镁二次电池正极材料谢芙尔相电子结构与电化学性能的构效关系研究

Mg rechargeable batteries are being considered as one of the most prospective alternatives for Li rechargeable batteries because of their merits including Mg abundance, absence of Mg dendrite and high theoretical specific capacity attributed to two-electron charge transfer reaction process. However，the achievable performance of Mg rechargeable batteries is still limited regarding their practical application. The key limitation to this lies in cathode. To date, Chevrel phase Mo6(SxSe1-x)8 is the only cathode material in which Mg2+ can be efficiently intercalated/deintercalated. Extensive theoretical and experimental studies have been done to screen out promising cathode materials by constructing the crystal structure similar to Chevrel phase. However, successful research has been rarely reported. It indicates that only taking into account of the crystal structure is not sufficient enough to explore the Mg /transport behavior in the electrode. Given the fact that the two-electron charge transfer process was involved in Mg batteries, the evolution of electronic structure in Chevrel phase might provide more important information to elucidate the physical nature of Mg storage/transport in the electrode. However, the experimental study on the evolution of electronic structure in Chevrel phase cathode is rare. In our project, Chevrel phase Mo6(SxSe1-x)8 will be focused on as the model material. Taking advantages of the unique and powerful technique of the tender X-ray absorption spectrum, Mo L2, 3 and S K edge will be in-situ studied with the charge/discharge of the Mg batteries. Detailed information includes the unoccupied state of Mo 4d orbital, ligand geometric type of S can be obtained through the in situ study. The obtained results would be used to figure out why its kinetics properties are excellent and reaction potential is low, and get the deep insight into the Mg electrochemistry. It is also expected to attract more attention to the spectroscopy study on electronic structure in the energy material field.

二次Mg电池凭借Mg储量丰富、无枝晶及2e-转移产生的高理论比容量等优势成为颇具前景的有效补充二次Li电池的储能装置之一。二次Mg电池的发展主要受到正极的限制。谢芙尔相是目前唯一可以高效脱嵌镁的正极材料。大量的理论和实验研究通过构建类谢芙尔相的晶体结构来筛选正极材料，但是鲜有成功的报道。这表明通过晶体结构解析储Mg行为可能不再足够有效。基于2e-的电荷转移特性，谢芙尔相的电子结构可能更有效地解析储Mg的关键影响因素。但实验上对于二次Mg电池电极材料的电子结构研究尚未见正式报道。本项目将以谢芙尔相Mo6(SxSe1-x)8为主要研究体系，依托稀有的tender X-ray 吸收谱研究Mo-L和S-K边，通过原位研究Mg2+脱嵌过程中Mo的4d轨道未占据态、S的配位体等变化信息，探究谢芙尔相正极动力学性能优良、反应电位低的机理，促进Mg电池基础研究的认识和商业化应用的发展。

二次Mg电池凭借Mg储量丰富、无枝晶及2e-转移产生的高理论比容量等优势成为颇具前景的有效补充二次Li电池的储能装置之一。二次Mg电池的发展主要受到正极的限制。由于谢芙尔相仍然是目前唯一可以高效脱嵌镁的正极材料，因而备受关注。已经有不少研究从电化学或结构角度对谢芙尔相的存储机理进行了研究，但仍然对谢芙尔相的反应电位、倍率特性及循环可逆性的起源缺乏深入的理解，不利于电极材料的理性设计和新材料的探索。本项目以典型的谢芙尔相Mo6S8为模型材料，利用柔性X射线吸收谱对谢芙尔相对小离子电化学存储的机制进行了非原位或原位研究。研究中首先化学合成了高质量的Mo6S8样品，配制了相应电解液，完成电池组装，进行了基本的电化学测试与特性比较；其次设计并加工了原位电化学反应池及相应测试支架，优化了电化学窗口，完成了原位电化学反应池的组装及测试；之后非原位研究了不同种类或不同数量的小离子电化学嵌入Mo6S8过程中Mo L2边和S K边吸收谱的演变，了解了S上电荷分布演变及团簇间键长演变的情况，对材料的稳定性、电化学可逆性等问题形成了一定认识；最后原位研究了Mo6S8在储Lic过程中局域结构的演变，利用等吸光点对反应区间的划分和特征峰的不对称演变发现了嵌锂和脱锂过程的反应路径不对称，基于特征峰的具体演变对反应过程的热力学和动力学进行了深入讨论。上述的研究从电子结构和局域结构角度一定程度上理解了谢芙尔相优异电化学性能的起源，促进了Mg正极的理性设计和新型正极材料的探索；原位柔性X射线吸收谱研究的开展填补了国内的空白，建立的测试方案被推广到更多的电池体系研究中，有助于加深电池工作机理的认识及推动同步辐射X射线吸收谱在电池机理研究中的应用。