s-Tetrazine-bridged 2,2'-Bipyrimidine as Superior Atom-economic Multi-charge Cathode Material for Lithium-ion Batteries

Nitrogen-containing heterocyclic molecules feature metal resource-independence, flexible conjugation structure and fast charge storage kinetics, making them promising to be used as the electrode material in lithium-ion batteries. However, an insufficiently high capacity (< 400 mAh g-1) seriously threatens their practical application. Herein, a novel molecule, viz. 3,6-bis(2-pyrimidinyl)-1,2,4,5-tetrazine (DPmT), is developed by inserting an electron-withdrawing π-bridge unit of s-tetrazine between two pyrimidine rings, in which a dense assembly of multiple active sites is realized. The DTmP exhibits a remarkable atom economy of 40 g (mol e)-1, which refers to the molecule mass per unit of charge transferred. The cathode material of DPmT yields a high capacity of 653 mAh g-1 and an energy density of 1188 Wh kg-1 at 50 mA g-1 at 70 ℃. And 80% capacity retention is achieved after 500 cycles at 500 mA g-1, confirming its superior cyclability. Spectroscopy studies and theoretical calculations are performed to investigate the charge storage process. First, the C=N and N=N are claimed as plausible binding sites for the lithium ions. Second, the introduction of s-tetrazine significantly enhances molecular planarity, thereby promoting charge delocalization and molecular stability. This work provides a novel strategy for designing atom-economic multi-charge electrode materials with high capacity.