J. Cent. South Univ. (2014) 21: 1279−1284 DOI: 10.1007/s11771-014-2063-8

Performances of lithium manganese oxide prepared by hydrothermal process KONG Long(孔龙)1, 2, LI Yun-jiao(李运姣)1, 2, ZHANG Peng(张鹏)3, HUANG Hai-hua(黄海花)3, YE Wang-qi(叶万奇)1, LI Chun-xia(李春霞)1, 2 1. School of Metallurgy and Environment, Central South University, Changsha 410083, China; 2. Citic Dameng Mining Industries Limited, Nanning 530028, China; 3. Changsha Research Institute of Mining and Metallurgy Corporation Limited, Changsha 410083, China © Central South University Press and Springer-Verlag Berlin Heidelberg 2014 Abstract: A simple hydrothermal process followed by heat treatment was applied to the preparation of spinel Li1.05Mn1.95O4. In this process, electrolytic manganese dioxide (EMD) and LiOH·H2O were used as starting materials. The physiochemical properties of the synthesized samples were investigated by thermogravimetry-differential scanning calorimetry (TG-DSC), X-ray diffractometry (XRD), and scanning electronic microscopy (SEM). The results show that the hydrothermally synthesized precursor is an essential amorphous. The precursor can be easily transferred to spinel powders with a homogeneous structure and a regularly-shaped morphology by heat treatment. Li1.05Mn1.95O4 powder obtained by heat treating the precursor at 430 °C for 12 h and then calcining at 800 °C for 12 h shows an excellent cycling performance with an initial charge capacity of 118.2 mA·h·g−1 obtained at 0.5C rate and 93.8% of its original value retained after 100 cycles. Key words: lithium ion batteries; LiMn2O4; hydrothermal method; heat treatment

1 Introduction Lithium-ion batteries are efficient, light-weight, and rechargeable power sources for consumer electronics, such as digital cameras, laptop computers, and cellular phones. Lithium cobalt oxide, LiCoO2, is currently the popularly used positive electrode material in commercial Li-ion batteries, due to its high reversible capacity (130−150 mA·h·g−1), high working voltage (3.6 V), long cycle-life (300−500 cycles) and easy preparation [1]. Unfortunately, the safety and high cost issues, the poor rate performance and the toxicity of cobalt associated with LiCoO2 limit its use in electrical hybrid vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs). This limitation has motivated the study of other compounds that contain less or no cobalt at all, such as Li-Ni-O [2], and Li-Mn-O systems [3−4]. LiNiO2, as a positive electrode material, possesses low cost and high rechargeable capacity [5]. However, its drawbacks, such as the difficulty to prepare a pure material [6], the low thermal stability in the charged state [7−8], and the poor capacity retention upon long range cycling [9], have to be overcome before commercialization. LiMn2O4 has been attracted as an important cathode material for rechargeable Li-ion batteries since it has

such advantages as high potential, low cost, good thermal stability and low toxicity, over compounds based on cobalt or nickel [10−12]. It is known that the quality of the LiMn2O4 powders is greatly related to the starting materials and the preparation route, including the precursor synthetic method and the following heat treatment procedure. The physical and chemical properties of the LiMn2O4 materials, such as lattice parameters, particle size, stoichiometry and average Mn valence as well as electrochemical performance, are highly associated with the preparation conditions. So far, LiMn2O4 is only commercially obtained by solid state reactions. However, the final products contain large irregular particles as well as impurity phases, which have negative effects on electrochemical performance. In addition, it is difficult to control the morphology, homogeneity, and microstructure of particles. Recently, soft methods such as microemulsion routes [15], topochemical methods [16], and rheological-phaseassisted microwave methods [17] could address those problems to some extent, but usually lead to complex processes and high cost of reagents. Hydrothermal process, as a useful method for the preparation of advanced materials, has lots of advantages [18]. It has been used for the preparation of the precursors with high purity, great homogeneity and good activity, which may

Foundation item: Project(50174058) supported by the National Natural Science Foundation of China; Project(2011A025) supported by the Glorious Laurel Scholar Program of Guangxi Zhuang Autonomous Region, China Received date: 2012−11−09; Accepted date: 2013−11−11 Corresponding author: LI Yun-jiao, Professor; Tel: +86−731−88830476; Fax: +86−731−88710171; E-mail: [email protected]

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affect the phase transformation with different routes in the subsequent heat treatment. In this work, a hydrothermal process was used for the synthesis of the lithium manganese oxide precursor using EMD and LiOH·H2O as raw materials. The precursor was subsequently subjected to heat treatment to obtain spinel lithium manganese oxides as positive materials for Li-ion batteries. The experimental results on the physiochemical properties and the cycle performances of the materials obtained by different heat treatment procedures were mainly reported in detail.

2 Experimental 2.1 Synthesis and characterization The lithium manganese oxide precursor was prepared by a simple hydrothermal process using EMD (w(Mn)≥59.36%, Xiangtan Chemical Industry, China) and LiOH·H2O (purity≥98.9%, Sichuan Tianqi Lithium Industries, Inc., China) as starting materials. A certain amount of EMD, typically 200 g, was pre-treated and then added to 1.0 L stainless steel autoclave (GCF-1 L, Weihai Jingda Chemical Machinery Co., Ltd.) with required amounts of LiOH·H2O and deionized water. The autoclave was then sealed and heated up to the designed temperature to fulfill the hydrothermal process. At the end of the test, the slurry was discharged and subjected to solid−liquid separation to obtain the precursor. The detailed procedure was described in our previous study [19]. The obtained lithium manganese oxide precursors were heat treated in a muffle furnace at a temperature between 400 and 900 °C for 5 h to get the information on the phase transformation. In order to obtain the spinel-structured Li1.05Mn1.95O4 powders with more uniform morphology and better electrochemical cyclic performance, the precursor was pre-heat treated at 430 °C for 12 h and then calcined at 700, 750, 800 and 850 °C for 12 h, respectively. The structure of the as-prepared powders was characterized by X-ray diffraction (XRD, Rigaku D/max-2500) with Cu Kα radiation. The scanning angle was varied from 10° to 80° and the scanning velocity was 1 (°)/min. Furthermore, the particle morphology of the products was examined by means of scanning electron microscopy (SEM, JEOL JSM-6360LV) operated at 20 kV. The thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) were performed using a NETZSCH STA 449 C instrument in the temperature range of 30−900 °C at a scan rate of 10 °C/min. 2.2 Electrochemical measurement For electrochemical testing, a slurry mixed with

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85% (mass fraction) active material, 10% acetylene black and 5% polyvinylidene fluoride (PVDF) in Nmethyl-2-pyrolidinone (NMP) was coated onto an aluminum foil and dried at 120 °C for 12 h under vacuum. The cathode was punched as a circular disk from the foil, and Li metal disk was used as the anode. The electrolyte was based on 1 mol/L LiPF6 in a 1:1 (volume ratio) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The thin polypropylene film was used as the separator. The cell assembly set-up was performed in an argon-filled glove box. Galvanstatic charge/discharge studies were carried out at 0.5C (1 C=148 mA·h·g−1) rate between 3.0 and 4.3 V at room temperature with Land (CT2001A) cell systems.

3 Results and discussion 3.1 Morphology and structure of materials Figure 1 shows the TG-DSC curves for the thermal decomposition of the complex precursor prepared by the hydrothermal method. These results display four mass loss temperature regions: 30−130.5, 130.5−431.7, 431.7− 786.4 and 786.4−900 °C. The little (about 1.02%) mass loss of the first region may be attributed to the superficial water loss due to the hygroscopic nature of the precursor complex [20]. The mass loss (about 3.64%) of the second region may be attributed to the loss of chemically bonded water which locates between the sheets of [MnO6] octahedra in the samples coupled with release of oxygen gas. The mass loss of the second (about 3.64%) and the third (about 1.86%) regions between 130.5 and 786.4 °C is a complex thermal process that includes the crucial formation step for spinel-structured Li1.05Mn1.95O4. Typical DSC data show an exothermic peak around 426.1 °C, which corresponds to the transformation of Mn(+4) to Mn(+3) with the release of oxygen gas. The mass loss of the last region may due to the formation of oxygen-lack Li1.05Mn2O4-δ [21]. The XRD patterns of the precursor and the samples obtained at different heat temperatures are shown in

Fig. 1 TG-DSC data of as-synthesized Li-Mn-O material

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Fig. 2. The powder XRD pattern data reveal that the precursor is an amorphous material with low crystallinity. Besides, some weak unidentified peaks appear at about 2θ=42° and 53°, which are consistent with Ref. [22]. A further increase in the reaction temperature to 500 °C induces a progressive reduction of Mn4+ to Mn3+. This results in the pure cubic spinel Li1.05Mn1.95O4 formed without any other impurity peaks detected. Notably, when the precursor synthesized by hydrothermal method is heat treated between 500 and 900 °C for 5 h, no impurities, such as Mn2O3 or Mn3O4, are detected. This result strongly suggests that the present hydrothermal method is much superior to conventional solid-state reaction for obtaining spinel lithium manganese oxide powders. The fact may be attributed to the following factors. First, the Li+ ion diffusion rate in such a liquid−solid reaction system is much higher than that in a solid state system, which results in the easy formation of the chemically combined lithiated manganese dioxide with uniform and atomic-scale distribution of metals, namely, Li-Mn-O precursor. Second, the as-prepared precursor has an amorphous structure with high activity, which makes it easily transform to designated spinel LiMn2O4 structure at relatively low temperature.

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Fig. 3 XRD patterns of precursor pre-heated at 430 °C for 12 h and then calcined at different temperatures for 12 h

Fig. 4 Lattice parameters of precursor pre-heated at 430 °C for 12 h and then calcined at different relatively higher temperatures for 12 h Fig. 2 XRD patterns of precursor and samples heat-treated at various temperatures for 5 h at rising rate of 3 °C/min: (a) Precursor; (b) 400 °C; (c) 500 °C; (d) 600 °C; (e) 700 °C; (f) 800 °C; (g) 900 °C (Insert: Expanded view of samples heat-treated at 400 °C and 500 °C detected at 2θ=40°−55°)

The representative X-ray diffraction patterns of each sample preheated at 430 °C for 12 h and then calcined at different temperatures for 12 h are shown in Fig. 3. All diffraction peaks can be indexed by the cubic spinel. In addition, the XRD patterns for the sample heated between 700 and 850 °C show high-intensity peaks corresponding to planes (111), (311) and (400). This confirms the occupancy of lithium ions in tetrahedral 8a sites, and the manganese ions in 16d sites as well as oxygen ions in 32e sites [23]. Figure 4 shows the effect of the heat treating temperature on the lattice constant,

obtained from the Rietvelt refinement on the XRD data. It can be seen that the values of the lattice parameters of the samples increase with increasing the temperature. The phenomena might be owing to the increase in Mn3+ concentration in the materials, which has a larger ionic radius (r=0.645 Å) than Mn4+ (r=0.530 Å). It has been reported that Mn3+ is more stable than Mn4+ at higher temperatures [22]. Figure 5 shows the surface morphology and the grain size of Li1.05Mn1.95O4 investigated by SEM. It is found that the spinel crystals can be observed from the SEM images of the powders obtained at 700 °C. With increasing the temperature, the spinel crystals become smooth and well-shaped, and the particle size becomes larger. The spinel crystals with a uniform, nearly cubic structure morphology and a narrow size distribution

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Fig. 5 Typical SEM photographs of Li1.05Mn1.95O4 powders preheated at 430 °C for 12 h and then calcined at different relatively higher temperatures for 12 h: (a) 700 °C; (b) 750 °C; (c) 800 °C; (d) 850 °C

perfectly form in the relatively higher temperature. This means that the morphology and the particle size of the Li1.05Mn2O4 spinel powder can be controlled by altering the calcination temperature. The results indicate that the formation temperature of spinel phase is rather low, but the growth of the crystalline and the well-shaped particles needs rather a high temperature [24]. The characteristics of the materials, such as a high homogeneity and a granular shape, can be attributed to the homogeneous distribution of the metal ions at an atomic scale in the precursor. 3.2. Electrochemical performance Figure 6 shows the initial discharge curves at 0.5C of the samples obtained at different calcined temperatures. The discharge curves display two plateaus, which is typical electrochemical characteristic of LiMn2O4 [25−26]. It can also be seen that the discharge capacity of all samples increases with temperature up to 800 °C and slightly decreases at 850 °C. The increase in initial capacity is attributed to the improvement of the homogeneity and the crystallinity of the materials. However, it should be observed that the capacity decreases from 118.2 mA·h·g−1 to 113.4 mA·h·g−1 as the temperature increases from 800 to 850 °C, indicating that there are other factors in the thermal treatment process that affect the electrochemical behavior in a negative way. Among them, altered structure of the powders that results

Fig. 6 Initial discharge curves of Li1.05Mn1.95O4 powders pre-heated at 430 °C for 12 h and then calcined at different temperatures for 12 h

from the loss of oxygen may play an important role and thus lead to the microstructure defects. In fact, we definitely detect a mass loss from the TG curve above 800 °C, as shown in Fig. 1. It is suggested that two factors which govern the micro-structural characteristics influence the electrochemical performances. The improvement of the homogeneity and the crystallinity of the materials taking place between 700 and 800 °C may give the first factor place, while the formation of oxygen-lack Li1.05Mn2O4-δ leading to crystal defects that result in the electrochemical performance deterioration at 850 °C may occupy the vital factor.

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The discharge characteristics, as a function of cycle number at a rate of 0.5C between 3.0 and 4.3 V, of the samples obtained under different heat treatment conditions are shown in Fig. 7. In general, all the products exhibit a high initial discharge capacity. However, Li1.05Mn1.95O4 heat treated at 800 °C shows the highest initial discharge capacity and the best cycleability. It initially delivers 118.2 mA·h·g−1 and retains 112.3 mA·h·g−1 after 100 cycles, which are better than those of the powders calcined at any other temperature between 700 and 850 ˚C.

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1) Spinel Li1.05Mn1.95O4 powder is prepared by a novel hydrothermal method followed by a simple heat treatment from pretreated electrolytic manganese dioxide and lithium hydroxide. 2) The spinel Li1.05Mn1.95O4 powders obtained at 800 °C, having a good morphology and an uniform particle size distribution, exhibit both a high initial discharge capacity of 118.2 mA·h·g−1 at 0.5C rate between 3.0 and 4.3 V and a good cycle performance with the discharge capacity of 112.3 mA·h·g−1 retained after 100 cycles. 3) It is believed that the homogeneous distribution of the metal ions on an atomic-scale in the precursor, prepared by the novel hydrothermal method, greatly contributes to the excellent performance of the materials in heat treatment and electrochemical cycling.

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(Edited by YANG Bing)