【作者】 魏义永；

【导师】 戴建明；

【作者基本信息】 中国科学技术大学 ， 材料物理与化学， 2020， 博士

【摘要】 最近,以MnO和MnF2为代表的锰基金属氧化物和氟化物负极材料,由于其具有较好的储能特性而引起了广泛的关注。MnO和MnF2具有理论比容量(755.6和577 mAh g-1)较高、电动势值相对较低和资源很丰富等优点。然而,MnO和MnF2的电导率都较低,电化学反应过程中体积变化较大使得电极材料粉化脱落严重,导致该类材料具有较差循环稳定性和倍率性能。针对这些不足,我们通过优化反应物浓度、磁场诱导、碳复合和元素掺杂等手段调控材料的形貌、微纳结构和导电性,研究该类材料电化学储能特性,主要内容如下:(1)采用水热法和磁场辅助水热法分别合成MnCO3前驱体,然后热解制备了多孔微米立方体和介孔微米链结构MnO材料。通过调控尿素含量和改变磁场强度优化了材料的形貌和微纳结构,从而获得良好的电化学性能。多孔微米立方结构MnO在0.5 C时经过200个循环后,放电比容量达到914.6 mAh g-1。而介孔微米链结构MnO在2 C时经过250个循环后,放电比容量达到768.7 mAh g-1;甚至在8 C时经过2000个循环后,放电比容量仍保持在198.3 mAh g-1。多孔微米立方和介孔微米链结构MnO的优异电化学储能特性都归因于它们的高比表面积和较稳定的微纳结构,这有利于锂离子和电子的快速传输,同时适应Li+在电化学过程中体积的急剧变化,从而提高该材料电化学性能。(2)通过水热法利用葡萄糖原位包覆合成前驱体,将其煅烧制备成介孔MnO@C微米立方体。研究了葡萄糖含量对MnO电化学性能的影响。结果表明,包覆0.2 g葡萄糖MnO具有最优的电化学储能特性,在1 C时经过200次循环后,放电比容量达到1006.5 mAh g-1,容量保持率是105.3%。即使在8 C下,仍可提供284.9 mAh g-1的放电比容量。其优异的电化学储能归因于多孔结构,良好的结晶度和适量的碳原位包覆。(3)通过水热法实现Mn位Co元素掺杂,有效调控MnO本征电导率,优化其电化学储能特性。Mn0.9Co0.1O表现出最佳的电化学性能,在电流密度1 C时经过250次循环后,放电比容量达到738.9 mAh g-1,甚至在8 C时可逆比容量仍可维持287.0 mAh g-1。该材料优越电化学储能特性明显改善主要归因于Mn位Co元素掺杂可有效增强材料的电导率,缓解材料中电极化现象,提高锂离子的扩散速率。(4)采用溶剂热法合成NH4MnF3前驱体,然后热解制备多孔MnF2。通过改变反应物浓度梯度调控最终产物的形貌,进而提高该材料的电化学性能。其中核壳结构MnF2微球显示出最好的电化学性能,在0.5 C时经过250个循环后,放电比容量达到721.5 mAh g-1,此外,在4C时可逆比容量仍具有211.8 mAh g-1。该材料优异电化学性能归因于其高的比表面积和稳定的核壳结构,这有助于锂离子和电子的快速输运,同时缓解电化学循环过程中的剧烈体积变化。(5)采用溶剂热法合成了介孔MnF2@CNT立方体。将高导电碳纳米管分散在MnF2立方体内部和表面,以提高其导电性。与纯MnF2相比,MnF2@CNT立方体明显改善电化学性能。在0.5 C时经过100次循环后,放电比容量达到了 292.9 mAh g-1,此外,它在8 C时还可维持151.7 mAh g-1的可逆比容量。碳纳米管的加入和分级结构的结合提高了 MnF2@CNT的倍率性能,使其循环性能相对稳定。更多还原

【Abstract】 Recently,MnO and MnF2,as the representative of manganese based metal oxides and fluorides anode materials,have attracted wide attention due to their good energy storage properties.MnO and MnF2 have the advantages of high theoretical specific capacity(755.6 and 577 mAh g-1),relatively low electromotive force value and abundant resources.However,the conductivity of such materials is low,and the large volume change in the electrochemical process makes the electrode materials pulverize and fall off,thus resulting in poor cycle stability and rate performance.In order to solve these problems,we adjusted the morphology,micro-nano structure and electrical conductivity of the materials by optimizing the reactant concentration,magnetic field induction,carbon composite and element doping,and studied the electrochemical performance of the material.The main results are as follows:(1)The MnCO3 precursors were synthesized by hydrothermal method and magnetic field assisted hydrothermal method respectively,and then porous structure microcube and mesoporous microchain structure MnO materials were prepared by thermal decomposition.By adjusting the urea content and changing the magnetic field strength,the morphology and micro-nano structure of the material are optimized,so as to obtain good electrochemical performance.The porous microcubic structure MnO has a specific discharge capacity of 914.6 mAh g-1 after 200 cycles at 0.5 C.However,after 250 cycles of mesoporous microchain structure MnO,the specific discharge capacity of 768.7 mAh g-1 was maintained at 2 C,even after 2000 cycles at 8 C,the specific discharge capacity of 198.3 mAh g-1 was still maintained.The superior electrochemical energy storage characteristics of porous microcubic and mesoporous microchains structure MnO are attributed to their high specific surface area and relatively stable micro-nano structure,which is conducive to the rapid transmission of lithium ions and electrons,while adapting to the change of volume in the lithium ion electrochemical process,thus effectively improving the electrochemical performance.(2)In-situ coated glucose precursors were synthesized by hydrothermal method,and then the precursors were calcined to prepare MnO@C microcube.The effect of glucose content on the electrochemical performance of MnO@C was studied.The results showed that 0.2 g glucose coated MnO had the best electrochemical energy storage characteristics.After 200 cycles at 1 C,its discharge specific capacity reached 1006.5 mAh g-1,and the capacity retention rate was 105.3%.Even at 8 C,it can still provide a specific discharge capacity of 284.9 mAh g-1.Its superior electrochemical energy storage characteristics may be attributed to porous structure,good crystallinity,and appropriate amount of carbon in situ coating.(3)The Co-doped Mn site is realized by hydrothermal method,which can effectively control the intrinsic conductivity of MnO and optimize its electrochemical energy storage characteristics.Mn0.9Co0.1O showed the best electrochemical performance.After 250 cycles at 1 C current density,the discharge specific capacity reached 738.9 mAh g-1,and even at 8 C the reversible specific capacity could still maintained 287.0 mAh g-1 even at 8 C ratio.The obvious improvement of the electrochemical performance of the material is mainly due to the fact that the doping of Co element at Mn position can effectively enhance the conductivity of the material,alleviate the polarization phenomenon in the material and improve the diffusion rate of Li+.(4)MnF2 were fabricated by thermal decomposition of the NH4MnF3 precursor synthesized by solvent-thermal method.The morphology of the final product was controlled by changing the reactant concentration gradient,so as to improve the electrochemical performance of the material.The core-shell structure MnF2 microspheres showed the best electrochemical performance.After 250 cycles at 0.5 C,the discharge specific capacity reached 721.5 mAh g-1.Besides,the reversible specific capacity at 4 C still had 211.8 mAh g-1.The outstand electrochemical performance of MnF2 may be attributed to its high specific surface area and stable core-shell structure,which helps the rapid transport of lithium ions and electrons,while adapting to the dramatic volume changes during the electrochemical cycle.(5)Mesopore MnF2@CNT cube was prepared by solvothermal method.Hhighly conductive CNT were dispersed on the inner and surface of MnF2 cube to improve their conductivity.Compared with pure MnF2,the electrochemical properties of MnF2@CNT composite were significantly improved.After 100 cycles at 0.5 C,the discharge specific capacity reached 292.9 mAh g-1.In addition,it also has a reversible discharge capacity of 151.7 mAh g-1 at 8 C.The addition of CNT and the combination of the hierarchical structure improve the rate performance of MnF2@CNT,and make its cycle performance relatively stable.更多还原