Charge compensation and capacity fading in LiCoO_2 at high voltage investigated by soft x-ray absorption spectroscopy

In order to obtain an in-depth insight into the mechanism of charge compensation and capacity fading in LiCoO_2, the evolution of electronic structure of LiCoO_2 at different cutoff voltages and after different cycles are studied by soft x-ray absorption spectroscopy in total electron(TEY) and fluorescence(TFY) detection modes, which provide surface and bulk information, respectively. The spectra of Co L2,3-edge indicate that Co contributes to charge compensation below 4.4 V.Combining with the spectra of O K-edge, it manifests that only O contributes to electron compensation above 4.4 V with the formation of local O 2 p holes both on the surface and in the bulk, where the surficial O evolves more remarkably. The evolution of the O 2 p holes gives an explanation to the origin of O_2~-or even O_2. A comparison between the TEY and TFY of O K-edge spectra of LiCoO_2 cycled in a range from 3 V to 4.6 V indicates both the structural change in the bulk and aggregation of lithium salts on the electrode surface are responsible for the capacity fading. However, the latter is found to play a more important role after many cycles.     

F8BT薄膜表面形貌及与Al形成界面的电子结构和反应

基于共轭聚合物光电器件的性能与聚合物的表面形貌、分子取向、以及与金属电极形成的界面结构密切相关. 本文利用原子力显微镜(AFM)、同步辐射光电子能谱(SRPES)和近边X射线吸收精细结构谱(NEXAFS)等, 研究了聚(9, 9-二辛基芴并苯噻二唑)(F8BT)薄膜的表面形貌、分子取向及其与Al 电极形成界面过程的结构变化. 结果表明, 在略低于F8BT玻璃转变温度(T_g=130 ℃)条件下对F8BT薄膜进行退火, 可明显增加薄膜的表面粗糙度, 薄膜中F8BT 的分子取向角约为49°, 9, 9-二辛基芴单元(F8)与苯噻唑单元(BT)几乎在同一平面. 在Al/F8BT 界面形成过程中, Al与F8BT中的C, N和S均发生不同程度的化学反应, 并导致价带结构和未占据分子轨道(LUMO)态密度的变化. Al对F8BT进行n型掺杂引起F8BT能带弯曲的同时, 未占据能级被部分占据, 更多的电子将被注入到LUMO+1中. 通过考察价带电子结构、芯能级位移及二次截止边的变化, 绘制了清晰的Al/F8BT界面能级图.