SOLID-STATE NMR STRATEGIES FOR THE STRUCTURAL INVESTIGATION OF CARBON-BASED ANODE MATERIALS

Multinuclear solid-state NMR spectroscopy is a well-suited technique f or structural analysis of amorphous carbon-based anode materials gener ated from pyrolysis of poly(methacrylonitrile/divinylbenzene) copolyme rs. Results are presented for the untreated polymeric precursor, the o xidatively stabilized material, and amorphous carbons prepared by high -temperature pyrolysis. In addition, structural effects of silicon dop ants and lithium intercalants are studied. The structural changes occu rring during the processes of oxidative stabilization and carbonizatio n up to 700 degrees C are effectively monitored by C-13 and N-15 cross -polarization/magic angle spinning methods. The peak assignments are a ssisted by short contact time and dipolar dephasing experiments. For c arbons prepared at higher pyrolysis temperatures this technique is lim ited by the low structural proton content. For such materials, the H-1 chemical shifts of sorbed water molecules are found to be linearly co rrelated with the pyrolysis temperature. This effect is attributed to surface ordering phenomena. Si-29 CPMAS spectra of carbons prepared wi th tetravinylsilane comonomers indicate that the silicon component is oxidized during the stabilization process. Li-7 MAS NMR is well suited to differentiate between electrochemically relevant intercalated spec ies and other species that are unable to participate in the intercalat ion due to parasitic processes. For the intercalated species, a linear correlation of the Li-7 chemical shift with the charging state of the carbon is observed.