Solid-State Nuclear Magnetic Resonance Investigations of the Lithium- and Sodium-Storage Mechanisms of Pyrolytic Phosphorus-Carbon Composites.

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 P, Li, and Na solid-state nuclear magnetic resonance analysis of the intercalation mechanism of novel, stable, dual-phase phosphorus-doped, and phosphorus-encapsulated turbostratic graphite microspheres is presented. Results indicate that lithium intercalation occurs through the formation of LiP from white phosphorus trapped within the graphitic layers, with the involvement of lithiated phosphorus atoms within the graphitic lattice. A dual-lithiated carbon 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 is observed. Upon removal of ions, carbon-encapsulated phosphorus with a local structure similar to red phosphorus forms, which subsequently allows effective reversible ion storage.