High-performance nickel manganese ferrite/oxidized graphene composites as flexible and binder-free anodes for Li-ion batteries

Abstract

This work demonstrates a new method for fabrication of mixed metal oxide/oxidized graphene (OGP) composites as flexible and binder-free anode materials for Li-ion batteries. The composites containing nickel manganese ferrite (Ni_0.5Mn_0.5Fe₂O₄ (NMFO)) nanoparticles grown on an OGP network structure are fabricated by a facile solvothermal method. In the synthesis, Ni(CH₃COO)₂, Mn(CH₃COO)₂, and FeCl₃ are used as the metal precursors; CH₃COONa, HOCH₂CH₂OH and distilled water as the mixed solvent. The flexible and binder-free electrodes are prepared by coating OGP and NMFO/OGP on polypropylene microporous film via vacuum filtration. The multilayer and porous structure of the NMFO/OGP film generate good contact between the electrode materials and the current collector (OGP film), which is essential for flexible devices. As anticipated, both the free-standing NMFO/OGP film and NMFO/OGP coated on polypropylene microporous film exhibit super-flexible properties without using any binder. The obtained flexible and binder-free electrodes show good electrochemical performance with high lithium storage capacity and excellent cycling stability. This work opens a new way for fabrication of flexible and binder-free anode materials for Li-ion batteries.

---

1. Introduction

In the last several years flexible lithium-ion batteries (LIBs) have attracted great attention as a promising power source in emerging flexible and wearable electronic devices such as touch screens, “smart skins”, biomedical devices, and roll-up displays,^1–5 which is mainly because of their high gravimetric and volumetric energy density and long cycle life.^6–9 Recently, a number of flexible electrode materials for flexible LIBs have been reported.^10–13 These electrode materials include nanoporous carbon nanotubes,¹⁴ holey graphene paper,¹⁵ cellulose/graphite nanocomposites,¹⁶ carbon nanotube/graphene composites,¹⁷ carbon nanotubes/TiO₂ nanofibres,¹⁸ mesoporous NiCo₂O₄ nanowire arrays grown on carbon textiles,¹⁹ carbon nanotube/LiNbO₃ nanoplate–polypyrrole hybrid,²⁰ single-walled carbon nanotube/SnO₂ paper,²¹ Si-conductive nanopaper,²² SnO₂/nitrogen-doped graphene nanocomposites,²³ and CuO nanosheets/reduced graphene oxide composites.²⁴ Among them, graphene-based materials are reported with good cycling performance due to their unique structure, excellent mechanical properties, and high electrical conductivity.^25,26 For instance, Kung et al. indicated that flexible holey graphene paper exhibited high-rate lithium-ion storage capabilities.¹⁵ On the other hand, there are a number of examples to preparation the binder-free electrodes,^27,28 e.g., nanostructured Fe₃O₄/single-walled nanotube binder-free anodes,²⁹ and homogeneous CoO on graphene binder-free anodes.³⁰ Also, great attention has been paid to the nanostructured mixed metal oxides, including ZnMn₂O₄ hollow microspheres,³¹ CoMn₂O₄ hollow microcubes,³² and ferrite materials including MnFe₂O₄ mesoporous microspheres,³³ NiFe₂O₄ nanoparticles,³⁴ and CoFe₂O₄ microspheres³⁵ for their applications as the LIBs anode materials. Compared with CoFe₂O₄ ferrite anodes, MnFe₂O₄ and NiFe₂O₄ ferrite anodes show lower cost and less toxicity. Among them, nickel-based, manganese-based, and iron-based oxides have received increased attention as very promising anode materials for LIBs because of their high theoretical capacity, low cost, non-toxicity, and improved safety. Furthermore, addition of conductive carbon into metal oxides has been proven to prevent interparticle agglomeration and enhance their electronic conductivity, thus significantly improving the cycling performance as compared with the pure metal oxides.^35–40 For example, Guo et al. reported a reversible capacity of 456 mA h g⁻¹ at a current density of 100 mA g⁻¹ after 100 cycles over the flexible electrode of SnO₂/nitrogen-doped graphene nanocomposites,²³ and Peng et al. obtained a specific capacity of 736.8 mA h g⁻¹ after 50 cycles over the flexible hybrid lamellar paper containing free-standing CuO nanosheets/reduced graphene oxide.²⁴

Inspired by but different from the previous work,²⁴ in which free-standing flexible electrodes were fabricated by directly filtering the anode materials suspension using a filtration system, in this work, we focus on increasing the flexible performance of the electrodes by alternately adding more oxidized graphene (OGP), and on enhancing the mechanical flexibility of the electrodes by coating mixed metal oxides/OGP on polypropylene microporous film. Recently, our group has successfully grown mesoporous mixed metal oxides nanospheres on graphene, which showed a much better electrochemical performance than that of pure mixed metal oxides nanospheres for LIBs.⁴¹ It seems that the graphene layers play dual functions: (i) enhancing electronic conductivity,⁴² and (ii) absorbing stress induced by volume expansion, similar to the role of elastic buffer.^43,44 However, flexible and binder-free mixed metal oxides/OGP and mixed metal oxides/OGP coated on polypropylene microporous anodes for LIBs have not been well explored, although they are anticipated to show super-flexible performance.

Herein, we demonstrate a solvothermal method and alternate filtration process to synthesize flexible and binder-free free-standing Ni_0.5Mn_0.5Fe₂O₄ (NMFO)/OGP film and NMFO/OGP coated polypropylene microporous film with high mechanical flexibility as anode materials. Differing from the bare NMFO nanoparticles, these flexible binder-free anodes for LIBs are able to maintain a capacity as high as 558.8 and 541.2 mA h g⁻¹ after 100 cycles at a current density of 100 mA g⁻¹. Also, this new approach for the fabrication of flexible and binder-free electrodes is applicable to synthesis of some other nanostructured metal oxides/OGP electrodes with higher mechanical flexibility for flexible and bendable energy storage devices.

2. Experimental

2.1 Material synthesis

2.1.1 Synthesis of oxidized graphene (OGP). All the chemicals were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. OGP was prepared by a modified Hummers method.⁴⁵ Graphene (GP) (Nanjing XFNANO Material Technology Co., Ltd, China) was washed with dilute HCl (10%). The purified GP (0.2 g) was 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. Subsequently, the flask was heated in an oil bath at 40 °C, and added with 0.5 g of NaNO₃ which was dissolved with ca. 10 min. This step was followed by the slow addition of 2.0 g of KMnO₄ while keeping the reaction temperature at 40 °C. After a continuous stirring for 30 min, 100 mL of water was slowly added to the flask. After another 30 min, the flask was removed out from the oil bath, and the reaction was ended by adding 300 mL of water and 30 mL of 30% H₂O₂ to the mixture. The suspension was stirred at room temperature for 15 min, then repeatedly centrifuged and washed with 10% HCl solution for several times, and finally rinsed with copious amounts of water. The obtained precipitate was dispersed in 20 mL of water and lyophilized to obtain a dry product of ∼0.19 g.

2.1.2 Synthesis of Ni_0.5Mn_0.5Fe₂O₄/oxidized graphene (NMFO/OGP). All the chemicals were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. In a typical synthesis, Ni(CH₃COO)₂·4H₂O (0.5 mmol), Mn(CH₃COO)₂·4H₂O (0.5 mmol), FeCl₃·6H₂O (2.0 mmol), CH₃COONa·3H₂O (10.0 mmol) and OGP (x g) were dissolved/dispersed in a mixture containing 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 washed with distilled water and absolute ethanol via centrifugation, and finally dried in vacuum at 100 °C for 24 h to obtain the desired products. When the mass of the used OGP (x) was 0.01, 0.02, and 0.04 g, the obtained NMFO/OGP nanocomposites were denoted NMFO/OGP-1, NMFO/OGP-2, and NMFO/OGP, respectively.

2.1.3 Synthesis of flexible and binder-free NMFO/OGP composites. In the preparation of OGP and NMFO/OGP aqueous dispersion, sodium deoxycholate was used as a surfactant to stabilize OGP and NMFO/OGP. At first, sodium deoxycholate was dissolved in deionized water at a weight percent of 0.15%.⁷ OGP and NMFO/OGP were added into the surfactant solution, then the suspension was sonicated by cell crushing ultrasonic irradiation. The concentration of OGP and NMFO/OGP in the aqueous dispersion was about 0.1 and 0.3 mg mL⁻¹, respectively. A typical preparation process for free-standing NMFO/OGP film and NMFO/OGP coated on polypropylene microporous film was shown in Fig. 1a. The OGP and NMFO/OGP aqueous dispersion were filtered by alternately adding OGP and NMFO/OGP aqueous dispersion through a polypropylene microporous film (Celgard 2400) filter, and then was washed with deionized water. The NMFO/OGP coated on polypropylene microporous film was dried at 80 °C in air for 48 h, and rolled by a roller machine. The free-standing NMFO/OGP film was peeled off from polypropylene microporous film filter. The free-standing NMFO/OGP film and NMFO/OGP coated on polypropylene microporous film were cut into disks with a diameter of 14 mm. The free-standing NMFO/OGP film (Fig. 1b) and NMFO/OGP coated on polypropylene microporous film (Fig. 1d) show good flexible performance after being bend 180° with hand, as shown in Fig. 1c and e.

Fig. 1 Schematic of the preparation processes of NMFO/OGP coated on polypropylene microporous film and free-standing NMFO/OGP film (a), photographs of NMFO/OGP coated on polypropylene microporous film (b), being bend 180° with hand (c), free-standing NMFO/OGP film (d), and being bend 180° with hand (e).

2.2 Materials characterization

X-Ray diffraction patterns (XRD) were recorded on a PANalytical 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 5 °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 ESCALAB 250Xi from Thermo Scientific Corporation using Al_Kα X-ray radiation.

2.3 Electrochemical measurement

The free-standing NMFO/OGP film and NMFO/OGP coated polypropylene microporous film were cut into disks with a diameter of 14 mm, then dried at 80 °C in vacuum for 48 h. The disks were used as anodes without binder and conductive carbon. The individual electrode disk for free-standing NMFO/OGP film is approximately 1.2 mg, the individual disk for polypropylene microporous film is about 2.4 mg, and the individual electrode disk for NMFO/OGP coated on polypropylene microporous film is approximately 3.6 mg. So the mass of active materials for individual electrode is about 1.2 mg. The working electrode for NMFO 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 by the CT2001A LAND testing instrument in the voltage range between 0.01 and 3.0 V at current densities of 100, 200, 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 a CHI660D potentiostat over a frequency range from 100 kHz to 10 mHz with an ac oscillation of 5 mV.

3. Results and discussion

The XRD patterns of Ni_0.5Mn_0.5Fe₂O₄ (NMFO) nanoparticles and NMFO/OGP nanocomposites are presented in Fig. 2a. 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_0.5Mn_0.5Fe₂O₄ (NMFO) in the samples of NMFO, NMFO/OGP-1, NMFO/OGP-2, and NMFO/OGP.⁴⁶ Fig. 2b displays the TG curves of NMFO nanoparticles and NMFO/OGP nanocomposites. It can be seen that the weight loss derived from carbon combustion is located between 200–500 °C for OGP. For the bare NMFO nanoparticles, the weight loss of about 0.9 wt% should be attributed to the loss of trace amount of organic residue on their surface. For NMFO/OGP nanocomposites, the weight loss of NMFO/OGP-1, NMFO/OGP-2, and NMFO/OGP is about 7.1, 11.3, and 15.6 wt% from 100 to 1000 °C, which can be due to the decomposition of organic species and the combustion of OGP, suggesting that the mass ratios of OGP are approximate 6.2, 10.4, and 14.7 wt% in the samples of NMFO/OGP-1, NMFO/OGP-2, and NMFO/OGP, respectively.

Fig. 2 XRD patterns (a) and TG curves (b) of all the samples.

Fig. 3a indicates the SEM image of NMFO nanoparticles, which have a size distribution from several to several tenth nanometers. Most of the NMFO nanoparticles are agglomerated together due to the high surface energy of the nanoparticles. The TEM image of NMFO nanoparticles in Fig. 3b shows that the particle size is also in the range of 5–30 nm, consistent with the above SEM result. The selected-area electron diffraction (SAED) pattern in inset of Fig. 3b displays the discontinuous diffraction rings, implying that the NMFO nanoparticles are polycrystalline. As shown in the HRTEM image of these nanoparticles (Fig. 3c), the lattice fringe spacing is about 0.294 nm, corresponding to the interplanar distance of (220) planes in NMFO.⁴⁷ The EDX spectrum of NMFO nanoparticles in Fig. 3d proves the presence of Ni, Mn, Fe, and O elements. The elemental mapping images based on the NMFO nanoparticles show the homogeneous distribution of all the four elements (Ni, Mn, Fe, and O) (Fig. S1†). The atomic ratio of Ni, Mn, and Fe elements is approximate 1 : 1 : 4 from ICP-OES analysis. In fact, it is possible to tune the atomic ratio of Ni_xMn_yFe₂O₄ (x + y = 1, 0 < x, y < 1) by controlling the ratio of raw materials (Ni(CH₃COO)₂ and Mn(CH₃COO)₂).

Fig. 3 SEM image (a), TEM image (b) (inset is its SAED patterns), high-magnification TEM image (c), and EDX spectrum (d) of NMFO nanoparticles.

Fig. 4a shows the wide XPS spectra of NMFO, revealing the presence of Ni, Mn, Fe, and O elements. In the Ni 2p spectrum shown in Fig. 4b, two strong peaks at 855.1 eV for Ni 2p_3/2 and 872.7 eV for Ni 2p_1/2, and one shakeup satellite (indicated as “Sat.”) are observed, indicating the oxidation state of Ni²⁺ in NMFO.⁴⁸ Fig. 4c displays the Mn 2p spectrum, in which two strong peaks at 641.2 eV for Mn 2p_3/2 and 653.1 eV for Mn 2p_1/2, and one shakeup satellite are assigned to the oxidation state of Mn²⁺ in NMFO;³³ while in the Fe 2p spectrum shown in Fig. 4d, two major peaks at 711.0 and 724.6 eV and two shakeup satellites are ascribed to Fe 2p_3/2 and Fe 2p_1/2 of Fe³⁺, respectively.³³ These results indicate that the chemical composition of NMFO nanoparticles contain Ni²⁺, Mn²⁺, and Fe³⁺, which are in good agreement with the above EDX and ICP-OES results. The formation process of NMFO nanoparticles is speculated that the Ni²⁺, Mn²⁺, and Fe³⁺ ions were nucleated under solvothermal conditions via the reaction (0.5Ni²⁺ + 0.5Mn²⁺ + 2Fe³⁺ + 4H₂O → Ni_0.5Mn_0.5Fe₂O₄ + 8H⁺) to form NMFO nanosized crystalline.

Fig. 4 XPS spectra for the NMFO: wide spectra (a), Ni 2p (b), Mn 2p (c), and Fe 2p (d).

The SEM images in Fig. 5a and b show that, after adding 0.01 g and 0.02 g OGP in NMFO/OGP-1 and NMFO/OGP-2 respectively, most of the OGP particles are embedded in NMFO nanoparticles leading to the formation of a network structure. When the OGP amount is further increased to 0.04 g (Fig. 5c and d), most of NMFO nanoparticles are coated on the surface of OGP forming the multilayer network composites. The surface of the OGP becomes coarser after the coating with the smaller NMFO nanoparticles. More detailed structural information can be obtained from the TEM results.

Fig. 5 SEM images of NMFO/OGP-1 (a), NMFO/OGP-2 (b), and NMFO/OGP (c and d).

The TEM images of NMFO/OGP-1 and NMFO/OGP-2 in Fig. 6a and b indicate that small NMCFO nanoparticles are dispersed on the surface of OGP after adding 0.01 g and 0.02 g OGP. Fig. 6c and d show that the NMFO nanoparticles with a size of around 5–30 nm are uniformly adhered to the surface of OGP. The crystal NMFO and OGP in NMFO/OGP composites could be easily differentiated by the brightness contrast in the TEM images. The HRTEM image in Fig. 6e shows the lattice with an interplanar spacing of about 0.294 nm corresponding to the (220) plane of NMFO, which is consistent with the observation in Fig. 3c. Similar to the above SEM observation, these crystal NMFO nanoparticles are attached to the surface of OGP to form composites with the network structure. The formation process of NMFO/OGP composites is speculated that the OGP are well-dispersed in the solution, which provides the substrate for the nucleation and in situ growth of NMFO nanocrystals.⁴⁹ The electrostatic attraction between the positively charged metal ions (Ni²⁺, Mn²⁺, and Fe³⁺) and the electron-rich OGP causes the anchoring of Ni²⁺, Mn²⁺, and Fe³⁺ on the OGP during the nucleation and crystallization process.⁵⁰ HOCH₂CH₂OH may act as structure-directing agents to regulate the surface state of the nanosized crystalline particles during nucleation and to avoid aggregation of the nanoparticles. Since these NMFO nanoparticles are grown up on the OGP surface, and they are naturally adhered to the OGP forming the multilayer network structure of the NMFO/OGP nanocomposite. The binding between oxidized graphene and Ni_0.5Mn_0.5Fe₂O₄ nanoparticles are formed by electrostatic interactions, noncovalent interactions, and covalent bonding through functionalization of graphene by oxidation.^51,52 In this process, the addition of more OGP leads to the additional nucleation sites, which the Ni²⁺, Mn²⁺, and Fe³⁺ of a given amount are absorbed. Thus, the alternately packed NMFO nanoparticles and OGP form a unique hierarchical network structure after adding more OGP, due to the dispersion effect of OGP. It is understandable that both sides of each OGP as nucleation centers play a key role for the growth of the NMFO/OGP network structure. Therefore, it is possible to tune the nanocomposites network structure and composition by varying the synthesis conditions such as the amount of OGP and raw chemicals.

Fig. 6 TEM images of NMFO/OGP-1 (a), NMFO/OGP-2 (b), NMFO/OGP (c and d), and high-magnification TEM image of NMFO/OGP (e).

The cross-sectional SEM images of the free-standing NMFO/OGP film are shown in Fig. 7a and b, which indicate that the film thickness is about 25 μm, and the OGP layer and NMFO/OGP layer are packed closely to form a dense multilayer structure, quite different from the loose NMFO/OGP multilayer structure film formed in the case without alternate addition of OGP (Fig. S2†). Moreover, the surface of NMFO/OGP film is quite smooth after rolling by a roller machine (Fig. 7c). Therefore, the OGP film can act as a current collector, whereas the free-standing NMFO/OGP film can be directly used as a flexible electrode. These films exhibit good flexible properties, the photographs of the processes for free-standing NMFO/OGP film by being bent 180° are shown in Fig. 8. Moreover, compared with the free-standing NMFO/OGP film before bent, these films don't show obvious difference after being bent 180° for 100 times (Fig. 7d and e), indicating the good flexible and structural stability of these materials (the Movie is shown in ESI-Blending Test-Movie†). However, the free-standing NMFO/OGP film becomes fragile when using sharp tweezer. In addition, the front surface of OGP film can act as a current collector, thus the NMFO/OGP film coated on polypropylene microporous film can also be directly used as a flexible electrode. These films are super-flexible, the photographs of the processes for NMFO/OGP coated on polypropylene microporous film by being bent 180° are shown in Fig. 9, and exhibit good structural stability after being bent 180° for 100 times (Fig. 7f and g) (the Movie is shown in ESI-Blending Test-Movie†). Moreover, the NMFO/OGP coated polypropylene microporous film doesn't become fragile when using sharp tweezer, proving the good mechanical flexibility.

Fig. 7 Cross-sectional SEM images of free-standing NMFO/OGP film (a and b) and the surface SEM image of NMFO/OGP film (c), photographs of the front side (d and f) and the other side (e and g) after being bent 100 times for free-standing NMFO/OGP film (d and e) and NMFO/OGP coated on polypropylene microporous film (f and g).

Fig. 8 Photographs of the processes for free-standing NMFO/OGP film by being bent 180° (a–e).

Fig. 9 Photographs of the processes for NMFO/OGP coated on polypropylene microporous film by being bent 180° (a–e).

As shown in Fig. 10a, the discharge and charge capacities in the first run are 1273.9 and 869.7 mA h g⁻¹ for free-standing NMFO/OGP film, and 1270.9 and 864.8 mA h g⁻¹ for NMFO/OGP coated polypropylene microporous film, and accordingly, their initial Coulombic efficiencies are around 68.3 and 69.8% in the first cycle, respectively. These irreversible capacity losses can be attributed mainly to the formation of a solid electrolyte interface (SEI) layer and the side reactions occurred during the electrochemical process.^53,54 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, Ni²⁺ to Ni, and Mn²⁺ to Mn during the initial discharge process. Meanwhile, a defined plateau is observed in the charge process at ca. 1.5–2.5 V, which is corresponding to the oxidation of Fe to Fe³⁺, Ni to Ni²⁺, and Mn to Mn²⁺ during the initial charge process, which based on the reduction–oxidation reaction: 4Li₂O + 0.5Ni + 0.5Mn + 2Fe ↔ Ni_0.5Mn_0.5Fe₂O₄ + 8Li⁺ + 8e⁻. The cycling performance in Fig. 10b indicates that the discharge capacity of the free-standing NMFO/OGP film and the NMFO/OGP coated polypropylene microporous film after 100 cycles is around 558.8 and 541.2 mA h g⁻¹, respectively, indicating the good cycling performance of the flexible and binder-free anodes compared with the bare NMFO (after 50 cycles it is around 423.5 mA h g⁻¹).

Fig. 10 Electrochemical properties: the first discharge–charge (a), and cycling property and Coulombic efficiency (b) of NMFO, free-standing NMFO/OGP film at current densities of 100 mA g⁻¹, and NMFO/OGP coated on polypropylene microporous film, rate performance at different current densities (c), and CV curves of the first three cycles (d), and 51st to 57th cycles after 50 rate discharge–charge cycles (e) for free-standing NMFO/OGP film at a scan rate of 0.1 mV s⁻¹, and Nyquist plots of all the samples at the electrode potentials from 0.70 to 0.10 V (f).

The average Coulombic efficiency after the second cycle and the average capacity fading rate during 100 cycles of free-standing NMFO/OGP film, and NMFO/OGP coated polypropylene microporous film are about 97.2 and 97.0%, and 0.36 and 0.37% per cycle, respectively. Fig. 10c reveals that the charge capacities of free-standing NMFO/OGP film are 859.2, 809.6, 590.2, 383.3, and 828.4 mA h g⁻¹ at the current densities of 100, 200, 500, 1000, and 100 mA g⁻¹, respectively. The charge capacities of free-standing NMFO/OGP film after 10 cycles (Fig. 10c) are 862.1, 823.5, 535.1, 321.4, and 806.6 mA h g⁻¹, respectively, and the retention of the capacity at 100, 200, 500, 1000, and 100 mA g⁻¹ is about 100.0, 100.0, 90.7, 83.9, and 97.4%, suggesting a good rate performance of free-standing NMFO/OGP film at different current densities. This may be attributed to the higher carbon content as well as the increased interfacial space formed between NMFO and OGP in free-standing NMFO/OGP film. Fig. 10d presents the CV curves of the free-standing NMFO/OGP film for the first three cycles at a scan rate of 0.1 mV s⁻¹, respectively. In the first scan, two cathodic peaks are observed at about 0.55 and 0.83 V, which correspond to the conversion reactions of Fe³⁺, Ni²⁺, and Mn²⁺ to their metallic states and the formation of Li₂O, respectively. The broad anodic peak from 1.25 to 2.30 V can be ascribed to the oxidation reactions of metallic Fe, Ni, and Mn. The metal oxide NMFO stores Li through reversible formation and decomposition of Li₂O. In the second scan, the reduction peaks are shifted to 0.75 and 1.50 V, which is different from the first scan, this is mainly due to the formation of a solid electrolyte interface (SEI) layer with irreversible capacity losses and the side reactions occurred during the first scan, consist with the first and second discharge–charge voltage plateau change in Fig. 10a. In addition, the peak intensity and integral areas of the 51st to 57th cycles after 50 cycles rate performance measurement are almost the same as those of the second and third cycles for free-standing NMFO/OGP film (Fig. 10e), which showed the good reversible oxidation–reduction reaction and reversible formation and decomposition of Li₂O. Because of the presence of conductive OGP, these network NMFO/OGP composites exhibit much lower electric resistance than the NMFO nanoparticles, as evidenced by the drastically reduced diameter of these semicircle at high-frequency region in the electrochemical impedance spectroscopy (EIS) patterns (Fig. 10f). As a result of the good charge-transfer impedances, lithium ion diffusion and electron transfer are facilitated to give the greatly enhanced electrochemical performance of the NMFO/OGP film composites. After 100 discharge–charge cycles, SEM images of electrode disks containing charged free-standing NMFO/OGP film (Fig. S3a and b†), and NMFO/OGP coated on polypropylene microporous film (Fig. S3c and d†) show that the disks are not broken, maintain its original textural properties, and SEI layers on the surface of the electrodes are observed, indicating the good structural stability of these electrode disks. These results indicate that NMFO/OGP film exhibits good electrochemical reversibility. The introduced OGP can highly disperse NMFO nanoparticles, preventing their aggregation, buffering the volume change, and providing efficient Li⁺ diffusion and electronic conduction pathway, which are also beneficial for better cycling stability and high rate performances.^55,56

4. Conclusions

In summary, we have prepared flexible and binder-free electrode materials using OGP and NMFO/OGP coated polypropylene microporous film via vacuum filtration of their aqueous dispersions. Comparing with the bare NMCFO nanoparticles, the obtained flexible and binder-free free-standing NMFO/OGP film and NMFO/OGP coated polypropylene microporous film have high mechanical flexibility as well as stable capacity retention (558.8 and 541.2 mA h g⁻¹ after 100 cycles at a current density of 100 mA g⁻¹) when used as anodes for Li-ion batteries. Therefore, this work paves a new method for fabrication of nanostructured metal oxide/OGP nanocomposites as binder-free and flexible anode materials for Li-ion batteries.

Acknowledgements

The authors gratefully acknowledge the supports from the National Natural Science Foundation of China (no. 51402299, and 51272252) and Hundred Talents Program of the Chinese Academy of Sciences.

Notes and references

1. J. A. Rogers, T. Someya and Y. G. Huang, Science, 2010, 327, 1603.
2. Z. Y. Fan, H. Razavi, J. W. Do, A. Moriwaki, O. Ergen, Y. L. Chueh, P. W. Leu, J. C. Ho, T. Takahashi, L. A. Reichertz, S. Neale, K. Yu, M. Wu, J. W. Ager and A. Javey, Nat. Mater., 2009, 8, 648.
3. L. B. Hu, H. Wu, F. La Mantia, Y. A. Yang and Y. Cui, ACS Nano, 2010, 4, 5843.
4. H. Nishide and K. Oyaizu, Science, 2008, 319, 737.
5. G. M. Zhou, F. Li and H. M. Cheng, Energy Environ. Sci., 2014, 7, 1307.
6. B. Dunn, H. Kamath and J. M. Tarascon, Science, 2011, 334, 928.
7. H. Wu, S. Shevlin, Q. Meng, W. Guo, Y. Meng, K. Lu, Z. Wei and Z. Guo, Adv. Mater., 2014, 26, 3338.
8. B. Liu, X. F. Wang, H. T. Chen, Z. R. Wang, D. Chen, Y. B. Cheng, C. W. Zhou and G. Z. Shen, Sci. Rep., 2013, 3, 1622.
9. B. Scrosati and J. Garche, J. Power Sources, 2010, 195, 2419.
10. H. Gwon, J. Hong, H. Kim, D. H. Seo, S. Jeon and K. Kang, Energy Environ. Sci., 2014, 7, 538.
11. L. Li, Z. Wu, S. Yuan and X. B. Zhang, Energy Environ. Sci., 2014, 7, 2101.
12. Y. H. Hu and X. L. Sun, J. Mater. Chem. A, 2014, 2, 10712.
13. E. Luais, J. Sakai, S. Desplobain, G. Gautier, F. Tran-Van and F. Ghamouss, J. Power Sources, 2013, 242, 166.
14. X. F. Li, J. L. Yang, Y. H. Hu, J. J. Wang, Y. L. Li, M. Cai, R. Y. Li and X. L. Sun, J. Mater. Chem., 2012, 22, 18847.
15. X. Zhao, C. M. Hayner, M. C. Kung and H. H. Kung, ACS Nano, 2011, 5, 8739.
16. L. Jabbour, C. Gerbaldi, D. Chaussy, E. Zeno, S. Bodoardo and D. Beneventi, J. Mater. Chem., 2010, 20, 7344.
17. C. Kang, R. Baskaran, J. Hwang, B. C. Ku and W. Choi, Carbon, 2014, 68, 493.
18. P. Zhang, J. X. Qiu, Z. F. Zheng, G. Liu, M. Ling, W. Martens, H. H. Wang, H. J. Zhao and S. Q. Zhang, Electrochim. Acta, 2013, 104, 41.
19. L. F. Shen, Q. Che, H. S. Li and X. G. Zhang, Adv. Funct. Mater., 2014, 24, 2630.
20. Q. Fan, L. X. Lei, G. Yin and Y. M. Sun, Chem. Commun., 2014, 50, 2370.
21. L. Noerochim, J. Z. Wang, S. L. Chou, D. Wexler and H. K. Liu, Carbon, 2012, 50, 1289.
22. L. B. Hu, N. Liu, M. Eskilsson, G. Y. Zheng, J. McDonough, L. Wagberg and Y. Cui, Nano Energy, 2013, 2, 138.
23. J. F. Liang, Z. Cai, Y. Tian, L. D. Li, J. X. Geng and L. Guo, ACS Appl. Mater. Interfaces, 2013, 5, 12148.
24. Y. Liu, W. Wang, L. Gu, Y. W. Wang, Y. L. Ying, Y. Y. Mao, L. W. Sun and X. S. Peng, ACS Appl. Mater. Interfaces, 2013, 5, 9850.
25. S. J. R. Prabakar, Y. H. Hwang, E. G. Bae, S. Shim, D. Kim, M. S. Lah, K. S. Sohn and M. Pyo, Adv. Mater., 2013, 25, 3307.
26. X. Wang, X. Q. Cao, L. Bourgeois, H. Guan, S. M. Chen, Y. T. Zhong, D. M. Tang, H. Q. Li, T. Y. Zhai, L. Li, Y. Bando and D. Golberg, Adv. Funct. Mater., 2012, 22, 2682.
27. X. L. Huang, D. Xu, S. Yuan, D. L. Ma, S. Wang, H. Y. Zheng and X. B. Zhang, Adv. Mater., 2014, 26, 7264.
28. D. L. Ma, Z. Y. Cao, H. G. Wang, X. L. Huang, L. M. Wang and X. B. Zhang, Energy Environ. Sci., 2012, 5, 8538.
29. C. M. Ban, Z. C. Wu, D. T. Gillaspie, L. Chen, Y. F. Yan, J. L. Blackburn and A. C. Dillon, Adv. Mater., 2010, 22, E145.
30. X. L. Huang, R. Z. Wang, D. Xu, Z. L. Wang, H. G. Wang, J. J. Xu, Z. Wu, Q. C. Liu, Y. Zhang and X. B. Zhang, Adv. Funct. Mater., 2013, 23, 4345.
31. L. Zhou, H. B. Wu, T. Zhu and X. W. Lou, J. Mater. Chem., 2012, 22, 827.
32. L. Zhou, D. Y. Zhao and X. W. Lou, Adv. Mater., 2012, 24, 745.
33. Z. Zhang, Y. Wang, Q. Tan, Z. Zhong and F. Su, J. Colloid Interface Sci., 2013, 398, 185.
34. Y. S. Fu, Y. H. Wan, H. Xia and X. Wang, J. Power Sources, 2012, 213, 338.
35. Z. Zhang, Y. Wang, M. Zhang, Q. Tan, X. Lv, Z. Zhong and F. Su, J. Mater. Chem. A, 2013, 1, 7444.
36. Z. Zhang, G. Kan, W. Ren, Q. Tan, Z. Zhong and F. Su, RSC Adv., 2014, 4, 33769.
37. W. H. Shi, X. H. Rui, J. X. Zhu and Q. Y. Yan, J. Phys. Chem. C, 2012, 116, 26685.
38. H. Xia, D. D. Zhu, Y. S. Fu and X. Wang, Electrochim. Acta, 2012, 83, 166.
39. M. C. Li, W. X. Wang, M. Y. Yang, F. C. Lv, L. J. Cao, Y. G. Tang, R. Sun and Z. G. Lu, RSC Adv., 2015, 5, 7356.
40. A. Dey, S. Panja, A. K. Sikder and S. Chattopadhyay, RSC Adv., 2015, 5, 10358.
41. Z. Zhang, Y. Wang, D. Li, Q. Tan, Y. Chen, Z. Zhong and F. Su, Ind. Eng. Chem. Res., 2013, 52, 14906.
42. S. S. Tao, W. B. Yue, M. Y. Zhong, Z. J. Chen and Y. Ren, ACS Appl. Mater. Interfaces, 2014, 6, 6332.
43. B. Rangasamy, J. Y. Hwang and W. Choi, Carbon, 2014, 77, 1065.
44. W. J. Zhu, H. Huang, Y. P. Gan, X. Y. Tao, Y. Xia and W. K. Zhang, Electrochim. Acta, 2014, 138, 376.
45. Y. Y. Liang, H. L. Wang, P. Diao, W. Chang, G. S. Hong, Y. G. Li, M. Gong, L. M. Xie, J. G. Zhou, J. Wang, T. Z. Regier, F. Wei and H. J. Dai, J. Am. Chem. Soc., 2012, 134, 15849.
46. M. K. Shobana and S. Sankar, J. Magn. Magn. Mater., 2009, 321, 2125.
47. Z. Zhang, Q. Tan, Y. Chen, J. Yang and F. Su, J. Mater. Chem. A, 2014, 2, 5041.
48. S. Verma, A. Kumar, D. Pravarthana, A. Deshpande, S. B. Ogale and S. M. Yusuf, J. Phys. Chem. C, 2014, 118, 16246.
49. H. L. Wang, J. T. Robinson, G. Diankov and H. J. Dai, J. Am. Chem. Soc., 2010, 132, 3270.
50. R. Zan, U. Bangert, Q. Ramasse and K. S. Novoselov, Nano Lett., 2011, 11, 1087.
51. J. Liu, A. G. Rinzler, H. J. Dai, J. H. Hafner, R. K. Bradley, P. J. Boul, A. Lu, T. Iverson, K. Shelimov, C. B. Huffman, F. Rodriguez-Macias, Y. S. Shon, T. R. Lee, D. T. Colbert and R. E. Smalley, Science, 1998, 280, 1253.
52. F. T. Edelmann, Angew. Chem., Int. Ed., 1999, 38, 1381.
53. Z. Zhang, W. Ren, Y. Wang, J. Yang, Q. Tan, Z. Zhong and F. Su, Nanoscale, 2014, 6, 6805.
54. S. Goriparti, E. Miele, F. De Angelis, E. Di Fabrizio, R. P. Zaccaria and C. Capiglia, J. Power Sources, 2014, 257, 421.
55. X. H. Wang, X. W. Li, X. L. Sun, F. Li, Q. M. Liu, Q. Wang and D. Y. He, J. Mater. Chem., 2011, 21, 3571.
56. L. Q. Tao, J. T. Zai, K. X. Wang, H. J. Zhang, M. Xu, J. Shen, Y. Z. Su and X. F. Qian, J. Power Sources, 2012, 202, 230.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra03556b

---