Electrochemical Changes in Lithium-Battery Electrodes Studied Using 7Li NMR and Enhanced 13C NMR of Graphene and Graphitic Carbons

An anode composed of tin-core, graphitic-carbon-shell nanoparticles distributed on graphene nanosheets, Sn@C-GNs, is studied during the lithiation process. 7Li NMR provides an accurate measure of the stepwise reduction of metallic Sn to lithium–tin alloys and reduction of the graphitic carbon. The metallic nanoparticle cores are observed to form ordered, crystalline phases at each step of the lithiation process. The 7Li 2D experiments presented provide insight into the proximity of the various phases, reflecting the mechanism of the electrochemical reaction. In particular, a sequential model of nanoparticle lithiation, rather than a simultaneous process, is suggested. Movement of lithium ions between two elements of the nanostructured Sn@C-GNs material, the metallic core and carbon shell, is also observed. Conventional 13C solid-state NMR, SSNMR, experiments on <5 mg of active material from electrochemical cells were found to be impossible, but signal enhancements (up to 18-fold) via the use of extended echo trains in conjunction with magic-angle spinning enabled NMR characterization of the carbon. We demonstrate that the 13C data is extremely sensitive to the added electron density when the graphitic carbon is reduced. We also investigate ex situ carbon electrodes from cycled Li–O2 cells, where we find no evidence of charge sharing between the electrochemically active species and the graphitic carbon in the 13C NMR spectroscopy.