The LiFePO₄ powder (Aleees Inc.) employed had an average grain size of 500 nm and a carbon content of 3.74wt%. Al₂O₃-coated LiFePO₄ powder was prepared by using the Sol-Gel method. The amount of Al₂O₃ coated was about 1wt%-5wt% of the LiFePO₄ powder. In the preparation, firstly LiFePO₄/Cpowder (Aleees Inc.) was mixed with Al(C₃H₇O)₃ in ethanol, then the slurry was stirred at 70℃ until totally dried. Finally, the as-prepared powder was calcined at 500℃ for 1 h in Ar gas flow to prevent the formation of Fe³⁺ compounds.

X-ray diffraction (XRD) was carried out on a diffractometer (Rigaku Co. D/MAX-2500, Japan) with Cu-Kα radiation. The lattice parameters were determined by Rietveld analysis of the diffraction data using the Maud program. The particle morphology was examined by scanning transition electron microscope (JEM-2011, JEOL Japan), and transition electron microscope (JEM-2011, JEOL, Japan). Energy dispersive X-ray spectroscope (EDS) was used to analyze the surface composition and element distribution of cathode materials.

The electrochemical performance was carried out in CR2016 coin-type cells. The cells consisted of a cathode and a lithium foil as anode separated by a microporous polypropylene sheet (Celgard 2400, Celgard Inc., USA). The electrolyte was 1 mol/L LiPF₆ in a mixture solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1. Composite cathode films were prepared by mixing of 80wt% LiFePO₄ active material with and without Al₂O₃ coating, 10wt% acetylene black as a conductive additive and 10wt% polyvinylidene fluoride (PVDF) as a binder, and nmethyl pyrrolidone (NMP) as a solvent. The paste was then coated onto an aluminum foil, and finally was dried under vacuum at 100℃ for 10 h. The coin cells were assembled in glove box filled with pure argon (H₂O, O₂ <0.1×10^-6, M. Braun). The cells were placed for 12h before testing. The charge-discharge tests were conducted on a battery test system (C2001A, LAND, China) with cut-off voltages of 2.5-4.3 V ( vs Li/Li⁺) at different rates at 25℃ and 55℃. The electrochemical impedance spectroscope (EIS) was employed to characterize the interfacial resistance of cathode using a Chenhua CHI760B Electrochemical Workstation over the frequency range from 1 MHz to 1 mHz with an amplitude of 10 mV at room temperature.

The XRD patterns of the Al₂O₃-coated LiFePO₄ powders are the same as that of the sample without Al₂O₃ coating, and no Al₂O₃ peaks occur as shown in Fig. 1. A possible explanation is that the lower processing temperature of 500℃ would result in low crystalline or amorphous-like Al₂O₃.This may be due to the little amount of the Al₂O₃coating, and the coating consists of nano amorphous particles. The lattice constants of uncoated LiFePO₄ and Al₂O₃-coated LiFePO₄ (in Table 1) are obtained by Rietveld refinement analysis. The Al₂O₃ coating does not cause the change of the lattice constants, which indicates that nano Al₂O₃ particles adhere on the surface of LiFePO₄particles instead of diffusing into LiFePO₄ lattice.

Fig. 1

	Figure Option ViewDownloadNew Window
	Fig. 1 XRD patterns of pristine LiFePO4 (a) and the coated LiFePO4 powders with Al2O3 content of 1wt% (b), 2wt% (c), 3wt% (d) and 5wt% (e), respectively

Table 1

Table 1

Table 1 Lattice constants of pristine LiFePO4 and Al2O3-coated LiFePO4 by Rietvelt refinement Sample Lattice constants/nm Volume of unit/nm3 Space group a b c Pristine LiFePO4 1.03301(3) 0.60049(2) 0.46932(2) 0.29112(4) Pmnb 1wt% Al2O3 1.03306(4) 0.60034(3) 0.46893(1) 0.29082(4) Pmnb 2wt% Al2O3 1.03323(2) 0.60035(1) 0.46912(2) 0.29099(4) Pmnb 3wt% Al2O3 1.03304(1) 0.60021(2) 0.46910(4) 0.29086(1) Pmnb 5wt% Al2O3 1.03330(1) 0.60035(1) 0.46920(2) 0.29106(4) Pmnb

Table 1 Lattice constants of pristine LiFePO4 and Al2O3-coated LiFePO4 by Rietvelt refinement

TEM images of the uncoated LiFePO₄ and 2wt% Al₂O₃-coated LiFePO₄ samples are shown in Fig. 2. The LiFePO₄ particles are of nearly orbicular shape, the uncoated particles exhibit almost smooth surfaces as shown in Fig. 2(a), whereas the surface of the Al₂O₃-coated particles are covered with a thin amorphous layer as shown in Fig. 2(b, c), which should be the Al₂O₃ coating.

Fig. 2

	Figure Option ViewDownloadNew Window
	Fig. 2 TEM images of pristine LiFePO4 (a) and 2wt% Al2O3- coated LiFePO4 (b), (c)

SEM and EDS mapping images of 2wt% Al₂O₃-coated LiFePO₄ show that the product particles consist of the uniform fine grains with the average size of about 300-500 nm. Al₂O₃ coating on the surface of LiFePO₄ particle could not be observed clearly (Fig. 3(a)) because of the low content and small particle size of Al₂O₃ powder. The EDS mappings of Al, Fe (Fig. 3(c) and Fig. 3(d)) match with the SEM micrograph of the corresponding particles shown in Fig. 3(b), which clearly reveal that Al₂O₃ is uniformly distributed on the surface of LiFePO₄ particles, which is consistent with the TEM analysis shown in Fig. 2.

Fig. 3

	Figure Option ViewDownloadNew Window
	Fig. 3 SEM images of Al2O3-coated LiFePO4(a, b) and the corresponding EDS mapping of Al (c) and Fe (d) element

The discharge capacity and cycling performance of the uncoated LiFePO₄ and the Al₂O₃-coated LiFePO₄ cathode materials were tested between 2.5 V and 4.3 V at 25℃. Figure 4(a, b) show the curves of discharge capacity vs cycle numbers at rate of 0.1 C and 5 C, respectively. The results show the discharge capacities of the uncoated LiFePO₄ are nearly 150 mAh/g at 0.1 C and 110 mAh/g at 5 C, whereas the capacity of the 2wt% coated LiFePO₄cell get nearly 155 mAh/g at 0.1 C and 123 mAh/g at 5 C. The higher capacities of Al₂O₃-coated LiFePO₄ derive from improving of the orderliness of lithium-ion intercalation/de-intercalation and accelerating the lithium-ion diffusion. This result also can be verified from the impedance measurement as shown in Fig. 5. As the coating amount increases, however, the capacity of 5.0wt% Al₂O₃ coated samples drops. This might result from the thick coating layer to obstruct from Li-ion diffusion.

Fig. 4

	Figure Option ViewDownloadNew Window
	Fig. 4 Comparison of cycling behaviors of pristine LiFePO4 and Al2O3-coated LiFePO4 at rate of 0.1 C (a) and 5 C (b)

The electrochemical impedance spectroscopy (EIS) of the uncoated LiFePO₄ and 2wt% Al₂O₃-coated LiFePO₄ are shown in Fig.5. After the first cycle, the cathode impedance shifts from 39 Ω to 24 Ω (Fig. 5(a)). After 100 cycles, the impedance of the uncoated LiFePO₄ cathode increases to 94 Ω, while the impedance of the Al₂O₃-coated LiFePO₄ cathodes only has 40 Ω. It indicates that the interfacial resistance is significantly suppressed for the Al₂O₃-coated LiFePO₄, as depicted in Fig. 5(a). The increase of the interfacial resistance with cycle numbers has also been restrained significantly. This means that the lithium-ion diffusion is enhanced in the Al₂O₃-coated LiFePO₄. Similar results were also found for SiO₂-coated LiFePO₄^{[ 16]}. This outcome suggests that the degradation at the LiFePO₄/electrolyte interface is restrained by the metal oxide coating, which seems to improve the interfacial properties on the LiFePO₄ surface. In order to investigate the cycling behavior of LiFePO₄with the Al₂O₃ coating at the elevated temperature, the cycling has been carried out between 2.5 V and 4.3 V at 55℃ and 1 C rate. The capacity retention curve is shown in Fig. 6. As a result, the capacity retention rate of the LiFePO₄without Al₂O₃ coating is only about 85% of the initial capacity after 100 cycles, while that of the LiFePO₄with 3wt% Al₂O₃coating increases significantly and reach 95%. But the capacity retention of 5wt% Al₂O₃ coated LiFePO₄ is less than that of 3wt% coated sample as shown in Fig. 6. The improvement of the cycle performance of the LiFePO₄ with Al₂O₃ coating is largely due to the nano Al₂O₃ coating, which helps to prevent LiFePO₄ particles from having direct contact to the electrolyte, and thus reduces iron dissolution of LiFePO₄ during cycle.

Fig. 5

	Figure Option ViewDownloadNew Window
	Fig. 5 Electrochemical impedance spectroscopy (EIS) of the cells discharged to 2.5 V after 1 and 100 cycles with testing frequency from 1 MHz to 1 mHz at room temperature

Fig. 6

	Figure Option ViewDownloadNew Window
	Fig. 6 Capacity retention rates of the cells vs weight ratio of Al2O3after 100 cycles at 55℃ and 1 C

But when the amount of Al₂O₃ coating layer is more than 3wt%, the increase of the interfacial resistance results to the capacity and capacity retention decrease.

The electrochemical performance of LiFePO₄ cathode can be improved by nano Al₂O₃ coating, which significantly increases the capacity and cycle ability of the coated LiFePO₄ electrode. About 2wt% Al₂O₃ coating results in a higher cycling capacity and lower cell impedance and excellent capacity retention of the cell. The reason for the improved electrochemical performance of the Al₂O₃-coated LiFePO₄ could be that the Al₂O₃ coating effectively prevents the LiFePO₄ particles from having direct contact with the electrolyte solution, improving the structural stability, increasing the order of lithium-ion diffusion, and reducing the interfacial resistance. Therefore, nano Al₂O₃ surface modification for the LiFePO₄ particle is a promising route for preparing LiFePO₄ based cathode materials.