Facile synthesis and electrochemical properties of CoMn2O4anodes for high capacitylithium-ion batteriesCoMn2O4Spinel Hierarchical Microspheres Assembled with Porous Nanosheets asStable Anodes for Lithium-ion BatteriesFigure 5b shows discharge-charge curves of the electrodes made from theCoMn2O4 hierarchical microspheres at a current density of 100 mA·g-1at room temperature in a potential window between 0.01 and 3.0 V (vs. Li+/Li). It can be seen that there is a large deviation in potential between charge and discharge curves. The gap between charge and discharge curves is important, which involves the energy efficiency. The large gap may be a feature of metal oxide anode due to the polarization related to ion transfer during charge-discharge cycles. This phenomenon is often observed in many metal oxide anodes due to poor electrical conductivity, including CoMn2O4 reported previously. It is reported that the incorporation of graphene sheets can partly reduce the voltage polarization. The solution can be further studied in the future. The initial discharge and charge capacities are 1442 and 937 mAh g-1, which is higher than the theoretical value (920mAh g-1) based on the conversion reaction (see equation 5). The excess capacities could be associated with the decomposition of the electrolyte at low voltages generating a solid electrolyte interphase (SEI) layer and the further lithium storage via interfacial charging at the metal/Li2O interface. The large irreversible capacity loss of the first cycle can be attributed to the phenomenon that the formed SEI film can not completely decompose during the first charge.Figure 5c shows the discharge/charge capacity versus cycle number for the electrodes made from the CoMn2O4 hierarchical microspheres at a current of 100 mAh g-1. It is interesting to observe that the discharge capacity decreases only slightlybefore 36 cycles. After that, more interestingly, an unusual phenomenon that the discharge capacity gradually increases to 900 mAh g-1 can be seen. This capacity is very close to the theoretical value of a fully reversible conversion reaction (920 mAh g-1). Generally, after charge and discharge at a low current, the electrolyte penetrates into inner part of active materials. In our case, the current density of 100 mAh g-1 is not very high. So, to some extent, the above situation may also happen, making inner part of CoMn2O4 also involved in the conversion reactions,and then results in the increase of capacity. On the other hand, the phenomenon that the discharge capacity gradually increases also attributes to progressive generation of electro-chemistry active polymeric gel-like films. Not all the surface layer is covered on the first discharge. The internal surface within the pores is more difficult to access. It would appear that the polymer layer builds-up slowly, over a number of cycles. Of course, the specific reasons need to be studied further. After 65 cycles, this discharge capacity can be retained at 894 mAh g-1, corresponding to 94.9% of the second discharge capacity (942 mAh g-1), indicating excellent cycling stability. These above results are better than those in previous reports. It has been reported that CoMn2O4 hollow microcubes achieved a capacity of 624 mAh g-1 with retention of about 75.5% at a current of 200 mAh g-1after 50 cycles, while CoMn2O4 powders only exhibited a capacity of 515 mAh g-1 with retention of about 64% at a current of 69 mAh g-1 after 50 cycles.Double-Shelled CoMn2O4 Hollow Microcubes as High-Capacity Anodes forLithium-Ion BatteriesThe as-prepared double-shelled CoMn2O4 hollow microcubes are then tested as anodes for LIBs. Representative chargedischarge profiles of the electrode made of the double-shelled CoMn2O4hollow microcubes at a current density of 200 mA g−1are shown in Figure 4b. The initial discharge and charge capacities are 1282 and 806 mA g−1, respectively. The irreversible capacity loss of the first cycle can be attributed to the formation of the SEI film. Interestingly, even the initial charge capacity is much higher than the theoretical value (691 mA g−1) based on the oxidation of metallic Co and Mn nanoparticles to CoO and MnO respectively: 3Li2O +Co +2Mn ↔2MnO +CoO + 6Li++ 6e−. There are two possible reasons for this discrepancy. Firstly, it may be attributed to the reversible formation/dissolution of polymeric gel-like film resulted from electrolyte degradation, which has also been observed in nanostructured ZnMn2O4[27]and other transition metal oxides. [28]Secondly, it may also be contributed from the further oxidation of MnO to Mn3O4 or even Mn2O3 as observed in Kim’s study on ZnMn2O4 nanowires.[29]However, no peaks corresponding to the Mn2+/Mn3+ redox couple can be observed in the CV curves in our study. Thus, the more plausible origin for the extra reversible capacity in the present study is the reversible formation/dissolution of polymeric gel-like film.Figure 4c shows the discharge capacity as a function of cycle number at current densities of 200 and 800 mA g−1in the voltage range of 0.01–3.0 V vs.Li/Li+. Discharge capacities obtained for the first and second cycles at 200 mA g−1 are 1282 and 827 mA g−1, respectively. From the second cycle onwards, the discharge capacity only decreases slightly. After 50 cycles at 200 mA g−1, a high discharge capacity of 624 mA g−1is still retained, corresponding to 75.5% of the second discharge capacity. Even at a high current density of 800 mA g−1, a discharge capacity of 406 mA g−1 is retained after 50 cycles.Previously, the electrochemical properties of sub-micrometer sized CoMn2O4 particles have been reported, where a reversible capacity of about 515mA g−1 with retention of about 64% after 50 cycles at a current density of 69 mA g−1is achieved.[27]To the best of our knowledge, this is the first report on utilizing CoMn2O4 hollow structures as an anode material for LIBs. A much higher specific capacity with enhanced capacity retention is achieved in the present study through engineering the microstructure. More specifically, the superior electrochemical performance of our CoMn2O4 material can be attributed to the following two factors: (1) The nanometer-sized subunits not only make the conversion reaction more feasible but also allow the reversible formation/dissolution of polymeric gel-like film at the surfaceof the active material, both of which contribute to the high specific capacity.[30,31](2) The hollow interior and the porosity in the shells can buffer the large volume change of anodes based on the conversion reaction during the repeated Li+insertion/extraction, thus alleviating the pulverization problem and enhancing the cycling performance.Electrochemical properties of yolk–shell and hollow CoMn2O4powders directly prepared by continuous spray pyrolysis as negative electrode materials for lithium ionbatteries3The cycling performances of the CoMn2O4powders prepared from spray solutions with different concentrations of sucrose are shown in Fig. 4c. The Y1 and Y2 samples had high discharge capacities of 519 and 573 mAh g 21 after 40 cycles, respectively. However, the H1 and H2 samples had low discharge capacities of 418 and 276 mAh g 21 after 40 cycles, respectively. The H1, Y1 and Y2 samples had good cycle performances at a high discharge rate of 800 mA g 21.On the Other hand,the H2 sample had poor cycle performances. The CoMn2O4 powders had high Coulombic efficiencies above 97% from the second cycles irrespective of their morphologies.Facile preparation of ZnMn2O4hollow microspheres as high-capacity anodes for lithium-ionbatteries†Hydrothermal synthesis and characterization of MnCo2O4in the low-temperature hydrothermal process: Their magnetism and electrochemical properties。
