Excellent Temperature Performance of Spherical LiFePO4/C Composites Modified with Composite Carbon and Metal Oxides

Nanosized spherical LiFePO4/C composite was synthesized from nanosized spherical FePO4 ·2H2O, Li2C2O4, aluminum oxide, titanium oxide, oxalic acid, and sucrose by binary sintering process. The phases and morphologies of LiFePO4/C were characterized using SEM, TEM, CV, EIS, EDS, and EDX as well as charging and discharging measurements. The results showed that the as-prepared LiFePO4/C composite with good conductive webs from nanosized spherical FePO4 ·2H2O exhibits excellent electrochemical performances, delivering an initial discharge capacity of 161.7 mAh·g−1 at a 0.1 C rate, 152.4 mAh·g−1 at a 1 C rate and 131.7 mAh·g−1 at a 5 C rate, and the capacity retention of 99.1%, 98.7%, and 95.8%, respectively, after 50 cycles. Meanwhile, the high and low temperature performance is excellent for 18650 battery, maintaining capacity retention of 101.7%, 95.0%, 88.3%, and 79.3% at 55°C, 0°C, −10°C, and −20°C by comparison withthat of room temperature (25°C) at the 0.5 C rate over a voltage range of 2.2 V to 3.6 V, respectively.


Introduction
Clean and rechargeable secondary energy sources are urgently needed for the development of modern technology and widespread application of portable electrical equipment. Among secondary energy sources, rechargeable lithium- (Li) ion battery is the most attractive and promising. In 1997, Padhi et al. [1] proved that olivine LiFePO 4 has an excellent performance in Li-ion intercalation and deintercalation. Its theoretical capacity is 170 mAh⋅g −1 with a flat discharge voltage at 3.4 V. Among polyanion cathodes, LiFePO 4 has the advantages of excellent cycle ability, good thermal stability, inexpensive raw materials, and environmental friendliness. LiFePO 4 is also attractive for use as a next generation cathode material for Li-ion batteries. However, the low electronic conductivity of LiFePO 4 leads to a poor charge-discharge performance at a high current rate. Therefore, modifications to LiFePO 4 to improve its properties have been developed. Such modifications include coating with nanocarbon [2][3][4][5][6][7][8][9], encapsulation with a conductive polymer [10,11], and doping with suitable metals [12][13][14][15][16][17][18].
FePO 4 ⋅ H 2 O is a promising precursor for preparing the Li-ion battery cathode material LiFePO 4 . The advantages of this precursor include innocuity, low cost, similar structure with LiFePO 4 , and oxidation avoidance of ferrous iron [19][20][21]. Traditional methods for the synthesis of FePO 4 ⋅ H 2 O and LiFePO 4 include solid phase synthesis [22,23], sonochemistry [24], coprecipitations [25][26][27][28][29], and hydrothermal synthesis [30][31][32]. These methods usually involve the use of expensive metal-organic compounds. In the solid-state synthetic process, high-energy consumption is generated and particles are relatively unevenly distributed. The sonochemistry process has some advantages in preparing iron phosphate, including nonrequirement of oxidant, less reaction time, and controllable particle size. However, the largescale production of iron phosphate is difficult to be realized [33,34]. Lee and Teja [35] Xu et al. [36] prepared LiFePO 4 nanoparticles in subcritical and superheated water. Yu et al. [37] investigated the rapid and continuous production of LiFePO 4 /C nanoparticles in superheated water. Chen et al. [38] reported the influences of carbon sources on the electrochemical performances of LiFePO 4 /C composites.
In this work, nanosized spherical LiFePO 4 /C composite was prepared from nanosized spherical FePO 4 ⋅2H 2 O, Li 2 C 2 O 4 , aluminum oxide, titanium oxide, oxalic acid, and sucrose by binary sintering process. The process is simple, requires uncomplicated equipment, and consumes low energy. Fine particles were obtained in homogenous distribution, appropriate for industrialized production.

Preparation of Materials.
The LiFePO 4 /C composite was prepared by mixing stoichiometric amounts of nanosized spherical FePO 4 ⋅2H 2 O, Li 2 C 2 O 4 , aluminum oxide, and titanium oxide dispersed in ethylene glycol with oxalic acid and sucrose, followed by grinding via ball milling. After evaporating the ethylene glycol, the mixture was firstly sintered in a horizontal quartz tube at 400 ∘ C for 6 h in an argon atmosphere. As the presintered product cooled to room temperature, the LiFePO 4 /C composites were obtained after being calcined at 650 ∘ C for 8 h.  with a perturbation signal of 5 mV over a Chi 660c setup. All electrochemical measurements were conducted at room temperature (25 ∘ C).

Electrode Fabrication and Electrochemical Measurements.
The as-prepared cathode was mixed with acetylene black and polyvinylidene difluoride at a mass ratio of 80 : 10 : 10. LiFePO 4 /C cathode was prepared by spreading the above mixture on an aluminum foil and drying in a vacuum oven at 120 ∘ C. Charge-discharge tests on LiFePO 4 /C were performed in coin cells using LiFePO 4 /C cathodes and Li anodes. A porous membrane (Celgard 2300) was used as a separator, and the electrolyte was 1 mol⋅L −1 LiPF 6 dissolved in a mixture of ethylene carbonate, dimethyl carbonate, and methyl-ethyl carbonate at a volume ratio of 1 : 1 : 1. Coin cells (CR 2025) were assembled in an argon-filled glove box. The cells were charged and discharged at the rates of 0.1 C, 1 C and 5 C over a voltage range of 2.5 V to 4.2 V, respectively, versus the Li/Li + electrode at ambient temperature using a battery testing system (Neware BTS-2000). The high and low temperature performance of LiFePO 4 /C was tested via fabricating 18650 battery at the rate of 0.5 C over a voltage range of 2.2 V to 3.6 V.

Morphology of LiFePO 4 /C Prepared with Nanosized
Spherical FePO 4 ⋅2H 2 O. Nanosized spherical FePO 4 ⋅2H 2 O has been used to prepare LiFePO 4 /C. As seen in Figure 1, the SEM images of LiFePO 4 /C from nanosized spherical FePO 4 ⋅2H 2 O were shown. The products had diverse morphology. The asprepared LiFePO 4 /C composite was spherical and the particles' size close to 100 nm in size was uniformly distributed. The shapes of particles were traced back, following each step of preparation (Figure 2(a) for mixture, Figure 2(b) for presintering product, and Figure 2(c) for sintered product). Oxalic acid as reductant was ultimately decomposed into CO 2 and H 2 O. Sucrose as carbon source was used for coating nanosized LiFePO 4 particles. Figure 3 showed the TEM images of LiFePO 4 /C. As shown in the image, LiFePO 4 particles were well surrounded by a thin surface layer of carbon. The thickness of the carboncoated layer was about 4 nm. There was a layer of carbon web, providing good electronic contact between LiFePO 4 particles.
As seen from Figure 4, the EDS and EDX results showed that as-prepared sample had the elements, such as Fe, P, O, C, Ti, and Al, and all of the elements were distributed uniformly, indicating that C, Ti, and Al dispersed in LiFePO 4 evenly.   Figure 5 showed the initial charge-discharge capacity and cycling performance of the LiFePO 4 /C composite cathodes at three different rates of 0.1 C, 1 C, and 5 C.

High and Low Temperature
Performance. The fatal disadvantage of commercial LiFePO 4 is that it is poor at low temperature performance. The general low temperature was referred to −20 ∘ C. Over a voltage range of 2.0 V to 3.65 V, commercial LiFePO 4 had a capacity retention ratio of 60%∼70% at 0.5 C at −20 ∘ C by comparison with that of room temperature (25 ∘ C). Therefore, to resolve the problem plays an important role in industrialized application. The performance of LiFePO 4 can be characterized by 18650 battery with designed capacity of 1000 mAh. As illustrated in Figure 6, the high and low temperature performance of the LiFePO 4 /C composite cathodes delivered the outstanding progress to the rate of 0.5 C over a voltage range of 2.2 V to 3.6 V for 18650 battery. Meanwhile, it was found that the battery exhibited a discharge of 1028.3 mAh, 1010.9 mAh, 960.4 mAh, 892.9 mAh, and 801.5 mAh at 55 ∘ C, 25 ∘ C, 0 ∘ C, −10 ∘ C, and −20 ∘ C, respectively. Compared with discharge performance at room temperature (25 ∘ C), the capacity retention maintained 101.7%, 95.0%, 88.3%, and 79.3% at 55 ∘ C, 0 ∘ C, −10 ∘ C, and −20 ∘ C, respectively. Moreover, the high discharge voltage plateaus at various temperatures demonstrated the asprepared sample to be the excellent material for industrialized application.

CV and EIS.
Freshly deposited LiFePO 4 /C composite has been examined by CV at a scan rate of 0.1 mV⋅s −1 , as shown in Figure 7. the extraction and insertion of Li + in the LiFePO 4 olivine structure, respectively.
Meanwhile, as we can seen from Figure 7(b), the electrochemical impedance of LiFePO 4 /C synthesized from nanosized spherical FePO 4 ⋅2H 2 O was shown. The curve was formed by a depressed semicircle in the high-to middlefrequency region and a straight line in the low-frequency range. According to the literature [39], the depressed semicircle represented the charge-transfer reaction between the active materials and the electrolyte ( ct ). From the result, ct had the charge-transfer reaction resistance of 82 Ω, indicating the excellent electrical conductivity and Li-ion diffusion of LiFePO 4 /C composites.

Conclusions
LiFePO 4 /C composites were then prepared from nanosized spherical FePO 4 ⋅2H 2 O, Li 2 C 2 O 4 , aluminum oxide, titanium oxide, oxalic acid, and sucrose by binary sintering process. The EDS and EDX showed that Ti and Al were distributed in LiFePO 4 /C uniformly. And the composites had good conductive carbon webs and delivered an initial discharge capacity of 161.7 mAh⋅g −1 at a 0.1 C rate, 152.4 mAh⋅g −1 at a 1 C rate, and 131.7 mAh⋅g −1 at a 5 C rate. And the capacity retention obtained 99.1%, 98.7%, and 95.8%, respectively, after 50 cycles. Meanwhile, the high and low temperature performance is excellent for 18650 battery, exhibiting a discharge of 1028.3 mAh, 1010.9 mAh, 960.4 mAh, 892.9 mAh, and 801.5 mAh at 55 ∘ C, 25 ∘ C, 0 ∘ C, −10 ∘ C, and −20 ∘ C, respectively.
In summary, the as-prepared LiFePO 4 /C materials had favorable properties for their commercial applications.