Trans. Nonferrous Met. Soc. China 22(2012) 2541-2547

Synthesis of spinel LiMn₂O₄ microspheres with durable high rate capability

ZHOU Yu-bo^{1, 3}, DENG Yuan-fu^{2, 3}, YUAN Wei-hao², SHI Zhi-cong³, CHEN Guo-hua³

1. School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China;

2. The Key Laboratory of Fuel Cell Technology of Guangdong Province,

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China;

3. Centre for Green Products and Processing Technologies,

Guangzhou HKUST Fok Ying Tung Research Institute, Guangzhou 511458, China

Received 9 July 2012; accepted 19 August 2012

Abstract: Spinel LiMn₂O₄ microspheres with durable high rate capability were synthesized by a facile route using spherical MnCO₃ precursors as the self-supported templates, combined with the calcinations of LiNO₃ at 700 ℃ for 8 h. The spherical MnCO₃ precursors were obtained from the control of the crystallizing process of Mn²⁺ ions and NH₄HCO₃ in aqueous solution. The effects of the mole ratio of the raw materials, reaction time, and reaction temperature on the morphology and yield of the MnCO₃ were investigated. The as-synthesized MnCO₃ and LiMn₂O₄ microspheres were characterized by powder X-ray diffractometry (XRD) and scanning electron microscopy (SEM). Galvanostatic charge/discharge tests indicate that the spinel LiMn₂O₄ microspheres deliver a discharge capacity of 90 mA·h/g at 10C rate show good capacity retention capability (75% of their initial capacity after 800 cycles at 10C rate). The durable high rate capability suggests that the as-synthesized LiMn₂O₄ microspheres are promising cathode materials for high power lithium ion batteries.

Key words: MnCO₃ microspheres; self-supported template; LiMn₂O₄ microspheres; rate capability

1 Introduction

Lithium ion batteries (LIBs) have become revolutionized portable electronic devices in the past two decades [1]. They are also expected to make a great impact on the pure electric vehicles (EV) and hybrid electric vehicles (HEV) [2]. The battery performance depends critically on the materials, and therefore the development of new materials is important for advancing battery technology. The current commercial lithium ion batteries commonly based on the layered cobalt oxides cathode materials can hardly fulfill the requirement of high power applications such as HEV and EV [3]. Thus, the novel cathode materials with high energy and power density have received considerable interest [4-6]. Spinel LiMn₂O₄ is a prospective candidate for the cathode material for EV and HEV because of its intrinsic advantages of suitable Mn⁴⁺/Mn³⁺ redox potential (4.0 V, vs Li⁺/Li), low cost, high abundance, better safety, and environmental compatibility. Especially, the three- dimensional lithium ion channels of the three- dimensional crystal structure LiMn₂O₄ is beneficial for the intercalation and deintercalation of lithium ion at high rate [7]. However, the application of spinel LiMn₂O₄ materials is hampered by the problem of capacity decay during cycling, which mainly results from Mn dissolution via a disproportionation reaction (2Mn³⁺→Mn²⁺+Mn⁴⁺) and structural transformation from cubic to tetragonal phase that is induced by Jahn– Teller distortion of Mn(III) with high spin [8]. Previous studies demonstrated that the discharge capacities, cycling life, and rate performance of rechargeable lithium ion batteries are dramatically affected by the particle size, surface morphology and synthesized method of the active materials [9]. It is commonly acknowledged that the nanostructured electrode materials can overcome the limits, especially the power density. Various nanostructured LiMn₂O₄, such as nanoparticles [10], mesoporous materials [11], double-shelled hollow micro- spheres [12], and one-dimensional (1-D) nanomaterials (nanowires [13], nanorods [3] and nanotube [14]) have been investigated aiming at the application in high-power LIBs. Although nanometer- sized LiMn₂O₄ has demonstrated improvement in rate capability, its tap density is very low compared with that of the conventional oxide electrode material. Low tap density results in low volumetric energy density, and consequently, nano-LiMn₂O₄ cathodes are not suitable for lithium-ion batteries designed for applications in plug-in hybrid vehicles (PHEVs) or electric vehicles. In addition, the electrodes prepared with nano-scaled cathode materials turn out to be easily loosened and slough off, which results in bad cycleability of the electrode. Therefore, it is of great importance, to develop a facile and scalable method to synthesize the electrode materials with spherical morphology (high tap-density) and reasonable and uniform size distribution (high rate performance).

Herein, the preparation of LiMn₂O₄ microspheres using spherical MnCO₃ precursors was reported, which were obtained from the control of the crystallizing process of Mn²⁺ ions and NH₄HCO₃ in aqueous solution, as the self-supporting templates. Without surface coating or cation doping, the as-prepared LiMn₂O₄ microspheres could deliver good durability at high charge/discharge rates after 800 cycles.

2 Experimental

2.1 Synthesis

2.1.1 Synthesis of spherical MnCO₃

The synthesis of spherical MnCO₃ was carried out by controlling the crystallizing process of the following reaction equation in aqueous solution.

MnCl₂·4H₂O+2NH₄HCO₃→

MnCO₃↓+2NH₄Cl+CO₂↑+5H₂O             (1)

In a typical experiment, NH₄HCO₃ (2.752 g, 40 mmol) was dissolved in 50 mL deionized water and added to 10 mL solution containing 3.444 g MnCl₂·4H₂O. After being stirred at 35 ℃ for 20 min, some pale yellow powders were precipitated from the reaction solution. After filtration, the pale yellow powders were washed with distilled water and ethanol three times and dried at 80 ℃ in vacuum. In addition, the effects of the mole ratio of the raw materials, reaction time, reaction temperature and surfactant on the morphology and yield of the MnCO₃ were investigated.

2.1.2 Synthesis of LiMn₂O₄ microspheres

The LiMn₂O₄ microspheres were prepared according to the following equation:

MnCO₃+LiNO₃→LiMn₂O₄+N_xO_y↑              (2)

In a typical experiment, the obtained spherical MnCO₃ powders were mixed with LiNO₃ in stoichiometric ratio of 2.00:1.05, and then the mixture was ground to obtain a homogeneous mixture. After that, the homogeneous mixture was calcined at 700 ℃ for 8 h to obtain black LiMn₂O₄ microspheres.

2.2 Characterization

The structures and morphologies of the as-prepared materials were characterized by powder X-ray diffractometry (XRD, Bruker D8 Advance powder diffractometer) using Cu K_α radiation at 40 kV, 40 mA and steps of 0.020 and field emission scanning electron microscopy (SEM, Jeol JSM—6390).

The electrochemical characterizations of the sample were performed using CR2025 coin-type cell. The electrode was prepared by mixing the as-prepared active material (80%, mass fraction), acetylene black (10%) and polyvinylidene fluoride (PVDF) binder (10%) with a mass ratio of 8:1:1. After these materials were thoroughly mixed in N-methyl-2-pyrrolidone (NMP) solution, the prepared slurry was coated on Al foil in the N-methyl pyrrolidinone (NMP). The slurry was coated on an aluminum foil and dried at 90 ℃ for 2 h in a vacuum oven. The electrode was incorporated into the coin cell with a lithium foil anode and a Celgard 2325 microporous membrane separator in a dry glove box under argon. The electrolyte used was 1.0 mol/L LiPF₆ ethylene carbonate (EC) solution and ethyl methyl carbonate (EMC) (3:7 in volume). The electrochemical charge-discharge measurements were carried out in the voltage range from 3.2 to 4.4 V (vs Li⁺/Li) at different rates.

3 Results and discussion

3.1 Effect of reaction temperature on MnCO₃ yield and morphology

Table 1 shows the yield of MnCO₃ obtained at different reaction temperatures (35, 45 and 55 ℃) when the reactions were kept in aqueous solution with the mixture of MnCl₂·4H₂O and NH₄HCO₃ at a mole ratio of 1:2. The reaction time was 20 min.

Table 1 Yield of MnCO₃ at different reaction temperatures

The above results clearly reveal that a little high yield of MnCO₃ can be obtained at 35 ℃, which probably can be attributed to the fast decomposition of NH₄HCO₃ at a high temperature. Figure 1 shows the typical morphologies of the MnCO₃ obtained at different temperatures. As seen in Fig. 1(a), the MnCO₃ precursors obtained at 35 ℃ are uniform spheres with average diameters of around 1 μm. However, the SEM images of MnCO₃ prepared at 45 and 55 ℃ (Figs. 1(b) and 1(c)) show square structure and some agglomeration. Thus, 35 ℃ was selected as the optimum temperature for the synthesis of MnCO₃ microspheres.

Fig. 1 SEM images of MnCO₃ obtained at different reaction temperatures: (a) 35 ℃; (b) 45 ℃; (c) 55 ℃

3.2 Effect of reaction time on MnCO₃ yield and morphology

When the reaction was kept at 35 ℃, the effect of reaction time on the MnCO₃ yields was investigated. As shown in Table 2, the MnCO₃ yields were all above 99% with no obvious difference when the reactions were kept at different reaction time. Because of some disadvantages including time consumption, energy wastage and some agglomeration of the obtained products, we selected 20 min as the most suitable condition for the synthesis of MnCO₃. Figure 2 shows the morphology of the MnCO₃ obtained for different reaction times.

Table 2 Yield of MnCO₃ obtained at 35 ℃ for different reaction times

Fig. 2 SEM images of MnCO₃ obtained at 35 ℃ for different reaction times: (a) 20 min; (b) 40 min; (c) 60 min

3.3 Effect of ratio of MnCl·4H₂O and NH₄HCO₃ on MnCO₃ yield and morphology

Table 3 lists the yield of MnCO₃ at different ratios of MnCl·4H₂O to NH₄HCO₃. The experiments were conducted at 35 ℃ for 20 min. The results reveal that the yield of the MnCO₃ increases with the increase of the mole ratio of MnCl·4H₂O to NH₄HCO₃. When the ratio is 1:1, the yield of MnCO₃ is only 45.75%. When the ratio is 1:2, the yield is nearly up to 100%. These results can be explained by the fact that the chemical equilibrium moves right if the concentration of reactants increases. However, when the ratio increases to 1:5, the yield is only 94.25%, which is probably attributed to the side reactions in the high pH value solution by       the addition of surplus NH₄HCO₃. Combining with the decrease of cost and the increase of purity of products, the ratio of 1:2 was selected as the optimal experimental condition.

Table 3 Yield and morphology of MnCO₃ at different mole ratios of MnCl·4H₂O to NH₄HCO₃

Figure 3 shows the morphologies of the MnCO₃ obtained from different mole ratios of MnCl·4H₂O to NH₄HCO₃. As shown in Fig. 3(a), the MnCO₃ prepared with the ratio of 1:1 shows the uniform squares with average diameters of around 1 μm. However, the MnCO₃ prepared at the ratios of 1:3 and 1:4 (Figs. 3(b) and 3(c)) clearly display to some degree agglomeration.

Based on the above observations, we selected the mole ratio of 1:2, temperature 35 ℃ and reaction time 20 min as the optimal parameters to obtain the MnCO₃ with good morphology and high yield.

3.4 Characterizations of MnCO₃ precursor and LiMn₂O₄ microspheres

Figure 4(a) shows the XRD pattern of the MnCO₃ obtained from the optimal experimental conditions. All the diffraction peaks are consistent with the standard card (JCPDS card 00-044-1472), which demonstrated that the as-synthesized MnCO₃ is with high purity and crystallinity.

Figure 4(b) displays the XRD pattern of the LiMn₂O₄ microspheres produced from the calcination of the mixture of LiNO₃ and spherical MnCO₃ precursors at 700 ℃ for 8 h. The diffraction peaks can be assigned to well-crystallized cubic spinel LiMn₂O₄ (JPSDS card 35-0782). The refinement cell parameter is 8.196 ? (calculated using the Jade Software), which is smaller than the standard value of 8.248 ?. This result demonstrates that the manganese ions in the octahedral sites are partially replaced by lithium ions in the spinel lattice [3].

Figure 5 shows the typical SEM image of the LiMn₂O₄ microspheres produced from the calcination of the mixture of LiNO₃ and spherical MnCO₃ precursors at 700 ℃ for 8 h. In Fig. 5, most of the obtained LiMn₂O₄ sample preserved the spherical morphology of the MnCO₃ precursor except for some aggregation because of the calcination at high temperature. In addition, some small particles were observed on the surface of the LiMn₂O₄ microspheres, which probably results from the gas evolution caused by the thermal decomposition of MnCO₃ and LiNO₃.

3.5 Electrochemical properties of LiMn₂O₄ micro- spheres

Figure 6 shows the typical galvanostatic charge- discharge profiles of LiMn₂O₄ microspheres tested at potential range of 3.2-4.4 V and current rate of 0.1C. Both of the charge and discharge curves exhibit two distinguishable pseudo plateaus, which are attributed to the two-phase transformation of λ-MnO₂/ Li_0.5Mn₂O₄ and Li_0.5Mn₂O₄/LiMn₂O₄ [14], respectively. The initial specific capacity is 109 mA·h/g at 0.1C, with a coulomb efficiency of 75.65%. Although the initial specific capacity of the as-synthesized material is lower than the most recently reported results [3, 9-14], the specific capacity of the as-synthesized material of 103.0 mA·h/g (as shown in Fig. 7) is comparable to some of the reported results [15-17]. Furthermore, the specific capacity of 96 mA·h/g and the retention capability of larger than 93% after 100 cycles at 1.0C of the LiMn₂O₄ microspheres demonstrate that they exhibit good cycling stability, which can be attributed to their reasonable particle size, stable spherical structure and small surface areas (0.6 m²/g).

Fig. 3 SEM images of MnCO₃ obtained of different mole ratios of MnCl·4H₂O to NH₄HCO₃: (a) 1:1; (b) 1:2; (c) 1:3; (d) 1:5

Fig. 4 XRD patterns of MnCO₃ obtained from optimal experimental condition (a) and LiMn₂O₄ microspheres produced from calcination of mixture of LiNO₃ and MnCO₃ (b)

Fig. 5 SEM image of LiMn₂O₄ microspheres

Fig. 6 Charge-discharge curves of LiMn₂O₄ microspheres at 0.1C

Fig. 7 Cycling performance of LiMn₂O₄ microspheres at 1.0C rate

To evaluate the high-power capability, the LiMn₂O₄ microspheres prepared at 700 ℃ were cycled at different discharge/charge current densities ranging from 1 to 20C. Figure 8 shows the discharge curves of the as-synthesized material at different discharge current densities. As anticipated, the profiles at low charge/ discharge rates clearly show two distinguished plateaus located at about 4.1 and 4.0 V (vs Li/Li⁺). When the charge/discharge rate is over 10C, the separation of the two discharge plateaus gradually becomes blurred and the plateau voltages shift lower due to the increased cell polarization at very high current rates.

Fig. 8 Discharge curves of LiMn₂O₄ microspheres at different rates

Figure 9 displays the rate performance of the electrode made from LiMn₂O₄ microspheres. As the current density increased from 1 to 20C, the discharge capacity decreased slightly from 103 to 86.4 mA·h/g. At a rate of 10C, a discharge capacity of 91 mA·h/g can be obtained. When the current density was decreased from 20 to 1C, the discharge capacity went back up to 101 mA·h/g, revealing the superior reversibility of the LiMn₂O₄ microspheres and their suitability as a high rate cathode material. In addition, the capacities remained at 68 mA·h/g even after 800 discharge/charge cycles at 10C (as shown in Fig. 10), and the corresponding capacity retentions were 75%. It is safe to say that the as-prepared LiMn₂O₄ microspheres exhibit good rate capability and cycleability.

Fig. 9 Rate and cycling performance of LiMn₂O₄ micro- spheres

Fig. 10 Cycling performance of LiMn₂O₄ microspheres at 10C rate

4 Conclusions

1) Spinel LiMn₂O₄ microspheres with durable high rate capability were synthesized by a facile route using spherical MnCO₃ precursors as the self-supported templates, combined with the calcinations of LiNO₃ at 700 ℃ for 8 h.

2) The reactant ratio, reaction time and reaction temperature have significant effects on the morphology and yield of the MnCO₃ precursors.

3) The obtained LiMn₂O₄ microspheres exhibited remarkable high rate capability with 85 mA·h/g at 20C current density and durable capacity retention capability at 1.0 and 10C (the capacity retention capabilities are larger than 93% at 1.0C for 100 cycles and 75% at 10C for 800 cycles, respectively).

4) The excellent electrochemical properties are attributed to the reasonable particle size, stable spherical structure and small surface areas.

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耐久型高倍率性能锰酸锂微球的合成

周玉波^{1, 3}，邓远富^{2, 3}，袁伟豪²，施志聪³，陈国华³

1. 大连理工大学 环境科学与工程学院，大连 116024；

2. 华南理工大学 化学与化工学院 广东省燃料电池技术重点实验室，广州 510640；

3. 广州市香港科大霍英东研究院，广州 511458

摘  要：以Mn²⁺和NH₄HCO₃为原料，通过控制结晶法合成球形MnCO₃前驱体模板。以LiNO₃和MnCO₃为原料，按照一定的摩尔比机械混合，在700 ℃下煅烧8 h，合成高倍率性能和长循环性能的球形尖晶石LiMn₂O₄材料。分别考查原料的摩尔比、反应时间以及反应温度对前驱体MnCO₃形貌和产率的影响。采用X射线粉末衍射和扫描电镜对合成的MnCO₃和LiMn₂O₄进行表征，对LiMn₂O₄样品进行室温条件下的充放电性能测试。电化学测试结果表明：尖晶石锰酸锂微球在10C的放电倍率下的首次放电容量达90 mA·h/g (1C放电容量为148 mA/g)，800次循环后容量保持率达到75%。该方法合成的LiMn₂O₄微球作为高功率型锂离子电池的正极材料有着较好的应用前景。

关键词：碳酸锰微球；模板法；锰酸锂微球；高倍率性能

(Edited by LONG Huai-zhong)

Foundation item: Project (2011M501090) supported by the China Postdoctoral Science Foundation; Project (SCUT2012ZZ0042) supported by the Fundamental Research Funds for the Central Universities; Project supported by the “SPR-2011”of South China University of Technology; Project (NRC07/08.EG01) supprted by the Fok Ying Tung Foundation

Corresponding author: DENG Yuan-fu; Tel: +86-20-87112053; Fax: +86-20-34685679; E-mail: chyfdeng@scut.edu.cn

DOI: 10.1016/S1003-6326(11)61498-2