Unique 1D Co3O4 crystallized nanofibers with (220) oriented facets as high-performance lithium ion battery anode material

A novel one-step hydrothermal and calcination strategy was developed to synthesize the unique 1D oriented Co3O4 crystal nanofibers with (220) facets on the carbon matrix derived from the natural, abundant and low cost wool fibers acting as both carbon precursor and template reagent. The resultant W2@Co3O4 nanocomposite exhibited very high specific capacity and favorable high-rate capability when used as anode material of lithium ion battery. The high reversible Li+ ion storage capacity of 986 mAh g−1 was obtained at 100 mA g−1 after 150 cycles, higher than the theoretical capacity of Co3O4 (890 mAh g−1). Even at the higher current density of 1 A g−1, the electrode could still deliver a remarkable discharge capacity of 720 mAh g−1 over 150 cycles.

Scientific RepoRts | 6:26460 | DOI: 10.1038/srep26460 nanofibers was facilely controlled by adjusting the calcination time for the precursor obtained from the hydrothermal treatment. The synthesis strategy is as follows: In the first step, Co 2+ ions were introduced into the solution containing wool fibers and deionized water with a thorough mixing process, which enables a full adsorption of Co 2+ ions on the surface of wool substrate through charge attraction between the positively charged cobalt ions and the negatively charged functional groups of proteins in the wool. In the second step, a certain amount of urea was added into the above mixture, resulting in the formation of cobalt oxide precursor with a fiber-like morphology. The hydrothermal reaction brought about the formation of a fiber-like precursor, and the subsequent heating treatment under air at 500 °C for 2 h involved thermal decomposition of the precursor, resulting in the fabrication of 1D W2@Co 3 O 4 nanofibers with hierarchically porous structure. SEM image (Fig. 1b) indicates that the resulting W2@Co 3 O 4 sample has an identical 1D fiber-shaped morphology. TEM image (Fig. 1c) shows that the W2@Co 3 O 4 sample consists of continuous and uniform 1D nanofibers, which are as long as about 1 μ m with almost constant diameter of about 20 nm (Fig. 1d). The lattice fringes from the HRTEM image ( Fig. 1e) 29 . Besides, a broad and weak diffraction peak appearing at 2θ = 24.7° could be observed for W2@Co 3 O 4 , which can be indexed into the (002) facet of the carbon matrix with the hexagonal graphitic lattice derived from the high temperature calcination of the wool fibers in the precursor.
FT-IR spectra in the range of 4000-500 cm −1 are given to illuminate the formation of the W2@Co 3 O 4 nanofibers (Fig. 2b). The distinct and sharp absorption peak at 661 cm −1 was observed, which may be assigned to the stretching vibration of the Co-O bond confirming the formation of the spinel Co 3   The high-resolution XPS spectrum of the Co 2p, which shows two major peaks with the binding energy at 780.7 and 795.7 eV, corresponding to the Co 2p 3/2 and Co 2p 1/2 spin-orbit peak of Co 3 O 4 , respectively. And (h) N 2 adsorption-desorption isotherms, indicating the high specific area and hierarchically porous structure. The inset is pore size distribution, showing a narrow pore size distribution with the pore size of about 1 nm and a broad pore size distribution mainly from 2-10 nm with the average pore size of 3.4 nm.
to the carboxylic O-H deformation vibration and the C-O-C stretching vibration, respectively [30][31][32] . The peak at 1632 cm −1 can be assigned to the carbon skeletal vibrations (aromatic C = C) of the furanic and aromatic groups 29 .
Raman spectrum of the W2@Co 3 O 4 nanofibers was measured to investigate the formation of the carbon matrix derived from wool fibers and Co 3 O 4 , showing five peaks at 189, 480, 520, 606 and 686 cm −1 (Fig. 2c). The peak at 189 cm −1 is attributed to the characteristic species of F 2g (3) symmetry in the tetrahedral sites (CoO 4 ). The peak with medium intensity located at 480 and 520 cm −1 is corresponding to the species of E g and F 2g (2) symmetry, respectively 33 . The weak peak located at 606 cm −1 is related to the species of F 2g (1) symmetry. The peak at 686 cm −1 is assigned to the species of A1g in the O h 7 spectroscopic symmetry, which is in consistent to the characteristic of the octahedral sites (CoO 6 ) 34,35 . The observed five peaks are characteristics of the cubic Co 3 O 4 phase, which is consistent with the XRD analyses. In addition, the broad peak displaying at 1565 cm −1 is assigned to the G-band of carbon, corresponding to the E 2g phonon of C sp 2 atoms. The high intensity of the G-band compared to the D-band indicates a good graphitization of the carbon matrix derived from wool fibers, where the more sp 2 -hybridized carbon atoms formed in stacked and highly ordered hexagonal rings may give rise to the enhanced conductivity of the carbon matrix.
XPS measurements were carried out to determine the chemical composition and elemental valence state of the W2@Co 3 O 4 nanofibers. XPS survey scan spectrum of W2@Co 3 O 4 (Fig. 2d) exhibits the characteristic peak at 284.6, 529.6 and 779.9 eV, corresponding to the C 1s, O 1s and Co 2p component, respectively. The C 1s (Fig. 2e) could be split into three peaks: the sp 2 bonded carbon at 284.8 eV (C-C/C = C), the epoxy at 286.4 eV (C-OH/C-OCo) and the carbonyls at 288.5 eV (HO-C = O), indicating the nonoxygenated carbon atom, hydroxyl groups and oxygen containing functional groups 29 . Figure 2f shows the XPS spectrum of the O 1s, comprising three peaks at 532.5 (C-OH), 531.5 (O-C) and 530.5 eV (O = C-OH). The peak at the lower energy of 529.8 eV is associated with the lattice oxygen in the spinel Co 3 O 4 36 . The Co 2p XPS spectrum (Fig. 2g) shows two major peaks with the binding energy at 780.7 and 795.7 eV, corresponding to the Co 2p 3/2 and Co 2p 1/2 spin-orbit peak of Co 3 O 4 , respectively 29 . The binding energy difference between the two peaks is 15 eV and their intensity ratio is almost 2:1, which are typical characteristics of the standard Co 3 O 4 spectrum. The linkage between Co-O and C-O brings about a large electron charge overlap in the interface between the Co 3 O 4 and carbon matrix derived from wool fibers, which may promote the electronic transports during the charge-discharge cycles. N 2 adsorption-desorption measurements were performed to give the porous characteristics of the W2@Co 3 O 4 nanofibers (Fig. 2h). The Brumauer-Emmett-Teller (BET) specific area and total pore volume was calculated to be 78.25 m 2 g −1 and 0.393 cm 3 g −1 , respectively (Fig. 2h). A narrow pore size distribution with the pore size of about 1 nm and a broad pore size distribution mainly from 2-10 nm with the average pore size of 3.4 nm were confirmed for the W2@Co 3 O 4 nanofibers by the pore size distribution curve (inset in Fig. 2h) analyses based on the density functional theory model. It can be clearly seen that the hierarchically porous W2@Co 3 O 4 delivers the highest surface area among all the investigated W@Co 3 O 4 samples ( Fig. S5 and Table S1).
The charge-discharge properties of the W2@Co 3 O 4 anode materials were determined at a current density of 100 mA g −1 (Fig. 3a,b). The first discharge and charge capacity was 1442 and 1092 mAh g −1 , respectively, corresponding to the Coulombic efficiency of 75.7%. The long discharge voltage plateau at 0.95-1.0 V could be associated with the reduction process from Co 3 O 4 to metallic cobalt 14 . The discharge specific capacity was 1112 mAh g −1 at the 2 nd cycle and gradually decreased to 937 mAh g −1 at the 50 th cycle (Fig. 3b), where the Coulombic efficiency of W2@Co 3 O 4 quickly increased to 95.6% for the 2 nd cycle and stabilized in the range of 96-99% in the subsequent cycles. The decreased capacity could be dominated by the decreasing conductivity caused by the phase transformation and pulverization of the electrode materials upon cycling 37 . After that, the specific capacity continuously increases with the high specific capacity of 986 mAh g −1 at the 150 th cycle with the stabilized Coulombic efficiency of 96-99%. As for W2@Co 3 O 4 , the unique 1D nanostructure with the high surface energy (220) oriented facets may result in easy transports of Li + ions, and the carbon matrix from wool fibers with the good graphitization could mitigate the volume expansion. Therefore, these factors jointly weakened phase transformation and particle pulverization, bringing about the unusual continuously increased capacity from the 50 th to the 150 th cycle. The CV curves of the W2@Co 3 O 4 electrode (Fig. S6) are in good agreement with the charge-discharge voltage profiles. For comparison, the specific capacities of W1@Co 3 O 4 and W3@ Co 3 O 4 electrodes were also determined and low specific capacities were obtained after the same cycling numbers (Figs S7 and S8). Apparently, the W2@Co 3 O 4 revealed the best electrochemical performance with high specific capacity and excellent cyclic stability in all the three composite electrodes.
Because the rate capability is an important parameter for the LIB anode material, the cyclic stability of the W2@Co 3 O 4 electrode was evaluated at a high current rate of 1 A g −1 (Fig. 3c). The specific capacity was still kept 720 mA h g −1 at 1 A g −1 for 150 cycles, indicating a superior rate capability of the W2@Co 3 O 4 electrode. The Coulombic efficiency of the W2@Co 3 O 4 electrode at 1 A g −1 corresponds to 100% (Fig. 3c), further indicating its high electrochemical stability. The rate performance of W2@Co 3 O 4 at various current rates was investigated (Fig. 3d). The cells cycled at 100 mA g −1 for 50 cycles were used for rate capability test in order to avoid the induced effect due to the activation of the electrode. A reversible capacity of 1042 mAh g −1 was achieved at 100 mA g −1 . When the charge-discharge rate increased to the 500 mA g −1 , the W2@Co 3 O 4 electrode could still exhibit a reversible capacity of 828 mAh g −1 . Even at high rate of 1 and 2 A g −1 , the electrode reserved a specific capacity of 733 and 580 mAh g −1 , respectively. Moreover, when the current density returned to the 100 mA g −1 , the discharge capacity recovered to the same levels initially shown at that rate. Notably, the result is much better than most of the reported Co 3 O 4 -based anode materials, such as peapod-like Co 3 Table S2. As for W2@Co 3 O 4 , the carbon matrix derived from natural, abundant and low cost wool, which is a readily available waste and could provide the potential to large scale production of the electrode.
The electrochemical impedance spectroscopies (EIS) were determined to understand the enhanced rate capability. The Nyquist plots for W2@Co 3 O 4 at different cycles from 0.1 MHz to 0.01 Hz are shown in Fig. 3e. All the curves describe a semicircle at high-medium frequency and an inclined line at low frequency, which respectively correspond to the charge transfer and diffusion. The equivalent circuit is indicated as the insert of Fig. 3e, where, R s is the ionic resistance of electrode (intrinsic resistance of substrate, and contact resistance at the active material/current collector interface), R ct is the charge transfer resistance (the semicircle diameter), Z W is the Warburg impedance (Li + ions diffusion into the active materials, slope of the curve at low frequency), and CPE is the constant phase-angle element which involves the double layer capacitance 31 . It can be clearly seen that the R ct became lower from 146.1, 113.7 to 40.7 Ω with the increased cycling after 1, 75 and 150 cycles, demonstrating the good electrolyte infiltration and charge-transport capability. In the low frequency region, the W2@Co 3 O 4 electrode showed less Z w with a line slope close to 45 o , which is a result of Li + ions diffusion dependence at electrolyte/ electrode interface 44 . For comparison, the Nyquist plots of the W1@Co 3 O 4 and W3@Co 3 O 4 electrodes are shown in Fig. S9. Apparently, the R ct of W2@Co 3 O 4 nancomposites was smaller than that of the other two electrodes, resulting from the 1D nanofiber porous structure. The above mentioned EIS results suggest that the W2@Co 3 O 4

Discussions
The unique 1D porous W2@Co 3 O 4 nanocomposite possessing of the oriented Co 3 O 4 crystal nanofibers with the (220) facets on the carbon matrix has been successfully synthesized via one-step hydrothermal method following with calcination. The formation of well-defined 1D nanosized morphology can be facilely controlled by adjusting the calcination time of the precursor obtained from the hydrothermal treatment. The natural, abundant and low-cost wool fibers act as both the precursor of the carbon matrix and template reagent of the W2@Co 3 O 4 nanofibers. The W2@Co 3 O 4 product can deliver excellent reversible capacity, high rate performance and long cyclic stability used as the anode material of LIBs due to the following reasons: (I) the oriented (220) facets with much higher energies could reduce the oxidation-reduction gaps, thus greatly accelerating the reaction rates; (II) the 1D nanostructure with hierarchical pores may effectively reduce the Li + ions diffusion lengths and provide space for volume shrinking and expansion during insertion/extraction processes resulting in improved Li + ion diffusion rate and high cyclic stability; (III) the carbon matrix from wool fibers with the good graphitization could enhance the electron transports and mitigate the volume expansion; and (IV) the close contact between Co 3 O 4 and C could be favorable for the structure stability of the W2@Co 3 O 4 composite. These factors simultaneously provide the advantages of the 1D W2@Co 3 O 4 nanofibers as the high-performance Li + ion storage materials and they are very important for the rational design of advanced electrode material of LIBs.

Methods
Materials preparation. The wool fibers obtained from Zhejiang province of China were thoroughly washed with isopropanol and dried at 80 °C. The cleaned fibers were cut into fine debris (~5 mm in length). All the chemicals used in the experiments are analytical grade and were used without further purification. The precursors are synthesized under hydrothermal condition. In a typical synthesis, 2.0 mmol of Co(CH 3 COO) 2 ·4H 2 O was dissolved in 40 mL of a mixture containing 3.0 mL of ethylene glycol and 37 mL of deionized water. After stirring for 15 min, a certain amount of urea was added into the above solution. The mixture was stirred for another 30 min. Then 0.5 g of cleaned wool fibers were added into the above solution and immersed for 1 h. The obtained mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 200 °C for 24 h in an electron oven. After that, the autoclave was cooled naturally to room temperature. The product was collected and washed with deionized water and ethanol for several times by centrifugation, followed by vacuum-drying at 60 °C. After calcinating the collected precursor at 500 °C in air for different time (1, 2 and 3  Characterization. Scanning electron microscopy (SEM) measurements were carried out on JSM-7500F (5 kV) instrument. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were examined on JEOL JEM-2100F at an acceleration voltage of 200 kV. Powder X-ray diffraction (XRD) patterns were determined on the X-ray diffractometor (X-ray 6000) with the 2θ angle region from 10° to 80° at a scan rate of 3° min −1 . Fourier transform infrared (FT-IR) spectra were obtained by the spectrophotometer (Nicolet iN10 MX, USA). Raman spectra were measured on a microscopic confocal Raman spectrometer (Lab RAM HR800) under a back scattering geometry (λ = 514 nm). X-ray photoelectron spectroscopy (XPS) analyses were performed using an Al K α (150 W) monochromatic X-ray source (ESCALAB 250, Thermo Fisher Scientific, USA). N 2 adsorption-desorption isotherms were examined at 77 K using a Micromeritics ASAP 2020. Thermogravimetric analyses (TGA) were determined at SDTQ600 (TA Instruments, USA) under an air atmosphere at a heating rate of 10 °C min −1 from room temperature to 700 °C. Cyclic voltammetry (CV) was performed by using CHI1040C electrochemical work station between 0.01 and 3.0 V at a scan rate of 0.2 mV s −1 . The galvanostatic charging/discharging test was conducted by using coin cells (CR2032) at room temperature on a multi-channel battery testing system (LAND CT2001A) with a cutoff voltage of 0.01-3.0 V vs Li + /Li. Working electrodes were prepared by mixing 80 wt.% the resulting W@Co 3 O 4 material, 10 wt.% acetylene black (Super-P) and 10 wt.% polyvinylidenefluoride (PVDF) binder dissolved in N-methyl-2-pyrrolidinone (NMP). 1.0 M LiPF 6 in mixed ethylene carbonate (EC) and diethyl carbonate (DEC) (EC: DEC = 1:1 by volume) was used as the electrolyte in the system.