- Nano Express
- Open Access
Cobalt Oxide Porous Nanofibers Directly Grown on Conductive Substrate as a Binder/Additive-Free Lithium-Ion Battery Anode with High Capacity
- Hao Liu†1, 4, 5Email authorView ORCID ID profile,
- Zheng Zheng†2,
- Bochao Chen3,
- Libing Liao3 and
- Xina Wang2Email author
© The Author(s). 2017
Received: 4 February 2017
Accepted: 7 April 2017
Published: 26 April 2017
In order to reduce the amount of inactive materials, such as binders and carbon additives in battery electrode, porous cobalt monoxide nanofibers were directly grown on conductive substrate as a binder/additive-free lithium-ion battery anode. This electrode exhibited very high specific discharging/charging capacities at various rates and good cycling stability. It was promising as high capacity anode materials for lithium-ion battery.
Transition metal oxides usually exhibit remarkably high specific capacities as lithium-ion battery anode materials, and they are believed to be promising anode materials to replace the commercial graphite anode [1–5]. Among various transition metal oxides, cobalt monoxide (CoO) has drawn great attention due to its high theoretical capacity of 716 mA h g−1 which is nearly twice than that of graphite anode [2, 6]. Nevertheless, this anode material suffers from large volumetric change during discharging/charging cycles, leading to pulverization and capacity fading of bulk electrode. Nanostructural design with specific architectures has been demonstrated as an effective way to alleviate the volumetric change of active materials, such as Si , Ge , and transition metal oxides [6, 9], because lots of voids or pores’ presence in the nanostructures can accommodate the volumetric expansion of the active materials during lithiation process. Specially, one-dimensional (1D) nanostructures such as nanowires or nanofibers are proved to be an optimized architecture for the electrochemical electrodes due to their well-defined charge transport path, high specific surface area, and high porosity features [6, 10–12]. Nevertheless, in traditional battery electrode preparation, these 1D nanostructures are usually mixed with binders and conductive additives before being tape casted on a flat current collector to enhance mechanical and electrical properties of electrodes [12, 13]. The introduction of binders and additives not only reduces the energy density of the electrode but also generates undesired interfaces between the active materials and the additives, which increases the complexity of charge transfer process .
In the present work, CoO porous nanofibers were directly grown on conductive substrate as a binder/additive-free anode for lithium-ion battery. These porous nanofibers exhibited high specific discharging/charging capacities at various rates and good cycling stability, suggesting its promising application as lithium-ion battery anode.
Fabrication of CoO Nanofibers on Conductive Substrate
Typically, 1 mmol cobalt dichloride, 5 mmol urea, and 10 mmol sodium chloride were dissolved in 30 mL of distilled water under vigorous stirring for 20 min to form homogeneous pink solution. The previous solution was transferred into a Teflon-lined stainless steel autoclave, and then a 2 cm × 2 cm titanium foil which had been cleaned by dilute hydrochloric acid was put into the autoclave, followed by heating for 10 h at the temperature of 100 °C. The products with titanium substrate were taken out, washed with distilled water, and then dried at 60 °C for 1 h. Finally, the as-prepared products were annealed at the temperature of 500 °C for 2 h under an N2 atmosphere.
The crystallinity of the products was examined using an X-ray diffraction (XRD) with Cu-Kα radiation (Bruker D8). The morphologies and compositions of products were characterized by a field-emission scanning electron microscope (SEM; JSM-7100F) and a transmission electron microscope (TEM; JOEL 2100) equipped with an energy-dispersive X-ray (EDX) spectrometer.
Electrochemical Properties of the CoO Nanofiber Anode
The electrochemical properties of the samples were examined using CR2032 coin-type cells with Li foil as counter electrode. No binder or conducting carbon was used during the cell assembly. The liquid electrolyte was 1.0 M LiPF6 in ethylene carbonate/diethyl carbonate solvent (Shenzhen Kejing Star Technology C., Ltd.). All coin cells were cycled between 0.01 and 3.0 V at different rates on a battery test system (CT4008W, Neware Co., Ltd.). The electrochemical impedance spectroscopy (EIS) of the batteries was collected in the frequency range from 100 kHz to 0.5 Hz under an alternating current (AC) stimulus with a 10 mV of amplitude (Vertex, Ivium Electrochemical Measurement System). After the cycling test, the coin cells were disassembled to characterize the morphologies change of the electrode.
Results and Discussion
The formation mechanism of the porous CoO fiber can be explained as the following . In the hydrothermal process, the hydrolysis-precipitation process of urea took place and numbers of CO3 2− and OH− anions were gradually produced. Then, cobalt-hyroxide-carbonate nucleus (Co(OH)x(CO3)0.5(2−x)·nH2O) were formed and continuously grew along specific orientation preferentially on the metal substrate. As a result, cobalt-hyroxide-carbonate nanowires were obtained. During the next annealing process under the Ar gas protection, the as-prepared nanowires decomposed to release CO2 and H2O and, then, porous CoO nanofibers were obtained.
Figure 3b shows the discharging/charging voltage profiles taken from the second cycle at various rates in the voltage range of 0.0–3.0 V vs. Li+/Li. The lithiation voltage shows a sloping profile below 1.25 V vs. Li+/Li, being consistent with the CV results. In addition, the CoO anode exhibits high discharging/charging capacities at different rates, such as 1205/1186, 1158/1156, 1141/1130, 1033/1021, and 423/410 mA h g−1 at rates of 0.1, 0.2, 0.5, 1.0, and 5.0 C. It is noticed that the specific capacities of the CoO anode at various rates are significantly higher than its theoretical capacity of 716 mA h g−1. Large additional capacities were always observed in transition metal oxide anode materials, which are probably contributed by pseudocapacitance of nano-sized materials  or the formation of oxygen-rich transition metal oxide materials . Furthermore, this electrode also shows good capacity retention, as its discharging/charging capacities can be recovered to ~1202/1196 mA h g−1 when the rate returns to 5.0 from 0.1 C rate (Fig. 3c). It is worth to note that the initial coulombic efficiency is ~81%, much higher than those in other reports [2, 18]. In addition, the coloumb efficiency remarkably decreases when the charging/discharging current density increased from 1 to 5 C rate. This may result from the instability of the electrode during the drastic change of current, e.g., new SEI layer formed on the electrode surface. After four charging/discharging cycles, the coloumbic efficiency increases to 99%, indicating the stabilization of the electrode. The cycling performance of the electrode is shown in Fig. 3d. A capacity of ~981 mA h g−1 is obtained at 1.0 C rate, and a slow decay to ~764 mA h g−1 is found after 130 cycles. This is to say, only 0.17% of the initial capacity is lost in each cycle. Moreover, the coulombic efficiency remains above 99% during the cycling test.
In summary, CoO porous nanofibers, being composed of lots of nanoparticles, were directly synthesized on conductive substrate and applied as bind-free Li-ion battery anode. This anode showed very high discharging/charging capacities at various rates, such as 1205/1186 and 1033/1021 mA h g−1 at rates of 0.1 and 1.0 C, respectively. In addition, the porous nanofiber structure was well maintained during the cycling, leading to good cycling stability of this CoO anode. Therefore, the CoO porous nanofibers grown on conductive substrate showed promise as bind/additive-free lithium-ion battery anode.
We would like to thank Rui Tong and Xi Feng for their taking of HRTEM images.
This work was supported by the National Natural Science Foundation of China (No. 51502271, 51472080), the Fundamental Research Funds for the Central Universities (No. 2652015186), and the Open Funds of Hubei Key Laboratory of Ferro and Piezoelectric Materials and Devices (No. 201604).
HL and XNW carried out the experimental design and wrote the manuscript. ZZ prepared the CoO samples and participated in the writing of the manuscript. BCC participated in the electrochemical test of these samples. LBL gave some advices to this work. All authors have read and approved the manuscript.
The authors declare that they have no competing interests.
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