Porous Mn₂O₃ microsphere as a superior anode material for lithium ion batteries

Abstract

Porous Mn₂O₃ microspheres have been synthesized by morphology-controlled decomposition of spherical MnCO₃ precursors at 600 °C. The porous Mn₂O₃ microspheres show a good rate capability and a high specific capacity of 796 mA h g⁻¹ after 50 cycles.

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Rechargeable lithium ion batteries (LIBs) have become the dominant power source for portable electronic devices in the past two decades.¹ They have also been considered to be the major player in powering pure electric or hybrid electric vehicles. Since it is important to have the specific energy density to approach that of gasoline, the ever-growing need for batteries with high energy density along with good safety have been the trend for intensive research of novel electrode materials.² Nanostructured metal oxides, such as FeO_x, Co₃O₄, NiO, and SnO₂, are among the most widely investigated alternative anode materials for LIBs because of their higher specific capacities than graphite.³ Particularly, Fe₃O₄ represents a significant advance because of its low price and good conductivity.⁴ Compared with iron, cobalt and Ni-based oxides, manganese-based oxides have lower operating voltages (average discharge and charge voltages of 0.5 and 1.2 V, respectively). In general, anode materials with lower charge (the discharge process of a battery) voltages versus Li/Li⁺ can deliver a higher energy density.⁵ Therefore, investigations of Mn-based anodes have received great interest recently.⁶ Although Mn₂O₃ has a high specific theoretical capacity (1018 mA h g⁻¹) as an anode material for LIBs, previous reports^6c,6d,6j have claimed that the capacity retention of this material is generally very poor, except for the straw-sheaf-shaped Mn₂O₃.^6k It is well known that the sizes and morphologies of the electrode materials have very important effects on the electrochemical performance of the battery.⁷ Furthermore, micron-sized spherical materials with suitable porous structures can not only increase their energy density, but also improve their rate capabilities.⁸ Combining porous spherical structures concept and simple synthetic technique, herein, we successfully synthesized porous Mn₂O₃ microspheres by morphology-controlled decomposition of spherical MnCO₃ precursors. The porous Mn₂O₃ microspheres show good electrochemical performance as anode materials for LIBs, as seen subsequently.

Uniform, micron-sized MnCO₃ spheres were first synthesized by using widely available MnCl₂·4H₂O and urea as raw materials, and ethylene glycol (EG) as the solvent. The reaction temperature was maintained at 200 °C for 24 h. After thermal decomposition of spherical MnCO₃ at 600 °C for 2 h in air, porous Mn₂O₃ microspheres were obtained. Based on the previous result,⁹ we chose 600 °C as the calcining temperature of the MnCO₃ sample to prepare Mn₂O₃. Fig. 1 shows the XRD patterns of the MnCO₃ microspheres and porous Mn₂O₃ microspheres. Pure rhombohedral MnCO₃ (JCPDS Card No. 44-1472) and orthorhombic Mn₂O₃ (JCPDS Card No. 24-0508) are obtained.

Fig. 1 XRD patterns of (a) MnCO₃ and (b) Mn₂O₃ obtained from thermal decomposition of MnCO₃ under air atmosphere at 600 °C for 2 h.

Fig. 2a and 2b are typical SEM images of the obtained MnCO₃ microspheres. They show a dense spherical morphology with a uniform size distribution ranging from 6 to 8 μm. The rough surface of as-prepared MnCO₃ microspheres is composed of tiny grains. After heating under air atmosphere at 600 °C for 2 h, MnCO₃ microspheres will be decomposed into porous Mn₂O₃ microspheres. The particle size is also around 6–8 μm. The features of the porous Mn₂O₃ microspheres are characterized by nitrogen sorption–desorption. The sample exhibits a type II isotherm with a hysteresis loop, which is characteristic of macroporous materials (Fig. S1†).

Fig. 2 Scanning electron micrographs of MnCO₃ (a and b) and Mn₂O₃ (c and d) obtained by thermal decomposition of MnCO₃ under air atmosphere at 600 °C for 2 h.

Fig. 3 shows the cyclic voltammetry (CV) profiles of the porous Mn₂O₃ microspheres for the first three cycles in the voltage range 0.01–3.0 V versus Li/Li⁺ at a scan rate of 0.1 mV s⁻¹. The CV curve for the first cycle is substantially different from those of the subsequent ones. The two peaks centred at 1.25 and 0.90 V in the first cathodic process could be associated with the reduction of Mn³⁺ to Mn²⁺ and an irreversible reaction related to the lithium ion insertion of the conductive agent AC (AC = acetylene black), respectively.⁶ Another main peak centred at 0.20 V is attributed to the reduction of MnO to Mn⁰. In the subsequent cycles, a broad peak at ∼ 1.25 V may be attributed to the incomplete reduction of Mn³⁺ to Mn²⁺, and the main cathodic peak shifts to about 0.30 V, which is ascribed to Mn²⁺ to Mn⁰. During the anodic process, two peaks are observed at 1.25 V and 2.35 V. These are presumably associated with the oxidation of Mn⁰ to Mn²⁺ and Mn²⁺ to Mn³⁺, respectively. Compared with the first discharge process, the peak current and the integrated area of this process decrease, indicating capacity loss during the charging process. However, in all of the three charging cycles, the integrated areas remain almost equivalent, indicating good capacity retention after the first discharge process.

Fig. 3 Cyclic voltammetry plots of the Mn₂O₃ electrode at a scan rate of 0.1 mV s⁻¹ in the voltage range 0.0–3.0 V versus Li⁺/Li.

Fig. 4a and 4b illustrate the discharge–charge profiles and cycling performance of the porous Mn₂O₃ microspheres as anode materials for LIBs. The voltage profile of Mn₂O₃ between 3.00 and 0.01 V in the first discharge can be divided into four distinct voltage regions. The first region starts from the open circuit voltage (∼3.0 V) and shows a continuous decrease in voltage to 1.25 V, resulting in a voltage plateau at 1.25 V and extending to a total of 0.67 moles of Li consumption. This could be associated with the reduction of Mn₂O₃ to Mn₃O₄. The voltage then decreases to ∼0.38 V with a new plateau observed. The capacity of 300 mA h g⁻¹ up to the voltage of 0.38 V in this region may be attributed to the reduction of Mn₃O₄ to MnO and irreversible reactions between Li⁺ and AC (the discharge–charge curves of AC are shown in Fig. S2†). In the third region, the phase transformation reaction happens with the complete reduction of MnO to Mn and the formation of amorphous Li₂O, extending up to a consumption of 4.0 moles of Li. The voltage then decreases gradually down to the deep discharge limit of 0.01V, with a total specific capacity of 1310 mA h g⁻¹, which may be associated with the irreversible decomposition of the solvent in the electrolyte to form a gel-like film and solid electrolyte interface on the surface.⁶ The first charge curve shows no obvious voltage plateau with an overall specific capacity of 882 mA h g⁻¹, corresponding to the reversible oxidation of Mn⁰ to Mn²⁺ and Mn³⁺. It is worth noting that there is an obvious turning point at the charge voltage of 2.1 V, delivering a charge capacity of ∼760 mA h g⁻¹. This corresponds to the theoretical reversible capacity of MnO. The formation MnO then reacted with Li₂O when the electrode was charged above 2.1 V, forming a mixture of MnO and MnO_x (1.0 < x < 1.5). This first discharge–charge profile of Mn₂O₃ has some differences from the reported data,^6j indicating that the morphologies of the oxide materials have an effect on their electrochemical behavior. After the first cycle, the slope at 1.5–0.5 V and the voltage plateau at 0.5 V should be ascribed to the Li-ion insertion of Mn³⁺ to Mn²⁺ and Mn²⁺ to Mn⁰, respectively. This agrees well with the one broad peak at 1.25 and 0.4 V in the CV curves. Corresponding to the discharge curves, there are two slopes at 0.01–2.1 and 2.1–3.0 V in the charge curves, which are ascribed to the oxidation of Mn⁰ to Mn²⁺ and incomplete oxidation of Mn²⁺ to Mn³⁺, respectively. Two peaks centred at 1.25 and 2.35 V in the anodic process agree well with the above results.

Fig. 4 (a) The discharge–charge profiles and (b) cyclic performance of porous Mn₂O₃ microspheres at a current density of 100 mA g⁻¹ in the voltage range 0.01–3.00 V versus Li⁺/Li.

The discharge reversible specific capacity of porous Mn₂O₃ microspheres gradually increases to 900 mA h g⁻¹ at the 8th cycle, corresponding to the activation process associated with the partial crystallinity degradation of the Mn₂O₃ spheres during initial cycling (Fig. 4b). Then the specific capacity decays very slowly and is maintained above 796 mA h g⁻¹ during 50 cycles at 0.1 C discharge current density, demonstrating the high capacity retention capability of these porous Mn₂O₃ spheres.

The rate capability of the porous Mn₂O₃ microspheres was also evaluated at different current densities as shown in Fig. 5. As expected, the specific capacity decreased with current density from ∼900 mA h g⁻¹ at 100 mA g⁻¹ to ∼600 mA h g⁻¹ at 1200 mA g⁻¹. Even at a current density of 2400 mA g⁻¹, the specific capacity was ∼470 mA h g⁻¹, still much higher than the theoretical capacity of graphite (372 mA h g⁻¹). When the current density was decreased from 2400 back to 100 mA g⁻¹, 95% of the initial specific capacity was recovered. This shows that the tested material has capacity retention capability and good rate performance. Such good performance of the Mn₂O₃ anode material is considered to be caused by its porous structure, reasonable primary particle size, and high crystallinity. This is the first report of such a high rate performance for Mn-based anode materials as far as we know.

Fig. 5 (a) Representative charge and discharge curves and (b) capacity retention of porous Mn₂O₃ microspheres at various current densities.

The ex-situ XRD analysis was conducted on the electrodes after discharging or charging to the desired voltage (Fig. S3†). After the first discharge to 1.25 V, the peaks of Mn₂O₃ have already disappeared, with a new broad peak centred at 37.5°(the strongest XRD peak of Mn₃O₄) appearing, which is attributed to the main peak of Mn₃O₄. This phase lasts until the discharge capacity is over 410 mA h g⁻¹ (0.38 V), indicating that a new process of lithium insertion has started. This XRD pattern may be ascribed to MnO (a broad peak between 30 and 42°). As the material is discharged to 0.01 V, the XRD pattern becomes very broad, and no obvious XRD peaks can be matched with known crystals. This could be attributed to the nanoparticle nature of the electrochemically-formed species. Similarly, no obvious diffraction peaks can be detected when the material is recharged to 2.10 and 3.00 V after the first cycle. Based on the CV curves, the discharge–charge curves and the ex-situ XRD analysis, the lithium storage mechanism of the synthesized porous Mn₂O₃ material is suggested as follows:

2Li⁺ + 3Mn₂O₃ + 2e → 2Mn₃O₄ + Li₂O (1)

2Li⁺ + Mn₃O₄ + 2e → 3MnO + Li₂O (2)

2Li⁺ + MnO + 2e → Mn + Li₂O (3)

Mn + xLi₂O ↔ 2xLi⁺ + MnO_x + 2xe (1.0 < x < 1.5) (4)

In conclusion, a cheap, scalable and highly reproducible process was developed for the preparation of porous Mn₂O₃ microspheres. The resulting Mn₂O₃ microspheres exhibit a good rate capability and highly reversible specific capacity of 796 mAh g⁻¹ at a current density of 100 mA g⁻¹ after 50 cycles. The improved electrochemical performance shows that the synthesized Mn₂O₃ could be a promising material for a high capacity, low cost, and environmentally friendly anode for LIBs.

This work was supported by the Fundamental Research Funds for the Central Universities (SCUT20120042), by the NNFSC (20801020, 21176045), by the China Postdoctoral Science Foundation (2011M501090), by the Guangdong Province Sci & Tech Bureau (Industry–Education–Research Project, Grant No. 2010B091000004, by the Key Strategic Project, Grant No. 2011B05030008), and by the Fok Ying Tung Foundation (NRC07/08.EG01).

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Footnote

† Electronic Supplementary Information (ESI) available: Experimental details. See DOI: 10.1039/c2ra20062g/

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