Copper nanofiber-networked cobalt oxide composites for high performance Li-ion batteries
© Nam et al; licensee Springer. 2011
Received: 27 November 2010
Accepted: 5 April 2011
Published: 5 April 2011
We prepared a composite electrode structure consisting of copper nanofiber-networked cobalt oxide (CuNFs@CoO x ). The copper nanofibers (CuNFs) were fabricated on a substrate with formation of a network structure, which may have potential for improving electron percolation and retarding film deformation during the discharging/charging process over the electroactive cobalt oxide. Compared to bare CoO x thin-film (CoO x TF) electrodes, the CuNFs@CoO x electrodes exhibited a significant enhancement of rate performance by at least six-fold at an input current density of 3C-rate. Such enhanced Li-ion storage performance may be associated with modified electrode structure at the nanoscale, improved charge transfer, and facile stress relaxation from the embedded CuNF network. Consequently, the CuNFs@CoO x composite structure demonstrated here can be used as a promising high-performance electrode for Li-ion batteries.
Cobalt oxide (CoO x ) is a high-capacity electrode material for Li-ion batteries with a theoretical capacity of at least two times greater than that of graphite (ca. 370 mAh g-1) . However, the cobalt oxides show large irreversible capacity and poor cycling performance caused by Li-alloying, agglomeration or growth of passivation layers . In addition, severe volume expansion during discharge/charge process accelerates fading of the capacity, and electrical contact between the electrode material and current collector eventually fails. To overcome these problems, several strategies that employ a secondary material , a chemically or physically prepared surface coating , size optimization [4, 5], and fabrication of a nanostructure  have been reported. These approaches generally provide a facile electrochemical reaction route, high conductivity, and structural stability. In particular, nanostuctured electrode materials are expected to be well-suited for next-generation Li-ion batteries due to their substantially increased reaction area and facilitated charge carrier transport through shortened Li-ion diffusion paths . For example, Kim et al.  proposed a core-shell nanorod array electrode, which consists of a metallic conducting core with a vanadium oxide (VO x ) shell layer. Such highly conducting core-embedded nanostructure was capable of enhancing the electrochemical properties of the VO x electrodes even though the electroactive materials have high electrical resistance. In addition, incorporation of metal into active material was found to increase the charge transfer in electrode materials along with facilitated Li-ion diffusion . Therefore, it is expected that the incorporation of highly conducting metal nanowires into cobalt oxide materials would be a promising way to increase electrical conductivity and mitigate the particle agglomeration of the cobalt oxide during Li-ion insertion/extraction.
In this report, we prepared cobalt oxide electrode that is composited with copper nanofiber network, and demonstrated that such embedded nanostructure is able to enhance electrical conductivity and mechanical stability for the CoO x electrode during repeated cyclings.
Fabrication of Cu nanofiber-embedded cobalt oxide composites
The composite nanostructure of copper nanofiber-networked cobalt oxide (CuNFs@CoO x ) was prepared by using an electrospinning process to produce the copper nanofibers and followed by a radio frequency magnetron sputtering (RF sputtering) to deposit the CoO x materials. The electrospinning solution was prepared by mixing copper(II) chloride dihydrate (CuCl2 2H2O, Sigma-Aldrich, Saint Louis, USA), methanol, and polyvinylpyrrolidone (PVP; M w = 1,300,000 g mol-1, Sigma-Aldrich, Saint Louis, USA). The solution was then immediately loaded into a syringe, which was attached to a 23-gauge stainless steel needle. A 10-kV electric field was applied between the needle tip and a grounded stainless steel disc at a distance of 10 cm. The stainless steel substrate was mechanically polished before use with a sandpaper and diamond paste (ca. 0.3 μm) until a mirror-like surface was obtained. Subsequently, the collected CuCl2/PVP composite on the substrate was heated at 300°C for 3 h in air. To obtain the metallic CuNFs, a reduction treatment was performed at 200°C in H2 atmosphere at a flow rate of 60 sccm. Next, CoO x was deposited onto the Cu nanofibers-formed substrate via RF sputtering with an cobalt oxide target under an inert Ar gas atmosphere at a working pressure of 1 × 10-3 Torr. The deposition thickness of the CoO x was controlled to ca. 100 nm. The mass ratio of the deposited CuNFs and CoO x was measured to be 2:3 using a micro-balance (Sartorius, M3P). The mass of the electrodes was controlled to have the similar quantity (ca. 0.125 mg) of CoO x as the active material.
The microstructures were characterized by field emission scanning electron microscopy (FESEM, Hitachi S-4700) and x-ray diffraction (XRD, Rigaku Ru-200B). To measure the thickness and investigate the cross section, the electrodes were deposited onto Si substrates instead of stainless steel substrates. The composition of the deposited CoO x was characterized by x-ray photoelectron spectroscopy (XPS, VG Multilab 2000) with a monochromic Al Kα x-ray source (E = 1486.6 eV). Data processing was performed using the Avantage 4.54 software program. The background was corrected using the Shirley method, and the binding energy of the C 1s peak from the support at 284.5 eV was taken as an internal standard.
The electrochemical tests were performed using a two-electrode system fabricated with the prepared materials for the working electrode and metallic Li for the counter electrode in an Ar-circulating glove box. A 1-M LiPF6 solution in a 1:1 volume mixture of ethylene carbonate and diethyl carbonate was used as the electrolyte. The galvanostatic discharge/charge mode at various C-rates from 0.15 to 3C was conducted with a potential window of 2.5 to 0.01 V (vs. Li/Li+) using a battery cycler (WonA tech, WBCS3000). 0.15C rate corresponds to a current rate of 0.135 A g-1 of Co3O4, in which the theoretically complete discharge could be achieved in 6.7 h, and 3C rate corresponds to 2.7 A g-1. The AC impedance measurement was performed using a Solartron 1260 frequency response analyzer. An amplitude voltage of 5 mV was applied over the frequency range from 100 kHz to 10 mHz.
Results and discussion
Morphology and microstructure
The first discharge process is an irreversible reaction of Co3O4 and Li, which forms metallic Co and Li2O phase. During the first charge process, the Co and Li2O forms CoO instead of Co3O4 owing to the similarity of oxygen lattice in the Li2O and CoO . In the subsequent discharge/charge processes, the modified oxygen lattice is continuously preserved, indicating that the reaction of CoO with Li develops into reversible cycles.
A CuNFs@CoO x composite electrode was fabricated to serve as an anode for rechargeable Li-ion batteries. As an efficient Li-ion battery anode material, CuNFs@CoO x exhibited a higher capacity and rate performance than bare CoO x TF without CuNFs; the capacity at 0.15C was increased by ca. 30%, and the capacity was maintained above 50% even at 3C. These enhancements could be attributed to an increased number of reaction sites, facilitated charge transport, a decreased electrochemical double-layer capacitance, and facile stress relaxation by embedded CuNF network within the CoO x . Consequently, this CuNFs@CoO x composite structure can be a promising candidate for use in the electrodes of high-performance Li-ion batteries.
- CuNFs@CoO x :
copper nanofiber-networked cobalt oxide
- CoO x TF:
cobalt oxide thin-film
- CoO x :
field emission scanning electron microscopy
- VO x :
x-ray photoelectron spectroscopy
This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (R15-2008-006-03002-0) and by the Korean government (MEST) (no. 2010000018) and by the Core Technology Development Program for Next-generation Solar Cells of Research Institute of Solar and Sustainable Energies (RISE), GIST.
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