⁷Li solid-state nuclear magnetic resonance as a probe of lithium species in microporous carbon anodes

Electrochemical insertion of lithium into pyrolyzed amorphous carbons prepared from polymeric precursors of methacrylonitrile and divinylbenzene has been investigated by ⁷Li variable temperature static and magic angle spinning (MAS) NMR. This method is able to characterize the chemical inventory of lithium, differentiating between reversible species that are truly inserted into the carbon and irreversible species that are lost to traps and parasitic processes. Furthermore, large chemical shift effects indicate that the structural and electronic properties of the inserted reversible lithium species are sensitively dependent on the extent of loading. At loading levels <90% relative capacity, the trend is monotonic, whereas in more highly charged samples, two types of electrochemically relevant sites are observed, distinguishable from one another only at lower temperatures. At higher temperatures, dynamic exchange effects between them are evident on the NMR time scale. In the initially prepared state, these highly charged materials contain regular intercalated lithium sites as well as clusters of atomic lithium species dispersed in the amorphous matrix. The cluster sites are depopulated with time, a process that can be accelerated by heating.