基于阴离子氧化还原化学的层状嵌入化合物

Secondary batteries technology has dominated the portable electronic market for more than two decades and should hold much promise as green-power sources for electric vehicles. The electrode’s material is one of the key components for perfecting secondary batteries. It plays a key role in establishing the overall properties of the secondary battery and is the main obstacle for the next generation of these batteries. Nearly all intercalation cathodes with high- energy density for these secondary batteries involve a cationic redox reaction, in which transition-metal provides the suitable redox chemistry. At the present time, it seems that scientists met with the bottleneck to design intercalation cathodes and the strategies for designing a new insertion electrode is mainly limited to the optimized transition metal redox properties and the utilization of the redox of oxygen. Here, a mind-blowing is made to design an anionic redox chemistry such as S2-/S22- in layered compounds without the redox of transition metal. The relationship between the redox chemistry of S2-/S22- and its electronic structure is firstly investigated. The motivation of this work is to enrich the reversible redox chemistry of layered intercalation compounds for understanding the nature of their electronic structure change during the deintercalation/intercalation of guest ions. Based on research, we will discuss the challenges in the design of single anionic redox chemistry of S2-/S22-, and the understanding of the unique typical physical chemistry by using ab initio computations and in situ X-ray diffraction, ex situ X-ray absorption spectroscopy, spherical-aberration-corrected scanning transmission electron microscopy, electron paramagnetic resonance and X-ray photoelectron spectroscopy techniques, and we will try to improve the performance of high-capacity electrode by tuning the hybridization of transition-metal d and S 3p bands or by designing double anion systems with O and S elements. The charge compansation for the sodium removal is predominantly carried out by the oxidation of sulfur, what should the transition metal in intercalation compound act as for the stability of the layered structure? Our experimental and theoretical studies will provide very valuable information for the research and development of intercalation compounds as electrode materials for secondary batteries.

二次电池技术已经占据可携带电子设备市场超过20多年，并且有望作为绿色能源为电动车提供电源。电极材料是二次电池关键组成部分，它将决定了整个二次电池性能，电池材料的创新成为新一代电池发展的主要引擎。目前，几乎应用在二次电池中所有具有高能量密度的嵌入化合物都涉及到阳离子的氧化还原化学。科学家在设计嵌入化合物时似乎面临一个瓶颈，仅限于优化基于过渡金属的氧化还原性质或涉及O2-。本课题申请拟开拓新的思维方式，试图设计基于阴离子的电荷补偿的嵌入化合物，研究如S2-/S22-氧化还原反应与体系电子结构变化的关系。此研究的意义在于丰富了层状嵌入脱出材料的可逆氧化还原反应，以了解层状化合物在充放电过程中的电子结构的变化规律。如果电荷补偿来自于阴离子，阳离子在不参加电荷补偿下取什么物理化学作用？另试图通过调整过渡金属d和S 3p轨道的杂化或设计具有O和S的双阴离子体系来寻找高能量密度的新型电极材料。

不断增长的储能需求要求改善正极材料的性能。在传统的层状正极中，金属3d在o2p轨道上的高能量导致单带阳离子氧化还原，单靠阳离子的容量不能满足更高能量密度的要求。新兴的阴离子氧化还原化学有望获得更高的容量。在最近的研究中，低能O非键2p轨道被设计用于激活单带氧氧化还原，但仍然伴随着可逆性问题，如氧损失、阳离子迁移和电压衰减。本课题拟探索基于阴离子可逆变价的层状嵌入化合物新体系，研究体系中基于阴离子电荷补偿如S2-/S22-氧化还原反应与体系电子结构变化的关系。此研究的意义在于丰富了层状嵌入脱出材料的可逆氧化还原反应，以了解层状化合物在充放电过程中的电子结构的变化规律。我们可以根据这种阴离子电荷补偿机理，再通过调整过渡金属TMn+1/n费米能级或引入氧硫双阴离子体系，从而设计出具有高能量密度的新型层状电极材料。在此，我们研究了O3-NaCr2/3Ti1/3S2阴极，其基于阳离子和阴离子氧化还原过程提供了约186 mAh/g（0.95 Na）的高可逆容量。在本研究中，研究了层状NaCr2/3Ti1/3S2中硫阴离子氧化还原过程的各种电荷补偿机制，包括形成类二硫化物物种、元素硫沉淀以及S-S二聚，特别是通过形成电子空穴。获得了电子空穴形成的直接结构证据和S-S距离缩短的（S2）n物种的存在。我们的结果表明，S–S二聚体的高度可逆形成可以通过Cr离子向Na位的迁移来驱动。当S-3p带底部与费米能级合并时，电子空穴和S-S二聚体可以通过Cr3+/Ti4+-（S2）n-共价相互作用稳定。这些结果为开发基于阴离子氧化还原反应的新材料提供了有价值的信息。此外，通过调节金属配体能级以形成高度重叠的d-p带，在NaCr1-xVxS2系统中实现了阴离子氧化还原和高度可逆的阴离子氧化还原过程提供的额外容量。观察到Cr、V和S的同时阳离子和阴离子氧化还原过程。Cr/V和S之间的强共价相互作用稳定了阴离子上的电子空穴，防止了二聚产生，抑制了电压滞后和电压衰减。在高共价条件下，通过简单调节Cr/V的结合比，可以将阴离子氧化还原电位从2.43v调节到2.7v。我们的工作提供了对层状化合物中高可逆阴离子氧化还原的基本理解，并提出了基于d-p共价杂交带的阴离子氧化还原化学的可行性。