A Facile Synthesis of ZnCo2O4 Nanocluster Particles and the Performance as Anode Materials for Lithium Ion Batteries
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ZnCo2O4 nanocluster particles (NCPs) were prepared through a designed hydrothermal method, with the assistance of a surfactant, sodium dodecyl benzene sulfonate. The crystalline structure and surface morphology of ZnCo2O4 were investigated by XRD, XPS, SEM, TEM, and BET analyses. The results of SEM and TEM suggest a clear nanocluster particle structure of cubic ZnCo2O4 (~100 nm in diameter), which consists of aggregated primary nanoparticles (~10 nm in diameter), is achieved. The electrochemical behavior of synthesized ZnCo2O4 NCPs was investigated by galvanostatic discharge/charge measurements and cyclic voltammetry. The ZnCo2O4 NCPs exhibit a high reversible capacity of 700 mAh g−1 over 100 cycles under a current density of 100 mA g−1 with an excellent coulombic efficiency of 98.9% and a considerable cycling stability. This work demonstrates a facile technique designed to synthesize ZnCo2O4 NCPs which show great potential as anode materials for lithium ion batteries.
KeywordsZnCo2O4 nanocluster particles Hydrothermal method Sodium dodecyl benzene sulfonate Lithium ion batteries
ZnCo2O4 nanocluster particles (NCPs) were prepared through a hydrothermal method with the assistance of sodium dodecyl benzene sulfonate (SDBS).
The ZnCo2O4 NCPs exhibit excellent rate performance. The initial lithiation-specific capacity of ZnCo2O4 NCPs with a current density of 100 mA g−1 reached 1110 mAh g−1 with a coulombic efficiency of 84.7 %, and a high delithiation capacity of 700 mAh g−1 was achieved over 100 cycles.
It is well known that novel renewable energy sources and energy storage materials are two major challenges in electrochemical technology. Rechargeable lithium ion batteries (LIBs), which have been recognized as vitally important devices of power sources, have attracted widespread attention. LIBs with high energy and power density, low cost, and short charging time are needed urgently to meet the rapid development of hybrid and electric vehicles. In principle, the electrochemical performance of safe LIBs depends largely on the electrode materials for lithium storage.
Among the array of promising anode materials for LIBs, transition metal oxides have been widely studied due to their higher specific capacities compared to traditional graphite with a specific capacity of 372 mAh g−1. Ternary oxides, AB2O4 (A=Mg, Mn, Fe, Co, Ni, Cu, or Zn; B=Mn, Fe, Co, Ni, or Cu; A≠B), with a variety of crystal structures (spinel, scheelite, brannerite, etc.) have been investigated as anode materials for LIBs [1, 2, 3, 4]. This class of materials contains at least one transition metal ion and one or more electrochemically active/inactive ions. AB2O4 in previous electrochemical studies were synthesized via molten salt method [5, 6, 7, 8], oxalate decomposition method [9, 10], combustion method [11, 12], solvothermal method , etc. And they were found to show good Li cyclability with relatively high specific capacities.
The typical ternary oxide, zinc cobaltite (ZnCo2O4), possesses a spinel structure, where the Zn2+ occupies the tetrahedral sites and the Co3+ occupies the octahedral sites. ZnCo2O4 has been demonstrated to be a promising candidate as anode materials for LIBs because of the outstanding electrochemical performance (the theoretical specific capacity of 975 mAh g−1) and the abundant source, low cost, and low toxicity of zinc. Generally, the electrochemical performance of electrode materials depends on the preparation technique, the size and shape of particles and the morphology. The strategies deployed to prepare ZnCo2O4 are similar to those designed to synthesize AB2O4 mentioned above [1, 14].
Hao  reported porous ZnCo2O4 microspheres synthesized by a solvothermal method, with a high reversible capacity of 940 mAh g−1 at 0.1 °C. In Huang’s work , core–shell ZnCo2O4 microspheres were fabricated by a hydrothermal method. They showed an initial discharge capacity of 1280 mAh g−1 at 200 mA g−1, and only 3.9% capacity was lost between the 2nd and the 5th cycles at 400 mA g−1. According to Zhao’s study , highly ordered mesoporous spinel ZnCo2O4 was prepared with SBA-15 as templates. It displayed a high reversible capacity of 1623 mAh g−1 at 2.0 A g−1. The capacity still remained at 1470 mAh g−1 with a high current density of 8.0 A g−1. Wang’s group  prepared hierarchical porous ZnCo2O4 microspheres by simply decomposing PBA followed by sintering at 550 °C, which showed an initial lithiation and delithiation capacity of 1737.1 and 1051.6 mAh g−1, respectively, after 100 cycles at 100 mA g−1. In general, nanosized ZnCo2O4 with uniquely designed structures showed promising results in enhancing the electrochemical performance due to the high surface-to-volume ratio and the excellent electronic transport property. However, the limitation for the industrial application of this anode material is the control in preparation of the active material.
Herein, a facile approach is designed to synthesize uniform ZnCo2O4 NCPs. The cycling stability study of our ZnCo2O4 NCPs shows a delithiation capacity of 700 mAh g−1 over 100 cycles under a current density of 100 mA g−1. Excellent electrochemical performance of ZnCo2O4 NCPs demonstrates that it is promising to employ this material in high-energy storage devices.
3.1 Preparation of ZnCo2O4 NCPs and Structure Characterization
The morphology and structure of ZnCo2O4 NCPs were examined by a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Thermal analysis of the pink precursor power was characterized by thermogravimetry–differential thermal analysis (TG–DTA). The specific surface area of pure ZnCo2O4 NCPs powder was measured on Micromeritics Instrument Corporation TriStar II 3020 using N2 adsorption–desorption isotherms at −196 °C.
3.2 Electrochemical Characterization
CR2032 coin cell was used to carry out the electrochemical experiments with Li foil serving as a reference and a counter electrode. Slurries of the active material (ZnCo2O4 NCPs), carbon black, and poly (vinyl difluoride) (PVDF; weight ratio of 70:20:10) in N-methyl-2-pyrrolidone were pasted on pure Cu foil with a thickness of 150 µm and dried under vacuum at 95 °C overnight to make working electrodes. The active material loading was 1.0–1.5 mg cm−2. 1.0 mol L−1 LiPF6 dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC; volume ratio of 1:1:1) was used as the electrolyte. The cells were assembled in an Ar-filled glove box, with one microporous polypropene film (Celgard 2400) and one glass fiber as separator. An electrochemical workstation (VMP3/Z, Bio-logic, France) and a battery test system (CT-3008-5 V/5 mA, Neware Technology Ltd., Shenzhen, China) were used to test the electrochemical performance of all the cells under different current densities from 0.005 to 3.0 V vs. Li+/Li.
4 Results and Discussion
4.1 Structure and Morphology of ZnCo2O4
Figure 1b shows the XRD pattern of synthesized ZnCo2O4 NCPs. The exhibited diffraction peaks can be indexed as a single cubic phase of ZnCo2O4 with the lattice constant a = 8.06 Å, in good agreement with the standard value of 8.09 Å (JCPDS card No. 23-1390). No peaks from other phases are detected, implying the high purity of synthesized ZnCo2O4 NCPs. Based on the Scherrer formula, the average diameter of ZnCo2O4 NCPs is around 13 nm calculated from the XRD pattern.
4.2 Electrochemical Performance of ZnCo2O4 NCPs
Impedance parameters of ZnCo2O4–Li after different cycles in the fully charged state
OCV (V vs. Li) open-circuit voltage
R e (Ω) electrolyte resistance
R (sf+ct) (Ω) surface film + charge transfer resistance
CPE(sf+dl) (µF) constant phase element due to surface film + double layer capacitance
W s (Ω) Warburg resistance
C i (µF) intercalation capacitance
In summary, ZnCo2O4 NCPs are synthesized successfully by a designed hydrothermal method with the assistance of SDBS. The characterizations by XRD, SEM, and TEM show uniform ZnCo2O4 NCPs around 100 nm in diameter, comprising aggregated primary ZnCo2O4 nanoparticles (~10 nm in diameter). The electrochemical measurements reveal that the first lithiation and delithiation capacities of ZnCo2O4 NCPs are 1110 and 941 mAh g−1, respectively. After 100 cycles, a high reversible delithiation capacity of 700 mAh g−1 is retained. The high capacities and good stability are attributed to the unique nanostructures of ZnCo2O4, which demonstrate the promising application of our synthesized ZnCo2O4 as anode materials for LIBs.
We gratefully acknowledge the financial support of this research by the National Natural Science Foundation of China (51572052), the Natural Science Foundation of Heilongjiang Province of China (LC2015004), the China Postdoctoral Science Special Foundation (2015T80329), the Major Project of Science and Technology of Heilongjiang Province (GA14A101), and the Project of Research and Development of Applied Technology of Harbin (2014DB4AG016).
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