Pyridinic-to-graphitic conformational change of nitrogen in graphitic carbon nitride by lithium coordination during lithium plating

Abstract

The reversibility of lithium plating/stripping should be guaranteed in lithium metal batteries. Seriously localized lithium growth during plating leads to the dendritic evolution of lithium metal due to the uneven current distribution on the electrically conductive surface. Artificial protective layers covering electrodes (e.g., polymer film on copper foil) have been used to narrow the gap of the current density between positions on the conductive surface. Herein, we incorporated an active ingredient to attract lithium ions into the dendrite-suppressing layer. Pyridinic nitrogen of graphitic carbon nitride (g-C3N4) served as the lithium ion affinity center. Conformation of the nitrogen changed from pyridinic to graphitic in the presence of lithium ions, which confirms the coordination of lithium ion to the pyridinic nitrogen. Moreover, lithium ion conduction was facilitated in the presence of g-C3N4 layer probably via a site-to-site hopping mechanism. Lithium metal was plated between the g-C3N4 layer and the copper current collector (or the lithium metal). The homogeneous lithium nucleation expected from the active role of the pyridinic nitrogen (lithium ion affinity and facilitated ionic conduction) suppressed the dendritic growth of lithium metal and decreased the overpotential required for the initial metal nucleation. Due to the top down ion flux regulation on the uppermost surface (or tip) of lithium metal, the reversibility of lithium plating/ stripping was dramatically improved.