Mesoporous ZnCo₂O₄ microspheres as an anode material for high-performance secondary lithium ion batteries

Abstract

Herein, we report mesoporous ZnCo₂O₄ microspheres fabricated by a facile hydrothermal method followed by pyrolysis of a Zn_0.33Co_0.67CO₃ precursor. The obtained ZnCo₂O₄ microspheres were made up of closely packed primary nanoparticles with a diameter of about 30 nm and a large number of pores that were sized between 10 to 40 nm, which results in a high BET surface area of 39.52 m² g⁻¹. The large surface area permits a high interfacial contact area with the electrolyte and provides more locations and channels for fast Li⁺ insertion/extraction into the electrode material. The porous structure may not only be beneficial for Li⁺ ions to diffuse efficiently to active material with less resistance but also to buffer the volume expansion during the discharging/charging processes. When used as an anode material, the specific capacity was maintained at a high value of 1256 mA h g⁻¹ after 100 cycles at a current density of 100 mA g⁻¹, which is about 3.4 times larger than that of the commercial graphite electrode (372 mA h g⁻¹). More interestingly, a reversible capacity as high as 774 mA h g⁻¹ could be retained at a high current density of 1000 mA g⁻¹ after 200 cycles, which indicates that the mesoporous ZnCo₂O₄ microspheres had excellent cycling performance at a high current density for use as anode materials for lithium-ion batteries (LIBs).

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Introduction

Nowadays, lithium ion batteries are the state of the art secondary batteries that serve as a rechargeable power source for portable electronics and high energy fields because of their high energy density, long lifespan and environmental friendliness.^1–4 However, the theoretical capacity of commercial graphite electrodes is only 372 mA h g⁻¹, which cannot meet the increasing demand for high energy density.⁵ As a result, a large amount of efforts are being made to design and prepare various materials as anode materials for LIBs, such as Si,⁶ Sn,⁷ carbon⁸ and transition metal oxides.^9,10 Among them, nanostructured transition metal oxides, such as NiO,¹¹ Fe₂O₃,¹² Co₃O₄,¹³ have attracted extreme interest due to their promising electrical properties. Based on previous reports on the electrochemical performance of transition metal oxides, Co₃O₄ has shown the best anodic performance.¹⁴ However, Co₃O₄ is not an ideal electrode material due to its toxicity and high cost. Thus, a combination of two transition metal oxides with the spinel structure of ACo₂O₄ (A = Zn, Cu, Fe, Mg, Ni),^15–20 has been investigated immensely and is considered to be an ideal platform for anodic materials because of the complementary behaviour and synergy of the properties between the metal elements in the cycle process. For example, NiCo₂O₄ shows better electronic conductivity and better electrochemical activity than either nickel oxide or cobalt oxide.²¹

However, it is well known that the huge volume changes arising from the conversion reaction mechanism during the cycle process leads to the pulverization of the electrode materials and detachment from the current collector, resulting in poor cycling ability. Therefore, various structures with different shapes, such as nanotubes, nanorods, nanowires, nanophase^22–30 were synthetized to resolve the volume expansion problem. It could be concluded that porous nanostructures with nano-sized regions or domains provide excellent electrochemical performance because they possess enough free space for efficient transport and there is a high specific surface area. The high specific surface area and nano-sized structure have many advantages, including improving cycling life and possessing highly effective contact areas and short diffusion path lengths.^31,32 Furthermore, the free space can serve as buffer space during lithium ion insertion and extraction.

In this paper, the mesoporous ZnCo₂O₄ microspheres were prepared directly by a typical hydrothermal method. The whole experimental flowchart is schematically illustrated in Fig. 1. First, Zn_0.33Co_0.67CO₃ microspheres as the precursor was obtained by a facile solvothermal process. Afterwards, the precursor was calcined under an air atmosphere to turn into the mesoporous ZnCo₂O₄ microspheres with escaping CO₂ gas. When used as electrode materials for LIBs, the Li⁺ ion could pass into the ZnCo₂O₄ microspheres with little resistance and a short distance. Thus, the mesoporous ZnCo₂O₄ microspheres are predicted to show high reversible specific capacities, good cyclability and excellent rate performance as anode materials for LIBs.

Fig. 1 Schematic illustration of the formation of the ZnCo₂O₄ microspheres.

Experimental

Material synthesis

The mesoporous ZnCo₂O₄ microspheres were prepared directly by a typical hydrothermal method. In a typical synthesis, 1 mmol zinc nitrate (Zn(NO₃)₂·6H₂O, Aladdin, 99.9%) and 2 mmol cobalt chloride (CoCl₂·6H₂O, Aladdin, 99.9%) were dissolved into a mixed solution containing ethylene glycol (EG 25 ml) and distilled water (25 ml) under magnetic stirring. Then 30 mmol of ammonium bicarbonate (NH₄HCO₃) as a complexing agent was added to the above solution with continual stirring for 40 min, which resulted in a clear solution. The clear solution was transferred into a Teflon lined stainless-steel autoclave (100 ml capacity) and maintained at 180 °C for 20 h. The pink precipitate was collected and washed with distilled water and absolute alcohol several times and dried at 60 °C for 6 h. To obtain the mesoporous ZnCo₂O₄ microspheres, the precursors were further calcined at 450 °C for 8 h at a temperate rate of 2 °C min⁻¹.

Material characteristics

The crystallographic information for the mesoporous ZnCo₂O₄ microspheres was investigated by powder X-ray diffraction (XRD, PANalytical X'Pert PRO) with Cu-Kα radiation (λ = 1.5604 nm) at a scanning rate of 0.02° s⁻¹ in the 2θ range of 10° to 80°. The surface elemental composition of the mesoporous ZnCo₂O₄ microspheres was examined using an energy dispersive spectrometer (EDS). The thermogravimetric analysis (TGA/DTG) measurements of the mesoporous ZnCo₂O₄ microspheres were carried out at a heating rate of 10 °C min⁻¹ from room temperature to 600 °C. The morphologies and microstructures of the samples were characterized by scanning electron microscopy (SEM, ZEISS ULTRA55) and transmission electron microscopy (TEM, JEM-2100HR), respectively. The Brunauer–Emmett–Teller (BET) specific surface area of the mesoporous ZnCo₂O₄ microspheres was calculated using the BET equation. A desorption isotherm was used to determine the pore size distribution using the Barrett–Joyner–Halenda (BJH) method.

The electrochemical performance of the samples was measured using a CR2430 button cell, with lithium serving as the counter and reference electrode, assembled in an argon-filled glove box. The working electrode had a composition of 80 wt% active material, 10 wt% acetylene black and 10 wt% PVDF on the copper foil of 10 μm thickness, dried at 60 °C for 8 h and subsequently pressed. The electrolyte consisted of a solution of 1 M LiPF₆ in ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) (1 : 1 : 1 volume ratio). The charge/discharge test was conducted with a NEWARE Battery Testing System in the voltage range of 0.01 to 3.0 V at room temperature. Cyclic voltammetry (CV) was measured by a Solartron 1470E potentiostat at a scan rate of 0.1 mV s⁻¹ between 0.01 V and 3.0 V.

Results and discussion

The composition of the precursors was examined by XRD, and the results (shown in Fig. 2a) reveal that all the peaks can be indexed to calcite CoCO₃ (JCPDS no. 11-0692) and ZnCO₃ (JCPDS no. 08-0449), which indicates the precursors were made up of CoCO₃ or ZnCO₃. Combined with the EDS result (Co : Zn = 2.04 : 1), we can confirm the precursor was Zn_0.33Co_0.67CO₃. The crystallinity and phase information of the samples were provided by XRD and confirm the formation of single-phase ZnCo₂O₄. As indicated in Fig. 2b, all the diffraction peaks of the sample observed from the XRD pattern are in good agreement with the cubic ZnCo₂O₄ spinel structure (α = 8.095 Å; space group Fd3̄m (227), JCPDS card no. 23-1390). The diffraction peaks located at 2 theta values of 18.93°, 31.23°, 36.81°, 38.19°, 44.79°, 55.59°, 59.55°, 65.25° and 77.19° correspond to the (111), (220), (311), (222), (400), (422), (511), (440) and (533) crystal planes, respectively. The absence of any diffraction peaks for cobalt oxide or ZnO confirms the high purity of the samples. The narrow diffraction peaks are indexed to the high crystallinity of the mesoporous ZnCo₂O₄ microspheres. Additionally, the EDS information of the as-prepared products is shown in Fig. 2c. The elements of Zn, Co, O could be seen clearly and the other signals come from the SEM substrate. The molar ratio of Zn and Co was determined to be about 1 : 2.04, which agrees well with the theoretical ratio. Thus, we reasonably speculate that the XRD pattern, combined with the EDS pattern, confirms the synthesis of ZnCo₂O₄.

Fig. 2 XRD patterns of the precursors and as-prepared ZnCo₂O₄; EDS microanalysis of the samples.

The thermal properties of the Zn_0.33Co_0.67CO₃ precursor was characterized by the TG measurement. As shown in Fig. 3, there are two major weight loss steps: a gradual weight loss below 280 °C that is attributed to the loss of free water, EG or other organic molecules. The main weight loss of 32.19% between 280 and 380 °C, corresponds to a sharp exothermic peak located at 356.5 °C in the DTG curve, originates from the thermal decomposition of the Zn_0.33Co_0.67CO₃ into ZnCo₂O₄ by the release of CO₂, as shown in Fig. 1. Furthermore, the calculated value is comparable to the theoretical value (31.95%) for weight loss. No weight loss and exothermic peaks are found over 380 °C, indicating that the Zn_0.33Co_0.67CO₃ is transformed into ZnCo₂O₄ completely. Thus, 450 °C was chosen as the calcination temperature for the synthesis of the mesoporous ZnCo₂O₄ microspheres.

Fig. 3 TG/DTG curves for the Zn_0.33Co_0.67CO₃ precursor.

The detailed morphology and microstructure of the mesoporous ZnCo₂O₄ microspheres were investigated by SEM and TEM. From the SEM images, shown in Fig. 4a and b, it can be discovered easily that the products were composed of uniform spheres with an average diameter of 6 to 8 μm. A large number of pores appear in the microspheres, which arise from the generation of CO₂ during the decomposition of the precursor. Fig. 4c and d show images of a broken microsphere. The images reveal that the microstructure of a ZnCo₂O₄ sphere is made up of closely packed primary nanoparticles with a diameter of about 30 nm. As marked with the red arrow, the calculated size of the pores ranged from 10 to 40 nm, which matches well with the results of the desorption pore size distribution (Fig. 5). When used as an anode material, as shown in Fig. 4c, the Li⁺ ions can diffuse into and out the ZnCo₂O₄ microspheres through these pores with little resistance and a short distance. Moreover, the adequate free space could buffer the volume changes during the charging and discharging processes. A representative high-resolution TEM (HRTEM) image is shown in Fig. 4e, which further proves the good crystallinity of the products for its distinct lattice fringes. The HRTEM image (inset pictures in Fig. 4e) reveals three sets of lattice fringes with interplanar spacing of 0.23, 0.24 and 0.28 nm, corresponding to the (222), (311) and (220) plane of spinel crystalline ZnCo₂O₄, respectively. The selected area election diffraction (SAED) pattern (Fig. 4f) indicates the polycrystalline nature of the mesoporous ZnCo₂O₄ microspheres, and all the diffraction can be indexed to the (111), (220), (311), (400), (511) and (440) planes from the inside to the outside, respectively.

Fig. 4 Characterization of the mesoporous ZnCo₂O₄ microspheres: (a to d) SEM images, (e) high-resolution TEM images, (f) SAED.

Fig. 5 Nitrogen adsorption–desorption isotherm and the corresponding pore size distribution (inset) of the ZnCo₂O₄ microspheres.

The specific surface area of the as-prepared ZnCo₂O₄ microspheres was characterized by BET with a nitrogen adsorption–desorption isotherm at 200 °C for 24 h. From the result (Fig. 5), the isotherm could be categorized as a type IV with a H1 hysteresis loop (according to the IUPAC classification) at the relative pressure range of 0.9 to 1.0, implying a mesoporous architectural style of the obtained ZnCo₂O₄ microspheres. According to the corresponding Barrett–Joyner–Halenda (BJH) plots (the inset of Fig. 5) calculated from the nitrogen isotherms, the pore size distribution is in the range of 10 to 40 nm with an average diameter of about 28 nm, which agrees with size distribution observed in the SEM image. In addition, the mesoporous ZnCo₂O₄ microspheres exhibited a relatively high BET surface area of 39.52 m² g⁻¹ and a pore volume of 0.214 cm³ g⁻¹. The high surface area is mainly attributed to the unique nanostructure and big total pore volume. It is well known that more locations and channels can be provided by a large surface area for fast Li⁺ ion insertion/extraction into the electrode material. Furthermore, there may be enough pore volume to be beneficial for the Li⁺ ion to diffuse efficiently to the active material with less resistance and also to buffer the volume expansion during the discharging and charging processes. Thus, the obtained ZnCo₂O₄ microspheres are expected to improve the electrochemical performance.

Motivated especially by the structure, the mesoporous ZnCo₂O₄ microspheres were configured in a CR2430 coin cell to investigate the electrochemical properties. The first five cyclic voltammetry (CV) curves of the ZnCo₂O₄ electrode at a scan rate of 0.1 mV s⁻¹ in the voltage range of 0.01 to 3.0 V are shown in Fig. 6a. In the first cycle, a broad reduction peak is recorded at 0.42 V, which corresponds to the reduction reaction of ZnCo₂O₄ to Zn and Co with the formation of Li₂O. In the anodic sweep, two main oxidation peaks located at around 1.7 V and 2.1 V are attributed to the oxidation of Zn and Co to Zn²⁺ and Co³⁺, respectively.³³ A very weak peak observed at 0.9 V (the inset in Fig. 6a) arises from the dealloying process of the Li–Zn alloy. The main reduction peak at 0.42 V in the initial cycle shifting to 0.87 V originates from the irreversible reduction reaction and little or no formation of an SEI film,^25,30 whereas the two main oxidation peaks were barely shifted. In addition, the reduction and oxidation peaks in the CV curves overlapped substantially from the second cycle onward, indicating that the electrode made from the mesoporous ZnCo₂O₄ microspheres showed outstanding cycling ability for LIBs. According to the previous analysis, the lithium insertion and extraction reaction for the ZnCo₂O₄ electrode is believed to proceed as follows:

ZnCo₂O₄ + 8Li⁺ + 8e⁻ → Zn + 2Co + 4Li₂O (1)

Zn + Li⁺ + e⁻ ↔ LiZn (2)

Zn + Li₂O ↔ 2Li + ZnO + 2e⁻ (3)

2Co + 2Li₂O ↔ 2CoO + 4Li⁺ + 4e⁻ (4)

2CoO + 2/3Li₂O ↔ 2/3Co₃O₄ + 4/3Li⁺ + 4/3e⁻ (5)

Fig. 6 (a) The first five CV cycles for the ZnCo₂O₄ microspheres at a scan rate of 0.1 mV s⁻¹ in the voltage of 0.01 to 3.0 V. (b) Discharge–charge profiles of the ZnCo₂O₄ microspheres cycled at a constant current of 100 mA g⁻¹ at 0.01 to 3.0 V. (c) Cycling performance of as-prepared ZnCo₂O₄ electrodes for the first 100 cycles and the corresponding coulombic efficiency at a constant current of 100 mA g⁻¹, (d) rate performance of the ZnCo₂O₄ electrode.

Fig. 6b presents the selected discharge/charge cycle profiles of the electrode made from the mesoporous ZnCo₂O₄ microspheres at a current density of 100 mA g⁻¹ in a potential window between 0.01 and 3.0 V (vs. Li⁺/Li). From the profiles, it can be discovered that there is a clear potential plateau at about 0.9 V in the first discharge curve, and this potential plateau shifts upward close to 1.2 V and becomes steeper in the subsequent discharge curves, which is associated with the above CV detection results. The initial discharge and charge capacities are 1600 and 1205 mA h g⁻¹, leading to a high initial coulombic efficiency of about 75.3%. Compared with the theoretical value (975 mA h g⁻¹) based on the conversion reaction (eqn (1)), the excess capacities (about 625 mA h g⁻¹) can be mainly ascribed to the formation of the SEI film and the organic polymeric/gel-like layer.^34,35 Furthermore, the interfacial storage originating from the porous architecture is also considered to be responsible for the extra capacity. The irreversible capacity loss of 24.7% in the initial cycle is assigned to the phenomenon that formed the SEI film that cannot completely decompose during the first charge or to the conversion of crystal structures to amorpous.^36–38 In addition, there is a large deviation in potential between the discharging and charging curves, which is a common feature of a large number of metal oxide anodes due to the polarization related to ion transfer during the cycling process.³⁹ The large gap between charge and discharge curves involves the energy efficiency and this phenomenon may result from the poor electrical conductivity of the metal oxide anode.⁴⁰

The cycling performance and the corresponding coulombic efficiency of the designed mesoporous ZnCo₂O₄ microspheres electrode at a current density of 100 mA g⁻¹ in the range of 0.01 to 3.0 V is given in Fig. 6c. The discharge capacities obtained for the first and second cycles are 1600 and 1215 mA h g⁻¹, respectively. Moreover, the as-prepared mesoporous ZnCo₂O₄ microspheres presented excellent cyclic capacity retention upon prolonged cycling after the second cycle. From the 5th cycle onward, it is interesting to note that the reversible capacities begin to increase slightly during the cycling and a reversible capacity as high as 1256 mA h g⁻¹ was retained at the end of 100 cycles, which is a bit higher than the reversible capacity of the second cycle (1215 mA h g⁻¹). This interesting phenomenon may be attributed to more electrolytes gradually penetrating into inner part of active materials through numerous pores in the sample during the Li⁺ insertion/extraction process. On the other hand, the progressive generation of electrode-chemistry active polymeric gel-like films may also contribute to the increasing reversible capacity. Beside the superior specific capacity and excellent cycling ability, the rate capability is another important parameter for many practical applications of LIBs such as electric vehicles and power tools. The rate capability of the mesoporous ZnCo₂O₄ microsphere electrode was evaluated at various current densities from 500 to 4000 mA h g⁻¹. As shown in Fig. 6d, the average discharge capacity decreased from 1103, 1000, 760 to 430 mA h g⁻¹ when the current density was gradually increased from 500 to 1000, 2000 and 4000 mA g⁻¹. In the subsequent cycle, the specific capacities rebounded to 530, 670 and 900 mA h g⁻¹ with the current densities back to 2000, 1000 and 500 mA g⁻¹, respectively. This result demonstrates that the mesoporous ZnCo₂O₄ microspheres have great potential as a high rate anode material for LIBs.

Because the mesoporous ZnCo₂O₄ microspheres show an excellent rate capacity, we further evaluated the electrochemical performance of the ZnCo₂O₄ electrode at a high current density. As shown in Fig. 7, the discharge capacity decreased from the initial capacity of 1366 mA h g⁻¹ rapidly to 645 mA h g⁻¹ after 75 cycles when a high density of 1000 mA g⁻¹ was applied to the ZnCo₂O₄ electrode. The reason for the capacity decreasing rapidly may be that the continuous loss of active materials would occur at a high rate due to the partial embedding of metallic cobalt and zinc in the Li₂O matrix. The structural strain of the mesoporous ZnCo₂O₄ microspheres would be inevitable during the discharging and charging processes even though the mesoporous ZnCo₂O₄ microspheres provide enough free space to buffer the volume expansion, which contributes to the active materials failure. However, the discharge capacity slowly increased to 774 mA h g⁻¹ after 200 cycles. The result is highly superior to the recent report, which demonstrated ZnCo₂O₄ with a discharge capacity of 432 mA h g⁻¹ after 40 cycles and 331 mA h g⁻¹, 202 mA h g⁻¹ after 500 cycles.^41,42 In addition, the mesoporous ZnCo₂O₄ microspheres also showed outstanding electrochemical performance as anode materials for LIBs compared to other cobalt-based spinel structures. For example, the discharge capacity of MnCo₂O₄ microspheres just retained 320 mA h g⁻¹ after 200 cycles at 900 mA g⁻¹ and the mesoporous NiCo₂O₄ microspheres stabilized at 705 mA h g⁻¹ after 500 cycles at a current density of 800 mA g⁻¹.^43,44 The excellent cycling performance of mesoporous ZnCo₂O₄ microspheres at a high density further demonstrates their great potential as a high rate anode material for LIBs.

Fig. 7 Cycling performance of the as-prepared ZnCo₂O₄ electrodes at a high current density of 1000 mA g⁻¹.

Conclusions

In summary, we have developed a facile hydrothermal method to prepare uniform mesoporous ZnCo₂O₄ microspheres by pyrolysis of the Zn_0.33Co_0.67CO₃ precursor at 450 °C in air. The obtained mesoporous ZnCo₂O₄ microspheres possessed a high surface area and provided a large amount of mesopores. The large surface area permits a high interfacial contact area with electrolyte and provides more locations and channels for fast insertion/extraction of Li⁺ into the electrode material. The porous structure may not only be beneficial for the Li⁺ ion to diffuse efficiently within active material with less resistance but also to buffer the volume expansion during the discharging and charging processes. When used as an anode material for LIBs, the specific capacity was still maintained at a high value of 1256 mA h g⁻¹ after 100 cycles at a current density of 100 mA g⁻¹ in the potential ranging from 0.01 to 3.0 V, which is about 3.4 times higher than that of the commercial graphite electrode (372 mA h g⁻¹). More interesting, a reversible capacity as high as 774 mA h g⁻¹ could be retained at a high current density of 1000 mA g⁻¹ after 200 cycles, which indicates that the mesoporous ZnCo₂O₄ microspheres has the potential to be a high rate anode material for LIBs. Based on the facile synthetic method and excellent electrochemical properties, the mesoporous ZnCo₂O₄ microspheres hold promise to be the next-generation high power LIBs.

Acknowledgements

This work was supported by National Natural Science Foundation of China (Grant no. 51101062 and 51171065), Science and Technology Project of Guangzhou City, China (Grant no. 2011J4100075), Foundation for Distinguished Young Talents in Higher Education of Guangdong, China (Grant no. LYM09052), China Scholarship Council (no. 201308440314), Extracurricular Science Foundation for Students in South China Normal University of Guangdong, China (Grant no. 13WDGB03), The Scientific Research Foundation of Graduate School of South China Normal University (Grant no. 2013KYJJ039), The Natural Science Foundation of Guangdong province (Grant no. S2012020010937, 10351063101000001, 1414050001547), and University-Industry Cooperation Projects of Guangdong province, Ministry of Education and Science & Technology (Grant no. 2011A091000014).

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