Nanocomposite and Its Electrochemical Properties as Cathode Material for Li-Ion Batteries

LiFePO 4 /graphene nanocomposite was successfully synthesized by rheological phase method and its electrochemical properties as the cathode materials for lithium ion batteries were measured. As the iron source in the synthesis, FeOOH nanorods anchored on graphene were first synthesized. The FeOOH nanorods precursors and the final LiFePO 4 /graphene nanocomposite products were characterized by XRD, SEM, and TEM.While the FeOOH precursors were nanorods with 5–10 nm in diameter and 10–50 nm in length, the LiFePO 4 were nanoparticles with 20–100 nm in size. Compared with the electrochemical properties of LiFePO 4


Introduction
Rechargeable lithium ion batteries have been considered as the next generation of power sources for electric vehicles, hybrid electric vehicles, and plug-in hybrid electric vehicles [1][2][3].Due to its low cost, long cycle life, environmental friendliness, and thermal stability, olivine phase LiFePO 4 , with a theoretical capacity of 170 mA h g −1 and a flat voltage plateau at 3.4 V (versus Li + /Li), has been considered as a promising electrode material for these rechargeable lithiumion batteries [4].However, along with the above-mentioned advantages, like another polyanion-type cathode materials, LiFePO 4 has also disadvantages of low electronic conductivity and slow diffusion rates of lithium ions.To overcome the low electronic conductivity of LiFePO 4 , a lot of methods have been made through different material processing approaches [5][6][7][8].Among these approaches, coating LiFePO 4 with carbonaceous conductors is the easiest and most used method to improve the electronic conductivity of LiFePO 4 particles.
Graphene is a monolayer of graphite, consisting of sp 2 hybridized carbon atoms arranged in a honeycomb crystal lattice.It is a nearly ideal two-dimensional (2D) material with extraordinary mechanical, electrical, thermal, and optical properties [9][10][11][12].Owing to the outstanding electrical conductivity, graphene has recently been used to improve the performances of the cathode electrode materials in supercapacitors, fuel cells, solar cells, and lithium ion batteries [13][14][15][16].In the application for lithium ion batteries, since graphene itself is electrochemically active as the anode materials, it has been used in most cases in the anode materials to restrict the volume expansion and to improve the electronic conductivity.Graphene has also been used to improve the performance of LiFePO 4 synthesized by solid-state reactions and sol-gel reaction [17][18][19].
In this paper, LiFePO 4 /graphene nanocomposite was synthesized by rheological phase method [20] with FeOOH nanorods as the precursor.The final products were LiFePO 4 nanoparticles anchored on graphene nanosheets.The electrochemical measurements showed that the 2D graphene nanosheets in the nanocomposites did improve the performances of LiFePO 4 as the cathode materials for lithium ion batteries.

Synthesis of LiFePO 4 /Graphene Nanocomposite
Synthesis of FeOOH Nanorods-Graphene Oxide Nanocomposite.Graphite oxide (GO) was obtained by ultrasonic treatment of graphite oxide, which was prepared using the traditional Hummers's method [21].After 200 mg GO was stirred in 100 mL water-ethanol system (H 2 O : C 2 H 5 OH = 1 : 1 in volume) for 1 h, 1.35 g FeCl 3 ⋅6H 2 O was dissolved in the solution and refluxed at 100 ∘ C for 2 h in oil bath.The mixture was then naturally cooled and centrifuged at 4800 r/min for 10 min.After being washed sequentially in deionized water and ethanol several times and dried in an oven at 60 ∘ C for 24 h, the brown yellow FeOOH powder was obtained.
Synthesis of LiFePO 4 -Graphene Nanocomposite.Firstly 200 mg PEG10000 was dissolved in 20 mL ethanol.Then 2 mmol FeOOH powder and 2 mmol LiH 2 PO 4 were added in the solution.With ultrasonic treatment for 5 min, the LiH 2 PO 4 was completely dissolved and the FeOOH powder was dispersed uniformly.After heating and stirring for overnight, the solvent was partially evaporated and the system was very viscous.After being adjusted into a rheological phase, the reactants were annealed at 650 ∘ C for 10 h.The final dark brown powder was obtained after being cooled to room temperature naturally.

Characterization.
The phase purity of the products was examined by powder X-Ray Diffraction on a Bruker D8 Advance X-ray diffractometer using Cu Κ radiation ( = 1.54056Å).The crystal size and morphology of the products were examined with a scanning electron microscope (SEM, QUANTA 200, Holland).The thermal analysis was determined by a Netzsch STA 449C (Germany) in oxygen at a heating rate of 10 ∘ C min −1 from room temperature to 800 ∘ C. Transmission electron microscope (TEM) images and high-resolution transmission electron microscope (HRTEM) images were taken on a JEM 2010-FEF (JEOL Ltd., Japan) operating at 200 kV.The carbon contents in the composites were determined by VarioEL III elemental analyzer (Elementar Analysen System GmbH, Germany).

Electrochemical Property Measurements.
The electrochemical measurements were carried out using CR2016 coin cells with lithium metal disks as the counter electrodes.The working electrodes were made by pressing mixtures of the synthesized LiFePO 4 /Graphene composites, acetylene black (AB), and polyvinylidene fluoride (PVDF) binder with a weight ratio of 80 : 15 : 5 on Al meshes which were used as the current collectors.The electrolyte was composed with 1 M LiPF 6 in ethylene carbonate/dimethyl carbonate (1 : 1 v/v) solvents and the separator was Celgard 2300 microporous film.The cells were assembled in a glovebox filled with high purity Ar gas.The electrochemical tests were performed galvanostatically in the voltage window of 2.0-4.4V on Neware battery test system (Shenzhen, China) at room temperature.All the charge-discharge specific capacities were calculated on the net mass of LiFePO 4 excluding carbon content (both the amorphous carbon and the graphene).

Results and Discussion
Figure 1 showed the XRD spectrum of the FeOOH precursors obtained by the reflux reaction between graphite oxide and FeCl 3 ⋅6H 2 O.All the reflection peaks can be indexed on the basis of FeOOH standard card without any reflection peak from impurity phases.Also, the low intensity and the big width of the reflection peaks in the XRD spectrum implied the nanoscale sizes of the FeOOH precursors.
Figure 2 shows the TEM images of the FeOOH precursors.It is clear that the FeOOH precursors have nanorods morphology, evenly distributed and anchored on the graphene nanosheets.The nanorods are 5-10 nm in diameter and 10-50 nm in length.
Figure 3 shows the XRD spectra of the LiFePO 4 /graphene composites synthesized at 600 ∘ C and 650 ∘ C, respectively.All the diffraction peaks can be indexed on the base of olivine phase LiFePO 4 without any peaks from impurities.Compared with the XRD spectrum of the composite obtained at 600 ∘ C, the peaks in the XRD spectrum of the composite obtained at 650 ∘ C have much higher intensities, implying that the LiFePO 4 synthesized at 650 ∘ C has better crystallinity.Since good crystallinity of LiFePO 4 is beneficial to the diffusion of Li + and can thus improve the electrochemical performances of the materials, 650 ∘ C was used as the annealing temperature in the experiment.
Figure 4 shows the SEM and TEM images of the LiFePO 4 /graphene composite.From the low magnification SEM image of the LiFePO 4 /C (Figure 4(a)), we can see that the LiFePO 4 /C powder is comprised of many aggregations of microplanes.When the image is magnified 10000 times (Figure 4(b)), it is clear that there are many nanoscale particles anchored on the microplanes.In the TEM image of the LiFePO 4 /C (Figure 4(c)), the graphene is obvious.And we can see that the phosphate particles are not very uniform, with large particles having about 100 nm in size and small    particles at about 20 nm in size.In the high-resolution TEM image (Figure 4(d)), the layers of the graphene nanosheets and the crystal planes of the LiFePO 4 nanocrystals can be observed.
Figure 5 shows the cyclic voltammograms curves of the LiFePO 4 with and without graphene in the first chargedischarge cycle.Both the electrodes exhibit the redox peaks of Fe 2+ /Fe 3+ at a scan rate of 0.1 mV s −1 .For the LiFePO 4 /graphene composite, the anodic peak at 3.5 V corresponds to the oxidation of Fe 2+ to Fe 3+ , and the cathodic peak at 3.3 V corresponds to the reduction of Fe 3+ to Fe 2+ , with a potential interval of 0.2 V between these two redox peaks.The separation observed between the oxidation and reduction peaks is often used to differentiate the electrochemical reversibilities of the electrode materials, with larger separation indicating lower reversibility.This narrow separation of the redox peaks implies that the LiFePO 4 /graphene composite has very good electrochemical kinetics.
Galvanostatic charge-discharge measurements were carried out with lithium cells at a current density of 0.1C (1C = 170 mA g −1 ) to evaluate the electrochemical properties of the LiFePO 4 with and without graphene nanosheets.Figure 6 shows the initial 10 cycling profiles of the phosphate electrodes with (Figure 6 at room temperature.In the charge-discharge curves of the LiFePO 4 /graphene composite (Figure 6(a)), the charge profile of the first cycle exhibits a voltage plateau at 3.5 V (versus Li + /Li), which should correspond to the Fe 2+ /Fe 3+ redox couple.The discharge profile of the first cycle shows a voltage plateau at 3.3 V (versus Li + /Li), with a very small separation (0.2 V) to the plateau in the charge profile.The specific discharge capacity in the first cycle is 158 mA h g −1 , which is around 93% of the theoretical specific capacity of LiFePO 4 .Also, the charge-discharge profiles have showed very stable cycle life.Although the charge-discharge curves for LiFePO 4 (Figure 6(b)) also show stable cycle life, the LiFePO 4 has much smaller discharge capacities when compared to the LiFePO 4 /graphene composite.The specific discharge capacity is only 125 mA h g −1 for LiFePO 4 when no carbon or graphene is present.Figure 7 shows the cycling performances of the LiFePO 4 / graphene composite (labelled as LFPG in the figure) at the rate of 0.1C.The discharge specific capacity in the first cycle is 158 mA h g −1 .And the discharge capacity in the fifteenth cycle is 149 mA h g −1 , which is about 95.5% of the capacity in the first cycle.It shows that the LiFePO 4 /graphene composite has very good cyclability.The LiFePO 4 without graphene (labelled as LFP in the figure), though having much lower discharge capacities than the LiFePO 4 /graphene composite, also shows very good cycling performances.This implies that the good cyclability of the material comes from the very stable crystal structure of LiFePO 4 and has no relations with graphene.
Figure 8 shows the discharge-cyclability profiles for the LFPG and LFP samples at different rates.At all the charge/ discharge rates, the LFPG shows better performances than the LFP samples.In particular, at the rate of 5C, while the LFP shows discharge capacity of 78 mA h g −1 , the LFPG shows discharge capacity of 107 mA h g −1 , which is about 37% more than the LFP material.Although both materials have very good cyclability, the LiFePO 4 with graphene always shows larger capacities than the pristine LiFePO 4 without graphene, especially at high charge/discharge rates.The phenomenon probably is due to the great electronic conductivity of the graphene monolayers.Also the existence of the graphene monolayer may also restrict the growing up of the LiFePO 4 particles and the small sizes of the LiFePO 4 are beneficial to the high-rate performances of the cathode materials.
To prove that the graphene monolayers in the LFPG composite increased the electronic conductivity of the material, we measured the electrochemical impedance spectroscopies (EIS) of both LFPG and LFP and showed the EIS profiles in Figure 9. From comparing the diameters of the semicircles of the EIS profiles, we can see that the charge-transfer resistance for the LFPG cathode (∼270 ohm) is much smaller than the resistance for the LFP cathode (∼550 ohm).This much smaller solid-electrolyte interface resistance should be owing to the existence of graphene layers in the LFPG composite, which has very good electronic conductivity.

Conclusion
In summary, LiFePO 4 -graphene composite was successfully synthesized by rheological phase method, with FeOOH nanoparticles anchored on graphene nanolayers as the iron source.The obtained materials were characterized by XRD, SEM, TEM, charge-discharge tests, and electrochemical impedance spectrum measurements.Although both the LiFePO 4 with and without graphene showed excellent cyclabilities, the LiFePO 4 -graphene composite showed higher capacities than the LiFePO4 without graphene at all the charge/discharge rates measured.These better performances should be owing to the good electronic conductivity of the graphene nanolayers in the LiFePO 4 -graphene composite.

Figure 1 :
Figure 1: The XRD spectrum of the FeOOH precursors.

Figure 2 :
Figure 2: The TEM images of the FeOOH nanorod precursors with different magnifications.The images showed that the FeOOH nanorods are evenly distributed and anchored on the graphene nanosheets.

Figure 6 :
Figure 6: The charge-discharge curves of the first ten cycles for the LiFePO 4 and the LiFePO 4 /graphene composite.

Figure 7 :
Figure 7: The cycling performances of the LiFePO 4 and the LiFePO 4 /graphene composite at the rate of 0.1C.