Trans. Nonferrous Met. Soc. China 23(2013) 1028-1032

Preparation and electrochemical performance of 2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C cathode material for lithium-ion batteries

Jia-feng ZHANG, Bao ZHANG, Xue-yi GUO, Xing OU, Jian-long WANG, Chun-li PENG, Jun-chao ZHENG, He-zhang CHEN, Chao SHEN

School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China

Received 7 May 2012; accepted 5 September 2012

Abstract: 2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C was synthesized using Fe_1-2xCo_2xVO₄ as precursor which was prepared by a simple co-precipitation method. 2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical measurements. All 2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C composites are of the similar crystal structure. The XRD analysis and SEM images show that 2LiFe_0.96Co_0.04PO₄-Li₃V₂(PO₄)₃/C sample has the best-ordered structure and the smallest particle size. The charge-discharge tests demonstrate that these powders have the best electrochemical properties with an initial discharge capacity of 144.1 mA·h/g and capacity retention of 95.6% after 100 cycles when cycled at a current density of 0.1C between 2.5 and 4.5 V.

Key words: LiFePO₄; Li₃V₂(PO₄)₃; Co doping; lithium-ion batteries

1 Introduction

The applications of electric vehicles (EVs) and hybrid electric vehicles (HEVs) require lithium-ion batteries to have the qualities of high energy density and power density, long cycling life, etc. Olivine-type LiFePO₄ [1], as a promising cathode material for lithium-ion rechargeable batteries, has been extensively studied owing to its environmental friendliness, low costs, and good cycling stability. However, its low-electronic conductivity and low lithium-ion diffusivity within particles [2,3] result in a poor rate capability, which prevents it from being applied more widely. Many efforts including metal doping [4,5], coating with the electronically conductive materials like carbon, metal, and metal oxide [6-9], and optimization of particles with suitable preparation procedures [10,11] have been made to improve the performance of LiFePO₄ cathode materials.

Compared with LiFePO₄, monoclinic Li₃V₂(PO₄)₃ possesses higher ionic diffusion coefficient, better rate capability, higher discharge voltage and energy density [12,13]. Moreover, Li₃V₂(PO₄)₃ exhibits two pairs of redox plateaus around 3.62, 3.68, 4.08, and 4.55 V. It can extract/insert two Li ions reversibly between 3.0 and 4.3 V based on the V³⁺/V⁴⁺ redox coupled with an average operation voltage of 3.8 V. Three Li ions may be completely extracted when charged to 4.8 V with a mean intercalation voltage up to 4.0 V [14,15].

WANG et al [16] found that 2LiFePO₄- Li₃V₂(PO₄)₃/C compound has great electrochemical performance, and it is highly desirable to investigate the valence of V in these LiFePO₄ compounds. Moreover, an increase in the conductivity by the chemical substitution of Co for Fe resulting in LiFe_xCo_1-xPO₄ compositions has been reported [17].

In this work, a series of 2LiFe_1-xCo_xPO₄- Li₃V₂(PO₄)₃/C samples were prepared using Fe_1-2xCo_2xVO₄ as precursor compounds, with which LiCoPO₄ formed together as an additive during the heating process. The electrochemical characterization of all the samples was carried out by galvanostatic charge– discharge experiments and cyclic voltammetry. Besides enhancement of conductivity and mean operation voltage, a high capacity was also expected for the LiFePO₄-based Li-ion batteries.

2 Experimental

2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C(x=0.01, 0.02, 0.03, 0.04, 0.05) samples were synthesized by means of co-precipitation method using Fe_1-2xCo_2xVO₄ as precursor. The Fe_1-2xCo_2xVO₄ precursor was prepared by a simple co-precipitation method, details of which are described as follows: a 0.2 mol/L NH₄VO₃ solution was pumped into a continuously stirred reactor at 80 °C, and 0.2 mol/L Fe(NO₃)₃ and Co(NO₃)₂ solution were added drop by drop at pH 6. The resultant slurry was aged in a sealed container at 25 °C for 12 h. The precipitate was collected by filtration, air-drying and then calcined at 600 °C for 6 h. Thereafter, the obtained Fe_1-2xCo_2xVO₄ powder was mixed with a stoichiometric amount of Li₂CO₃, NH₄H₂PO₄, glucose in acetone according to the carbon contents in 2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C composite, and then ball milled for 4 h. After evaporating the acetone at 100 °C, the mixture was heat-treated in a horizontal quartz tube from ambient temperature to 750 °C with the heating rate of 5 °C/min and kept at that temperature for 12 h in N₂ atmosphere. Eventually, 2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C composite was synthesized.

The samples were characterized by X-ray diffraction (XRD, Rigaka D/MAX 2500V) using Cu K_α radiation (K_α=0.154 nm) to identify the crystalline phase of the prepared Fe_1-2xCo_2xVO₄ and 2LiFe_1-xCo_xPO₄- Li₃V₂(PO₄)₃/C samples. The data were collected in the range of 10°-90° in scanning rate of 8 (°)/min. The surface morphologies of the samples were observed with a scanning electron microscope (SEM, JSM-6380LV).

Electrochemical properties of 2LiFe_1-xCo_xPO₄- Li₃V₂(PO₄)₃/C electrodes were measured after assembling them into coin cells (type CR2032) in an argon-filled glove box. The cathode was prepared by spreading a mixture of active material (80% in mass fraction), acetylene black (10%), and poly(vinylidene fluoride) binder (10%) dissolved in N-methyl pyrrolidone onto an aluminum foil current collector with thickness of 0.2 mm and dried in a vacuum oven at 120 °C. The cathode was separated from the lithium anode by a separator (Celgard 2300). The electrolyte was 1 mol/L LiPF₆ dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and methyl–ethyl carbonate (EMC) with a volume ratio of 1:1:1. The cells were cycled between 2.5 and 4.5 V at ambient temperature using a battery testing system (Neware BTS-2000) at 0.1C current rate.

3 Results and discussion

3.1 Characterization of Fe_1-2xCo_2xVO₄

The XRD patterns of the Fe_1-2xCo_2xVO₄ samples are shown in Fig. 1. As can be seen, all the peaks can be readily indexed as a triclinic structure corresponding to FeVO₄ and no impurity-phase peaks exist, indicating that the structure of the samples has not been changed with Co doping. The diffraction peaks are found to be very narrow, revealing that the samples have good crystallinity.

Fig. 1 XRD patterns of Fe_1-2xCo_2xVO₄ samples

SEM micrographs of Fe_1-2xCo_2xVO₄ samples with different Co contents are presented in Fig. 2. It can be clearly observed that all the samples consist of irregular primary particles. Besides, the size of irregular primary particles increases when the Co is doped. However, there is no difference in the micrographs of Fe_1-2xCo_2xVO₄ with different Co contents. It is indicated that Co doping has little influence on the structure and micrographs of Fe_1-2xCo_2xVO₄.

3.2 Structure and morphology of 2LiFe_1-xCo_xPO₄- Li₃V₂(PO₄)₃/C

To prepare 2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C, the Fe_1-xCo_xVO₄ precursor was mixed with Li₂CO₃, NH₄H₂PO₄ and glucose and then the mixture was treated by a high-temperature calcination process.

The XRD patterns of the 2LiFe_1-xCo_xPO₄- Li₃V₂(PO₄)₃/C powders are shown in Fig. 3. All main peaks can be indexed as crystallized LiFePO₄ phase and crystallized Li₃V₂(PO₄)₃ phase. The narrow diffraction peaks reveal that the samples have good crystallinity. These results indicate that solid-state reaction method and experimental conditions can produce 2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C with high crystallinity and purity, which are key factors for a high performance electrode material. It has also been found that there is no visible difference between the XRD patterns of pristine 2LiFePO₄-Li₃V₂(PO₄)₃/C and low Co-doped samples (x=0.01, 0.02), which indicates that Co has been doped into the host lattice or the amount of the composite formed from cobalt is too small. In contrast, for those high-level Co-doped samples (x=0.03, 0.04, 0.05), the diffraction intensity of peaks of LiFePO₄ decreases to a certain extent, indicating that the structure of the material has a small change.

Fig. 2 SEM images of Fe_1-2xCo_2xVO₄ samples prepared

Fig. 3 XRD patterns of 2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C samples

Figure 4 shows SEM images of 2LiFe_1-xCo_xPO₄- Li₃V₂(PO₄)₃/C composites. The powders consist of agglomerates of primary particles. The morphology of composites is slightly changed with different amounts of Co doping. The particle size of pristine decreases when x is below 0.04, whereas that of pristine with x=0.05 increases and it presents irregular blocks with a wide size distribution.

3.3 Electrochemical performance of 2LiFe_1-xCo_xPO₄- Li₃V₂(PO₄)₃/C

Figure 5 shows the initial charge-discharge voltage profiles of 2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C composites synthesized with different Co doping amounts in the cut-off potential range of 2.5-4.5 V at 0.1C rate. The initial discharge capacities of composites with different cobalt contents of 0.01, 0.02, 0.03, 0.04 and 0.05 are 135.7, 140.1, 142.4, 144.1 and 139.4 mA·h/g, respectively. During the calculation of the discharge capacity of 2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C samples, the active materials includes 10% carbon in the composites, but does not include the acetylene black added as electric conductor in the electrode fabrication.

Fig. 4 SEM images of 2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C samples

Fig. 5 Initial charge-discharge curves of 2LiFe_1-xCo_xPO₄- Li₃V₂(PO₄)₃/C composites

Obviously, all the charge-discharge profiles exhibit four redox plateaus around 3.38, 3.62, 3.68 and 4.08 V, which are ascribed to one Li⁺ extracted based on the Fe²⁺/Fe³⁺ redox potential for LiFePO₄ and three Li⁺ reversibly extracted based on the V³⁺/V⁴⁺ redox couple for Li₃V₂(PO₄)₃. Moreover, the redox plateaus around 3.38, 3.62 and 3.68 V of 2LiFe_1-xCo_xPO₄- Li₃V₂(PO₄)₃/C (x=0.03, 0.04, 0.05) are higher than those of the samples with x=0.01 or 0.02. So, we can conclude that the proper content of Co-doping can effectively improve the discharge capacity and discharge potential plateau.

Specific capability is one of the important electrochemical characteristics of a lithium secondary battery for storage application. Figure 6 shows the cycling performance of 2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C composites synthesized with different Co contents at 0.1C rate between 2.5 and 4.5 V at room temperature. 2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C samples with x of 0.01, 0.02, 0.03, 0.04 and 0.05 keep the capacity retention of 90.3%, 91.4%, 92.2%, 95.6% and 92.8%, respectively after 100 cycles.

Fig. 6 Cycle performance of 2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C with various amounts of Co doping at 0.1C

When Co content is less than 0.04, 2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C composites show better cycling performance with the increase of carbon content because addition of cobalt enhances the conductivity of the composites [17]. Meanwhile, addition of cobalt can decrease the sizes of particles and thus reduce the volume change of composite particles during the charge-discharge cycles. So, addition of cobalt enhances the structural stability of composites in charge-discharge process and improves the cycling performance. However, when Co content is 0.05, the composites show a worse cycling performance. Among these cathodes, 2LiFe_0.96Co_0.04PO₄-Li₃V₂(PO₄)₃/C shows the highest initial discharge capacity of 144.1 mA·h/g, and 95.6% capacity after 100 cycles.

4 Conclusions

1) 2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C samples with different Co doping contents can be synthesized using Fe_1-2xCo_2xVO₄ as precursor.

2) Co doping has little effect on structure and morphology of the precursor. All 2LiFe_1-xCo_xPO₄- Li₃V₂(PO₄)₃/C composites are of the similar crystal structure with crystallized LiFePO₄ phase and crystallized Li₃V₂(PO₄)₃ phase.

3) The maximum capacity of 144.1 mA·h/g is observed in 2LiFe_0.96Co_0.04PO₄-Li₃V₂(PO₄)₃/C sample at 0.1C. The capacity retention of the sample remains at 95.6% after 100 cycles at 0.1C.

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2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C锂离子电池正极材料的制备和性能

张佳峰，张 宝，郭学益，欧 星，王健龙，彭春丽，郑俊超，陈核章，沈 超

中南大学 冶金科学与工程学院，长沙 410083

摘  要:用共沉淀法合成的Fe_1-2xCo_2xVO₄为原料制备2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C。采用X射线衍射、扫描电镜以及电化学测试对2LiFe_1-xCo_xPO₄-Li₃V₂(PO₄)₃/C样品进行表征。结果表明，所有2LiFe_1-xCo_xPO₄- Li₃V₂(PO₄)₃/C 复合材料样品均有相似的晶体结构。X射线衍射和扫描电镜表明，2LiFe_0.96Co_0.04PO₄-Li₃V₂(PO₄)₃/C具有最有序的晶体结构和细小、均匀的颗粒。充放电测试表明，2LiFe_0.96Co_0.04PO₄-Li₃V₂(PO₄)₃/C具有最佳的电化学性能，在电压2.5~4.5 V，0.1C条件下首次放电比容量可达144.1 mA·h/g，循环100次后容量保持率仍可达到95.6%。

关键词: 磷酸铁锂；磷酸钒锂；钴掺杂；锂离子电池

(Edited by Hua YANG)

Foundation item: Project (51072233) supported by National Natural Science Foundation of China

Corresponding author: Bao ZHANG; Tel: +86-731-88836357; E-mail: csuzb@vip.163.com

DOI: 10.1016/S1003-6326(13)62562-5