PSA modified 3 D flower-like NiCo₂O₄ nanorod clusters as anode materials for lithium ion batteries

Abstract

Novel 3-dimensional (3 D) flower-like NiCo₂O₄ (NCO) nanorod clusters are fabricated by a facile hydrothermal process using styrene–acrylonitrile copolymer (PSA) nanospheres as a complex agent. The structure and morphology of NCO are characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the PSA modified NCO (PNCO) exhibits excellent electrochemical performance. Compared with pure NCO, the flower-like PNCO materials with enough free space as anodes in lithium ion batteries (LIBs) deliver an initial discharge capacity of 1519.1, 1447.3 and 1337.3 mA h g⁻¹ at the current densities of 500, 1000 and 2000 mA g⁻¹, as well as 1417.5, 819.0 and 719.5 mA h g⁻¹ after 100 cycles. Meanwhile, they display improved rate performance at elevated current rates, such as 1247.3, 1193.5 and 944.5 mA h g⁻¹ at current densities of 1000, 2000 and 4000 mA g⁻¹, respectively. They have great prospects for the application of anode materials for lithium-ion batteries.

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1. Introduction

With the rapid growth of the global economy, the depletion of fossil fuels, and increasing environmental pollution problems are becoming more and more serious. Thus, there is an urgent requirement for exploring efficient and sustainable energy sources. In recent years, LIBs as energy storage devices dominating the power sources for portable electronics and electrical/hybrid vehicles, have been attracting considerable attention in scientific and industrial communities by virtue of their high energy density, long cycling life and environmental friendliness.^1,2 However, as is well known to all, the vast majority of commercial anode materials used for LIBs are the conventional graphite with a low theoretical specific capacity of 372 mA h g⁻¹. Meanwhile, it displays a relatively poor rate capability on account of the formation of LiC₆ in the Li intercalation process.^3,4 Therefore, it is essential to search for alternative anode materials with high reversible capacity, stable cycle performance and high rate capability.

Nanostructured binary transition metal oxides (TMOs), such as NiO,^5,6 FeO_x,^7,8 SnO₂,^9,10 CoO_x,^11,12 ZnO^13,14 and MnO₂,^15,16 have been long studied as promising electrode materials owing to their advantages of high theoretical specific capacity (500–1000 mA h g⁻¹), high specific surface area and short path length for Li-ion diffusion in comparison with their bulk counterparts.¹⁷ According to the previous studies of several transition metal oxides, the cobalt oxides exhibit improved anodic performance.¹⁸ However, Co₃O₄ is not a perfect electrode material for the fact that cobalt is toxic and expensive. Thus, a large amount of efforts are aimed at partially taking place of Co₃O₄ by less expensive and more eco-friendly alternative metals. More recently, some ternary transition metal oxides such as MeCo₂O₄ (Me = Ni, Cu, Zn, Fe, Mn)^19–27 have displayed a higher electrochemical performance than single component cobalt oxides because of their high feasible oxidation state and high electrical conductivity. Among the variety of the ternary metal oxides, spinel nickel cobaltite is considered as an ideal and attractive candidate to substitute for traditional graphite anode in view of its many advantages such as high theoretical capacitance (890 mA h g⁻¹), low cost, environmental friendly and abundant resources. Unfortunately, the huge volume changes due to Li⁺ insertion/extraction process cause the limitation for the commercial applications of those anode materials. Much effort has been devoted to solve the problem of poor cycling of anode materials. One of the advanced approaches is to modify the morphology and microstructure. It is well known that the electrochemical performance of LIBs highly depends on the unique structural properties of the electrode materials. Until now, various microstructures such as nanosheet,^28–30 nanosphere,^31,32 nanocage,³³ nanotube³⁴ and nanowire^35–37 have been reported, exhibiting enhanced electrochemical performance.

In this work, a novel class of 3 D flower-like NCO nanorod clusters supported on polymer spheres have been successfully synthesized via a facile hydrothermal method followed by a simple thermal treatment in air. Styrene–acrylonitrile copolymer (PSA) is a thermoplastic polymer and has been used as an ideal reinforcing polymer since it is not very responsive to chemical stress cracking.³⁸ Regarding the several useful properties like high heat distortion temperature and dimensional stability, PSA could be used as a matrix material for composite fabrication. Meanwhile, the negatively charged PSA combined with positively charged Ni²⁺ and Co²⁺ through electrostatic interaction and evenly distributed on the PSA spheres. Interestingly, the PSA is important in controlling the morphology of the as-prepared composites. To the best of our knowledge, this special structure is rarely reported as an anode material for LIBs. The flower-shaped nanorod clusters with large surface area which is benefit for Li⁺ ion insertion and electrolyte penetration. When evaluated as anode materials for LIBs, they deliver good cycling stability and excellent rate capacity. These excellent results reveal that such distinctively structured PSA modified NCO nanorod clusters are promising in the electrochemical energy storage applications.

2. Experimental

2.1 Preparation of flower-like PNCO nanorod clusters

The PSA-assisted synthetic route for hierarchical NCO nanorod clusters is illustrated in Fig. 1. 0.2 g of styrene–acrylonitrile copolymer (PSA) nanosphere with a size of 200–400 nm was firstly dissolved in 6 M HCl solution in an ultrasound bath for 15 min, and then centrifuged, washed with deionized (DI) water for several times. 2 mmol of Ni(NO₃)₂·6H₂O and 4 mmol of Co(NO₃)₂·6H₂O were dissolved in 70 mL of DI water to form a homogeneous pink solution, followed by the addition of 10 mmol of urea and 4 mmol of NH₄F, respectively. After stirring for 15 min, the obtained intermediate PSA solution was transferred into the above mixed solution and stirred for another 30 min. Finally, all the mixture solution was transferred into a 100 mL Teflon-lined stainless steel autoclave, sealed and heated in an oven at 95 °C for 10 h. After cooling to room temperature, the as synthesized composite was centrifuged, washed with DI water and ethyl alcohol for several times, and dried at 60 °C in air overnight. Finally, the as-fabricated precursor was further calcined at 350 °C with a temperature rate of 2 °C min⁻¹ and kept at the same temperature for 5 h in air to get the final products.

Fig. 1 Schematic illustration of the formation of the PNCO nanorod clusters.

Meanwhile, a controlled experiment was also carried to synthesize the electrode materials of pure NCO at the same experimental conditions, which was employed for the comparison in the electrochemical measurement.

2.2 Material characteristics and electrochemical measurements

The crystal structure and morphology of as-prepared products were characterized with powder X-ray diffraction (XRD; PANalytical X'Pert PRO, Cu/Kα radiation, λ = 0.15418 nm), field emission scanning electron microscopy (FESEM; ZEISS ULTRA 55), and transmission electron microscopy (TEM; JEM-2100HR). Raman spectra were recorded on an HR800UV Raman microspectrometer (Jobin Yvon; France) with 514 nm laser excitation. Thermogravimetric (TG) curves of the final products were carried out on a TGA-2050 (TA Corp.). The specific Brunauer–Emmett–Teller (BET) surface area was determined by N₂ adsorption–desorption on a Coulter SA 3100 surface area analyzer.

The electrochemical performance was tested by using coin-type half-cells (CR2430) assembled in an argon-filled glove box. Working electrodes, with a composition of 80 wt% active materials, 10 wt% acetylene black, and 10 wt% PVDF, were fabricated on the copper foil of 13 μm thickness and dried at 80 °C for 12 h under vacuum subsequently. The electrolyte was 1.0 M LiPF₆ in a mixture of ethylene carbonate, diethyl carbonate, and ethyl methyl carbonate (1 : 1 : 1 by volume, provided by Chei Industries Inc., South Korea). The separator was made of a Celgard 2400 film. The galvanostatic discharge–charge performance and the rate performance were tested by LAND CT2001A batteries testing system in the voltage range of 0.01–3.0 V at room temperature. The cyclic voltammetry (CV) was carried out on Solartron 1470E electrochemistry system.

3. Results and discussion

XRD patterns of as-prepared samples are shown in Fig. 2(a). All the diffraction peaks of the NCO and PNCO are perfectly consistent with the cubic NCO spinel structure (JCPDS card no. 20-0781), where the bivalent Ni ions occupy the tetrahedral sites and the trivalent Co ions occupy the octahedral sites.³⁹ The diffraction peaks located at 18.91°, 31.15°, 36.69°, 44.62°, 55.43°, 59.09° and 64.98° correspond to the (111), (220), (311), (400), (511) and (440) crystal planes, respectively. Meanwhile, no other peaks from impurities were observed, indicating the higher phase purity of both samples.

Fig. 2 (a) XRD patterns of NCO and PNCO; (b) Raman spectra of the PNCO; (c) the TG curve for the PNCO.

In order to study the carbon features of PSA after calcination process, such as disorder and defect structures, the Raman spectroscopy technique is carried out. As showed in Fig. 2(b), the 3 D flower-like PNCO exhibits a strong G line around 1582 cm⁻¹, representing the first order scattering of the E_2g phonon of sp² carbon atoms, and a relatively weak G line around 1362 cm⁻¹, which should be assigned to a breathing mode of K-point photons of A_1g symmetry, respectively.⁴⁰ The intensity ratio of the D band to G band (R(I_D/I_G)) is 0.76, which indicates a partially disordered carbon in PNCO after thermal treatment.

To make sure of the amount of NCO in the composite, we have tested the TG profile of the as-prepared PNCO as displayed in Fig. 2(c), which was conducted to follow the heat treatment process from 60 °C to 750 °C at a heating rate of 10 °C min⁻¹ in oxygen atmosphere. It can be seen clearly that the TG curve exhibits two distinct weight loss steps due to dehydration and oxidization of the PNCO. The initial weight loss of 2.9% at the low temperature (60–250 °C) can be attributed to the loss of the evaporation of moisture in the as-prepared PNCO. The following 15.1% weight loss occurs at 400–700 °C with a big step, which is mainly assigned to the burning of amorphous carbon.

The morphologies of the pure NCO and PNCO composites are investigated by SEM. As shown in Fig. 3(a) and (b), the NCO products are composed of nanorods with agglomeration and irregular arrangement, which is not good for electrode cycling. After adding PSA, there is no obvious aggregation, radical flower-like cluster are formed uniformly in which NCO nanorods (30–50 nm diameter) connect into an even tighter bundle. It can be concluded that PSA plays the role in controlling the regular shape due to the coordination effects. It is well known that PSA consists of –CN functional group, which is in favour of bonding with Ni²⁺ and Co²⁺ ions via electrostatic forces and greatly significant for formatting the special PNCO nanorod bundles. The unique hierarchical nanostructure will play an important role in the electron transfer and ion diffusion. What is important, this three dimensional network structure with enough free space is beneficial to improve the electrochemical performance. Nanosized radial sphere could decrease the diffusion lengths for both Li⁺ ion and electron and the adequate free spaces can alleviate the volume changes during continuous rapid charge/discharge processes.

Fig. 3 SEM images of (a and b) NCO and (c and d) PNCO nanorod clusters.

To further understand the morphology and structural characteristics of the PNCO, TEM, HRTEM images and SAED patterns are also employed, and the corresponding results are illustrated in Fig. 4. It shows the typical flower-like clusters made of several bundles. In Fig. 4(b), close examination reveals that bundles are composed of numerous ∼10 nm nanorods instead of the conventional single crystalline nanorod, in which adjacent nanorods loosely connect forming porous morphology. This hierarchical structure actually is characterized with three dimensions due to the release of CO₂ and H₂O gas under the annealing process, which provides a large specific surface and enough space, thus leading to relatively short ion and charge diffusion pathways and high transport rates of both lithium ions and electrons. The typical HRTEM image of PNCO in Fig. 4(c) reveals two sets of lattice spacing of 0.288 and 0.204 nm, corresponding to the (220) and (400) planes of NCO, respectively. The SAED pattern of PNCO is presented in Fig. 4(d). It indicates the polycrystalline nature of the composites and the first four sets of rings can be indexed to the (111), (220), (311) and (422) planes, which is in accordance with the XRD results.

Fig. 4 TEM and HRTEM analysis of PNCO: (a and b) morphology at low magnification; (c) morphology at high magnification and (d) HRTEM image with the corresponding SAED pattern.

To further investigate the specific surface area and the porous structure of the pure NCO and PNCO, N₂ adsorption–desorption isotherm at 77 K is shown in Fig. 5(a) and (b), with the inset showing its corresponding Barrett–Joyner–Halenda (BJH) pore size distribution. The N₂ adsorption–desorption isotherm is characteristic of type IV with a H3 hysteresis loop in the relative pressure range of 0.6–0.95, indicating mesoporous structure of the as-prepared samples. The Brunauer–Emmett–Teller (BET) specific surface area value of the pure NCO and PNCO is calculated to be 87.13 and 80.58 m² g⁻¹ with a pore volume of 0.213 and 0.237 cm³ g⁻¹, respectively. The high surface area is mainly ascribed to the specific nanostructure and its large total pore volume. However, the pore size distribution of the PNCO is more uniform than NCO as shown in the insert of Fig. 5. As is well known to all, a huge surface area can offer more space and channels for fast Li⁺ ion insertion and extraction. Meanwhile, the sufficient pore volume can be in favour of diffusing effectively to the active material with less resistance for Li⁺ ion and buffering the volume expansion during discharge–charge process. Thus, the hierarchical PNCO nanorod clusters are expected to improve the electrochemical performance.

Fig. 5 Nitrogen adsorption–desorption isotherm and the corresponding pore size distribution (inset) of (a) NCO and (b) PNCO.

To identify the lithiation mechanism of NCO and PNCO, cyclic voltammograms and galvanostatic charge/discharge tests were carried out. Fig. 6(a) shows the first two CV curves of the NCO and PNCO electrode at a scanning rate of 0.2 mV s⁻¹ in the voltage of 0.01 V to 3.0 V. As shown in Fig. 6(a), the Li⁺ ion storage behavior of PNCO is similar to that of NCO. Based on the previous reports, the electrochemical process for NCO can be expressed as follows:^41–44

NiCo₂O₄ + 8Li⁺ + 8e⁻ ↔ Ni + 2Co + 4Li₂O (1)

Ni + Li₂O ↔ NiO + 2Li⁺ + 2e⁻ (2)

Co + Li₂O ↔ CoO + 2Li⁺ + 2e⁻ (3)

CoO + 1/3Li₂O ↔ 1/3Co₃O₄ + 2/3Li⁺ + 2/3e⁻ (4)

Fig. 6 (a) The first two CV curves of NCO and PNCO electrodes at a scanning rate of 0.2 mV s⁻¹ in the voltage range of 0.01–3.0 V; (b) discharge–charge profiles of NCO electrode cycles at a current density of 500 mA g⁻¹ in the voltage range of 0.01–3.0 V; (c) discharge–charge profiles of PNCO electrode cycles at a current density of 500 mA g⁻¹ in the voltage range of 0.01–3.0 V.

During the first cycle, there is a brand and irreversible reduction peak located at 0.72 V for the NCO while there is the slightly left shift (∼0.12 V) of reduction peak for PNCO. The reduction of PNCO can be ascribed to the decomposition of NCO to Ni and Co by lithium insertion and the formation of Li₂O. At the same time, two peaks are recorded at about 1.6 V and 2.2 V both for PNCO and NCO in the anodic polarization process, which can be attributed to the oxidation of Ni to Ni²⁺ and Co to Co³⁺, respectively. In the 2nd cycle, the reduction peak at around 0.6 V in the 1st cycle moved to about 0.9 V due to less or no formation of SEI film and irreversible reduction reaction. Meanwhile, the two oxidation peaks are almost no shift and overlap very well from 1st to 2nd.

Fig. 6 shows the representative voltage profiles of (b) NCO and (c) PNCO electrodes for the 1st, 2nd, 10th and 100th during discharge and charge processes at a current density of 500 mA g⁻¹ in the voltage range of 0.01–3.0 V. As revealed in Fig. 6(b), there is a plateau at 0.72 V at the 1st discharge curve followed by sloping down to the cutoff voltage of 0.01 V, the initial discharge specific capacities are 1587.9 mA h g⁻¹ for NCO, corresponding to a coulombic efficiency of 70.39%, the potential curve becomes steeper along with lower capacities since 2nd cycle because of the irreversible intercalation reaction. As for PNCO, there is a more stable potential plateau about 0.9 V at the 1st discharge and the initial discharge specific capacities is 1519.1 mA h g⁻¹, corresponding to the higher efficiency of 73.62%. The lower value of the initial coulombic efficiency for the NCO is mainly due to the formation of more irreversible SEI on the surface of the NCO electrode during the first cycle, which is also in accord with the observed CV images as shown in Fig. 6(a). Obviously, the SEI film on NCO electrode accounts for much larger peak area than PNCO, which leads to the lower initial coulombic efficiency. According to the previous reports, the irreversible capacity loss is related to the formation of SEI film at the active materials interface,⁴⁵ the reduction of metal oxide to metal with Li₂O formation,⁴⁶ the polymeric layer on the metal nanoparticles and the decomposition of the electrolyte. Compared with the theoretical capacity (890 mA h g⁻¹), the excess capacities can be owing to the formation of SEI film and the organic polymeric/gel-like layer.^47,48 In comparison with the pure NCO, the PNCO depicts a higher initial coulombic efficiency and better cycling stability attributed to the regular arrangement of nanorod clusters, which efficiently endures the volume expansion/contraction during the Li⁺ insertion/extraction.

Fig. 7(a) shows the cycling performance of NCO and PNCO electrodes at a current density of 500 mA g⁻¹. Obviously, it can be observed that the reversible capacity of the two as-prepared electrodes is slowly increasing during the initial 22 cycles except for the first cycle. This phenomenon can be explained by the reason that a polymeric surface film attach to the active material.^49–51 However, the discharge capacity of the NCO decreases rapidly, which can be attributed to the damage of the NCO structure related to the expansion of volume during the discharge–charge process. While the PNCO electrode keeps gradually successively ascending on account of the reasons mentioned above. With the cycle number increased, the reversible capacity of the PNCO electrode manifests a little decline during 40–75 cycles. The phenomenon may be ascribed to the loss of the active material because of the embedding of metallic cobalt and nickel in the Li₂O matrix partially⁵² and the aggregation of the active species into clusters.⁵³ After 100 cycles, the PNCO shows the much higher reversible discharge capacity of 1417.5 mA h g⁻¹ as compared to the NCO with the discharge capacity of 300.1 mA h g⁻¹. The main reason is that the high discharge/charge current density might induce drastic structure reorganization accompanied by decomposition and reformation of the electrolyte. Similar results were also observed by previous reports on monodisperse NCO mesoporous microspheres³ at 800 mA g⁻¹ and Mn_1.5Co_1.5O₄ core–shell microspheres⁵⁴ at 400 mA g⁻¹. In addition, the coulombic efficiency of PNCO could achieve as high as 96% from 3rd cycle to 100th cycle. These results prove that the NCO modified by PSA exhibits excellent performance for the particular flower-like structure of PNCO has high specific surface area and enough free space.

Fig. 7 (a) Cycling performance of NCO and PNCO electrodes and the corresponding coulombic efficiency at a current density of 500 mA g⁻¹; (b) electrochemical impedance spectra for the samples of NCO and PNCO electrodes after cycling for 100 cycles at the current density of 500 mA g⁻¹; (c) rate capability of the electrode made from the NCO and PNCO; (d) cycling performance of the PNCO electrode and the corresponding coulombic efficiency at a current density of 1000 mA g⁻¹ and 2000 mA g⁻¹, respectively; (e) schematic illustration of the operating principles of PNCO nanorod clusters electrodes.

The charge transfer resistance of electrodes is another crucial factor for influencing their electrical performance. We investigated the charge transfer resistance of the NCO and the PNCO electrodes after cycling for 100 cycles at the current density of 500 mA g⁻¹. As showed in Fig. 7(b), the Nyquist plots of the electrodes are consisted of a single semicircle in the high-medium frequency region and a straight line in the low frequency region. The inset in Fig. 7(b) also demonstrates the equivalent electrical circuit which is made up of the bulk solution resistance (R_s), the electrolyte (R_ct), a constant phase element accounting for a double-layer capacitor (CPE(ct)), surface film resistance (R_f) and Warburg impedance (Z_w).^55,56 As presented in Fig. 7, it indicates that the value of R_s and R_ct for PNCO are approximated to 3.23 and 13.81 Ω, which are much smaller than those of the NCO (6.01 and 14.50 Ω), suggesting the former possesses a faster charge-transfer process. Meanwhile, a significant slope differences is observed from the vertical diffusion lines, implying the superior capacitive performance of the flower-like PNCO nanorod clusters electrodes.

In order to better understand the good cycling performance of PNCO, the rate capability is studied by multiple-step charge–discharge at different current density ranging from 500 to 4000 mA g⁻¹ and the results are shown in Fig. 7(c). It can be seen that both electrodes shown good cycling stability at the low current density rang of 500 and 1000 mA h g⁻¹. The average discharge capacity of the NCO and PNCO is 1259.8 and 1229.0 mA h g⁻¹ at the current of 500 mA g⁻¹, 1233.7 and 1247.3 mA h g⁻¹ at the current of 1000 mA g⁻¹, respectively. While the current rate gradually increases from 2000 to 4000 mA g⁻¹, there are significant differences of cycling stability and capacity between NCO and PNCO, such as 1193.5 and 944.5 mA h g⁻¹ for PNCO, 972.8 and 490.8 mA h g⁻¹ for NCO. When the current density is reversed back the initial value of 500 mA g⁻¹, the capacity of the PNCO can easily recover to 1332.7 mA h g⁻¹ after 70 cycles of charge/discharge, which is excessed the capacity of the initial 10 cycles at the current density of 500 mA g⁻¹. However, the capacity of the pure NCO only can rebound to 584.2 mA h g⁻¹, which indicates that the PNCO has a better rate performance. This excellent rate performance can be accounted to the unique flower-like structure which provides extra active position for Li⁺ ion storage and effectively shortens the pathway for Li⁺ ion diffusion.

In order to further investigate the long cycling performance at a large current density, Fig. 7(d) shows the cycling performance of the PNCO electrode and the corresponding coulombic efficiency at a large current density of 1000 mA g⁻¹ and 2000 mA g⁻¹ in the voltage window of 0.01–3.0 V for 100 cycles. It is obvious that the curves of the as prepared electrodes at high current density are similar to at a current density of 500 mA g⁻¹ in shape, which indicates good cycling stability for PNCO. Meanwhile, the retained reversible discharge capacity still reached up to approximately 819.0 and 719.5 mA h g⁻¹ at a rate of 1000 mA g⁻¹ and 2000 over 100 cycles, respectively, which is higher than that of commercially used graphite (372 mA h g⁻¹) even at the high current density. The good cycling stability can be ascribed to the outstanding nanostructures which not only can be beneficial for Li⁺ ions to diffuse efficiently to active material with less resistance but also to buffer the volume expansion during the discharging/charging processes.

Fig. 7(e) shows the schematic illustration of the operating principles of PNCO nanorod clusters electrodes. The high capacity, excellent cycling stability, and good rate capability can be attributed to the unique morphology and structure of the current PNCO electrodes which provide ideal conditions for facile diffusion of the electrolyte and accommodation of the strain induced by the volume change during electrochemical reactions, thus leading to a higher efficiency of lithiation and delithiation under the electrolyte penetration. Meanwhile, the novel 3D flower-like structure shortens the Li⁺ ion diffusion paths in the nanowires and enhances the rate capability. In order to evaluate intrinsic electrochemical performance of the PNCO electrode for LIBs, we made a comparison of 3D flower-like PNCO and other binary cobalt-based metal oxides with different morphologies recently reported, as summarized in Table 1. It is obvious that the PNCO exhibits excellent cycle performance and enhanced rate capacity at a high current density, which confirms its great potential as a high rate anode material for LIBs.

Table 1 Cycling performance of the as-obtained PNCO and previously reported NCO nanomaterials

Type of materials	Reversible capacity (mA h g^−1)	Current density (mA g^−1)	Ref.
Fe2O3@NCO	1079.6/100th	100	33
NCO@SnO2	765/100th	100	20
NCO nanobelt	981/100th	500	57
NCO nanosheets	767/50th	100	58
NCO/RGO	806/70th	100	59
	396/30th	800
NCO hollow spheres	706/100th	200	60
	533/50th	2000
PNCO	1417.5/100th	500	This work
	719.5/100th	2000

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To further explore the excellent electrochemical performance of the flower-like PNCO nanorods, the morphologies of the pure NCO and PNCO electrodes are characterized by SEM after 100 cycles at the current density of 500 mA g⁻¹. All the cracks in the SEM imagines can be attributed to the repeated volume change between metals and metal oxides. As demonstrated in Fig. 8(a) and (b), the pure NCO was seriously destroyed owing to the expansion of volume during the charge/discharge process. Meanwhile, the NCO tends to aggregate, which reduces the effective contact areas of the electrolyte and active materials. The morphology of the flower-like PNCO nanorods is shown in the Fig. 8(c) and (d). Evidently, some narrow cracks present on the surface of the electrode. However, these cracks can offer a short pathway for Li⁺ ion and increase the opportunities for the contact between Li⁺ ion and active materials. Compared with the cycling and SEM images after cycling of the two samples, it implies that the PSA modified NCO nanorod clusters electrodes could greatly improve the electrochemical performance.

Fig. 8 (a and b) SEM images of NCO electrodes after 100 cycles at a current density of 500 mA g⁻¹; (c and d) SEM images of PNCO electrodes after 100 cycles at a current density of 500 mA g⁻¹.

4. Conclusions

In summary, a facile hydrothermal method combined with a simple thermal treatment in air is applied to synthesize a novel class of 3 D flower-like PNCO nanorod clusters modified by polymer spheres. When applied as anode materials in LIBs, the hierarchical PNCO nanorod clusters show superior cycling ability and excellent rate performance. For example, the specific capacity still retains 1417.5 mA h g⁻¹ after 100 cycles at the current density of 500 mA g⁻¹ in the potential range of 0.01 to 3.0 V and an average discharge capacity as high as 846.7 mA h g⁻¹ can be reached at current density of 4000 mA g⁻¹. Meanwhile, the reversible capacities as high as 819.0 and 719.5 mA h g⁻¹ could be obtained at the current density of 1000 and 2000 mA g⁻¹ after 100 cycles. What is important, owing to the facile synthesized method and excellent electrochemical properties, the flower-like PNCO nanorod clusters have the prospect of becoming the next-generation high power LIBs.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 51101062 and 51171065), Science and Technology Project of Guangzhou City, China (Grant No. 2011J4100075), Foundation for Distinguished Young Talents in Higher Education of Guangdong, China (Grant No. LYM09052), China Scholarship Council (Grant No. 201308440314), the Scientific Research Foundation of Graduate School of South China Normal University (Grant No. 2014ssxm13), and Guangdong Natural Science Foundation (Grant No. S2012020010937, 10351063101000001, 2014A030313436).

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