Electrospray deposition of a Co₃O₄ nanoparticles–graphene composite for a binder-free lithium ion battery electrode

Abstract

A binder-free anode was fabricated from a Co₃O₄–graphene composite by direct electrospray deposition on current collectors. The highly-interconnected mesoporous structure enables fast ion transport and accommodates the volume expansion of Co₃O₄ upon Li⁺ insertion/extraction. The incorporation of highly conductive graphene also improves the charge transport and thus the electrochemical performance.

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Lithium ion batteries (LIBs) require a high energy capacity and a long cycle life for portable electronic devices, electric vehicles and implantable medical devices.¹ Currently, commercial LIBs usually use graphite as the anode material owing to its excellent stability. However, it has a low theoretical specific capacity of only 372 mA h g⁻¹.² During the fabrication of commercial battery electrodes, carbon-based conductive materials and polymeric binders are introduced,³ adding parasite weight and ultimately reducing the specific capacity of the electrode. Moreover, the physical mechanisms of these additives are still obfuscated by the inhomogeneities routinely introduced in the battery assembly process, leading to difficulties in modelling and characterizing the diffusion paths of the ions and electrons.⁴ At elevated temperatures, the most-widely used binder, poly(vinylidene fluoride) (PVdF), can react with lithiated graphite (Li_xC₆) and metal lithium to form stable LiF and unsaturated [double bond splayed left]C=CF– bonds, and the accompanying thermal release (as high as 7180 J g⁻¹ for PVdF) may cause safety concerns. The incorporation of the PVdF binder also covers the surface of the electrode material, decreasing the electrochemically active surface area.⁵

Transition metal oxides (MO_x, where M = Cu, Fe, Ni, Co, etc.) display high theoretical capacities (>600 mA h g⁻¹) and are alternative anode candidates for high performance LIBs.⁶ However, electrodes made of MO_x nanoparticles have not been used in industry because of their low electrical conductivities and the large volume expansion/contraction associated with Li-ion insertion/extraction, leading to irreversible capacity loss and poor cycling stability.⁷ Extensive efforts have been devoted to developing binder-free LIBs based on MO_x.⁸ Different approaches have been used to fabricate binder-free electrodes, such as the direct growth of active materials of nanowires, nanotubes or nanoparticles on substrates using chemical vapor deposition, hydrothermal reactions or electrophoretic deposition.^4,8

Herein, we report a simple and cost effective approach for fabricating binder-free electrodes for LIBs using direct electrostatic spray deposition (ESD) on conductive substrates. The ESD technique has been widely employed to synthesize porous structures with high surface areas for various applications such as catalysis, chemical sensors, inorganic membranes, and electrode materials for solid oxide fuel cells (SOFCs) and LIBs.⁹ In this work, a Co₃O₄–graphene composite coating was uniformly deposited on copper foils using the ESD technique without any binders to form a 3D mesoporous structure. The 3D porous architecture enables fast ion transport during the charge and discharge processes and accommodates the large volume expansion of Co₃O₄ when Li⁺ inserts or extracts into its lattice.

Fig. 1 is a schematic diagram showing the fabrication of the binder-free and porous Co₃O₄–graphene composite electrode on the Cu foils using the ESD process. Pure Co₃O₄ nanoparticles (12 mg, 10–30 nm, 99.8% purity, Nanostructured & Amorphous Materials, Inc.) were first mixed with 1 g L⁻¹ of polycyclic aromatic hydrocarbons (PAH) solution, followed by ultra-sonication for 1 hour using an ultrasonic bath in order to form positively-charged amine end groups on the surface.¹⁰ A graphene oxide (GO) solution (30 ml, 0.2 g L⁻¹), prepared using the Hummer's method, was added into the Co₃O₄–PAH mixture, which caused the negatively-charged GO nanosheets to wrap up on the positively-charged surfaces of the amine-modified Co₃O₄ nanoparticles due to charge attraction. The GO–Co₃O₄ sheet suspension was then pumped into a stainless steel nozzle (inner diameter 0.8 mm) for ESD. The distance and DC voltage between the nozzle and substrate (copper foil) were 2.5 cm and 9–10 kV, respectively. The high voltage applied during the ESD process charged the GO–Co₃O₄ suspension and forced them into a conical shape, which was deposited on the copper foil by the electrostatic force. The deposited GO–Co₃O₄ hybrid film on the Cu foil was heated in a furnace at 250 °C for 2 hours in protective N₂ to generate the Co₃O₄–graphene film (named G–Co₃O₄).

Fig. 1 (a) A schematic diagram showing the wrapping up of the GO nanosheets on the Co₃O₄ nanoparticles; and (b) the fabrication of the GO–Co₃O₄ hybrid film by a direct ESD process on Cu foils. The optical image shows a uniform coating of the ESD deposited composite film.

A uniform film coating was achieved by the direct ESD process on the copper foils, as demonstrated in the optical image in Fig. 1b. Note that the ESD technique can be applied to continuous large scale productions in which active materials with different mass loadings can be easily sprayed onto the substrates without scalability constrains. For comparison, the GO–Co₃O₄ film was also prepared by normal dip coating on the Cu foil (see Fig. S1, ESI†). The composite film formed from the dip coating method appeared like clutter stains, in contrast to the smooth counterpart formed using the ESD method.

The morphology and microstructure of the ESD coated GO–Co₃O₄ and G–Co₃O₄ films were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with a Carl Zeiss Supra 55 SEM and a JEOL 2010 TEM operated at 200 kV, respectively. As shown in Fig. 2a and b, a highly porous interconnected 3D structure with various mesopores was obtained. This porous structure was thermally stable even after thermal treatment without pore collapse (see Fig. 2c and d). Co₃O₄ nanoparticles with an average diameter of 10–30 nm were anchored on the GO sheets as a result of the strong charge attraction, as shown in Fig. 2d. The TEM images (Fig. 2e and f) showed that the Co₃O₄ nanoparticles were anchored on the graphene sheets. The 3D porous structure fabricated using the ESD process displayed distinct advantages for use as a LIB anode, such as improving the electron mobility by incorporating highly conductive graphene, promoting Li⁺ ion diffusion through the open pore structure and greatly accommodating the volume change in the electrode.

Fig. 2 SEM and TEM images of the GO–Co₃O₄ and G–Co₃O₄ films: (a and b) SEM images of the GO–Co₃O₄ film and (c and d) SEM images of the G–Co₃O₄ film; (e) TEM image of the GO–Co₃O₄ film and (f) TEM image of the G–Co₃O₄ film.

The phase composition and crystal structure of the ESD deposited composite electrode were analyzed by X-ray diffraction using a PANalytical X-ray diffractometer with Cu radiation. As shown in Fig. 3a, the crystal structures of the Co₃O₄ and Cu foil remained unchanged upon heat treatment. Strong X-ray diffraction peaks from the Cu substrate were observed. The diffraction peaks with weaker intensities can be indexed to cubic Co₃O₄ (JCPDS no. 42-1467, see Fig. S2, ESI†). A high resolution TEM image (see Fig. S3, ESI†) showed the lattice image of the Co₃O₄ nanoparticles with an interplanar spacing of 0.46 nm, corresponding to an interlayer spacing of (111) for the cubic phase of Co₃O₄.

Fig. 3 Structural characterization of the ESD composite films: (a) XRD patterns and (b) Raman spectra of the Cu foil, GO–Co₃O₄ film and G–Co₃O₄ film. The small squares in the XRD pattern denote the diffraction peaks from Co₃O₄.

Raman spectra were acquired using a Jobin-Yvon HR300 Raman spectrometer with a 532 nm green laser source to further investigate the structure of the ESD composite electrode. The main Raman G and D peaks from graphene were identified at 1580 and 1350 cm⁻¹, respectively. As shown in Fig. 3b, both the GO–Co₃O₄ and G–Co₃O₄ films displayed strong D peaks located at 1580 cm⁻¹, suggesting that various defects and domain boundaries existed on the graphene sheets even after the GO–Co₃O₄ hybrid film was thermally treated for 2 hours at 250 °C. The intensity ratio between the D peak and G peak (I(D)/I(G)) decreased from 1.2 to 0.8, implying that the oxygenated groups of the GO sheets decomposed¹¹ after thermal reduction, leading to various porous structures consistent with the SEM results (Fig. 2). An additional Raman peak at ∼686 cm⁻¹ corresponds to the A_1g vibration mode of Co₃O₄.¹²

The electrochemical performance of the composite electrode formed by direct ESD was evaluated using a coin cell configuration, in which the hybrid G–Co₃O₄ film without any binder or carbon black was used as the working electrode and lithium metal foil was used as the counter electrode. For comparison, a traditional working electrode of bulk Co₃O₄ was prepared using a slurry coating procedure. The slurry was obtained by mixing Co₃O₄, carbon black and PVdF at a weight ratio of 80 : 10 : 10 in N-methylpyrrolidinone (NMP), which was then coated on the Cu foil and dried at 80 °C overnight under vacuum. The electrolyte used was 1 M LiPF₆ dissolved in a solution with 1 : 1 ethylene carbonate and diethyl carbonate by volume (Novolyte Technologies). The mass loadings were between 1.13 and 1.21 mg cm⁻². The cyclic voltammetry (CV) and Nyquist plots were analyzed using a potentiostat VersaSTAT 4 (Princeton Applied Research). The electrochemical behavior of the binder-free electrode was characterized using an Arbin BT 2000 testing station. All the electrochemical measurements were carried out at room temperature.

Fig. 4a shows the cyclic voltammograms of the G–Co₃O₄ electrode at a scan rate of 0.5 mV s⁻¹ between 0.01 and 3.0 V vs. Li⁺/Li. The peaks in the cyclic voltammograms of the G–Co₃O₄ film are nearly identical to other Co₃O₄ and graphene composites.^4,13 The first three CV cycles all showed a broad oxidation peak at ∼2.15 V; while the two reduction peaks at 0.41 and 0.81 V in the first cycle disappeared in the next two cycles. This can be attributed to the irreversible formation of the solid electrolyte interphase (SEI) and the decomposition of the electrolyte.¹⁴ Additionally, the reduction peaks became two peaks in the third cycle around 1.31 and 0.98 V, indicating the occurrence of some irreversible processes in the electrode material.¹⁵

Fig. 4 Electrochemical performance of the G–Co₃O₄ composite electrode: (a) cyclic voltammograms of the G–Co₃O₄ electrode at a scan rate of 0.5 mV s⁻¹ for three cycles between 0.01 and 3.0 V vs. Li⁺/Li; (b) charge and discharge curves after the first and 10^th cycles at a current rate of 100 mA g⁻¹; (c) cycling performance at different rates from 100 mA g⁻¹ to 5 A g⁻¹; and (d) capacity and Coulombic efficiency at 1 A g⁻¹.

The charge–discharge voltage profiles at a low current rate of 100 mA g⁻¹ after the first and 10^th cycles are shown in Fig. 4b. The first discharge curve presented two long voltage plateaus around 1.09 and 0.84 V, which are generally attributed to the reduction processes of Co₃O₄ to an intermediate-phase CoO then to metallic Co, respectively.^15,16a The discharge and charge capacities after the first cycle were 1627 and 1032 mA h g⁻¹, respectively. It is worth noting that these capacities are much higher than the theoretical value for Co₃O₄ (890 mA h g⁻¹), which is probably a result of the SEI formation. Similar phenomena have been observed for other metal oxide anodes.^13a,16 After 10 cycles, a high discharge capacity of 1248 mA h g⁻¹ was reached, with a corresponding charge capacity of 1225 mA h g⁻¹. Compared to the theoretical capacity of bulk Co₃O₄, the extra capacity in these cycles can probably be attributed to the larger electrochemical active surface area of graphene and the grain boundary area of the Co₃O₄ nanoparticles.^13a,16a

Fig. 4c shows the cycle performance of the G–Co₃O₄ film electrode at various current densities from 100 mA g⁻¹ to 5 A g⁻¹. The G–Co₃O₄ composite exhibited an excellent rate capability. At a current density of 1 A g⁻¹, the binder-free G–Co₃O₄ composite electrode could still reach a high capacity of 631 mA h g⁻¹ and the columbic efficiency was greater than 97% after 58 cycles (see Fig. 4d). At a higher current density of 5 A g⁻¹, a high capacity of 321 mA h g⁻¹ could still be retained. These results demonstrate that the 3-D, binder-free G–Co₃O₄ film displayed an outstanding lithium storage capability under high current rates, owing to the open structure and buffer capability of the graphene sheets during the charge–discharge process. It was interesting to observe that a much higher capacity could be reached when the current rate decreased back to 100 mA g⁻¹, suggesting that electrode still maintained its structural integrity and stable framework upon cycling at different change current rates, and retained its capacity for lithium storage.

For comparison, the bulk conventional Co₃O₄ slurry was tested over a similar current density range as a control experiment. As shown in Fig. S4, ESI,† much lower capacities and rate performances were observed for the control samples compared to the ESD G–Co₃O₄ composite electrode. Specifically, both the charge and discharge capacities were reduced to nearly zero under a high current density of 5 A g⁻¹ due to the absence of highly conductive graphene sheets.

In order to validate the influence of graphene incorporation and being binder-free on the electronic conductivity of the G–Co₃O₄ electrode, electrochemical impedance spectroscopy (EIS) was utilized and the Nyquist plots obtained were modelled and interpreted with the help of an appropriate electric equivalent circuit (see Fig. 5). It was revealed that the G–Co₃O₄ and Co₃O₄ powder electrodes had almost similar electrolyte resistances, but the former had a much lower charge transfer resistance (R_ct = 71.3 and 174.1 Ω, respectively). This is due to the improved electronic conductivity profile provided by the graphene nanosheets. This result indicates a distinguished synergistic effect between the cross-linked pores in the graphene architecture and the Co₃O₄ nanoparticles in the composite in improving the charge transfer efficiency and thus enhancing the electrochemical performance.

Fig. 5 Nyquist plots of the G–Co₃O₄ film and Co₃O₄ electrodes. Inset is a modelled equivalent circuit of EIS. R_e is the electrolyte resistance, and R_s and C_PE1 are the resistance and capacitance of the surface film formed on the electrodes, respectively. R_ct and C_PE2 are the charge-transfer resistance and double-layer capacitance, respectively. Z_W is the Warburg impedance related to the diffusion of lithium ions into the electrodes.

Conclusions

A binder-free G–Co₃O₄ film electrode was deposited directly on Cu foil using the ESD technique. Synergistic effects from spraying and heat treatment resulted in various 3D cross-linked graphene–Co₃O₄ porous structures, allowing for volume change variation of the active materials upon cycling and improving the charge transfer efficiency and thus the electrochemical performance. This approach can also be applied to fabricate other binder-free metal oxide–graphene films for electrochemical energy storage.

Acknowledgements

This work was supported by a NSF award under the award number of DMR 1151028. T. Hu and C. Liu also acknowledge the State Scholarship Fund of the China Scholarship Council (File no. 2011608043) and the Fundamental Research Funds for the Central Universities (N100702001).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Optical images, XRD pattern, TEM image and electrochemical analysis. See DOI: 10.1039/c3ra45571h

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