surface of the GO will result in successful reactions which producing in SnO₂ particles strongly bonded to the surface of the graphene. The random and high frequency nature of the impacts on the surface of GO ensure a relatively even SnO₂ coating. It is expected that a hierarchical SnO₂@graphene structure would form in this process which is beneficial to electrochemical performances of SIBs. Moreover, with the uniform coating, graphene sheets could be avoided from stacking together, enabling the efficient transportation in the electrode of Na ions.

Experimental

SnCl₂·2H₂O (0.45 g, Merck Pty. Ltd.) and graphene oxide (GO, 0.1 g, Tianjin Plannano Technology Co. Ltd.) are firstly dispersed in 10 mL deionized water. The resultant mixture is added to a planetary zirconia ball miller at room temperature at a speed of 500 rpm for 3 h. Then the as-prepared product SnO₂@graphene is washed in water and ethanol in sequence, and subsequently dried in a vacuum oven at 60 °C. Commercial tin oxide nanopowder (SnO₂, Nanostructured & Amorphous Materials Inc.) was used as a control sample.

The morphology was examined by a scanning electron microscope (SEM, JSM-7001F) and transmission electron microscopy (TEM, FEI Model Tecnai G20). X-ray diffraction (XRD) was also tested using CuKα radiation over the 2θ range of 10–80° (Model LabX-6000, Shimadzu, Japan). The multipoint Brunauer–Emmett–Teller (BET) surface area was estimated using adsorption data obtained from a surface area analyser (Micromeritics Tristar 3020). For X-ray photoelectron spectroscopy (XPS, Kratos Axis ULTRA incorporating a 165 mm hemispherical electron energy analyzer) test, all binding energies were referenced to the C1s peak (284.8 eV). Raman spectra were examined at room temperature by a Renishaw 100 system (Raman spectrometer using 514 nm Argon green laser as light source).

As active materials, the samples are mixed with 10 wt% carbon black and 10 wt% polyvinylidene difluoride (PVDF, Aldrich) in N-methyl-2-pyrrolidone (NMP, Aldrich) solvent to form homogeneous slurries. The resultant slurries are uniformly coated onto Cu foils with an area of 1 cm². The loading of the electrode material is ca. 1–2 mg. The pasted Cu foils are dried in a vacuum oven at 60 °C and then pressed by a double-roll compressor. CR2032 coin-type cells are assembled in an argon-filled M-Braun glove box. A glass fiber (GA-55) membrane is used as the separator, a sodium metal sheet as the counter electrode, and 1 M NaClO₄ in a 1 : 1 (v/v) mixture of ethylene carbonate (EC) and propylene carbonate (PC) as the electrolyte. To measure the electrochemical capacity and cycle life of the working electrodes, the cells are charged and discharged using LANDCT 2001A battery tester (Wuhan, PRC) in a voltage range from 0.01 to 2.5 V vs. Na/Na⁺. Cyclic voltammetry (CVs) were performed using a CHI 660D electrochemical workstation (CH Instrument, Shanghai, PRC). CVs were recorded between 2.5 V and 0.01 V at a scan rate of 0.1 mV s⁻¹, using the composite as the working electrode and a sodium sheet as both counter electrode and reference electrode. Electrochemical impedance spectroscopy (EIS) was also carried out in this two-electrode system with amplitude of 5 mV over the frequency range from 100 kHz to 0.01 Hz.

Results and discussion

The SEM images in Fig. 1a and b display the morphologies of the GO and SnO₂@graphene samples, respectively. After wet ball-milling, the sheet-like shape of the graphene in SnO₂@graphene samples is maintained, but are now less-transparent, suggesting that the sheet thickness has been significantly increased, seemingly due to the SnO₂ nanoparticles anchored onto the graphene sheets. The anchored SnO₂ nano particles prevents the graphene sheets from stacking together, and the interlaced sheets would create some voids/channels in the electrode which are tolerant for the dramatic volume change during the charge/discharge process. The corresponding selected-area electron diffraction (SAED) pattern in Fig. 1c (inset) shows that the rings from inside to outside are well-indexed to the (100), (101), and (211) planes of SnO₂, respectively, confirming the formation of SnO₂. It can be observed that the SnO₂ particles are fine and uniform, densely attached on graphene sheets. The HRTEM images in Fig. 1d and S1† clearly show that the regular inter-plenary spacing of 0.34 nm can be ascribed to the (110) planes of SnO₂ and the average SnO₂ particle size is only ca. 3 nm.

Fig. 1 (a) SEM image of the GO; (b) SEM image of SnO₂@graphene; (c) TEM image with the SAED pattern (inset) of SnO₂@graphene and (d) HR-TEM image of SnO₂@graphene sample.

The XRD pattern in Fig. 2a confirms the formation of SnO₂ (PDF 41-1445). According to the Scherrer equation, the crystal diameter can be approximated as ca. 3 nm, agreeing well with the results of HRTEM. Achieving such a small particle size can be considered as a unique advantage of the proposed wet mechanochemical method as the dry mechanochemical method can commonly achieve as small as several micrometres.²⁷ This ultra-fine size would be beneficial to Na ions mass transport as it shortens the diffusion path length of the Na ions. The adsorption–desorption isotherms belong to type IV based on the hysteresis loop (Fig. 2b), which indicates the existence of 3-D channels and the network structure. The SnO₂@graphene composites exhibit a high surface area of 273 m² g⁻¹ according to the BET measurement. This also implies successful inhibition to the aggregation of nanoparticles and restacking of graphene sheets, which may enable the insertion/extraction of Na ions for the graphene sheets. The weight percentage of SnO₂ of the composites is characterizes by TGA test as ca. 70%.

Fig. 2 (a) XRD image and (b) BET adsorption measurement of the SnO₂@graphene; (c) XPS of C 1s for GO and SnO₂@graphene samples; (d) Raman spectra of SnO₂@graphene sample and GO. (e) The reaction mechanism and the colours of reactants and products.

The reduction degree of GO by reaction (1) was investigated by XPS measurements. Fig. 2c displays the data of C 1s for GO and SnO₂@graphene samples. The oxygen functional groups could be clearly observed from GO profile, which are corresponding to C*–CO, C–O, C=O and COOH, respectively.²⁴ In comparison, for SnO₂@graphene, after the wet ball-milling process, the oxygen functional groups on the surface of graphene decreased dramatically, indicating the successful reduction from GO to graphene. The color of the samples also changes from brown to black as shown in Fig. 2e, suggesting the successful reduction of GO. D band and G band peaks could be observed in Raman spectra (Fig. 2d) for both the SnO₂@graphene composite and GO. It is established that the ratio of the D band and G band intensities (I_D/I_G) reflects the extent for the disruption of the symmetrical hexagonal graphitic lattice.^28–30 In this case, the I_D/I_G for SnO₂@graphene is higher than that of GO due to the interaction of the between SnO₂ and graphene leads to the disruption. This implies that the SnO₂ nano particles are successfully anchored on graphene sheets.

Electrochemical characteristics of the commercial SnO₂ and SnO₂@graphene samples were investigated systematically as anodes of SIBs. Fig. 3a and b present the CV curves of the commercial SnO₂ and SnO₂@graphene samples with a scan rate of 0.1 mV s⁻¹. At the first cycle of SnO₂ electrode, three obvious irreversible cathodic peaks could be observed at 0.75 V, 0.4 V and 0.01 V, which are attributed to the formation of NaSn₅, NaSn and Na_xSn₄ (x ≥ 9), respectively, according to the density functional theory (DFT) calculation results for the Sn sodiation voltages of SIBs.³¹

SnO₂ → Sn → NaSn₅ → NaSn → Na₉Sn₄ → Na₁₅Sn₄. (2)

Fig. 3 (a) CV profiles for the first 3 cycles of commercial SnO₂ sample and (b) SnO₂@graohene sample as the electrodes materials for SIBs. (c) The charge/discharge profiles for the SnO₂@graphene SIB; and (d) the charge/discharge capacities and the coulombic efficiency of the SnO₂@graphene and the capacities of the commercial SnO₂ at a current density of 0.1 A g⁻¹.

In contrast, the CV curve of the first cycle for SnO₂@graphene (Fig. 3b) shows only a cathodic broad peak at 0.01–0.4 V which is contributed by the reduction for SnO₂ nanoparticles to Na₁₅Sn₄, the formation of solid electrolyte interface (SEI) and the insertion of Na ions into graphene 3-D network. The shapes of the CV curves in the following two cycles are similar as the first one. It can be concluded as the capacity of SnO₂@graphene for SIBs is a combination of two parts: the insertion/extraction of Na ion in graphene 3-D framework and the alloying/dealloying between Sn and Na₁₅Sn₄. It can be described as eqn (3)–(5):^8,22

SnO₂ + 4Na⁺ + 4e⁻ → Sn + 2Na₂O (3)

4Sn + 15Na⁺ + 15e⁻ ↔ 4Na₁₅Sn₄ (4)

C + xNa⁺ + xe⁻ ↔ Na_xC (5)

The shapes of CV profile for SnO₂@graphene sample is quite similar as the shapes of carbon material,³² but the capacity is much higher than that of graphene sample (about 176 mA h g⁻¹ at 0.1 A g⁻¹ (ref. 33)).

The charge/discharge voltage profiles of the commercial SnO₂ and SnO₂@graphene samples are evaluated at 0.1 A g⁻¹ in the range of 0.1–2.5 V as shown in Fig. 3c. A characteristic irreversible discharge plateau from 0.8 V vs. Na⁺/Na is observed and the resultant electrode delivers a high specific capacity at 750 mA h g⁻¹ at the 1st cycle while it maintains only 407 mA h g⁻¹ at the 2nd cycle due to the irreversible reactions at the first cycle, agreeing well with the CV result. From the 2nd cycle, this plateau disappears, and the charge/discharge curves almost overlap, indicating that the electrochemical reduction process is completed since the reduction reaction of SnO₂ is an irreversible reaction¹⁸ and the SEI is stabilized.

In comparison with the control sample, the SnO₂@graphene shows a lot better cycling stability as shown in Fig. 3d. After 50 cycles, the capacity of the SnO₂@graphene is 286 mA h g⁻¹ and maintains at 270 mA h g⁻¹ after 100 cycles (86% retention for the 5th cycle). Interestingly, the extraordinary coulombic efficiencies (>100%) are observed in first 3 cycles mainly due to due to there is an irreversible discharge process, (reaction (3)) in addition to the reversible discharge process (reaction (4)). After 3 cycles, the charge/discharge capacities stabilize and the coulombic efficiency continuously maintain at ca. 100%, showing a good reversibility. The commercial SnO₂ only delivers a low initial irreversible discharge capacity (315 mA h g⁻¹) and then drops rapidly to zero.

The rate capacities of the SnO₂@graphene samples are further investigated in Fig. 4a. The SnO₂@graphene delivers a discharge capacity of 320 mA h g⁻¹ at a current density of 0.05 A g⁻¹. When the rate rises to 0.1, 0.2, 0.4 and even 0.8 A g⁻¹, the specific capacities are retained and are quantified as 290, 274, 246 and 207 mA h g⁻¹, respectively. An excellent rate capability and stability was observed in that only 28.6% capacity loss when the rates increase from 0.1 to 0.8 A g⁻¹. The high rate capacity (0.8 A g⁻¹) for the SnO₂@graphene composites is among the best of current literatures,^8,18,25 which is due to the small particle size of SnO₂ and the special networks structure with a large specific area. After the varied rate cycles, the capacity of the electrode could nearly recover to the initial capacity of over 285 mA h g⁻¹ at 0.1 A g⁻¹. Fig. 4b presents the charge and discharge profiles of SnO₂@graphene for SIB at different current densities. It could be observed that with the increase of the current density, the curves shape does not change, suggesting the SnO₂@graphene electrode can enable a fast transportation of Na ions for SIBs. The structure and the chemistry during the charge and discharge process are all stabilized, which is benefited from the framework structure of the electrode.

Fig. 4 (a) the rate charge/discharge capability for SnO₂@graphene and (b) their charge and discharge curves from 0.05 A g⁻¹ to 0.8 A g⁻¹ (from the 15^th, 25^th, 35^th, 55^th cycles, respectively), (c) Nyquist plots of the SnO₂@graphene and commercial SnO₂. (d) Schematic representation of the hierarchical structure of the SnO₂@graphene nanocomposite for SIBs.

EIS measurements are conducted for the SnO₂@graphene and SnO₂ control samples after 3 cycles charge/discharge. Both of the Nyquist plots shown in Fig. 4c display depressed semicircles in high-middle frequency region, which are ascribed to the charge transfer resistance (R_ct, directly proportional to the radius of the semicircles); and a 45° inclined line in the low frequency region, which can be considered to be Warburg impedance. A much smaller R_ct (∼320 Ω for SnO₂@graphene; ∼2338 for commercial SnO₂) could be observed for the SnO₂@graphene, which can be attributed to robust and strong links between the SnO₂ and graphene sheets as well as the highly conductive nature of the graphene.

The mechanism responsible for the exceptional electrochemical performance and resilient stability of the resultant SIBs can be concluded in Fig. 4d. Firstly, the achievement of ultra-small size of the SnO₂ particles (ca. 3 nm), the successful prevention of the graphene stacking and the strong links between the SnO₂ and graphene sheets facilitate thorough electrochemical reduction of SnO₂ for Sn production. Secondly, the superior electronic conductivity of graphene, produced by the thorough reduction reaction of GO in the wet-mechanochemical process, which guarantees high electron collection percentage within the 3-D network. Last but not least, such robust 3-D conductive networks and the hierarchical structure of the nanocomposites would accommodate the dramatic volume change of SnO₂ and Sn during charge/discharge progress. Large surface areas and channels in the electrode not only enhance rapid mass transport of the Na ions within the resultant 3-D networks and the alloying/dealloying process, but also enables the insertion and extraction of Na ions in graphene sheets.

Conclusions

SnO₂@graphene nanocomposites are successfully synthesized via a facile wet-mechanochemical route. The SnO₂ nanoparticles are uniformly anchored onto graphene sheets, forming a hierarchical 3-D network on the electrode. This process prohibits the aggregation of SnO₂, eliminates the restacking of graphene sheets and therefore enhances the mass transport of Na ions within the 3-D conductive graphene network. Consequently, the SnO₂@graphene composite could overcome the problems of pulverization and high resistance of SnO₂, delivering outstanding energy capacity and cycling stabilities in SIBs. This process is a promising way for mass production of low cost and high quality anode materials for SIBs.

Acknowledgements

The authors acknowledge the financial supports from Australia Research Council and Griffith University, Australia, and the facilities and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Center for Microscopy and Microanalysis (CMM) at the University of Queensland, Australia.

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Footnote

† Electronic supplementary information (ESI) available: Further characterizations of TEM and Raman tests. See DOI: 10.1039/c4ra09699a

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