A hierarchical NiCo₂O₄ spinel nanowire array as an electrocatalyst for rechargeable Li–air batteries

Abstract

Hierarchical NiCo₂O₄ spinel nanowire array (H-NCO-NWA) electrocatalysts have been prepared through a facile template-free co-precipitation route. The as-prepared H-NCO-NWA exhibits a mesoporous (ca. 8 nm) structure and a high specific surface area of 124 m² g⁻¹. The assembled Li–air batteries presented lower overpotentials, reasonable specific capacity, and enhanced cyclability.

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Nowadays, rechargeable nonaqueous lithium–air batteries have received worldwide attention due to their extremely high theoretical energy density (5200 W h kg⁻¹, including oxygen) which is comparable to that of gasoline and is 5–10 times higher than that of current lithium-ion batteries.¹ The outstanding energy densities of nonaqueous Li–air batteries make them the most promising power source candidates for future electrical vehicles, provided that the existing challenges can be resolved. These challenges include the lower kinetics of both discharge and charge reactions, the bad cycle stability of discharge and charge, the instability of the organic electrolyte, the negative effect of impurities from air during discharge–charge cycles, etc.² The air electrode is crucial to solve these challenges and improve the electrochemical performance of rechargeable nonaqueous lithium–air batteries.

Up to days, two main strategies have been explored to optimize the air electrode performance. One, exploring high efficiency bifunctional catalyst for both the oxygen reduction reaction (ORR) and the evolution reaction (OER); the other, designing new air electrode structure.³ In the former case, lots of works have been reported, including platinum (Pt) or palladium (Pd) based noble catalysts,^3a,b transition metal oxide catalysts,^3c,d carbon materials^3e and some heterogeneous composite catalysts,^3f etc. An ideal bifunctional catalyst can effectively accelerate the ORR and OER processes, lower the overpotential and thus improve the performance of Li–air batteries. In the latter case, the design of new air electrodes requires enough void volumes and open frameworks in their architecture structures for accommodating the insoluble discharge products, which should be helpful to improve discharge capacity as well as cycling performance. Various air electrode structures have been explored, such as a conventional porous composite electrode,^4a carbon fiber/porous anodized aluminium oxide (AAO),^4b vertically aligned carbon nanotubes,^4c and so on. However, a criterion has not been established yet.

As we all know that the ORR is an interfacial reaction or surface reaction, the adsorption and dissociation of oxygen molecules mainly occur at the interface or surface of catalysts. So, the more effective active sites of the catalysts have, the better catalytic activities they will give.⁵ Self-standing nanowire array electrodes possess plentiful electrochemical active sits, high utilization efficiency of active materials, and superior mass transport property.⁶ The unique structure of this type of electrode is particularly beneficial for the ORR, and especially for the OER which involves a large volume of gas evolution. Furthermore, controlled distance between nanowire and nanowire help to accommodate more insoluble discharge products. To the best of the authors' knowledge, there are not too much reports using nanowire array electrode in the Li–air batteries.

NiCo₂O₄, a well-known cobalt-nickel spinel oxide, owns two solid-state redox couples (Co³⁺/Co²⁺ and Ni³⁺/Ni²⁺) in its structure, which enables it to exhibit a remarkable electrocatalytic activity. Recently, nanostructured NiCo₂O₄ has gained much research interest owing to its electrochemical applications in lithium ion cell, supercapacitor and metal air battery.⁷ Although Zhang assembled and tested Li–O₂ batteries with NiCo₂O₄ nanoflakes as cathode catalyst,^7c hierarchical NiCo₂O₄ nanowire arrays (H-NCO-NWA) have not been considered as cathode catalysts for Li–air batteries yet. Herein, we develop a facile template-free co-precipitation route to design and fabricate well-ordered H-NCO-NWA with high specific surface area, and typical coin Li–air cells have been first assembled and tested with H-NCO-NWA as cathode catalyst. The as-prepared NiCo₂O₄ has a specific nanostructure with numerous catalytic active sites. As a consequence, the as-assembled Li–air batteries exhibited lower overpotentials, reasonable specific capacity and enhanced cycle stability.

The structure of the final H-NCO-NWA product was firstly investigated using XRD as shown in Fig. 1(a). Five distinct peaks are observed at 2θ values of 31.24, 36.61, 44.48, 59.24, and 64.89, which can be assigned to (220), (311), (400), (511), and (440) plane reflections of the spinel NiCo₂O₄ crystalline structure, respectively (JCPDF file no. 20-0781; space group: F*3 (202)). This result clearly confirms the formation of NiCo₂O₄ spinel.^5b

Fig. 1 (a) XRD pattern of the H-NCO-NWA sample. (b) Schematic diagram of the H-NCO-NWA. (c) SEM image of the H-NCO-NWA. (d) TEM image of two single nanowires of the H-NCO-NWA sample.

Several synthesis methods have been reported to control NiCo₂O₄ morphology, such as a simple electrochemical method to synthesize NiCo₂O₄ nanosheets,^8a a simultaneously coordinating etching and precipitation reactions to prepare hollow crossed NiCo₂O₄ nanocubes,^8b and a hydrothermal reaction to prepare flower-like NiCo₂O₄ nanocubes,^8c etc. However, there have few reports about H-NCO-NWA as catalyst for Li–air battery. In this work, we develop a facile template-free co-precipitation route to design and fabricate well-ordered H-NCO-NWA with high specific surface area. As shown in Fig. 1(b), the single nanowire is composed of smaller nanoparticles, the diameter of nanoparticles in nanowire root is bigger than that of in nanowire tail because of the longer aging time, all of the nanowires arranged in array according to certain direction, and the loose internal structure easily form mesoporous. This unique porous structure could be an ideal design for an O₂ electrode. During the process of discharge, the robust large tunnels distributed between NiCo₂O₄ nanowires can function as “highways” to supply oxygen to the interior parts of the cathode and help to accommodate more insoluble discharge products, while smaller mesopores on each nanowire are the “exits” to provide the tri-phase (solid–liquid–gas) regions required for oxygen reduction and evolution reactions.

The morphologies the synthesized H-NCO-NWA were investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). Fig. 1(c) shows the SEM image of the final H-NCO-NWA powder. It can be seen that well-ordered NiCo₂O₄ nanowire arrays have been formed, and the diameter of a single nanowire is about 50 nm and the length of which is about 400 nm. The surface of these nanowires looks rough, which is due to the fact that NiCo₂O₄ nanowires are composed of a large number of nanoparticles with a diameter of less than 10 nm, and the particle in the root is bigger than that in the tail. Fig. 1(d) displays the TEM image of two single NiCo₂O₄ nanowires. It can be clearly observed that the nanowire is composed of smaller nanoparticles, and the loose internal structure demonstrates the presence of mesopores in the sample.

This H-NCO-NWA was further characterized using an N₂ adsorption–desorption experiment (as shown in Fig. S1, ESI†). The isotherm of the NiCo₂O₄ sample is of type IV, and exhibits a clear H3 hysteresis loop, which is indicative of mesoporosity. The H3 hysteresis loops are typically observed in the case of aggregates of plate-like particles giving rise to slit-shaped mesopores. According to the N₂ adsorption–desorption experiment, calculated BET surface area of the hierarchical NiCo₂O₄ spinel nanowire arrays is 124 m² g⁻¹, which is the maximum values of reported about NiCo₂O₄ material so far (Table S1, ESI†). The pore size distribution was also calculated and displayed inset in Fig. S1.† It was clearly observed that there were main mesopores with a wide size range of about 8 nm, in agreement with the TEM results.

The electrocatalytic activity of hierarchical NiCo₂O₄ spinel nanowire arrays catalyst for the ORR and the OER is then examined in Li–air batteries and compared with pure acetylene black and regular NiCo₂O₄ catalyst synthesized through a sol–gel process at the same testing conditions. Firstly, we utilized cyclic voltammetry (CV) to explore the ORR catalytic activity of the H-NCO-NWA catalyst in the LiTFSI–TEGDME electrolyte from 4.5 to 2.0 V (Fig. 2(a)). Compared with the pure acetylene black based and regular NiCo₂O₄ based electrodes, the H-NCO-NWA based electrode exhibited a higher ORR onset potential (∼2.7 V) and a more positive current peak, which indicated that the H-NCO-NWA catalyst owns better catalytic activity. The first discharge–charge profiles at a discharge and charge current density of 200 mA g⁻¹ of Li–air batteries with the three different air electrode catalysts are displayed in Fig. 2(b). It can be clearly seen that the first discharge capacity of the H-NCO-NWA based Li–air battery is 7600 mA h g_(c+catalyst)⁻¹, while that of regular NiCo₂O₄ and pure acetylene black based Li–air batteries are 1720 and 720 mA h g_(c+catalyst)⁻¹, respectively. Interestingly, it can be found that the discharge and especially the charge voltage of Li–air batteries can be improved with the help of the H-NCO-NWA catalyst, which would consequently enhance the round-trip efficiency. In detail, although the discharge voltage of Li–air batteries with H-NCO-NWA electrode is higher than that of regular NiCo₂O₄ electrode by about 100 mV and pure acetylene black electrode by about 150 mV, its charge voltage is much lower than that of Li–air batteries with regular NiCo₂O₄ electrode by 70 mV and pure acetylene black electrode by 100 mV. It should be noted that these differences in discharge and charge voltages might be due to the different catalytic activities of these three electrodes, H-NCO-NWA electrode shows better catalytic activity.

Fig. 2 (a) CV curves of Li–air batteries with pure acetylene black, H-NCO-NWA, and regular NiCo₂O₄ catalysts at a scan rate of 0.2 mV s⁻¹. (b) First discharge–charge curves of Li–air batteries with pure acetylene black, H-NCO-NWA, and regular NiCo₂O₄ catalysts at a current density of 200 mA g⁻¹. (c) The rate capacities of Li–air batteries using H-NCO-NWA based electrodes under different current densities. (d) Typical discharge–charge voltage profiles and (e) cycling performances of the initial 9 cycles of H-NCO-NWA based electrodes at a constant current of 200 mA g⁻¹ between 2.0 and 4.4 V. (f) Cycling response of the Li–air battery with H-NCO-NWA based electrodes under specific capacity limit of 1000 mA h g⁻¹ at a current density of 200 mA g⁻¹.

In order to identify the effect of the H-NCO-NWA catalysts on the reaction kinetics of Li–air batteries, the discharge and charge profiles of the H-NCO-NWA electrodes in the first cycle at different current densities of 100 mA g⁻¹, 200 mA g⁻¹ and 500 mA g⁻¹ have been obtained and shown in Fig. 2(c). When the current density decreases from 500 to 100 mA g⁻¹, the discharge capacity of the electrode increases from 2680 to 7600 mA h g_(c+catalyst)⁻¹ and the voltage gap between charge and discharge plateaus declines, showing a good rate capability. Discharge and charge stability is one of the most critical aspects of nonaqueous Li–air batteries, however, it is typically difficult to realize due to the accumulation of the discharge product, and the instability of the electrolyte. It has been reported that improved cycle performance can be achieved by using optimized catalysts. Under the circumstance of deep discharge and charge depth at 200 mA g⁻¹, Li–air battery based on H-NCO-NWA catalysts shows a stable cycle performance for 9 cycles, as shown in Fig. 2(d). After 9 cycles of fully discharge and charge, the specific capacities can keep over 3700 mA h g_(c+catalyst)⁻¹, with a retention of 70%. More interestingly, there appears a increasing trend for specific capacities of 2, 3 and 4 circles compared with the first circle, implying that there is an activated process because of hierarchical structure of single nanowire. As a comparison, a fully discharge and charge cyclic experiment of Li–air battery based on regular NiCo₂O₄ electrode was also carried out. The result was shown in Fig. S2, ESI.† It can be seen that the specific capacity kept only 49% after five cycles. Furthermore, columbic efficiency of Li–air battery based on H-NCO-NWA electrode was also calculated according to discharged and charged data as shown in Fig. 2(d). From Fig. 2(e), it can be observed that the charge capacity is quite close to the discharge capacity and the columbic efficiency is over 88%, which means the battery has a good reversibility. We also did circle stability tests for five circles at a bigger discharge and charge current density of 500 mA g⁻¹, the results were displayed in Fig. S3, ESI.† It can be seen that the discharge specific capacities is still near 1500 mA h g_(c+catalyst)⁻¹ after five circles.

To further confirm the stability of Li–air batteries with H-NCO-NWA catalysts, we then tested the batteries at 200 mA g⁻¹ under a specific capacity limit of 1000 mA h g⁻¹. As depicted in Fig. 2(f), the terminal discharge voltage is over 2.60 V after 50 cycles, suggesting the good stability of H-NCO-NWA catalysts. Also, we can observe that there is a increasing trend for specific capacities of the fifth cycle compared with the first circle from the enlarged plots in Fig. 2(f). This result is consistent with the fully discharge and charge experiments, as shown in Fig. 2(d).

The structure of the H-NCO-NWA electrodes during the discharge and charge course is also examined through SEM to visualize the formation of Li₂O₂ and their morphological evolution in hierarchically porous structures. The Li–air battery is discharged and charged at 200 mA g⁻¹ from open circuit voltage (OCV) to 2.2 V and 4.4 V, respectively. Fig. 3(a)–(c) display SEM images of the H-NCO-NWA electrode at OCV, 2.2 V and 4.4 V, respectively. Before discharge, the porous NiCo₂O₄ and acetylene black are loosely dispersed in the initial electrode as shown in Fig. 3(a). After 1st discharge, the insoluble species precipitate on the surface of the cathode. Fig. 3(b) shows the generation of Li₂O₂ particle, which is the typical morphology of the discharged product.^3b,8 The Li₂O₂ particles are packed to form a film. However, the open framework of the electrode is still well maintained for O₂ transport into the inner side of the electrode. After 1st charge, the porous structure and nanoparticulated morphology are essentially regained for the porous NiCo₂O₄ electrode even for there are some undecomposed Li₂O₂ particles (Fig. 3(c)). These results again suggest the superior activity of H-NCO-NWA in catalysing the oxidation of Li₂O₂ during the charging process.

Fig. 3 SEM images of the H-NCO-NWA based electrode: (a) pristine, (b) after 1st discharge and (c) after 1st recharge.

Conclusions

In summary, we reported the hierarchical NiCo₂O₄ nanowire arrays electrocatalyst for the rechargeable Li–air battery for the first time. SEM, TEM and BET results show that the H-NCO-NWA has a mesoporous (ca. 8 nm) structure and a high specific surface area of 124 m² g⁻¹. Li–air batteries based on the H-NCO-NWA electrodes exhibit improved performances, including lower overpotential, reasonable specific capacity, and enhanced cyclability. This work might provide an alternative approach by structure design of electrocatalyst to improve Li–air battery performances.

Acknowledgements

The project was supported by National Natural Science Foundation of China (51272167, 21206101), Natural Science Foundation of Jiangsu Province, China (BK20141199), Natural Science Foundation of the Higher Education Institutions of Jiangsu Province, China (14KJB480005), Specialized Research Fund for the Doctoral Program of Higher Education (20133201120005), and the ministry of education to study abroad returned funds.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06300g

‡ These authors contributed equally to this work.

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