Solid chemical reaction in microwave and millimeter-wave fields for the syntheses of LiMn2O4 compound

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Abstract

Absorbing electromagnetic energies and transferring to heat energies solid-state chemical reaction in microwave field and millimeter-wave field would happen. In present paper, the reactions for synthesis of LiMn2O4 compound as a cathode electrode material for Li-ion batteries in electromagnetic field were studied. The reaction finished in a shorter time comparing with conventional method. The microstructure of reaction product including phase of reaction products, solid-state reaction course in electromagnetic fields, and conventional solid-state syntheses were analyzed by using X-ray diffraction (XRD) and scanning electron microscope (SEM). The results show the chemical reactions in electromagnetic fields were influenced by several factors including soaking time, pre-heating and coupled agent, and it has a simpler route to obtain LiMn2O4 compounds due to the different reaction mechanism comparing with the conventional reaction method.

Introduction

As most peoples known, replacing the negative electrode, pure lithium metal by lithium containing compound, lead to safety rechargeable Li batteries, the so-called rocking-chair batteries. Up to now, oxidizing compounds including LiCoO2, LiNiO2 and LixMn2O4 were widely studied as cathode electrodes. Due to the high cost and low yield of Co and Ni, many researchers concentrate their research work on exploring the structure and properties of the spinel phase for LiMn2O4 as the positive electrode to lower the batteries cost.

Spinel LiMn2O4 is being extensively studied because manganese-based cathode materials have economic and environmental advantages [1]. The spinel LiMn2O4 is normally synthesized by solid-state reaction of lithium salts such as Li2CO3, LiNO3, etc., mixed with manganese oxides, or acetates. The mixtures are heated at different temperatures according to the synthesis method. The temperatures usually are during 450 and 850 °C [2], [3], [4], [5], [6], [7]. Conventional solid-state reactions processing, usually being employed to synthesize the spinel phase LiMn2O4, a much complicated, and there exist of intermediate products (Li2MnO3, Mn2O3, Mn3O4, etc.), which have great influence on the properties of Li-ion battery. There are many methods for the synthesis of spinel LiMn2O4, such as solid-state reaction [8], [9], melt-impregnation [10], the Pechini process [11], the emulsion-drying method [12], the citric acid method [13], microwave synthesis [2], [7], etc.

Microwave synthesis is one of the new methods overcoming the problems occurred in conventional solid-state reaction [14], [15], [16], [17], [18], [19]. It could shorten the reaction time greatly due to the different reaction mechanisms [14], [15]. Microwave techniques have been extended to different research areas including material preparation. Compared with the conventional synthesis, microwave synthesis has many advantages such as higher efficiency, lower consumption, shorter reaction time and so on. Microwave synthesis at frequency 2.45 GHz could also shorten reaction time, enhance grain growth, but due to the poor uniformity of microwave field, so the uniform heating area for the samples could set up difficulty. Recently, extensive application of millimeter-wave heating has been proceeded on the sintering of some ceramics by millimeter-wave instead of 2.45 GHz microwave [20], [21], [22]. In comparison with 2.45 GHz microwave heating, higher power absorption and weaker temperature dependence of dielectric loss are brought by use of millimeter-wave, and the millimeter-wave electromagnetic field in a multi-mode cavity is more uniform.

Section snippets

Experimental

In the present work, the chemical reaction was performed with different methods, including solid-state reaction, sol–gel and microwave (millimeter-wave) heating. The microwave heating system, which was designed by Wuhan University of Technology, was composed of microwave source, water load, piston monitor, single mode cavity, directional microwave couplers, infrared detector thermometer and data collection system (shown in Fig. 1). Microwave synthesis was performed in TE103 single mode cavity

Results and discussion

The heating in a microwave field and a millimeter-wave field was different from the conventional heating due to the interaction between the synthesizing materials and the electromagnetic field. The heating principles in microwave field and in millimeter-wave field were almost the same, but different from the frequency of the electromagnetic field. The reaction in a microwave field and a millimeter-wave field would be similar. The reaction and the microstructure of products will be analyzed in

Conclusions

In this work, the reactions for synthesis of Li–Mn–O compound as a cathode electrode materials for Li-ion batteries in electromagnetic field were studied. Pure spinel LiMn2O4 was obtained using microwave synthesis and millimeter-wave synthesis. The reaction finished in 20 min which is a shorter time comparing with the conventional method. The results showed the chemical reactions in electromagnetic fields were influenced by several factors including soaking time, pre-heating and coupled agent.

Acknowledgments

The authors would like to thank the National Natural Science Foundation of China (Grant No. 20371038), and the Ministry of Education of PR China, for supporting this research project.

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