Controlled Synthesis Of Micro/nano-structured Manganese Oxide Materials For Lithium Ion Batteries

With the rapid development of global economy, the fossil fuels are consumed quickly and the environment is heavily polluted. There is an urgent need for e?cient, clean, and sustainable sources of energy. Lithium ion batteries(LIBs) become one of the most suitable candidates to satisfy the energy requirements due to their high energy and power densities and long cycle lifetime. Nowadays, LIBs have been widely used in portable electronic devices and become increasingly popular in the fields of electric vehicles(EVs) and hybrid electric vehicles(HEVs). As an important component of LIBs, electrode material is a key factor that restrains the developments of LIBs. Therefore, exploring electrode materials with high capacity, high energy density and long cycle lifetime is the most important issue in the field of LIBs.As anode electrodes for LIBs, manganese oxides have attracted considerable attention because of their thermal stability, high theoretical capacity, low price and natural abundance. However, the poor electrical conductivity and large volume expansion during the cycling process result in the poor cyclability and rate capability, restricting their practical utilization in LIBs. To resolve the above problems, this dissertation mainly focuses on the research on buffering the serious volume expansion/contraction during the discharge/charge process and improving the electronic conductivity and ion conductivity of the electrode materials, so as to enhance the cyclability and rate capability of manganese oxides.In this dissertation, we have prepared several micro/nano-structured manganese oxide materials via a systematic, facile and environment friendly liquid phase reduction method, and deeply studied their synthesis mechanisms and electrochemical performances, as well as the relationship between the structure and performance. Some significant results have been obtained in this dissertation and are listed as followed:(1) Well shaped single crystalline Mn3O4 nano-octahedra with different sizes have been successfully prepared. Our results reveal that the electrochemical performances of Mn3O4 nano-octahedra are closely related with the exposed active {011} facets: the exposed active {011} facets with alternating Mn and O atom layers facilitate the conversion reaction between Mn3O4 and Li, resulting in high Li-ion storage capacity, excellent rate capability and low charge transfer resistance. In addition, the smallest sized Mn3O4 nano-octahedra demonstrate the best electrochemical properties because the character of nanopatticlrs can ensure full contact with the electrolyte, facilitate the rapid Li-ion diffusion at the electrode/electrolyte interface and provide short distances for fast lithium-ion transportation within the particles. The synergy of high surface area and exposed {011} facets ensures the excellent electrochemical performance of Mn3O4 nano-octahedra for LIBs.(2) Two types of hierarchical mesoporous Mn3O4/carbon microspheres have been fabricated via an in situ strategy, where the mesoporous structure is directly carbonized from the DEG ligand in Mn-DEG along with the crystallization of Mn3O4. Such unique hierarchical mesoporous structure of the Mn3O4/carbon sample provides large electrode-electrolyte contact area, short Li+ transport path, low charge transfer resistance, and superior structural stability upon prolonged cycling, leading to the high lithium storage capacity(915 mA h g-1 at 100 mA g-1), great cycling stability and excellent rate capability in the voltage range of 0.01- 3 V. Even in a very narrower voltage range of 0.01- 1.5 V, the Mn3O4/carbon can deliver a high lithium storage capacity of 556 mA h g-1 at 100 mA g-1 and an excellent rate capability of 269 mA h g-1 at 1000 mA g-1.(3) Three dimensional bicontinuous bimodal mesoporous Mn2O3 single crystals with cube geometry have been successfully synthesized via the thermal decomposition of MnCO3 single crystals. The Mn2O3 single crystals exhibit a three dimensional interconnected bicontinuous porous system with bimodal mesoporosities throughout the whole crystal and a high surface area. Three differently sized Mn2O3 cubes(500 nm, 700 nm and 1.2 ?m) were prepared to investigate the relationship between the pore structure or crystal size and the electrochemical performance. The results show that the Mn2O3 cubes around 700 nm display superior electrochemical performances with a large reversible capacity(845 m A h g-1 at 100 mA g-1), high coulombic efficiency(over 95% after the second cycle), excellent cycling stability and good rate capability(410 mA h g-1 at a current density of 1 A g-1). This can be attributed to the special hierarchically porous structure of the Mn2O3 single crystals with high crystallinity and high porosity.(4) Unique uniform walnut-shaped porous MnO2/carbon nanospheres have been fabricated via an in situ strategy. The polyvinylpyrrolidone(PVP) is used as surfactant and stabilizer, and also as carbon source to form the carbon scaffold along with the crystallization of MnO2. When evaluated as anode material for LIBs, the novel walnut-shaped porous MnO2/carbon nanospheres with carbon coating on the nanocrystallites surface exhibit super-high reversible capacity(1176 mA h g-1 at 100 mA g-1), good cycling stability and excellent rate capability(540 mA h g-1 at 1 A g-1). It is extremely noteworthy that the further deep oxidation of Mn2+ to Mn3+ in MnO2/C electrode results in such a high reversible capacity. Especially an extraordinarily high capacity of 1192 mA h g-1, very close to the theoretical reversible capacity of MnO2(1230 mA h g-1), can be achieved at a high current density of 1000 mA g-1 after long period cycling. This is the highest value observed for MnO2 type material based electrodes. The high lithium storage capacity and rate capability can be attributed to the enhanced electrochemical kinetics of porous MnO2/C nanospheres.(5) A novel manganese alkoxide that blending with nitrogen-doped graphene(Mn-EG/NG) has been synthesized via a facile one-pot solution-phase reaction. For the first time, the Mn-EG/NG hybrid is employed as the anode material for LIBs, which shows featuring high reversible capacity(946.6 mA h g-1 at 100 mA g-1), excellent rate capability(704 mA h g-1 at 2000 mA g-1) and long cycle life(97.4% capacity retention after 300 cycles at 1000 mA g-1). The superior electrochemical performances are unprecedented and among the best of the manganese oxides,which can be ascribed to the synergistic couple of high capacity Mn-EG and N-doped conductive graphene. This novel hybrid can be employed to design advanced anode electrode with high energy density, power density, and long lifespan in LIBs.