Hydrothermal-template synthesis and electrochemical properties of Co₃O₄/nitrogen-doped hemisphere-porous graphene composites with 3D heterogeneous structure

Abstract

Despite the high capacity of Co₃O₄ employed in lithium-ion battery anodes, the reduced conductivity and grievous volume change of Co₃O₄ during long cycling of insertion/extraction of lithium-ions remain a challenge. Herein, an optimized nanocomposite, Co₃O₄/nitrogen-doped hemisphere-porous graphene composite (Co₃O₄/N-HPGC), is synthesized by a facile hydrothermal-template approach with polystyrene (PS) microspheres as a template. The characterization results demonstrate that Co₃O₄ nanoparticles are densely anchored onto graphene layers, nitrogen elements are successfully introduced by carbamide and the nanocomposites maintain the hemispherical porous structure. As an anode material for lithium-ion batteries, the composite material not only maintains a relatively high lithium storage capacity (the first discharge specific capacity can reach 2696 mA h g⁻¹), but also shows significantly improved rate performance (1188 mA h g⁻¹ at 0.1 A g⁻¹, 344 mA h g⁻¹ at 5 A g⁻¹) and enhanced cycling stability (683 mA h g⁻¹ after 500 cycles at 1 A g⁻¹). The enhanced electrochemical properties of Co₃O₄/N-HPGC nanocomposites can be ascribed to the synergistic effects of Co₃O₄ nanoparticles, novel hierarchical structure with hemisphere-pores and nitrogen-containing functional groups of the nanomaterials. Therefore, the developed strategy can be extended as a universal and scalable approach for integrating various metal oxides into graphene-based materials for energy storage and conversion applications.

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1. Introduction

To address the problems of depleting fossil fuels and global warming, there is an urgent need to develop energy storage devices with high-power density that can store and supply electricity.¹ Lithium-ion batteries (LIBs), as a new generation of environmental-friendly electrochemical energy storage and conversion device, have been widely used in portable electronic devices, static energy storage systems and electric bicycles due to their advantages of high energy density, long cycle life and high discharge voltage.^2–4 However, traditional LIBs using graphite electrodes with a low theoretical specific capacity (372 mA h g⁻¹) cannot meet the ever-growing demands for high performance.⁵ Recently, organic materials (azobenzene-4,4′-dicarboxylic acid lithium salt, and dithianon) with the advantages of light weight, abundance, low cost, sustainability, and recyclability have become desirable for green and sustainable LIBs but the poor cycle stability and low power density limit their large-scale application.^6,7 Therefore, persistent efforts have been devoted to exploiting new anode materials, which can enhance their rate capability, energy density and cycling stability.

As anode materials of next generation LIBs, transition metal oxides (TMOs) have attracted significant attention due to their higher theorical capacity than traditional graphite.^8,9 Among these TMOs, Co₃O₄ has been recognized as a potential candidate of anode material for LIBs owing to its high theoretical specific capacity of 890 mA h g⁻¹ than that of TiNb₂O₇ (387.6 mA h g⁻¹), CuO (674 mA h g⁻¹) and SnO₂ (780 mA h g⁻¹),¹⁰ which is based on the following reaction equation:^11–13

Co₃O₄ + 8Li⁺ + 8e⁻ ⇌ 3Co + 4Li₂O (1)

However, the practical application of Co₃O₄ has been limited because of its low inherent electrical conductivity, poor rate capability and severe particle agglomeration during the repeated charge/discharge cycling, which can result in the pulverization of electrodes, poorer cycling stability and more capacity loss.^14,15 Hence, several efforts have been devoted to circumvent these troublesome drawbacks. For example, the hybrid nanomaterials consisting of nanostructured Co₃O₄ and carbonaceous matrix (graphene,^16,17 carbon nanotubes^18,19 and carbon nanowires²⁰) were synthesized. It is noteworthy that graphene-based nanomaterials can be a promising candidate for the recombination with Co₃O₄ because they can serve as buffer against volume expansion and enhance the electrical conductivity of the anode electrodes.²¹ Additionally, the graphene-based materials possess high thermal conductivity and chemical stability, which offer significant accessional advantages.²² Nulu Venugopal²³ et al. exhibited that Co₃O₄ hybridized with carbon nanotubes can act as an anode material of LIBs with a lithium ion storage capacity of 602 mA h g⁻¹ at 500 mA g⁻¹. Dou²⁴ et al. revealed that Co₃O₄/graphene composite displayed a superior endurance during thousands of charge/discharge cycles when it was used as an active material of LIBs. Sun²⁵ et al. indicated that mesoporous Co₃O₄ nanosheets-3D graphene networks (3DGN) hybrid materials have been synthesized and investigated as anode materials for Li-ion batteries (LIBs). Consequently, coupling Co₃O₄ with other functional materials has been confirmed to be an effective strategy to improve the electrochemical properties.

Besides, doping is another efficient approach for enhancing electrochemical performance. Recently, nitrogen-doped graphene-based composite has been investigated as a significant research hotspot, because the incorporation of nitrogen elements into graphene can effectively enhance the electrochemical performance of carbon-based materials. The doping of nitrogen can not only induce numerous defects, but also form a disordered structure of carbon-based materials, which can provide more active sites to improve the lithium ions storage capacity.²⁶ Additionally, the presence of nitrogen heteroatoms also can promote the electrical conductivity, interfacial stability and reactivity of the carbon-based nanomaterials, which is beneficial to enhancing the rate capability and facilitating its reaction kinetics of working electrodes in storing energy.²⁷ Moreover, for the carbon-based anode nanomaterials, the porous structure can shorten the transport length of lithium-ions and promote the reaction of charge-transfer on the electrode/electrolyte interface, which is capable of offering excess high lithiation capability.²⁸ Hence, it is expected that compounding Co₃O₄ with N-doped carbon materials should be a promising strategy to increase the electrical conductivity, promoting the reaction kinetics and maintain the structure, thus reaching a significant storage of lithium ions.

Herein, anode electrodes (Co₃O₄/N-HPGC) consisting of Co₃O₄ and nitrogen-doped hemispherical porous graphene were synthesized by a hydrothermal-template approach with polystyrene (PS) microspheres as the template. Several characterization techniques have been employed to explore the structure, morphology and physicochemical properties of the Co₃O₄/N-HPGC composites. Benefiting from the desirable unique structure and advanced nitrogen doping, the as-synthesized Co₃O₄/N-HPGC nanocomposite, examined as electrode materials for LIBs, exhibits high lithium storage capacity, good rate performance and excellent cycling stability, which can be one of the most promising candidates for LIBs anode materials.

2. Results and discussion

2.1. Structural characterization

The structure and morphology of HPGC and Co₃O₄/N-HPGC nanocomposite were investigated by TEM (Fig. 1). As can be seen, the TEM image of composite without Co₃O₄ loads (Fig. 1a) reveals a hemispherical superimposed structure, formed due to the removed PS microsphere template, resulting in the self-assembly process of RGO. According to Fig. 1b and d–f, the nanocomposites loaded with Co₃O₄ particles still maintain the hemispherical pore structure after removing the PS microsphere template, which undoubtedly increases the specific surface area of the complex. Additionally, the Co₃O₄ nanoparticles are tightly anchored onto the surface of RGO sheets. However, the increase of Co₃O₄ loadings renders the nanoparticles more densely dispersed and simultaneously results in partial agglomeration. The HRTEM image (Fig. 1c) of Co₃O₄/N-HPGC-2 nanocomposite indicates that the presence of 0.36 nm and 0.24 nm interlayers spacing coincides with the (400) and (311) lattices spacing of Co₃O₄, meanwhile, the size of particles is measured to be between 5 and 12 nm. Furthermore, the SAED patterns (Fig. 1c) indicates that the diffraction rings sequentially corresponds to the (111), (220), (311) and (400) crystal planes of Co₃O₄, which provides further evidence that Co₃O₄ nanoparticles are successfully loaded onto RGO layers without noticeable impurity formation.²⁹

Fig. 1 TEM images of (a) HPGC and (b and d–f) Co₃O₄/N-HPGC-2, 1, 3 and 4, (c) HRTEM and SAED images of Co₃O₄/N-HPGC-2.

The XRD patterns of composites with different Co₃O₄ loads are illustrated in Fig. 2. For HPGC and Co₃O₄/N-HPGC-1, 2, 3 composites, the broad peak appears at about 26.5°, corresponding to (002) diffraction mode of graphene structure, which is the characteristic of nitrogen-doped hemisphere-porous graphene. It can be found that the diffraction peaks of the Co₃O₄/N-HPGC nanocomposites at 2θ = 19°, 31.2°, 36.7°, 44.2°, 59.4° and 65.6° are recognized to the diffraction of (111), (220), (311), (400), (511) and (440) planes of the spinel Co₃O₄ with face-centered cubic phase (JCPDS no. 42-1467),¹³ respectively. Additionally, compared with HPGC, the characteristic peaks coincided with Co₃O₄ in the sample become more obvious; meanwhile, the (002) diffraction peak of graphene decreases gradually with the amount of cobalt nitrate increasing. Therefore, the XRD analysis confirms the Co₃O₄ nanoparticles were uniformly immobilized on the hemisphere-porous graphene sheets, which corresponds to the TEM analysis.

Fig. 2 XRD patterns of HPGC, Co₃O₄/N-HPGC-1, 2, 3 and 4.

Raman spectroscopy was performed to study the nature of graphitization of as-prepared samples. As shown in Fig. 3a, the peaks appeared at 1349 cm⁻¹ and 1592 cm⁻¹ are corresponded to the D band (disordered carbon) and G band (graphitized carbon), which is attributed to the disordered structure or edge defects and sp²-hybridized carbon atoms of crystalline graphite, respectively.³⁰ Moreover, the peak at 621 cm⁻¹ in Co₃O₄/N-HPGC-2 nanohybrids is observed, which can be assigned to the characteristic peak of crystalline of Co₃O₄.³¹ Generally, the intensity ratio (I_D/I_G) of D and G bands can provide the structural change information of nanocarbon materials, and a high I_D/I_G ratio indicates a high extent of disordered structure.³² The I_D/I_G of Co₃O₄/N-HPGC-2 is 1.066, higher than 0.996 of HPGC, indicating a higher density of defect sites due to the porous structure and nitrogen doping, which confirms the coexistence of Co₃O₄ amorphous carbon and N-doping.³³ The TGA curve of Co₃O₄/N-HPGC-2 was shown in Fig. 3b. The first low-temperature weight loss under 200 °C is primarily due to the desorption of physically and adsorbed water.³⁴ The second weight loss step in the range of 200–600 °C is mostly ascribed to the burning of N-doped graphene and the remnants after 600 °C should be attributed to Co₃O₄. Based on the TGA curve, the weight percentage of HPGC in Co₃O₄/N-HPGC-2 is around 23.44 wt%.

Fig. 3 (a) Raman spectra of HPGC and Co₃O₄/N-HPGC-2 (b) TGA curve of Co₃O₄/N-HPGC-2.

The specific surface area and pore size distribution of Co₃O₄/N-HPGC nanocomposite were calculated by N₂ adsorption–desorption measurements and BET analysis. The N₂ adsorption–desorption isotherm is shown in Fig. 4 and the pore structure parameters are listed in Table 1. According to Fig. 3a, it is observed that the isotherms of all composites can be classified as type IV isotherms with H3 hysteresis loop, which is mainly attributed to wedge-shaped holes formed by layers of graphene and the presence of hierarchical pores.³⁵ Additionally, as shown in Fig. 4b, the pore size distribution based on BJH method reveals that the existence of some micropores, mesopores and a few hemisphere macropores formed by the removed polystyrene (PS) microspheres templates, and the aperture distribution range gradually narrows due to the overmuch Co₃O₄ loadings. The excessive loads will easily induce the agglomeration of Co₃O₄ particles and clog the pores of samples, which can reduce the specific surface area of the composites.³⁶ Moreover, the BET specific surface area of Co₃O₄/N-HPGC-1, 2, 3, 4 are listed in Table 1. With the increase of the Co₃O₄ loading capacities, the specific surface area of Co₃O₄/N-HPGC-2 nanomaterials can reach the maximum of 348 m² g⁻¹. While when the loadings further increase, the surface area gradually decreases to the minimum of 117 m² g⁻¹. The higher surface area of Co₃O₄/N-HPGC-2 nanohybrids may be ascribed to the distribution of these hierarchical pores. Thus, it is conductive to electrolyte diffusion to the active sites, and can effectively enhance the storage of lithium-ions during charge/discharge, contributing to the application of Co₃O₄/N-HPGC composite materials in LIBs.

Fig. 4 (a) N₂ adsorption and desorption curve and (b) pore size distribution curve of Co₃O₄/N-HPGC-1, 2, 3, 4.

Table 1 Pore structure parameters of Co₃O₄/N-HPGC composites

Sample	SBET (m^2 g^−1)	Vt (cm^3 g^−1)	Vmic (cm^3 g^−1)	Vmes/Vt (%)
Co3O4/N-HPGC-1	337	1.284	0.004	99.69
Co3O4/N-HPGC-2	348	1.555	0.314	79.81
Co3O4/N-HPGC-3	226	0.357	0.079	78.87
Co3O4/N-HPGC-4	117	0.285	0.101	64.60

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According to N₂ adsorption–desorption results, the Co₃O₄/N-HPGC-2 possesses the highest specific surface area, thus the electronic structure and chemical composition of Co₃O₄/N-HPGC-2 composites are further analyzed by XPS, and the results are shown in Fig. 5. From the full spectrum (Fig. 5a) of Co₃O₄/N-HPGC-2 nanocomposite, it can be observed that in addition to the three peaks of O 1s, N 1s and C 1s, there is also the characteristic peak of Co 2p at 780.5 eV, indicating that Co has been successfully doped into graphene. According to the analysis of the Co 2p high-resolution energy level spectrum (Fig. 5b), the two main peaks at 795.9 eV and 780.5 eV can be attributed to Co 2p_1/2 and Co 2p_3/2 spin orbital peaks of Co₃O₄, respectively, which illustrates Co element exists in the form of Co₃O₄ and is compounded with HPGC material.^37,38 The spin-energy separation is 15 eV with the characteristic of a Co₃O₄ phase, which indicates the presence of Co²⁺ and Co³⁺ species in the composites and is in accordance with the previous reports.³⁹ As shown in Fig. 4c, the main peak centered at 284.7 eV is related to the C–C group of graphite carbon. The peak at 285.2 eV reflects the bonding structure of the C–N bonds, corresponding to the N–sp² C bonds, which may originate from substitution of N atoms and defects or the edge of the nitrogen doped graphene.⁴⁰ And the weak peak appeared at 286.1 eV is ascribed to C–O group of the nanocomposites. Additionally, the high-resolution N 1s spectra (Fig. 5d) displayed four peaks located at 398.7 eV, 400.2 eV, 401.1 eV and 406.2 eV, which are attributed to pyridinic-N (a N atom bonding with two C atoms), pyrrolic-N, graphite-N (a nitrogen atom replacing a graphitic carbon atom) and oxidized-N. Moreover, the pyridinic-N can directly bond with metal atoms and the graphite-N can mediate their neighboring carbon atoms to bond with metal atoms, which can facilitate the deposition of Co₃O₄. The results affirmed that the composite material contains nitrogen elements introduced by carbamide, accounting for about 1.74% of the atomic weight (Table 2). Due to the higher electronegativity and smaller diameter of nitrogen than that of carbon, doping of nitrogen in graphene is beneficial for the introduced defects and more active sites toward lithium storage.⁴²

Fig. 5 (a) XPS survey spectra of Co₃O₄/N-HPGC-2 and high-resolution XPS spectra of (b) Co 2p, (c) C 1s, (d) N 1s of Co₃O₄/N-HPGC-2.

Table 2 Relative atomic percentages in Co₃O₄/N-HPGF-3 analyzed based on the XPS spectra

Sample	C 1s (%)	O 1s (%)	N 1s (%)	Co 2p (%)
Co3O4/N-HPGC-2	89.89	7.24	1.74	1.13

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2.2. Electrochemical performance analysis

To investigate the electrochemical performance of the Co₃O₄/N-HPGC nanocomposite as an anode for LIBs, the galvanostatic charge–discharge curves at a current density of 0.1 A g⁻¹ in the voltage range from 0.01 to 3.0 V have been measured. The first charge and discharge curves of Co₃O₄/N-HPGC composite electrodes with different Co₃O₄ loads are shown in Fig. 6a, the specific capacity and coulomb efficiency are listed in Table 3. According to results, the first charge and discharge capacity of Co₃O₄/N-HPGC electrodes gradually decreases with the increasing of Co₃O₄, while the first coulomb efficiency continuously increases, which indicates that the introduction of Co₃O₄ can improve the reversibility of lithium. However, the agglomeration of overmuch Co₃O₄ particles will lead to the decrease of lithium storage capacity. Hence, only the right amount of Co₃O₄ can enhance the electrochemical performance of Co₃O₄/N-HPGC. And the hemispherical pore structure can not only inhibit the agglomeration of nanoparticles, but also accelerate the transfer of lithium ions, which can effectively enhance its lithium storage capacity. As shown in Fig. 6b, the rate capability of Co₃O₄/N-HPGC electrode in the range of 0.1–5 A g⁻¹ is further tested, and the Co₃O₄/N-HPGC-2 has a higher capacity and an excellent rate capability. When the current density is set as 0.1, 0.3, 0.5, 0.7, 1.0 and 2.0 A g⁻¹, the reversible specific capacity of Co₃O₄/N-HPGC-2 stabilizes at 1188, 901, 740, 678, 623 and 488 mA h g⁻¹, respectively, which are shown in Table 4. Even at the high current density of 5 A g⁻¹, the electrode still remains at 344 mA h g⁻¹, which is significantly higher than HPGC (254 mA h g⁻¹), Co₃O₄/N-HPGC-1 (262 mA h g⁻¹), Co₃O₄/N-HPGC-3 (259 mA h g⁻¹) and Co₃O₄/N-HPGC-4 (116 mA h g⁻¹). Moreover, with the current density reset to 0.1 A g⁻¹, the specific capacity of Co₃O₄/N-HPGC-2 can maintain at 1194 mA h g⁻¹ higher than HPGC (913 mA h g⁻¹), quite reversibly.

Fig. 6 (a) First charge/discharge curves at a current density of 0.1 A g⁻¹ and (b) rate performances at various current densities for Co₃O₄/N-HPGC composites with different Co₃O₄ loadings.

Table 3 First charge/discharge capacities and coulombic efficiency at a current density of 0.1 A g⁻¹ for Co₃O₄/HPGC-1, 2, 3 and 4

Sample	Discharge capacity (mA h g^−1)	Charge capacity (mA h g^−1)	Coulomb efficiency (%)
Co3O4/N-HPGC-1	3356	1400	41.71
Co3O4/N-HPGC-2	2696	1316	44.33
Co3O4/N-HPGC-3	2055	1093	53.20
Co3O4/N-HPGC-4	1378	754	54.73

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Table 4 Reversible capacities at different current densities for HPGC and Co₃O₄/N-HPGC-1, 2, 3, 4

Sample	Reversible specific capacity (mA h g^−1)
	0.1 A g^−1	0.3 A g^−1	0.5 A g^−1	0.7 A g^−1	1 A g^−1	2 A g^−1	5 A g^−1
HPGC	1142	744	620	549	509	398	254
Co3O4/N-HPGC-1	1282	828	682	628	561	420	262
Co3O4/N-HPGC-2	1188	901	740	678	623	488	344
Co3O4/N-HPGC-3	978	734	660	624	496	398	259
Co3O4/N-HPGC-4	729	628	603	561	478	308	116

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The first three cyclic voltammetry (CV) curves of HPGC and Co₃O₄/N-HPGC composite materials are recorded in Fig. 7(a–d) at the scanning rate of 0.3 mV s⁻¹. In the first cathodic curve, the broad reduction peak at about 0.53 V can be attributed to the formation of solid electrolyte interphase (SEI) film,⁴³ but it disappears in the subsequent cycle. And compared with HPGC and Co₃O₄/N-HPGC-1, there are two new reduction peaks appearing near 0.83 V and 1.30 V in the other three nanocomposites, implying the production of CoO and metal cobalt during the reduction process, respectively.⁴⁴ Furthermore, two oxidation peaks are observed around 1.40 V and 2.22 V, which corresponds to the reaction of lithium ion with Co₃O₄ at different stages. And the reversible reaction of lithium is as follows:

Fig. 7 Cycle voltammetry curves of HPGC, Co₃O₄/N-HPGC-1, 2, 3 and 4 (a–d) with different Co₃O₄ loadings.

The peaks intensity and the integral areas of the third cycle are very close to that of second cycle. These results show that the electrochemically reversibility of the Co₃O₄/N-HPGC composite is gradually improve after the first cycle.

In order to get further comprehension about the electrochemical process, the electrochemical impedance spectroscopy (EIS) is performed, which can evaluate the electrical conductivity and lithium ions transfer of the fabricated electrodes. Fig. 8a indicates that all EIS spectra contain a broad semicircle at high frequencies demonstrating the charge transfer at the electrode/electrolyte interface, and consist of a sloped line at low frequencies, which is assigned to the lithium ions diffusion at the solid state in the electrodes.⁴⁴ Obviously, the radius of the semicircle at high frequency region for the Co₃O₄/N-HPGC-2 nanocomposite is much smaller than that for the other composite materials. The diminution of the semicircle demonstrates a substantial decline of charge transfer resistance due to the presence of hierarchical pore structure,⁴⁵ and the doping of N element is beneficial to the transfer of lithium ions owing to its higher electronegativity. Moreover, EIS spectra exhibit an increase of the slopes of the lines in low-frequency region, illustrating an improvement of the lithium lithiation/delithiation kinetics in the Co₃O₄/N-HPGC-2 anode. According to the equivalent circuit fitting values (Table 5), the R_SEI (29.2 Ω) and R_ct (5.3 Ω) of Co₃O₄/N-HPGC-2 are significantly below the HPGC (52.0 and 6.0 Ω, Fig. S3†) Co₃O₄/N-HPGC-1 (32.6 and 13.5 Ω), Co₃O₄/N-HPGC-3 (50.6 and 22.5 Ω), and Co₃O₄/N-HPGC-4 (51.1 and 27.4 Ω), which affirms that the Co₃O₄/N-HPGC-2 nanocomposite possess better electrical conductivity. The equivalent circuit used for fitting impedance spectra is shown in Fig. S2,† where R_e refers to the electrolyte resistance, R_SEI and C_SEI are the resistance and capacitor of the surface film formed on the electrodes, R_ct and C_dl are the double-layer charge-transfer resistance and capacitance, and Z_w is the Warburg impedance related to the diffusion of lithium ions into the bulk of the electrodes.⁴⁶

Table 5 Values of the equivalent circuit components used for fitting the experimental curve

Sample	Re (Ω)	RSEI (Ω)	Rct (Ω)
HPGC	2.4	52.0	6.0
Co3O4/N-HPGC-1	1.8	32.6	13.5
Co3O4/N-HPGC-2	2.2	29.2	5.3
Co3O4/N-HPGC-3	1.9	50.6	22.5
Co3O4/N-HPGC-4	2.1	51.1	27.4

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Fig. 8 (a) Nyquist plots of Co₃O₄/N-HPGC composites with different Co₃O₄ loadings (b) cycle performance at a current density of 1 A g⁻¹ for HPGC and Co₃O₄/N-HPGC-2.

As shown in Fig. 8b, in order to further validate the excellent electrochemical performance of Co₃O₄/N-HPGC-2 electrode, its cycling performance is evaluated under the current density of 1 A g⁻¹. It can be observed that Co₃O₄/N-HPGC-2 has a high initial charge–discharge capacity of 758 mA h g⁻¹ and 1464 mA h g⁻¹ respectively, and the coulomb efficiency is about 52%. At the 16th cycle, the reversible specific capacity decreases to the minimum of 644 mA h g⁻¹, which can be attributed to the formation of SEI membrane and the irreversible transformation of Co₃O₄. The reversible specific capacity increases to the maximum of 768 mA h g⁻¹ at the 195th cycle due to the continuous activation of the composite electrode. Additionally, at the 500th cycle, its reversible specific capacity stabilizes at 683 mA h g⁻¹, much higher than that of HPGC electrode (568 mA h g⁻¹) after 500 cycles at the current density of 1 A g⁻¹. Moreover, the coulombic efficiency of Co₃O₄/N-HPGC-2 electrode rise to more than 98% from the 18th cycle, suggesting the right amount of Co₃O₄ loads can significantly enhance its electrochemical properties, which is mainly ascribed to the synergistical effects of the Co₃O₄ nanoparticles, the hemispherical porous structure and the N-containing functional groups on the Co₃O₄/N-HPGC nanocomposites.⁴⁷ The 3D hemispherical superimposed structure of graphene with uniformly loaded Co₃O₄ nanoparticles not only can prevent further stack of graphene, but also can restrain the combination of Co₃O₄ particle, inhibit the circulation volume change and accelerate electron transfer during the charge–discharge process, thereby enhancing the lithium storage capacity and cycle stability of the composite electrode.⁴⁸

Fig. 9 Schematic illustration of the preparation approach to Co₃O₄/N-HPGC.

Recently, many electrode nanomaterials have been explored based on carbon-based materials and Co₃O₄, and all of them have some advantages and limitations. A comparison of the performance of our newly designed Co₃O₄/N-HPGC with those already reported in literature is shown in Table 6. Through comparison, it can be clearly seen that the as-prepared nanocomposites result in a relatively high reversible capacity (683 mA h g⁻¹ after 500 cycles) based on the large current density at 0.1 A g⁻¹.

Table 6 Comparison of the performance of Co₃O₄/N-doped carbon-based nanocomposites as anode materials for lithium ion batteries

Active materials	Current density A g^−1	Reversible capacity mA h g^−1	Cycle numbers	Initial coulombic efficiency (%)	Ref.
Co3O4/N-HPGC-2	0.1	683	500	52.0	This work
Co3O4@N–C nanotubes	0.1	598	50	72	41
Porous Co3O4/N–C	1	579.3	200	51	49
N–C/Co3O4	0.1	983.9	100	54.7	35
Co3O4/N-doped GN	0.2	950	100	No	45

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In addition, the nitrogen-containing functional groups of the nanocomposites also play a significant role in enhancing the electrochemical properties of the Co₃O₄/N-HPGC electrodes. And the doped nitrogen element can introduce the donor states near the Fermi level to generate n-type conductive materials, the N element configuration provides much more available active sites and nucleation sites to strengthen the binding energy between the metal oxides and carbon-based materials, which is beneficial to promoting the adhesion of electrolyte ions onto the electrode surface and decreasing the charge transfer resistance.⁵⁰ Therefore, a conductive network is generated because of the homogeneous dispersion of the Co₃O₄ particles on the surface and internal hemispheric pore sites of the Co₃O₄/N-HPGC nanomaterials, which can effectively enhance its capacitance. Furthermore, the nitrogen-containing functional groups on the nanocomposite react with lithium by forming Li₃N:

3Li + N → Li₃N, (3)

and the reaction is quasi-reversible:^51,52

4Co + Li₃N ⇌ Co + Co₃N + 3Li. (4)

Meanwhile, the anchored Co₃O₄ on the composites can generate metallic cobalt owing to the electrochemical reduction reaction, which can further catalyze the decomposition of Li₃N and produce lithiated cobalt nitride possessing high lithium storage capacity. Thus, the nitrogen-containing functional groups and Co₃O₄ nanoparticles on the Co₃O₄/N-HPGC can synergistically facilitate the lithium-ions storage capacity.³⁵

3. Experimental

3.1. Chemicals and reagents

Flake graphite of chemical grade (≤30 μm) were from Sinopharm Chemical Reagent Co. Ltd., China. Styrene, ethanol and sodium p-styrene sulfonate were obtained from Kelong chemical reagent company. Cobalt nitrate hexahydrate was bought from Shanghai Macklin Biochemical Co. Ltd. All other reagents used in this study, including ethylenediamine, sodium bicarbonate, potassium persulfate, sodium chloride, sodium hydroxide, sodium nitrate, concentrated sulfuric acid, potassium permanganate, concentrate hydrochloric acid, hydrogen peroxide, absolute ethyl ethanol and carbamide were purchased from Luoyang Chemical Reagent Co. Ltd., China. All these chemicals were of analytical grade and were used without further purification. Water used in the experiments was double-distilled.

3.2. Preparation of PS colloidal microspheres

The first step was the pretreatment of styrene monomer in order to obtain the purified styrene. In a typical procedure, sodium hydroxide (5 wt%) was added into the styrene solution in a separatory funnel to remove the polymerization inhibitor recognized as a phenolic compound through shaking the mixed solution, which was defined as extraction separation. The above operation was repeated three times. The solution treated by extraction separation was then placed into a distilling flask for further purification through decompressing distillation, and eventually the purified styrene was obtained. The PS colloidal microspheres were fabricated through emulsion polymerization as described below. The deionized water was first added into the jacketed reactor connected with the thermostatic water bath for controlling the reaction temperature. Styrene as the monomer, sodium bicarbonate as the stabilizer, and sodium p-styrene sulfonate as the emulsifier were then added into the reactor maintaining stirring at 80 °C for 1 h. Subsequently, the potassium persulfate served as the initiator was added to the emulsion and kept the reaction for another 18 h. Finally, monodispersed PS colloidal microspheres were prepared, which appeared bright milky white.

3.3. Preparation of Co₃O₄/N-HPGC nanocomposites

Graphene oxide (GO) was prepared through the modified Hummers method. Then, the prepared GO suspension (3 mg mL⁻¹) and PS microspheres emulsion (60 mg mL⁻¹, about 112 nm) were mixed and ultrasonically dispersed for 30 min. Took 120 mL of the above mixed suspension into 250 mL beaker, then slowly added cobalt nitrate solution (1 mol L⁻¹), 0.36 g of carbamide and 6 mL PS microsphere emulsion were slowly added into the mixture suspension successively, continuously stirring for 15 min and ultrasonicated for 15 min. Subsequently, the mixed solution was transferred to a Teflon lined stainless steel reaction vessel, heated up to 120 °C at the heating rate of 2 °C min⁻¹, and then maintained at this temperature for 12 h before naturally cooled to room temperature. Afterwards the content was washed with ethanol solution for three times and the product was freeze-dried at −60 °C. Finally, the prepared precursor was calcined at 500 °C for 2 h in a tube furnace under nitrogen protection at the heating rate of 5 °C min⁻¹ to obtain the Co₃O₄/N-HPGC composite. The above operation was repeated to obtain the nanocomposites with different volumes of cobalt nitrate solution (0, 0.375, 0.75, 1.5 and 3.0 mL), which were denoted as HPGC, Co₃O₄/N-HPGC-1, 2, 3, 4, respectively. Furthermore, the fabrication process for Co₃O₄/N-HPGC is illustrated in Fig. 9.

3.4. Characterization

The structure and morphology of Co₃O₄/N-HPGC were investigated using a JEM-2100 transmission electron microscope (TEM, JEOL, Japan). The elemental analysis of samples was determined by X-ray photoelectron spectra (XPS) on a VG Escalab 250 spectrometer (Thermo Scientific, USA). The X-ray diffraction (XRD) patterns were obtained by a D8 Advance X-ray diffractometer (Bruker, Germany) equipped with a Cu Kα radiation source (λ = 0.15418 nm) at a scanning rate of 5° min⁻¹. N₂ adsorption–desorption isotherms were measured using a Quantachrome Autosorb-iQ-MP (USA) analyzer at −196 °C with a relative pressure ranging from 0.0112 to 0.9948. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) model.

3.5. Electrochemical measurements

The electrochemical performances of the Co₃O₄/N-HPGC nanocomposites were evaluated by assembling them into CR2016 button-type cells. The working electrodes were prepared as follows: a slurry was synthesized by blending Co₃O₄/N-HPGC active materials (0.08 g, 80 wt%), conductive acetylene black (0.01 g, 10 wt%) and polyvinylidene fluoride (PVDF) adhesive (0.01 g, 10 wt%) in N-methyl-2-pyrrolidone (NMP) solvent. Then the prepared slurry was coated on the surface of copper foils, the foliated coppers were dried at 120 °C under vacuum for 10 h to remove the excess solvent, and the obtained electrodes were cut into disks with the diameter of 14 nm. The cells were assembled in an argon-filled glovebox (with less than 1 ppm of oxygen and water vapor), using lithium foils as the counter electrode and reference electrode, Celgard 2400 microporous polypropylene membrane as the separator, and a solution of 1 M LiPF₆ in a mixture of ethylene carbonate (EC)–dimethyl carbonate (DMC) (1 : 1 by volume) as the electrolyte. The galvanostatic charge–discharge (GCD) experiments were tested by Neware BTS 4000 batteries testing system at cycling rates ranging from 0.1 A g⁻¹ to 5 A g⁻¹, between 0.01 and 3.0 V at ambient temperature. Cyclic voltammetry (CV) measurement was performed at a scan rate of 0.3 mV s⁻¹ with a voltage of 0.01–3.0 V (vs. Li⁺/Li) using an electrochemical working station (RST5200F, Shiruisi Co. Zhengzhou). And the electrochemical impedance spectroscopy (ESI) experiment of the as-pared composites was carried out using a Princeton PARSTAT 2273 (USA) electrochemical workstation in the frequency range of 50 to 10⁵ Hz.

4. Conclusions

In summary, the Co₃O₄/N-HPGC nanocomposites were synthesized by a hydrothermal-template approach with polystyrene (PS) microspheres as the template. As shown in the aforementioned results, the Co₃O₄ nanoparticles are uniformly loaded on the graphene sheets of Co₃O₄/N-HPGC composite material, and the prepared Co₃O₄/N-HPGC nanohybrid has a large specific area (348 m² g⁻¹), rich hemisphere-porous structure, successful nitrogen doping and high electrical conductivity. It has been observed that the Co₃O₄/N-HPGC nanocomplex is a prospective anode material for LIBs due to its enhancive reversible specific capacity, outstanding cyclic performance and excellent rate capability. Meanwhile, the improved electrochemical performance is ascribed to the synergistic effects of the nanoscale Co₃O₄ particles, the hemispherical porous structure and the nitrogen-containing functional groups of the nanocomposites. Under the current density of 0.1 A g⁻¹, the reversible specific capacity of Co₃O₄/N-HPGC-2 with suitable loads of Co₃O₄ is as high as 1188 mA h g⁻¹. And its capacity remains at 344 mA h g⁻¹ under 5 A g⁻¹, even can maintain 683 mA h g⁻¹ after 500 cycles the current density of 1 A g⁻¹, which offers a possibility to synthesize nanostructure electrode materials for energy storage.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (U1803114), the Education Department Science Foundation of Henan Province (19A530002, 19A440002), the National Key Research Plan (2018YFB0604500) and the China Postdoctoral Science Foundation (207500).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra06897g

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