Preparation And Electrochemical Performances For High Energy Manganese-based Lithium/Sodium Ion Batteries

As renewable energy sources,lithium ion batteries(LIBs)are widely used in the consumer electronics,electric vehicles and other energy storage fields.Advanced LIB technology requires even higher energy density.Despite the continuous improvement in recent years,there is no significant breakthrough in the energy density compared to the-state-of-the-art LIBs.The reasons are deeply rooted in two parts.Firstly,it is limited by the existing battery preparation process technology.More important point is the inherent theoretical energy density of the existing cathode and anode materials.As we all know,the energy density of the electrode is the product of its average discharge voltage and mass(or volume)specific capacity.In addition,in recent years,the further development of large-scale energy storage technology gives much remarkable advantages for sodium ion batteries(SIBs)due to the abundant resources of electrode materials and significantly lower battery cost.The research on high energy density SIBs is still lacking.In this thesis,the abundant and cheap manganese-based compounds are elected as the starting raw materials to synthesize LIB electrodes.On one hand,we focus on the higher voltage plateau(4V)cathodes such as olivine-type structure lithium manganese iron phosphate(LiMn1-xFexPO4)and spinel-type structure lithium manganate oxide(LiMn2O4).On the other hand,layered-type lithium-rich cathode and manganese oxide(MnO)anode with high specific capacity are also studied.Finally,a high energy density full LIB cell based on manganese-based electrodes is constructed.Moreover,the electrochemically converted "Sodium manganese iron phosphate" and "Sodium-rich" SIB cathodes are synthesized,which opens up a new direction on exploring high energy density SIB cathodes.In Chapter 1,a general introduction of the working mechanism and key components of LIBs are given.Then a detailed research progress of typical cathode materials and transition metal oxide anode materials for LIBs is presented.Additionally,the background of this thesis is expounded.In Chapter 2,the experimental reagents used in the course of the thesis experiments are listed.The equipments used to prepare and characterize the samples are introduced.A detailed procedure to fabricate coin-cells for lithium and sodium ion batteries is also presented.The instruments for structural and electrochemical analyses are also described.In Chapter 3,mixed-carbon coated LiMn1-xFexPO4(x=1,0.6,0.5,0.2,0)nano-particles are synthesized by a novel solvothermal approach.The mechanism of ascorbic acid in the solvothermal process and the influences of Mn/Fe ratio on samples' morphology and electrochemical performance are explored.Results show that ascorbic acid can act as antioxidant,surfactant and a first-time carbon coating source to obtain a uniform carbon coating nano-particles,The synthesized LiMn0.4Fe0.6PO4(Mn/Fe=2/3)sample has the optimal properties with a high energy density(593 Wh kg-1),good cycleability(96.8%,130 cycles)and rate capability(128.5 mA h g-1,20C).Even at-20?,the specific capacity is still remain 106.6 mAh g-1 at 0.2C.In Chapter 4,a simple two-step solid-state-reaction process is adopted to synthesize carbon-coated Mg-doped LiMg0.02Mn0.98-xFexP04(x=0.2,0.5)nano-particles,which show excellent electrochemical properties in a temperature range from-20 to 55?.The LiMg0.02Mn0.98-xFexPO4 electrodes can be electrochemically converted into Mg doped NaMg0.02Mn0.98-xFexP04 electrodes,which also possess impressive electrochemical performance for energy storage and open up a new research direction on olivine-type cathode materials for SIBs.In Chapter 5,nanoplate-assembled MnO2 spheres with a high surface area are synthesized and act as a manganese precursor for the hydrothermal process to synthesize MnO/reduced graphite oxide(MnO/RGO).Two methods of morphology design and carbon coating are adopted to improve the electrochemical performance of MnO,the effect of reducing agent N2H4 on the quality of synthesized RGO is investigated.Besides,the effect of RGO content on the electrochemical properties of the composite is also studied.In Chapter 6,a two-step hydrothermal approach is employed to synthesize LiMn2O4/CNT composite powders using the nanoplate-assembled MnO2 spheres as a manganese precursor.The role and content of the CNT in the hydrothermal process are studied.The hydrothermal synthesized composite powders do not require high temperature treatment;the CNT can also act as conductive additives.The optimal CNT content in the LiMn2O4/CNT composite shows significantly improved long cycle and high rate electrochemical performance.In Chapter 7,a high temperture solid-state-reaction process is adopted to synthesize Li1.2Mn0.6Ni0.2O2 powder using the nanoplate-assembled MnO2 spheres as a manganese precursor.The sphere-shaped Li-rich cathode is assembled with primary nanoplates with enhanced growth of nanocrystal planes(parallel to to the c-axis),which is in favor of Li+ intercalation/deintercalation along the a-b planes.The special morphology of the Li-rich phase has excellent electrochemical performance,especially the rate performance.In addition,The Li-rich electrodes can be electrochemically converted into 03-type "Na-rich" electrodes,which also possess impressive electrochemical performance for energy storage and open up a new research direction on high energy density cathode materials for SIBs.In Chapter 8,Nano-spherical Li-rich cathodes and MnxCo1-xO anodes are synthesized from as-solvothermal MnxCo1-xCO3 precursor,Based on the half-cell studies of these materials,Li-rich 0.5Li2MnO3·0.5LiMn0.5Ni0.5O2 and binary transition metal oxide Mn0.8Co0.2O are selected respectively as the optimal positive and negative electrodes to construct a full cell,which allows no need for pre-activation of the electrodes.The special coulombic efficiency match-up and tailored microstructures and compositions of the electrode materials are all crucial to achieve such a high energy density.At last,an overview of the main innovative work and deficiencies of this thesis are presented in Chapter 9.Some expectations and suggestions for the further research are also given.