Morphology-controlled Synthesis And The Electrochemical Performance Of Mn-based Compounds

Among various cathode materials for lithium ion battery, Mn-based compounds (especially for spinel LiMn2O4) are considered as one of the most promising candidates due to its advantages such as low-cost, environmental friendliness and high abundance. However, the application of LiMn2O4is confined by the capacity decay and cycling instability caused by manganese dissolution, oxygen vacancy and Jahn-Teller effect. Therefore, it’s necessary to explore an efficient system with stable structure to improve the electrochemical property. In present work, the morphology-controlled synthesis and electrochemical performance of LiMn2O4and β-MnO2were studied. The main contents and conclusions are as follows:1. Single-crystalline LiMn2O4nanorods with a diameter of-100nm were synthesized via a template-engaged reaction using tetragonal β-MnO2nanorods as starting material. The investigations on the structures and morphologies of both the precursor and the final product reveal that a minimal structure reconstruction can be responsible for the chemical transformation from tetragonal β-MnO2nanorods to cubic LiMn2O4nanorods. The obtained LiMn2O4nanorods as cathode material for Li-ion battery exhibits superior high-rate capability and good cycling stability in a potential range of3.5-4.3V vs. Li+/Li, which can deliver an initial discharge capacity of125mAh g-1(>84%of the theoretical capacity of LiMn2O4) at a current rate of0.5C, and about75%of its initial capacity can be remained after500charge-discharge cycles at a current rate of3C. Importantly, the rod-like nanostructure and single-crystalline nature are also well preserved after prolonged the charge/discharge cycling time at a relatively high current rate, indicating good structural stability of the single-crystalline nanorods during the lithium intercalation/deintercalation processes, and such high-rate capacity and cycling performance can be ascribed to the favorable morphology and the high crystallinity of the obtained LiMn2O4nanorods.2. Al-doped LiMn2O4nanorods were synthesized base on the above route, which almost have the same size with the above LiMn2O4nanorods. With the increasing doped amount of Al, the initial discharge capacity of the products decrease, but the cycling performance is gradually improved. It is found that the LiAl0.1Mn1.9O4nanorods has the best capacity retention through the comparison of the rate capability at0.5～5C rate for10cycles successively. The LiAl0.1Mn1.9O4nanorods also have excellent cycling performance at an elevated temperature, which still retain70%of the initial capacity at3C after500cycle at50℃, while the LiMn2O4nanorods has only33%of the initial capacity at the same condition. The Al-doped LiMn2O4nanorods with excellent cycle performance at high temperature will be promising cathode materials for high-power lithium ion batteries to promote the development of lithium ion battery.3. LiMn2O4nanorods were synthesized using y-MnO(OH) nanorods as self-template, which was prepared via solvothermal reaction. The electrochemical performance of the products obtained at different calcination temperature was studied. It is found that the LiMn2O4nanorods obtained at600℃has the best electrochemical performance, which delivers the initial capacity of125.6mAh g-1and the capacity retention of95.6%after100cycles at3C. Considering that the low cost of raw materials, simple operation and the low calcination temperature, it is a promising route to synthesize one-dimensional LiMn2O4nanorods with good electrochemical performance using γ-MnO(OH) as precursor.4. Single-crystal β-MnO2hollow bipyramids (HB-β-MnO2) were synthesized via a template-free hydrothermal method. The as-synthesized hollow bipyramids with100-300nm pores along the axis direction of β-MnO2bipyramids are formed through self-assembly and phase transformation processes from α-MnO2nanowires to β-MnO2bipyramids followed by chemical etching of its metastable crystal faces. Compared to the commercial bulk β-MnO2(c-β-MnO2), the as-synthesized HB-β-MnO2exhibits more excellent electrochemical performance with an initial discharge capacity of269mAh g-1, which means up to0.87Li+intercalation per β-MnO2unit, while only0.2Li+intercalation per β-MnO2unit for the commercial c-β-MnO2, The excellent electrochemical activity of the as-synthesized HB-β-MnO2can be attributed to its hollow structure and single crystal nature. The former can provide higher contact area with the electrolyte and buffer ability against volume change during the charge/discharge processes, while the latter contributes to good electronic conductivity and structural integrity stability. Considering the low cost, simple synthesis and the excellent electrochemical activity of the HB-β-MnO2, it will play an important role in the field of lithium battery.