Solid-State NMR Investigations of the Lithium- and Sodium-Storage Mechanisms of Pyrolytic Phosphorus-Carbon Composites

Abstract

Phosphorus-doped carbons provide a balance between the electrochemical stability of graphitic lattices and the high energy density of phosphorus materials when used in lithium and sodium-ion batteries. Herein, a comprehensive ex-situ 31P, 7Li and 23Na solid-state NMR analysis of the intercalation mechanism of novel stable, dualphase phosphorus-doped and phosphorus-encapsulated turbostratic graphite microspheres is presented. Results indicate lithium intercalation occurs through the formation of Li3P from white phosphorus trapped within the graphitic layers, with the involvement of lithiated phosphorus atoms within the graphitic lattice. A duallithiated doped-phosphorus environment is tentatively proposed at low voltages. Sodiation occurs through a similar mechanism, however, no evidence of a dual sodiated doped-phosphorus environment was observed. Upon removal of ions, carbon-encapsulated phosphorus with a local structure similar to red phosphorus forms, which subsequently allows effective reversible ion storage.