A Co₃O₄@MnO₂/Ni nanocomposite as a carbon- and binder-free cathode for rechargeable Li–O₂ batteries

Abstract

Rational design and synthesis of cathode catalysts are crucial for enhancing the performance of rechargeable Li–O₂ batteries. Here, we report a controlled synthesis of the nanocomposite, Co₃O₄ nanohorns coated with MnO₂ nanosheets on a Ni foam (Co₃O₄@MnO₂/Ni). It integrates the catalytic activities of oxygen reduction and oxygen evolution reactions and functions as a carbon- and binder-free cathode catalyst for rechargeable Li–O₂ batteries. This nanocomposite catalyst presents a small discharge/charge voltage gap of 0.76 V (a low polarization) and a long cycle life of 170 cycles at 300 mA g⁻¹, coupled with an ionic liquid-based electrolyte, 0.5 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI), which are much better than those based on the individual Co₃O₄ and MnO₂ cathodes. The enhanced electrochemical performance is ascribed to the integrated bifunctional catalytic activities and the porous micro/nanostructure of the Co₃O₄@MnO₂/Ni nanocomposite, as well as the ionic liquid-based electrolyte, indicating its promising application in rechargeable Li–O₂ batteries.

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Introduction

Rechargeable Li–O₂ batteries, which show a high theoretical energy density of ∼11 680 Wh kg⁻¹ (based on the mass of Li), have attracted much attention in recent years.^1–4 However, one critical issue of such batteries is to keep the high catalytic activities of cathode catalysts towards both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER).^5,6 Among various bifunctional catalysts,^7–11 transition metal oxides with micro/nanostructures have been applied as ORR or OER electrocatalysts owing to their low cost and easy preparation.^12–17 Specifically, Mn³⁺ and Mn⁴⁺ species were found to be more active for ORR, and Co³⁺ was demonstrated to be the active site for OER.^18,19 However, the two separate functions have seldom been attempted in one system. Meanwhile, MnO_x suffers from its intrinsic poor conductivity (e.g., 10⁻⁵–10⁻⁶ S cm⁻¹ for MnO₂).^20,21 Integrating cobalt oxide with manganese oxide to construct a CoO_x–MnO_x composite is one of the strategies to achieve the synergistic effects towards both ORR and OER. Furthermore, CoO_x can serve as a conductive network for MnO_x. To the best of our knowledge, the application of a nanostructured CoO_x–MnO_x hybrid has rarely been explored in rechargeable Li–O₂ batteries.

Conventional air cathodes consist of a catalyst supported on carbon black and a polymer binder. Unfortunately, carbon was revealed to be unstable in a highly oxidizing environment of cathodes, especially at high voltages. It can spontaneously react with the discharge product Li₂O₂ to form lithium carbonates, which are difficult to be decomposed, and thus induce high discharge/charge overpotentials.^22–24 In addition, the polymer binders, such as poly(vinylidene fluoride) and polyethylene oxide, are vulnerable to O₂⁻ anions, which are intermediates of the ORR process.^25,26 Therefore, employment of carbon- and binder-free air cathodes in place of the carbon counterparts is essential to circumvent or suppress side reactions for rechargeable Li–O₂ batteries with long cycle life.

Herein, we report a controlled synthesis of the nanocomposite of Co₃O₄ nanohorns coated with MnO₂ nanosheets on a Ni foam (Co₃O₄@MnO₂/Ni) via a facile hydrothermal method. This nanocomposite integrates the respective ORR and OER catalytic activities, and works as a carbon- and binder-free cathode catalyst for Li–O₂ batteries. In combination with an ionic liquid (IL)-based electrolyte, 0.5 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMITFS), which has a low vapor pressure, wide electrochemical potential window and high chemical stability,²⁷ the resultant Li–O₂ batteries present a small discharge/charge voltage gap of 0.76 V and a long cycle life of 170 cycles of discharge and charge at 300 mA g⁻¹. This suggests the feasibility of a catalyst design strategy for integrating the functionalities of the individual materials and its potential application as a cathode catalyst for Li–O₂ batteries.

Experimental

Syntheses of materials

Co₃O₄ nanohorns on a Ni foam (Co₃O₄/Ni) were fabricated by a hydrothermal method. Typically, 1.45 g of Co(NO₃)₂·6H₂O was dissolved in 50 mL of deionized water to form a homogeneous pink solution, and then 0.37 g of NH₄F and 1.5 g of urea were added under constant stirring. One piece of Ni foam (3.0 × 3.0 cm in a square shape) was washed with 0.1 M HCl solution to remove the possible skin oxide and later with deionized water several times. Subsequently, the treated Ni foam and the above aqueous solution were transferred into an 80 mL Teflon-lined stainless autoclave, which was maintained at 90 °C for 5, 8, 10, and 12 h, and 120 °C for 10 h, respectively. The product-loaded Ni foam was taken out, washed, dried, and then annealed at 250 °C in air for 4 h to obtain crystallized Co₃O₄.

The nanocomposite of Co₃O₄ nanohorns coated with MnO₂ nanosheets on a Ni foam (Co₃O₄@MnO₂/Ni) was prepared by the growth of MnO₂ nanosheets on Co₃O₄ nanohorns via a carbon assisted reaction route. The synthetic procedures are schematically described in Scheme 1. The Co₂(OH)₂CO₃ precursor was firstly obtained by a hydrothermal process at 90 °C, and its further annealing at 250 °C for 4 h resulted in the formation of porous Co₃O₄ nanohorns on a Ni foam. The as-synthesized Co₃O₄/Ni was immersed into a 0.3 M aqueous glucose solution for 24 h. After carbonization of the coated glucose in an Ar atmosphere at 450 °C for 2 h, a thin carbon layer was produced on Co₃O₄ nanohorns in Co₃O₄/Ni. Then, the carbon coated Co₃O₄/Ni was transferred into an 80 mL Teflon-lined stainless autoclave containing 50 mL of 0.04 M KMnO₄ solution, which was heated to 120 °C and kept for 3 h. Thus, the thin carbon layer as a sacrificial template reacted with KMnO₄ to generate MnO₂ nanosheets on Co₃O₄ nanohorns. Finally, the nanocomposite of Co₃O₄@MnO₂/Ni was obtained by washing with distilled water and ethanol three times and dried at 60 °C in a vacuum oven. With similar procedures, the bare MnO₂ nanosheets on Ni foam (MnO₂/Ni) were prepared by coating glucose onto the Ni foam, carbonization, and then hydrothermal treatment in the presence of KMnO₄ at 120 °C. Simultaneously, Co₃O₄ nanomaterials were synthesized with similar procedures to those supported on a Ni foam. MnO₂ nanomaterials were obtained via a hydrothermal treatment of 20 mg carbon black (Vulcan XC-72) and 50 ml of 0.04 M KMnO₄ at 120 °C for 3 h after ultrasonic dispersion.

Scheme 1 Schematic illustration of the synthetic procedures of the Co₃O₄@MnO₂/Ni nanocomposite.

Material characterization

Micro/nanostructures and morphologies of the as-synthesized materials and the discharged/charged cathodes were characterized by powder X-ray diffraction (XRD) using a Rigaku/MiniFlex600 diffractometer with Cu Kα radiation, scanning electron microscopy (SEM) on JEOL JSM 7500F, and transmission electron microscopy (TEM) on a Philips Tecnai F20 (200 kV) equipped with an energy dispersive spectroscope (EDS). A Raman spectroscope from Thermo-Fisher Scientific was applied at 532 nm excitation (DXR) to identify the discharge/charge products during the cycles.

Battery assembly

Electrocatalytic activity of the as-synthesized materials was evaluated in non-aqueous Li–O₂ batteries based on CR2032 coin cells. The battery assembly was carried out in an Ar-filled glove box (Mikrouna Universal 2440/750, H₂O < 0.1 ppm and O₂ < 0.1 ppm). The cell consisted of a newly polished lithium foil anode (Φ14 × H0.3 mm), a prepared air cathode (Φ10 mm), a glassy fibre filter separator (Φ16 × H0.3 mm) soaked in the electrolyte 0.5 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI) or tetraethyleneglycoldimethyl ether (TEGDME). The mechanically mixed cathode of Co₃O₄/MnO₂/C was prepared by casting a mixture of 25 wt% Co₃O₄, 25 wt% MnO₂, 45 wt% Vulcan carbon-72 and 5 wt% poly(vinylidene fluoride) (PVdF) onto a nickel foam and then dried at 110 °C overnight in a vacuum oven. The assembled battery was then transferred into a sealed container, which was later flushed with 1 atm high-purity oxygen.

Electrochemical tests

The galvanostatic discharging and charging were carried out in a 1 atm O₂ atmosphere at room temperature using a LAND-CT2001A battery-testing system after at least 5 h rest. The discharge/charge cycles of Li–O₂ batteries were successively conducted without interruptions. All the current densities and specific capacities were calculated based on the weight of the loaded oxide catalysts. The mass loading of active materials in each cathode is around 0.8 mg cm⁻².

Results and discussion

Fig. 1 shows the XRD patterns and Raman spectra of the as-synthesized Co₃O₄/Ni and Co₃O₄@MnO₂/Ni nanocomposites. The diffraction peaks of bare Co₃O₄ can be well indexed to its cubic spinel phase apart from the three strong diffraction peaks of the Ni substrate in Fig. 1a, according to the JCPDS card no. 42-1467. The enlarged XRD profiles in Fig. 1b indicate the formation of the birnessite-type MnO₂ phase (JCPDS card no. 42-1317) in the Co₃O₄@MnO₂/Ni nanocomposite. The relatively weak diffraction peaks of MnO₂ may be ascribed to its poor crystallinity and nanosize. The structural features of the nanocomposite were further confirmed by Raman spectroscopy as shown in Fig. 1c. The vibration bands located at around 191, 480, 525 and 682 cm⁻¹ are assigned to the F_2g³, E_g, F_2g¹ and A_1g modes of Co₃O₄,²⁰ respectively. The other two Raman bands as indicated at 575 and 645 cm⁻¹ are in accordance with the Mn–O stretching vibration and the symmetric stretching vibration of the MnO₆ groups in the birnessite-type MnO₂, respectively.^28–30 These results indicate the formation of the Co₃O₄@MnO₂/Ni nanocomposite.

Fig. 1 XRD patterns (a, b) and Raman spectra (c) of the as-synthesized Co₃O₄/Ni and Co₃O₄@MnO₂/Ni nanocomposites.

The Co₃O₄ nanohorns on a Ni foam (Co₃O₄/Ni) and the precursor of Co₃O₄@MnO₂/Ni as shown in Scheme 1 are presented in the SEM image in Fig. 2a. It can be seen that the large-scale, dense and uniform Co₃O₄ nanohorns are vertically grown on the skeleton of the Ni foam. These nanohorns are several micrometers in length, as displayed in Fig. 2a and Fig. S1 (ESI†) under various magnifications. The micro/nanostructure and morphology of the Co₃O₄ nanohorns are dependent on the initial hydrothermal conditions, as revealed in Fig. S2, ESI.† The optimal temperature and duration are determined to be 90 °C and 10 h. The cross sectional image of the Co₃O₄/Ni nanocomposite in Fig. 2b displays the Co₃O₄ nanohorns of 1.5 μm in length and 100 nm in diameter. It is noted that the Co₃O₄ nanohorn is composed of numerous nanoparticles, which are connected to each other to form a highly porous structure, as depicted in the TEM image in Fig. S1, ESI.† The lattice spacing of 0.28 nm in the high-resolution TEM (HRTEM) image in Fig. S1 (ESI†) is in accordance with the (220) facet of Co₃O₄ in the XRD patterns in Fig. 1a. These suggest that the dense Co₃O₄ nanohorns are in situ formed on the Ni foam in the Co₃O₄/Ni nanocomposite via hydrothermal treatment.

Fig. 2 SEM images of Co₃O₄ NAs (a) and its cross section (b); and SEM (c), TEM (d) and HRTEM (e) images and EDS elemental mappings (f) of Co₃O₄@MnO₂/Ni. The inset in (f) shows the selected area of the EDS mappings.

With the formation of thin layers of MnO₂ nanosheets on the Co₃O₄ nanohorns in Co₃O₄/Ni via the reaction of carbon and KMnO₄, the Co₃O₄@MnO₂/Ni nanocomposite is obtained, as depicted in Scheme 1. The top view of the Co₃O₄@MnO₂/Ni nanocomposite is shown in Fig. 2c. The diameter of Co₃O₄@MnO₂ increases and its surface becomes porous after coating with MnO₂. The TEM observation in Fig. 2d shows that the MnO₂ is in the form of thin layers of nanosheets on a Co₃O₄ nanohorn. This kind of nanosheet morphology is related to the preferential growth of MnO₂ with the layered brucite crystallographic structure.^28,31 The interplanar spacing of the MnO₂ nanosheets is estimated to be 0.67 nm in the HRTEM image in Fig. 2e, which corresponds to the (001) plane of the birnessite-type MnO₂. The nanosheets of thin-layered MnO₂ are in agreement with the weak diffraction peaks in the XRD pattern in Fig. 1b. Moreover, the EDS mappings of the Co₃O₄@MnO₂ nanocomposite in Fig. 2f unambiguously confirm that MnO₂ is coated on the surface of a Co₃O₄ nanohorn. No carbon residue can be detected in the EDS mappings of the Co₃O₄@MnO₂/Ni nanocomposite, which is consistent with the Raman spectra in Fig. 1c and the EDS spectrum in Fig. S3, ESI.† The weight ratio of Co₃O₄ to MnO₂ in the Co₃O₄@MnO₂/Ni nanocomposite is estimated to be 1 : 1, based on the EDS spectrum in Fig. S3, ESI.† Under similar hydrothermal conditions to Co₃O₄@MnO₂/Ni, the MnO₂ nanosheets on a Ni foam (MnO₂/Ni) were also synthesized for comparison and are presented in Fig. S4, ESI.† The oxide nanocomposite structures can be generated on Ni foams by the post conversion strategy.

The electrocatalytic properties of the Co₃O₄@MnO₂/Ni nanocomposite as a bifunctional catalyst for Li–O₂ batteries were evaluated. Fig. 3a shows the first discharge/charge curves of the Li–O₂ batteries with the Co₃O₄@MnO₂/Ni, Co₃O₄/Ni, and MnO₂/Ni cathodes at a current density of 100 mA g⁻¹. The Co₃O₄@MnO₂/Ni nanocomposite displays a significantly improved discharge voltage and a reduced charge voltage as shown in Fig. 3a and b. This results in a small discharge/charge voltage gap of 0.76 V, which is ca. 250 and 370 mV lower than those of the Co₃O₄/Ni and MnO₂/Ni cathodes, respectively. The charge plateaus are corresponding to the decomposition of the discharge products, since the EMITFSI IL-based electrolyte has good electrochemical stability of up to 5.3 V.³² The round-trip efficiency based on the ratio of the discharge voltage to charge voltage is calculated to be 78% with a capacity limit of 1000 mA h g⁻¹, higher than 74 and 69% of the Co₃O₄/Ni and MnO₂/Ni cathodes as shown in Fig. 3b, respectively. It is notable that both the discharge and charge voltages of MnO₂/Ni are higher than those of Co₃O₄/Ni. This suggests that the MnO₂ is more active in the ORR process and Co₃O₄ is beneficial for the OER process. The integration of MnO₂ and Co₃O₄ into the Co₃O₄@MnO₂/Ni nanocomposite therefore presents a synergistic effect by showing higher discharge and lower charge voltages than the individual MnO₂/Ni and Co₃O₄/Ni cathodes.

Fig. 3 (a) Discharge/charge profiles of the Li–O₂ batteries based on MnO₂/Ni, Co₃O₄/Ni and Co₃O₄@MnO₂/Ni at 100 mA g⁻¹ in the first cycle; (b) enlarged comparison of the discharge–charge voltage gaps of the profiles in (a); and (c) terminal discharge/charge voltages versus cycle number with a limited capacity of 1000 mA h g⁻¹ at 300 mA g⁻¹.

The cycling stability of the Li–O₂ battery with the Co₃O₄@MnO₂/Ni nanocomposite is displayed in Fig. 3c. It can be operated for 170 cycles with stable terminal discharge voltages above 2.0 V and charge voltages below 4.7 V at 300 mA g⁻¹. This is in sharp contrast to the cycle life of 88 and 64 cycles for the individual Co₃O₄ and MnO₂ cathodes, respectively. For references, the mechanical mixture of Co₃O₄ and MnO₂ nanomaterials, which are composed of nanohorns and nanosheets, respectively, is displayed in Fig. S5, ESI.† They present similar morphologies to those in the Co₃O₄@MnO₂/Ni nanocomposite, and have the same size dimensions. The as-synthesized Co₃O₄ and MnO₂ with a weight ratio of 1 : 1 were mechanically mixed with carbon (designated as Co₃O₄/MnO₂/C) and coated onto a Ni foam. It leads to reduced reversibility (61 cycles), as presented in Fig. S6, ESI.† Compared to MnO₂/Ni in Fig. 3, Co₃O₄/MnO₂/C shows comparable discharge voltages and slightly lower charge overpotentials during the cycles. This suggests the function of Co₃O₄ in catalyzing the OER. The battery performance based on the presented Co₃O₄@MnO₂/Ni nanocomposite is competitive by referring to the reported results on Co and/or Mn-based oxide electrocatalysts, as shown in Table S1.† This indicates the structural advantages of the Co₃O₄@MnO₂/Ni nanocomposite: (i) homogeneous integration of the catalytically active sites of both MnO₂ and Co₃O₄ for ORR/OER ensures good rechargeability, (ii) its vertical growth on a Ni foam enables facile electron transport and its function as a carbon- and binder-free cathode to restrain the carbon and binder involved side reactions, (iii) the void space between the Co₃O₄@MnO₂ nanocomposites benefits electrolyte permeation and accommodation of the discharge product. Furthermore, an ether-based electrolyte, 1 M LiTFSI in tetraethylene glycol dimethyl ether (TEGDME), was also employed in the Li–O₂ batteries with the Co₃O₄@MnO₂/Ni nanocomposite. It presents 100 cycles of discharge and charge versus 170 cycles in the IL-based electrolyte counterpart, as depicted in Fig. S6, ESI.† The inferior performance of the TEGDME-based electrolyte may be attributed to slow decomposition of the electrolyte, which leads to formation of irreversible by-products, like lithium carbonate or carboxylates.^33,34 The micro/nanostructure and bifunctional electrocatalytic activities of the carbon- and binder-free Co₃O₄@MnO₂/Ni nanocomposite, as well as the stable IL-based electrolyte, ensure good electrochemical performance of the Li–O₂ batteries.

During the cycles, the discharged and charged Co₃O₄@MnO₂/Ni nanocomposite cathodes were characterized by SEM and Raman spectroscopy, as displayed in Fig. 4. The Co₃O₄@MnO₂/Ni cathode after the 10th discharge is shown to be covered by the mossy-like discharge product shown in Fig. 4a. It tends to aggregate in the void space between Co₃O₄@MnO₂. In the following charge, the discharge product is reversibly decomposed to show the void space, and the porous structure of the Co₃O₄@MnO₂/Ni cathode is recovered, as presented in Fig. 4b. The discharge product is identified to be Li₂O₂ by the Raman spectrum in Fig. 4c, and decomposed in the following charge with the disappearance of the characteristic Raman vibration band of Li₂O₂. In the 50th cycle of discharge and charge, the reversible formation and decomposition of Li₂O₂ at the Co₃O₄@MnO₂/Ni cathode are monitored in the Raman spectra in Fig. 4c. But, at the Co₃O₄/MnO₂/C cathode, in addition to the discharge product Li₂O₂, Li₂CO₃ is also detected by Raman spectroscopy in the 50th cycle as shown in Fig. 4d. In contrast, almost no Li₂CO₃ can be found at the Co₃O₄@MnO₂/Ni cathode during the cycles as shown in Fig. 4c. Thus, the carbon- and binder-free in situ formed Co₃O₄@MnO₂/Ni nanocomposite can effectively suppress the formation of lithium carbonate. The remarkable electrochemical performance of the Li–O₂ battery based on the Co₃O₄@MnO₂/Ni cathode originates from the integrated ORR and OER catalytic activities, efficient electron transfer through the networks of Co₃O₄ nanohorns grown on the Ni foam, and the porous micro/nanostructure that facilitates oxygen transport, electrolyte permeation, and accommodation of the discharge product Li₂O₂.

Fig. 4 SEM images of the discharged (a) and charged (b) Co₃O₄@MnO₂/Ni cathodes after the 10th cycle; Raman spectra of the pristine, the 10th and 50th cycles of discharge/charge of Co₃O₄@MnO₂/Ni (c) and Co₃O₄/MnO₂/C (d) cathodes.

Conclusions

The porous Co₃O₄@MnO₂/Ni nanocomposite was synthesized via a hydrothermal process and employed as a carbon- and binder-free cathode in rechargeable Li–O₂ batteries by coupling with an ionic liquid-based electrolyte, 0.5 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide. The as-synthesized Co₃O₄@MnO₂/Ni nanocomposite is featured as Co₃O₄ nanohorns coated with MnO₂ nanosheets on a Ni foam, and ensures a small discharge/charge voltage gap of 0.76 V and a long cycle life of 170 cycles. This is derived from the integrated bifunctional catalytic activities towards both ORR and OER and the porous micro/nanostructures of the Co₃O₄@MnO₂/Ni nanocomposite, which benefit the electron and oxygen transport and accommodation of Li₂O₂ as well as the ionic liquid-based electrolyte. The investigation suggests the promising application of this nanocomposite in Li–O₂ batteries, and will give implications on the rational design of cheap, carbon- and binder-free catalysts for high-performance Li–O₂ batteries.

Acknowledgements

This work was supported by the NSFC (21231005 and 21322101) and the 111 Project (B12015).

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Footnote

† Electronic supplementary information (ESI) available: Synthesis and electrochemical characterization details, additional SEM and TEM images, EDS data and cyclic stability in different electrolytes. See DOI: 10.1039/c6qi00066e

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