s-Tetrazine-Bridged 2,2′-Bipyrimidine as Superior Atom-Economic Multi-Charge Cathode Material for Lithium-Ion Batteries

Abstract

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 °C. 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.