Rational Design of 1-D Co3O4 Nanofibers@Low content Graphene Composite Anode for High Performance Li-Ion Batteries

Cobalt oxide that has high energy density, is the next-generation candidate as the anode material for LIBs. However, the practical use of Co3O4 as anode material has been hindered by limitations, especially, low electrical conductivity and pulverization from large volume change upon cycling. These features lead to hindrance to its electrochemical properties for lithium-ion batteries. To improve electrochemical properties, we synthesized one-dimensional (1-D) Co3O4 nanofibers (NFs) overed with reduced graphene oxide (rGO) sheets by electrostatic self-assembly (Co3O4 NFs@rGO). The flexible graphene oxide sheets not only prevent volume changes of active materials upon cycling as a clamping layer but also provide efficient electrical pathways by three-dimensional (3-D) network architecture. When applied as an anode for LIBs, the Co3O4 NFs@rGO exhibits superior electrochemical performance: (i) high reversible capacity (615 mAh g−1 and 92% capacity retention after 400 cycles at 4.0 A g−1) and (ii) excellent rate capability. Herein, we highlighted that the enhanced conversion reaction of the Co3O4 NFs@rGO is attributed to effective combination of 1-D nanostructure and low content of rGO (~3.5 wt%) in hybrid composite.

Scientific RepoRts | 7:45105 | DOI: 10.1038/srep45105 sacrifices the actual capacity of Co 3 O 4 . Thus, we suggested desirable design for hybrid nanocomposite architecture of Co 3 O 4 and graphene.
One-dimensional (1-D) NFs, in common with other nanostructures, have large electrode/electrolyte interface area with volume-strain relaxation which can prevent the volume expansion during charge/discharge process. Moreover, these NFs have larger electrochemical active area because they are less likely to undergo aggregation than nanoparticles [14][15][16][17] . In general, large amount of graphene is required to cover nanoparticles with large specific surface area. Meanwhile, it is possible to coat entire surfaces of NFs with very low-content of graphene.
In this work, the hybrid Co 3 O 4 NFs@rGO offer significant benefits that are enumerated briefly as follows: 1-D reinforcement scaffold of the Co 3 O 4 NFs for fast Li + diffusion, large area for electrode/electrolyte interface, continuous electric contact and volume accommodation, and flexible yet robust graphene sheets directly coated on the Co 3 O 4 NFs for highly electrically conductive networks before and after conversion reactions. Small amount of the rGO greatly improved the performance of nanocomposite, synergistic effect of which on conversion reaction is emphasized. Fig. 1, the 1-D Co 3 O 4 NFs were fabricated via electrospinning method. As-spun Co precursor/polymer composite NFs (as-spun Co(Ac) 2 /PVP NFs) were achieved after the electrospinning. In subsequent calcination step at 600 °C in air, residual solvent and polymer were burned out and Co precursor was oxidized to Co 3 O 4 , resulting in the formation of 1-D Co 3 O 4 NFs. Finally, to cover the surface of the Co 3 O 4 NFs with highly conducting graphene sheets, we employed chemical solution method based on electrostatic interaction and chemical bonds 18 Fig. 2e,f, showing that polycrystalline Co 3 O 4 grains connected together were uniformly covered by the ultrathin rGO sheets (~3 nm) (Fig. 2d,e). The lattice fringe of the Co 3 O 4 in the Co 3 O 4 NFs@rGO is 4.66 Å, which is well matched with the spacing of Co 3 O 4 (111) planes (JCPDS PDF#43-1467) (Fig. 2f). The scanning TEM-energy dispersive spectroscopic (STEM-EDS) mapping analysis for the Co 3 O 4 NFs@rGO confirms homogeneous atomic distribution of Co and C in the 1-D scaffold (Fig. 2g,h).

Synthesis of 1-D Co 3 O 4 NFs and Co 3 O 4 NFs@rGO. As shown in
To check exact content of carbon in the Co 3 O 4 NFs@rGO, we conducted element analysis (EA). From the EA results, the concentration of carbon was measured to be only 3.56 wt% (Table S1). This value indicates very small amount of carbon in the Co 3 O 4 NFs@rGO. Even using minimum amount of carbon to anode, rGO directly coated on Co 3 O 4 NFs enhances the electrical conductivity and provides high capacity for the Co 3 O 4 NFs. X-ray diffraction (XRD) patterns show that the Co 3 O 4 NFs and the Co 3 O 4 NFs@rGO have the single phase spinel crystal structure for Co 3 O 4 phase (Fig. 2i). The XRD patterns exhibit additional peaks at 2θ = 31.3°, 36.8°, 38.5°, 44.8°, 55.7°, 59.4° and 65.2°, which are attributed to scattering from the (220), (311), (222), (400), (422), (511) and (440) lattice planes of cubic spinel Fd-3m Co 3 O 4 phase (JCPDS PDF#43-1467), respectively. Moreover, any peak change was not observed in the Co 3 O 4 NFs@rGO, indicating that the crystal structure of the Co 3 O 4 NFs is not affected during chemical-solution based graphene wrapping process.
In order to investigate the characteristics of two-dimensional (2-D) graphene layer, Raman spectroscopy analysis was performed. In Raman spectra, five characteristic bands at 195, 478, 517, 618, and 687 cm −1 are discovered in common with three samples (the Co 3 O 4 NFs, PAH-treated Co 3 O 4 NFs and the Co 3 O 4 NFs@rGO) (Fig. 3). The band at 195 cm −1 (F 2g ) exhibits CoO 6 scissoring vibration and other bands at 478, 517, 618 and 687 cm −1 are assigned to E g , 2F 2g and A 1g , that exhibit Co-O symmetric stretching vibration, respectively 19 . In the Co 3 O 4 NFs@   rGO, two specific bands were detected at 1356 and 1590 cm −1 , which correspond to the D and G band, respectively, confirming that the rGO sheets were formed in the Co 3 O 4 NFs@rGO 20 . Also, the Raman analysis clearly shows that any structural changes of the Co 3 O 4 NFs did not occur during graphene wrapping step through any changes in peaks of Co 3 O 4 . . These reaction peaks were well-matched with potentials represented with the plateaus in charge-discharge curves (Fig. 4c,d). The charge-discharge behaviors in the formation cycle, 1st, 10th and 20th cycle for the Co 3 O 4 NFs and the Co 3 O 4 NFs@rGO were observed with voltage window between 0.01 and 3.0 V at current densities of 0.1 A g −1 for formation cycle and 1.0 A g −1 after formation cycle. The discharge capacity in formation cycle of the Co 3 O 4 NFs@rGO shows higher capacity of 1474 mAh g −1 , than the Co 3 O 4 NFs (1202 mAh g −1 ). The irreversible capacity losses of both the Co 3 O 4 NFs and the Co 3 O 4 NFs@rGO were estimated to be 25% of the initial discharge capacity.

Discussion
In general, Co 3 O 4 converts to the Co metal nanograins dispersed in Li 2 O matrix after conversion reaction with 8 moles of Li + per one mole of Co 3 O 4 . To investigate stability of the the Co 3 O 4 NFs and the Co 3 O 4 NFs@rGO toward such severe reaction with Li + , cycling performances and rate-capability were evaluated (Fig. 5a-c). In  NFs. In addition, the Co 3 O 4 NFs@rGO exhibit high electrochemical performance that shows first discharge capacity of 669 mAh g −1 and 400th discharge capacity of 615 mAh g −1 (cycle retention of 92%) at a high current density of 4.0 A g −1 (Fig. 5c). It is noticeable that a low amount of rGO (~3.5 wt% in composite) is enough to improve electrochemical performance along with 1-D structural effect in a complementary manner. This amount of carbon is overwhelmingly lower than previously reported values (Table S2).
In Fig. 5, the Co 3 O 4 NFs and the Co 3 O 4 NFs@rGO tend to increase the capacity as the cycle progresses. Such increase in capacity of the Co 3 O 4 NFs may be due to an activated formation of gel-like polymeric film on Co metal surface 23 . Interestingly, as shown in Fig. 5, the capacity of Co 3 O 4 NFs@rGO has been increased through the graphene wrapping process, and also its capacity is higher than the theoretical capacity of Co 3  ). These extra capacities can be explained by the interfacial Li + storage that represents the formation of gel-like polymeric film and the pseudo-capacitive property on Co metal grains. The gel-like polymeric film is the reversible product from the side reaction of electrolyte decomposition on the surface of Co metal grain at low voltage during discharge process, and this film provides extra capacity for lithium storage 24,25 . In addition, the Co metal grain which came from conversion reaction can allow extra Li + adsorption site by metallic pseudo-capacitive property [26][27][28] . In consideration of such extra charge, unlike the pristine Co 3 O 4 NFs, the Co 3 O 4 NFs@rGO provide the fast electron pathway to form negatively charged Co metal grains for effective Li + adsorption on their surface. Figure 6a shows the differential capacity plots for Co 3 O 4 NFs and Co 3 O 4 NFs@rGO after 20 th cycle at a current density of 1.0 A g −1 . The broad cathodic peak at around 2.2 V is caused by Li + insertion to Co 3 O 4 NFs, which corresponds to region I in Fig. 6b,c. The peak at 1.23 V is the reduction of Co 3 O 4 to Co metal nanograins (conversion reaction). These peaks are positioned at same potential for both samples, because the plateau appears at the same voltage in Fig. 6b,c. In Fig. 6b,c there are the regions I and II that are related to Li + insertion and conversion of Co 3 O 4 with Li + and the region III is relevant to the interfacial Li + storage ability of active materials occurred in solid-liquid interface with Li + adsorption [29][30][31] . Furthermore, the dQ/dV value of Co 3 O 4 NFs@rGO at 1.23 V is twice that of Co 3 O 4 NFs. The broad cathodic peak at around 1.0 V is caused by the interfacial lithium ion storage of electrolyte decomposition on Co metal nanograin and pseudo-capacitive property of metal grains. For the peak at around 1.0 V, the dQ/dV value of Co 3 O 4 NFs@rGO is 1.5 times higher compared with the Co 3 O 4 NFs. Based on these results, the Co 3 O 4 NFs@rGO proceed the same reaction compared to Co 3 O 4 NFs, but the rGO layers promote the degree of Li + interfacial storage reaction in all cases of Li + insertion, conversion, and interfacial adsorption.
To further understand the effect of graphene layer wrapping, ex-situ SEM analysis for the Co 3 O 4 NFs@rGO and the Co 3 O 4 NFs were carried out after 100th cycle (Fig. 7a,b). The surface state of the delithiated-Co 3 O 4 NFs@ rGO was observed, indicating that the nanocomposite structure could be intactly preserved with formation of stable SEI layer (Fig. 7a), whereas the SEI layer of the Co 3 O 4 NFs was conspicuously and irregularly generated (Fig. 7b). To further investigate resistance in the nanocomposites, electrochemical impedance spectroscopy (EIS) measurement was conducted. Figure 7c shows the Nyquist plots of the Co 3 O 4 NFs and the Co 3 O 4 NFs@rGO after the 100th cycle and the equivalent model corresponding to the EIS model. The impedance values of each impedance component calculated from the EIS data are represented in Table S3. R E , R SEI , R CT-1 , R CT-2 and R P are the ohmic resistance of the cell, the SEI resistance, the charge transfer resistances, and the phase transformation resistance, respectively. In the case of the Co 3 O 4 NFs@rGO, the SEI resistance was reduced by 37.7%, compared to the Co 3 O 4 NFs. Also, the charge transfer resistance of Co 3 O 4 NFs@rGO (18.26 Ω) is much smaller than that of Co 3 O 4 NFs (68.68 Ω). Therefore, it is believed that the low-content rGO layer on Co 3 O 4 NFs is significantly beneficial to enable smaller polarization for LIBs. Here, we proposed the simple reaction models during discharge process for the Co 3 O 4 NFs@rGO and the Co 3 O 4 NFs (Fig. 7d,e). In the case of the Co 3 O 4 NFs surrounded by SEI and super P (carbon additive), electrons may not efficiently transfer between electrode materials (Fig. 7e). Meanwhile, for the Co 3 O 4 NFs@rGO, the 2-D rGO layers not only greatly suppress the formation of insulating SEI layer, but also give facile passage for electron. More importantly, after the lithiation, the rGO can be more effective electron bridge between the discharge products (Co NPs in Li 2 O matrix) (Fig. 7d). From the point of view of the Li + interfacial storage, the Co 3 O 4 NFs@rGO is much suitable compared to Co 3 O 4 NFs without such conducting route. These envisioned mechanisms can be supported by higher capacity of the Co 3 O 4 NFs@rGO than that of the Co 3 O 4 NFs in Fig. 5a,b. Based on the data above and our interpretation, it can be thought that combination of 1-D nanostructure and rGO-wrapping is appropriate for desirably designed electrode architecture, strategy of which will be an explicit direction for conversion-based LIBs.
In summary, we designed the ultra-thin rGO layer wrapped 1-D Co 3 O 4 NFs as high performance anode for LIBs, which were simply synthesized via an electrospinning and subsequent self-assembly wrapping of graphene sheets on Co 3 O 4 NFs. 1-D nanostructure of Co 3 O 4 NFs not only overcome the alleviation of pulverization upon cycling, but also provide the fast Li + diffusion and continuous electric contact. Moreover, the rGO layer, which exists as small percentage of carbon within Co 3 O 4 , provides high electric conductivity to Co 3 O 4 NFs for high specific capacity, rendering favorable conversion reaction and Li + interfacial storage. The Li anode electrode with Co 3 O 4 NFs@rGO delivered a relatively high reversible capacity of 615 mAh g −1 with stable capacity retention of 92% after 400th cycle at the high current density of 4.0 A g −1 . Through above results, the reduced graphene oxide sheets wrapped Co 3 O 4 NFs were suggested as high performance anode for LIBs.  After electrostatic assembly between the PAH-treated Co 3 O 4 NFs and the GO, to reduce the GO, 0.5 g of hydrazine monohydrate was added into the solution and stirred at 200 rpm for 2 h. Finally, several washing and drying steps were conducted.

Material characterization. The morphologies of the Co 3 O 4 NFs and the Co 3 O 4 NFs@rGO were confirmed
via scanning electron microscopy (SEM, XL-30 SFEG, Philips). The field emission transmission electron microscope (FE-TEM, Tecnai G2 F30 S-Twin, FEI, Netherlands) was used to check the lattice spacing of Co 3 O 4 and the thickness of graphene layer. The energy-dispersive spectrometry (EDS) mapping of scanning transmission electron microscope (STEM) shows the distribution of carbon, cobalt and oxygen atoms on NFs. The x-ray diffraction (XRD) pattern of Co 3 O 4 NFs and Co 3 O 4 NFs@rGO was carried out using X-Ray diffraction-meter (D/MAX-2500, Rigaku, Japan). The range of measured X-ray diffraction angle was 2θ = 20-70° using Cu-Kα (λ = 1.54 Å ) radiation. The amount of carbon component of the Co 3 O 4 NFs@rGO was analyzed by element analyzer (FLASH 2000 series, Thermo Scientific). To investigate the graphene sheets, Raman spectroscopy (Aramis, Horiba Jobin Yvon, France) was used.
Electrochemical measurements. To measure electrochemical performance, active materials were mixed with Super P and binders. The binders were used with CMC solution and PAA solution. The ratio of active material (Co 3 O 4 NFs or Co 3 O 4 NFs@rGO), super P, CMC solution and PAA solution was 75:15:5:5 in weight. Mixed slurry was coated on Cu-foil with a thickness of 90 μ m. Sequentially, the coated Cu-foil was dried at 50 °C for 20 min for removing solvents. For evaporation of moisture and activation of binder, the coated Cu-foil was dried in vacuum oven at 150 °C for 2 h. The obtained anode was assembled to CR2032 coin-type cell in an argon gas filled glove box. The lithium metal was used as counter electrode. The other components of coin cell were Celgard ® 2400 as separator, 1.3 M of LiPF 6 dissolved in EC/DEC = 3/7 (v/v) + 10.0 wt% FEC as electrolyte where EC is ethylene carbonate, DEC is diethyl carbonate, and FEC is fluoroethylene carbonate. Loading mass of active material on Cu-foil was 0.88 ± 0.01 mg cm −2 . Current-Voltage curve was measured at Wonatech WBCS3000 in the voltage window of 0.01-3.0 V and a scan rate of 0.5 mV s −1 . Galvanostatic charge/discharge process was carried out by Maccor series 4000 in the voltage window of 0.01-3.0 V. Electrochemical impedance spectroscopy (EIS) was performed to investigate the impedance change after 100th cycle with one channel potentiostat (ZIVE SP1 potentiostat, Wonatech, Korea) by applying a sine wave of 10 mV in the frequency range of 10 5 -0.01 Hz.