INTRODUCTION

With the increasing concerns about environmental protection and energy saving, rechargeable lithium-ion batteries have been extensively used in a wide variety of portable electric devices owing to their high energy densities, high voltage and portability [1–3]. In recent years, there has been a dramatic increase in research and commercialization activities [3–7]. However, these batteries still need to be improved before they can be used in special applications which need high energy and high power, such as portable power tools, hybrid electric vehicles and other power supplies [8–12]. Olivine-structured lithium iron phosphate (LiFePO4) has many advantages compared with conventional cathode materials, such as LiCoO2, LiNiO2, and LiMn2O4. The insertion and extraction related to the LiFePO4 electrode involves a two phase mechanism with the reversible reaction expressed as [13–15]:

$${\text{LiFeP}}{{{\text{O}}}_{{\text{4}}}} \leftrightarrow {\text{FeP}}{{{\text{O}}}_{{\text{4}}}} + {\text{L}}{{{\text{i}}}^{ + }} + {\text{e}}{\text{.}}$$

It shows a plateau at around 3.5 V relative to lithium metal and high theoretical discharge capacity of 170 mA h g–1. And it has excellent structural and chemical stability during intercalation and thermal cycling. However, a main obstacle of LiFePO4 is its poor electronic conductivity and low lithium-ion diffusion coefficient, which is the most unfavorable issue for the rate capability of batteries. And these hinder the application of LiFePO4 in high power field, such as electric vehicles.

As the drawbacks mentioned above, various methods have been adopted to improve the conductivity of electrons in the materials, such as nanonization [8, 16–20], surface coating with carbon [11, 19–25], conductive organic polymer, and Li+ site doping with Mg2+, Mn3+, Ce3+, Zr4+, Ti4+, Ni2+ [12, 26]. In addition, some researches have focused on the electrochemical kinetics and mechanism studies of the lithium intercalation process of LiFePO4 cathode materials, which makes the investigation more deep and extensive. Among these methods, coating nano-sized LiFePO4 crystal with carbon can enhance conductivity and confine the crystal size of LiFePO4 for solving the problem of high power performance [10, 11, 25, 27]. There are many literatures that reported the synthesis of LiFePO4/C via various methods. However, these methods generally involve a high-temperature treatment to ensure the conductivity of the resulting carbon materials. But the increase of crystallite size is inevitable during the high temperature condition. And the carbon coated on the surface of the material is also incomplete, just like the description at the literature [19, 20, 22, 25, 28, 29].

In addition, to obtain phase-pure LiFePO4, oxidation of Fe2+ to Fe3+ during the synthesis must be avoided. Since Fe2+ can be easily oxidized during the prepare process, it is typically hard to obtain phase-pure LiFePO4 by applying Fe2+ raw material. And these salts are much more expensive than Fe3+ salts. According to [30], H2 and CO can reduce the iron to a lower oxidation state during the prepare process. This suggests that much cheaper iron (III) material may be used directly for the synthesis of LiFePO4. In 2015, R. Sehrawat and A. Sil have developed a polymer gel combustion method, which use different amount of aniline as the monomer of gel formation. The final particle size of the LiFePO4/C depends on the initial monomer content used in the synthesis process. Prepared LiFePO4/C cathode material performs a higher rate capability, due to small particle size, low charge transfer resistance, and higher Li+ diffusion coefficient [31].

In this work, a spherical carbon coated nano-structured LiFePO4 cathode composite is synthesized by a polymer-assisted method from inexpensive iron(III) raw material. The synthesis process of the composite includes two steps, in which the aniline plays an important role. There are two different amorphous carbons in the as-synthesized LiFePO4/C composite, which greatly improve the electronic conductivity of the materials. The composite showed a high rate capacity (about 118 mA h g–1) at 10 C rate and excellent cycling performance. That made it a promising cathode material for advanced electrochemical devices such as power Li-ion battery and supercapacitor.

EXPERIMENTAL

Synthesis of FePO4/PANI and LiFePO4/C Cathode Material

NH4H2PO4 (Aldrich, 99%) and (NH4)2HPO4 (Aldrich, 99%) with molar ratio of 1 : 1 were dissolved in deionized water. Then the aniline (Aldrich, 99.5%) and a little mount of tween-60 were added to the solution, marked as Solution A. The Fe(NO3)3 (Aldrich, 99.9%) in equimolar amount with [PO4]3–, were dissolved in deionized water, marked as Solution B. Then Solution B was slowly added to Solution A with stirring. Saturated NH4HCO3 (Sinopharm Group Co. LTD., 99%) solution was used as pH regulator. The reactive mixture was then stirred for 4 h at room temperature. And the resulting FePO4/PANI was filtered and washed several times with deionized water.

An equimolar mixture of CH3COOLi · 2H2O (Alfa Aesar, 99.9%) and FePO4/PANI, with a contain amount of sucrose (Sigma-Aldrich, 99.5%) were dispersed in the ethanol as the precursor. After milling for about 1 h, the precursors were heated at 400°C for 4 h under Ar containing 5% H2 (Jingxiang, 99.999%) in order to decompose the organic composition. The precursors were then remilled for about 1 h and finally calcined at 700°C for 10 h under argon containing 5% H2 to obtain the composite.

Structural Characterization

The DSC–TGA of the precursor was performed by thermal gravimetric analysis (TA SDT Q600, American) under the mixture atmosphere (Ar : H2 = 95 : 5). Raman spectrum of the samples was performed with Raman Spectrometers (Bruker RAM, Germany).The crystal structure of the obtained LiFePO4/C powder was identified by X-ray diffraction (XRD) (D/Max-2500, Japan) using CuKα radiation scanned in the range 20°–45°. The particle size and morphology were observed by a field emission scanning electron microscopy (FESEM) (S-4800, Hitachi, Japan). The nature and thickness of the coated carbon was measured using the images from high resolution transmission electron microscope (HRTEM) (JEM-2100, JEOL, Japan).

Electrochemical Measurements

The electrochemical properties of the products were investigated using a two-electrode test cell with lithium foil as the negative. A positive electrode was made by coating a paste of the nano-structured LiFePO4 composite, acetylene black and polyvinylidene fluoride (PVdF) binder (80 : 10 : 10 wt %) on an aluminum-foil collector. The positive film was subjected to roll press and the electrodes of 10 mm diameter were punched out. The positive electrodes were dried at 120°C for 12 h in a vacuum oven. For comparison, a commercial LiFePO4 powder with micron size (Marked as sample A), provided by Hao Run Technology Co. Ltd., is also used to prepare positive electrode with the same surface density as the synthesized LiFePO4/C composite. The cells were assembled in an argon filled glove box with an electrolyte of 1 mol L–1 LiPF6 in DC–DMC (1 : 1, volume ratio) solution and a separator of Celgard 2400. The charge–discharge measurements were performed using a multi-channel battery tester (LAND CT2001A, China) operating in galvanostatic mode at various rates, between 2.0 and 3.85 V.

RESULTS AND DISCUSSION

Structure, Morphology and Physical Property

Figure 1 shows the DSC–TGA curve of the precursor under the mixture atmosphere (Ar : H2 = 95 : 5). It can be seen that the weight loss is about 12 wt % from room temperature to 120°C because of the evaporation of water. Accompanied with the decalescence, there is a weight loss of 29 wt % from 120 to 150°C, which is caused by the pyrolysis of sucrose and polyaniline. The Fe3+ is reduced to Fe2+ and forms the LiFePO4 with a weight loss of 26 wt % from 360 to 450°C. At the same time, there appears an exothermic peak in the DSC curve during 400 to 450°C. It indicates that the LiFePO4 can be formed when the temperature is higher than 450°C. The XRD patterns shows that the element iron exist mainly as the form of FePO4, Fe7(PO4)6, Fe3(PO4)2 and a small quantity exist as the form of LiFePO4. The crystalline LiFePO4 is formed with the increase of the temperature. The carbon, which forms because of the pyrolysis of sucrose and polyaniline, has two effects during the process. It can reduce Fe3+ into Fe2+. At the same time, it can restrain the increase of the LiFePO4 crystal.

Fig. 1.
figure 1

The DSC–TGA curve of the precursor under the mixture atmosphere (Ar : H2 = 95 : 5).

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Figure 2 shows XRD patterns of commercial sample A and nano-structured LiFePO4/C composite. It can be found clearly that there are four strong diffraction peaks at 2θ = 20.68° (101), 25.50° (111), 29.69° (211), and 35.54° (311) in the XRD patterns of sample A and nano-structured LiFePO4/C composite. And there appears another weak peaks at 2θ = 22.63°, 23.99°, 32.18°, 36.46°, 37.83°, 39.24°, and 42.16°. All the peaks are consistent with the standard LiFePO4 (JCPDS 81-1173), which indicates the presence of the LiFePO4 phase as an ordered olivine structure. According to the Scherrer’s equation, the crystal size of the nano-structured LiFePO4/C composite is about 50.2 nm, indicating the samples have a high crystal degree. At the same time, there is no diffraction peaks attributed to carbon, most likely because there may be amorphous or a low content of crystalline carbon in the nano-structured LiFePO4/C composite.

Fig. 2.
figure 2

XRD patterns of LiFePO4 powder and nano-structured LiFePO4/C composite.

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Figure 3 shows the Raman spectrum of the nano-structured LiFePO4/C composite. Raman spectroscopy is an important method for the investigation of the properties of the carbon. The typical characteristic of carbon in the Raman spectrum are the two broad bands at 1350 and 1582 cm–1, which are called as D‑band (disorder/defect band) and G-band (graphitic band), respectively. These two bands are observed in Raman spectrum of the nano-structured LiFePO4/C composite. It indicates that the carbon is composed of amorphous phase and a little amount of graphitic carbon. The electronic conductivity of the composite has a complicated correlation with the peak intensity ratio and the bands are a ratio of the D vs. G band. The ration of ID/IG of the synthesized powder is 0.92, indicating that the coated carbon might be amorphous phase. The result is consistent with that of XRD analysis, which shows that no diffraction lines caused by graphite have been found in XRD patterns of the nano-structured LiFePO4/C composite.

Fig. 3.
figure 3

Raman spectrum of nano-structured LiFePO4/C composite.

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The surface morphology of the precursor particles and the LiFePO4/C sample as well as the shape of the carbon coating is investigated by SEM and HRTEM, as shown in Fig. 4. Figure 4a shows that the precursor powder FePO4/PANI has a spherical morphology and its diameter is about 30 nm. As shown in Fig. 4b, the diameter of nano-structured LiFePO4/C composite is about 55 nm. In contrast to the precursor FePO4/PANI, the diameter of nano-structured LiFePO4/C composite becomes bigger. The HRTEM images of Fig. 4c show that the FePO4 is amorphous and there is no clear interface between the FePO4 and PANI. In Fig. 4d, it is clearly founded that regular crystal plane of LiFePO4 particle and coating layer with little crystallinity. It shows that the thickness of the coated carbon layer is about 2 nm. These findings combined both XRD and Raman spectra can help us to understand these grains have a core-shell structure with LiFePO4 crystallite as the core and carbon coating as the shell. The LiFePO4 grains are sphere-like and surrounded completely by carbon layers. The resultant LiFePO4/C composites show an enhanced rate property due to the improvement of electronic conductivity from the effective carbon coating.

Fig. 4.
figure 4

SEM images of (a) the precursor particles and (b) the nano-structured LiFePO4/C composite, HRTEM images of (c) the precursor powder FePO4/PANI, and (d) the LiFePO4 particle and carbon layer.

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The EDX analysis shows that the content of the carbon is different in the different region of the nano-structured LiFePO4/C composite. As shown in Fig. 4b, the carbon content of in the dot b region is much higher than that in the dot a region. The ratio is about 4 : 1, most likely because the carbon in the dot b region come from the decomposition of sucrose and the carbon in the dot a region come from the decomposition of PANI. These results can help us to understand the carbon distribution in nano-structured LiFePO4 and the reason why the composites having excellent rate properties.

Electrochemical Performance

In order to evaluate the potential application of the nano-structured LiFePO4/C composite as cathode materials for lithium-ion batteries, the electrochemical performances with respect to Li insertion/extraction have been investigated. Figure 5 shows the cyclic voltametry test of nano-structured LiFePO4/C composite at different scanning rate. It is can be seen that a couple of redox peaks are observed between 3.3 and 3.5 V (vs. Li/Li+) in the cyclic voltametry curve obtained at a scanning rate of 0.1 mV s–1. These peaks correspond to the extraction and insertion of lithium ions. These redox peaks can still be clearly observed at a scanning rate of 0.9 mV s–1. It indicates that the composite has good high power performance. The result is consistent with that of the two-electrode test cell as shown in Fig. 6 and Table 1.

Fig. 5.
figure 5

Cyclic voltametry test of nano-structured LiFePO4/C composite at different scanning rate.

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Fig. 6.
figure 6

The 4th charge–discharge curves of nano-structured LiFePO4/C composite at different rates.

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Table 1. Discharge capacities and the discharge capacity retention s at various values of QnC/Q0.2C for nano-structured LiFePO4/C composite at different rates
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To test the working voltage and rate properties, the LiFePO4/C electrodes have been discharged at different rates, as shown in Fig. 6. The electrode is charged at 0.2 C for each charging step and then discharged at gradually increased rates. At low discharge rate (0.2, 1 C), the nano-structured LiFePO4/C composites has the similar charge–discharge curves with close discharge capacity (138.0, 142.5 mA h g–1) and flat plateaus. And the electrode exhibits flat voltage plateaus at each C-rate. Even at a 20 C rate, the discharge voltage plateau is still higher than 3.0 V (vs. Li+/Li), and the delivered capacity is more than 103 mA h g–1. Moreover, the charge–discharge plateau voltage differences are 0.1, 0.15, 0.5 and 0.7 V at 0.2, 1.0, 10, and 20 C-rate, respectively. The results indicate that the nano-structured LiFePO4/C electrode has low polarization, good electrical conductivity, as well as good rate capability.

The retention of discharge capacity (QnC/Q0.2C, n = 0.2, 1, 10, 20, 30) for LiFePO4/C at different discharge currents are listed on Table 1. It can be seen that the nano-structured LiFePO4/C composites has good rate capability. The discharge capacity retention is 85.5, 74.6, 66.7% at 10, 20, and 30 C-rate respectively, while the discharge capacity is 118, 103 and 92 mA h g–1. From the results, it could be concluded that the nano-structured LiFePO4/C composites present better rate performance, which can be attributed to the structure of the LiFePO4/C composites.

Figure 7 shows the discharge capacity at various rates (0.2, 1.0, 10, 20, 30, and 40 C-rate) as a function of cycle number for the nano-structured LiFePO4/C composites and a commercial sample A. At 0.2 C rate, the nano-structured LiFePO4/C composites and sample A reached a reversible capacity of 138 and 158 mA h g–1, respectively. After 50 cycles at various rates, the reversible capacity of the two materials maintained at about 138 and 158 mA h g–1, which indicated a favorable reversible capacity and cycling stability retention even under high-rate discharge conditions. But from the figure, it can be seen that the reversible capacity of the nano-structured LiFePO4/C composites is higher than that of sample A at high discharge rate. It can be seen the nano-structured LiFePO4/C composites has good rate-cycle performance.

Fig. 7.
figure 7

Cycling performance of the nano-structured LiFePO4/C composite at different rates compared to a commercial sample A.

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This excellent high rate performance of the nano-structured LiFePO4/C composite is ascribed to the carbon coated on the surface of the LiFePO4 and the carbon interlinked between the particles, as shown in Fig. 8. According to the literature, although the conductive carbon is added to the electrode film during the preparation process, it cannot form a convenient conductive network because of dispersion reasons. Thus, during the intercalation process of LiFePO4, the electrons cannot reach all the positions where Li+ ion intercalation takes place, resulting in polarization of the electrode, especially at high rate circumstance. In the composite prepared by a polymer-assisted method, the LiFePO4 is completely coated with conductive carbon that comes from PANI. Lithium ions can easily intercalate into the LiFePO4 through the carbon shell (about 2 nm). Moreover, the completely carbon coated-LiFePO4 is interlinked by the carbon that comes from sucrose, which form a convenient conductive network in the composite. When the Li+ ion intercalation takes place, the electrons can reach from all the positions with the conductive network. Thus it could further alleviate this polarization phenomenon and present excellent high rate performance.

Fig. 8.
figure 8

(a) HRTEM images of the carbon coated LiFePO4 particles and (b) electron-transfer pathway for LiFePO4 particles with nano-size and two type of carbon.

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CONCLUSIONS

In summary, a spherical carbon coated nano-structured LiFePO4 composite is synthesized by a polymer-assisted method from inexpensive iron(III) raw material. The cathode material shows a discharge capacity of 138, 136, 118, 103 and 92 mA h g–1 at 0.2, 1, 10, 20, and 30 C-rates. The nano-structured LiFePO4 composite provides an excellent rate performance and a good cycling stability at high rates. This excellent behavior of the nano-structured LiFePO4 composite is ascribed to the carbon coated on the surface of the LiFePO4 and the carbon interlinked between the particles. Such a synthesis using Fe(III) cheap raw material in combination with sucrose as an organic carbon source is a facile and energy-saving way for the synthesis of high-rate performance LiFePO4/C composite for high power lithium-ion batteries.