NiSnO₃ nanoparticles/reduced graphene oxide composite with enhanced performance as a lithium-ion battery anode material

Abstract

NiSnO₃ nanoparticles (NPs) were loaded on reduced graphene oxide (RGO) by a facile hydrothermal technique as a anode material for lithium ion batteries (LIBs). It was found that the NiSnO₃/RGO anode exhibits improved LIBs performance compared to bare NiSnO₃ or RGO. The NiSnO₃/RGO anode can maintain a reversible capacity of 792 mA h g⁻¹, tested at 1200 mA g⁻¹ after 60 cycles. When the current density was lowered in a test of rate capacity, the charge capacity was completely restored after high rate cycling at 6000 mA g⁻¹ and maintained 889 mA h g⁻¹ at 200 mA g⁻¹ after 115 cycles. The enhanced LIBs performance of the NiSnO₃/RGO nanocomposites can be attributed to the synergistic effects between a highly loaded NiSnO₃ NPs and graphene network.

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Introduction

For lithium ion battery (LIBs) anodes, it is hard for commercial graphite to meet the increasing requirements for high energy density, due to its limited theoretical capacity of 372 mA h g⁻¹.¹ Therefore, many efforts have been made to research alternative anode materials with higher lithium storage performances.² As one of the most promising anode materials, Sn-based materials have attracted much attention due to the high theoretical capacity of 994 mA h g⁻¹ of metal Sn, and a well-suited discharge potential versus Li/Li⁺.^3–5 However, the major drawback of such materials is large volume changes of above 250% during electrochemical reactions, which decrease the performance of high-capacity anodes.⁵

The synthesis of novel nanostructured Sn-based oxides has been confirmed to reduce the mechanical stress and avoid the pulverization of electrodes during the lithiation/delithiation.^6–10 Among them, the nanostructured binary transition metal oxides of MSnO_x (M = Co, Cd, Ni, and Zn) have been studied as anode materials for their low cost, various morphologies and appealing electrochemical performances.^7–10 As the composite oxide of nickel and tin, nanostructured NiSnO₃ is comprised of two cations. The tin cation forms the desired lithium alloy, while nickel is the inactive buffering component. It was recently reported that NiSnO₃ has better electrochemical performance than that of the NiO/SnO₂ mixture, which is due to the ‘self-matrix’ of the discharge products buffering the volume change.^9,10 However, the rate capability and cyclability of such nanostructured Sn-based oxides are still limited, which may be attributed to an inevitable aggregation of NPs, and a poor conductivity induced by high interparticle resistance.^6–10

Introducing a carbon matrix is regarded as an effective method to solve the above problems. Graphene is very appealing as a matrix to anchor the nanostructured metal oxides because of its high surface area, outstanding electrical conductivities, and superior mechanical strength.^11,12 It was recently reported that the Sn-based oxide/graphene exhibits much better electrochemical properties, compared to bare Sn-based oxide.^13–18 The graphene can improve the electrical conduction of the composite material, act as a buffer of the volume change, and prevent the aggregation of the NPs during electrochemical reactions.^13–18 Thus, it is valuable to try loading nanostructured NiSnO₃ on graphene as a probable high performance anode material of LIBs. Although the nanostructured graphene-based electrodes have obviously enhanced gravimetric energy densities, they may have low packing densities because of the sparsity of the matrices, which can reduce the volumetric capacity. From a practical viewpoint, the volumetric capacity is important to consider for a battery.^19,20 Therefore, it is necessary to develop closely-packed graphene networks and a high-loading of metal oxide NPs, since such frameworks can have a high tap density.²¹ For active materials, the size and loading of metal oxide NPs on graphene are crucial in composite electrodes to achieve satisfactory performances.¹⁵

Herein, we report a facile hydrothermal method to synthesize a NiSnO₃/RGO nanocomposite, with ultrasmall and highly-loaded NPs, as an advanced anode material. Compared to bare NiSnO₃ or RGO, the as-prepared NiSnO₃/RGO anode displays enhanced LIBs performance with higher specific capacity and better rate capability. Otherwise, the NiSnO₃/RGO electrode also has a high volumetric energy density because of its high tap density. It is definite that the small particle size of the highly loaded NiSnO₃ NPs and their relatively uniform distribution on the RGO network are the critical factors that can introduce a large number of accessible active sites with Li-ion insertion, and thus increase the electrochemical performance as an anode material for LIBs.

Experimental

Sample preparation

Graphene oxide (GO) was synthesized using a modified Hummers' method; the NiSnO₃/RGO composite was prepared as follows. Firstly, 75 mg of GO was dispersed in 75 mL of ultrapure water by sonication for several hours to form a homogeneous solution. Next, 10 mL of ultrapure aqueous solution of NiSO₄ (0.5 mmol) was added little by little into the above dispersion with magnetic stirring at room temperature, followed by adding 10 mL ultrapure water solution of K₂SnO₃ (0.5 mmol). The mixed dispersion was then transferred into a 100 mL Teflon-lined stainless steel autoclave and heated in an electric oven at 180 °C for 20 h. The obtained precursor was collected and washed several times with ultrapure water and absolute ethanol by centrifugation. The obtained precursor was then dried under vacuum at 60 °C overnight and annealed under argon flow at 300 °C for 4 h to obtain the NiSnO₃/RGO composite. For comparison, bare NiSnO₃ NPs and RGO nanosheets were also prepared under the same conditions, without the presence of GO and NiSnO₃.

Material characterization

The prepared samples were characterized by X-ray diffraction (XRD, Rigaku MiniFlex II), X-ray photoelectron spectroscopy (XPS, ULVAC-PHI, Japan), thermogravimetric analysis (TGA, NETZSCH STA449C), scanning electron microscopy (SEM, JSM-7500F, Japan) with energy-dispersive X-ray spectrometry (EDX), and transmission electron microscopy (TEM, Tecnai G2 F20).

Electrochemical measurements

Electrochemical performances were measured via CR2025 coin-type cells with a metallic lithium film as the counter electrode. The anode electrodes consisted of 80 wt% active material (NiSnO₃/RGO, NiSnO₃ or RGO), 10 wt% polyvinylidenefluoride (PVDF) and 10 wt% acetylene black, homogeneously coated on Cu foil. The electrolyte was composed of 1 M LiPF₆ in the ethyl carbonate (EC) and dimethyl carbonate (DMC) (1 : 1 in volume) mixture. The electrodes were dried under vacuum at 80 °C and then assembled into the coin cells in an argon-filled glove box. Galvanostatic cycling was performed using a LAND CT2001A battery test system at various current densities of 200–6000 mA g⁻¹ in the voltage range of 0.02–3.00 V. Cyclic voltammograms (CV) were performed using an electrochemical workstation (CHI660C) in the voltage range of 0.02–3.00 V at a scanning rate of 0.3 mV s⁻¹. Electrochemical impedance spectra (EIS) measurements were tested in a frequency range from 0.01 Hz to 100 kHz with amplitude of 5 mV. The electrochemical properties were all calculated based on the overall active material of the electrodes.

Results and discussion

Fig. 1a shows the XRD patterns of the as-prepared RGO, NiSnO₃ and NiSnO₃/RGO. The XRD pattern of the prepared NiSnO₃ is in accordance with that of the commercial NiSnO₃ (Sigma Aldrich) in previous reports,^9,10 indicating the crystalline nature of the NiSnO₃. For RGO, the appearance of the (002) diffraction peak at around 24.8° and disappearance of the diffraction peak at 11.8° in the XRD pattern mean that GO nanosheets were reduced to graphene nanosheets and restored to an ordered crystal structure.²² In the XRD pattern of the NiSnO₃/RGO composite, the major diffraction peaks correspond well with those of bare NiSnO₃,¹⁰ while the low and broad (002) diffraction peak indicates the disordered stacking of the RGO nanosheets.²³ Fig. 1b gives the XPS survey spectra of RGO and NiSnO₃/RGO. The dominant peak at 284.5 eV of the XPS spectra is assigned to graphitic C 1s. The O 1s peak (530 eV) appearing in both NiSnO₃/RGO and RGO XPS spectra indicates the existence of residual oxygen groups on RGO nanosheets. Other peaks in the survey XPS spectra of NiSnO₃/RGO are also found in Fig. 1b, which indicate the existence of Sn and Ni elements as expected. The peaks at 25 eV, 715 eV and 755 eV are related to Sn 4d, Sn 3p_3/2 and Sn 3p_1/2, respectively.¹⁶ The peaks at 485.9 and 494.5 eV correspond to Sn 3d_5/2 and Sn 3d_3/2, respectively.¹⁶ The peak at 855.4 eV is attributed to Ni 2p_3/2, and the peak at 872.8 eV corresponds to Ni 2p_1/2.²⁴ The Sn 3d XPS and Ni 2p XPS spectra are also shown in Fig. S1.† The results reveal that the Sn exists in the 4⁺ oxidation state and Ni exists in the 2⁺ oxidation state in the NiSnO₃/RGO composite.^16,24 To evaluate the RGO weight content in the composites, TG analysis was carried out from room temperature to 800 °C at a heating rate of 5 °C min⁻¹ in air. Based on the TG curve given in Fig. 1c, the weight contents of RGO and NiSnO₃ in the NiSnO₃/RGO composite were calculated to be about 25.7% and 74.3%, respectively.

Fig. 1 (a) XRD patterns of RGO, NiSnO₃ and NiSnO₃/RGO. (b) XPS survey spectra of RGO and NiSnO₃/RGO. (c) TG analysis of NiSnO₃/RGO nanocomposite.

The morphologies of the NiSnO₃/RGO composite were examined by SEM and TEM. Fig. 2a shows a representative SEM image of the highly loaded NiSnO₃/RGO composite. Instead of being exposed on the outer surface, most of the NiSnO₃ particles were found to be wrapped in the graphene matrix. The high loading benefited from the residual oxygen groups on the RGO nanosheets, which have a positive effect on anchoring the metal NPs on RGO.¹⁶ According to ref. 25, the tap density of NiSnO₃/RGO was measured when the volume of powder no longer changed after gently tapping for at least over 300 times. The high loading of NiSnO₃ NPs can provide a high tap density (1.23 g cm⁻³, powder form), enhancing the overall energy density per cubic centimeter of the NiSnO₃/RGO anode. Fig. 2b gives the EDX mapping (marked by the rectangular area in Fig. 2a) of the NiSnO₃/RGO composite. It demonstrates that the composites are composed of Sn, Ni, C, and O elements, which are homogeneously distributed throughout the samples. The structure of the composite was also examined by TEM analysis, as shown in Fig. 2c and d. It was observed that uniform NiSnO₃ NPs with subdued NPS aggregation are homogeneously distributed on the RGO nanosheets. Fig. S2† gives the particle size distribution, which is calculated based on the TEM image of Fig. 2d with the help of the Image J software; the NiSnO₃ NPs are relatively uniform with diameters of ca. 3–9 nm. At the same time, we noticed that there are no free NiSnO₃ NPs from the RGO, in spite of undergoing vigorous sonication, which implies a strong interaction between RGO and NiSnO₃ NPs; i.e., the RGO can not only act as an ideal matrix to load NiSnO₃ NPs and prevent their aggregation, but also avoid the exfoliation of NPs during the Li⁺ insertion/extraction, preserving the structural stability of the whole electrode. In contrast, the bare NiSnO₃ is composed of ultrafine particles, most of them stacked with each other in the absence of the immobilizing effect of RGO (Fig. S3†).

Fig. 2 (a) SEM image and (b) corresponding EDX mapping images for C, O, Sn, and Ni of NiSnO₃/RGO nanocomposites. (c) and (d) TEM images of NiSnO₃/RGO nanocomposites at various magnifications.

Cyclic voltammograms (CV) were obtained to study the reaction mechanism of the electrodes. Fig. 3 presents CV curves of the as-prepared NiSnO₃/RGO composite and bare NiSnO₃ electrodes for the first ten cycles scanned at 0.3 mV s⁻¹ between 0.02 and 3 V vs. Li/Li⁺. The first cycle CV curve of NiSnO₃ is almost identical to the previous report of the NiSnO₃ electrode (Fig. 3a).^9,10 However, the plots of the NiSnO₃ electrode shrink clearly with cycling, indicating the degenerative electrode kinetics, which may be caused by the uncontrolled formation of a solid electrolyte interface (SEI). Otherwise, the serious aggregation of NiSnO₃ NPs can reduce the contact area between the electrode and electrolyte, and lead to the non-reversing volumetric change during lithiation/delithiation.^6,9 For NiSnO₃/RGO, the first circle of the CV curves is also similar to that of tin oxides (Fig. 3b).^10,16 The cathodic peaks located at about 1.0, 0.6 and 0 V (vs. Li/Li⁺) are ascribed to the reduction reaction of NiSnO₃ with lithium, and peaks at around 0.5, 1.5 and 2.2 V in the anodic process correspond to the extraction of Li⁺ in the dealloying reaction.¹⁰ The difference between the initial two cycles may be due to the formation of SEI film and local structural rearrangement of the electrode to buffer the stress induced during the Li⁺ insertion/extraction.²⁶ Furthermore, for NiSnO₃/RGO, the plots are almost overlapped in the succedent scans, highlighting the better reversibility of the NiSnO₃/RGO electrode, which is due to the introduction of RGO, not only acting as a buffer to accommodate the volume changes, but also relieving the aggregation of NiSnO₃ NPs during long-term cycling. The mechanisms of the whole process can be described by the following electrochemical conversion reaction:

NiSnO₃ + 4Li⁺ + 4e⁻ → NiO + Sn + 2Li₂O (1)

NiO + 2Li⁺ + 2e⁻ ↔ Ni + Li₂O (2)

Sn + xLi⁺ + xe⁻ ↔ Li_xSn (3)

Fig. 3 Cyclic voltammograms of (a) NiSnO₃ and (b) NiSnO₃/RGO in the potential range of 0.02–3 V at a scan rate of 0.3 mV s⁻¹.

Fig. 4a–c show the discharge/charge (D/C) profiles of RGO, NiSnO₃ and NiSnO₃/RGO electrodes at a current density of 200 mA g⁻¹ between 0.02 V and 3.00 V. These voltage plateaus shown in the D/C profiles of the NiSnO₃ and NiSnO₃/RGO electrodes are consistent with the CVs observation in Fig. 3, but the D/C profile of the RGO shows an inconspicuous voltage plateau, as previously reported.²⁷ More importantly, it was found that the first coulombic efficiencies (CE) for bare NiSnO₃, RGO and NiSnO₃/RGO were about 32.2% (i.e., the charge/discharge capacity was 464 mA h g⁻¹/1443 mA h g⁻¹), 48.6% (i.e., 692 mA h g⁻¹/1424 mA h g⁻¹), and 65.9% (i.e., 974 mA h g⁻¹/1479 mA h g⁻¹), respectively. The enhancement of the first CE originated from the synergistic effect of the NiSnO₃ NPs and graphene network structure. In addition, it was observed that the NiSnO₃/RGO electrode has a slower shrinkage of charge/discharge curves after the first cycle, implying a higher cycling stability. However, the curves of NiSnO₃ contract quickly upon cycling, meaning an ineffective contact and then capacity decay.

Fig. 4 Galvanostatic charge/discharge curves of (a) RGO, (b) NiSnO₃, and (c) the NiSnO₃/RGO nanocomposites at the 1st, 2nd, 5th, 10th, and 30th cycles, between 3 and 0.02 V (vs. Li/Li⁺) at a current density of 200 mA g⁻¹. (d) Comparison of the cycling performance of RGO, NiSnO₃, and NiSnO₃/RGO electrodes at a current density of 200 mA g⁻¹.

Accordingly, Fig. 4d shows the cycling performance of RGO, NiSnO₃ and NiSnO₃/RGO electrodes at a current density of 200 mA g⁻¹. For bare NiSnO₃, the reversible charge capacity decreases to only 61 mA h g⁻¹ at the sixtieth cycle. We find that it decays more obvious than that of NiSnO₃ reported before,^9,10 which may be due to the higher aggregation degree of the ultrasmall NiSnO₃ NPs we prepared. The RGO electrode also shows considerable capacity decay and the reversible charge capacity decreases to 382 mA h g⁻¹ after 60 cycles, which may be attributed to the restacking of RGO nanosheets with hydrophobic nature. On the other hand, it is obvious to see that the NiSnO₃/RGO electrode exhibits much better cycling stability than those of RGO and NiSnO₃ electrodes. The CE of the composite rapidly rises from 65.9% in the first cycle to 94.6% in the second cycle, and 96.2% in the sixtieth cycle (Fig. S4†). The charge capacity of the NiSnO₃/RGO electrode maintains a value of 787 mA h g⁻¹ at the sixtieth cycle. This is much higher than the performance of NiSnO₃ in ref. 9, which maintains a capacity of around 450 mA h g⁻¹ at 100 mA g⁻¹. The significantly enhanced electrochemical performance is due to the high loading of NiSnO₃ NPs (3–9 nm) on the graphene matrix with large specific surface area. The introduced RGO can highly load the ultrasmall NiSnO₃ NPs and uniformly disperse them, which enables the full electrochemical utilization of the active material and offers plenty of lithium storage sites. At the same time, the NiSnO₃ NPs restrain the direct stacking of the RGO nanosheets and increase the contact area of RGO with the electrolyte.

The cycle performances of the NiSnO₃/RGO electrode were tested at a relatively higher current density after being activated at 150 mA g⁻¹ in the first three cycles, as shown in Fig. 5a. It was found that the reversible capacity of the NiSnO₃/RGO electrode was 881 mA h g⁻¹, tested at 600 mA g⁻¹ after 60 cycles, and the reversible capacity can remain at 792 mA h g⁻¹ even at a high current density of 1200 mA g⁻¹ after 60 cycles. The CE is kept at approximately 98.0% at 600 mA g⁻¹ or 1200 mA g⁻¹ at the 60th cycle (Fig. S4†). It was also found that the reversible capacities of NiSnO₃/RGO show a slight increase during cycling. This phenomenon may be attributed to a gradual activation process, and the reversible reaction between the NiSnO₃ NPs and the electrolyte, which also appeared for metal-oxide composite materials in the previous reports.^28,29 The nanometer size, good dispersity of metal oxide NPs, and the compact contact of metal oxide NPs/supports are beneficial for such activation processes.³⁰

Fig. 5 (a) Cycling performances of NiSnO₃/RGO nanocomposites at current densities of 600 and 1200 mA g⁻¹, (b) rate capability of RGO, bare NiSnO₃, and NiSnO₃/RGO nanocomposites at various current densities between 200 and 6000 mA g⁻¹.

To explore the root of the enhanced cycle performance of the as-synthesized composite compared to the NiSnO₃, TEM images of the two electrodes after 60 cycles at 600 mA g⁻¹ are shown in Fig. S5.† For the NiSnO₃/RGO electrode, it was seen that NiSnO₃ NPs were still uniformly distributed within RGO during cycling (Fig. S5c and d†). However, for the NiSnO₃ electrode, it was observed that the NiSnO₃ NPs had significant aggregation, which could lead to a rapid decay of charge capacity (Fig. S5a and b†). From the TEM images, it can be verified that one of the important mechanisms to explain the superior performance of the NiSnO₃/RGO, compared to the NiSnO₃ only, is that the NiSnO₃ within RGO is hard to aggregate during cycling.

Because of the particular nanostructure, the NiSnO₃/RGO composite also exhibits greatly improved rate capacity at different current densities between 200 and 6000 mA g⁻¹, compared with the bare NiSnO₃ and RGO electrodes, as is shown in Fig. 5b. For the NiSnO₃/RGO composite, capacities are relatively stable at various current densities. The high reversible charge capacities of 725 mA h g⁻¹ at 1200 mA g⁻¹, 590 mA h g⁻¹ at 2400 mA g⁻¹, 449 mA h g⁻¹ at 4800 mA g⁻¹ can be achieved, demonstrating an excellent high-rate performance of the nanocomposite electrode. Even at a high current density of 6000 mA g⁻¹, the NiSnO₃/RGO composite still delivers a reversible charge capacity of 350 mA h g⁻¹. However, the reversible charge capacity of RGO only remains at the value of 100 mA h g⁻¹ at a current density of 6000 mA g⁻¹. As for bare NiSnO₃, the large current density was almost too much to tolerate, and only a small capacity remained. As is known, RGO can endure the rapid charging and discharging process.¹⁵ The enhanced rate capability of the NiSnO₃/RGO composite originated from the good electric conductivity of RGO, and the short diffusion distance for both electrons and ions provided by the uniform, ultrasmall NiSnO₃ particles and flexible carbon matrix. More importantly, as the current density returned to 200 mA g⁻¹, a high capacity of 889 mA h g⁻¹ was recovered after 115 cycles for the NiSnO₃/RGO composite, much better than that of RGO (351 mA h g⁻¹). This indicates that the integrity of the electrode after high rate cycling remains, due to the interaction of disordered RGO nanosheets and NiSnO₃ NPs, which is beneficial for bearing varied charge and discharge currents. In order to verify whether the better rates are also a consequence of the sparsity of the matrices, we carried out SEM on the NiSnO₃/RGO electrode (Fig. S6†). The NiSnO₃/RGO electrode has a dense packing morphology. In fact, the close-packing of graphene and metal oxide NPs reduces the surface area without losing the superiority of the multi-porous nanostructure, but sets up the conducting network for enhancing the electrical conductivity. On the whole, the NiSnO₃/RGO electrode shows a better rate capability, which is due to a good synergistic effect between NiSnO₃ NPs and the RGO network. The good rate capability is important for rechargeable batteries in high power applications.

The volumetric capacity is another important parameter in practical applications of LIBs. The tap density of the NiSnO₃/RGO electrode powder is 1.23 g cm⁻³, which can yield a good volumetric performance. Fig. S7† shows the volumetric rate capacities of the NiSnO₃/RGO electrode. The volumetric charge capacities were 892 mA h cm⁻³ at 1200 mA g⁻¹, 726 mA h cm⁻³ at 2400 mA g⁻¹, 552 mA h cm⁻³ at 4800 mA g⁻¹, and 431 mA h cm⁻³ at 6000 mA g⁻¹. In addition, the volumetric capacity was well recovered to 1093 mA h cm⁻³ when the current density returned to 200 mA g⁻¹ after 115 cycles. Most of these values were higher than those of a graphite electrode with capacities of 652 and 242 mA h cm⁻³ at 100 and 1000 mA g⁻¹, respectively.¹⁹ The high volumetric energy density of the NiSnO₃/RGO composite enables it to be a suitable candidate when small volume and high rate are required in practical application.

Fig. 6 presents the electrochemical impedance spectroscopy (EIS) of bare NiSnO₃ and NiSnO₃/RGO electrodes after 8 charge/discharge processes at room temperature. Both impedance curves consist of one semicircle in the medium frequency region assigned to internal resistances in the electrode, and the sloped line assigned to Li⁺ diffusion into the bulk of the electrode materials. It can be seen that the semicircle of NiSnO₃/RGO is much lower than that of bare NiSnO₃ NPs, which indicates that NiSnO₃/RGO possesses lower charge transfer resistance during electrochemical reaction. The impedance spectra can be well fitted with an equivalent circuit, shown in the inset of Fig. 6. Here, the symbols, R_e, R_ct, R_s, and Z_w, relate to the electrolyte resistance, the charge-transfer resistance, the SEI film resistance, and Warburg impedance, respectively.³¹ As indicated in Table S1,† the R_ct of NiSnO₃/RGO is 32.7 Ω, which is smaller than that of NiSnO₃ NPs (163.7 Ω). The results clearly indicate enhanced electrochemical activity when introduced in a highly conductive RGO network. The lower charge transfer resistance can lead to rapid reaction kinetics of the NiSnO₃/RGO composite and further enhance their electrochemical performance.

Fig. 6 EIS profiles of the NiSnO₃ NPs and the NiSnO₃/RGO nanocomposites tested after 8 cycles in a frequency range from 0.01 Hz to 100 kHz with amplitude of 5 mV. The inset shows the corresponding equivalent circuit.

Conclusions

NiSnO₃ NPs were uniformly loaded on reduced graphene oxide by a facile hydrothermal technique and used as anodes for LIBs. Compared to bare NiSnO₃ NPs and RGO, the NiSnO₃/RGO composite exhibits better electrochemical performance. A high specific capacity of 792 mA h g⁻¹ was maintained after 60 cycles at 1200 mA g⁻¹. Even at a high current density of 6000 mA g⁻¹, the NiSnO₃/RGO anode still delivered a high reversible capacity of 350 mA h g⁻¹, and it was well recovered to 889 mA h g⁻¹, while the current density reverted to 200 mA g⁻¹ after 115 cycles. The NiSnO₃/RGO composite has a high tap density (1.23 g cm⁻³), and it also offers the benefit of a correspondingly high volumetric capacity. The increased LIBs performances of the NiSnO₃/RGO composite can be attributed to the synergistic effect of NiSnO₃ NPs and the conductive RGO network. Obviously, ultrasmall particle size, uniform distribution, and highly loading are the key factors for the NiSnO₃/RGO composite to have enhanced electrochemical performance.

Acknowledgements

We acknowledge the financial support by the Natural Science Foundations of China (No. 11404059 and No. 51202031), the Natural Science Foundation of Fujian Province (No. 2014J01175 and No. 2016J01010), the Fund of Education Committee of Fujian Province (JK2013010 and JA13064).

Notes and references

1. M. H. Park, M. G. Kim, J. Joo, K. Kim, J. Kim, S. Ahn, Y. Cui and J. Cho, Nano Lett., 2009, 9, 3844–3847.
2. V. Etacheri, R. Marom, R. Elazari, G. Salitra and D. Aurbach, Energy Environ. Sci., 2011, 4, 3243–3262.
3. Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa and T. Miyasaka, Science, 1997, 276, 1395–1397.
4. N. Tamura, R. Ohshita, M. Fujimoto, S. Fujitani, M. Kamino and I. Yonezu, J. Power Sources, 2002, 107, 48–55.
5. J. Y. Huang, L. Zhong, C. M. Wang, J. P. Sullivan, W. Xu, L. Q. Zhang, S. X. Mao, N. S. Hudak, X. H. Liu, A. Subramanian, H. Fan, L. Qi, A. Kushima and J. Li, Science, 2010, 330, 1515–1520.
6. J. S. Chen and X. W. Lou, Small, 2013, 9, 1877–1893.
7. Y. Zhao, X. Li, B. Yan, D. Xiong, D. Li, S. Lawes and X. Sun, Adv. Energy Mater., 2016, 6, 1502175.
8. Y. Sharma, N. Sharma, G. V. Subba Rao and B. V. R. Chowdari, J. Power Sources, 2009, 192, 627–635.
9. X. Li and C. Wang, RSC Adv., 2012, 2, 6150–6154.
10. L. Fu, K. Song, X. Li, P. A. van Aken, C. Wang, J. Maier and Y. Yu, RSC Adv., 2014, 4, 36301–36306.
11. A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183–191.
12. D. Lin, Y. Liu, Z. Liang, H. Lee, J. Sun, H. Wang, K. Yan, J. Xie and Y. Cui, Nat. Nanotechnol., 2016, 11, 626–632.
13. Y. Deng, C. Fang and G. Chen, J. Power Sources, 2016, 304, 81–101.
14. H. Zhang, L. Gao and S. Yang, RSC Adv., 2015, 5, 43798–43804.
15. B. Y. Wang, H. Y. Wang, Y. L. Ma, X. H. Zhao, W. Qi and Q. C. Jiang, J. Power Sources, 2015, 281, 341–349.
16. Y. Cao, L. Zhang, D. Tao, D. Huo and K. Su, Electrochim. Acta, 2014, 132, 483–489.
17. C. Chen, Q. Ru, S. Hu, B. An, X. Song and X. Hou, Electrochim. Acta, 2015, 151, 203–213.
18. Y. Wang, D. Li, Y. Liu and J. Zhang, Electrochim. Acta, 2016, 203, 84–90.
19. Y. Xu, Z. Lin, X. Zhong, B. Papandrea, Y. Huang and X. Duan, Angew. Chem., 2015, 127, 5435–5440.
20. H. R. Byon, B. M. Gallant, S. W. Lee and Y. Shao-Horn, Adv. Funct. Mater., 2013, 23, 1037–1045.
21. F. Ye, B. Zhao, R. Ran and Z. Shao, Chem.–Eur. J., 2014, 20, 4055–4063.
22. G. Wang, J. Yang, J. Park, X. Gou, B. Wang, H. Liu and J. Yao, J. Phys. Chem. C, 2008, 112, 8192–8195.
23. Z. S. Wu, W. Ren, L. Wen, L. Gao, J. Zhao, Z. Chen, G. Zhou, F. Li and H. M. Cheng, ACS Nano, 2010, 4, 3187–3194.
24. C. Kong, Z. Li and G. Lu, Int. J. Hydrogen Energy, 2015, 40, 9634–9641.
25. Z. S. Wu, W. Ren, L. Xu, F. Li and H. M. Cheng, ACS Nano, 2011, 5, 5463–5471.
26. X. Zhou, Z. Dai, S. Liu, J. Bao and Y. G. Guo, Adv. Mater., 2014, 26, 3943–3949.
27. D. Pan, S. Wang, B. Zhao, M. Wu, H. Zhang, Y. Wang and Z. Jiao, Chem. Mater., 2009, 21, 3136–3142.
28. F. Zou, X. Hu, Z. Li, L. Qie, C. Hu, R. Zeng, Y. Jiang and Y. Huang, Adv. Mater., 2014, 26, 6622–6628.
29. Y. Sun, X. Hu, W. Luo, F. Xia and Y. Huang, Adv. Funct. Mater., 2013, 23, 2436–2444.
30. C. Peng, B. Chen, Y. Qin, S. Yang, C. Li, Y. Zuo, S. Liu and J. Yang, ACS Nano, 2012, 6, 1074–1081.
31. M. Zou, J. Li, W. Wen, L. Chen, L. Guan, H. Lai and Z. Huang, J. Power Sources, 2014, 270, 468–474.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13186g

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