Introduction

Lithium ferrous phosphate (LiFePO4) cathode material is considered to be one of the most promising cathode materials for lithium-ion power batteries due to the abundant raw materials, high safety, environmental friendliness. However, low electron conductivity and ion diffusion limit the electrochemical properties of LiFePO4 materials, hindering the commercialization process [1]. Effective strategy to improve the conductivity of LiFePO4 is proven to be carbon coating along with preferential nanocrystallization [2,3,4,5,6] and ion doping [7,8,9], which can make the electrochemical performances of LiFePO4 materials reach a level of commercial application. At present, the cost and stability of its preparation process are still a bottleneck of limiting its large-scale application compared to conventional lead-acid and nickel-hydrogen batteries [10, 11]. Therefore, it is essential for a wide industrial application to develop an inexpensive and facile solid-state method to produce a uniform LiFePO4 material without particle agglomeration.

Although many researchers have reported many procedures of synthesizing LiFePO4, such as spray pyrolysis [12], combination of spray pyrolysis and ball-milling [13], spray drying [14], sol–gel [15], coprecipitation [16] and conventional solid-state reaction method [17], the synthesis method of LiFePO4 materials can be generally divided into solid state reaction and solution method. Solution method usually is accompanied with complex preparation process, leading to the high cost of preparation [10]. Accordingly, the large-scale preparation mainly adopts solid state method, although the solid state method encountered some application issues such as particle agglomeration with irregular morphology [5, 18]. In a conventional solid-state process of preparation, an iron source, phosphorus source and Li source are used as starting materials for synthesizing LiFePO4 materials. Before it is decomposed at a temperature ranging from 600 to 700 °C for 8–15 h, a precursor is usually obtained via mixing the starting materials through ball milling. The ball milling can help both homogeneity and reduction of the particle size. However, the ball milling often brings about irregular morphology and particle agglomeration, resulting in unstable electrochemical performances [19]. In addition, ball milling also increases the cost of material preparation. To reduce irregular morphology and the particle agglomeration of LiFePO4 materials in conventional solid-state method, the introduction of integrated materials containing the Li, Fe, P elements as raw materials, such as LiFePO4(OH), is very necessary. The LiFePO4(OH) was first reported by Whittingham to be formed through the reaction of FePO4⋅H2O and CH3COOLi in hydrothermal process [20]. Subsequently it was found that nano-sized LiFePO4 reacts with H2O to form LiFePO4(OH) in a wet environment [21, 22], and the LiFePO4(OH) could still be reverse-reacted to LiFePO4 after high-temperature heat treatment [22]. Next, LiFePO4(OH) or LiFe(PO4)(OH)xF1−x was synthesized by solvothermal method [23,24,25]. After mixing it with carbon, the LiFePO4(OH) was electrochemically active, with a specific capacity of 130 mA h g−1 at 0.1C. Another research on LiFePO4(OH) has suggested that it can also be obtained by FePO4 H2O through H+/Li+ exchange [26]. In addition, the effective strategy to relieve the sluggish kinetics of LiFePO4 is to integrate LiFePO4 and graphitized carbonaceous materials with large contact area such as graphene and carbon nanotubes [27,28,29], which is widely applied for synthesizing high-rate hybrid electrodes with the least carbon content. For instance, the graphene-modified LiFePO4 materials were fabricated and promoted the improvement of its overall electrochemical performance [28, 30]. To the best of our knowledge, no research on using the integrated LiFePO4(OH) as a precursor for synthesizing LiFePO4 material has been reported. Based on the above considerations, we applied the integrated LiFePO4(OH) compound containing Li, Fe, P elements as precursor, so that a morphology-inherited LiFePO4 material with uniform element distribution can be obtained without ball milling.

Herein, a graphene-supported LiFePO4/C@G composite with uniform element distribution is successfully synthesized using the integrated LiFePO4(OH) and the sucrose in graphene oxide (GO) suspension as raw materials. The as-prepared LiFePO4/C@G exhibits excellent electrochemical performances due to uniform particle and electronic conducting network formed with graphene and surface coating carbon layer. This provides a reference for large-scale preparation route of LiFePO4/C materials.

Experimental

The LiFePO4OH precursor was prepared by hydrothermal method using FePO4⋅2H2O and CH3COOLi as raw materials. Specifically, 0.006 mol of CH3COOLi was dissolved in 120 ml of deionized water. Then, stoichiometric FePO4⋅2H2O (0.006 mol) was added into the above solution under continuous stirring for 30 min. Then, the total solution was transferred into a 200 ml of stainless-steel autoclave, sealed and heated at 180 °C for 72 h. The resultant product was washed with deionized water for several times and was dried in a vacuum at 80 °C, before the LiFePO4OH precursor was obtained. After it was mixed with 14 wt% of sucrose in 3 wt% graphene oxide (GO) suspension, the obtained mixture was freeze-dried and calcined at 700 °C for 10 h under N2 atmosphere. Finally, the LiFePO4/C@G composite was obtained. The LiFePO4/C was synthesized via the same procedure except without adding GO for comparison.

The crystalline phases of the samples were identified by a PANalytical X’PertPro X-ray diffractometer (Cu Kα radiation, 40 kV). The particle morphologies of the samples were examined by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800). X-ray photoelectron spectroscopy (XPS) data were collected by a PHI Quantera SXM spectrometer using the C 1 s peak with 284.8 eV, and the corresponding fitted spectra were carried out by using XPSPEAK 4.1 software. Thermogravimetric analysis (TGA) was measured using STA 449F3 analyzer (NETZSCH Co.) under air atmosphere to obtain carbon content of composites at a heating rate of 10 °C min−1.

The electrochemical tests were investigated by assembling CR2016 coin-type cells. The cathode electrode was fabricated by coating aluminum foil with the slurry composed of 80 wt% of active material LiFePO4/C, 10 wt% of polyvinylidene binder and 10 wt% of Super-P in N-methyl-2-pyrrolidone. After drying at 120 °C for 12 h in a vacuum, the electrode was punched into disc with the diameter of 14 mm as the cathode. Lithium foil served as the counter electrode, and Celgard 2400 membrane was used as the separator. The electrolyte is consisted of a solution of 1 M LiPF6 in EC and DMC (volume ratio of 1:1). The buttoned batteries (CR2016) were assembled in a glove box filled with argon. After the assembled batteries were laid aside for 3 h, the electrical performance of batteries were tested on the batteries tester (Land CT 2001A, Land Co. China) under a voltage range of 2.0 ~ 4.5 V. Cyclic voltammetry and AC impedance spectroscopy measurements were performed at electrochemical workstation (CHI604E, Chenhua, China). The amplitude and frequency range of AC impedance are 5 mV and 0.1 ~ 100 kHz, respectively.

Results and discussion

The preparation process is shown in Scheme 1. To solve component segregation of the raw materials and improve the consistency of the LiFePO4 materials in mixing in solid-state preparation method of LiFePO4 materials, a strategy was proposed that the reaction phases were reduced in the mixing of raw materials through using LiFePO4(OH) containing Li, Fe and P as precursor. LiFePO4(OH) precursor was prepared by hydrothermal method using FePO4 and CH3COOLi as raw materials, followed by thermal conversion of LiFePO4OH in the presence of sucrose in graphene oxide (GO) suspension as carbon sources for graphene-supported LiFePO4/C composite.

Scheme 1
scheme 1

Schematic illustration for synthesis of the LFP/C@G sample

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Figure 1 presents XRD patterns of the LiFePO4(OH) precursor, LFP/C and LFP/C@G samples. As shown in Fig. 1, the diffraction peaks of the as-prepared LiFePO4(OH) samples match well with the standard diffraction peaks of LiFePO4(OH) (JCPDS No. 041–1376), indicating the high purity of the prepared LiFePO4(OH) sample. 2θ = 17.82°, 18.59°, 19.02°, 22.53°, 25.90°, 26.80°, 27.23°, 29.38° and 36.47° positions correspond to the characteristic diffraction peaks of (100), (011), (001), (− 101), (111), (021), (− 120), (101) and (− 201) crystal planes, respectively. In addition, the diffraction peaks of the obtained LFP/C and LFP/C@G samples from LiFePO4(OH) also are well indexed to olivine structured LiFePO4 with space group Pnma (JCPSD 83–2092). This indicates that LiFePO4/C composites with high purity can be synthesized using LiFePO4(OH) with Li, Fe and P elements as raw material. XPS spectrum was obtained to verify the elemental compositions and chemical states, shown in Fig. 1b–d. The XPS survey spectrums of the LiFePO4/C@G sample show the typical peaks for Fe 2p, C 1 s, P 1 s and O 1 s, respectively. In the high-resolution spectrum of the Fe 2p, two peaks located at about 711 and 724 eV, matching to Fe 2p3/2 and Fe 2p1/2, which may be ascribed to Fe (II) state [31]. In Fig. 1d, the peak for C 1 s at 284.7, 286.1 and 288.7 eV can be regarded as C–C band, C–O band and O–C = O band, which evidences the presence of the modified graphene [30].

Fig. 1
figure 1

XRD patterns for the as-prepared samples (a) as well as XPS survey (b) and high-resolution elemental of Fe 2p (c) and C 1 s (d) for the LFP/C@G

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Figure 2 shows SEM images of the LiFePO4(OH) precursor and carbon-coated LiFePO4/C material. As seen from Fig. 2a, LiFePO4(OH) precursor possesses uniform spindle-shaped morphology with 2 μm in length and 300 nm in width. There is a smooth surface with no other impurity phase on the material particles. In Fig. 2b, SEM images of the carbon-coated LFP/C sample shows little different morphology compared with the LiFePO4(OH) precursor. This indicates that the morphology of LFP/C materials can be regulated by LiFePO4(OH) precursor. In order to further improve the rate performance of LFP/C, GO was added to the LFP/C sample, and its SEM was shown in Fig. 2c. In Fig. 2c, graphene can be observed in the LFP/C@G sample, indicating that the graphene is successfully coated on the surface of LFP/C particles. The EDS results of LFP/C@G in Fig. 2d show that C, P and Fe are uniformly distributed in the LFP/C@G sample. Therefore, the LiFePO4(OH) precursor can be used as raw materials for synthesizing uniform LiFePO4/C materials without the ball milling, which is beneficial for engineering application [32, 33].

Fig. 2
figure 2

SEM image of LiFePO4(OH) raw material (a), LFP/C (b) and LFP/C@G (c) as well as EDS images (d) of the LFP/C@G

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Figure 3 shows rate capability and cycling performances of the LFP/C and LFP/C@G samples synthesized from LiFePO4(OH) precursor. In Fig. 3a, the LFP/C sample shows an initial specific capacity of 158 mA h g−1 at rate of 0.1 C (close to theoretical capacity of 170 mA h g−1), indicating a high purity of the as-prepared LFP/C sample, while the LFP/C sample just maintains a specific capacity of 135 and 106 mA h g−1 at a high rate of 2 and 5 C, respectively and shows a great polarization with merely a voltage platform of 2.5 V at 5 C, indicating that the rate performance of the LFP/C sample needs to be further improved. It is worthy of mentioning that the carbon content of the LFP/C and LFP/C@G samples is calculated to be 1.49 and 4.37 wt% from the thermogravimetric (TGA) curves (Fig. S1), respectively [34]. Therefore, the inferior rate performance of LFP/C sample may be mainly due to the low electronic conductivity resulting from less carbon content of only 1.49 wt%. To further improve the rate performance of the LFP/C sample, GO was added to the LFP/C sample for synthesizing the graphene-supported LiFePO4/C composite (LFP/C@G). In Fig. 3b, the LFP/C@G delivers a specific capacity of 156, 154, 150, 145, 139, 132 mA h g−1 at 0.1, 0.2, 0.5, 1, 2 and 5 C with a higher voltage platform of 3.3 V at 5 C, showing excellent rate performance (Fig. 3c). As shown in Fig. 3d, the specific capacity of the LFP/C@G sample drops from 131 to 127 mA h g−1 in the 200 cycles at 2 C, showing a longer capacity retention of 97.0% compared with the LFP/C (capacity retention of 89.2%), indicating better cycling stability for LFP/C@G. The above electrochemical results demonstrate that a LiFePO4 material with excellent electrochemical performances can be obtained using LiFePO4(OH) as precursor combined with graphene modification.

Fig. 3
figure 3

Charge/discharge curves for LFP/C (a) and LFP/C@G (b) as well as their rate capability (c) and cycling performance (d)

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Electrochemical impedance spectroscopy (EIS) of the as-prepared samples was performed in the fully discharged (lithiation) state, as shown in Fig. 4. The EIS profiles consist of a partially overlapped semicircle in the high-frequency region followed by a sloping line in the low-frequency region. The semicircle in the high- and middle-frequency regions is due to the charge-transfer resistance (Rct). The sloping line in the lower frequency represents the Warburg impedance (Ws) associated with lithium-ion diffusion in the bulk of the electrode [35, 36]. By fitting data, the charge-transfer resistance of LFP/C and LFP/C@G samples was approximately 68 and 130 Ω, respectively, indicating better kinetic behavior of the LFP/C@G sample. The EIS result validates that the superior rate performance of LFP/C@G sample results from the improved electrochemical reaction kinetics.

Fig. 4
figure 4

EIS plots of LFP/C and LFP/C@G samples

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Conclusion

In summary, we prepared a graphene-supported LiFePO4/C@G composite with uniform morphology and electronic conducting network via a freeze-drying assisted solid-phase method using the integrated LiFePO4(OH) as precursor. The as-prepared LFP/C samples show a similar spindle-shaped morphology as the LiFePO4(OH) precursor, indicating a feasibility for regulation of the LiFePO4/C morphology by LiFePO4(OH). Moreover, the graphene-supported LiFePO4/C@G shows better electrochemical properties with a specific capacity of 156, 154, 150, 145, 139, 132 mA h g−1 at 0.1, 0.2, 0.5, 1, 2, 5 C, which benefits from the synergetic effect of uniform LiFePO4(OH) as precursor and continuous graphene electronic conducting network. The freeze-drying assisted solid-phase method using LiFePO4(OH) as precursor without ball milling provides a new insight for solving the problem of particle agglomeration caused by ball milling in solid-state method. The strategy would apply to olivine materials with other transition metals (Mn, Co or Ni).