Elsevier

Electrochimica Acta

Volume 106, 1 September 2013, Pages 360-371
Electrochimica Acta

Synthesis of Mn2O3 microstructures and their energy storage ability studies

https://doi.org/10.1016/j.electacta.2013.06.001Get rights and content

Highlights

  • MnF2 precursor was hydrothermally synthesized and then pyrolyzed to α-Mn2O3 microstructures.

  • Spherical α-Mn2O3 exhibited excellent lithium storage capacity of 2899 mAh g−1 at first cycle and 265 mAh g−1 after 15 cycles.

  • As supercapacitor electrode materials, α-Mn2O3 was transformed into burserite in charge/discharge process.

  • Small particle facilitated the transformation of α-Mn2O3, and 202 F g−1 was obtained after many cycles of activation.

Abstract

α-Mn2O3 microstructures, including spheres and polyhedrons, were fabricated through a two-step process: MnF2 precursor was first hydrothermally synthesized using manganese acetate and hydrofluoric acid in ethanol, and then pyrolyzed to α-Mn2O3 at 350 °C. α-Mn2O3 morphologies were controlled through MnF2 precursors by adjusting HF/Mn(CH3COO)2 molar ratio and solvents. Spherical α-Mn2O3 particles were formed when HF/Mn(CH3COO)2 molar ratio was 2:1, and polyhedral α-Mn2O3 particles were prepared and particle size increased when the molar ratio increased to 12:1. Solvent viscosity affected Mn2O3 morphologies and particle size. Irregular particles of α-Mn2O3 with larger size were formed as aqueous solvent was substituted for ethanol. Smaller particles of α-Mn2O3 were formed when glycerol was used instead. The discharge mechanism and cycling stability of α-Mn2O3 electrode materials were studied. Spherical α-Mn2O3 exhibited excellent lithium storage capacity of 2899 mAh g−1 at first cycle and 265 mAh g−1 after 15 cycles. The formation of LiAl alloy did much contribution to the discharge capacity of first cycle. As for supercapacitor electrode materials, α-Mn2O3 was transformed into burserite during charge/discharge process, and capacitance increased with the increase of surface area. The highest specific capacitance was 202 F g−1 and kept steady after 400 cycles. The as-prepared α-Mn2O3 with various microstructures might be applied as rechargeable electrode materials for lithium-ion battery and supercapacitor.

Graphical abstract

Mn2O3 powder with various microstructures was fabricated by pyrolysis of MnF2 precursors hydrothermally synthesized using Mn(CH3COO)2 and hydrofluoric acid in ethanol, water, and glycerol, respectively. Mn2O3 exhibited high discharge capacity for lithium battery anode materials, and showed acceptable capacitance for supercapacitor electrode and transformed into burserite in charge/discharge process.

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Introduction

As one of the largest families of transition metal oxides, manganese oxides have received growing attention due to their unique fundamental properties and potential applications in the field of catalysis [1], chemical sensing [2], magnetism [3], supercapacitors [1], [4], and lithium ion batteries [5], [6], [7] because of the low cost of raw materials, diversity of structures, low toxicity, and environmentally friendly characteristics. Lower valence state manganese oxides, particularly Mn2O3 and Mn3O4, have been widely investigated as promising electrode materials for supercapacitors, lithium ion batteries, and catalysts [4], [7], [8], [9], [10], [11], [12], [13]. Studies have focused on the optimization of microstructure and electrochemical performance of Mn2O3 likely due to that microstructures and morphologies, which were normally controlled by preparation methods and routes, affected its physicochemical properties [12].

Direct chemical reactions were seldom utilized to control the morphologies of Mn2O3. A facile solvothermal approach was once developed to manipulate the growth of hierarchically ordered architectures of Mn2O3 catalyst, and various solvents were also employed to control the product morphologies and structures [11]. Cubic and chain-like structures of α-Mn2O3 with large surface areas were prepared by air oxidation of manganese chloride through sol process [14]. Two-step reactions were usually conducted to prepare α-Mn2O3: precursors were firstly synthesized and then decomposed to target materials while maintaining the similar morphologies. High specific surface area and uniform microporous structures of Mn2O3 with rhombus-like polyhedron morphology have been obtained from the synthesized manganese oxalate with two crystallization water, and the degree of crystallinity was controlled by adjusting the calcination temperatures, and surface area remarkably positively influenced the specific capacitance [12]. Representative manganese oxide nanorods including Mn2O3 were fabricated by annealing of amorphous MnO2 synthesized by hydrothermal reaction, exhibiting a lithium storage capacity of 679 mAh g−1 [8]. The preformed MnCO3 nanomaterials were used to prepare Mn2O3 nanorods by thermal decomposition, delivered a specific capacity of 998 mAh g−1 in the initial discharging and a reversible capacity of 349 mAh g−1 in the first charging [7]. Uniform and micron-sized MnCO3 spheres were firstly synthesized and then thermally decomposed to spherical Mn2O3, which exhibited a good rate capability and highly reversible specific capacity of 796 mAh g−1 after 50 cycles at a current density of 100 mA g−1 [15]. Various Mn2O3 mesoporous nanostructures and hollow structures, such as spheres, cubes, ellipsoids, and dumbbells, have been obtained using MnCO3 with different morphologies [16], [17]. Mn-NTA (nitrilotriacetate acid) was firstly formed and sequentially thermally decomposed to porous Mn2O3 nanofibers, nanowires, and nanorods, which demonstrated large initial discharge capacity [18]. The morphologies of Mn2O3 were likely affected by those of MnCO3, MnO2 and other precursors [4], [7], [12], [13], [14], [15], [16], [17], [18].

Fluorides could be thermally decomposed to metal oxides, such as CuO and CoO, but the morphology changing process has not been studied [19]. Single crystalline TiOF2 nanocubes were hydrothermally synthesized, and TiO2 hollow nanocages were then obtained using heat-treatment for lithium-ion battery anode materials [20]. The growth of ZnO thin films from pure ZnF2 was once investigated as a function of the oxygen partial pressure and substrate temperature using pulsed laser deposition [21]. Homogeneous α-Al2O3 platelets were synthesized by introducing AlF3 precursor to alumina and the effects of additive on phase transformation and morphology change were investigated [22]. MnF2 nanocrystals were solvothermally synthesized using manganese acetate and excess hydrofluoric acid aqueous solution in ethylene glycol [23]. To the authors’ knowledge, the feasibility for the transformation of MnF2 to manganese oxides has not yet been reported. Thermal decomposition and oxidation are possibly involved in this process. The structure types and morphologies of manganese oxides should be studied during the phase transformation process of MnF2, which remarkably affect their physical and chemical properties.

Mn2O3 nanomaterials have been studied as anode materials for rechargeable lithium ion batteries, and their electrochemical properties were investigated by constant current discharge/charge test and cyclic voltammetry. The reduction of Mn2O3 and lithium-ion insertion processes involved in a multistep electrochemical reaction according to the redox current peaks and discharge/charge voltage platform [7], [8], [15], [16], [18]. The influence of morphology and surface area on lithium storage performance of Mn2O3 was studied [16], [18]. However, the intermediate products should be characterized and the redox process needs to be further verified. Mn2O3 and its composites have been synthesized for supercapacitor electrode materials [4], [9], [12]. Lower valence state manganese oxides including Mn3O4 and Mn2O3 would convert into layered birnessite during supercapacitive studies [4], [24], [25]. However, the intermediate products in charge–discharge process should be characterized as soon as the electrochemical reaction was finished, and the influence of particle size on the transformation process of Mn2O3 into birnessite was rarely investigated.

In this work, α-Mn2O3 microstructures were prepared through a two-step process: MnF2 precursors were synthesized by a facile hydrothermal reaction in ethanol, aqueous solution, or glycerol, and then thermally decomposed to target materials at 350 °C in air. The influence of HF/Mn(CH3COO)2 molar ratio on the morphologies of MnF2 precursor was investigated. The lithium storage capacity and capacitive properties of α-Mn2O3 with different morphologies were examined, and their electrochemical reaction mechanism and morphology conversion process were further studied using galvanostatic charge/discharge test, cyclic voltammetry, and electrochemical impedance spectroscopy.

Section snippets

Materials and chemicals

Manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O, ≥99.0%), hydrofluoric acid aqueous solution (HF, 40%), anhydrous alcohol (≥99.7%), glycerol (C3H8O3, ≥99.0%), potassium chloride (≥99.5%), and sodium sulfate (≥99.0%) were all purchased from China National Medicine Group Shanghai Chemical Reagent Company. MnO (≥99.5%) powder was supplied by Aladdin Industrial Corporation. Metallic lithium pellets were supplied by Wuhan Xinsirui Tech. Co., Ltd., China. All reagents used were of analytical grade,

Synthesis and characterization of Mn2O3

Metal fluorides have been previously prepared in organic solvent systems, such as isopropanol and ethylene glycol [22], [23]. As shown in Fig. 1a, MnF2 (JCPDS card ID: 24-0727) could be synthesized in alcohol solvent system using Mn(CH3COO)2 and hydrofluoric acid at 120 °C for 12 h. It was observed that the relative diffraction peak intensity gradually increased with an increase of molar ratio of HF/Mn(CH3COO)2, suggesting that MnF2 crystallinity was enhanced likely due to that increasing

Conclusions

A series of α-Mn2O3 microstructures with different morphologies have been successfully prepared from the transformation of MnF2 using heat treatment at 350 °C for 12 h. MnF2 precursors were fabricated using hydrothermal reactions of manganese acetate and hydrofluoric acid solution in ethanol, pure water, and glycerol, respectively. The molar ratio of HF/Mn(CH3COO)2 and the viscosity of solvents affected the morphologies of α-Mn2O3. When HF/Mn(CH3COO)2 molar ratio was 2:1 in ethanol to synthesize

Acknowledgments

The authors thank the National Natural Science Foundation of China (Grant Nos.: 41171375 and 20807019), the Program for New Century Excellent Talents in University of China (No. NCET-12-0862), the Young Outstanding Talent Foundation of Hubei Province of China (No. 2012FFA031), and the Fundamental Research Funds for the Central Universities (Program No.: 2011SC23, 2009SC007, 2012SC31, 2011PY015, 2013PY029 and 2013PY030) for financial support. The authors also gratefully acknowledge Dr. Deli Wang

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