Formation of NiFe2O4/Expanded Graphite Nanocomposites with Superior Lithium Storage Properties
A NiFe2O4/expanded graphite (NiFe2O4/EG) nanocomposite was prepared via a simple and inexpensive synthesis method. Its lithium storage properties were studied with the goal of applying it as an anode in a lithium-ion battery. The obtained nanocomposite exhibited a good cycle performance, with a capacity of 601 mAh g−1 at a current of 1 A g−1 after 800 cycles. This good performance may be attributed to the enhanced electrical conductivity and layered structure of the EG. Its high mechanical strength could postpone the disintegration of the nanocomposite structure, efficiently accommodate volume changes in the NiFe2O4-based anodes, and alleviate aggregation of NiFe2O4 nanoparticles.
KeywordsNiFe2O4 Expanded graphite Anode materials Lithium-ion batteries
A NiFe2O4/expanded graphite (NiFe2O4/EG) nanocomposite was synthesized via a simple grinding and mixing process followed by annealing at a high temperature. The obtained NiFe2O4/EG nanocomposite showed superior lithium storage properties, with a capacity of 601 mAh g−1 at a current density of 1 A g−1 after 800 cycles.
The hybridNiFe2O4/EG nanostructure could efficiently improve the electrical conductivity and maintain structure stability, and its disintegration was delayed during discharge–charge processes, which led to a good cycling performance.
To meet the demands of high-energy storage for practical applications in electric vehicles (EVs) and hybrid electric vehicles (HEVs), much attention has been paid to lithium-ion batteries (LIBs) because of their distinct advantages, e.g., their large specific capacity, high energy density, long cycle life, and environmental friendliness [1, 2, 3]. Today, there is a great demand to design and develop novel and high-performance electrode materials to achieve LIBs with higher energy density, longer cycle life, improved safety, and lower cost. However, it is still urgency to improve the reversible charge capacity of anode materials in commercial LIBs, i.e., graphite [4, 5]. Among various anode materials with higher specific capacities than that of the material used in commercial batteries, ternary compounds (e.g., CuFe2O4 [6, 7], ZnFe2O4 [8, 9], CoFe2O4 [10, 11, 12], Zn2SnO4 [9, 13], ZnMn2O4 [14, 15, 16, 17]) have been considered promising candidates because of their high theoretical capacities, low cost, and safety . Nevertheless, their poor electrical conductivities and huge volume changes during continuous charge–discharge processes would lead to electrode pulverization. Rapid disintegration of these electrode materials caused by induced mechanical stress is responsible for a decrease in the capacity upon cycling and further hinders their practical applications [18, 19]. Expanded graphite (EG) possesses many advantageous properties, including fewer functional groups, better conductivity, higher mechanical strength , and lower cost . A layered NiFe2O4/EG composite structure can be fabricated by incorporating nanostructured NiFe2O4 material with EG. This provides outstanding electron conductivity [22, 23, 24, 25] and an ideal solution to the aforementioned inherent drawbacks of ternary compounds. However, to the best of our knowledge, there have been few reports on the construction of nanostructured material/EG composites with superior lithium storage properties because it is a challenge to homogenously disperse nanostructured materials into EG nanosheets.
In this work, a NiFe2O4/EG nanocomposite was easily fabricated via a grinding and mixing process, followed by annealing at a high temperature. This NiFe2O4/EG nanocomposite showed superior lithium storage properties, with a capacity of 601 mAh g−1 at a current density of 1 A g−1 after 800 cycles. In general, the combination of several structural features of the NiFe2O4/EG nanocomposite may contribute to the enhanced capacity and cycling performance. First, the layered structure of the NiFe2O4/EG nanocomposite can alleviate agglomeration of materials and improve the cycling performance. Second, the higher mechanical strength of EG can postpone disintegration of the nanocomposite structure. Third, EG can provide an enhanced conductivity performance, which is critical for the lithium storage performance. Fourth, EG has a high reversible capacity and good cycling performance, which can assist in enhancing the capacity and cycling performance of NiFe2O4/EG nanocomposite.
3.1 Synthesis of NiFe2O4 and NiFe2O4/EG Nanocomposites
NiFe2O4 was synthesized using the procedure reported in a previous paper . Typically, 1 mmol of nickel chloride hexahydrate (NiCl2·6H2O), 2 mmol of ferric chloride hexahydrate (FeCl3·6H2O), and 3 mL of ammonia were dissolved in 37 mL of alcohol. The mixture was sonicated for 30 min, transferred to a Teflon-lined autoclave, and then maintained at 180 °C for 12 h. The final product was separated by centrifugation and dried at 60 °C.
The NiFe2O4/EG nanocomposite was synthesized using the following procedure: 0.5 g of NiFe2O4, 0.2 g of expandable graphite, and 5 mL of alcohol were mixed and ground until the alcohol was completely volatilized. After that, the mixture was annealed at 1000 °C for 5 min in an Ar atmosphere, and the final product was ground and used as the active material for lithium-ion batteries.
The crystal structures of the powder samples were characterized using X-ray diffraction (XRD, Shimadzu XRD-6000, CuKα, 40 kV, 30 mA, 20° ≤ 2θ ≤ 70°). A thermogravimetric (TG) analysis was performed on a PerkinElmer 7 instrument to determine the weight ratio of EG to NiFe2O4. The morphology of each sample was studied using a transmission electron microscopy (TEM) system (JEOL, JEM-2100).
3.3 Electrochemical Testing
The working electrodes were fabricated from a slurry containing 80% active material, 10% polymer binder (polyvinylidene difluoride, PVDF), and 10% acetylene black on a copper foil using the procedure outlined in previous work  and dried in a vacuum oven at 80 °C for 12 h. The electrochemical performances were determined using a LAND battery tester (CT2001A model, Wuhan Jinnuo Electronics, Ltd.) between 0.01 and 3 V versus Li+/Li. All of the tests were performed at room temperature, with an electrolyte composed of 1 mol L−1 of LiPF6 in a mixed solvent of ethylene carbonate (EC)/diethylene carbonate (DEC) (1:1 vol%) and a Li cathode placed in the cell. Cyclic voltammetry measurements were performed using a CHI 660C potentiostat between 0.01 and 3 V at a scan speed of 0.5 mV s−1. A frequency range of 10 kHz to 0.1 Hz was used for electrochemical impedance spectroscopy (EIS) at an open-circuit potential with an alternating spectrum (AC) perturbation of 10 mV on a Zennium electrochemistry workstation.
4 Results and Discussion
Figure 5b shows the cyclic performances of NiFe2O4 and NiFe2O4/EG at a current density of 1 A g−1. A discharge capacity of 986 mAh g−1 and a charge capacity of 741 mAh g−1 in the first cycle were observed on the NiFe2O4/EG electrode. The large initial irreversible discharge capacity could be attributed to the formation of the SEI. The capacities continued to decrease until the 50th cycle. This capacity decrease was partially ascribed to the decomposition of the electrolyte , along with the incompletely reversible reaction of the NiFe2O4/EG nanocomposite. It is interesting to note that after the 100th cycle, the reversible capacities significantly increased with the further activation of NiFe2O4, which was also observed in earlier work . Furthermore, the reversible capacities of the NiFe2O4/EG continued to increase from 443 mAh g−1 in the 450th cycle to 601 mAh g−1 in the 800th cycle. These increasing capacities can be attributed to the reversible polymeric/gel film on the nanocomposite. A similar phenomenon has been observed in other transition metal oxides [37, 41]. The cyclic stability of the NiFe2O4/EG nanocomposite electrode was better than those reported in previous papers [42, 43]. The results indicated that the excellent mechanical properties of the EG contributed to the cyclic stability of the obtained NiFe2O4/EG composite.
Based on the above discussion, the advantage of the NiFe2O4/EG nanocomposite involves its unique structural and electrochemical nature. First, the layered structure of the NiFe2O4/EG nanocomposite can alleviate the agglomeration of materials and improve the cycling performance. Second, EG has fewer functional groups and excellent mechanical properties, which can maintain the structure stability and postpone the disintegration of the nanocomposite structure during the discharge–charge processes, leading to a good cycle performance. Third, the high electrical conductivity of EG can increase the conductivity of the electrodes, ensuring fast electron transportation. Finally, EG has a high reversible capacity and good cycle performance, which is conducive to enhancing the capacity and cycling performance .
We developed a simple method for the direct homogeneous dispersion of NiFe2O4 nanoparticles onto EG nanosheets for use as a superior anode material for lithium-ion batteries. This hybrid nanostructure showed improved electrical conductivity, maintained structure stability, and exhibited delayed disintegration during the discharge–charge processes, which led to a good cycle performance. As a result, the fabricated NiFe2O4/EG composite demonstrated a high reversible capacity of 601 mAh g−1 over 800 cycles at a current density of 1 A g−1. Our synthesis approach could easily be extended to combine other ternary compounds (MFe2O4, MCo2O4, MMn2O4, etc.) with EG, offering promising routes for the low-cost mass production of advanced electrode materials for the next generation of LIBs.
The authors acknowledge the support from the National Basic Research Program of China (2014CB239702), National Natural Science Foundation of China (Grant Nos. 21371121, 21506126 and 51502174) and Shenzhen Science and Technology Research Foundation (Grant Nos. JCYJ20150324141711645,JCYJ20150324141711616 and JCYJ20150626090504916), and China Postdoctoral Science Foundation (2015 M582401 and 2015 M572349).
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