Synthesis of graphene-wrapped ZnMn₂O₄ hollow microspheres as high performance anode materials for lithium ion batteries

Abstract

Hollow microspheres of ZnMn₂O₄ wrapped by graphene have been successfully synthesized via a facile APS aided method. Characterization results certify that the reduced graphene sheets have been wrapped around the hollow ZnMn₂O₄ microspheres. Charge–discharge testing reveals that ZnMn₂O₄/RGO delivers superior electrochemical properties in terms of specific capacity, cycle stability (1082 mA h g⁻¹ at 100 mA g⁻¹ after 90 cycles) and high rate capability (580 mA h g⁻¹ at 1 A g⁻¹ after 150 cycles). The improved rate capability and cycling performance of the modified ZnMn₂O₄ microspheres are attributed to the incorporated RGO sheets’ perfect synergy collaborating with the hollow structures, which can provide higher electronic conductivity, a shorter Li⁺ diffusion path and also buffer the volume change during Li⁺ insertion and extraction.

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Introduction

Energy storage devices have attracted great attention due to the increasing demands for sustainable and renewable energy owing to the limited supply of fossil fuels and environmental problems. Therefore, rechargeable lithium-ion batteries with high storage capacity and cycling stability are considered to be versatile, clean and promising power sources.^1–6 Over the years, metal oxides, such as nickel oxides, cobalt oxides, iron oxides and tin oxides, have been considered as potential substitutes for the traditional graphite anode due to their higher theoretical specific capacities (600–1000 mA h g⁻¹).^7–12 Recently, ZnMn₂O₄ with a spinel structure has been found to possess obvious advantages because of its lower cost, lower toxicity and lower working voltage (as a general rule, anode materials with lower charge/discharge voltages versus Li/Li⁺ can deliver a higher energy density).^13,14 As a consequence, ZnMn₂O₄ has attracted much attention as a high performance anode material for lithium-ion batteries.^15–18 However, the crucial problems of the material are the poor cyclability and rate performance during Li ion insertion and extraction due to the large volume change inducing electrode pulverization, and its poor electronic conductivity.^19,20 Therefore, developing a high performance ZnMn₂O₄ electrode material with both outstanding cycling stability and rate capability remains a great challenge. As we know, the introduction of conductive interconnected networks is one of the most common approaches to promote electrical conductivity, leading to enhancement of the electrochemical performance of electrode materials. For instance, Yin et al. synthesized a ZnMn₂O₄/carbon aerogel hybrid, which displayed a reversible capacity of 833 mA h g⁻¹ at a current density of 100 mA g⁻¹.²¹ Li et al. reported that the 3D ZnMn₂O₄/PCF exhibited a high capacity of 760 mA h g⁻¹ at 100 mA g⁻¹ and superior rate capability.²²

Graphene is a two-dimensional mono-atom thick lattice of carbon atoms. It has excellent electronic conductivity, a large specific surface area and chemical stability.²³ To date, great progress has been made on the synthesis of graphene-modified materials for lithium ion batteries.^24–26 The superior electrochemical performance is due to a three-dimensional conductive network offered by the graphene substrate. Furthermore, graphene can be used as a “buffer backbone” to support the electrode material and slow down the electrode pulverization, thus suppressing the capacity fading to a certain extent.^27–29 Recently, various hybrid ZnMn₂O₄/graphene composites have been reported, such as ZnMn₂O₄ nanoparticles/graphene,³⁰ ZnMn₂O₄ nanorods/graphene,³¹ ZnMn₂O₄ porous spheres/graphene,³² and as expected, a high reversible capacity and excellent rate capability were achieved.

Compared to other morphologies, hollow microspheres exhibit unique advantages such as increasing the electrolyte material contact area, buffering the large volume change and alleviating the strain caused during repeated Li⁺ insertion/extraction, thus improving the cycling stability and rate capability. Recently, a series of anode materials with hollow spherical architectures with improved capacities and cycle performances have been explored.^5,33–37

Herein, in order to obtain an improved electrochemical performance of the ZnMn₂O₄ anode, we report a method to prepare hollow ZnMn₂O₄ microspheres supported by reduced graphene oxide (RGO). Compared with the pure ZnMn₂O₄ material, the ZnMn₂O₄/RGO composite exhibits higher reversible capacity and enhanced rate capability. It is due to the synergistic effect of the stable hollow structures and graphene nanosheets on the accommodation of the volume change, the reduction of the diffusion path of lithium ions and the enhancement of electronic conductivity.

Experimental

Synthesis of ZnMn₂O₄ hollow microspheres

0.5948 g of Zn(NO₃)₂·6H₂O, 1.4316 g of Mn(NO₃)₂·4H₂O, and 0.42028 g of citric acid were dissolved in a solution consisting of 9 mL of deionized water and 25 mL of ethanol. The solution, after being magnetically stirred for 10 min in air at room temperature, was transferred to a 50 mL Teflon-lined stainless steel autoclave. The autoclave was then tightly sealed and left in an oven at 140 °C for 6 h. After the reaction, the autoclave was cooled to room temperature naturally. The products were obtained by centrifuging and sequentially washing with water and ethanol several times and then dried in a vacuum oven at 60 °C for 10 h.

Synthesis of RGO-modified ZnMn₂O₄ hollow microspheres

The GO suspension was prepared from natural graphite according to a modified Hummers method, as reported elsewhere.³⁸ To prepare the RGO-modified ZnMn₂O₄ hollow microspheres, typically, 0.4 g of ZnMn₂O₄ hollow microspheres was dispersed into 250 mL of ethanol. Then, 8 mL of 3-aminopropyltrimethoxysilane (APS) was added and refluxed at 80 °C for 10 h, followed by a sufficient wash with ethanol. Afterwards, the APS-treated ZnMn₂O₄ hollow microspheres were added into a certain amount of graphene oxide (GO) suspension under sonication. The suspension was then washed with deionized water and dried at 70 °C overnight. Finally, the ZnMn₂O₄/RGO composite was obtained by annealing the as-prepared powder at 350 °C for 3 h in air. The possible evolution process is illustrated in Fig. 1.

Fig. 1 Schematic diagram of the formation mechanism of ZnMn₂O₄/RGO.

Sample characterization

X-ray diffraction (XRD) of the samples was measured using a Bruker AXS D8 X-ray diffractometer using a Cu-Kα X-ray source operating at 40 kV and 100 mA. The diffraction data was recorded in the 2θ range of 10–80° with a scan rate of 5° min⁻¹. The elemental analysis was carried out using an Elementar Vario EL cube. X-ray photoelectron spectroscopy (XPS) tests were performed using an ESCALAB spectrometer with a Mg-Kα light source. The morphology of the material was studied using a JSM-6700F scanning electron microscope (SEM). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were taken using an FEI Tacnai G2 electron microscope operated at 200 kV. Raman scattering spectroscopy was recorded using a Renishaw inVia Raman microscope with Ar-ion laser excitation (λ = 663 nm).

Electrochemical measurements

The electrochemical characterization was performed using 2032-type coin cells while using metallic lithium foil as the counter electrode. The working electrode was composed of 80 wt% of the active material, 10 wt% of super P (Sinopharm) conductive additive and 10 wt% of CMC (Sigma-Aldrich)–SBR (NIPPON A&L INC) (CMC : SBR = 1 : 1 by weight ratio) binder, which was then pasted onto a copper current collector and cut into pieces of 1 cm × 1 cm. The loading mass of the active material was about 1–2 mg cm⁻². The counter electrode and working electrode were separated by a Celgard 2320 membrane. A 1 mol L⁻¹ LiPF₆ (lithium hexafluorophosphate) solution dissolved in EC (ethylene carbonate) and DMC (dimethyl carbonate) (EC : DMC = 3 : 7) was used as the electrolyte. The battery cells were assembled in an argon-filled glove box. The charge–discharge test was measured using a LAND-2100 (Wuhan, China) battery tester in the voltage range between 0.01–3.0 V versus Li/Li⁺. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were performed using a VSP multichannel galvanostatic-potentiostatic system (Bio-Logic SAS, France). The impedance spectra were carried out by applying an ac voltage of 5 mV in the frequency range from 1 MHz to 5 mHz.

Results and discussion

Structure and morphology analysis

Fig. 2 shows the XRD patterns of the ZnMn₂O₄ and ZnMn₂O₄/RGO samples. The materials can be indexed well on the basis of the tetragonal structure of ZnMn₂O₄ with a space group of I4₁/amd (JCPDS card no. 14-7311). The lattice parameters of both samples are the same despite the modification (a = 5.720 Å and c = 9.243 Å), which is consistent with the literature^21,39 and the values given in the standard card. The similar average crystallite sizes of the ZnMn₂O₄ and ZnMn₂O₄/RGO samples are calculated as about 30 nm from the (211) reflection according to the Scherrer formula D = kλ/β cos θ,⁴⁰ where β is the angular line width at half of the maximum intensity in the (211) crystal facet, λ is the wavelength of the X-ray radiation, k is a constant which is 0.9 and θ is the Bragg angle. There are no visible impurities present, indicating the formation of a highly pure ZnMn₂O₄ compound. It is noticed that no typical diffraction peak of GO (001) (normally appears at 10°) or graphene (002) (normally appears at 26.5°) can be observed in the XRD pattern of the ZnMn₂O₄/RGO composite which may be ascribed to the fact that GO was reduced to graphene during the heat treatment process and the RGO content in the composite is too low to be detected by XRD.⁴¹ The RGO content in the ZnMn₂O₄/RGO sample was determined to be merely 6.5 wt% via the C/H/N elemental analysis.

Fig. 2 XRD patterns of ZnMn₂O₄/RGO and ZnMn₂O₄.

Fig. 3 shows the Raman spectra of the as-synthesized ZnMn₂O₄, GO and ZnMn₂O₄/RGO samples. As we can see, both ZnMn₂O₄ and ZnMn₂O₄/RGO exhibit three obvious peaks located at 320 cm⁻¹, 385 cm⁻¹ and 680 cm⁻¹, which are consistent with the characteristic vibration modes of ZnMn₂O₄ reported in the literature.^42–46 In the cubic spinel oxides, the modes above 600 cm⁻¹ usually correspond to the motion of oxygen in the tetrahedral AO₄ group with A_1g symmetry.⁴⁷ The other two low frequency modes are characteristic of the octahedral site (BO₆).^48,49 Two additional Raman bands at 1356 cm⁻¹ and 1603 cm⁻¹ associated with the disordered (D) and graphitic (G) bands of carbon-based materials are present in the spectrum of the ZnMn₂O₄/RGO sample. The intensity ratio of the D-to-G bands is usually an indication of graphitization degree in carbon-based materials. A larger D-to-G intensity ratio of ZnMn₂O₄/RGO compared with that of GO strongly suggests the reduction of GO to graphene,^29,50 caused by the increased number of defects and edges generated during the heat treatment process.

Fig. 3 Raman spectra of ZnMn₂O₄, ZnMn₂O₄/RGO and GO.

XPS analysis reveals the surface composition and chemical state of ZnMn₂O₄/RGO. The survey spectrum in Fig. 4(a) shows that the sample consists of manganese, zinc, oxygen, and carbon elements.⁵¹ The Mn 2p spectrum shown in Fig. 4(b) exhibits two peaks at binding energies of 653.6 and 641.8 eV, corresponding to Mn 2p_1/2 and 2p_3/2, respectively. This characteristic indicates that the oxidation state of Mn in ZnMn₂O₄/RGO is Mn³⁺ and there are no signs of Mn²⁺ or Mn⁴⁺ ions detected.^52,53 The peaks at binding energies of 1044.9 and 1021.7 eV in the Zn 2p spectrum shown in Fig. 4(c) are attributed to Zn 2p_1/2 and Zn 2p_3/2, respectively, demonstrating the presence of Zn²⁺ in the samples.⁵⁴ The wide and asymmetric O 1s spectrum can be fitted into two peaks, as shown in Fig. 4(d). One stronger peak with the binding energy of 530.2 eV implies the characteristics of lattice oxygen in the metal (Mn, Zn) oxide. The peak of 532.2 eV indicates the contributions of the remaining unreduced GO and adsorbed oxygen, such as –OH and H₂O on the surface.⁵⁴ The C 1s XPS spectra of ZnMn₂O₄/RGO and GO are given in Fig. 4(e) and (f). The spectra of GO consists of four different peaks with binding energies centered at 284.6, 286.3, 287.0 and 288.1 eV, which can be attributed to graphitic C=C/C–C, C–O, C=O and COOH bonds, respectively.³⁸ After reduction, the peaks from the oxygen-containing functional groups are significantly weakened which confirms the successful reduction of GO (Fig. 4(f)). Therefore, good electrical conductivity is expected through undergoing this remarkable deoxygenation reaction.

Fig. 4 XPS survey spectrum of the ZnMn₂O₄/RGO composite (a), Mn 2p (b), Zn 2p (c), O 1s (d) and C 1s spectrum of the ZnMn₂O₄/RGO composite (e), and C 1s spectrum of GO (f).

Fig. 5(a)–(c) show the SEM images of the bare ZnMn₂O₄ sample. The uniform microspheres with diameters of 2–3 μm are detected and the hollow interior can be identified unambiguously from the broken part. Careful observation of the surface of the spheres reveals that each sphere consists of spinel-like primary particles with sizes ranging from 100–200 nm. The similar in size but less coarse surfaces of the ZnMn₂O₄ microspheres are shown in Fig. 5(d) after they are embedded in RGO. Since the surface of GO is negatively charged in an aqueous solution, opposite to that of ZnMn₂O₄, a spontaneous assembly of the two components occurred upon mixing. The wrapping structure can be recognized from Fig. 5(e) and (f), where the RGO films are clearly seen to exist and fill the space of the material to effectively link the adjacent ZnMn₂O₄ spheres. This connection can facilitate the transport of electrons in an effective percolating network and the contact area between the ZnMn₂O₄ spheres as well, thus giving rise to high electronic conductivity. The RGO sheets could not wrap over the surface of the ZnMn₂O₄ spheres entirely due to the relatively large dimensions of the microspheres. TEM was used to further analyze the detailed structural information of the ZnMn₂O₄/RGO sample. As shown in Fig. 6(a), TEM provides the best evidence for the hollow structure of the microspheres by showing a notable contrast difference between the hollow and solid parts, and the RGO sheets are successfully wrapped on the surfaces of the ZnMn₂O₄ spheres. Fig. 6(b) shows the high magnification TEM image of the sphere edge where the RGO coating layer can be clearly distinguished. The thickness of the RGO layer is measured to be 5–20 nm, which illustrates the multi-layer feature of graphene. Fig. 6(c) is a high resolution TEM (HRTEM) image recorded within the region marked by a rectangle in Fig. 6(a), indicating the amorphous state of graphene and the polycrystalline nature of the ZnMn₂O₄ spheres. Lattice fringes with d-spacings of 0.246 nm and 0.486 nm are observed, which is consistent with the (211) and (101) interplanar spacings of the spinel ZnMn₂O₄.^55,56 Hence, the presence of the flexible RGO wrapping layer on the ZnMn₂O₄ spheres can significantly improve the electronic conductivity of the material. DC electronic conductivity measurement shows that the electronic conductivity of the pristine ZnMn₂O₄ is 5.75 × 10⁻¹¹ S cm⁻¹ while that of the RGO-wrapped ZnMn₂O₄ is increased to 8.55 × 10⁻⁸ S cm⁻¹, which is three orders of magnitude higher than that of the pristine material.

Fig. 5 SEM image of the bare ZnMn₂O₄ (a–c) and ZnMn₂O₄/RGO (d–f).

Fig. 6 TEM images of the ZnMn₂O₄/RGO composite (a and b), HRTEM images of the ZnMn₂O₄/RGO composite (c).

Electrochemical measurements

The discharge/charge curves of the ZnMn₂O₄ and ZnMn₂O₄/RGO samples are shown in Fig. 7(a) and (b), which were recorded at a current density of 100 mA g⁻¹ in the voltage range of 0.01 to 3.00 V (versus Li/Li⁺). The specific capacity of ZnMn₂O₄/RGO is calculated based on the total mass of ZnMn₂O₄ and graphene. Despite their different specific capacities, both materials show similar discharge/charge curves indicating the same electrochemical reactions occurring in the Li⁺ insertion/de-insertion processes. In the first discharge step, there is a long and steady voltage plateau around 0.4 V which is replaced by a sloping curve in the following discharge, suggesting the reduction of ZnMn₂O₄ to metallic Zn and Mn embedded in the Li₂O matrix and the formation of the Li–Zn alloy. The related reaction can be depicted as ZnMn₂O₄ + 9Li⁺ + 9e⁻ → ZnLi + 2Mn + 4Li₂O.^57,58 For the subsequent discharge–charge process, the curves are quite similar and can be described as ZnLi + 2Mn + 3Li₂O ↔ ZnO + 2MnO + 7Li⁺ + 7e⁻.^32,59 However, both of the two samples have a higher practical capacity than the theoretical value in the first discharge stage which can be attributed to the formation of the solid electrolyte interface (SEI) film.^13,57,60 In addition, the first discharge and charge capacities of ZnMn₂O₄/RGO are 957 and 1346 mA h g⁻¹ with a coulombic efficiency of 71.1%, while those of the bare ZnMn₂O₄ are 650 and 1148 mA h g⁻¹, and the coulombic efficiency is 56.7%. There are two probable reasons that result in the low coulombic efficiencies, one is the irreversible reaction during the first discharge process, while the other is the formation of a thick irreversible SEI layer on the surface of the material associated with the first cycle. Nevertheless, we can obviously find that the participation of graphene considerably increases the charge/discharge capacities and boosts the initial coulombic efficiency of ZnMn₂O₄. The cycle performance of ZnMn₂O₄ and ZnMn₂O₄/RGO samples is displayed in Fig. 7(c).

Fig. 7 The charging–discharging curves of ZnMn₂O₄ (a) and ZnMn₂O₄/RGO (b), the corresponding cycling performances at a current density of 100 mA g⁻¹ (c), and the rate properties of ZnMn₂O₄ and ZnMn₂O₄/RGO (d) at various current densities between 0.01 and 3.0 V.

Obviously, the bare ZnMn₂O₄ exhibits gradual capacity fading with charge/discharge cycling. It only shows a discharge capacity of 281 mA h g⁻¹ after 90 cycles, corresponding to a capacity retention of 43.2%. In comparison, the ZnMn₂O₄/RGO electrode shows excellent capacity retention. The material can still deliver a high discharge capacity of 1082 mA h g⁻¹ after 50 cycles which is even better than that of the first cycle. The rate capability of the ZnMn₂O₄ and ZnMn₂O₄/RGO materials is presented in Fig. 7(d), the ZnMn₂O₄/RGO electrode also exhibits a greatly enhanced rate capability compared to the pure ZnMn₂O₄ electrode. Even at a high current density of 2 A g⁻¹, the ZnMn₂O₄/RGO electrode can still deliver a reversible capacity of about 280 mA h g⁻¹, which is much higher than that of the pure ZnMn₂O₄ (55 mA h g⁻¹). The high rate cycling performance of the ZnMn₂O₄/RGO composites was tested at the current density of 1 A g⁻¹. As shown in Fig. 8, the hybrid electrode shows an initial discharge specific capacity of 844 mA h g⁻¹, and the specific discharge capacity of this composite could still remain at 580 mA h g⁻¹ after 150 cycles. The superior electrochemical performance of ZnMn₂O₄/RGO can be attributed to the participation of graphene. Firstly, RGO sheets significantly improved the electronic conductivity of the material, thus reducing the ohmic polarization of the electrode and resulting in good rate capability. Secondly, the large interface area of graphene provides more Li⁺ insertion/extraction sites, which facilitate Li⁺ transfer between the electrode and the electrolyte, thus leading to a large reversible capacity. In addition, the graphene nanosheets can work as an elastic buffer to accommodate larger volume expansion/contraction during Li⁺ insertion/de-insertion, thus leading to excellent cycling stability.

Fig. 8 Cycling performance of ZnMn₂O₄/RGO composites at 1 A g⁻¹.

The morphologies of the pure ZnMn₂O₄ and ZnMn₂O₄/RGO electrodes after the 100^th high rate cycle were investigated using SEM. As shown in Fig. 9(a) and (b), the electrode surface of ZnMn₂O₄ exhibits some severe big gaps, while a dense and smooth surface film could be maintained in the cycled ZnMn₂O₄/RGO electrode, indicating its good maintainability during cycling. This is probably due to the strong cohesive force among the material particles offered by the graphene substrate. From larger magnification images shown in Fig. 9(c) and (d), it is hard to find integral spheres in the ZnMn₂O₄ electrode, and the gaps can be observed even clearer. As a comparison, the spherical character has been maintained very well in the ZnMn₂O₄/RGO electrode. The above results are also the rational explanation of the enhanced cycle and rate performance of the ZnMn₂O₄/RGO material.

Fig. 9 SEM images of the pure ZnMn₂O₄ (a and c) and ZnMn₂O₄/RGO (b and d) electrodes after the 100^th high rate cycle.

AC impedance measurements were carried out in order to gain insight into the remarkable electrochemical performance of ZnMn₂O₄/RGO. Fig. 10(a) and (b) provide the Nyquist plots of the ZnMn₂O₄ and ZnMn₂O₄/RGO samples collected at the open circuit voltage (OCV) and after three discharge/charge cycles. As shown in the figures, the impedance plots consist of a depressed semicircle as well as a straight line. It is well known that the intercept at the highest frequency is attributed to the internal resistance (R_s) of the cell arising from the electrolyte, separator, current collector, etc. The semicircle in the middle frequency range is due to the charge-transfer resistance (R_ct). The straight line in the low frequency (Warburg) is associated with lithium ion diffusion in the electrode bulk. Based on this, the Nyquist plots are simulated using the equivalent circuit, as shown in the insets of Fig. 10(a) and (b). The Nyquist plots at the OCV state reflect the electrochemical kinetic properties of the block electrodes. It is seen that the ZnMn₂O₄/RGO electrode has a smaller semicircle (366.4 Ω) than the bare ZnMn₂O₄ electrode (771 Ω). This indicates that the charge transfer resistance of the electrode is decreased by the participation of RGO. The charge transfer resistance significantly decreases after three cycles due to the effective activation of the electrodes. One can see that the charge transfer resistance of ZnMn₂O₄/RGO (31.25 Ω) is still smaller than that of the bare ZnMn₂O₄ electrode (139.4 Ω). This result validates that the RGO in ZnMn₂O₄ microspheres enables lower charge transfer resistance and enhances the electrochemical activity of the ZnMn₂O₄ active material.

Fig. 10 Nyquist plots of ZnMn₂O₄ and ZnMn₂O₄/RGO before cycling (a) and after 3 cycles (b) using EIS measurement at 0.1 mV s⁻¹ and linear fit of the Z_real versus w^−1/2 relationship of ZnMn₂O₄ and the ZnMn₂O₄/RGO composite (c).

The Warburg region in the Nyquist plots has been used to determine the chemical diffusion coefficient of Li⁺ in electrode materials. By using the model proposed by Ho et al., D_Li of ZnMn₂O₄ and ZnMn₂O₄/RGO can be calculated using the equation:⁶¹

where V_m is the molar volume of ZnMn₂O₄, F is the Faraday constant, S is the active surface area of the electrode, and is the first-order derivative of the discharge profile. σ is the Warburg factor which obeys the following relationship:

Fig. 10(c) displays the linear fitting of Z_real vs. w^−1/2, from which the slope σ can be obtained. Based on these, the chemical diffusion coefficients of ZnMn₂O₄ and ZnMn₂O₄/RGO were calculated as 1.362 × 10⁻¹¹ cm² s⁻¹ and 2.08 × 10⁻¹¹ cm² s⁻¹, respectively. The lithium diffusion coefficient D_Li is a transport parameter that reflects both the ionic and the electron transports of the active material. It is deduced from Fick’s first law and can be expressed as the product of a self-diffusion coefficient D_Li(self) and a thermodynamic factor Φ, i.e., D_Li = D_Li(self) × Φ. D_Li(self) and Φ are very sensitive to the structural and electronic properties of the material, respectively. Briefly, D_Li(self) is a measure of the diffusion that takes place even in the absence of a chemical potential gradient and corresponds to the ionic mobility or viscosity in a solution. The Φ factor measures the chemical potential deviation from that of an ideal solution. It depends on both kinetic parameters (transport number, mobility) and thermodynamic properties (stoichiometry, activity). Sometimes it indicates that the ionic flux density intensifies due to the simultaneous transport of electrons through the active material.^62,63 As an intrinsic parameter of intercalation materials, the self-diffusion coefficient of ZnMn₂O₄ should not be changed by GO wrapping. Thus, the larger lithium diffusion coefficient of ZnMn₂O₄/RGO should be closely related with higher electronic conductivity. The enhancement of electronic conductivity, no matter what the intrinsic electronic conductivity or the surface electronic conductivity, will lower the internal resistance of the electrode, and therefore increase the internal electrical field inside the battery. As a result, the diffusion of Li ions is accelerated by this stronger internal electrical field thus resulting in a higher lithium diffusion coefficient.

To consummate our investigation, the high rate electrochemical tests of ZnMn₂O₄/RGO with various RGO content have been carried out. As displayed in Fig. 11, the result verifies again that the incorporation of RGO indeed improves the electrochemical performance of ZnMn₂O₄. Moreover, an appropriate amount of RGO is helpful to significantly improve the electrochemical performance of ZnMn₂O₄.

Fig. 11 Cycling performance of the ZnMn₂O₄/RGO composites at various percentages of RGO content.

Conclusions

In summary, we presented a facile synthetic approach to fabricate spinel ZnMn₂O₄/RGO hollow microspheres. As expected, ZnMn₂O₄/RGO shows improved capacity, cycling stability and rate capability compared to bare ZnMn₂O₄. The superior performance of ZnMn₂O₄/RGO is due to the synergistic effect between the conducting graphene nanosheets and the ZnMn₂O₄ hollow microspheres. The unique carbon framework not only provides an expressway for electron transfer during Li⁺ insertion/de-insertion, but also improves the lithium diffusion in the electrode bulk. The good electrochemical properties of ZnMn₂O₄/RGO make it a promising anode material for Li-ion batteries.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (No. 51272088 and 21201073), the Science and Technology Development Planning of Jilin Province (No. 20150204030GX), the graduate Innovation Fund of Jilin University Project (No. 2015012).

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