【作者】 曹杨；

【导师】 李会巧；

【作者基本信息】 华中科技大学 ， 材料学， 2020， 硕士

【摘要】 随着经济、可持续的新能源取代传统化石燃料的需求日益增长,锂资源的消耗不断增加,开发可替代锂离子电池（LIBs）的电化学储能技术引起了人们的广泛关注。其中,与锂离子电池具有相似存储机制的钠离子电池（SIBs）,由于具有丰富的资源储量和低廉的成本,被认为是最有希望的下一代电化学储能技术。电极材料作为电池的关键部分很大程度上决定了电池的性能。因此,设计和构建合适的、稳定的、高性能的电极材料对推动钠离子电池的发展具有重要的研究意义。在已经报道的各种储钠负极材料中,层状钛基氧化物因其较大的层间距、原材料丰富、优异的电化学性能以及环境友好等优点,成为最具吸引力的电极材料之一。层状结构Na₂Ti₃O₇作为钛酸钠材料中电位最低、容量最高的化合物,与合适的正极材料匹配将有潜力获得最高的能量密度。但是其循环稳定性差,充放电过程容易发生相变,这导致性能衰减严重。此外,层状结构材料长时间暴露于空气中,空气中的H₂O、CO₂等小分子容易嵌入到层间结构,使得碱金属发生溶解、生成水合物等,加剧了电化学性能的衰减。因此,本文以层状钛基材料为研究对象,围绕提高电池稳定性和储钠动力学的结构设计,从晶体结构出发提出多种调控策略,制备了不同结构的层状钛基材料用于钠离子电池负极应用。具体开展了以下两方面的工作:1.采用固相法在不同温度驱动力下制备不同相结构的层状Na₂Ti₃O₇材料,我们首次合成了三斜晶系的Na₂Ti₃O₇化合物。这表明Na₂Ti₃O₇化合物包含单斜和三斜两种相结构,两种相结构分别具有P 2₁/m1和P-1空间群。从晶体结构中我们发现两种相化合物中的TiO₆八面体呈现不同的扭曲变形程度,具有不同的晶体结构和生长特点。此外,两种相结构表现不同的电化学行为和循环性能,单斜相Na₂Ti₃O₇材料前20圈迅速衰减,而高温下得到的三斜相Na₂Ti₃O₇材料保持相对稳定,具有更佳的循环性能。原位XRD检测到两种材料在充放电过程中发生结构变化,表现出差异化的储钠行为。其中三斜相Na₂Ti₃O₇电极循环后保持更完整,其层状结构更加稳定。2.针对层状结构材料容易相变和对水分子敏感的特点,我们提出了掺杂的合成策略,在层状Na_xTi O₂中引入原子半径小、质量轻的锂元素,采用固相法制备层状Na_0.73+xLi_0.36-xTi_0.73O₂材料。其中,Li同时占据过渡金属层和钠离子层的位点,在x=0.0365时获得纯相的(Na_0.73Li_0.09)(Li_0.27Ti_0.73)O₂材料,具有O3相的结构特点。对O3-(Na_0.73Li_0.09)(Li_0.27Ti_0.73)O₂进行储钠机理研究,发现其表现出接近零应变的结构特点和超6000圈的长循环稳定性能。通过模拟空气中水分子加速老化实验对O3-(Na_0.73Li_0.09)(Li_0.27Ti_0.73)O₂材料进行泡水实验,泡水后的O3-(Na_0.73Li_0.09)(Li_0.27Ti_0.73)O₂具有优异的空气稳定性,明显优于包括P2-Na_0.66Li_0.22Ti_0.78O₂在内的大部分层状钛基材料,这得益于层状结构中较少的空位和较小的层间距。更多还原

【Abstract】 With the increasing demand for economically sustainable new energy to replace traditional fossil fuels,together with accelerated consumption of lithium source,the development of electrochemical energy storage technology that can replace lithium ion batteries（LIBs）has aroused extensive research.Sodium ion batteries（SIBs）that possess similar storage mechanism to LIBs,are considered as the most promising due to the abundant resource reserves and low cost.The electrode material as the key component greatly determines the battery performance.Therefore,it is of great significance to design and prepare suitable and high-performance electrode materials to promote the development of SIBs.Among the various reported anode materials,layered titanium-based oxides have become one of the most attractive electrode materials owing to their large layer spacing,abundant raw materials,excellent electrochemical properties and environmental friendliness.Thereinto,layered Na₂Ti₃O₇ shows the lowest potential and highest capacity,implying the highest energy density when matched with appropriate cathode materials.However,considering its poor cycling stability and severe structural evolutions,the electrochemical performance of Na₂Ti₃O₇would experience serious degradation.In addition,when exposed to air for a long time,small molecules such as H₂O and CO₂ in air are easily inserted into the interlayer,which might dissolve the alkali metals and generate hydrates.Such phenomena could result further performance degradation.Therefore,this paper focuses on improving the stability of batteries and the kinetics of sodium storage by layered titanium-based materials.Based on the crystal structure,a variety of structural control strategies were proposed to prepare layered titanium-based materials for sodium ion batteries.The main research contents are as follows:1.Layered Na₂Ti₃O₇ materials with different phase structures were prepared by solid-phase method under different calcination temperature.A triclinic Na₂Ti₃O₇ compound was synthesized for the first time,indicating that Na₂Ti₃O₇ compound has monoclinic and triclinic phase structures.These two phase structures have different space groups of P-1 and P2₁/m1,respectively.According to the crystal structures,it can be observed that the distortion of TiO₆octahedron in the two phase is different with varied crystal parameters and growth characteristics.In addition,monoclinic and triclinic Na₂Ti₃O₇ compounds also exhibit different electrochemical behaviors and cycling properties.The monoclinic Na₂Ti₃O₇compound delivers a rapid performance decay in the initial 20 cycles while the triclinic Na₂Ti₃O₇ compound presents relatively stable charge/discharge curve and better cycling performance.In-situ XRD results revealed that both two compounds experienced structural evolution upon sodiation.However,the XRD pattern of the cycled electrodes show that the layered structure of triclinic Na₂Ti₃O₇ electrode remains more integral,indicating a more stable layered structure.2.In view of the fact that layered materials are prone to phase transition and air instability,we proposed a doping strategy for structural construction.Li,with small atomic radius and low density was introduced.Layered Na_0.73+xLi_0.36-xTi_0.73O₂ was prepared by solid-phase method and the pure(Na_0.73Li_0.09)(Li_0.27Ti_0.73)O₂ compound with O3-type layered structure is obtained.Li ion occupies both the transition metal layer and sodium ion layer.This compound delivers a reversible capacity of 108 m Ah·g^-1,and shows a near-zero structural strain upon sodiation,ensuring a prolonged cycling performance over 6000 cycles.In order to simulate the accelerated aging in humid air,the O3-(Na_0.73Li_0.09)(Li_0.27Ti_0.73)O₂ material was soaked in water.The result indicates that aged O3-(Na_0.73Li_0.09)(Li_0.27Ti_0.73)O₂ material has excellent air stability due to fewer vacancies and smaller layer spacing in the layered structure.更多还原