Solid-state NMR strategies for the structural investigation of carbon-based anode materials

Multinuclear solid-state NMR spectroscopy is a well-suited technique for structural analysis of amorphous carbon-based anode materials generated from pyrolysis of poly(methacrylonitrile/divinylbenzene) copolymers. Results are presented for the untreated polymeric precursor, the oxidatively stabilized material, and amorphous carbons prepared by high-temperature pyrolysis. In addition, structural effects of silicon dopants and lithium intercalants are studied. The structural changes occurring during the processes of oxidative stabilization and carbonization up to 700 degrees C are effectively monitored by C-13 and N-15 cross-polarization/magic angle spinning methods. The peak assignments are assisted by short contact time and dipolar dephasing experiments. For carbons prepared at higher pyrolysis temperatures this technique is limited by the low structural proton content. For such materials, the H-1 chemical shifts of sorbed water molecules are found to be linearly correlated with the pyrolysis temperature. This effect is attributed to surface ordering phenomena. Si-29 CPMAS spectra of carbons prepared with 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 species and other species that are unable to participate in the intercalation 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.