Preparation of three-dimensional free-standing nano-LiFePO₄/graphene composite for high performance lithium ion battery

Abstract

Electric vehicles and high-power electrical appliances demand batteries of a high rate discharge performance, but it is still a challenge due to large electric resistance for very fast charge transport. In this paper, a nano-LiFePO₄/graphene (G/LiFePO₄) composite is synthesized by one-pot in situ hydrothermal method. The LiFePO₄ nanoparticles are wrapped by graphene sheets on a three-dimensional (3D) free-standing graphene foam obtained from the self-assembling of graphene oxide, which is used as a highly conductive current collector. The as-prepared G/LiFePO₄ composite has a 3D conductive network and can be directly used for the cathode without any conductive carbon black, binder or aluminum current collector. The electrochemical measurements show it has excellent rate performance, the 10C-rate specific discharge capacity is 115 mA h g⁻¹. The overall electrochemical performance from this 3D self-assembly free-standing structure is superior in the similar reported system.

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1. Introduction

Since the rechargeable lithium ion batteries were developed by Sony in the early 1990s, they are widely applied to consumer electronics and large electrical appliances such as power stations, electric vehicles and hybrid electric vehicles to solve the serious risk of the depletion of non-renewable resources and the air pollution in large urban areas.^1–3 The olivine LiFePO₄ has emerged as the critical cathode material with many appealing features, such as long cycle life, high capacity (170 mA h g⁻¹), low cost, suitable voltage (3.4 V versus Li⁺/Li), environmental benignity and high thermal stability, has been considered as a promising cathode for lithium ion batteries (LIBs).^4–7 However, the low electronic conductivity (10⁻⁹ to 10⁻¹⁰ S cm⁻²) and the low diffusion of Li⁺ ions across the LiFePO₄/FePO₄ boundary limiting its widespread application in LIBs.⁸

To overcome the demerits, numerous efforts have been made to enhance the conductivity of LiFePO₄ particles, including coating or admixing with electronically conductive materials,^9–11 reducing particle size and morphology control.^12–14 Therein, graphene is a favorite additive because it's an ideal substrate used for growing and anchoring insulating materials due to its high charge carrier mobility (20 m² V⁻¹ s⁻¹), high theoretical surface area of 2630 m² g⁻¹, and a broad elector chemical window.^15,16 Macromolecular graphene sheets coated onto LiFePO₄ help to form an interparticle conductive matrix and lead to good electronic interparticle connection.^17–19

This paper reports a facile in situ one-pot hydrothermal method for preparing a conductive G/LiFePO₄ composite. LiFePO₄ nanoparticles loading on 3D graphene network own rapid electronic transmission and short lithium ion transfer pathway. Furthermore, the free-standing composite can be used for cathode directly without carbon black, poly (vinylidene fluoride) (PVDF) and aluminum current collector, which can improve the energy density of battery. When used in LIBs, it shows an excellent rate performance of 115 mA h g⁻¹ at 10C.

2. Experimental

2.1 Synthesis of graphene oxide (GO)

Graphene oxide were prepared by a modified Hummers method. Specifically, 2 g graphite flakes and 2 g NaNO₃ were added into a beaker and 96 mL H₂SO₄ was slowly dropped in at 0 °C. Then 12 g KMnO₄ was slowly added to the mixture accompanied by continuous magnetic stirring. After stirring for 2 h, the reactants were heated to 35 °C and keeping for 2 h. Then 80 mL deionized water was slowly introduced and heated to 90–95 °C for 10 min. Following 10 mL H₂O₂ (30%) and 2000 mL deionized water were added to the mixture after it cooling to room temperature. The GO paste were dialyzed until its pH reached 7.0. Then added moderate water dilutes the graphite oxide paste and slightly stirring. The resulting graphene oxide solution was collected by centrifugation and keeping the limpid solution. The complete transformation of exfoliated graphite oxide into GO was achieved by ultrasonic treatment.

2.2 Synthesis of the G/LiFePO₄

The G/LiFePO₄ composite was synthesized by a one-pot in situ hydrothermal method, followed by modification through heat treatment. Typically, 2 mmol FeSO₄ and LiH₂PO₄ (the molar ratio of Li : Fe : PO₄ was 1 : 1 : 1) and 1 mL anhydrous ethylenediamine were orderly dissolved into 40 mL (1.5 mg mL⁻¹) as prepared GO solution. This mixture was then transferred into a 50 mL Teflon-lined stainless steel autoclave and maintained at 200 °C for 6 h. The as-prepared products were obtained by freeze drying and heat treatment at 700 °C for 6 h under flowing H₂/Ar₂ atmosphere (Ar₂ 200 mL min⁻¹, H₂ 10 mL min⁻¹). When assembling the coin cell, at first a part of the as-prepared G/LiFePO₄ composite was cut into a disc, and pressed into an electrode slice, then the electrode slice was assembled into coin cell.

2.3 Characterization and electrochemical measurements

The phase purity and crystalline structure of the samples were characterized by a X-ray diffraction (XRD) equipped with Cu Kα radiation operated at 45 kV and 200 mA. The particle size and morphology were observed by scanning electron microscope (SEM, JSM-6510). The GSs were characterized with Raman spectroscopy having laser excitation energy of 514 nm in Renishaw in Via Reflex laser Raman. The graphene content in the samples was revealed by thermogravimetric analysis (TGA) with a NETZSCHSTA 449F3 simultaneous thermogravimetric analyzer under air in the temperature range from 30 to 800 °C with a heating rate of 10 °C min⁻¹. The charge and discharge capacities were measured with coin cells in which a lithium metal foil was used as the counter electrode. The electrolyte employed was 1 M solution of LiPF₆ in ethylene carbonate and dimethyl carbonate (EC + DMC) (1 : 1 in volume). Electrochemical impedance spectroscopy tests were carried out with the frequency ranging from 10⁻² to 10⁵ Hz by CHI760b electrochemical work station, and cyclic voltammetry (CV) tests were examined in the voltage range of 2.5–4.2 V at a scan rate of 0.1 mV s⁻¹.

3. Results and discussion

Scheme 1 shows the illustration of one-pot in situ hydrothermal method for the synthesis of LiFePO₄. In this reaction, the molar ratio of LiH₂PO₄ : FeSO₄ is 1 : 1, the ethylenediamine can make the GO solution been alkaline, and it also can reduced the GO by dislodge the hydroxyl and carboxyl from the GO surface due to its reducibility.²⁰ During the hydrothermal process, it's easy for GO sheets to assemble into 3D cylinder. Meanwhile LiFePO₄ dissolve out and embedded into the graphene 3D-assembly because of the strong absorption of GO. After freeze drying and annealing, the as-prepared G/LiFePO₄ shows a cylindrical free-standing 3D structure, and has a definite strength as shown in Fig. 1a. The GO was converted to graphene during the annealing at 700 °C for 6 h under flowing H₂/Ar₂ atmosphere.²¹

Scheme 1 Illustration of the formation process of G/LiFePO₄ 3D-assembled cylinder.

Fig. 1 The 3D-assemble cylinder (a), SEM images of G/LiFePO₄ (b–d).

Fig. 1b shows that the 3D-assemble graphene framework has a well-defined interconnected 3D porous network and the pore size is in the submicrometer to several micrometer ranges. This 3D structure is formed by crosslinking of intramolecular and intermolecular dehydration reaction during the process of GO reduction under the hydrothermal treatment. Meanwhile, the LiFePO₄ nucleation occurred at the GO sheets. Fig. 1c indicates that LiFePO₄ particles are uniformly dispersed into the 3D graphene cylinder. The particles have a small size less than 100 nm, which are wrapped by graphene layers tightly, as shown in Fig. 1d.

From XRD pattern of G/LiFePO₄ (Fig. 2), all Bragg peaks were indexed to orthorhombic olivine LiFePO₄ (JCPDF no. 77-0179), indicating that the obtained samples are of a high purity and a high crystallinity. Moreover, the G/LiFePO₄ shows well-crystallized which can speculated that the 16 percents of graphene has on effects on the crystal structure of LiFePO₄. The Raman spectra of G/LiFePO₄ are shown in Fig. 3, two strong Raman peaks around 1355 cm⁻¹, 1600 cm⁻¹ are attributed to the D band and G band of graphene, respectively. The D band indicates the disordered graphitic crystal stacking of the graphene nanosheets, and the G band indicates the localized in plane sp² domains of the graphene nanosheets, while the I_D/I_G value generally provides a useful index for comparing the degree of disordering in the carbon found in samples, where a higher I_D/I_G ratio indicates a greater degree of disorder in carbon arrangement.^22,23 The I_D/I_G value of G/LiFePO₄ is 1.47, indicating that the presence of graphite carbon. There is a weak Raman peak at 2700 cm⁻¹ indicates the 2D band of graphene, which means that GO was well reduced after heat treatment under H₂/Ar₂ mixed atmosphere.

Fig. 2 XRD pattern of G/LiFePO₄.

Fig. 3 Raman spectra of G/LiFePO₄.

The carbon content of the G/LiFePO₄ composites can be calculated by the TGA curves (Fig. 4). The LiFePO₄ heated in air can be oxidized to Li₃Fe₂(PO₄)₃ and Fe₂O₃, according to the following equation

LiFePO₄ + ¼O₂ → ⅓Li₃Fe₂PO₄ + ⅙Fe₂O₃

Fig. 4 TGA curves of the G/LiFePO₄.

And the weight increased by 5.07 wt% theoretically.²⁴ After heated, the weight percent of the G/LiFePO₄ is 88.6%. The weight loss is caused by the carbon material. Therefore, the graphene content of the G/LiFePO₄ is about 16 wt%.

In order to evaluate the conductivity of G/LiFePO₄, the AC impedance was performed as shown in Fig. 5. The semicircle at high frequency corresponded to the charge transfer impedance at the interface between cathode and electrolyte and the linear tail at low frequency was attributed to the Warburg impedance, which restricted the lithium ion diffusivity within LiFePO₄ particles. From comparing the diameters of the semicircles, the impedance of the LiFePO₄ electrode is larger than that of the G/LiFePO₄ electrode. The charge transfer resistance (R_ct) of G/LiFePO₄ is ∼66, which was much lower than pristine LiFePO₄ (∼150), indicating that the inner 3D graphene network enhances the conductivity of G/LiFePO₄ electrode. Concretely, the decrease of the impedance means the developing of its kinetics, which may result from the nanoparticle of LiFePO₄ and the 3D graphene electric network reduce the charge transfer resistance. The R_ct decreased to 30 after 50 cycles can ascribe to the nanosizing of LiFePO₄ particles, which achieve a higher surface area and reduce the charge transfer resistance.²⁵

Fig. 5 Electrochemical impedance spectroscopy of the pure LiFePO₄ and G/LiFePO₄.

Fig. 6 shows the CV curves of G/LiFePO₄ and pure LiFePO₄ electrode, both the curves shows a couple of reduction and oxidation peaks, which corresponds to the two-phase charge/discharge reaction of Fe²⁺/Fe³⁺ redox. Compared to the two curves, the G/LiFePO₄ shows the sharper redox peaks and higher current peaks than pure LiFePO₄, which implying the improved electrode kinetics and the better electrical conductivity of G/LiFePO₄. It was further supported by the decreasing size of voltage separation between the redox peaks, which suggests better reversibility of lithium insertion/extraction related to LiFePO₄ electrode. And the large integral area of G/LiFePO₄ curve (0.375 W) means a higher capacity than pure LiFePO₄ (0.367 W). All the above results suggest that the G/LiFePO₄ has better kinetics due to the improved of electrical conductivity by 3D graphene network.

Fig. 6 Cyclic voltammetry characteristics of pure LiFePO₄ and G/LiFePO₄ electrode at a scanning rate of 0.1 mV s⁻¹.

Fig. 7a displays the charge/discharge curves of the G/LiFePO₄ from 0.2C to 10C. It can be seen that the voltage different (0.35 V at 10C) between the charge and discharge plateaus has obvious decrease especially at high rates comparing to the pure LFP (0.65 V) in our reported work,²⁶ which can owing to the kinetics of electron transport and lithium ion diffusion in the electrode system are improved by the 3D porous graphene network. The cell voltage is 3.45 V at 0.2C and decreases with the increasing current density, the cell voltage drops to about 3.2 V at 10C and still have a flat voltage plateau.

Fig. 7 (a) Charge–discharge curves for G/LiFePO₄ composite between 2.5 and 4.2 V at different current densities (b) rate performance of pure LiFePO₄ and G/LiFePO₄ composite. The specific capacity is calculated based on the mass of the LiFePO₄.

Fig. 7b shows the comparison of the rate performance between the pure LiFePO₄ and the G/LiFePO₄. Under all charge/discharge rates, the specific capacities of G/LiFePO₄ are higher than those of the pure LiFePO₄. For G/LiFePO₄, rate capacity reached to 160, 152, 145 and 130 mA h g⁻¹ at 0.2, 1, 2, and 5C, respectively. Even at the rate of 10C, the specific discharge capacity is still 115 mA h g⁻¹ 71.9% of the initial capacity. All the rate capacity is much higher than the pure LiFePO₄ (∼110, ∼90, ∼78 and ∼63 mA h g⁻¹ at 0.2, 1, 2, and 5C respectively, ∼38 mA h g⁻¹ at 10C only 34.5% of the initial capacity).²⁶ In addition, the capacity can be recovered to 154 mA h g⁻¹ when the current density is decreased to 0.2C, implying that the G/LiFePO₄ has good electrochemical reversibility and structural stability. The improved capacity indicate that the 3D graphene network offers the possibility of increasing the speed of electron migration, and the embedded form make it stability for LiFePO₄ and also act as a bridge to increase the area to electrons and lithium ions thus reducing the inert zones, which is a great help to maintaining high capacity even at high charge–discharge rate.

Fig. 8 shows the cycling performance of pure LiFePO₄ and the G/LiFePO₄ at 1C. The capacity of the G/LiFePO₄ is much higher than pure LiFePO₄, which is because the prepared LiFePO₄ nanoparticles anchoring on the surface of graphene, forming a highly conductive network, improved the electronic conductivity of the cathode. It is clearly seen that the cyclic performance of the G/LiFePO₄ is more stable than that of the pure LiFePO₄. It can be seen that the G/LiFePO₄ sample exhibited good capacity retention with the continuous charge–discharge processes, and its capacity retention was 94.2% after 100 cycles, which is obvious better than the pure LiFePO₄ sample, its capacity declined from 100 mA h g⁻¹ at the first cycle to 76.4 mA h g⁻¹ at the 100th cycle, with a total loss in capacity of 23.6% between the first and hundredth cycle. Obviously, modification of LiFePO₄ with 3D graphene coating can improve its cycling performance significantly. It is believed that the cycling performance was enhanced by the 3D graphene coating which might protect the core material from direct contact with the acidic electrolyte, in addition to improving the electronic conductivity of LiFePO₄.

Fig. 8 The cycling performance of pure LiFePO₄ and G/LiFePO₄ composite at 1C.

4. Conclusions

In summary, we have successfully synthesized the 3D self-assemble free-standing G/LiFePO₄ cylinder composite by an in situ one-pot hydrothermal method. Above results indicate that this 3D self-assemble free-standing structure is superior in the similar system because the 3D structure offer a conductive network which can improve the conductivity of the composite and the rapid of electronic transmission. The GO coating also favors of decrease the size of LiFePO₄ nanoparticles, which is another reason for the improving performance. And the electrochemical performance measurements have indicated that the 3D G/LiFePO₄ composite exhibit high capacity of 160 mA h g⁻¹ at 0.2C (94.12% of its theoretical capacity 170 mA h g⁻¹), good rate performance of 115 mA h g⁻¹ at 10C (71.9% of its initial capacity) and good cycling performance of 94.2% capacity retention after 100 cycles, implying potential application in high rate lithium ion battery.

Acknowledgements

This work is financially supported by NSF of China (Grant No. 61376056), and Science and Technology Commission of Shanghai (Grant No. 14520722000, 14520503100). Education Commission of Shanghai (Grant No. 15ZZ095).

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Footnote

† Y. Du and Y. Tang contributed equally to this work.

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