Facile solvothermal synthesis and superior lithium storage capability of Co₃O₄ nanoflowers with multi-scale dimensions

Abstract

In this study, Co₃O₄ nanoflowers (Co₃O₄-NFs) have been successfully synthesized by a facile ammonia-assisted solvothermal process and subsequent heat treatment. By suitably increasing ammonia solution added during solvothermal synthesis, flower-like Co₃O₄ structures were tailored from nanoflowers to microflowers (Co₃O₄-MFs). Materials characterization indicated that Co₃O₄-NFs with a specific surface area as high as 103.9 m² g⁻¹ were composed of multi-scale dimensions, including nanoparticles (0D) of about 10 nm in size, nanosheets (2D) of about 10 nm in thickness and nanoflowers (3D) of 290 ± 25 nm in diameter. The unique structural characteristics were beneficial for fast lithium ion diffusion and efficient electron transport. In addition, the mesoporous structure (22.6 nm) and the large pore volume (0.587 cm³ g⁻¹) of Co₃O₄-NFs were favorable for effectively alleviating volume expansion problems of high-capacity anode materials during charge–discharge cycles. When Co₃O₄-NFs were investigated as anode materials for lithium ion batteries, superior lithium storage capability, excellent cycling stability and C-rate performance were achieved, in comparison with Co₃O₄-MFs and commercial Co₃O₄ micro-/nanoparticles (Co₃O₄-NPs). Specifically, when tested at a current density of 500 mA g⁻¹ for 100 cycles, the reversible specific capacity of 1323 mA h g⁻¹ was achieved for Co₃O₄-NFs, much higher than Co₃O₄-MFs (1281 mA h g⁻¹) and Co₃O₄-NPs (107 mA h g⁻¹). When current densities of C-rate tests were increased to 1000, 2000 and 3000 mA g⁻¹, Co₃O₄-NFs could still deliver average specific capacities of around 1081, 925.9 and 570.8 mA h g⁻¹, respectively. Owing to the intriguing merits from the unique characteristics, Co₃O₄-NFs with a nanoflower structure could achieve remarkable lithium storage capability, demonstrating great potential in next-generation lithium ion batteries.

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Introduction

Rechargeable lithium ion batteries (LIBs) have achieved great success in portable electronic devices, such as cell phones, digital cameras and notebooks, owing to their high energy density, memory-free effect, long life span and environmental benignity.^1–5 Currently, LIBs also have promising widespread applications in hybrid electric vehicles (HEVs) and electric vehicles (EVs) to improve roadside air quality by reducing the emission of fine particulate matter (e.g. PM 2.5) and various types of greenhouse gases (e.g. CO₂ and NO_x). However, with the ever-increasing demands in energy density, high performance LIBs have encountered technical challenges for future applications in HEVs and EVs. For instance, the energy density of LIBs in EVs needs to be 2–5 times higher than the existing LIB technology (ca. 200 W h kg⁻¹).⁶ At present, graphite is the most frequently used anode material in LIBs. However, its specific capacity (372 mA h g⁻¹) is comparatively low to meet the increasing demand in high energy density.⁷ Therefore, alternative anode materials with various compositions and architectures are extensively explored in order to achieve excellent specific capacity, rate capability and cycling stability.^8–11

Recently, transition metal oxides (TMOs) have received much attention as promising anode materials for next generation LIBs, due to their high theoretical capacities (e.g. Co₃O₄: 890 mA h g⁻¹, CoO: 715 mA h g⁻¹, NiO: 718 mA h g⁻¹).^6,12,13 Particularly, Co₃O₄ has aroused considerable interest as its theoretical specific capacity is around 2–3 times higher than graphite. Unfortunately, Co₃O₄ suffers from poor cycling stability and poor rate capability in repeated charge–discharge cycles, due to huge volume expansion (about 100%) and low electrical conductivity.^14,15 The rational design of nanostructures was proven to be one of the effective strategies to mitigate the problems arising from large volume variation and poor electron transport.^16,17 So far, nanostructured Co₃O₄ anode materials with different morphologies have been examined, such as zero-dimensional nanostructures (e.g. nanoparticles),¹⁸ one-dimensional nanostructures (e.g. nanorods, nanotubes),^19,20 two-dimensional nanostructures (e.g. nanosheets)^21–23 and three-dimensional (3D) nanostructures (e.g. flower-like, urchin-like and pompon-like spheres).^24–26 Generally, the reversible and fast transport of Li⁺ between the anode and the cathode is one of the main concerns for high performance electrode materials. Evidently, a flower-like structure, endowed with a 3D architecture, micro-/nanoscale dimension and continuous networks, is advantageous for alleviating volume expansion and enhancing charge (Li⁺ and electrons) transport in electrochemical processes. For example, Chen et al. compared lithium storage properties of cobalt-based nanocubes, nanodiscs and nanoflowers and confirmed the superior electrochemical performances of nanoflowers delivering a reversible capacity of 649 mA h g⁻¹ after 100 cycles.²⁷ Also, many other anode materials with a flower-like structure have already demonstrated excellent lithium storage performance, such as NiO, TiO₂, SnO₂ and NiCo₂O₄.^28–31

Typically, the pursuit of high performance electrode materials has triggered the research interest in exploring flower-like Co₃O₄. For example, Jadhav et al. reported the chemical co-precipitation synthesis of flower-like Co₃O₄ composed of many porous nanorods of several micrometers in length.³² Also, Han et al. reported the hydrothermal synthesis of Co₃O₄ microscale flowers with sodium dodecyl sulfate (SDS) as a soft-template. The interaction between Co²⁺ ions and the hydrophilic terminus of the SDS surfactant was proposed to explain the nucleation and growth of flower-like microstructures.³³ In addition, flower-like Co₃O₄ microspheres (ca. 3–4 µm in diameter) with nanosheets as building units were fabricated via a solvothermal route in the presence of succinic acid.²⁴ Unfortunately, for previous studies, flower-like Co₃O₄ spheres applied in lithium storage performance evaluation were usually at the microscale.²⁴ Based on the directly proportional relationship between diffusion time (τ) and the second power of diffusion length (λ²), the microscale diffusion length of Li⁺ was definitely detrimental to lithium storage capability.¹⁷ In this regard, it is of great significance to develop Co₃O₄ materials with smaller dimensions for next generation LIBs, such as nanoflower architectures.

Herein, mesoporous Co₃O₄ with nanoflower morphology (Co₃O₄-NF) was successfully fabricated via a facile ammonia-assisted solvothermal strategy. Ammonia played a crucial role as a complexing reagent in controlling nanoscale or microscale dimensions of flower-like Co₃O₄. By suitably increasing the amount of ammonia solution added, the dimension of a flower-like structure was tailored from nanoscale to microscale (Co₃O₄-MFs). Material characterization indicated that Co₃O₄-NFs were endowed with intriguing features for achieving excellent lithium storage, such as multi-scale dimensions, a high specific surface area (104 m² g⁻¹), a mesoporous architecture (22.6 nm) and a large pore volume (0.587 cm³ g⁻¹). When evaluated at a high current density of 500 mA g⁻¹ for 100 cycles, the performance results demonstrated the superior lithium storage of Co₃O₄-NFs in comparison with Co₃O₄-MFs and Co₃O₄-NPs. The specific discharge capacity of Co₃O₄-NFs after 100 cycles was about 1323 mA h g⁻¹ without noticeable variation. The excellent C-rate performances of Co₃O₄-NFs also confirmed the great potential application in high-performance LIBs.

Experimental section

Synthesis of flower-like Co₃O₄-NFs and Co₃O₄-MFs

Flower-like Co₃O₄ precursors were first synthesized from cobalt sulphate in the presence of ethylene glycol, ammonia and sodium hydroxide by the solvothermal approach. In a typical process, 1.4 g of CoSO₄·7H₂O (≥99.0%, Sigma-Aldrich) was first dissolved in 30 mL of ethylene glycol (99.8%, Sigma-Aldrich), followed by adding 4 mL of concentrated ammonia solution (25–28%, Acros Organics). Meanwhile, 0.4 g of NaOH (≥98%, Sigma-Aldrich) was dissolved in 30 mL of ethylene glycol by magnetic stirring. The above solution was homogeneously mixed in a 50 mL Teflon-lined stainless-steel autoclave, followed by solvothermal treatment at 120 °C for 12 hours. After natural cooling, the as-prepared precursors were collected by vacuum filtration and rinsed 3 times with absolute ethanol. Finally, the as-prepared Co₃O₄ precursors were heat treated at 450 °C for 2 hours to obtain nanoflower Co₃O₄ products, denoted as Co₃O₄-NFs or Co₃O₄-NF. For comparison, Co₃O₄ products with a microflower structure, abbreviated as Co₃O₄-MFs or Co₃O₄-MF, were also prepared by increasing the added ammonia solution to 8 mL in the above solvothermal process. The as-prepared Co₃O₄-NFs and Co₃O₄-MFs were directly applied in material characterization and electrochemical tests.

Material characterization

Morphologies and microstructures of flower-like Co₃O₄ precursors and products were identified using a field emission scanning electron microscope (FE-SEM, Hitachi S4800) and a transmission electron microscope (TEM, FEI Tecnai G² 20 scanning). The crystal phases of flower-like Co₃O₄ products were characterized using an X-ray diffractometer (XRD, Bruker) equipped with a LynxEye detector and Cu K_α12 X-ray radiation. The specific surface area and pore size distribution of Co₃O₄ products were obtained using Micromeritics ASAP 2020 by the Brunauer–Emmet–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods. Thermal conversion from precursors to Co₃O₄ products was investigated by thermogravimetric analysis (TGA 1, Mettler Toledo, STAR^e System) and differential scanning calorimetry (DSC 1, Mettler Toledo, STAR^e System) under an oxygen atmosphere with a heating rate of 10 °C min⁻¹. The oxidation states of elements in precursors and products were investigated by X-ray photoelectron spectroscopy (XPS, Model PHI5600). The binding energy value in XPS was referenced to the C 1s peak at 284.6 eV.

Electrochemical tests

Electrochemical performances of the as-prepared Co₃O₄ anode materials (Co₃O₄-NFs and Co₃O₄-MFs) were evaluated in a 2025 type coin cell at room temperature. In a typical process, Co₃O₄ anodes were prepared by mixing 70 wt% Co₃O₄ active materials with a 15 wt% conductive agent (acetylene black, Super P®) and a 15 wt% binder (polyvinylidene fluoride, PVDF) in N-methyl-2-pyrrolidinone (NMP) solvent. The as-prepared slurry was uniformly coated on a copper foil by employing an automatic film applicator (AFA-II, Shanghai Pushen Chemical Machinery Co. Ltd, China), followed by vacuum drying at 120 °C for 12 hours. A compact disc cutter (MSK-T-10, MTI Corporation) was used to prepare a Co₃O₄ anode disc (diameter: 15 mm). The mass loading of active anode materials was about 1 mg cm⁻². The electrolyte was composed of a mixture of 50 vol% ethylene carbon and 50 vol% diethyl carbonate in 1 mol L⁻¹ LiPF₆. With the Co₃O₄ disc as the working electrode, lithium foil (diameter: 15 mm) as the counter electrode, and a polypropylene microporous membrane (Celgard 2400, diameter: 18 mm) as the separator, coin cells were assembled using a hydraulic crimping machine (MSK-110, MIT Corporation) in a glove box (MB-10, MBRAUN). The concentrations of oxygen and water were controlled at <0.5 ppm. Electrochemical impedance spectroscopy (EIS) of Co₃O₄ was conducted on an electrochemical work station (Bio-Logic, DHS) by providing a sine wave with an amplitude of 5 mV in the frequency range of 200 kHz–0.01 Hz. Battery performances of Co₃O₄ anodes were evaluated using a battery testing system LAND CT2001A (Wuhan Jinnuo Electronics, Ltd, China) with a cut-off voltage range of 0.01 V to 3.0 V vs. Li⁺/Li. A constant current density of 500 mA g⁻¹ was applied to battery cycling performance evaluation for 100 cycles. For comparison, commercial Co₃O₄ micro-/nanoparticles (99.5%, Aladdin) abbreviated as Co₃O₄-NPs or Co₃O₄-NP were also tested under the same conditions. In addition, the C-rate performance of Co₃O₄-NF anode materials was studied by charge–discharge cycles with current densities of 100, 200, 500, 1000, 2000 and 3000 mA g⁻¹.

Results and discussion

The morphologies and dimensions of the as-prepared cobalt-based precursors and products were investigated by FE-SEM. As shown in Fig. 1a and b, when 4 mL of ammonia solution were added for the solvothermal synthesis, nanoflower precursors of 580 ± 80 nm diameter could be clearly observed. The petals of each nanoflower were composed of thin nanosheets with around 10 nm thickness, and inter-connected with each other at the bottom of the nanosheets. When the precursors were calcined at 450 °C for 2 hours, nanoflower products (Co₃O₄-NFs) with 290 ± 25 nm diameter were still retained. The shrinkage of the diameter should be associated with the decomposition of precursors at high temperature. In Fig. 1c and d, nanosheets of around 10 nm thickness were clearly found in the 3D nanoflower products. This suggested that three-dimensional Co₃O₄ nanoflowers (Co₃O₄-NFs) were successfully prepared using the ammonia-assisted solvothermal method followed by heat treatment. With the well-connected structure, the multi-scale dimensions may lead to efficient electron transportation and lithium ion diffusion, indicating an improved charge transfer process in an electrochemical environment. Interestingly, when 8 mL of ammonia was added during the synthesis, flower-like precursors of larger dimensions were obtained. As illustrated in Fig. S1 (ESI†), the primary structure was nanosheets of a few nanometers and the secondary structure was spheres of a few micrometers (ca. 8 µm). Similarly, these flower-like precursors were converted into Co₃O₄ microflowers (Co₃O₄-MFs) by heat treatment at 450 °C. However, as seen in Fig. S2 (ESI†), further increasing ammonia amounts resulted in the formation of an irregular structure, probably due to dissolution of cobalt-based precursors under high pH conditions. As indicated in Fig. S3 (ESI†), by increasing the amount of ammonia from 2 mL to 12 mL, the pH values of the reactant solution before and after solvothermal treatment were gradually increased from 10.2–10.4 to 10.8–11.2. For the given ammonia amount, the decrease of the pH value was ascribed to the consumption of hydroxyl ions by the precipitation reaction. Note that the petal-like structure started to form when the amount of added ammonia solution was 2 mL. Based on the above concentration-dependent morphological study, ammonia was assumed to play a pivotal role as a complexing agent in controlling the dimensions of flower-like structures. Yang et al. previously reported the synthesis of flower-like cobalt-based precursors of 5–15 µm diameter by hydrothermal treatment of cobalt acetate in the presence of ethylene glycol.³⁴ In this study, additional ammonia added during material synthesis could lead to a complexation reaction between cobalt ions and ammonia ions, resulting in the formation of stable [Co(NH₃)₆]²⁺.²³ This species could effectively slow down material growth kinetics, thus reducing sphere dimensions from the microscale to nanoscale. Therefore, Co₃O₄-NFs and Co₃O₄-MFs could be easily tailored by varying the added amount of ammonia from 4 mL to 8 mL, respectively.

Fig. 1 Typical FE-SEM morphologies of flower-like precursors and products when 4 mL ammonia solution was used in solvothermal synthesis (a and b) flower-like precursors; (c and d) Co₃O₄ nanoflowers (Co₃O₄-NFs).

The microstructure and crystallinity of Co₃O₄-NFs were investigated by TEM. Many Co₃O₄-NF spheres were found in Fig. 2a. As depicted in Fig. 2b, a typical Co₃O₄-NF sphere exhibited a diameter of about 260 nm, comparable with the FE-SEM result. Plenty of inter-connected nanoparticles of about 10 nm were found in the Co₃O₄-NFs. Uniform pores of around 20 nm in size were also observed. The mesoporous feature of Co₃O₄-NFs could effectively alleviate the problem of volume expansion during charge–discharge processes, meanwhile improving Li⁺ diffusion from the electrolyte to active sites for electrochemical reactions, resulting in superior cycling stability and rate capability.⁴ In addition, many inter-connected nanoparticles were present in Co₃O₄-NFs, which might be advantageous for effective electron transport. Note that Co₃O₄-NF products exhibited numerous nanoparticles by TEM characterization, which was different from the flower-like morphology by FE-SEM characterization. The reason for such a difference should be attributed to the different image formation principles and magnifications. The formation of TEM images was based on the transmitted electrons and could provide detailed two-dimensional microstructure information with a high magnification (195 000 in Fig. 2b), while FE-SEM images were capable of giving three-dimensional surface morphology information but with a lower magnification (60 000 in Fig. 1c) on the basis of the scattered electrons. The electron diffraction pattern in Fig. 2c implied that the as-prepared Co₃O₄-NFs exhibited a polycrystalline nature. The diffraction rings from inside to outside were indexed to the (111), (220), (311), (400), (422), (511) and (440) crystal planes of Co₃O₄. In Fig. 2d, the lattice spacing was measured to be 0.46 nm, corresponding to the (111) crystal plane of Co₃O₄. Therefore, TEM analysis further confirmed high crystallinity and mesoporous features of Co₃O₄-NFs.

Fig. 2 Typical TEM images of flower-like Co₃O₄-NFs and corresponding SAED pattern (a and b) TEM images of Co₃O₄-NFs; (c) SAED pattern; (d) HRTEM image.

The crystalline structures of Co₃O₄ nanoflowers and microflowers were also characterized by XRD analysis. As shown in Fig. 3, XRD patterns of Co₃O₄-NFs and Co₃O₄-MFs were similar, as evidenced by the same diffraction peak positions. The observed 2θ peaks at 31.3°, 36.8°, 44.8°, 59.4° and 65.2° should be assigned to the (220), (311), (400), (511) and (440) crystal planes, respectively (JCPDS No. 43-1003). In addition, no significant impurities were detected in the final products, suggesting the complete conversion from the precursors to high purity Co₃O₄. Thus, the amounts of NH₃ added had no influential effects on the crystal phase of the final products. The width of the Co₃O₄-NF (311) peak at half-height was much broader than that of Co₃O₄-MFs, suggesting the smaller dimension of crystallites in Co₃O₄-NFs. Based on the (311) diffraction peaks, the crystallite sizes of Co₃O₄-NFs and Co₃O₄-MFs were estimated by the Scherrer formula to be about 9 nm and 12 nm, respectively.³⁵ In addition, the XRD results of cobalt-based flower-like precursors in Fig. S4 (ESI†) indicated a broad peak at about 35°, consistent with previous studies.³⁶ Note that background noise in XRD patterns should be ascribed to the formation of fluoresced X-ray signals from transitional metal elements (e.g. Co) when Cu K_α12 was used for X-ray radiation. A similar noisy XRD background was also observed in previous studies on transition metal oxide materials.^37,38

Fig. 3 XRD patterns of flower-like Co₃O₄-NFs and Co₃O₄-MFs.

As shown in Fig. 4, N₂ adsorption–desorption isotherms of Co₃O₄-NFs implied a typical IV isotherm with a hysteresis loop located in a relative pressure range (P/P₀) of 0.5 to 0.97, indicating the existence of the mesoporous structure. In the insert of Fig. 4, the BJH pore size distribution curve of Co₃O₄-NFs implied that the average pore size was estimated to be 22.6 nm, comparable with TEM observation. However, no obvious pore size distribution was found in Co₃O₄-MFs, probably caused by the collapse of the sheet-like structure during heat treatment. As shown in Table S1 (ESI†), Co₃O₄-NFs showed a larger specific surface area (104 m² g⁻¹) and a larger pore volume (0.587 cm³ g⁻¹), in comparison with Co₃O₄-MFs (83.6 m² g⁻¹ and 0.308 cm³ g⁻¹). The large specific surface area of Co₃O₄-NFs should be ascribed to the multi-scale dimensions, including small nanoparticles (0D) of about 10 nm size, ultrathin nanosheets (2D) of about 10 nm thickness and nanoflowers (3D) of 290 ± 25 nm diameter. Not only the high specific surface area could provide more active sites for electrochemical reactions, but also enlarge the contact area between the anode and the electrolyte during lithiation–delithiation cycles.³⁹ As a result, due to the presence of mesopores and their large pore volume, Co₃O₄-NFs were capable of accommodating drastic volume variation during electrochemical cycling.

Fig. 4 N₂ adsorption–desorption isotherms of flower-like Co₃O₄-NFs and Co₃O₄-MFs (inset: pore size distributions of Co₃O₄-NFs and Co₃O₄-MFs).

Thermal conversion from precursors to Co₃O₄ products was examined by TGA and DSC analysis. Representative TGA-DSC curves of flower-like precursors are displayed in Fig. 5. The initial weight loss of about 5% was probably attributed to the evaporation of water and ethylene glycol molecules trapped by the 3D flower-like precursors. With the increment in temperature, a significant weight loss of about 20% was observed at 100–300 °C. This could be ascribed to the decomposition of cobalt-based precursors, resulting in the formation of Co₃O₄ products and gaseous by-products (e.g. CO₂ and H₂O). No extra weight loss was found after 400 °C, indicating that 450 °C used in this study was sufficient to convert cobalt-based precursors into Co₃O₄. The sharp peak in the DSC curve suggested that the decomposition reaction occurred at about 250 °C. In addition, the TGA curve of Co₃O₄-NF products in Fig. S5 (ESI†) signified a small weight loss of around 2.5% upon heat treatment. This should be attributed to the removal of adsorbed water molecules from Co₃O₄-NFs, suggesting excellent thermal stability.

Fig. 5 TGA-DSC curves of Co₃O₄-NF precursors tested with a heating rate of 10 °C min⁻¹.

Element analysis and cobalt valences of Co₃O₄-NF products and precursors were analysed by XPS. The wide scan spectra in Fig. 6 suggested the existence of Co, O and C in both precursors and products. For Co₃O₄-NF products, the Co 2p_3/2 peak could be split into two peaks at 781.2 and 779.4 eV, indicating the existence of Co²⁺ and Co³⁺, respectively.²³ For flower-like precursors, the major peak was located at 780.8 eV, accompanied by a satellite peak of 785.8 eV, suggesting Co²⁺ in the precursors. It should be noted that no nitrogen elements was detected in these two samples, implying no chemical interactions between ammonia molecules and precursors (Fig. S6, ESI†). These results confirmed the formation of high purity Co₃O₄ materials using the ammonia-assisted solvothermal method followed by heat treatment.

Fig. 6 XPS analysis of flower-like precursors and Co₃O₄ nanoflowers (Co₃O₄-NFs) (a) wide scan; (b) high resolution Co 2p spectra.

Electrochemical behaviours of Co₃O₄-NFs were investigated by charge–discharge profiles, as shown in Fig. 7a. The first four cycles were tested at a current density of 500 mA g⁻¹ in a voltage range of 0.01–3.0 V (vs. Li⁺/Li). In this study, discharging and charging processes mean lithiation and delithiation, respectively. For the 1st discharge process, the voltage rapidly declined from the open-circuit voltage to approximately 1.0 V and a long plateau remained at around 0.9–1.0 V. The long plateau was mainly attributed to the redox conversion reaction involving Li and Co₃O₄. Co₃O₄ underwent the reduction process, which was being converted to an intermediate phase CoO, and finally metallic Co during the lithium insertion process. In the meantime, Li₂O was formed by the oxidation process. The discharge curve further dropped to a cut-off voltage of 0.01 V. The sloping feature should be attributed to the formation of polymeric layers (e.g. solid electrolyte interphase, SEI) on Co₃O₄ active materials. In the reverse process, a charge plateau located at about 2.0 V should be ascribed to the reversible oxidation of metallic Co into Co₃O₄, accompanied by the release of Li⁺. The overall electrochemical reactions involved could be summarized into Co₃O₄ + 8Li⁺ + 8e⁻ ↔ 4Li₂O + 3Co.^40,41 The initial discharge capacity and charge capacity were determined to be 1311.6 and 992.3 mA h g⁻¹, respectively. For the 2nd, 3rd and 4th cycles, all of these cycles revealed similar profiles with a gradual drop. Their charge capacities were about 1036.5, 1051.9 and 1066.6 mA h g⁻¹ respectively, while all of their discharge capacities were almost identical at around 1100 mA h g⁻¹. The overlapping of charge–discharge profile suggested the identical electrochemical reaction pathways in subsequent cycles, indicating that stable SEI films were formed in the first charge–discharge cycle. It should be mentioned that the first cycle Coulombic efficiency of Co₃O₄-NFs was 75.6%, comparable or higher than previous studies on nanostructured Co₃O₄.^42–45 For example, Li et al. reported mesoporous Co₃O₄ derived from a cobalt-based metal organic framework (MOF) exhibited a low Coulombic efficiency of 68%.⁴² Du et al. reported porous Co₃O₄ nanotubes templated from carbon nanotubes showed an initial Coulombic efficiency of about 70%.⁴³ In addition, Fu et al. reported that lemongrass-like Co₃O₄ delivered a Coulombic efficiency of 73.2%.⁴⁴ The capacity loss might be ascribed to the irreversible formation of lithium-containing SEI films (e.g. ROLi and ROCO₂Li, R = alkyl group) in the first cycle.⁴⁶ In addition, the obtained specific charge and discharge capacities in this study were much higher than the theoretical value of 890 mA h g⁻¹. In fact, this phenomenon was common for cobalt-based and transition metal-based materials with a mesoporous structure.^47–50 The extra capacities were probably attributed to the reversible formation of an electrochemically active gel-like film or interfacial lithium storage.^51,52

Fig. 7 Electrochemical behaviors of Co₃O₄ anode materials (a) charge–discharge profiles of Co₃O₄-NFs in the range of 0.01–3.0 V vs. Li⁺/Li at a current density of 500 mA g⁻¹; (b) cycling performances of Co₃O₄-NFs, Co₃O₄-MFs and Co₃O₄-NPs tested at a current density of 500 mA g⁻¹; (c) C-rate performance of Co₃O₄-NFs.

As illustrated in Fig. 7b, lithium storage performances of different Co₃O₄ materials (Co₃O₄-NFs, Co₃O₄-MFs and Co₃O₄-NPs) were compared at a current density of 500 mA g⁻¹ for 100 cycles. For Co₃O₄-NFs, both specific discharge and charge capacities were slightly increased for 40 cycles and then stabilized at about 1355 mA h g⁻¹. After 100 cycles, the specific discharge capacity was approximately 1323 mA h g⁻¹ and capacity retention was nearly 100%. For the cycling performance of Co₃O₄-MFs, the first specific discharge and charge capacities were 1160.9 and 876.6 mA h g⁻¹ respectively, equivalent to a similar Coulombic efficiency of 75.5%. In the course of the 100 cycles, the charge and discharge capacities of Co₃O₄-MFs were gradually increased and after 100 cycles the final discharge was 1281 mA h g⁻¹ with no obvious decay. The gradual increase of capacity for Co₃O₄-MFs was possibly due to the microscale dimension. In the initial charge–discharge cycles, partial lithiation and delithiation of Co₃O₄-MFs with a microscale dimension was possible at a high current density of 500 mA g⁻¹, owing to the limited diffusion time for both lithium ions and electrons. With the proceeding of charge–discharge cycles, the activation of pores by repeated volume variation enabled the penetration of electrolytes into more active electrochemical sites, thus leading to the gradual increase of specific capacity.⁴⁹ Actually, the gradual increase of specific capacity at the beginning of the charge–discharge cycles is a common phenomenon for Co₃O₄-based anode materials.^27,49 In contrast, commercial Co₃O₄-NPs showed worse cycling performance in lithium storage. For the first cycle, the charge and discharge capacities were about 393 and 1112 mA h g⁻¹, respectively. As indicated by the low Coulombic efficiency (ca. 35%), Co₃O₄-NPs exhibited serious irreversible electrochemical reactions in the first cycles, probably due to the significant formation of thick and irreversible SEI films. After 100 cycles, the specific discharge capacity was significantly decreased to about 107 mA h g⁻¹. The typical FE-SEM image of commercial Co₃O₄ suggested the coexistence of nanoparticles of about 100 nm and microparticles of 1–2 µm in size (Fig. S7, ESI†). The apparent capacity fading of Co₃O₄-NPs was probably ascribed to the serious side reactions and agglomeration of Co₃O₄ micro-/nanoparticles in charge–discharge cycles.⁵³ Wang et al. also reported that the specific capacity of spherical Co₃O₄ nanoparticles was severely declined from about 700 to 450 mA h g⁻¹ after 50 cycles when tested at 100 mA g⁻¹.⁵⁴ The above results clearly demonstrated that Co₃O₄-NFs endowed with a unique structure and dimension were promising lithium storage materials for lithium ion batteries, in comparison with Co₃O₄-MFs and Co₃O₄-NPs. As shown in Table S2 (ESI†), from the view of capacity retention and discharge capacity, cycling performance of Co₃O₄-NFs was much better than most of the previous studies. For example, Co₃O₄ octahedra synthesized by Gao et al. delivered a capacity retention of 62.5% after 60 cycles at 500 mA g⁻¹ and ordered mesoporous frameworks prepared by Fan et al. showed a capacity retention of 72.6% after 50 cycles at 890 mA g⁻¹.^55,56 In addition, after the cycling test, the specific discharge capacity of Co₃O₄-NFs was comparable or even higher than other nanostructures, such as nanospheres (820 mA h g⁻¹@100 cycles) and snowflake-shaped Co₃O₄ (1044 mA h g⁻¹@100 cycles).⁵⁷ It should be mentioned that the pore size distribution of Co₃O₄-NFs (22.6 nm) was relatively larger than most of the previous studies (Table S2, ESI†). Too small pore size might be detrimental to fast electrolyte penetration and ionic diffusion.^23,58 Note that Co₃O₄-NF materials demonstrated a robust structure after the charge–discharge of 100 cycles, as indicated in Fig. S8 (ESI†). This suggested that the mesoporous nature and large pore volume (0.587 cm³ g⁻¹) of Co₃O₄-NFs could effectively alleviate the volume variation during lithiation/delithiation cycles, thereby resulting in excellent structural integrity.

In order to verify the excellent lithium storage capability of Co₃O₄-NFs at high current densities, the C-rate performance of Co₃O₄-NFs is shown in Fig. 7c. The average discharge capacity of Co₃O₄-NFs at a current density of 100, 200, 500 and 1000 mA g⁻¹ was around 1084, 1140, 1122 and 1081 mA h g⁻¹, respectively. It was clear that Co₃O₄-NFs maintained a stable and high specific capacity at current densities ranging from 100 to 1000 mA g⁻¹. However, when the current density was further increased to 2000 and 3000 mA g⁻¹, the average discharge capacity of Co₃O₄-NFs decreased to 925.9 and 570.8 mA h g⁻¹, respectively. Although lithium storage performance was slightly decreased at high current densities, the achieved specific capacity was still much higher than the theoretical capacity of the existing graphite materials. Again, when Co₃O₄-NFs were tested at a current density of 100 mA g⁻¹, an average discharge capacity of 1360 mA h g⁻¹ was obtained, slightly higher than the previous cycles. The improved reversibility of Co₃O₄-NFs may be attributed to the activation of mesopores and nanostructured building units. By contrast, Co₃O₄-NPs exhibited poor rate capability in the range of 100 mA h g⁻¹ to 3000 mA h g⁻¹. As shown in Fig. S9 (ESI†), when current densities were 200, 500, 1000, 2000 and 3000 mA g⁻¹, the specific discharge capacities were about 174, 127, 104, 85 and 74 mA h g⁻¹, respectively. The lithium storage capacity of Co₃O₄-NPs even at a low current density (e.g. 200 mA g⁻¹) was still lower than that of graphite. Previous work suggested that lithium storage capability at a high current density was determined by transport behaviours of both lithium ions and electrons.²³ Although Co₃O₄-NPs containing small dimensions (nanoparticles of about 100 nm in size) could shorten the lithium ion diffusion length, electron transport between individual nanoparticles was previously suggested to follow a random walk in electrochemical processes, thus becoming the rate-determining step.²² Instead, owing to many inter-connected nanoparticles (about 10 nm) and a 3D flower-like structure, Co₃O₄-NFs possessed not only a short lithium ion diffusion length but also improved 3D inter-connected electron transport networks. Thus, the C-rate performances of the Co₃O₄ electrode could be manipulated by material morphology and structure dimension, which had strong influences on lithium diffusion and electron transport networks.

In order to understand the superior lithium storage performance of Co₃O₄-NFs, EIS was conducted on fresh coin cells assembled with different Co₃O₄ architectures, including nanoflowers, microflowers and micro-/nanoparticles. As indicated in Fig. 8, Nyquist plots of Co₃O₄-NFs, Co₃O₄-MFs and Co₃O₄-NPs showed a depressed semicircle in the high-middle frequency region and an inclined line in the low frequency region, consistent with previous studies on Co₃O₄-based anode materials.^39,59 The equivalent circuit for fitting the measured impedance data is also given in the inset of Fig. 8. The electrolyte resistances (R₁) of Co₃O₄-NFs, Co₃O₄-MFs and Co₃O₄-NPs calculated from the intersection point at a high frequency real axis were about 4, 10 and 4 ohm, respectively. In addition, the charge transfer resistances (R₂) of Co₃O₄-NFs, Co₃O₄-MFs and Co₃O₄-NPs determined by the diameters of the semicircles were about 35, 89 and 141 ohm, respectively. Apparently, Co₃O₄-NFs exhibited the smallest charge transfer resistance, indicating fast transportation of lithium ions and electrons in an electrochemical environment. The inclined line in the lower frequency region was characteristic of Warburg impedance, which should be relevant to lithium ion diffusion. The improvement of charge transfer and lithium ion diffusion should be probably attributed to the intriguing structural characteristics of Co₃O₄-NFs.

Fig. 8 Nyquist plots of Co₃O₄-NFs, Co₃O₄-MFs and Co₃O₄-NPs (inset: the equivalent circuit adopted in the fitting of all the measured impedance data; solid line: fitting data).

In conclusion, Co₃O₄-NFs endowed with a nanoflower structure manifested better lithium storage performances, compared to Co₃O₄-MFs with a microflower structure and Co₃O₄-NPs with a micro-/nanoparticle structure. The remarkable lithium storage performances should be related to the unique structural characteristics, which contributed to the enhanced transportation of electrons and lithium ions during charge–discharge cycles. To be specific, first, multi-scale dimensions of Co₃O₄-NFs, including 0D nanoparticles, 2D nanosheets and 3D nanoflowers could greatly shorten the lithium ion diffusion length and improve electron transport networks. Second, the large specific surface area could provide more active sites and enlarge the contact area between anode materials and electrolyte for electrochemical reactions. And third, the mesoporous nature and the large pore volume could ensure excellent electrode integrity by providing sufficient space to alleviate volume variations in lithiation–delithiation cycles. These intriguing merits of Co₃O₄-NFs guaranteed the superior lithium storage capability, suggesting that the rational structure design was a promising solution to address the challenges of high-capacity anode materials.

Conclusions

In summary, Co₃O₄ nanoflowers (Co₃O₄-NFs) with multi-scale dimensions and a mesoporous structure were successfully synthesized using a facile ammonia-assisted solvothermal method, followed by heat treatment at 450 °C for 2 h. By increasing the amount of added ammonia solution in solvothermal synthesis, the nanoflower structure was correspondingly tailored into microflowers (Co₃O₄-MFs). When investigated as anode materials at a current density of 500 mA g⁻¹ for 100 cycles, Co₃O₄-NFs delivered a reversible specific capacity of 1323 mA h g⁻¹, much better than Co₃O₄-MFs (1281 mA h g⁻¹) and Co₃O₄-NPs (107 mA h g⁻¹). In addition, Co₃O₄-NFs manifested better C-rate performances at high current densities. Specifically, when current densities used in charge–discharge cycles were further increased to 1000, 2000 and 3000 mA g⁻¹, the average reversible specific capacities of Co₃O₄-NFs were around 1081, 925.9 and 570.8 mA h g⁻¹, respectively. The enhanced lithium storage capability should be attributed to the unique characteristics of Co₃O₄-NFs, including multi-scale dimensions, a large specific surface area, mesoporous nature and a large pore volume. Overall, this work demonstrated great potential of Co₃O₄ nanoflowers as promising anode materials for applications in next generation lithium ion batteries.

Acknowledgements

The research work presented in this paper was financially supported by Technological and Higher Education Institute of Hong Kong Seed Grant Scheme (No. 1415103 and 14151112).

Notes and references

1. J. M. Tarascon and M. Armand, Nature, 2001, 414, 359–367.
2. B. Dunn, H. Kamath and J.-M. Tarascon, Science, 2011, 334, 928–935.
3. J. B. Goodenough and K.-S. Park, J. Am. Chem. Soc., 2013, 135, 1167–1176.
4. P. G. Bruce, B. Scrosati and J. M. Tarascon, Angew. Chem., Int. Ed., 2008, 47, 2930–2946.
5. J. B. Goodenough and Y. Kim, Chem. Mater., 2010, 22, 587–603.
6. S. Goriparti, E. Miele, F. De Angelis, E. Di Fabrizio, R. Proietti Zaccaria and C. Capiglia, J. Power Sources, 2014, 257, 421–443.
7. A. Manthiram, J. Phys. Chem. Lett., 2011, 2, 176–184.
8. H. B. Wu, J. S. Chen, H. H. Hng and X. Wen Lou, Nanoscale, 2012, 4, 2526–2542.
9. P. Roy and S. K. Srivastava, J. Mater. Chem. A, 2015, 3, 2454–2484.
10. J. Liu and X.-W. Liu, Adv. Mater., 2012, 24, 4097–4111.
11. L. Lai, J. Zhu, Z. Li, D. Y. W. Yu, S. Jiang, X. Cai, Q. Yan, Y. M. Lam, Z. Shen and J. Lin, Nano Energy, 2014, 3, 134–143.
12. P. Poizot, S. Laruelle, S. Grugeon, L. Dupont and J. M. Tarascon, Nature, 2000, 407, 496–499.
13. L. Liu, Y. Guo, Y. Wang, X. Yang, S. Wang and H. Guo, Electrochim. Acta, 2013, 114, 42–47.
14. J. Guo, L. Chen, X. Zhang, B. Jiang and L. Ma, Electrochim. Acta, 2014, 129, 410–415.
15. Y. M. Chen, L. Yu and X. W. Lou, Angew. Chem., Int. Ed., 2016, 55, 5990–5993.
16. Y. Wang, H. Xia, L. Lu and J. Lin, ACS Nano, 2010, 4, 1425–1432.
17. Y. Tang, Y. Zhang, W. Li, B. Ma and X. Chen, Chem. Soc. Rev., 2015, 44, 5926–5940.
18. L. Shen and C. Wang, Electrochim. Acta, 2014, 133, 16–22.
19. R. Xu, J. Wang, Q. Li, G. Sun, E. Wang, S. Li, J. Gu and M. Ju, J. Solid State Chem., 2009, 182, 3177–3182.
20. X. W. Lou, D. Deng, J. Y. Lee, J. Feng and L. A. Archer, Adv. Mater., 2008, 20, 258–262.
21. F. Zhan, B. Geng and Y. Guo, Chem. – Eur. J., 2009, 15, 6169–6174.
22. B. Wang, X.-Y. Lu, Y. Tang and W. Ben, ChemElectroChem, 2016, 3, 55–65.
23. B. Wang, X.-Y. Lu and Y. Tang, J. Mater. Chem. A, 2015, 3, 9689–9699.
24. J. Zheng, J. Liu, D. Lv, Q. Kuang, Z. Jiang, Z. Xie, R. Huang and L. Zheng, J. Solid State Chem., 2010, 183, 600–605.
25. X. Rui, H. Tan, D. Sim, W. Liu, C. Xu, H. H. Hng, R. Yazami, T. M. Lim and Q. Yan, J. Power Sources, 2013, 222, 97–102.
26. W. Hao, S. Chen, Y. Cai, L. Zhang, Z. Li and S. Zhang, J. Mater. Chem. A, 2014, 2, 13801–13804.
27. J. S. Chen, T. Zhu, Q. H. Hu, J. Gao, F. Su, S. Z. Qiao and X. W. Lou, ACS Appl. Mater. Interfaces, 2010, 2, 3628–3635.
28. G. H. Yue, Y. C. Zhao, C. G. Wang, X. X. Zhang, X. Q. Zhang and Q. S. Xie, Electrochim. Acta, 2015, 152, 315–322.
29. Z. Zhang, Z. Zhou, S. Nie, H. Wang, H. Peng, G. Li and K. Chen, J. Power Sources, 2014, 267, 388–393.
30. H. Wang, Q. Liang, W. Wang, Y. An, J. Li and L. Guo, Cryst. Growth Des., 2011, 11, 2942–2947.
31. L. Li, Y. Cheah, Y. Ko, P. Teh, G. Wee, C. Wong, S. Peng and M. Srinivasan, J. Mater. Chem. A, 2013, 1, 10935–10941.
32. H. S. Jadhav, A. K. Rai, J. Y. Lee, J. Kim and C.-J. Park, Electrochim. Acta, 2014, 146, 270–277.
33. L. Han, Y. Cai, P. Tang and L. Zhang, Mater. Today, 2015, 18, 410–411.
34. L.-X. Yang, Y.-J. Zhu, L. Li, L. Zhang, H. Tong, W.-W. Wang, G.-F. Cheng and J.-F. Zhu, Eur. J. Inorg. Chem., 2006, 4787–4792.
35. B. Wang, X.-Y. Lu, L. K. Yu, J. Xuan, M. K. H. Leung and H. Guo, CrystEngComm, 2014, 16, 10046–10055.
36. J. Jiang, W. Shi, S. Song, Q. Hao, W. Fan, X. Xia, X. Zhang, Q. Wang, C. Liu and D. Yan, J. Power Sources, 2014, 248, 1281–1289.
37. Y. Wang, A. Pan, Q. Zhu, Z. Nie, Y. Zhang, Y. Tang, S. Liang and G. Cao, J. Power Sources, 2014, 272, 107–112.
38. J. Xu, L. He, W. Xu, H. Tang, H. Liu, T. Han, C. Zhang and Y. Zhang, Electrochim. Acta, 2014, 145, 185–192.
39. Y. Xiao, C. Hu and M. Cao, J. Power Sources, 2014, 247, 49–56.
40. H. Song, L. Shen and C. Wang, J. Mater. Chem. A, 2014, 2, 20597–20604.
41. H. Sun, X. Sun, T. Hu, M. Yu, F. Lu and J. Lian, J. Phys. Chem. C, 2014, 118, 2263–2272.
42. C. Li, T. Chen, W. Xu, X. Lou, L. Pan, Q. Chen and B. Hu, J. Mater. Chem. A, 2015, 3, 5585–5591.
43. N. Du, H. Zhang, B. D. Chen, J. B. Wu, X. Y. Ma, Z. H. Liu, Y. Q. Zhang, D. R. Yang, X. H. Huang and J. P. Tu, Adv. Mater., 2007, 19, 4505–4509.
44. Y. Fu, X. Li, X. Sun, X. Wang, D. Liu and D. He, J. Mater. Chem., 2012, 22, 17429–17431.
45. H. Chen, Q. Zhang, J. Wang, D. Xu, X. Li, Y. Yang and K. Zhang, J. Mater. Chem. A, 2014, 2, 8483–8490.
46. S. J. An, J. Li, C. Daniel, D. Mohanty, S. Nagpure and D. L. Wood Iii, Carbon, 2016, 105, 52–76.
47. H. Huang, W. Zhu, X. Tao, Y. Xia, Z. Yu, J. Fang, Y. Gan and W. Zhang, ACS Appl. Mater. Interfaces, 2012, 4, 5974–5980.
48. K. M. Shaju, F. Jiao, A. Debart and P. G. Bruce, Phys. Chem. Chem. Phys., 2007, 9, 1837–1842.
49. B. Wang, Y. Tang, X.-Y. Lu, S. L. Fung, K. Y. Wong, W. K. Au and P. Wu, Phys. Chem. Chem. Phys., 2016, 18, 4911–4923.
50. J. S. Chen, Y. Zhang and X. W. Lou, ACS Appl. Mater. Interfaces, 2011, 3, 3276–3279.
51. M. Y. Son, J. H. Kim and Y. C. Kang, Electrochim. Acta, 2014, 116, 44–50.
52. J. Shao, Z. Wan, H. Liu, H. Zheng, T. Gao, M. Shen, Q. Qu and H. Zheng, J. Mater. Chem. A, 2014, 2, 12194–12200.
53. W. W. Lee and J.-M. Lee, J. Mater. Chem. A, 2014, 2, 1589–1626.
54. D. Wang, Y. Yu, H. He, J. Wang, W. Zhou and H. D. Abruña, ACS Nano, 2015, 9, 1775–1781.
55. J. Guo, L. Chen, X. Zhang, B. Jiang and L. Ma, Electrochim. Acta, 2014, 129, 410–415.
56. S. Fan, X. Liu, Y. Li, E. Yan, C. Wang, J. Liu and Y. Zhang, Mater. Lett., 2013, 91, 291–293.
57. D. Ge, H. Geng, J. Wang, J. Zheng, Y. Pan, X. Cao and H. Gu, Nanoscale, 2014, 6, 9689–9694.
58. D. Li, L.-X. Ding, H. Chen, S. Wang, Z. Li, M. Zhu and H. Wang, J. Mater. Chem. A, 2014, 2, 16617–16622.
59. Y. Wang, B. Wang, F. Xiao, Z. Huang, Y. Wang, C. Richardson, Z. Chen, L. Jiao and H. Yuan, J. Power Sources, 2015, 298, 203–208.

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Footnote

† Electronic supplementary information (ESI) available: FE-SEM images, pH values as a function of added ammonia, XRD patterns of cobalt-based precursors, the TGA curve of Co₃O₄-NFs, XPS high resolution N 1s spectra, C-rate performance of Co₃O₄-NPs, specific surface areas, pore properties and performance comparison table. See DOI: 10.1039/c6qm00114a

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