Due to the relatively high energy density and outstanding electrochemical performance, the rechargeable Li-ion battery has become the mainstream power source in the electrical vehicle (EV), the electrical storage system (ESS), and consumer electronic products (CE) in recent years   . For commercialized lithium battery, the anode material of graphite has a relatively low theoretical specific capacity of 372 mAh/g, which cannot meet the increasing demands of energy density for lithium batteries. Recently, a lot of research has been devoted to the electrochemical active transition metal oxides (TMOs) as promising anode materials (SnO2 , TiO2 , ZnO , Fe3O4  and α-Fe2O3 , Co3O4  -  ), due to their high theoretical capacities (>700 mAh/g) and abundance in nature. Among these TMOs, Co3O4 is one of the most promising candidates, which owns a large theoretical capacity (about 890 mAh/g) and remarkable electrochemical properties  - . Before commercializing the transition metal oxides anode materials, at least three obstacles need to be overcome. The first one is the massive volume expansion when the lithium-ion is charging, combined with the insertion of the Li-ion into the anode material crystal structure, resulting in the electrode pulverization with fast reversible capacity fading during cycling. The other is the poor ionic and electronic conductivity of the TMOs, leading to the high AC and DC resistance of the battery and the low discharge capacity under the large discharge current. The electrochemical reaction below occurs for Co3O4 nanoparticles during the initial charging process, which formed the Li2O with the loss of reversible lithium ions:
Owning to the irreversible reaction between the lithium-ion and Co3O4, the low 1st-coulomb efficiency is a crucial factor which hinders its commercialization.
Up to now, the composites of transition metal oxides with carbon materials, such as graphene , carbon nanotube , carbon fibers , and carbon aerogel   has attracted more and more researchers’ attention. It is suggested that these carbon materials have a relatively large surface area, good electronic conductivity, and mechanical flexibility, which could increase the kinetics of the electrochemical reaction and inhibit the pulverization of the electrode material. 
Co3O4/carbonaceous composite anode materials exhibit the improvement of electrochemical performance compared with the pure Co3O4 particles. These composites include the Co3O4/acetylene black composites , the Co3O4/carbon aerogel composites  , and Co3O4/graphene composites  . The common feature of all the composites above is just modified from one dimension, to stabilize material structure from both inner and outside; we introduce the carbon coating layer on the Carbon aerogel (CA)/Co3O4 composite. In this article, we have reported a composite material Carbon aerogel (CA)/Co3O4/Carbon (C), which consists of the macroporous architecture of three-dimensional carbon aerogel (CA) with the nano-sized spherical Co3O4 particle, and the nano-thickness carbon layer. This structure is designed as a double protection structure, aiming to protect the Co3O4 from exposure of the electrolyte and alleviate the pulverization of anode materials from both the inner and outside. Compared with pure Co3O4 particles, this double protection Carbon Aerogel (CA)/Co3O4/Carbon (C) composites exhibit improved electrochemical performance, including the higher specific capacity, electrochemical kinetic and cycling stability.
2.1. Material and Chemicals
All the chemical reagents were analytical grade. Carbon aerogel (CA) was synthesized through a typical Sol-gel method . Co(NO3)2∙6H2O, octanol, anhydrous alcohol, sodium dodecyl sulfate were obtained from Sinopharm Chemical Reagent Cooperation (Beijing China).
A three-step process preparation of the Carbon aerogel (CA)/Co3O4/Carbon (C) composite has been applied, which includes the synthesis of Carbon aerogel (CA) by supercritical drying firstly, then the loading of active material nano-sized Co3O4 particles on CA by solvothermal method in octanol solution, and the coating of polypyrrole by in-situ chemical polymerization and the final carbonation under high temperature.
The 0.05 g Carbon aerogel was dispersed into 50 mL octanol by ultrasonic dispersion for 1 h treatment, after that, 0.5 g Co(NO3)2∙6H2O was added in to the solution. After stirring for 15 min, the solution was heated at 180˚C for 16 h in a Teflon-lined stainless-steel autoclave. The hydrothermal reaction products were washed with distilled water for 3 times, and then dried at 60˚C under vacuum conditions for 12 h. Finally, the pure Carbon aerogel (CA)/Co3O4 composite powder was obtained.
The Carbon aerogel (CA)/Co3O4/Carbon (C) composite was synthesized through an in-situ chemical polymerization. The Carbon aerogel (CA)/Co3O4 composite powder was dispersed in 100 mL deionized water. After adding 10 mL sodium dodecyl sulfate, the solution was stirred for 4 h for a homogenous dispersion. Then, the 50 uL pyrrole monomer and 0.2 g FeCl3∙6H2O were added and the mixture was stirred for 4 h at room temperature. The as-obtained Carbon aerogel (CA)/Co3O4/PPy composite precipitates were washed several times with deionized water and then dried at 80˚C under vacuum for 6 h.
The obtained Carbon aerogel (CA)/Co3O4/PPy composite was heat-treated under N2 atmosphere at 450˚C for 45 min and then cooled to room temperature. Finally, the composite of Carbon aerogel (CA)/Co3O4/Carbon (C) was obtained.
For comparison, the pure Co3O4 was fabricated in the absence of Carbon aerogel (CA) and Carbon (C) under the same condition.
2.2. Material Characterization
The crystal structure information of these samples were characterized by X-ray diffractometer (XRD, Rigaku D/MAX-2550) with CuKα radiation in the 2θ range of 10˚ - 80˚, and Fourier transform infrared spectroscopy (FTIR, MAGNAIR750) in the wave-numbered range of 1000 - 4000 cm−1. The morphology of these samples were characterized by a (FESEM, SEM, HITACHI, SU-70) and a (TEM, JEOL 2100F) at an acceleration voltage of 200 kV. The mass content loading of the active materials Co3O4 was characterized by the thermogravimetric (TG) analysis under airflow at a rate of 10˚C min−1 in the range of 30˚C - 900˚C. The specific surface area (BET) of the composite’s material was evaluated by N2 adsorption/desorption isotherm at 77K (V-sorb 2800TP), and the pore size distribution was determined by the adsorption branch of isotherms based on the Barrett-Joyner-Halenda (BJH) model.
2.3. Electrochemical Measurement
The electrochemical properties of the composites were evaluated using the CR2032-type coin cells assembled in a dry argon-filled glove box. The working electrodes were prepared by mixing 80 wt% of the active composite Carbon aerogel (CA)/Co3O4/Carbon (C), 10 wt% conductive carbon black (Super-P), and 10 wt% poly (vinylidene fluoride) binder dissolved in N-methyl-pyrrolidinone (NMP) solvent. The active material slurry was coated on the Cu foil with a slot die. After coating and drying, the active material electrode was cut into small discs with a diameter of 12 mm. The Li metal foil with the diameter of 16 mm was used as the counter electrode, and the Celgard 2400 PP separator with the diameter of 18 mm was used as separator. The electrolyte was obtained by dissolving 1 mol∙L−1 LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 v/v) .
The galvanostatic charge/discharge and cycling performance were tested by a CT2001A Land battery system at room temperature in the voltage range of 0.01 - 3.0 V. The galvanostatic charge/discharge tests were measured at a scan rate of 0.1C, and the cycling performance as tested at a scan rate of 0.5C. The electrochemical impedance spectroscopy (EIS) measurement were tested in the frequency range from 0.01 Hz to 100 kHz, using a CHI660E (Shanghai Chenhua, China) electrochemical workstation.
3. Results and Discussion
3.1. Structural Characterization Results
Figure 1 shows the XRD patterns of the bare Co3O4 and Carbon aerogel (CA)/ Co3O4/Carbon (C) composites samples. The (111), (200), (311), (400), (422), (511), and (440) peaks in the XRD patterns indicates the face-centered cubic Co3O4 (fcc, Fd3m, JCPDS No.42-1467). Compared with the Co3O4, the pattern of Carbon aerogel (CA)/Co3O4/Carbon (C) is inclined to the baseline, which indicates that the Carbon aerogel (CA)/Co3O4 is coated by amorphous carbon. Also, the broader peak of the Carbon aerogel (CA)/Co3O4/Carbon (C) composite suggests the smaller crystal size of the Co3O4. No other diffraction peaks appear, indicating the amorphous carbon coating does not change the crystal structure of Co3O4.
Figure 1. XRD pattern of the Co3O4 and CA/Co3O4/C composite.
The morphology and micro-structure of Carbon aerogel (CA)/Co3O4/Carbon (C) composites were observed through SEM and HR-TEM, as shown in Figure 2. The pure Co3O4 particles have spherical morphology with a diameter about 800 nm - 1500 nm, some particles are adhering to each other as shown in Figure 2(a). Due to high specific surface area of Carbon aerogel (CA) and its 3D network structure composed of interconnected carbon spheres, the Co3O4 nanoparticles are adsorbed on Carbon’s surface aerogel (CA), and the distribution of Co3O4 is uniform and random. As shown in Figure 2(c) and Figure 2(d), the morphology of the Carbon aerogel (CA)/Co3O4/PPy and Carbon aerogel (CA)/ Co3O4/Carbon (C) composites varies a little, suggesting that the coating layer is thin and do not change the structure and morphology of the composite of Carbon aerogel (CA)/Co3O4. Further investigation of HR-TEM reveals that the carbon layer with the thickness of 40 nm is equally coated on the surface of Co3O4 as shown in Figure 2(e), demonstrating the nanoparticles of Co3O4 are well protected from the erosion of the electrolyte by the carbon layer, which is also acting as the buffer layer inhibiting the pulverization of the anode electrode when charging and discharging repeatly. As show in Figure 2(f), the lattice image from the CA/Co3O4/PPy could be clearly seen in the high resolution TEM (HRTEM) lattice image. The lattice spacing is about 0.24 nm, which could be assigned to (113) inter planar of the crystal structure for Co3O4.
To further study the structure of carbon coating on the composite of Carbon aerogel (CA)/Co3O4/Carbon (C). The Fourier transform infrared spectroscopy (FTIR, MAGNAIR750) in the wave-numbered range of 1000 - 4000 cm−1 was tested for both Carbon aerogel (CA)/Co3O4/Carbon (C) composites and the pure Co3O4. As shown in Figure 3, compared with the pure Co3O4, no prominent new peaks were observed for the composite of Carbon aerogel (CA)/Co3O4/Carbon (C), which indicates the polypyrrole (PPy) was fully carbonized. The broad and strong peak at 3404 cm−1 may attributed to the O-H stretching vibration, and the peaks at 2927 cm−1 can be assigned to the vibration mode of –CH group  .
Figure 2. SEM images of (a) Co3O4, and (b) (c) (d) CA/Co3O4/C composite at different magnifications, high-resolution TEM images (e) and (f) of CA/Co3O4/C composite.
Figure 3. FTIR of the Co3O4 and CA/Co3O4/C composite.
To investigate the mass content of the Co3O4 in the composites of Carbon aerogel (CA)/Co3O4/Carbon (C), the thermogravimetric analysis (TGA) was conducted in the air from RT to 900˚C. As shown in Figure 4, the weight loss of ~2 wt% below 250˚C could be ascribed to removing the water adsorbed on the surface. The weight loss of ~44.9 wt% between 250˚C and 700˚C could be ascribed to the combustion of Carbon aerogel (CA) and Carbon (C) in the air. The results could be deduced that the amount of active material Co3O4 in the composites of Carbon aerogel (CA)/Co3O4/Carbon (C) is about 53.1%.
The specific surface area (BET) of the composites material was measured to further investigate the porous structure of the composite. As shown in Figure 5, the test result exhibits a hysteresis loop in the adsorption/desorption curve, indicating the existence of well-developed porous structure. The specific surface area (BET) of the Carbon aerogel (CA) is 219.25 m2/g, indicating the excellent characteristic for loading Co3O4 active material. After loading the active material of Co3O4 nanoparticles and the carbon coating, the specific surface area (BET) of Carbon aerogel (CA)/Co3O4/Carbon (C) composite decrease to 10.8 m2/g. The pore structure of Carbon aerogel (CA)/Co3O4/Carbon (C) composite was further verified by the Barrett-Joyner-Halendathe (BJH) calculation in the inset of Figure 5. The average size of the pores is about 5 nm, indicating the abundant mesopores in the composite structure. These pores could improve the electrolyte’s infiltration in the cathode and anode electrode and improve electrochemical reaction activity; the unique porous structure could also relieve the inner stress generation by the volume changes of the Co3O4 during the lithiation-delithiation process.
3.2. Electrochemical Characterization Results
A standard method based on Carbon aerogel (CA)/Co3O4/Carbon (C) composite—Li half cell was used to evaluate the electric-chemical performance. Figure 6
Figure 4. Thermal gravity result of the CA/Co3O4/C composite.
Figure 5. Nitrogen adsorption isotherms of the CA/Co3O4/C composite.
Figure 6. Galvanostatic charge and discharge profile of the CA/Co3O4/C composite.
shows the galvanostatic charge/discharge profile at a constant current rate of 100 mA∙g−1 in the 1st, 25th and 50th charge-discharge curves, respectively. The first discharge specific capacity of the composite is about 1600 mAh/g. The first charge specific capacity is about 1280 mAh/g, which is much higher than both the theoretical specific capacity of Co3O4 and Carbon aerogel’s theoretical prediction value (CA)/Co3O4/Carbon (C) composite. This phenomenon has been observed in many transition oxide composites, and the underlying mechanism is still under debate   . Some of the researches ascribe it to the formation of a solid electrolyte interface (SEI) layer, which can store the interfacial Li ions , some ascribe it to the identical porous structure of CA, which can provide host-sites for the reduction of lithium ions , the recent study indicates the spin-polarized electrons can be stored in the already-reduced metallic nano-particles and contributes to the charge surface capacity . The first discharge curve exhibits a platform between 0.9 V to 1.1 V, followed by a declining curve to a discharge cutoff voltage, which is the typical character for the pure Co3O4 material . In the 50th charge-discharge curve, the voltage platform slightly decreases, indicating the increase of the Ohmic and electrochemical resistance after cycling.
Figure 7(a) shows the cycling performance of the Carbon aerogel (CA)/ Co3O4/Carbon (C) electrode and pure Co3O4 particle at a constant current rate of 100 mA∙g−1 up to 50 cls at room temperature. The discharge capacity of pure Co3O4 decreased to 380 mAh/g with the capacity retention of 63.3% at the 50th cycle. While for the Carbon aerogel (CA)/Co3O4/Carbon (C) composite, the capacity retention is about 88.3% at the 50th cycle, much higher than the pure Co3O4 particle. This phenomenon indicates that the porous CA structure and the carbon coating benefit the structure stability of the Co3O4 anode materials with high cycling performance. The Coulomb efficiency of 98.5% could be achieved after 50 discharge/charge cycles in Carbon aerogel (CA)/Co3O4/Carbon (C) composite, higher than the pure Co3O4 particle of 94.8%, suggesting that more reversible oxidation-reduction reaction with less losing of Li-ion in cycling for composite. Like many other transition metal oxides, for pure Co3O4, the huge volume swelling and destruction of the structure may result in SEI formation and losing reversible Li ion repeating in each charge/discharge cycle. For Carbon aerogel (CA)/Co3O4/Carbon (C) composite, the carbon coating and porous CA structure could improve the electrode electronic conductivity and relieve the volume swelling, improving the quality of the carbon the cycling performance .
Figure 7. Cycling performance and rate capability performance of the Co3O4 and CA/Co3O4/C composite.
As shown in Figure 7(b), the rate performances of the composite electrode compare with are evaluated at different current density from 0.1C to 2.5C. When the current density is 0.1 A/g, the discharge capacity is 1080 mAh/g for the Carbon aerogel (CA)/Co3O4/Carbon (C) composite, and 780 mAh/g for the pure Co3O4. When the current density increases to 0.25 A/g, 0.5 A/g, 1 A/g, 2.5 A/g, the ratio of discharge capacity to that at 0.1 A/g is 81.94%, 44.4%, 23.7% and 16.24% for the Carbon aerogel (CA)/Co3O4/Carbon (C) composite, higher than that of 63%, 32.1%, 15.7% and 10.4% for the pure Co3O4. The better rate performance for the composite of Carbon aerogel (CA)/Co3O4/Carbon (C) indicates the unique double protection strategy for the composite of Carbon aerogel (CA)/Co3O4/Carbon (C) may improve its electronic conductivity and the electrochemical reaction kinetic. Some reported Co3O4-based nanostructures in the last two years are listed in Table 1. By contrast, the cycling performance of Carbon aerogel (CA)/Co3O4/Carbon (C) is higher than those of Co3O4 nanocage/Co3O4 nanoframework/TiO2 , Co3O4 nanocubes , Hollow Co-Co3O4/CNTs , Co3O4/nitrogen-doped carbon composite  and Co3O4 nanocomposite . But, it is admirable that Co3O4/nitrogen-doped carbon composite in reference , Co/Co3O4 nanoparticles in reference  and porous Mn-Co3O4/C microspheresin reference  show super cyclability and rate capability. Nevertheless, the cycling stability and rate capability of Carbon aerogel (CA)/Co3O4/Carbon (C) composite anode are also superior to most of previous reports, indicating that the Carbon aerogel (CA)/Co3O4/Carbon (C) composite obtained in this study is a promising anode materials for LIBs.
Table 1. Specific capacities of Co3O4-based nanostructures in the literature.
To further compare and study the electrode reaction kinetics of the Carbon aerogel (CA)/Co3O4/Carbon (C) composite and the pure Co3O4 active materials, electrochemical impedance spectroscopy (EIS) performance was tested. The impedance spectra were collected by applying an 5 mV AC voltage perturbation over the frequency from to 0.1 Hz to 100 kHz, and the result could provide detail information of each component of resistance by different reaction process, including the resistance of the electrolyte, solid electrolyte interface (SEI), charge transfer, and Li+ diffusion in the electrode   . As presented in Figure 8, for both samples, depressed semicircles are observed in the middle frequency range in the Nyquist plots, which is often considered as the response of charge transfer resistance (Rct) between the electrolyte and the electrode. The charge transfer resistance is well fitted by an equivalent circuit model as shown in the inset of Figure 8, the fitting results of the Rct values are 350.2Ω and 275.7Ω for Co3O4 and the (CA)/Co3O4/Carbon (C) composites electrode, respectively, indicating the increasing of electrochemically active sites and kinetic after introducing the CA and Carbon coating layer on the Co3O4 particles. The fitting result of the Rs value is 4.16Ω for the (CA)/Co3O4/Carbon (C) composites electrode, which is smaller than 7.81Ω of the pure Co3O4. This phenomenon indicates that the carbon coating could improve electronic conductivity and reduce the Ohmic impedance. The EIS and equivalent circuit model fitting results display that the double protective structure could effectively facilitate both the electronic conductivity and electrochemical kinetic of the active material of Co3O4.
To investigate the structural stability of the Carbon aerogel (CA)/Co3O4/Carbon (C) composites during the charging and discharging process, the composite electrode after cycled 50 circles was carefully cleaned and observed using SEM. As shown in Figure 9, the microstructure of the Co3O4 particles varies little compared to the fresh electrode, no serious damage or agglomeration of the Co3O4 nanoparticles is observed, indicating the high structure stability when
Figure 8. Nyquist plots of the Co3O4 and CA/Co3O4/C composite.
Figure 9. SEM image of CA/Co3O4/C composite after 50 cycle.
charge and discharge repeatly. Similar results have been reported in other transition metal anode materials (TMOs) like Fe3O4 , indicating the supportive effect of the carbonaceous materials on the structure of TMO anode materials. The excellent structure stability may attribute to the unique double protective structure of Carbon aerogel (CA)/Co3O4/Carbon (C) composites as follows: The Carbon aerogels played an important role in supporting and loading the active materials of Co3O4 nonoparticles with its identical porous skeleton structure; the nano thickness outer layer of carbon adhering to the Co3O4 surface firmly, which could relieve the volume expansion and agglomeration when charging.
In this work, a double protection Carbon aerogel (CA)/Co3O4/Carbon (C) composites have been successfully synthesized by a simple solvothermal method combined with subsequent in-situ polymerization of polypyrrole (PPy) composite and then heat-treatment under N2 atmosphere. The composite with 53.1% loading of Co3O4 possessed a specific discharge capacity of 1280 mAh/g in the first cycle, higher than both the theoretical specific capacity of Co3O4 (890 mAh/g). The cycle and rate performance were determined with excellent cycle stability and high-rate capability. The further in the investigation of the electrode after cycling indicates the more stable structural stability. The unique structure with inner Carbon aerogel and outer Carbon coating layer provides double protection against the swelling and crack of the nano-particles and provides double protection against the swelling and crack of the nano-particles and enhances the electronic conductivity and electrochemical kinetic. These results suggest that the Carbon aerogel (CA)/Co3O4/Carbon (C) composite is a very promising candidate for high energy density anode materials of the next-generation LIBs.
This work is supported by the Nature Science Foundation of Guangdong Province (2018A030313371), Youth Innovative Talent Project of Guangdong Province College (2017GkQNCX061) and Innovation project of Shenzhen Polytechnic (CXGC2020C0009).
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