Trans. Nonferrous Met. Soc. China 23(2013) 451-455

Synthesis and characterization of LiFePO₄ coating with aluminum doped zinc oxide

Hao TANG¹, Long TAN², Jun XU¹

1. Department of Materials Science, Fudan University, Shanghai 200433, China;

2. Felie Li-ion Battery Manufacture Co., Ltd., Wuxi 214028, China

Received 10 September 2012; accepted 18 December 2012

Abstract: Aluminum doped zinc oxide (AZO), as an electrically conductive material, was applied to coating on the surface of olivine-type LiFePO₄ synthesized by solid-state method. The charge-discharge test results show that the rate performance and low-temperature performance of LiFePO₄ are greatly improved by the surface treatment. Even at 20C rate, the discharge specific capacity of 100.9 mA·h/g was obtained by the AZO-coated LiFePO₄ at room temperature. At -20 °C, the discharge specific capacity at 0.2C for un-coated LiFePO₄ and the coated one are 50.3 mA·h/g and 119.4 mA·h/g, respectively. It should be attributed to the electrically conductive AZO-coating which increases the electronic conductivity of LiFePO₄. Furthermore, the surface-coating increases the tap-density of LiFePO₄. The results indicate that the AZO-coated LiFePO₄ is a good candidate of cathode material for applying in lithium power batteries.

Key words: lithium ion battery; LiFePO₄; coating; cathode material; aluminum doped zinc oxide

1 Introduction

Olivine-type LiFePO₄, as a replacement of LiCoO₂, has attracted attention since the significant report in 1997 by good enough [1] in which it was proposed to be used as cathode material for lithium ion batteries. It has the advantages of environment benign, inexpensive, thermal stable and good cycling stability [2-4]. However, its immanent disadvantages of low electronic conductivity and low diffusion rate of lithium ion during the charge-discharge process destroy the rate performance [5-7], and the capacity decreases rapidly even at moderate current density.

Many reports have discovered that the electrical performance of LiFePO₄ can be improved by ion doping [8], carbon coating [9,10] and metal oxide coating [11-13]. Especially, coating carbon which is an electrically conductive material has been wildly applied in industry with advantages of facie, inexpensive and good effect to control the particle size of LiFePO₄. However, the electrically conductive carbon reduces the tap-density of LiFePO₄, which is being harmful to the energy density of the lithium ion battery [14]. Further- more, the low-temperature performance of carbon-coated LiFePO₄ is also needed to be improved [11].

In this work, the electrically conductive material of aluminum doped zinc oxide(AZO) is firstly applied to coating on the surface of LiFePO₄. It is reported that the insulated pure zinc oxide can be changed to be electrically conductive with doping appropriate metals such as indium, gallium and aluminum [15,16]. And the resistivity of the AZO film even can be decreased to 10^-4 Ω·cm [16]. Besides, the solid density of zinc oxide (5.7 g/cm³) is even higher than that of the LiFePO₄ (3.6 g/cm³). Therefore, both the electrochemical properties and tap density of LiFePO₄ would be greatly improved by coating with the electrically conductive AZO. The low-temperature performance of the AZO- coated LiFePO₄ powders is also investigated.

2 Experimental

LiFePO₄ was synthesized by solid-state method using FeC₂O₄·2H₂O, NH₄H₂PO₄ and Li₂CO₃ as the raw materials. Stoichiometric FeC₂O₄·2H₂O, NH₄H₂PO₄ and Li₂CO₃ were full-mixed in isopropanol by ball milling for 2 h. Then the obtained slurry was dried at 80 °C. The resulted powders were calcined at 700 °C for 8 h in a purified N₂ flow and the uncoated LiFePO₄ was obtained, which is signed as Un-LFP.

C₄H₆ZnO₄·2H₂O and C₈H₁₂A_l2O₉·4H₂O in the molar ratio of 43.7:1 were initially dissolved in distilled water. The Un-LFP was then added and the mixtures were stirred for 1 h in order to make the Un-LFP disperse homogeneously in the solution. The resultant solution was then dried at 120 °C to get the powder precursor. The powder precursor was calcined at 500 °C for 1 h in a purified N₂ flow and the LiFePO₄ coating with AZO was finally obtained (m(AZO):[m(LiFePO₄)+ m(AZO)]=2%), signed as AZO-LFP.

All the reagents used in the experiment were of analytical purity and were used without further purification. Powder X-ray diffraction (XRD) analysis using Cu K_α radiation was employed to identify the crystalline phase of the prepared powder with a Bruker D8-advance (German) at room temperature in the range of 10°≤2θ≤70°. The morphology was investigated with a field emission scan electron microscope (SEM, S-4800, Japan). The tap-density meter (BT-300, Dandong Bettersize Instruments Ltd., China) was applied to measuring the tap-density of the prepared powder. The coin cells were assembled by using lithium foil as anode in an argon-filled glove box, the as-prepared powders mixed with 12% acetylene black and 8% PVDF as the cathode and 1 mol/L LiPF₆ in a 1:1(V/V) mixture of ethylene carbonate (EC) and dimethylcarbonate(DMC) as the electrolyte, Celgard 2300 membrane as the cell separator. The charge-discharge cycle was performed in a voltage range of 2.0-3.8 V using the coin cells. All the electrical measurements were carried out by a battery testing system (Landet-5 V/10 mA, Landet Electronic Corporation, China) at room temperature.

3 Results and discussion

Figure 1 shows the XRD patterns of the Un-LFP and AZO-LFP powders. All diffraction lines can be index for the orthorhombic olive structure with pmnb space group (JCPDS card No. 40—1499) [11]. No peaks corresponding to AZO were observed, indicating that the low content of AZO compound cannot be detected by XRD. The lattice constants of the Un-LFP and AZO-LFP powders have been refined and tabulated in Table 1. It is consistent with that of the standard LiFePO₄ and the results obtained in other reports [11,13]. The result also confirms that the surface modification with AZO did not cause the change in lattice constants. This indicates that the Al³⁺ and Zn²⁺ ions are adhere on the surface of the LiFePO₄ powders as AZO coating rather than diffuse into LiFePO₄ lattice. And more, the tap-densities of the Un-LFP and AZO-LFP powders were also measured and tabulated in Table 1. It is obvious that the tap density of LiFePO₄ is improved by the AZO coating. The result is different from that of the carbon coating and is significant for applying in lithium power batteries.

Fig. 1 XRD patterns of Un-LFP and AZO-LFP powders collected by steps of 0.02° in 10°≤2θ≤70°

Table 1 Calculated structure parameters and tap-densities of Un-LFP and AZO-LFP powders

The SEM images of the Un-LFP and AZO-LFP powders are shown in Fig. 2. It can be clearly seen in Fig. 2(a) that the surface of Un-LFP powder is smooth with little sponge-like material coating on and between the particles. It can be attributed to the decomposition of  in FeC₂O₄·2H₂O in purified N₂ flow, which causes little carbon remain. As seen in Fig. 2(b), the surfaces of LiFePO₄ particles are covered by some well-distributed nano-particles, which should be the AZO material. It proves that the AZO material has been successfully coated on the surface of LiFePO₄, which is consistent with the XRD result mentioned above.

The initial charge-discharge curves of the Un-LFP and AZO-LFP powders tested at different current densities are shown in Fig. 3(a). The cells were firstly charged to 3.8 V at the constant current density, then charged until the charge current density decreased to 0.05C, and finally discharged to 2.0 V at the constant current density. All these curves have the smooth plateaus, indicating the well-crystallized structure of the both samples. The initial charge-discharge specific capacities of the Un-LFP powders at 0.1C and 1C are 156.3/150.3 mA·h/g and 145.5/130.9 mA·h/g, respectively. For the AZO-LFP powders, 165.8/160.1 mA·h/g and 158.1/148.0 mA·h/g were obtained at 0.1C

Fig. 2 SEM images of Un-LFP (a) and AZO-LFP powders (b)

and 1C, respectively. Furthermore, it is obvious that lower discharge plateau is obtained for the curves of the Un-LFP powders, which attributes to the more sever polarization caused by the poor electronic conductivity of LiFePO₄ particles. The results well prove that the electrically conductive AZO coating increases the electronic conductivity of LiFePO₄, and then improves the specific capacities especially at a high rate.

Figure 3(b) shows the cycle performance of the Un-LFP and AZO-LFP powders. Good results at 0.1C are obtained for the both samples. About 99.5% and 97.4% of the initial capacities are retained for the AZO-LFP and Un-LFP powders. When the current density is increased to 1C, 96.5% of the initial specific capacity is retained for the AZO-LFP powders, but only 91.3% for the Un-LFP powders. This result demonstrates that the AZO coating can improve the cycling stability of LiFePO₄. This improvement is largely due to the presence of AZO which could impede the reaction between the cathode particles and electrolyte [10].

Fig. 3 Initial charge-discharge curves (a) and cycling properties (b) of Un-LFP and AZO-LFP powders tested at room temperature

To evaluate the rate capability of the Un-LFP and AZO-LFP, the discharge curves tested at different rates are shown in Fig. 4. For the Un-LFP (Fig. 4(a)), the discharged specific capacity decreases dramatically with increasing the discharge rate. When the rate increases to 5C, only 72.0 mA·h/g of the specific capacity is retained for the Un-LFP. However, good rate performance is obtained for AZO-LFP, as shown in Fig. 4(b). The discharge specific capacity can reach 100.9 mA·h/g even at 20C rate, indicating that the AZO-LFP is a good candidate of cathode material for lithium power batteries.

Fig. 4 Rate performance of Un-LFP (a) and AZO-LFP (b) powders at room temperature

Figure 5 displays the initial discharge curves of the Un-LFP and AZO-LFP (0.2C) tested at -20 °C. Apparently, the Un-LFP has a bad low-temperature performance shown in Fig. 5. After coating the AZO material, a smooth plateau appears in the discharged process and the specific capacity increases to 119.4 mA·h/g, which indicates that the reaction of Li ions extration/insertion and electron transfer can well proceed at -20 °C. It was reported that the surface reaction kinetics is slowed down with the drop of operation temperature [11]. The obtained results of AZO-LFP should be attributed to the surface treatment which increases the electrode kinetics at -20 °C. Moreover, the improved electrode kinetics can also be beneficial to the rate performance of LiFePO₄, which is consistent with the result in Fig. 4.

Fig. 5 Initial discharge curves of Un-LFP and AZO-LFP powders tested at -20°C

4 Conclusions

AZO was successfully coated on the surface of LiFePO₄ synthesized by solid-state method. Discharge specific capacity of 100.9 mA·h/g can be retained by AZO-coated LiFePO₄ testing in the current density of 20C at room temperature, whereas the un-coated LiFePO₄ can only reach 72.0 mA·h/g even at 5C rate. At -20 °C, the discharge specific capacities at 0.2C for un-coated LiFePO₄ and the coated one are 50.3 mA·h/g and 119.4 mA·h/g, respectively. The surface treatment of AZO-coating greatly improves the rate performance and low-temperature performance of LiFePO₄, which indicates that the AZO-LFP is a good candidate of cathode material for lithium power batteries.

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掺铝氧化锌包覆LiFePO₄的合成与性能

汤 昊¹，谭 龙²，许 军¹

1. 复旦大学 材料系，上海 200433；

2. 江苏恒益锂电池制造有限公司，无锡 214028

摘  要：以简单的固相法合成了橄榄石结构LiFePO₄，并以导电掺铝氧化锌材料(AZO)对其表面进行包覆。充放电结果显示，表面包覆大幅度改善了LiFePO₄材料的倍率和低温性能。在20C高倍率条件下，AZO包覆LiFePO₄的放电比容量可达100.9 mA·h/g；在低温-20 °C时进行0.2C充放电，未包覆LiFePO₄和AZO包覆LiFePO₄的放电比容量分别为50.3 mA·h/g和119.4 mA·h/g。经分析，这可能是由于采用导电AZO包覆措施而增加了LiFePO₄材料的电导率，从而极大地提高了其比容量。另外，导电AZO包覆措施还增加了LiFePO₄材料的振实密度。这些结果表明AZO包覆LiFePO₄材料是一种很好的适用于锂离子动力电池的正极材料。

关键词：锂离子电池；LiFePO₄；包覆；正极材料；掺铝氧化锌

(Edited by Hua YANG)

Corresponding author: Jun XU; Tel/Fax: +86-21-55664563; E-mail: junxu@fudan.edu.cn

DOI: 10.1016/S1003-6326(13)62484-X