Reconstruction of Mini‐Hollow Polyhedron Mn2O3 Derived from MOFs as a High‐Performance Lithium Anode Material

A mini‐hollow polyhedron Mn2O3 is used as the anode material for lithium‐ion batteries. Benefiting from the small interior cavity and intrinsic nanosize effect, a stable reconstructed hierarchical nanostructure is formed. It has excellent energy storage properties, exhibiting a capacity of 760 mAh g−1 at 2 A g−1 after 1000 cycles. This finding offers a new perspective for the design of electrodes with large energy storage.


DOI: 10.1002/advs.201500185
volume changes and will inevitably destroy the unstable electrode structure, leading to performance degradation. [ 23,24 ] Many merits, however, such as higher specifi c area, more active sites, and enhanced kinetics of the electrochemical activity, are also induced by the nanosize effect. [25][26][27][28] As reported by Poizot et al., [ 23 ] a nanosized electrode was created after the fi rst discharge and preserved on the following charge. This phenomenon is confi rmed in many reports. [ 28,29 ] Constructing hollow structure is believed to be an effective way to alleviate the structural strain. [ 30,31 ] However, the conventional hollow structure electrodes with high capacity achieved usually is not with satisfi ed long cyclic stability. Thus, it is reasonable to design an appropriate structure to improve the electrochemical performance by utilizing these merits and inhibiting the disadvantages simultaneously, achieving high capacity and long cyclic stability.
As multifunctional materials, metal organic frameworks (MOFs) have been used as templates or precursors to fabricate functional materials, recently. [ 32 ] For example, Co 3 O 4 -carbon nanowire arrays derived from MOFs exhibit high oxygen evolution reaction (OER) activity, [ 33 ] spindle-like and microboxes α-Fe 2 O 3 , CuO nanostructures, Zn x Co 3− x O 4 hollow polyhedron, and Co 3 O 4 nanoparticles have been prepared by MOFs and showed high lithium storages. [34][35][36] However, no reports about the utilization of MnO x derived from Mn-based MOFs for LIBs applications, although they have been used for OER. [ 2 ] In this work, polyhedron Mn 2 O 3 with small interior cavity (mini-hollow polyhedron Mn 2 O 3 for short) was derived from Mn-based MOFs (denoted as Mn-MOF). Different from the bulk material lacking room to hold the inward volume expansion and the conversional large hollow structure owning too large room for the inward volume expansion without confi ne, the small interior cavity of a mini-hollow structure is fi lled by the reformatted nanoparticles caused by nanosize effect, leading to the formation of a hierarchical nanostructure with homogeneous dispersion of the nanoparticles. When used as LIBs anode material, this mini-hollow polyhedron Mn 2 O 3 electrode shows excellent electrochemical performance: high specifi c capacity, long cycling stability, and superior rate capability (capacity of 819.8 mAh g −1 at 1 A g −1 after 1200 cycles and 760 mAh g −1 at 2 A g −1 after 1000 cycles). Further investigations revealed that the nanosize effect plays a key role in improving the electrochemical performance.
Mini-hollow polyhedrons Mn 2 O 3 were obtained after the Mn-MOF being annealed at 750 °C for 4 h with a temperate ramp of 5 °C min −1 . Typically, 3.75 mmol MnCl 2 ·4H 2 O (manganese (II) chloride tetrahydrate) and 10 mmol NH 4 Cl (ammonium chloride) were dissolved in the mixture of 37.5 mL Manganese oxides materials have shown great potential application as energy materials, such as water oxidation and oxygen reduction, [1][2][3] electrochemical capacitors, [ 4,5 ] lithium-O 2 batteries, [ 6 ] and lithium-ion batteries (LIBs). [7][8][9][10][11][12][13][14][15] Owing to their high specifi c theoretical capacity, low cost, and environmentally benign nature, manganese oxides (MnO x , 1 ≤ x ≤ 2) are believed to be the most promising alternative anode materials for next generation LIBs. [ 11,13,16,17 ] Moreover, the low operating voltages (1.3-1.5 V for lithium extraction) and small voltage hysteresis (<0.8 V) endow these materials with higher output voltage and higher energy density. [ 14,15,18 ] These two characteristics are important for developing better batteries to meet the increasing demands of our society. [ 19 ] Compared to its counterparts, Mn 2 O 3 has not been fully investigated, although it features a high theoretical capacity (1018 mAh g −1 ). Currently, the capacity is far lower than its theoretical value and the rate capability is not satisfactory. [8][9][10] What is more, the fully charged production is ambiguous. As reported by many works, the fi nal oxidation product is MnO x ( x = 1 or 1 < x < 1.5). [ 8,12,17,20 ] However, nanosized MnO electrode, which features high capacity, could be reoxidated to Mn (IV), [ 14,21 ] and our previous work also confi rmed that the nanosized active material possesses enhanced electrochemical kinetics and it is easy to gain a high oxidation product. [ 22 ] Therefore, it is worthwhile to synthesize Mn 2 O 3 LIBs anode with enhanced capacity and better rate capability by a new route. Meanwhile, gaining further understanding of the conversion mechanism of this material is also an urgent task.
As a characteristic of transition metal oxides electrodes, the nanosize effect is caused by the conversion mechanism (  [ 23 ] This process induces CH 3 CN, 17.5 mL HCOOH, and 17.5 mL CH 3 COOH. The solution was transferred to a 100 mL Tefl on-lined stainless steel autoclave and maintained at 100 °C for 24 h. After cooling down to room temperature naturally, the pink product was collected by suction fi ltration and washed with ethanol for three times. The pink Mn-MOF was obtained after drying at 60 °C for 4 h (detailed information can be seen in Figure S1, and Tables S1 and S2 in the Supporting Information). In the calcination process, there is a large temperature gradient, which leads to the transformation of microscale bulk single crystal Mn-MOF ( Figure S1d, Supporting Information) to submicroscale polyhedron manganese oxides (Figures S1e and S2, Supporting Information). [ 34 ] The structure of the product was investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM). As is shown in Figure 1 a, all the peaks can be well indexed to cubic Mn 2 O 3 (JCPDS no. . [ 8,9,37 ] Figure 1 b,c depicts that the polyhedron Mn 2 O 3 is well dispersed without any aggregation, reaching a length of ≈0.6-1.2 µm. The smooth surface suggests its single-crystal character and that is further confi rmed by the selected-area electron diffraction (SAED) patterns in  Figure S3a, Supporting Information) with a slight light on the core of polyhedron indicates the small hollow structure. The high-angle annular dark-fi eld scanning transmission electron microscope (HAADF-STEM) image (Figure 1 d) confi rms the hollow structure with a thick shell and mini-hollow cavity, different from the conventional hollow structure with a thin shell and large-hollow cavity. [ 31,[38][39][40] Obviously, the mini-hollow cavity could provide the space to hold the inward volume expansion. [ 13 ] The distances of the lattice fringes in Figure 1 e are around 2.72 and 3.84 Å, corresponding to the (222) and (211) planes of cubic Mn 2 O 3 . [ 9 ] As Figure 2 a shows, the capacity of this mini-hollow polyhedrons Mn 2 O 3 electrode shows a slow decrease during the fi rst 60 cycles followed by an increase in subsequent cycles. A capacity of 1370 mAh g −1 is stabilized after 450 cycles, exhibiting a high cycling stability and large capacity. All the discharge-charge curves at 0.4 A g −1 (Figure 2 b) show two plateaus and are highly overlapping, suggesting a two-step conversion mechanism with high electrochemical reversibility. [ 41 ] The rate capability also is excellent (Figure 2 d) and with excellent repeatability ( Figure S4a, Supporting Information). At high current densities of 1.0 and 2.0 A g −1 , stable capacity of 795.3 and 686.7 mAh g −1 can be obtained, respectively, and then recovered to 1164.1 mAh g −1 at 0.2 A g −1 . The discharge-charge curves (Figure 2 c) are similar at different current densities with limited voltage hysteresis changes, also indicating the high stability of this material. [ 13 ] In addition, the capacity at 2.0 A g −1 can be stabilized at ≈760 mAh g −1 , even after 1000 cycles. What is more, after such an extensive cycling at this aggressive current, an improved capacity of ≈1910 mAh g −1 is achieved at 0.2 A g −1 , which is much larger than both of initial value and the theoretical capacity of Mn 2 O 3 . The over-theoretical capacity may due to pseudocapacitive charge and partial reversible formation and decomposition of solid-electrolyte interphase (SEI) layer. [ 22,28,42 ] To the best of our knowledge, this is the best Mn-based LIBs anodes with such high reversible capacity and super-long cycling stability (the comparative results are in Table S3, Supporting Information). [ 11,13,15,17,21,27,41,43 ] For a comparison, a bulk polyhedron Mn 2 O 3 electrode (XRD and SEM are shown in Figure S5, Supporting Information) was prepared. [ 37 ] As is shown in Figure 2 e, the mini-hollow poly hedron Mn 2 O 3 electrode shows super-long cycling stability and a capacity of 819.8 mAh g −1 is obtained after 1200 cycles at 1.0 A g −1 , while the bulk polyhedron Mn 2 O 3 electrode presents a limited cycle performance and only a capacity of ≈160 mAh g −1 is retained after 250 cycles (this inferior performance is comparable to reported works). [ 8,10 ] The rate capability (Figure 2 d) and the cycling performance at 0.4 A g −1 (Figure S4b, Supporting Information) further confi rmed this inferior electrochemical performance compared to its mini-hollow counterpart.
The structural evolution of these two electrodes during cycling was monitored by SEM and the schematics illustration is shown in Figure 3 a,b, respectively. In the mini-hollow polyhedron Mn 2 O 3 electrode ( Figure S6a nanosize effect of the conversion mechanism and the volume expansion cannot be avoided. [ 23 ] Fortunately, the original small interior cavity offers room for the inward volume expansion, so that it is fi lled by the reformatted nanoparticles, leading to the formation of a hierarchical nanostructure with homogeneous dispersion of the nanoparticles. This formed hierarchical nanostructure keeps its structure even after 500 cycles ( Figure S6c,d, Supporting Information) without any serious "electrochemical sintering," [ 14 ] indicating its stability. Similarly, the nanosize effects also occur on the bulk polyhedron electrode ( Figure S7, Supporting Information). Due to the lack of room to contain the inward volume expansion, the unbalanced expansion tension may lead to the formation of a hierarchical nanostructure with congested core. Obviously, this reconstructed structure is not on equilibrium stage and will aggregate easily and fi nally collapse. In contrast to our mini-hollow electrode, the www.MaterialsViews.com www.advancedscience.com conventional large-hollow electrodes reported in previous works mostly did not show comparable cyclic stability though their capacity and rate capacity are excellent. [ 40,44 ] As illustrated in Figure 3 c, a hollow reconstructed hierarchical nanostructure was formed after the fi rst cycle. Owing to the lack of confi ne from the interaction with each other, the nanoparticles near the inner cavity may keep expanding and lead to the structure collapse. Thus, those hollow electrodes show limited cycles. This phenomenon can be seen in other conversion mechanism electrodes, such as cobalt oxide. [ 39,45 ] Honestly speaking, the size of the hollow cavity in our system may not be optimal. However, the comparing results indicate that it is important and practical to design and control the hollow size to improve the cycling stability of the conversion mechanism electrodes. Encouraged by the excellent electrochemical performance of this mini-hollow Mn 2 O 3 electrode, a further investigation on its conversion mechanism was carried out. Consisting with many works, [ 8,10,17 ] the peaks located at 0.98 and 0.56 V are considered to be the conversion of Mn(III) to Mn(II) and the formation of the SEI, while the intensive peak at 0.22 V is ascribed to the further reduction of Mn oxide (Mn 3 O 4 ) to metallic Mn in the fi rst cathodic process (Figure 4 a). However, in contrast to these reported works, there are two peaks located at about 1.25 and 2.5 V on the anodic process, which suggest that the oxidation products at 2.  Table S4 in the Supporting Information). X-ray photoelectron spectroscopy (XPS) in Figure S8 (Supporting Information) also confi rm the different oxidation states of the products at 2.0 and 3.0 V. [3][4][5]46 ] This means that the oxidation peaks at around 1.25 V should be ascribed to the oxidation of metallic Mn to Mn 3 O 4 , and the peak around 2.5 V should be the incomplete oxidation of Mn 3 O 4 to Mn 2 O 3 and MnO 2 . The differential charge capacity versus voltage curves (Figure 4 b) give further evidence. Obviously, the peak at 2.5 V becomes more and more intensive, shifts to lower potential, fi nally stabilizes at about 2.1 V. This phenomenon also is refl ected by the changes of the two plateaus of the charge curves at 0.4 A g −1 (Figure 2 b). These results indicate that the oxidation becomes easier as the cycles go on, and the electrochemical kinetics is improved. [ 14,21 ] The subsequent cyclic voltammetry (CV) curves ( Figure S9a, Supporting Information) with two pairs of anodic and cathodic peaks give further evidence and the improved kinetics is verifi ed by the reduced charge-transfer resistance ( Figure S9b, Supporting Information). [ 47 ] Based on the above discussion, the lithium storage mechanism of this mini-hollow polyhedral Mn 2 O 3 electrode is believed to proceed as follows: In order to further confi rm the superior electrochemical property induced by the reconstruction of mini-hollow structure, the corresponding discharge-charge curves at various current densities, differential charge capacity versus voltage curves, XPS of the products at different charged state, CV curves, and electrochemical impedance spectroscopy (EIS) plots of the bulk polyhedron Mn 2 O 3 electrode were preformed and presented in Figure S10 (Supporting Information). The discharge-charge curves ( Figure S10a,b, Supporting Information) shows that only one pair of plateau on them and the corresponding differential charge capacity versus voltage curves at 0.4 A g −1 (Figure S10c, Supporting Information) also shows only one oxidation peak at around 1.25 V and no peaks appears above 2.0 V. The CV curves at different cycles give visualized evidence, and furtherly reveal that the intensity of the peaks lowered with the cycling, as shown in Figure S10d    Based on the above analysis, the outstanding electrochemical properties exhibited by this mini-hollow polyhedron Mn 2 O 3 electrode are mainly induced by the nanosize effect by utilizing its merits and inhibiting its disadvantages simultaneously. First, the small interior cavity offers room for the inward volume expansion, forming a hierarchical nanostructure with homogeneous dispersion of the reformatted nanoparticles. Second, the reconstructed hierarchical nanostructure after the fi rst cycle remains steady for long cycling stability. Third, the nanostructure not only induces more active sites to join in the electrochemistry activity but also enhances the kinetics to easily produce higher oxidation products, achieving a large capacity and good rate capability.
In summary, mini-hollow polyhedron Mn 2 O 3 derived from Mn-based MOFs has been synthesized. This material exhibits promising Li storage property by utilizing the nanosize effect ingeniously. A high capacity and excellent rate capability were achieved. Meanwhile, the reasons for the improved www.MaterialsViews.com www.advancedscience.com www.MaterialsViews.com www.advancedscience.com electrochemical activities were studied and a new mechanism is proposed. What is more, the super-long cycling stability (exceeding 1200 cycles at 1.0 A g −1 ) and high capacity endow this material with a competitive prospects for application. The discussion on the mini-hollow structure suggests that it is important and practical to design and control the hollow size to improve the cycling stability of conversion mechanism electrodes, which offers a new perspective to design the structure of an electrode material with high-performance energy storage.

Experimental Section
All the chemicals were purchased from J&K and used without further purifi cation.
Materials Synthesis : Typically, 3.75 mmol MnCl 2 ·4H 2 O and 10 mmol NH 4 Cl were dissolved in the mixture of 37.5 mL CH 3 CN (acetonitrile), 17.5 mL HCOOH (methanoic acid), and 17.5 mL CH 3 COOH (acetic acid). The solution was transferred to a 100 mL Tefl on-lined stainless steel autoclave and maintained at 100 °C for 24 h. After cooling down to room temperature naturally, the pink product was collected by suction fi ltration and washed with ethanol for three times. The pink Mn-based MOF was obtained after drying at 60 °C for 4 h. In order to obtain minihollow polyhedron Mn 2 O 3 , the prepared Mn-based MOF was heated at 750 °C with a temperate ramp of 5 °C min −1 for 4 h in air.
The bulk polyhedron Mn 2 O 3 was synthesized according to the reported work. [ 37 ] Typically, 16 mmol of Mn(NO 3 ) 2 was dissolved in 50 mL 1-butanol solvent followed by a vigorous stirring for half an hour at room temperature. Then the mixture was transferred to a 100 mL Tefl on liner and sealed in an autoclave for solvothermal treatment at 120 °C for 20 h. The black bulk polyhedron Mn 2 O 3 was collected by suction fi ltration and washed with ethanol for three times.
Single-Crystal X-Ray crystallography : Suitable crystal Mn-MOF was placed in a cooled N 2 gas stream at ≈130 K for crystallographic data collection on a SuperNova Single Crystal Diffractometer equipped with graphite-monochromatic Mo Kα radiation ( λ = 0.71073 Å). Data reduction included absorption was performed by using the SAINT program. [ 48 ] The structures were solved by direct methods and refi ned by full-matrix least squares on F2 with SHELXS-97 and SHELXL-97 programs. [ 49 ] Materials Characterization : The crystal structures of the products were characterized by XRD (Rigaku MiniFlexII, with Cu Kα radiation, λ = 1.5408 Å), The morphology and microstructure were characterized by scanning electron microscopy (SEM; JEOL JSM-7500F) and transmission electron microscopy (TEM; Philips Tecnai G2 F20). Thermogravimetric analysis (NETZSCH, TG209) was carried out under air fl ow with a temperature ramp of 5 °C min −1 . Elemental analyses (for C, N, and H) were performed by using an Elementar analyzer (vario EL CUBE). The binding energy of Mn was investigated by XPS (Kratos Axis Ultra DLD spectrometer).
Electrochemical Measurements : Electrochemical measurements were carried out using a two-electrode cell assembled in an argonfi lled glove box. The working electrodes consist of 75 wt% active material (Mn 2 O 3 ), 15 wt% Super P carbon black, and 10 wt% sodium carboxymethyl cellulose. The loading amount of the electrode material was measured ≈0.60 mg cm −2 by a microbalance (Mettler, XS105DU) with an accuracy of 0.01 mg. The electrolyte is 1 M LiPF 6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DMC) (EC:DMC = 1:1 by volume). Pure lithium foil was used for both the counter and reference electrode. And the separator was Celgard 2320 membrane. CV testing with a cutoff voltage window of 0.01-3.00 V (vs Li + -Li, 0.1 mV s −1 ) and EIS (0.1-100k Hz) measurements were both performed on a CHI660b electrochemical workstation (Chenhua, Shanghai, China). Galvanostatic charge-discharge tests were carried out on a Land Battery Measurement system (Land CT2001A, Wuhan, China) under various current densities in the fi xed range of 3.00-0.01 V at room temperature.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.