Enhanced electrochemical properties of graphene-wrapped ZnMn₂O₄ nanorods for lithium-ion batteries

Abstract

Thermally reduced graphene oxide (rGO)-wrapped ZnMn₂O₄ nanorods have been successfully fabricated via a facile bottom-up approach. Characterization results show that porous ZnMn₂O₄ nanorods are uniformly wrapped by ultrathin rGO sheets. The unique structure of this rGO–ZnMn₂O₄ composite could facilitate both ion and electron diffusion, thus providing suitable characteristics of an anode material for high performance lithium-ion batteries. Specifically, the conductive rGO sheets could act as an efficient buffer to relax the volume changes from Li⁺ insertion/extraction, and enable the structural and interfacial stabilization of ZnMn₂O₄ crystals. As a consequence, a high and stable reversible capacity (707 mA h g⁻¹ at 100 mA g⁻¹ over 50 cycles) and an excellent rate capability (440 mA h g⁻¹ at 2000 mA g⁻¹) are achieved with this composite material.

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Introduction

Lithium-ion batteries (LIBs) have achieved large-scale applications in portable electronic devices because of their high energy density, long lifespan and environmental friendliness.^1,2 However, for applications in large-scale systems such as electric vehicles (EVs), great efforts are still needed to explore ideal electrode materials with higher energy density as well as power density.^3,4 In the area of anode materials, manganese-based oxides have attracted particular attention due to their higher theoretical specific capacities (600–1000 mA h g⁻¹) compared with that of commercial graphite (372 mA h g⁻¹).^5–10 Particularly, ZnMn₂O₄ is one of the most attractive anode materials due to its high theoretical specific capacity (784 mA h g⁻¹, based on the reversible reaction: ZnLi + 2Mn + 3Li₂O ↔ ZnO + 2MnO + 7Li⁺ + 7e⁻) and low operating voltage (average discharging and charging voltages of 0.5 V and 1.2 V, respectively).^11,12 According to recent reports, ZnMn₂O₄ nanoparticles could easily achieve a stable and high capacity at a low current density. However, the rate properties of these ZnMn₂O₄ nanoparticles are relatively poor, mainly owing to the large volume expansion and slow ion/electron diffusion of the non-connected particles during the charging–discharging process.^11–13 An effective strategy to circumvent this problem is to construct interconnected nanoparticle networks to relieve volume expansion stress and facilitate fast Li⁺ diffusion.^14,15 In this regard, ZnMn₂O₄ materials with loaf-like structure, flower-like superstructure, hollow microsphere and one-dimensional (1D) nanostructures have been synthesized and have shown great improvements in reversible capacity at high current densities.^16–21 However, their rate properties are still limited owing to the inherent poor electrical conductivity of ZnMn₂O₄. As we know, the introduction of conductive carbon layers is one of the most common approaches to enhance the electrical conductivity of electrode materials.^22–24 Recently, graphene has been widely used as an efficient additive in various electrode materials for energy storage owing to its excellent electrical conductivity, high chemical stability and light weight.^25–29 Therefore, to simultaneously achieve high reversible capacity and good rate performances, it should be promising to explore a hybrid structure composed of conductive graphene and ZnMn₂O₄ with interconnected nanoparticle networks.

In this paper, in an attempt to improve the electrochemical properties of ZnMn₂O₄ we report the preparation of porous ZnMn₂O₄ nanorods and the subsequent wrapping of reduced graphene oxide (rGO) sheets on these nanorods. This unique architecture can provide 1D interconnected ZnMn₂O₄ nanoparticle networks to facilitate rapid diffusion of lithium ions within the electrode material. Meanwhile, the rGO sheets act as a conductive layer for electron transfer and an efficient buffer layer to enable the structural stabilization of ZnMn₂O₄ crystals. As a consequence, a high and stable reversible capacity and excellent rate capability are achieved using the as-synthesized rGO–ZnMn₂O₄ composite material.

Experimental section

Materials

Graphene oxide (GO) sheets were prepared by a modified Hummer's method.³⁰ GO powders were dispersed in water to form a homogeneous aqueous suspension with a concentration of 2 mg mL⁻¹ as described earlier by our group.³¹

Porous ZnMn₂O₄ nanorods were prepared by a simple electrospinning method followed by calcination under an air atmosphere. In a typical procedure, 1.25 mmol of Zn(NO₃)₂·6H₂O, 2.5 mmol of Mn(CH₃COO)₂·4H₂O and 2.0 g of polyvinylpyrrolidone (PVP, K30, M_w = 30 000) were added to 5 mL of solvent containing isometric ethanol and dimethylformamide (DMF). The mixture was vigorously stirred for 12 h and then ejected from a stainless steel capillary with a voltage of 15 kV. The distance between the capillary and the collector was 15 cm. The collected precursor film was calcined at 400, 500 and 600 °C respectively for 4 h in air with a heating rate of 2 °C min⁻¹.

To prepare rGO wrapped-ZnMn₂O₄ nanorods (denoted as rGO–ZnMn₂O₄), 50 mg of the as-prepared ZnMn₂O₄ nanorods were firstly dispersed in 150 mL of ethanol containing 5 g of 3-aminopropyltrimethoxysilane (APS). The mixture was refluxed at 80 °C in an oil bath for 10 h. After being filtered and dried, the APS-modified ZnMn₂O₄ nanorods were dispersed in 100 mL of water, and then 2.5 mL of the aqueous dispersion of GO was added. After being well dispersed with the aid of ultrasonic processing, the suspension was filtered and dried to obtain the GO–ZnMn₂O₄ composite. Finally, the rGO–ZnMn₂O₄ composite was obtained by annealing GO–ZnMn₂O₄ at 500 °C for 1 h under an argon (Ar) atmosphere. For comparison, as-made pure ZnMn₂O₄ nanorods were also subjected to the same post-thermal treatment, and the resulting sample was denoted as ZnMn₂O₄–Ar.

Characterizations

The structure of the as-synthesized samples was characterized by powder X-ray diffraction (XRD, Cu Kα radiation, Panalytical X'Pert Pro). The morphology of the samples was investigated using a field emission scanning electron microscope (FESEM, JSM 6701F) and a transmission electron microscope (TEM, Tecnai G20). The weight ratio of carbon in the rGO–ZnMn₂O₄ composite material was tested by an Elements analyzer (EA, VarioEL). Zeta potential measurements were carried out with a Zetasizer nano3600 instrument.

Electrochemical measurements

The working electrodes and lithium-ion half-cells were prepared as follows. Typically, a mixture consisting of 70 wt% active material, 20 wt% conducting agent (AB, acetylene black) and 10 wt% binder (PVDF, polyvinylidene difluoride) was milled with N-methyl pyrrolidone (NMP) to form a homogeneous slurry, which was then coated onto a copper foil. The as-prepared electrodes were dried under vacuum at 110 °C for 10 h. After being pressed, the electrodes were assembled into coin cells (CR2032) in an argon-filled glove box using 1 mol L⁻¹ LiPF₆ in ethylenecarbonate (EC) and diethylenecarbonate (DEC) (1 : 1, v/v) as the electrolyte and Li metal as the counter electrode. The assembled coin cells were tested in the voltage range of 0.01–3.0 V on a CT2001A cell test instrument (LAND Electronic Co.). Electrochemical impedance spectroscopy (EIS) measurements were carried out over a frequency range from 100 kHz to 0.1 Hz using an Autolab electrochemical workstation (PGSTATμ(III) lab, Matrohm Co.). All the electrochemical measurements were carried out at 25 °C in a digital biochemical incubator, and the specific capacity was calculated based on the weight of ZnMn₂O₄.

Results and discussion

In our study, ZnMn₂O₄ nanorods were prepared by calcining the electrospun precursor film at high temperatures under an air atmosphere. When the calcination temperature was 500 °C, as-prepared ZnMn₂O₄ nanorods had the right combination of pure ZnMn₂O₄ crystals and the best half-cell properties (see the ESI, Fig. S1 and S2†). Thus, ZnMn₂O₄ nanorods synthesized at this temperature were used to prepare the rGO–ZnMn₂O₄ composite. To this end, as-made ZnMn₂O₄ nanorods were modified with APS molecules in order to produce some positive charge on their surfaces. In our system, the average zeta potentials of GO and APS modified ZnMn₂O₄ nanorods are −22.9, and 18.6 mV respectively. Therefore, GO wrapped ZnMn₂O₄ nanorods can be easily obtained through the mutual electrostatic interaction between negatively charged GO sheets and positively charged ZnMn₂O₄ nanorods.²⁶ After a subsequent thermal reduction at 500 °C, a large amount of the oxygen-containing functional groups in the GO sheets were removed, demonstrating that the GO had been thermally reduced to rGO (see the ESI, Fig. S3†). A unique rGO–ZnMn₂O₄ composite was finally fabricated, in which the ZnMn₂O₄ nanorods are uniformly wrapped by ultrathin rGO sheets. The whole preparation process is illustrated in Fig. 1.

Fig. 1 Schematic diagram of the synthetic route to the rGO–ZnMn₂O₄ composite.

XRD was used to analyse the crystalline phase of the as-prepared materials, and the structure of ZnMn₂O₄ was identified by a Rietveld refinement method.³² As shown in Fig. 2a, the XRD pattern of the as-prepared ZnMn₂O₄ fits well with the tetragonal structure of ZnMn₂O₄ with a space group of I4₁/amd (JCPDS card no. 14-7311), and there are no visible impurities present, indicating the formation of highly pure ZnMn₂O₄. The inset image in Fig. 2a shows the normal spinel structure of ZnMn₂O₄, with ZnO₄ tetrahedra (green) occupying the 4a sites and MnO₄ octahedra (gray) occupying the 8d sites.³³ The unique spinel structure of ZnMn₂O₄ may be closely associated with its good electrochemical properties. As shown in Fig. 2b, no conventional stacking peak of graphene sheets at 2θ = 26.6° is detected in the pattern of rGO–ZnMn₂O₄, suggesting that rGO sheets are homogeneously dispersed onto the surfaces of the ZnMn₂O₄ nanorods.²⁷ However, in the patterns of rGO–ZnMn₂O₄ and ZnMn₂O₄–Ar samples, there are some new diffraction peaks which can be indexed to ZnO (JCPDS card no. 65-3411) and MnO (JCPDS card no. 07-0230). It is worth noting that there are fewer impurities in the rGO–ZnMn₂O₄ sample than in ZnMn₂O₄–Ar. Generally, ZnMn₂O₄ crystals can easily decompose into ZnO and MnO with the evolution of oxygen, because Mn³⁺ ions are quite unstable when they are thermally treated in an inert atmosphere.³⁴ Nevertheless, for the GO–ZnMn₂O₄ composite, it is believed that GO could be easily reduced to rGO and was wrapped tightly onto the surfaces of the ZnMn₂O₄ nanorods during the heat treatment under Ar, preventing the serious decomposition of the inner ZnMn₂O₄ crystals. Thus, the structures of most of the ZnMn₂O₄ crystals in the rGO–ZnMn₂O₄ composite remained unchanged.

Fig. 2 (a) Rietveld refined XRD pattern (green line) of the as-prepared porous ZnMn₂O₄ nanorods with experimental data (red dots), Bragg positions (green markers) and difference curve (blue line). The inset is the determined single cell structure for ZnMn₂O₄, with ZnO₄ tetrahedra (green) occupying the 4a sites and MnO₄ octahedra (gray) occupying the 8d sites. (b) XRD patterns of as-prepared ZnMn₂O₄, rGO–ZnMn₂O₄ and ZnMn₂O₄–Ar samples.

SEM and TEM were used to characterize the morphology of ZnMn₂O₄ nanorods and the distribution of rGO in the rGO–ZnMn₂O₄ composite sample. As shown in Fig. 3a and b, ZnMn₂O₄ nanorods are about 100–200 nm in diameter, and consist of ZnMn₂O₄ crystal grains with interpores as a result of the thermolysis of the polymer precursor. The interconnected nanoparticles and interpores could provide numerous channels for the access of electrolyte as well as the rapid diffusion of lithium ions within the electrode material. As shown in Fig. 3c, rGO sheets are uniformly wrapped around the surfaces of ZnMn₂O₄ nanorods, forming a homogeneous architecture. The TEM image (Fig. 3d) further shows that the separate ZnMn₂O₄ nanorods are firmly connected by the ultrathin rGO sheets. In addition, elemental analysis reveals that the weight fraction of carbon in the rGO–ZnMn₂O₄ composite is about 3.4 wt%. It is believed that the wrapping of rGO can prevent the direct exposure of the inner ZnMn₂O₄ crystal grains to the electrolyte and facilitate electron transfer on the surfaces of ZnMn₂O₄ particles, resulting in the improvement of the rate properties of the rGO–ZnMn₂O₄ composite.

Fig. 3 SEM and TEM images of as-prepared ZnMn₂O₄ (a and b) and rGO–ZnMn₂O₄ (c and d) samples.

To verify the effect of rGO sheets on the electrochemical properties of the rGO–ZnMn₂O₄ composite, we investigated the capacities and cycling performances of three different samples (ZnMn₂O₄, rGO–ZnMn₂O₄ and ZnMn₂O₄–Ar), and the corresponding results are illustrated in Fig. 4. As shown in Fig. 4a–c, in the first discharge step there is a well-defined long voltage plateau around 0.4 V which can be related to the irreversible reaction: ZnMn₂O₄ + 2Li⁺ + 2e⁻ → ZnO + 2MnO + Li₂O. Clearly, the voltage plateau of ZnMn₂O₄ is longer than in the other two samples, in which there are a few impurities such as ZnO and MnO. In the following charging–discharging process, the flat plateau is substituted by a sloping curve, which is mainly due to the reversible reduction: ZnO + 2MnO + 7Li⁺ + 7e⁻ ↔ ZnLi + 2Mn + 3Li₂O. It is amazing that as-synthesized ZnMn₂O₄ nanorods deliver an extremely high initial discharge capacity of 1690 mA h g⁻¹, which is much higher than the theoretical value in the first discharge process. The extra capacity may be attributed to the irreversible formation of a solid electrolyte interface (SEI) film at the ZnMn₂O₄ crystal–electrolyte interface, which could be enlarged by the pores of the ZnMn₂O₄ nanorods.¹² However, as a result of the large volume expansion of ZnMn₂O₄ crystals during the charging–discharging process, the reversible capacity fades seriously in the first 10 cycles, and is only 560 mA h g⁻¹ after 50 cycles. In comparison, as shown in Fig. 4b and d, rGO–ZnMn₂O₄ achieves a high initial charging capacity of 960 mA h g⁻¹. After 50 cycles, there is still a reversible capacity of 707 mA h g⁻¹ remaining, and the capacity retention is 73.6%. The reversible adsorption–desorption of lithium ions on the SEI surface and the addition of rGO may contribute to the high reversible lithium storage capacity of rGO–ZnMn₂O₄.³⁵ In particular, it was found that the initial Coulombic efficiency of rGO–ZnMn₂O₄ was 68%, while that of ZnMn₂O₄ was only 55%. On the one hand, the presence of ZnO and MnO in the rGO–ZnMn₂O₄ composite can avoid the irreversible formation of an amorphous Li₂O matrix in the first discharging step (ZnMn₂O₄ + 4Li⁺ → ZnO + 2MnO + 2Li₂O). On the other hand, the wrapped rGO sheets serve as a buffer as well as a conductive layer and could promote the transfer of lithium ions and electrons during the charging–discharging process. According to the results of the ZnMn₂O₄–Ar sample (Fig. 4c and d), it is clear that the ZnO and MnO impurities do not contribute to the total capacity, and that the capacity fades seriously. During the discharging process, impurities such as ZnO or MnO can cause heterogeneous agglomeration of Li₂O, which may hinder the transfer of electrons and ions. Therefore, the high and stable reversible capacity of rGO–ZnMn₂O₄ is mainly due to the protection of the wrapped rGO sheets and the enhanced conductivity.

Fig. 4 The charging–discharging curves of ZnMn₂O₄ (a), rGO–ZnMn₂O₄ (b) and ZnMn₂O₄–Ar (c), and the corresponding cycling performances (d) at a current density of 100 mA g⁻¹.

The rate properties of ZnMn₂O₄ and rGO–ZnMn₂O₄ are illustrated in Fig. 5. Pure ZnMn₂O₄ nanorods can achieve a stable specific capacity of nearly 600 mA h g⁻¹ at a current density of 100 mA g⁻¹, but only 200 mA h g⁻¹ at 2000 mA g⁻¹. In comparison, the reversible capacities of the rGO–ZnMn₂O₄ composite can reach 980, 916, 800, 677 and 440 mA h g⁻¹ at current densities of 100, 200, 500, 1000 and 2000 mA g⁻¹, respectively. When the current density is again reduced to 100 mA g⁻¹, an increased capacity of 1000 mA h g⁻¹ is achieved. The synergistic effect between the conductive rGO sheets and porous ZnMn₂O₄ nanorods contributes to the excellent rate properties of the rGO–ZnMn₂O₄ composite. In this architecture, the ultrathin rGO sheets could act as a buffer to relax the volume changes due to Li⁺ insertion/extraction, and as a conductive layer to enable the interfacial stabilization of ZnMn₂O₄ crystals. These excellent rate properties make the rGO–ZnMn₂O₄ composite a more promising anode material compared with previously reported ZnMn₂O₄ nanostructures (the corresponding results are listed in Table 1).

Fig. 5 Rate properties of ZnMn₂O₄ and rGO–ZnMn₂O₄ at various current densities between 0.01 and 3.0 V.

Table 1 A summary of recent studies on ZnMn₂O₄ electrode materials for LIBs

Material	Preparation method	1st discharging–charging capacity (mA h g^−1)	Capacity retention (mA h g^−1)	Rate properties (mA h g^−1)	Reference
Nanocrystalline ZnMn2O4	Polymer pyrolysis	1302/766 at 100 mA g^−1	569 (50 cycles)		11
ZnMn2O4 nanoparticles	A single-source precursor route	1088/680 at 100 mA g^−1	650 (200 cycles)	405 at 600 mA g^−1, 280 at 2400 mA g^−1	12
ZnMn2O4 nanoparticles	Hydrothermal	1200/680 at C/10	430 (50 cycles)	75 at 2 C	13
ZnMn2O4 nanoparticles	Coprecipitation	1145/∼800 at C/10	690 (70 cycles)	365 at 1 C, 250 at 2 C	7
Loaf-like ZnMn2O4	Hydrothermal + solid state reaction	1357/839 at 100 mA g^−1	517 (100 cycles) at 500 mA g^−1	810 at 50 mA g^−1, 457 at 1000 mA g^−1	16
Flower-like ZnMn2O4	Solvothermal	763 at 100 mA g^−1	626 (50 cycles)		17
ZnMn2O4 hollow microspheres	Coprecipitation	1325/772 at 400 mA g^−1	607 (100 cycles)	361 at 1600 mA g^−1	18
ZnMn2O4 hollow microspheres	Solvethermal	1335/∼750 at 100 mA g^−1	433 (50 cycles)	245 at 400 mA g^−1	19
Ball-in-ball ZnMn2O4, hollow microspheres	Thermally driven contraction process	945/662 at 400 mA g^−1	490 (50 cycles), 790 (120 cycles)	480 at 1000 mA g^−1, 396 at 1200 mA g^−1	20
ZnMn2O4 nanorods	Electrospinning	1257/680 at 60 mA g^−1	318 (50 cycles)		21
ZnMn2O4 nanofibers		1469/716 at 60 mA g^−1	705 (50 cycles)
ZnMn2O4 nanowires		1526/891 at 60 mA g^−1	530 (50 cycles)
ZnMn2O4 nanorods	Electrospinning	1690/930 at 100 mA g^−1	560 (50 cycles)	300 at 1000 mA g^−1	This work
rGO–ZnMn2O4 composite	Bottom-up wrapping	1412/960 at 100 mA g^−1	707 (50 cycles)	677 at 1000 mA g^−1, 450 at 2000 mA g^−1
ZnMn2O4 nanowires	Solid state reaction	1400/891 at 60 mA g^−1	650 (40 cycles)	350 at 1000 mA g^−1	36
ZnMn2O4 nanoplates	An “escape-by-crafty-scheme” method	1277/730 at 100 mA g^−1	502 (30 cycles)	426 at 900 mA g^−1, 324 at 1800 mA g^−1	37

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EIS measurements were carried out to further investigate the effect of rGO on the charge transfer characteristics of the rGO–ZnMn₂O₄ composite. Fig. 6 shows that the impedance spectra consist of a semicircle in the high to medium frequency region and an inclined line in the low frequency region. It is clear that the diameter of the semicircle for the cycled electrode is smaller than that of the fresh electrode, indicating that the cycled electrode exhibits faster charge transfer than the fresh electrode. Additionally, similar EIS changes can be observed for both ZnMn₂O₄ and rGO–ZnMn₂O₄ electrodes before and after CV measurements. This partially indicates that the charge transfer would be greatly hindered during the formation of the SEI film.³⁸ It is worth noting that the rGO–ZnMn₂O₄ electrode shows a smaller semicircle diameter after CV measurement compared with the pure ZnMn₂O₄ electrode, indicating a lower charge transfer impedance. These results demonstrate that the introduction of rGO sheets enhances the electrical conductivity, and thus significantly improves the rate capability of the rGO–ZnMn₂O₄ composite.

Fig. 6 EIS plots of ZnMn₂O₄ (a) and rGO–ZnMn₂O₄ (b) electrodes over the frequency range 100 kHz–0.1 Hz before and after 4 cycles by CV measurement at 0.5 mV s⁻¹.

Conclusions

In summary, we have developed a facile route to prepare rGO–ZnMn₂O₄ composite anode material for LIBs, which involves the electrospinning preparation of porous ZnMn₂O₄ nanorods followed by wrapping them with rGO sheets. This unique structure provides suitable characteristics of anode materials for high-performance LIBs. Compared with pure porous ZnMn₂O₄ nanorods, the rGO–ZnMn₂O₄ composite electrode exhibits higher reversible capacity and better rate properties. The excellent Li-storage performance indicates that this rGO–ZnMn₂O₄ composite has great potential for use in LIBs in the future. Furthermore, our approach for the fabrication of rGO-wrapped metal oxide nanorods can be applied to various other electrode materials, which may be useful for high performance energy storage devices.

Acknowledgements

We are grateful for financial support from the Top Hundred Talents Program of Chinese Academy of Sciences, and the National Natural Science Foundation of China (21103205, 21201171).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ta13511j

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