A polymer-coated Co3O4 composite was electrochemically induced to be a high-performance lithium-storage material. The structure of the composite before and after electrochemical charge/discharge cycling was determined by extended X-ray absorption fine structure spectroscopy (EXAFS) and density functional theory (DFT) calculations. It is found that polymer coating does not disturb the structure of commercial Co3O4; no coordinates are recognized between Co and the polymer in the as-prepared composite. In contrast, strong coordination occurs between Co and N atoms in the discharged composite. Data fitting to the experimental EXAFS signal yields an average atomic distance of RCo-N = 1.91 Å and a Co-N coordination number of 2 for the first shell. The near edge absorption (XANES) signal also indicates the change of oxidation state of cobalt after discharge. A two-layer geometry of the Co-polymer coordinate is optimized from DFT calculations. The simulated EXAFS signal from this structure can fit the experimental data very well. It is suggested that the strong Co-N coordination weakens the C-H bonds of the polymer during discharge and makes them electrochemically active to store lithium ions, transferring the polymer to a high performance lithium-storage material.
The authors would like to thanks Dr. Philippe Martinez for his help with Gaussian calculations, Valerie BRIOIS and Amelie ROCHET for measuring the EXAFS spectra of metallic Co and pure Co3O4. This work was financially supported by the National 973 Program (No.2002CB211800 and 2009CB220100) and the National Science Foundation of China (NSFC No.50472072 and 20974120).
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