Rationally designed hierarchical MnO₂@NiO nanostructures for improved lithium ion storage

Abstract

The design of hierarchical nanostructures to be used as anodes (involving higher rate capabilities and better cycle lives) and meet further lithium ion battery applications has attracted wide attention. Herein, a hierarchical MnO₂@NiO core–shell nanostructure with a MnO₂ nanorod as the core and NiO flakes as the shell has been synthesized by combining a hydrothermal treatment and an annealing process. MnO₂ nanorods serve as a high theoretical capacity (1233 mA h g⁻¹) material, and they allow efficient electrical and ionic transport owing to their one-dimensional structure. The porous NiO flakes used as the shell would enlarge the contact area across the electrode/electrolyte, and can also serve as volume spacers between neighboring MnO₂ nanorods to maintain electrolyte penetration as well as reducing the aggregation during Li⁺ insertion/extraction. As a result, the MnO₂@NiO core–shell structure exhibits improved cycling stability (939 mA h g⁻¹ after 200 cycles at a current density of 1 A g⁻¹) and outstanding rate performance, suggesting that the synergetic effect and characteristics of the core–shell nanostructure would benefit the electrochemical performance of lithium ion batteries.

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Introduction

Lithium ion batteries (LIBs) are regarded as the dominant choice for many applications such as portable telecommunication equipment and electric vehicles.^1,2 Currently, commercial graphite is used as common anodes but its low capacity (372 mA h g⁻¹) and safety issues cannot keep pace with the rate of progress on new energy economy.³ Thus, numerous efforts have been devoted to explore alternative anode materials to meet the demand for advanced batteries with high energy density, long lifespan and lightweight design. In this respect, transitional metal oxides (TMOs) have attracted much attention due to their high theoretical capacities which are about two or three times higher than graphite.⁴ Unfortunately, their practical applications are restricted because of poor cycling performances and bad rate properties which are due to the huge volume variation during Li insertion/extraction.⁵

Many strategies have been investigated to overcome the above challenges, involving designing various nanostructures such as nanorods,⁶ cubes,⁷ tubes,⁸ and spheres,⁹ because nanostructured electrodes can accommodate the expanded volume caused by repeated Li intercalation/deintercalation and avoid fast capacity decay. Most recently, rational hybridization of two or more active materials has drawn much more attention, because it can make the best use of each material to achieve better LIB performance by adjusting the morphologies, compositions, and assembling organization of the elementary nanobuilding blocks. Especially, hybridized hierarchical nanostructure composites are more attractive owing to the large surface areas and integral configurations, which can not only enlarge the contact area between the electrode and electrolyte, but also provide short and continuous diffusion paths for electron/ion transfer.^10,11 For example, the six-fold-symmetry branched α-Fe₂O₃/SnO₂ nanoheterostructure was prepared by a two-step method and it showed a significantly improved initial discharge capacity of 1167 mA h g⁻¹, which was much better than bare SnO₂ nanowires (612 mA h g⁻¹) and α-Fe₂O₃ nanorods (598 mA h g⁻¹), but its cycling stability was not good enough.¹² Branched nanorods of β-MnO₂/α-Fe₂O₃ synthesized by Gu et al. displayed a reversible capacity of 1028 mA h g⁻¹ after 200 cycles, which was also much higher than those of β-MnO₂ nanorods (283 mA h g⁻¹) and porous α-Fe₂O₃ nanorods (314 mA h g⁻¹).¹³ Besides, branched Co₃O₄/Fe₂O₃ nanowires also showed improved electrochemical properties (cycling stability and good rate performance) compared to their single components (Co₃O₄ and Fe₂O₃).¹⁴ Despite those remarkable progresses, TMOs are still far from commercialization. Therefore, continued efforts are still necessary for constructing advanced TMO electrodes with high power rates and long cycling stability.

As previously reported, the selected components have a great effect on the final performance of the hybridized materials.^12–19 Reasonable components will generate excellent properties of electrode materials. MnO₂ is an attractive anode material when considering its environmental benignity, natural abundance and it has the highest theoretical capacity among the TMOs (1233 mA h g⁻¹).^13,20 Nevertheless, due to its large volume expansion and low electrical conductivity, individual MnO₂ shows a rapid capacity fading upon galvanostatic cycling.²¹ Based on the concept of the designed hybridization of active materials into a micro-nano structure, NiO was selected owing to its relatively high theoretical capacity (718 mA h g⁻¹) and low toxicity.²⁰ In particular, NiO shows relatively better capacity retention and cycling stability than MnO₂ (presented in previous reports).^22–27 Herein, hierarchical MnO₂@NiO core–shell nanostructures have been successfully constructed via a hydrothermal reaction and then a calcination process. Interestingly, hierarchical MnO₂@NiO core–shell nanostructures present nearly two-fold-symmetry and the porous NiO flakes are first developed on the surface of the MnO₂ nanorods. The porous NiO flakes shell can provide good access of the electrolyte to the electrode surface, reduce ion diffusion lengths and constrain the volume change during cycling.¹⁰ The inner component (MnO₂ nanorods) can provide high capacity and the one dimensional structure could afford continuous electron transfer channels.¹¹ Benefiting from the advantages of such rationally designed structures, MnO₂@NiO delivers superior electrochemical performance when compared to both individual MnO₂ and NiO. The hierarchical MnO₂@NiO core–shell is characterized by X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM) images, transmission electron microscopy (TEM) images and high-resolution TEM (HRTEM) images.

Experimental section

Synthesis of MnOOH

The synthesis of the MnOOH nanorods was modified from a previous report.²⁸ Briefly, potassium permanganate (KMnO₄, 0.1 g) and polyethylene glycol 400 (2 mL) were dissolved in deionized water (40 mL) under magnetic stirring. After being stirred for 30 min, the resulting solution was transferred into a Teflon-lined autoclave and then heated to 160 °C for 5 h. The product was collected by centrifugation and washed with deionized water and ethanol three times.

Synthesis of MnO₂@NiO

The obtained MnOOH nanorods (0.05 g) were redispersed into 20 mL ethanol and sonicated for 30 min to assure good dispersity. Then, 1.5 mmol of Ni(NO₃)₂·6H₂O and 3 mmol of urea were dissolved into 20 mL deionized water. The above two solutions were then mixed and transferred into a Teflon-lined autoclave. After being heated at 120 °C for 6 h, the product MnOOH@Ni₂CO₃(OH)₂·2H₂O was collected and washed with deionized water. Afterwards, MnOOH@Ni₂CO₃(OH)₂·2H₂O was annealed under air at 400 °C for 2 h to obtain MnO₂@NiO.

Sample characterization

X-ray powder diffraction (XRD) patterns were collected on a Bruker D8 advanced X-ray diffractometer using CuK_α radiation. The morphology of the as-prepared nanostructures was obtained from a transmission electron microscope (JEOL JEM 1011) and a field-emission scanning electron microscope (SUPRA™55). HRTEM images were achieved with an analytic transmission electron microscope (JEOL-2100). Nitrogen sorption isotherms were recorded on a Micromeritics ASAP-2020HD88 instrument. An Inductive Coupled Plasma Atomic Emission Spectrometer (ICP-AES, IRIS Intrepid II XSP) was used to analyze the chemical composition.

Electrochemical measurements

The working electrode consisted of the active material, conductive carbon black and the sodium salt of carboxymethyl cellulose (CMC) in a weight ratio of 70 : 20 : 10. The materials were dispersed in water and milled for 30 minutes to form a homogenous slurry. The slurry was then pasted onto copper foil and then dried at 60 °C for 12 h under vacuum. The typical mass loading of the active material was in the range of 1.5–2 mg cm⁻¹. CR2032-type coin cells were assembled in an argon filled glovebox (Mikrouna, Super 1220/750/900) with a Celgard 2400 membrane used as the separator and Li-metal circular foil as the reference/counter electrode. The electrolyte used consisted of 1 M LiPF₆ dissolved into a mixture of diethyl carbonate (DEC), dimethyl carbonate (DMC) and ethylene carbonate (EC) (1 : 1 : 1, vol. ratio). Galvanostatic charge–discharge tests were performed on Land-CT2001A battery cyclers (Wuhan, China). Cyclic voltammograms (CV) profiles were measured by a LK2005A electrochemical workstation (Tianjin, China). Electrochemical impedance spectra (EIS) were recorded on an electrochemical workstation (AUTOLAB PGSTAT204).

Results and discussion

The synthesis procedure of the hierarchical MnO₂@NiO core–shell nanostructures involves the three steps summarized in Scheme 1. First, uniform MnOOH nanorods prepared by a hydrothermal method possess a mean diameter of 100 nm and length of 5 μm (Fig. S1†). Second, the MnOOH nanorods can serve as backbones to support the subsequent growth of the Ni(NO₃)₂·6H₂O nanostructures through a facile hydrothermal reaction, because of the abundant oxhydryl groups anchored on the surface of MnOOH which promote the subsequent coating. Finally, the obtained precursor MnOOH@Ni₂CO₃(OH)₂·2H₂O was annealed in air, leading to the formation of hierarchical MnO₂@NiO core–shell nanostructures.

Scheme 1 Schematic illustration of the synthetic process of hierarchical MnO₂@NiO core–shell nanostructures.

Fig. 1 shows the XRD patterns of the samples recorded at different stages to verify the compositional transformation. In Fig. 1a, all of the diffraction peaks can be assigned to monoclinic MnOOH (JCPDS card no. 41-1379). The XRD patterns (Fig. 1b) of the precursor obtained via the hydrothermal reaction can be indexed to orthorhombic Ni₂CO₃(OH)₂·2H₂O (JCPDS card no. 29-0868) which is accompanied with MnOOH.²⁹ After annealing in an air atmosphere, the precursor was completely transformed into tetragonal phase β-MnO₂ (JCPDS card no. 24-0735) and cubic phase NiO (JCPDS card no. 47-1049).

Fig. 1 XRD patterns of (a) MnOOH nanorods, (b) the precursor MnOOH@Ni₂CO₃(OH)₂·2H₂O, (c) hierarchical MnO₂@NiO core–shell nanostructures. The referential JCPDS card spectra for MnO₂ and NiO are shown at the bottom.

Fig. 2 shows the typical SEM images of the as-prepared products. The low-magnification SEM images (Fig. 2a and b) indicate that the precursors exhibit hierarchical structures with numerous flakes grown on the surface of the MnOOH nanorods. Interestingly, the height of these flakes is not uniform but their thickness is similar (∼18 nm, Fig. 2c). The hierarchical structure exhibits a two-fold symmetry while there is an angle of 180° between adjoining flakes. After the annealing process, the morphology of the product is basically maintained (Fig. 2d and e), indicating that the hierarchical structures exhibit good thermal stability. However, it is clearly observed that the surface of the NiO flakes becomes rough and highly porous due to the release of gases (CO₂, H₂O etc.) (Fig. 2f). Such a porous shell offers the important merits of the hierarchical MnO₂@NiO core–shell nanostructures including larger interfacial area and more reactive sites which facilitate charge transfer and reduce the diffusion length of ions and electrons. These properties show that the material deserves and promises a higher rate capability and better cycle life.¹⁰

Fig. 2 SEM images of (a–c) the precursors and (d–f) hierarchical MnO₂@NiO core–shell nanostructures.

Fig. 3 displays the TEM and HRTEM characterization of the hierarchical MnO₂@NiO structures. The TEM images (Fig. 3a and b) confirm the core–shell structure by the contrasting light and dark areas. As indicated by the white lines, it is clearly observed that the height of the NiO flakes is about 50 nm, and it can also be revealed that the NiO flakes are highly porous. The HRTEM images provide the lattice fringes with their spacing about 0.24 nm and 0.21 nm, corresponding to the (101) planes of MnO₂ and (200) planes of NiO (Fig. 3c and d). The chemical composition has been tested by the ICP-AES technique, showing an atomic Mn/Ni ratio of 3.7 : 1 which is consistent with EDX spectra (Fig. S2†).

Fig. 3 (a and b) TEM images and (c and d) HRTEM images of the hierarchical MnO₂@NiO core–shell nanostructures.

Nitrogen sorption isotherms and pore size distribution were provided to investigate the porous properties of the hierarchical MnO₂@NiO core–shell nanostructures and they are shown in Fig. 4. The isotherm of MnO₂@NiO can be classified as Type IV with an obvious Type H1 hysteresis loop, indicating the mesoporous characteristic of the materials.³⁰ The Brunauer–Emmett–Teller (BET) specific surface area and pore volume of MnO₂@NiO are 59.3 m² g⁻¹ and 0.3 cm³ g⁻¹, respectively. The pore size distributions, calculated from the adsorption isotherms, have two wide peaks centered at ∼7 nm and ∼24 nm. The nitrogen sorption isotherms of the MnO₂ nanorods are shown in Fig. S4,† where type III isotherms can be detected, demonstrating the nonporous feature of the sample.³⁰ This is consistent with the enlarged TEM images of MnO₂ (Fig. 3b and S3†). After the introduction of the porous NiO flakes, the surface area of the composite increased remarkably (Fig. 4 and S5†), which will benefit the electrochemical properties of the composite because this can enlarge the contact area between the electrolyte and electrode and provide a buffer zone for volume variation by the pores.¹⁰

Fig. 4 Nitrogen adsorption/desorption isotherms and the corresponding pore size distribution (inset) of the hierarchical MnO₂@NiO core–shell nanostructures.

The electrochemical performances of MnO₂@NiO such as the CVs, specific capacities, long cycling stability and rate property, were systematic studied. Fig. 5a demonstrates the first five CV curves of MnO₂@NiO as an anode material using Li foil as the reference and counter electrode. Four cathodic peaks in the first cycle are observed. The weak peak at 0.71 V can be ascribed to the decomposition of the electrolyte forming the solid–electrolyte interface (SEI) layer and this peak vanished in the following cycles.³¹ The peaks located at 0.96 V and 0.25 V may be attributed to the reduction of MnO₂ to Mn²⁺ and Mn²⁺ to metal Mn, respectively.^13,26 The intense peak centered at 0.50 V is due to the reduction of NiO to metal Ni.^32,33 In the subsequent cycles, the main reduction peaks changed to 1.01 V and 0.29 V because of the structural rearrangement after the first cycle.^34,35 The anodic peaks at 1.29 V is assigned to the oxidation of metal Mn to Mn²⁺. The peak at 2.08 V is caused by the oxidation of Mn²⁺ and Ni to MnO₂ and NiO, respectively.^13,33 It is worth noting that the CV peaks of MnO₂@NiO during the anodic processes overlap very well in comparison to pure MnO₂ and NiO (Fig. S7†), indicating that the metal Ni (or Ni²⁺) and Mn²⁺ can facilitate sufficient oxidation on each other. Thus, the cycling performance of MnO₂@NiO would be enhanced in comparison to individual MnO₂ and NiO.

Fig. 5 (a) CV curves of the MnO₂@NiO electrode at a scanning rate of 0.1 mV s⁻¹ in the range of 0.01–3 V, (b) first discharge–charge curves at a current density of 1 A g⁻¹, (c) cycling performance at a current density of 1 A g⁻¹ and (d) cycling performance at various current rates of pure MnO₂, NiO and hierarchical MnO₂@NiO core–shell nanostructures. Coulombic efficiency for MnO₂@NiO is provided in (c).

Fig. 5b displays the first discharge–charge profiles of the MnO₂ nanorods, NiO nanoflowers and the hierarchical MnO₂@NiO core–shell nanostructures at a current density of 1 A g⁻¹ in the range of 0.01–3.0 V. The discharge/charge curves of the MnO₂@NiO core–shell nanostructures can be considered as a blend of β-MnO₂ and NiO, which also agrees with the CV examination. The MnO₂@NiO core–shell nanostructures exhibit a discharge capacity of 1495 mA h g⁻¹ and a charge capacity of 1150 mA h g⁻¹, corresponding to an initial capacity loss of 23.1%, which is better than MnO₂ nanorods (41.2%) and NiO nanoflowers (27.8%). The improved performance may be attributed to the synergetic effect of those components, which is consistent with the CV measurement. The first cycle capacities are higher than the theoretical capacities (MnO₂: 1233 mA h g⁻¹, NiO: 718 mA h g⁻¹, MnO₂@NiO (81.3 wt% MnO₂, 18.7 wt% NiO): 1135 mA h g⁻¹), which might arise from electrolyte decomposition and the interfacial storage of lithium ions in acetylene black, which is common in TMO electrodes.^36–39

The cycling performance of the MnO₂@NiO core–shell nanostructures is shown in Fig. 5c. After 200 cycles, the sample can still maintain 939 mA h g⁻¹ at the high current density of 1 A g⁻¹, which is much better than single MnO₂ nanorods (134 mA h g⁻¹) and NiO nanoflowers (402 mA h g⁻¹) under the same conditions. Moreover, the coulombic efficiency for MnO₂@NiO is maintained above 98% after the initial cycle. This phenomenon can be also discovered in other hierarchical hybridized materials because the hybridization can take full advantage of the primary nanobuilding blocks by controlling their morphologies, compositions, and assembling organization. In addition, the important rate performance was also studied and depicted in Fig. 5d. The MnO₂@NiO core–shell nanostructures show 1170, 1210, 1020, 920, 650, and 420, mA h g⁻¹ at the corresponding current densities of 0.05, 0.2, 0.5, 1, 2, and 4 A g⁻¹, which is superior to that of the single MnO₂ nanorods (970, 750, 550, 290, 90, and 46 mA h g⁻¹) and the NiO nanoflowers (540, 345, 171, 60, 28, and 8.7 mA h g⁻¹) in the same circumstances. It is noted that the specific capacity of MnO₂@NiO is still retained at 420 mA h g⁻¹ even at the high rate of 4 A g⁻¹, which is much higher than the MnO₂ nanorods (46 mA h g⁻¹) and NiO (8.7 mA h g⁻¹). More importantly, after high rate charge/discharge, the capacity of the MnO₂@NiO core–shell nanostructures (1180 mA h g⁻¹) is recovered when the current density was back to 0.05 A g⁻¹, indicating their good structural stability. However, the capacities of MnO₂ and NiO could not be recovered completely after testing at higher rates. The superior rate performance and cycling stability made the MnO₂@NiO core–shell nanostructure a very promising candidate for LIB applications.

Electrochemical impedance spectroscopy (EIS) measurements were studied to further explain the excellent electrochemical performance of MnO₂@NiO (Fig. 6a). The Nyquist plots consist of a sloping line in the low frequency region which is ascribed to the mass transfer process, and a semicircle in the high frequency region which is attributed to the charge transfer impedance.¹⁷ The equivalent electrical circuit fitting is analyzed and is shown in the inset of Fig. 6a. The charge transfer resistance of NiO (67 Ω) and MnO₂@NiO (89 Ω) is much lower than MnO₂ (217 Ω), indicating improved conductivity. Simultaneously, NiO and MnO₂@NiO display more vertical straight lines when compared to MnO₂, implying faster Li-ion diffusion behavior. These results indicate that the combination of MnO₂ with NiO can improve ion diffusion and charge transfer distinctly, which would take full advantage of the high theoretical capacity of MnO₂. Fig. 6b shows the SEM images of the MnO₂@NiO electrode after 20 cycles at a current density of 1 A g⁻¹. The hierarchical structures can still be maintained when the MnO₂ nanorods curved to a certain extent due to repeated lithiation/delithiation, implying that one dimensional structure can effectively buffer the volume expansion. Furthermore, the NiO flakes were still tightly connected to the surface of the MnO₂ nanorods without any structural collapse, which promises outstanding structural stability.

Fig. 6 (a) Nyquist plots for the electrodes based on MnO₂, NiO and MnO₂@NiO after 5 cycles at the same voltage of 2.8 V during the charge process. The figure inserted is the equivalent circuit to fit the plots. (b) Low and high magnification SEM images of the MnO₂@NiO electrode after 20 cycles.

The improved electrochemical properties involving the high rate capability and excellent cycle performance can be ascribed to the rationally designed hierarchical structure and synergistic effects of MnO₂ and NiO. First, abundant pores extend the electrodes/electrolyte contact area and reduce the path lengths for electron/ion diffusion, promising increased utilization of active materials. Meanwhile, MnO₂@NiO exhibits better conductivity and faster Li-ion diffusion behavior. Furthermore, MnO₂ and NiO promote each other’s sufficient reaction with Li/Li⁺, enabling high capacity without reduction. Last, plentiful free spaces between the flakes help accommodate severe volume changes during cycling and avoid disintegration as well as aggregation of components, and this ensures structural integrity after long cycling. Thus, high capacity along with better cycling stability of the electrodes were obtained.

Conclusion

The hierarchical MnO₂@NiO core–shell nanostructures were successfully synthesized via hydrothermal growth of NiO precursor flakes on MnOOH nanorods, presenting a near two-fold symmetry. BET measurements and TEM images indicate that the main pores are on the NiO flakes due to the decomposition of the precursor. The CVs and EIS tests confirm that the synergetic effect promotes electrical conductivity, charge transfer and ion diffusion. Such a hierarchical structure demonstrates lower initial irreversible capacity loss and it maintains 939 mA h g⁻¹ after 200 cycles at a current density of 1 A g⁻¹. The electrochemical performance of the complex structure is better than both of the single components which can also be attributed to the heterostructure. The superior rate performance and cycling stability make hierarchical MnO₂@NiO core–shell nanostructures a very promising candidate for LIB applications.

Acknowledgements

This work was supported by the 973 Project of China (No. 2011CB935901), the National Nature Science Foundation of China (No. 91022033, 51172076 and 21471090), Shandong Provincial Natural Science Foundation for Distinguished Young Scholar (JQ201205), Independent Innovation Foundations of Shandong University (2012ZD007), and new-faculty start-up funding in Shandong University.

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Footnote

† Electronic supplementary information (ESI) available: XRD patterns, TEM, SEM images, BET measurements and CV curves of MnO₂ and NiO. See DOI: 10.1039/c5ra11267b

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