Ni_0.33Mn_0.33Co_0.33Fe₂O₄ nanoparticles anchored on oxidized carbon nanotubes as advanced anode materials in Li-ion batteries

Abstract

We report a solvothermal synthesis of Ni_0.33Mn_0.33Co_0.33Fe₂O₄ (NMCFO) nanoparticles anchored on the surface of oxidized carbon nanotubes (OCNT) to form NMCFO/OCNT nanocomposites as advanced anode materials in Li-ion batteries. Ni(CH₃COO)₂, Mn(CH₃COO)₂, Co(CH₃COO)₂, and FeCl₃ were employed as the metal precursors and CH₃COONa, HOCH₂CH₂OH and H₂O as the mixed solvent in the synthesis. The obtained samples were characterized by X-ray diffraction, thermogravimetric analysis, inductively coupled plasma optical emission spectrometry, X-ray photoelectron spectroscopy, transmission electron microscopy, and scanning electron microscopy. It is found that the OCNT provided functional groups on the outer walls to nucleate and anchor NMCFO nanoparticles (5–20 nm), while retaining the inner walls intact with high conductivity. Compared with the bare NMCFO nanoparticles, NMCFO/OCNT composites showed a significantly improved electrochemical performance because OCNT can substantially inhibit the aggregation of NMCFO nanoparticles and buffer the volume change, and moreover, the inner walls of OCNT provide excellent electronic conduction pathways. This work opens an effective way for the fabrication of ferrite-based metal oxide/OCNT hybrids as promising anode materials for Li-ion batteries.

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1. Introduction

To achieve higher specific capacities than that of the conventional graphite (372 mA h g⁻¹), in recent years nanosized metal oxides such as NiO,¹ Fe₂O₃,² Co₃O₄,^3,4 ZnMn₂O₄,⁵ CoMn₂O₄,⁶ and MnFe₂O₄,⁷ have been intensively studied as anode materials in Li-ion batteries (LIBs). However, severe aggregation of the particles and volume change of the electrode materials during discharge–charge processes often lead to poor cycling stability, sluggish kinetics in charge transfer and ionic diffusion, as well as high intrinsic resistance.^8–11 These shortcomings can induce additional performance degradation of the metal oxide based electrodes, especially at high current densities, seriously limiting their practical application.¹² To circumvent the above technical barriers, conductive carbon materials are often incorporated with metal oxides to form metal oxides/carbon composites, such as Fe₂O₃@carbon,¹³ Fe₃O₄ nanoparticles embedded in porous carbon,¹⁴ Co₃O₄ coated on carbon nanotube,¹⁵ mesoporous carbon-encapsulated NiO,¹⁶ CoO coated on graphene,¹⁷ carbon nanofibers@CoMn₂O₄ coaxial nanocables,¹⁸ and mesoporous Mn_0.5Co_0.5Fe₂O₄ nanospheres grown on grapheme,¹⁹ which have partially improved the cycling stability and rate capability.^20,21 These nanostructured metal-oxide-containing electrodes with small size are able to partially buffer the stress and strain effects caused by the particle volume expansion/contraction during the Li-ion insertion/extraction, thus improving the cycling and rate capability.²² Meanwhile, the instructive guidelines for designing high-performance nanostructured hybrid materials based on metal oxides and carbon for LIBs have been established via these research activities.²³

Iron-based oxides have recently received increased attention as very promising anode materials for rechargeable LIBs because of their high theoretical capacity, non-toxicity, low cost, and improved safety.²⁴ Meanwhile, nanostructured carbon nanotubes (CNT) are also promising anode materials for LIBs due to their large electrode/electrolyte contact area and active surface, high chemical stability and conductivity, and good mechanical strength.^25–31 Therefore, in the past decade efforts were made to synthesize CNT–metal oxides nanoparticle hybrids or composites.^32,33 The binding between CNT and metal oxides nanoparticles are quite diverse, e.g., by noncovalent interactions (such as π–π stacking and hydrophobic wrapping), electrostatic interactions, or by covalent bonding through functionalization of outer wall by oxidation.^34–36 Xu et al. prepared NiO/CNT composites as anode material using nitric acid treated CNT, which maintained the capacity at ∼800 mA h g⁻¹ after 50 discharge–charge cycles at a current density of 50 mA g⁻¹.³⁷ Zhuo et al. prepared Co₃O₄/CNT composites using oxidative treatment on CNT, which left a large number of acidic groups on the surface, showing 776 mA h g⁻¹ at the 100th cycle at a current density of 200 mA g⁻¹.¹⁵ In addition, Wang et al. found that the carbon coated α-Fe₂O₃ hollow nanohorns supported on nitric acid treated CNT backbones exhibited a very stable capacity retention of 800 mA h g⁻¹ over 100 cycles at a current density of 500 mA g⁻¹.¹² From these literature results, it is found that metal oxides/CNT hybrids, in which the CNT are pre-oxidized partially, exhibit good electrochemical performance by virtue of their advantageous structural features. However, so far there has no report showing attractive hybrid containing multicomponent ferrite-based metal oxides nanoparticles and CNT for lithium storage.

Herein, we report a novel synthesis of Ni_0.33Mn_0.33Co_0.33Fe₂O₄ (NMCFO)/oxidized carbon nanotubes (OCNT) composites by directly growing NMCFO nanoparticles on OCNT. In this strategy, NMCFO nanoparticles are uniformly dispersed on the surface of OCNT to form NMCFO/OCNT network nanocomposites. Compared with the bare NMCFO nanoparticles, NMCFO/OCNT exhibits significantly improved electrochemical performance with excellent cycling stability and high rate performance, suggesting a potential application as anodes in future LIBs with a high energy density.

2. Experimental

2.1 Material synthesis

2.1.1 Synthesis of oxidized carbon nanotubes (OCNT). All the chemicals were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. The OCNT were made by a modified Hummers method.³⁸ Carbon nanotubes (CNT) (CNano Technology (Beijing) Ltd) were first purified by calcination at 500 °C for 2 h and washed with diluted HCl (10%). The purified CNT (2 g) were put into a 250 mL round-bottom flask, followed with addition of 50 mL of concentrated sulfuric acid, and the mixture was stirred at room temperature overnight. Next, the flask was heated in an oil bath at 40 °C, and 0.5 g of NaNO₃ was added and dissolved in the suspension. This step was followed with a slow addition of 2.0 g of KMnO₄, and the reaction temperature was maintained at 40 °C. The solution was stirred for 30 min. Afterward, 100 mL of water was slowly added to the flask. After 30 min, the flask was removed from the oil bath, followed with addition of 300 mL of water and 30 mL of 30% H₂O₂ to terminate the reaction. After stirred at room temperature for 15 min, the sample was then repeatedly centrifuged and washed with 10% HCl solution for several times, rinsed with copious amounts of water. The final precipitate was dispersed in 20 mL of water and lyophilized. Finally, a dry product of ∼1.9 g was collected.

2.1.2 Synthesis of Ni_0.33Mn_0.33Co_0.33Fe₂O₄/oxidized carbon nanotubes (NMCFO/OCNT). All the chemicals were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. In a typical synthesis, Ni(CH₃COO)₂·4H₂O (0.33 mmol), Mn(CH₃COO)₂·4H₂O (0.33 mmol), Co(CH₃COO)₂·4H₂O (0.33 mmol), FeCl₃·6H₂O (2.0 mmol), CH₃COONa·3H₂O (10.0 mmol) and OCNT (x g) were dissolved/dispersed in distilled water (40 mL) and HOCH₂CH₂OH (40 mL) to form a homogeneous slurry, which was then refluxed for 2 h at 110 °C under constant stirring in a three-neck flask. Subsequently, the mixture was sealed in a stainless-steel autoclave and heated at 200 °C for 48 h. The resulting precipitates were collected by centrifugation, washed with distilled water and absolute ethanol, and finally dried in vacuum at 120 °C for 24 h. The obtained NMCFO/OCNT nanocomposites containing OCNT (x g) 0.20, 0.15, 0.10, and 0.05 g of OCNT were denoted NMCFO/OCNT-1, NMCFO/OCNT-2, NMCFO/OCNT-3, and NMCFO/OCNT-4, respectively. Similarly, bare NMCFO was prepared without adding OCNT, and NMCFO/CNT prepared by adding un-oxidized 0.10 g CNT. In addition, amorphous NMCFO/OCNT precursor was prepared as a comparison as follows: Ni(CH₃COO)₂·4H₂O (0.33 mmol), Mn(CH₃COO)₂·4H₂O (0.33 mmol), Co(CH₃COO)₂·4H₂O (0.33 mmol), FeCl₃·6H₂O (2.0 mmol), CH₃COONa·3H₂O (10.0 mmol) and OCNT (0.10 g) were dissolved/dispersed in distilled water (40 mL) and HOCH₂CH₂OH (40 mL) to form a homogeneous slurry, which was then refluxed for 2 h at 110 °C under constant stirring in a three-neck flask. The resulting precipitates were collected by centrifugation, washed with distilled water and absolute ethanol, and finally dried in vacuum at 120 °C for 24 h to obtain the amorphous NMCFO/OCNT precursors.

2.2 Materials characterization

X-ray diffraction patterns (XRD) were recorded on a PAN analytical X'Pert PRO MPD using the Cu Kα radiation of (λ = 1.5418 Å). The microscopic feature of the samples was characterized by field-emission scanning electron microscopy (FESEM) with an energy-dispersive X-ray spectrometer (EDX) (JSM-7001F, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM) with an energy-dispersive X-ray spectrometer (EDX) (JEM-2010F, JEOL, Tokyo, Japan) operated at 300 V. Thermogravimetric (TG) analysis was carried out on an EXSTAR TG/DTA 6300 (Seiko Instruments, Japan) at a heating rate of 10 °C min⁻¹ in air (200 mL min⁻¹). The elemental analysis was conducted by inductively coupled plasma optical emission spectrometry (ICP-OES, Optima 5300DV, Pekin Elmer, US). X-ray photoelectron spectroscopy (XPS) analysis was carried out on an ESCALab250electron spectrometer from Thermo Scientific Corporation using monochromatic 150 W Al Kα radiation. Pass energy for then arrow scan was 30 eV. The chamber pressure was about 6.5 × 10⁻¹⁰ mbar. The binding energies were referenced to the C1s line at 284.8 eV.

2.3 Electrochemical measurement

The working electrode was prepared by mixing the active materials, acetylene black, and polyvinylidene fluoride (PVDF) in a weight ratio of 80 : 10 : 10 with N-methylpyrrolidone (NMP) as a solvent. The resulting slurries were cast onto a common Cu foil (current collector). The film composed of Cu foil and slurries were rolled into 25 μm thin sheets, and then dried at 50 °C for 24 h. The film were cut into disks with a diameter of 14 mm, and then dried at 120 °C in vacuum for 24 h. CR2016 coin-type cells were assembled in an Ar-filled glove box with lithium foils as the counter electrodes and polypropylene microporous films (Celgard 2400) as separators. The liquid electrolyte is 1 mol L⁻¹ LiPF₆ in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1 : 1, v/v). The galvanostatic charge and discharge tests were carried out on a NEWARE CT-3008-5V10mA testing instrument in a voltage range between 0.01 and 3.0 V at current densities of 50, 100, 500, and 1000 mA g⁻¹. Cyclic voltammetry (CV) was carried out using a CHI660D potentiostat in the voltage range of 0–3 V at a scanning rate of 0.1 mV s⁻¹ at room temperature. Electrochemical impedance spectroscopy (EIS) measurements were also conducted using the CHI660D potentiostat over a frequency range from 100 kHz to 10 mHz with an ac oscillation of 5 mV.

3. Results and discussion

The carbon nanotubes (CNT) used in this work are about 12 nm in diameter and several micrometers in length.³⁹ A modified Hummers method³⁸ was used to prepare OCNT under mild oxidation. As determined by X-ray photoelectron spectroscopy (XPS), the OCNT contains ∼15 at% oxygen (Fig. S1†). Fig. 1a shows the XRD patterns of all the samples. The observation of the strong (002) peak at 2θ values of about 26.3° in OCNT indicates its high graphite crystallinity (JCPDS no. 25-0284). The observed diffraction peaks at 2θ values of 30.1, 35.6, 43.4, 53.7, 57.3, and 62.8° correspond to the lattice planes of (220), (311), (400), (422), (511), and (440) respectively, indicating the formation of crystal Ni_0.33Mn_0.33Co_0.33Fe₂O₄ (NMCFO) in the samples of NMCFO, NMCFO/OCNT-1, NMCFO/OCNT-2, NMCFO/OCNT-3, and NMCFO/OCNT-4.⁴⁰

Fig. 1 XRD patterns (a) and TG curves (b) of all the samples.

Fig. 1b shows the TG curves of all the samples. The impurity of CNT is around 4.2 wt%, probably due to the presence of metal catalyst (Fe/Co/Ni) residue after the synthesis of CNT. However, these metal nanoparticles have been removed by calcination followed with washing by dilute hydrochloric acid for OCNT. It can be seen that the weight loss derived from carbon combustion is located between 360 and 600 °C for OCNT, lower than that for CNT (400–650 °C), indicating the lowered combustion temperature of OCNT after the oxidation treatment. For the bare NMCFO nanoparticles, the weight loss of about 1.2 wt% may be derived from the trace amount of organic residue on their surface. For NMCFO/OCNT composites, the weight loss starts at about 230 °C, lowering than that of OCNT (360 °C). This is because the presence of metal oxides (NMCFO) can catalytically promote the carbon combustion process at a lower temperature.⁴¹ The measured weight loss of NMCFO/OCNT-1, NMCFO/OCNT-2, NMCFO/OCNT-3, and NMCFO/OCNT-4 is about 41.7, 36.2, 28.9, and 16.7 wt% respectively in the temperature range from 100 to 1000 °C, which can be ascribed to the decomposition of organic species and the combustion of OCNT, suggesting that the weight percentage of OCNT is approximately at 40.5, 35.0, 27.7, and 15.5 wt% in the samples of NMCFO/OCNT-1, NMCFO/OCNT-2, NMCFO/OCNT-3, and NMCFO/OCNT-4, respectively.

Fig. 2a shows the TEM image of NMCFO nanoparticles with a size range of 5–20 nm. Most of the NMCFO nanoparticles are agglomerated together due to the surface effect of nanoparticles.⁴² Meanwhile, the HRTEM image of these nanoparticles (Fig. 2b) reveals that the lattice fringe spacing is about 0.293 nm, corresponding to the interplanar distance of (220) planes in NMCFO,⁴⁰ and each nanoparticle is actually a single crystal. The EDX spectrum of NMCFO nanoparticles in Fig. 2c demonstrates the presence of Ni, Mn, Co, Fe, and O elements. The elemental mapping images based on the NMCFO nanoparticles show the homogeneous distribution of all the five elements (Ni, Mn, Co, Fe, and O) (Fig. S2†). The atomic ratio of Ni, Mn, Co, and Fe elements is approximate 1 : 1 : 1 : 6 from ICP-OES analysis. In fact, it is possible to tune the atomic ratio of Ni_xMn_yCo_zFe₂O₄ (x + y + z = 1, 0 < x, y, z < 1) by controlling the ratio of raw materials (Ni(CH₃COO)₂, Mn(CH₃COO)₂, and Co(CH₃COO)₂). For example, for Ni_0.2Mn_0.4Co_0.4Fe₂O₄, the used raw materials amount is 0.2 mmol Ni(CH₃COO)₂, 0.4 mmol Mn(CH₃COO)₂, and 0.4 mmol Co(CH₃COO)₂ (Fig. S3a†); while for Ni_0.4Mn_0.4Co_0.2Fe₂O₄, the corresponding amount is 0.4 mmol Ni(CH₃COO)₂, 0.4 mmol Mn(CH₃COO)₂, and 0.2 mmol Co(CH₃COO)₂ (Fig. S3b†). The SEM image in Fig. 2d shows that the formed NMCFO/OCNT precursors have network structure. NMCFO and OCNT in NMCFO/OCNT precursors can be easily differentiated by the brightness contrast in the TEM images (Fig. S4b and c†), which shows that the NMCFO are amorphous and coated on the surface of OCNT, well agreement with the XRD result (Fig. S4a†). Also, it is very important to identify the amorphous nature of NMCFO precursors, as it helps to understand the formation process of the crystallized NMCFO.

Fig. 2 TEM images (a and b), and EDX spectrum of NMCFO nanoparticles (c), and SEM of amorphous NMCFO/OCNT precursors (d).

The SEM image of Fig. 3a shows that the OCNT are coated with NMCFO nanoparticles forming the network structure after adding 0.20 g OCNT in NMCFO/OCNT-1. The surface of the OCNT becomes coarser after coating with the smaller NMCFO nanoparticles. When the OCNT amount is decreased to 0.15 (Fig. 3b) and 0.10 g (Fig. 3c and d), most of NMCFO nanoparticles are coated on the surface of OCNT forming the NMCFO/OCNT-2 and NMCFO/OCNT-3 network composites. With further decrease of the OCNT amount to 0.05 g (Fig. S5a†), larger portion of NMCFO nanoparticles is aggregated together but still coated on the surface of OCNT. Therefore, it is possible to tune the nanocomposites network structure and composition by varying the synthesis conditions such as the amount of OCNT and raw chemicals, as well as the reaction time and temperature.

Fig. 3 SEM images of NMCFO/OCNT-1 (a), NMCFO/OCNT-2 (b), NMCFO/OCNT-3 (c), and the high-magnification of NMCFO/OCNT-3 (d).

More detailed structural information can be obtained from the TEM results. The TEM images of NMCFO/OCNT-1 in Fig. 4a and b show that smaller NMCFO nanoparticles are highly dispersed on the surface of OCNT after adding 0.20 g OCNT. Fig. 4c shows the high resolution TEM image of NMCFO/OCNT-1, in which, the NMCFO nanoparticles with a size of around 15 nm are attached to the surface of OCNT and the lattice with an interplanar spacing of about 0.293 nm corresponds to the (220) plane of NMCFO, consistent with the observation in Fig. 2b. The HRTEM image also reveals that the inner walls of graphitic carbon nanotubes in the hybrid are still well maintained or not broken (Fig. 4c), suggesting that the inner walls of OCNT were not destroyed by the oxidation treatment and the subsequent reactions during the hybrid synthesis. After addition of 0.15 g and 0.10 g OCNT, the samples of NMCFO/OCNT-2 (Fig. 4d and e) and NMCFO/OCNT-3 (Fig. 4f and g) with uniformly coated NMCFO nanoparticles on the surface of OCNT composites are obtained. However, with further decrease of the OCNT amount to 0.05 g, most of NMCFO nanoparticles and OCNT are aggregated together (Fig. S5b and c†). The crystal NMCFO and OCNT in NMCFO/OCNT composites also could be easily differentiated by the brightness contrast in the TEM images (Fig. 4, S5b and c†). These crystal NMCFO nanoparticles are attached to the surface of OCNT to form network composites, consistent with the above SEM observation.

Fig. 4 TEM images of NMCFO/OCNT-1 (a–c), NMCFO/OCNT-2 (d and e), and NMCFO/OCNT-3 (f and g).

The formation process of NMCFO/OCNT composites is illustrated in Fig. 5. Firstly, Ni²⁺, Mn²⁺, Co²⁺, and Fe³⁺ dissolved in the solvent are adsorbed on the external surface of the OCNT, because OCNT with functional groups on the outer walls is a good metal ion adsorbent (Fig. 5a).^15,38 With the increase of the temperature to 110 °C, these metal ions are accumulated specifically on the outer wall by the oxidative functional groups on OCNT.³⁵ These oxidative functional groups can act as heterogeneous nucleation sites in the early reaction stage to facilitate the formation of small amorphous nanoparticles during the precipitation.⁴³ Thereafter, the small amorphous NMCFO nanoparticles are generated and aggregated on the surface of OCNT via the reaction (0.33Ni²⁺ + 0.33Mn²⁺ + 0.33Co²⁺ + 2Fe³⁺ + 4H₂O → Ni_0.33Mn_0.33Co_0.33Fe₂O₄ + 8H⁺) with the water generated from the metal precursors (Fig. 5b). With the further increase of the temperature to 200 °C, the amorphous NMCFO nanoparticles are crystallized to form crystal NMCFO nanoparticles (Fig. 5c), which are also coated on the surface of the OCNT. And in the last stage, the crystal NMCFO/OCNT network composites are developed and formed (Fig. 5d). In case with addition of more OCNT, it will have more oxidative functional groups acting as the adsorption sites for the given amounts of Ni²⁺, Mn²⁺, Co²⁺, and Fe³⁺. As a result, the formed NMCFO will become smaller in size and be dispersed more uniformly on the OCNT surface as shown in Fig. 3, 4, and S5.† This is because of the dispersing effect of OCNT. In a control experiment, it was failed to get the uniform distribution of NMCFO nanoparticles and CNT network composites by using 0.10 g CNT without oxidization. Therefore, it is clear that the oxidative functional groups on the external surface of OCNT are the nucleation centers and play the key role in the growth of NMCFO nanoparticles.

Fig. 5 Schematic illustration of the formation process of NMCFO/OCNT network composites.

Fig. 6a presents the CV curves of NMCFO/OCNT-1 in the first two cycles, and in the 51st cycle after 50 rate discharge–charge cycles at a scan rate of 0.1 mV s⁻¹. In the first scan, two cathodic peaks are observed at about 0.57 and 0.78 V, which correspond to the conversion reactions of Fe³⁺, Co²⁺, Ni²⁺, and Mn²⁺ to their metallic states and the formation of Li₂O, respectively. The broad anodic peak from 1.0 V to 2.0 V can be ascribed to the oxidation reactions of metallic Fe, Co, Ni, and Mn. The quaternary metal oxide NMCFO stores Li through reversible formation and decomposition of Li₂O. In the second scan, the reduction peaks are shifted to 0.73 and 1.51 V respectively. The peak intensity and integral areas of the 51st cycle after 50 rate discharge–charge cycles measurement are almost the same as those of the second cycle for NMCFO/OCNT-1, which demonstrates the good reversible oxidation–reduction reaction, and reversible formation and decomposition of Li₂O.⁴⁴ The discharge and charge capacities in the first run, and their initial coulombic efficiencies are shown in Fig. S6† and Table 1. The discharge–charge capacity for NMCFO component may be based on the oxidation–reduction of metallic Fe, Co, Ni, and Mn nanoparticles to Ni_0.33Mn_0.33Co_0.33Fe₂O₄ respectively via the reaction: 4Li₂O + 0.33Ni + 0.33Mn + 0.33Co + 2Fe ↔ Ni_0.33Mn_0.33Co_0.33Fe₂O₄ + 8Li⁺ + 8e⁻. A distinct voltage plateau can be clearly identified at ca. 0.8–0.9 V, which is corresponding to the reduction of Fe³⁺ to Fe, Co²⁺ to Co, Ni²⁺ to Ni, and Mn²⁺ to Mn during the initial discharge process (Fig. S6†). Meanwhile, a clear plateau is observed in the charge process at ca. 1.5–2.5 V, which is corresponding to the oxidation of Fe to Fe³⁺, Co to Co²⁺, Ni to Ni²⁺, and Mn to Mn²⁺ during the initial charge process (Fig. S6†). The carbon lithiation and delithiation voltage plateau are not observed, because of the small contribution of carbon to the overall anode capacity in term of the previous reports (Fig. S6†).³⁹

Fig. 6 Electrochemical properties: CV curves of NMCFO/OCNT-1 for the first two cycles, and 51st cycle after 50 rate discharge–charge cycles at a scan rate of 0.1 mV s⁻¹ (a), cycling property and coulombic efficiency of NMCFO, NMCFO/OCNT-1, NMCFO/OCNT-2, NMCFO/OCNT-3, and NMCFO/OCNT-4 (b), the discharge–charge curves (c), and rate performance (d) of NMCFO/OCNT-1 at different current densities.

Table 1 The electrochemical performance of all the samples

Samples	C1st-di^a (mA h g^−1)	C1st-ch^b (mA h g^−1)	CEin^c (%)	ACE2nd^d (%)	ACF100th^e (% per cycle)	C100th^f (mA h g^−1)
a C1st-di, the first discharge capacity.b C1st-ch, the first charge capacity.c CEin, the initial coulombic efficiency.d ACE2nd, the average coulombic efficiency after second cycles.e ACF100th, average capacity fading rate during 100 cycles.f C100th, the discharge capacity after 100 cycles.
NMCFO	1541.3	723.2	46.9	94.5	0.804	141.8
NMCFO/OCNT-1	1092.1	692.2	63.4	97.4	0.028	673.7
NMCFO/OCNT-2	1252.2	771.5	61.6	97.2	0.050	733.1
NMCFO/OCNT-3	1452.2	909.1	62.7	97.0	0.219	710.3
NMCFO/OCNT-4	1495.1	928.5	62.1	96.8	0.455	506.3

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The cycling performance in Fig. 6b and Table 1 shows that the discharge capacity of NMCFO/OCNT-1, NMCFO/OCNT-2, NMCFO/OCNT-3, NMCFO/OCNT-4 after 100 cycles is around 673.7, 733.1, 710.3, and 506.3 mA h g⁻¹, respectively, which is higher than that of NMCFO (141.8 mA h g⁻¹) and OCNT (296.4 mA h g⁻¹) (Fig. S7†). It should be mentioned that the capacity of NMCFO/OCNT composites after 100 cycles is lower than that of the CoNiO nanowire arrays loaded on TiO₂ nanotubes (1097 mA h g⁻¹ after 40 cycles at a current density of about 606 mA g⁻¹),⁴⁵ and carbon-coated graphene/Fe₃O₄ nanosheets (920 mA h g⁻¹ after 100 cycles at a current density of 200 mA g⁻¹).⁴⁶ As reported in ref. 40, the discharge capacity after 60 cycles was about 487.2 mA h g⁻¹ for NMCFO mesoporous nanospheres, which is higher than that of the NMCFO nanoparticles (141.8 mA h g⁻¹ after 100 cycles). Probably the porosity structure of NMCFO nanospheres could buffer the large volume change of anodes caused by the conversion reaction during the repeated Li⁺ insertion/extraction,⁴⁷ and be an advantage for lithium-storage applications that require rapid ion transport and large contact area between the electrode and the electrolyte. In addition, the more added acetylene black in ref. 40 could inhibit the aggregation of NMCFO nanospheres, and provide effective electronic conduction pathways. After 200 cycles, the discharge capacity of NMCFO/OCNT-1 at the current densities of 50 and 500 mA g⁻¹ still maintains at about 624.8, and 421.0 mA h g⁻¹, as shown in Fig. S8,† indicating the good cycling performance of the composite anode. The average coulombic efficiency after the second cycle and average capacity fading rate during the 100 cycles of all above NMCFO/OCNT samples is shown in Table 1. Clearly, in a certain range, the more the OCNT added, the better the electrochemical performances will be for the formed network structure composites.

Fig. 6c showed the charge–discharge profiles of NMCFO/OCNT-1 of the first cycles at different current densities. NMCFO/OCNT-1 has a better rate performance than NMCFO/OCNT-1 as shown in Fig. 6d and S9.† In favor of the presence of conductive OCNT, these network composite materials exhibit much lower resistance than the NMCFO nanoparticles, as evidenced by the drastically reduced diameter of these semicircle at high-frequency region in the electrochemical impedance spectroscopy (EIS) patterns (Fig. S10†). As a result of lower contact and charge-transfer impedances, lithium ion diffusion and electron transfer are facilitated to give the greatly enhanced electrochemical performance of the NMCFO/OCNT composites. These results indicate that NMCFO/OCNT exhibits high electrochemical reversibility. The introduced OCNT can highly disperse NMCFO nanoparticles, prevent their aggregation, buffer the volume change, and promote efficient Li⁺ diffusion and electronic conduction, leading to better cycling stability and high rate performances.^48,49 It should be pointed out that this preparation method is extendable to the preparation of other ferrite-based metal oxide (such as Zn_aNi_bMn_cCo_dFe₂O₄, 0 ≤ a, b, c, d ≤ 1, a + b + c + d = 1 (ref. 40))/OCNT hybrid, and these materials can also be used as catalysis^50,51 and supercapacitor, etc.⁵²

4. Conclusions

In summary, we have prepared Ni_0.33Mn_0.33Co_0.33Fe₂O₄ (NMCFO) nanoparticles strongly coupled with oxidized carbon nanotubes (OCNT) network as anodes for Li-ion batteries. OCNT provides functional groups on the outer walls to nucleate and anchor NMCFO nanoparticles, leading to the hydrothermal formation of the NMCFO/OCNT composites. This unique hybrid structure remains a very stable capacity of 624.8 mA h g⁻¹ after 200 cycles at a current density of 50 mA g⁻¹, and a high-rate capability at high current densities of 100–1000 mA g⁻¹. The super electrochemical properties of the NMCFO/OCNT composites should originate from the formed network structure with the intimate interconnection between NMCFO nanoparticles and OCNT, which not only provides stable electrical and ionic transfer channels, but also significantly shortens the diffusion length of the lithium ions. This work opens a new way for fabrication of ferrite-based metal oxide/OCNT nanocomposites as anode materials for Li-ion batteries.

Acknowledgements

The authors gratefully acknowledge the supports from the National Natural Science Foundation of China (no. 51272252) and Hundred Talents Program of the Chinese Academy of Sciences.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra04483e

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