碳酸锰/石墨烯及其派生的氧化锰/石墨烯纳米复合电极的表界面活性和界面储锂机制的研究

With the ever increasing demand of high-energy and high-power lithium-ion batteries (LIBs), the low theoretical capacity of graphite (LiC6~372 mAh g-1) and its poor Li+-ion intercalation kinetics have been recognized as limiting factors. In recent years, transition metal oxides have been extensively explored as LIB anode materials and have been proved that they can hardly satisfy the challenges in terms of performance, cost and safety simultaneously. As a precursor of transition metal oxides, transition metal carbonate have also attracted scientists considerable attentions because of the high reversible capacity, environmental friendship and low cost. However, both the oxides and carbonates possess a fatal drawback of poorer conductivity, which can be improved by the doping or coating of carbonaceous substances such as functionalized graphene oxide (GO) and/or its derivatives (e.g., rGO). Additive GO or rGO can stabilize active substances through the hydrothermally cross-linking reaction derived from the rich functional groups on its surfaces, and the resulting composite electrode provides its sufficient contact with electrolyte and exhibits rapid channels for Li+ diffusion and hinders the agglomeration of anode materials occurring during the continuous charge-discharge cycles. . In this proposal, functionalized additive-assisted hydrothermal synthesis method is proposed to prepare crystalline MnCO3 nanoparticles with a manifest high-activity crystal plane and to obtain nanostructured MnCO3 with a hierarchical feature, then a high-temperature decomposition is adopted for the solid-state transformation of the nanostructured MnCO3 to porous MnxOy (i.e., x = 1, 2 or 3; y = 1, 2, 3 or 4), and then a well-dispersed suspension containing GO and active substances is hydrothermally treated for the final production of high-surface-area MnCO3/rGO and/or MnxOy/rGO nanocomposites. By focusing on the stabilized surface area and/or the correspondingly high surface/interface electrochemical activity, what we will investigate deals with the following issues: (i) the selective nucleation and dynamical stabilization of nanocrystalline MnCO3 with a manifest high-activity crystal plane, the solution-mediated self-assembly mechanism of tiny MnCO3 nanoparticles, and especially the specific adsorption, template effectiveness and interfacial recognition of organic additives with special functional groups therein; (ii) repeating the initially decreasing and then increasing phenomena of reversible lithium-storage capacity upon the continuously cycling; (iii) the possible mechanisms of modified lithium-storage dynamics of these nanocomposite electrodes and their improved faradic/non-faradic charge-storage capacity generated at the surface/interface therein; (iv) the influencing factors of irreversible capacity loss and the improved high-rate cycling stability of these nanocomposite electrodes. In a word, this project is of great importance theoretically, and the accomplishment of which could supply experimental data for the potential application of transition metal carbonates and their derivate oxides as LIB anodes.

商用石墨负极的理论容量较低(372 mAh g-1)，难以满足混合电动汽车和纯电动汽车等对高能量密度锂离子电池的需求，过渡金属碳酸盐及其氧化物的高容量储锂特性成为近年来的研究热点。围绕“纳米结构电极的表界面稳定性及其对电荷存储的贡献”这一关键性的科学问题，本申请拟在MnCO3/rGO及其派生的MnxOy/rGO纳米复合电极制备和结构表征的基础上，探讨其在可逆循环初始阶段的可逆比容量逐渐降低而后逐渐升高甚至在几百次循环后超出首次放电容量的规律。研究纳米复合电极的表界面稳定性和电化学活性；使用传统电化学方法测试分析表界面存储能力的大小及其随循环次数的变化；研究纳米复合电极的可逆储锂动力学和表界面电荷存储机理。分析总结纳米复合电极表界面存储能力的抑制或应用的可能性；希望能为过渡金属碳酸盐或其派生的多孔结构氧化物在锂离子电池领域的应用积累实验数据并提供理论指导。

商用锂离子电池的正极(如磷酸亚铁锂)和负极石墨的理论容量较低，难以满足混合电动汽车和纯电动汽车等对高能量密度储能设备的要求，研究开发新型的、具有高理论容量的电极材料是近年来的研究热点。. 围绕“纳米结构电极的表界面稳定性及其对电荷存储的贡献”这一关键性的问题，开展“过渡金属碳酸盐及其氧化物的高容量储锂机制”的研究，是本项目的主要研究内容之一。围绕“高能量密度锂/硫电池中单质硫的较差的电导率及其充放电过程中多硫化物中间体的穿梭效应”等关键问题，开展“多功能性的碳/硫复合物纳米结构电极的制备及其性能提升机制”的研究，是本项目的另一个主要研究内容。. 部分过渡金属碳酸盐或其氧化物作为锂离子电池负极活性物质时，其可逆容量随着循环次数的增加而逐渐增加、甚至超出首次放电比容量；文献只把此反常现象简单归结于其较大的比表面积及其对界面存储的贡献，其涉及的界面存储机理尚不清晰，更没一个普适的方法能够用于测试分析表界面存储能力的大小。通过制备具有不同纳米结构特征的过渡金属锰、镍等碳酸盐MCO3，然后进一步构筑“过渡金属氧化物及其与部分还原氧化石墨烯的复合纳米结构” ；分别将其用作锂离子电池负极时，将其电化学恒流充放电行为和其微分结果(dQ/dV)分析相结合，在区分本体能量存储和表界面存储的基础上；探讨这一表界面电荷存储的电化学机制，建立了界面能量存储能力的定量测试和分析方法。. 锂/硫电池的充放电过程是绝缘性S8环状分子经过一系列结构和形态变化，形成可溶性多硫化物和不溶性聚硫化物的过程；其中，电解液可溶的多硫化物在正负极之间的穿梭效应，直接导致了硫利用率的降低，大大降低了电池的循环稳定性及其库仑效率。研究开发具有丰富介孔结构的导电性的碳基骨架，研究其对单质硫的负载、对多硫化物的捕集及其电化学催化性能；希望这些理论结果能为锂/硫电池的商业化积累实验数据。

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