Elsevier

Electrochimica Acta

Volume 224, 10 January 2017, Pages 285-294
Electrochimica Acta

Rapid Microwave Synthesis of Self-Assembled Hierarchical Mn2O3 Microspheres as Advanced Anode Material for Lithium Ion Batteries

https://doi.org/10.1016/j.electacta.2016.12.080Get rights and content

Highlights

  • Hierarchical Mn2O3 microsphere is obtained by rapid microwave hydrothermal synthesis.

  • Large specific surface area with mesoporous is characterized.

  • High specific capacity of 921.6 mAh g−1 is reached at 200 mA g−1.

  • Continual enhancement of interface Li-storage brings an abnormal high capacity.

  • High capacity of 525 mAh g−1 is obtained at 1000 mA g−1 after 500 cycles.

Abstract

Rapid microwave hydrothermal synthesis is carried out to obtain hierarchical Mn2O3 microsphere through a self-assembled process within 30 minutes. All the raw materials used is nontoxicity without any template or polymer and the side-products are all small molecules which abide by the pricnciple of environmental protection. After the following solid state reaction, the as-prepared Mn2O3 powders can still keep the hierarchical morphology of micro sphere which is consisted of small primary particles and secondary slices. The sample obtained at 600 °C with high crystallinity, large specific surface area and mesoporous exhibits best electrochemical performances. High specific capacity of 921.6 mAh g−1 could be reached at a current density of 200 mA g−1. An interesting phenomenon with abnormal capacity increasing appears which could be ascribed to the continual enhancement of interface Li-storage. Thus, high specific capacity of about 525 mAh g−1 is obtained at high current density of 1000 mA g−1 after 500 cycles. Furthermore, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are performed to further study the hierarchical Mn2O3 anode material. It is revealed that such rapid microwave synthesis without any contamination is considered as a promising way to obtain hierarchical manganese oxides as anode material for applications.

Graphical abstract

Synopsis: Self-assembled hierarchical Mn2O3 microspheres is synthesized as anode material for lithium ion battery through rapid microwave hydrothermal processes within 30 minutes. All the raw materials and products are under the principle of environmental protection.

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Introduction

Recently, transition metal oxide nanomaterial has attracted great interest due to its practical value in many domains, especially in energy storage including supercapacitor and lithium ion batteries (LIBs) [1], [2], [3], [4], [5], [6], [7], [8], [9]. Serving as anode material for LIBs, transition metal oxides exhibit much higher specific capacity than the traditional carbon materials with excellent rate capability and cycle stability, if particular hierarchical nano-structure could be obtained [10], [11], [12], [13]. Among the common transition metal oxides, numerous researches have fused on manganese oxides with multi-valences due to its merit of nontoxicity and low cost [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. Mn2O3 as one of the members can deliver high theoretical capacity of 1018 mAh g−1 as anode material for LIBs [25], [26]. The operating voltage of Mn2O3 electrode materials is also very low, about 0.5 V for discharge and 1.2 V for charge [27]. Nevertheless, its rate capability and cycle stability are generally poor as reported resulting from its poor electronic conductivity itself and volume effect occurring during cycling [25], [27], [28], [29], [30].

As of now, various special hierarchical nano-structures are constructed to enhance the electrochemical property of manganese oxides [27], [31], [32], [33], such as hierarchical microsphere, nanorods/nanowires, 3D hollow porous nanostructures, straw-sheaf-shaped microstructure, and so on. These nano-structures composing of primary nano-particles can efficiently overcome the disadvantage caused by conversion reactions of transition metal oxides during charge and discharge [31], [32]. However, most of them are synthesized through long time growth of the crystal with original template under a solv/hydrothermal condition [12], [30], [33], [34], such as mesoporous silica or organized polymers [35], [36]. Although, such template method is efficient to obtain hierarchical manganese oxides with mesoporous, the processes are usually complicated, low-yield, time-consuming and high-cost. The template itself may be difficult to get and remove. In addition, the template, its derivative and some side-products during the reactions may cause serious contamination. Thus, it is urgent to find a new synthesis which is much facile, rapid, high-yield, low-cost and environmental benignant under the derivative of excellent electrochemical performances.

Microwave assisted hydrothermal method has been recently used to obtain transition metal oxides due to its fast reaction between reagents and uniform heat field of microwave [37], [38], [39], [40], [41], [42], [43], [44], [45]. Here, in this present work, rapid microwave hydrothermal method is carried out to obtain hierarchical Mn2O3 micro sphere as advanced anode material for LIBs. The method we performed is quite original with fast synthesis of the MnCO3 precursor within only 30 min. Furthermore, all the raw materials used is nontoxicity without any template or cpolymer and facile to be recycled. After fast reactions, the side-products are all small molecules, such as acetic acid, methanol which are dissolved in a mixed solution of ethanol and de-ion water. It is rather easy to separate them by distillation without any contamination. The as prepared precursors were then decomposed at 500–700 °C in air to obtain the finally Mn2O3 which keeps the hierarchical structure of microsphere. Remarkable electrochemical performances are obtained with great rate capability and cycle stability. It is concluded that such rapid microwave synthesis with high environmental benign characterization is quite promising applied to obtain transition metal oxides as anode materials for LIBs.

Section snippets

Synthesis of manganese oxides

Generally, the typical synthesis of microwave hydrothermal method to obtain manganese based anode material is quite facile. First of all, 10 mmol manganese acetate tetrahydrate and 20 mmol of dimethyl carbonate (DMC) were mixed with 20 mL of distilled water and 30 mL of ethyl alcohol. Then the totally 50 mL as-obtained solution was transferred into a 100 mL Teflon liner. The Teflon liner was sealed and treated at 200 °C for 30 min in a microwave hydrothermal system (WX-6000). Then, the precipitate was

The Synthesis, Structure and Morphology of hierarchical Mn2O3

The precursor design of the microwave synthesis is displayed in Fig. 1. The whole reaction is facile using DMC as the precipitant, manganese acetate as the manganese source. Ethanol and distilled water are the only solvent for the whole processes. The reaction mechanism is as follows:CH3OCOOCH3+2H2O2CH3OH+CO32+2H+CO32+Mn2+MnCO3H++CH3COOCH3COOH

Microwave technology is carried out to finish the above reactions within 30 min at 200 °C, which is enough for the formation of hierarchical MnCO3

conclusions

In order to satisfy the demand of energy storage and the principle of environmental protection, rapid self-assembled synthesis of hierarchical Mn2O3 anode material is performed through a microwave hydrothermal process within 30 minutes. The as prepared MnCO3 precursor exhibits a hierarchical morphology consisted of small primary particles and secondary slices which could be remained after high temperature solid state reactions. Only the size of the primary particle for the final Mn2O3 increases

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