A hybrid gel of hypergravity prepared NiO and polyaniline as Li-ion battery anodes

Abstract

The hybrid gel of nickel oxide (NiO) and polyaniline (PANI) was prepared and applied as Li-ion battery anodes. Hypergravity NiO nanoparticles was obtained by the combination of hydrothermal treatment with a high gravity field of 1000g in the 1,2-dichlorobenzene/water system at 120 °C for 0.5 h. The porous PANI gel around the surface of the NiO nanoparticles can help the electrons and ions transfer between the NiO anode and electrolyte. Electrochemical measurements show that the NiO/PANI gel electrode composites exhibit a high reversible capacity of 455 mA h g⁻¹ after 100 cycles at a charge–discharge rate of 0.5 C. The hybrid gel electrode may provide a favorable approach to optimize the electrochemical performance of electrode materials with large volume expansion and low electronic conductivity.

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1. Introduction

Developing rechargeable lithium-ion batteries with high energy density and long cycle life is very meaningful for applications in portable electronics, and hybrid and electric vehicles.^1–4 Nickel oxide (NiO) has attracted a great deal of attention as an alternative anode material for Li-ion batteries due to its high theoretical gravimetric capacity, good safety and environmental benignity.^5–10 However, nickel oxide suffers from the problems of cycle performance due to large volume expansion during charge–discharge cycles and low electronic conductivity. One method to increase the performance of NiO as anode material is to optimize its morphology. Nano-structured anode materials are seemingly the most promising candidates to improve the cycle and rate capability due to the short electron/ion diffusion length and the large specific surface area.^11–15 Hydrothermal method is one of the most conventional ways to prepare nanostructured materials. Recently, the combination of hydrothermal method and high gravity field has been employed to prepare nanomaterials with low sizes. Chen group has prepared CaCO₃ nanoparticles with 17–36 nm size, nanofibrils of aluminum hydroxide of 1–10 nm in diameter and 50–300 nm in length as well as nanoparticles of SrCO₃ with a mean size of 40 nm.¹⁶ Our group also synthesized CdS polycrystalline microspheres and flake-like MnCO₃ film at the various gravity fields.^17,18 To the best of our knowledge, synthesis materials by hydrothermal method with high gravity field for the Li-ion battery anode have not been reported before.

Another method to increase the performance of NiO as anode material is to immobilize them within conducing polymer matrix. Up to now, polypyrrole (PPy), poly(9,9-dioctylfluorene-co-suorenone-co-methylbenzoic acid) (PFFOMB), polyaniline and sodium alginate, have been used to improve the cycle life of electrodes materials.^19–25 The electronically conductive battery binders other than the conventional polyvinylidene difluoride (PVDF) can improve battery performance due to the good adhesion between the groups of conducting the polymers and the metal oxide.^26–28

In this experiment, the conducting polyaniline (PANI) hydrogel was selected to combine with NiO nanoparticles as the Li-ion battery anodes. Our system consists of three elements: the hypergravity prepared NiO, precursor of PANI and water. The polyaniline hydrogels were coating on the surface of NiO nanoparticles by the in situ polymerization. The 3D network of polyaniline hydrogels can assist the electrons and ions transfer between NiO anode and electrolyte. The facile preparation of NiO/PANI gel was shown in Fig. 1.

Fig. 1 Schematic diagram of the preparation of NiO/PANI gel electrodes.

2. Experimental

2.1. Materials

1,2-Dichlorobenzene, ammonium persulfate, nickel chloride hexahydrate and urea were purchased from Sinopharm Chemical Reagent Co. Ltd (China). Aniline and phytic acid were obtained from TCI Shanghai Development Co. Ltd (China). All the other reagents were of analytical purity and used without further purification. Deionized water was used throughout the experiments.

2.2. Preparation of NiO and NiO/PANI nanocomposite hydrogel

The nickel bicarbonate powders were prepared by the hydrothermal method with 1000g gravity field in the 1,2-dichlorobenzene/water system at 120 °C for 0.5 h. A turbine-type ultracentrifuge capable of generating a gravitational field of over 1000g (1g = 9.8 m s⁻²) at high temperatures up to over 200 °C for a period of over 10 h was used for this study. In a typical process of preparation, 2.0 mL 1,2-dichlorobenzene was firstly added into the Teflon-lined stainless steel autoclave of 12.0 mL capacity. And then 6 mL nickel chloride and urea mixture solution were added into the autoclave. The autoclave was sealed and then mounted in a titanium alloy rotor with an outer diameter of 50 mm. The autoclave was heated at 120 °C for 0.5 h under high gravity, and then cooled to room temperature. Because of the different density between oil phase and solution phase, under the effect of high gravity field, the Ni(HCO₃)₂ nanoparticles were formed on the interface of 1,2-dichlorobenzene and aqueous solution finally (Fig. S1, ESI†). The preparation mechanism of the ultracentrifuge apparatus and the Ni(HCO₃)₂ nanoparticles growth process was shown in Fig. S2.† The green precipitates were washed with distilled water and absolute alcohol several times to remove the excessive reactants and 1,2-dichlorobenzene, followed by thermal treatment at 400 °C for 3 h on the air condition. The non-hypergravity NiO nanoparticles (without of high gravity) was prepared by the same way.

The NiO/PANI gel for Li-ion battery anode was prepared by a very simple method: 0.09 mmol aniline monomer, 80 mg NiO nanoparticles, 0.03 mmol phytic acid and 0.04 mmol ammonium persulfate were added into 1.2 mL distilled water in sequence and subjected to 1 min bath sonication. The NiO/PANI gel was formed after 2 minutes later, and then the viscous gel was bladed onto a copper foil current collector and dried to form a uniform film over a large area. The NiO/PANI gel composite film was washed with distilled water to remove the excessive reactants, followed by vacuum drying overnight.

2.3. Material characterization

The X-ray diffraction (XRD) analysis was performed on a D8 Advance testing machine (Bruker). In continuous scan mode between the 2θ range of 10–80° with a step size of 0.02 with Cu Kα radiation (λ = 0.15406 nm). The samples were grind sufficiently before test and keep the dryness condition.

The rheological properties analysis was performed on the RS6000 rheometer (Thermo Scientific, Germany). For the dynamic frequency sweep, the stress value is 1 Pa and the scan range between 0.01 Hz to 1 Hz.

The transmission electron microscopy (TEM) measurement was performed on the JEM-2100 TEM of JEOL (Japan). The NiO/PANI nanocomposite hydrogel was dispersed in ethanol and drops on the copper support grid, vacuum dried the sample 12 h before the measurement.

Scanning electron microscope (SEM) and energy dispersive X-ray spectroscopic (EDX) mapping tests were performed on the Hitachi S4800 field emission SEM. The sample was sprayed gold before test.

Fourier transform infrared spectroscopy (FTIR) characterization was performed on the Nicolet FTIR. The sample was mixed with potassium bromide (KBr) and pressed into sheet. Both the NiO/PANI nanocomposite hydrogel sample and KBr were dried for 12 hours at 120 °C.

2.4. Electrochemical measurements

The electrochemical performance was evaluated using CR2016 coin cell. CR2016 coin-type half-cell was assembled in an argon-filled glove box using the lithium foil as counter electrodes. The electrolyte consisted of a solution of 1 M LiPF₆ in ethylene carbonate and dimethyl carbonate (EC : DMC : EMC = 1 : 1 : 1 in volume). The NiO/PVDF composite electrodes were consists of NiO (80 wt%), acetylene black (10 wt%) and polyvinylidene fluoride (PVDF) binder (10 wt%), uniformly mixed in N-methyl pyrrolidinone (NMP) solvent and then bladed onto a copper foil current collector and dried to form a uniform film. The mass loading of the NiO within the prepared NiO/PANI gel electrode was about 3 mg cm⁻², same to the NiO/PVDF.

The discharge–charge measurements were performed on a Land CT2001A tester (Wuhan, China) within the voltage window of 0.01–3.0 V. The cyclic voltammetry (CV) profile of the electrodes were tested in the voltage range of 0.01–3.0 V versus Li/Li⁺ at a scanning rate of 0.5 mV s⁻¹ using an Autolab PGSTAT 302N. Electrochemical impedance spectroscopy (EIS) of the electrode was also performed with Autolab PGSTAT 302N electrochemical workstation in the frequency range of 100 kHz to 0.01 Hz.

3. Results and discussion

3.1. Crystal structure of hypergravity NiO and NiO/PANI composites

Fig. 2 illustrates the X-ray diffraction (XRD) patterns of the NiO/PANI composites and hypergravity NiO. Typical diffraction peaks can be observed at 37.2°, 43.2°, 62.7°, 75.3° and 79.3°, corresponding to NiO (101), (012), (110), (113) and (202) faces, respectively (JCPDS 44-1159). No other diffraction peaks were obtained in the XRD pattern. It's demonstrated the pure cubic NiO phase was acquired by the hydrothermal method with high gravity field. The NiO/PANI pattern shows a broad peak at 30°, which was attributed to the PANI crystal characteristic peak.²⁹

Fig. 2 XRD patterns of hypergravity NiO and NiO/PANI gel.

3.2. Rheological properties of PANI and NiO/PANI composites

To understand the gelation kinetics of our system, we conducted frequency sweep measurements using a rheometer, for monitoring the storage modulus (G′) and loss modulus (G′′), as a function of frequency. Fig. 3A shows the values of storage modulus (G′) of PANI and NiO/PANI are significantly larger than their loss modulus (G′′) from 0.01 Hz to 1 Hz, confirming the substantial gel-like state for the PANI and NiO/PANI hybrid gel. The G′ value of NiO/PANI gel is larger than that of PANI due to the non-covalent interactions of PANI and NiO.

Fig. 3 (A) Dynamic frequency sweep between 0.01 Hz to 1 Hz of the PANI and NiO/PANI gel. (B) FTIR spectra of NiO/PANI gel.

Fig. 3B shows the Fourier transform infrared spectroscopy (FTIR) spectra of NiO/PANI gel. The peak at 3430 cm⁻¹ can be attributed to –OH group of adsorbed water molecules.³⁰ The band at 3234 cm⁻¹ and 2922 cm⁻¹ are corresponds to N–H and C–H stretching vibration.³¹ The characteristic peaks at 1580 and 1490 cm⁻¹ are attributed to C=C deformation of quinoid ring (Q) and benzenic ring (B). Band of 1300 cm⁻¹ is corresponds to C–N stretching of aromatic amine. The peak at 1138 cm⁻¹ is attributed to N=Q=N stretching of polyaniline.³² The band 400–800 cm⁻¹ can be attributed to Ni–O.³⁰

3.3. The morphology of the Ni(HCO₃)₂, NiO and NiO/PANI composites

The scanning electron microscope (SEM) images of the hypergravity Ni(HCO₃)₂ (with high gravity) and non-hypergravity Ni(HCO₃)₂ (without of high gravity) are shown in Fig. 4A and B. The hypergravity Ni(HCO₃)₂ was flower-like shape and constructed by nanoflakes (Fig. 4A). The Ni(HCO₃)₂ which is synthesized by traditional hydrothermal method has no typical shape and formed the bulk more than 500 nm in diameters (Fig. 4B). Morphology of the hypergravity NiO and non-hypergravity NiO were illustrate in Fig. 4C and D. The SEM image in Fig. 4C clearly indicates that the particle size of hypergravity NiO is about 20–40 nm. In comparison, the non-hypergravity NiO has a significantly aggregation and the bulk diameter is more than 200 nm (Fig. 4D). Compared with the traditional hydrothermal method, combination of hydrothermal method with high gravity field has obvious effects on the control of particle size and morphology.

Fig. 4 (A–D) SEM images of hypergravity Ni(HCO₃)₂, non-hypergravity Ni(HCO₃)₂, hypergravity NiO and non-hypergravity NiO (E) porous network structure SEM image of the NiO/PANI gel. (F) TEM image shows that the NiO nanoparticles are coated with PANI polymer layer.

Fig. 4E and F show the SEM and TEM images of the NiO/PANI nanocomposite gel. This conducting nanocomposite gel exist a number of porous network structure, which can help the ion and electron transfer. The TEM image demonstrates the typical core–shell structure that the PANI coated on the outside of NiO and formed the conducting nanocomposite gel. To further confirm the existing of NiO/PANI hybrid gel, energy dispersive X-ray (EDX) spectroscopic mapping is employed. The existing of C element (Fig. 5B), Ni element (Fig. 5C) and N element (Fig. 5D) in the mapping indicated that NiO and PNAI were intimate coexistence in the final gel.

Fig. 5 (A) SEM image of the NiO/PANI gel. (B–D) Energy dispersive X-ray (EDX) spectroscopy mapping images of the C, Ni and N elements.

3.4. Electrochemical performance

The formed NiO/PANI gels with a conductive three-dimensional network have an extremely important role for the electrochemical performance of the anode materials. A coin-type cell was used to evaluate the electrochemical properties of the material as anode electrode. The cyclic voltammetry (CV) profile of the NiO/PANI gel electrodes were tested in the voltage range of 0.01–3.0 V versus Li/Li⁺ at a scanning rate of 0.5 mV s⁻¹. As shown in Fig. 6, a strong redox peak at around 0.3 V can be noticed in the first cathodic scan, it's owe to the initial reduction of NiO to Ni and the formation of amorphous Li₂O and solid electrolyte interface.⁸ The oxidation peaks at around 1.5 V and 2.1 V in the first anodic scan corresponds to the NiO formation and Li₂O decomposition.⁷ The second and third cycle show the more stable redox peak around 0.9 V, 1.5 V and 2.1 V due to the steady electrochemical reaction process. The reversible reaction mechanism can be summarized as follows:³³

NiO + 2Li⁺ + 2e⁻ ↔ Ni + Li₂O

Fig. 6 Cyclic voltammetry (CV) curves of the NiO/PANI electrode at a scan rate of 0.5 mV s⁻¹.

The cycling behavior of hypergravity NiO/PANI electrode and non-hypergravity NiO/PANI electrode between 0.01 and 3 V were displayed in Fig. 7A, respectively. As indicated in Fig. 7A, the hypergravity NiO/PANI electrode shows better cycle performance in comparison with the non-hypergravity NiO/PANI electrode at the rate of 0.1C. After 30 charge and discharge cycles, hypergravity NiO/PANI electrode has still 544.4 mA h g⁻¹ capacities which are more than 3.7 times higher than the non-hypergravity NiO/PANI electrode (145.9 mA h g⁻¹). The smaller size of hypergravity-NiO nanoparticles should be the main cause for the better stability due to the shorter diffusion path for lithium ions and lower pulverization ratio during charge/discharge cycles.

Fig. 7 (A) Cycling performance of hypergravity NiO/PANI and non-hypergravity NiO/PANI electrode at the rate of 0.1C. (B) Cycling performance of NiO/PANI and NiO/PVDF electrode at the rate of 0.5C.

Fig. 7B shows the cycle performance of NiO/PANI and NiO/PVDF electrode. At a charge/discharge current of 0.5C, the NiO/PANI composite electrode exhibits a relatively stable reversible lithium capacity of 455 mA h g⁻¹ for 100 cycles. As a control, the NiO electrode with traditional PVDF binder exhibits the quickly decreased capacity after 35 cycles, which has only 100 mA h g⁻¹ capacity after 35 cycles. The excellent cycle stability of NiO/PANI gel electrode should be ascribed to the conducting PANI network, which prevents the aggregation of NiO nanoparticles during charge and recharge, improving the cycle property effectively.

We had also constructed the NiO/PANI mixture electrode by simply mixed the NiO and PANI powders. The impedance of the NiO/PANI mixture electrode is much larger than NiO/PANI composite electrode before and after 100 cycles which is indicated that the in situ polymerization method could overcome the low electronic conductivity of metal oxides efficiently (Fig. 8A). The cell impedance measurements of NiO/PANI gel electrode also proved the cycle stability, which has no obvious change even after 100 cycles. The rate capability of our electrodes was shown in Fig. 8B. The good performances are primarily attributing to the smaller size of the hypergravity prepared NiO and the conducting PANI network.

Fig. 8 (A) EIS spectra of the NiO/PANI composite electrode and NiO/PANI mixture electrode in the frequency range between 0.01 Hz and 100 kHz. (B) Rate capability of NiO/PANI electrode.

4. Conclusions

In summary, we obtained the hybrid gel by the combination of the hypergravity prepared NiO and the conducting polyaniline. The non-covalent interactions of NiO and PANI can significantly increase the strength of the gel. The gel electrode exhibited the significantly increased cycle performance as the electrode materials due to the three dimensional conducting network of PANI. The immobilization within the conducting gel matrix is a promising approach to overcome the unexpected volume expansion and the low electronic conductivity of metal oxides as anode materials.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21274111, 51473123) and the Recruitment Program of Global Experts.

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Footnote

† Electronic supplementary information (ESI) available: Mechanism image and XRD pattern. See DOI: 10.1039/c5ra17929g

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