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李荐, 姚书恒, 周宏明, 耿文俊. 用固相和水热结合法制备LiMn0.4Fe0.6PO4/C复合材料 . 无机材料学报, 2014, 29(4): 443-448
LI Jian, YAO Shu-Heng, ZHOU Hong-Ming, GENG Wen-Jun. Preparation of LiMn0.4Fe0.6PO4/C Composite by A New Route Combining Solid-state Reaction with Hydrothermal Synthesis . Journal of Inorganic Materials, 2014, 29(4): 443-448  
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用固相和水热结合法制备LiMn0.4Fe0.6PO4/C复合材料
李荐1,2,3, 姚书恒1, 周宏明1,2,3, 耿文俊1
1. 中南大学 材料科学与工程学院, 长沙410083
2. 湖南省正源储能材料与器件研究所, 长沙410083
3. 中南大学 有色金属材料科学与工程教育部重点实验室, 长沙410083
修回日期:2014-01-08
摘要

以Li2CO3、FeC2O4•2H2O、MnCO3和 NH4H2PO4为原料, 按5:6:4:10的摩尔比混合, 采用固相反应和水热法结合的新方法制备得到LiMn0.4Fe0.6PO4。通过XRD、SEM、TEM以及循环伏安(CV)和充放电测试对材料进行结构、形貌以及电化学性能表征。结果表明, 此方法合成的产物具有单一的橄榄石晶体结构, 颗粒尺寸约为120 nm, 且表面均匀包覆一层无定形碳。电化学测试结果表明, 样品的循环伏安曲线中有两对氧化还原峰, 分别对应Fe3+/Fe2+(3.5 V)和Mn3+/Mn2+(4.0 V)。LiMn0.4Fe0.6PO4/C在0.1C下的初始放电比容量为160 mAh/g, 0.5C下的初始放电比容量为143 mAh/g, 且具有较好的循环性能。

关键词: 锂离子电池; 固相反应; 水热合成; 电化学性能; 电子显微技术
中图分类号:TQ174   文献标志码:A    文章编号:1000-324X(2014)04-0443-06
Preparation of LiMn0.4Fe0.6PO4/C Composite by A New Route Combining Solid-state Reaction with Hydrothermal Synthesis
LI Jian1,2,3, YAO Shu-Heng1, ZHOU Hong-Ming1,2,3, GENG Wen-Jun1
1. School of Materials Science and Engineering, Central South University, Changsha 410083, China
2. Hunan Zhengyuan Institute for Energy Storage Materials and Devices, Changsha 410083, China
3. Key Laboratory of Nonferrous Metal Materials Science and Engineering of Ministry of Education, Central South University, Changsha 410083, China
LI Jian(1969–), male, PhD. E-mail:ziliao2000@126.com
Corresponding author: ZHOU Hong-Ming, PhD. E-mail:ipezhm@163.com
Fund:Hunan Science and Technology Project(2013GK3002); Changsha Science and Technology Project (K1202039-11);
Abstract

The olivine type LiMn0.4Fe0.6PO4 was synthesizedvia a new route that combining solid-state reaction with hydrothermal synthesis from Li2CO3, FeC2O4·2H2O, MnCO3 and NH4H2PO4 (at molar ratio of 5:6:4:10). The structure, particle size and surface morphology of these cathode active materials were investigated by XRD, SEM and TEM techniques. The electrochemical properties of LiMn0.4Fe0.6PO4/C as cathode in lithium-ion cells were testedvia cyclic voltammetry and galvanostatic charge-discharge measurements. Phase-pure particles of size ~120 nm are prepared with a conductive, thin web of carbon surrounding them. Cyclic voltammetry shows the active voltage range of the material which consists of two pairs of redox peaks: 3.5 V corresponding to Fe3+/Fe2+ reaction and 4.0 V corresponding to Mn3+/Mn2+ reaction. An initial discharge capacity of 160 mAh/g at 0.1C, 143 mAh/g and a stable cycling property at 0.5C are obtained for the LiMn0.4Fe0.6PO4/C composite.

Keyword: Li-ion battery; solid-state reaction; hydrothermal synthesis; electrochemical property; electron microscopy

In recent years, lithium-ion battery have developed rapidly due to the increased market demand for portable electronics, transportation and energy storage. Among the families of cathode materials, LiFePO4 is considered to be one of the most promising candidates for high- power batteries. Many efforts have been made over the past few years to improve the power performance of LiFePO4 by reducing particle size and improving the conductivity of the solid phase[ 1, 2, 3, 4, 5, 6, 7, 8]. High-performance LiFePO4 has appeared now in the field of commercial power Li-ion battery[ 9].

Since the success of LiFePO4, LiMnPO4 with the same structure and theoretical capacity (170 mAh/g) also receives extensive attention as attractive cathode material. LiMnPO4 has a redox potential of 4.1 V vs Li+/Li, is compatible with existing electrolytic liquid system[ 10, 11]. The voltage platform of 4.1 V vs Li+/Li makes LiMnPO4 has a higher theoretical energy density and lower energy costs than LiFePO4. However, along with low ionic conductivity, the electronic conductivity of LiMnP0.4 (<10-10s/cm) is much lower than that of LiFePo4 (1.8×10-9 s/cm) causing it difficult to obtain decent electrochemical activity[ 12, 13, 14]. Furthermore, electrochemical performance of LiMnPO4 becomes worse due to Jahn-Teller effect in Mn3+and interface strain owing to mutual transformation between LiMnPO4and MnPO4 during cycling[ 15]. So, LiMnPO4 materials with capacities more than 120 mAh/g are rarely attained in contrast to LiFePO4[ 14, 15, 16]. But higher discharge capacity could be attained when Fe and Mn coexist at the octahedral 4c site in the olivine structure. Therefore, LiMn yFe1- yPO4 has received greater attention in recent years compared with LiMnPO4[ 10, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28].

Among the different synthesis methods, solid-state reaction[ 16, 17, 18, 19, 20, 21]and Sol-Gel method[ 23]have been widely adopted for preparing carbon-coated LiMn yFe1- yPO4. However, LiMn yFe1- yPO4 samples synthesized by conventional solid-state reaction have large micron-sized particles and decreased capacity at higher Mn/Fe ratios. Literatures[ 18, 19, 21] show that the initial discharge capacity at 0.1 C rate for LiMn0.4Fe0.6PO4/C prepared by the traditional solid phase method is about 145 mAh/g. The particle size can be reduced by low sintering temperature, but it will lead to low crystallinity of material. Low temperature solid-state reaction and hydrothermal synthesis could be co-adopted to prepare uniform LiMn yFe1- yPO4 particles with small size. The raw materials were decomposed and reacted with each other to form olivine crystal nucleus during the low temperature solid-state reaction, and the uniform high-pressure low-temperature environment suppressed the growth of LiMn0.4Fe0.6PO4 particles during the hydrothermal synthesis process. To the best of our knowledge, very few literatures reported in this regard. In this work, we chose the yvalue of 0.4 and develop a new synthesis route for LiMn0.4Fe0.6PO4/C composites by the low temperature solid-state reaction combined with hydrothermal synthesis. We report the details of this new synthesis method of carbon-coated LiMn0.4Fe0.6PO4 samples and their impressive cathode performance.

1 Experimental
1.1 Synthesis of LiMn0.4Fe0.6PO4/C

The carbon-coated LiMn0.4Fe0.6PO4 composite was synthesized by the low temperature solid-state reaction combined with hydrothermal synthesis from the precursors Li2CO3, FeC2O4·2H2O, MnCO3 and NH4H2PO4. The method essentially consisted of: (i) blending of all ingredients together in alcohol, then ball milling of the powder at room temperature for 8 h; (ii) drying to get a fine solid powder, then thermal treatment at 400 ℃ and 500 ℃ for 4 h in argon gas atmosphere, respectively; (iii) heating the powder at 180 ℃ for 24 h in a Teflon-lined stainless steel autoclave; (iv) then drying the materials and further sintering at 700 ℃ for 3 h in argon gas atmosphere. About 18wt% of sugar was added along with other ingredients for getting a conductive coating. According to different thermal temperatures, the final products were labeled as T-400 and T-500, respectively.

1.2 Physical characterization

Thermal analysis of the precursor was investigated on a METTLER TOLEDO TGA/SDTA851e apparatus in the temperature range from 25 ℃ to 900 ℃ in flowing nitrogen at a heating rate of 10 ℃/min. The crystallographic structures of the synthesized samples were analyzed by X-ray powder diffraction on a Rigaku-D/ MAX-2500VB diffractometer with Cu Kα radiation ( λ=0.15406 nm). Scanning electron microscopy (SEM) imaging was carried by using field-emission scanning microscope (FE-SEM, Sirion 200). The nature of carbon coating was observed with high-resolution transmission electron microscope (HR-TEM, HEM-2100F/UHR).

1.3 Electrochemical evaluation

The working electrode was prepared by mixing the active material, conductive material (Super-P carbon black, Alfa) and binder (PVDF, Aldrich) in the weight ratio of 80:10:10. The viscous slurry of N-methylpyrrolidone (NMP) was then cast on the aluminum foil and dried at 100 ℃ in vacuum for 12 h. The film was cut into circular discs with area of 1.53 cm2and mass of 3.2 mg for using as cathode.

The coin type LiMn0.4Fe0.6PO4/Li cell was assembled with lithium metal as anode, Celgard®-2200 as diaphragm and 1 mol/L LiPF6 in ethylene carbonate (EC) /dimethyl carbonate (DMC) (1:1 by volume) as liquid electrolyte. Cyclic voltammetry (CV) was carried out between 2.5 V and 4.5 V to ensure both Fe and Mn redox reactions could take place. Electrochemical performance tests of LiMn0.4Fe0.6PO4/Li cells were evaluated between 2.5 V and 4.5 V vs Li/Li+at 0.1 C and 0.5 C current using an automatic galvanostatic charge-discharge unit at room temperature.

2 Results and discussion

Figure 1(a) and (b) show the XRD patterns for mixtures of raw materials sintered at different temperatures. At 400 ℃ as shown in Fig. 1(a), LiMPO4 ( M=Fe, Mn) becomes the dominant phase, but two small impurity peaks marked with the red arrow still exist, corresponding to Li4(P2O7) and Mn2P2O7, respectively. As the sintering temperature is increased to 500 ℃, single-phase LiMn0.4Fe0.6PO4 particles are obtained as shown in Fig. 1(b). After sintered at 400 ℃ for 4 h, the mixture was then heated at 180 ℃ for 24 h in purified water, and Fig. 1(c) is its XRD pattern. It shows that two small impurity peaks disappear and single-phase LiMn0.4Fe0.6PO4 particles are obtained with well-defined olivine structure (orthorhombic Pnma). The reason of disappearance of the impurity peaks has yet to be in-depth study.

Fig. 1 XRD patterns for a mixture of raw materials sintered at 400 ℃ for 4 h (a), at 500 ℃ for 4 h (b) and at 400 ℃ for 4 h, and then heated at 180 ℃ for 24 h in distilled water (c)

The TG/DTA measurement of the starting materials composed of Li2CO3, FeC2O4·2H2O, MnCO3 and NH4H2PO4 is presented in Fig. 2. Two endothermic peaks at 180 ℃ and 230 ℃ in the DTA curve are observed, corresponding to the decomposition of NH4H2PO4 and losing lattice water of FeC2O4·2H2O, respectively[ 29]. No other endothermic peaks are found in DTA curves under 445 ℃, but continual weight loss is observed in TG curve. The weight loss under 445 ℃ is about 47.5%. It indicates that FeC2O4 should be decomposed and reacted with the decomposed product of NH4H2PO4. From 230 ℃ to 500 ℃, an exothermic peak appears between 230 ℃ and 425 ℃, implying the formation of LiFePO4 as confirmed by XRD data. Another exothermic peak at 445 ℃ can be ascribed to crystallization of the amorphous LiMPO4[ 30].

Fig. 2 TG and DTA curves of the precursors synthesized from Li2CO3, FeC2O4·2H2O, MnCO3and NH4H2PO4 (5:6:4:10 in mole)

The morphology of samples was further examined by SEM. Figure 3 shows SEM images of LiMn0.4Fe0.6PO4/C synthesized by the new route. It can be easily seen that the temperature of solid-state reaction has a significant influence on the morphology of the final prepared samples. The particles of T-400 prepared at 400 ℃ are homogeneous with the particle size of about 120 nm. When the temperature is changed to 500 ℃, the primary particle size of T-500 becomes bigger and falls into the range of 120-500 nm, which is much larger than that of T-400.

Fig. 3 SEM images of T-400 (the solid-state reaction temperature is 400 ℃) (a) and T-500 (the solid-state reaction temperature is 500 ℃) LiMn0.4Fe0.6PO4/C (b)

The TEM and SAED were applied to further investigate the microstructure and confirm the carbon distribution and crystallinity of the prepared samples. Figure 4 shows the TEM image and SAED pattern of T-400. As shown in Fig. 4(a), the surface of core LiMn0.4Fe0.6PO4 is wrapped with a thin film, which is favorable to the electronic connection of the inter-particles. In addition, the high- resolution TEM (HRTEM) image (Fig. 4(b)) demonstrates that the coating is about 10 nm in thickness. And there are no lattice fringes on the surface of LiMn0.4Fe0.6PO4, indicating that the surface carbon is in an amorphous state. The related SAED (Fig. 4(c)) pattern indicates that the core particle is single-crystalline and can be indexed as LiMn0.4Fe0.6PO4 orthorhombic phase. These results further confirm the successful synthesis of crystallized LiMn0.4Fe0.6PO4 coated with amorphous carbon.

Fig. 4 TEM image (a), high-resolution TEM image (b) and the selected area electron diffraction (SAED) pattern (c) of T-400

The active voltage range of the cathode material (T-400) as valuated by CV is shown in Fig. 5. There are two pairs of redox peaks: peaks A and B centered around 3.5 V corresponding to the redox process of Fe3/Fe2+, and peaks C and D centered around 4.0 V corresponding to the redox process of Mn3+/Mn2+. When the sweep rate is 0.1 mV/s, the difference between the anodic and cathodic peaks’ potential △ Ep is 0.45 V for Fe2+/Fe3+ and 0.4 V for Mn2+/Mn3+. The literature[ 27]reports that the potential separation between the anodic and cathodic current peaks is 0.39 V for LiFePO4 and 0.5 V for LiMnPO4. Substitution of Fe2+ by Mn2+ causes a little increase of the potential separation of Fe peaks and a great decrease of Mn peaks. Padli, et al[ 31] explained this phenomenon on a basis of a super-exchange interaction between Fe-O-Mn ions. As the sweep rate is increased to 0.2 mV/s, the intensity and the potential separation of Fe peaks and Mn peaks become greater at the first cycling, which is in agreement with the results of Hashambhoy[ 28]. CV curves on repeated cycling indicate almost no change for the redox voltage or current with cycling, which indicates that the Fe and Mn oxidation/reduction processes in T-400 are reversible and they can obtain high reversible capacity.

Fig. 5 Cyclic voltammogram curves of T-400

To test the electrochemical performance of the prepared LiMn0.4Fe0.6PO4/C, coin cells using a LiMn0.4Fe0.6PO4/C cathode and lithium anode are charged and discharged between 2.5 V and 4.5 V. Figure 6 displays the initial charge/discharge voltage profiles of the LiMn0.4Fe0.6PO4/C synthesized by the new route discussed in this paper. The charging curve shows two distinct plateaus at ~ 3.5 and ~ 4.0 V, which is corresponding to the extraction of lithium ions from LiMn0.4Fe0.6PO4 at these voltage regions for Fe3+/Fe2+and Mn3+/Mn2+couples, respectively. Similarly,the discharge curve shows two corresponding voltage plateaus at ~3.4 and ~3.9 V. At 0.1 C, an initial discharge capacity of 160 and 155 mAh/g are obtained for T-400 and T-500, respectively, which are close to the theoretical capacity of 170 mAh/g. As the current is increased to 0.5 C, the initial discharge capacity falls into 143 and 132 mAh/g. These results indicate that the LiMn0.4Fe0.6PO4/C prepared by the new route discussed in this paper exhibits a good charge and discharge properties. This is ascribed to the smaller particle size allowing rapid diffusion of Li ions.

Fig. 6 Charge-discharge curves of lithium cells with T-400 and T-500 as cathode material at 25 ℃

Figure 7 depicts the excellent cycle performance of LiMn0.4Fe0.6PO4/C at 0.5 C rate which was tested over 50 cycles. As can be seen for T-400, although the discharge capacity monotonously decreases from 143 to 132 mAh/g during the first 10 cycles, it then fluctuates between 130 and 138 mAh/g during the successive cycles, and for T-500, the same phenomenon also exists, although the discharge capacity is a little lower. These results demonstrate the well cycling stability of the LiMn0.4Fe0.6PO4/C prepared by this new route. The good cycling capabilities of LiMn0.4Fe0.6PO4/C are attributed to the formation of small, uniform particles with much reduced diffusion length for lithium ions and the efficient conductive carbon coating in the material.

Fig. 7 Cycle performance of lithium cells with T-400 and T-500 as cathode materials at 0.5 C at 25 ℃

3 Conclusions

Carbon-coated phospho-olivine based on Mn, Fe mixed metals of structural formula LiMn0.4Fe0.6PO4 was synthesized by a new method that combining solid-state reaction with hydrothermal synthesis. The sample T-400 obtained from this method has uniform morphology with the particle size of about 120 nm and a nanometer thick coating of conductive carbon surrounding the particles. The carbon- coated LiMn0.4Fe0.6PO4 composite shows two well-defined redox peaks in 3.3-4.2 V range corresponding to Fe and Mn phases in the sample. On evaluation as the cathode materials in lithium metal battery at room temperature, an initial discharge capacity of 160 mAh/g at 0.1 C, 143 and 133 mAh/g after 50 cyclies at 0.5 C is achieved, demonstrating the suitability of the new method for synthesizing the material.

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