In Situ XRD and XAS Investigation for LiMn2O4, LiCoO2 and LiNiCoMnO2 Cathode Materialsin Li-ion Batteries

Jenq-Gong Duh (杜正恭), Hong-Wei Chan (詹宏偉), Ting Fang (方婷), and Pei-Yun Liao (廖珮芸)

Department of Materials Science and Engineering,NationalTsingHuaUniversity, Hsinchu, Taiwan

There has been a growing interest in lithium ion battery for portable electronic device because of itslow cost, long life, and high energy density.Each cell has a negative and a positive electrode, as the sources of chemical reactions, separated by the non-aqueous electrolyte. Among them,the cathode plays a critical role to determinate the performance such as stability, lifetime and reversibility. As lithium ions intercalate and deintercalate between electrodes, the cathode materials might undergo phase transitions during charge/discharge process which give rise to capacity fading [1-2]. In addition, different ion additives would influence the appearance of new phase. Therefore, it is crucial to have an in-depth understanding of structural changes. X-ray diffraction (XRD) is one of the most powerful tools to study the structural properties. Recently, cathodes materials with spinel and layered structure were successful synthesized and exhibited favorable electrochemical performance [3-6]. In this study, in-situ synchrotron XRD studies were conducted to investigate the correlations between the electrochemical characteristics and phase conversions. Three material systemswere employed: (a) the Cu-doped spinel lithium manganese oxide (i.e. LiCuxMn2-xO4-coated LiMn2O4 composite cathode), (b) ZnO-coated LiCoO2, and (c) LiNi1-x-yCoyMnxO2 with various ion concentrations.

Besides, X-ray absorption spectroscopy (XAS) can be used to examine the electronic and local environment of the cathode materials. Unlike the in-situ XRD which gives a good picture of the long range structural changes, the XAS can provide a better understanding of the ionic environment as a short range ordering. The XANES of Cu and Mn K-edge spectrum for LiCuxMn2-xO4-coated LiMn2O4 showed that the valence of Cu and Mn was close to Cu2+ and Mn4+. Furthermore, the oxidation state of Mn was reversibly increased and decreased during charge. Combined with these two techniques, an overall picture of reaction mechanisms and effects of dopants in cathodes for Li-ion battery could be depicted.

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