Molecular Design Strategy for High-Redox-Potential and Poorly Soluble n-Type Phenazine Derivatives as Cathode Materials for Lithium Batteries.

The n-type phenazine (PZ) derivatives represent an emerging class of cathode materials in lithium batteries for low-cost and sustainable energy storage. However, their low redox potential (<2 V) and high solubility hinder their application to battery systems. To explore and solve such problems in lithium batteries, we investigate the redox characteristics of 13 n-type PZ derivatives and their dissolution behavior in seven organic electrolytes systematically by using DFT calculations. Two decisive factors are observed to tune the redox potentials for these molecules: the first is the electron density around the N active sites and the second is the chelation on lithium by both the active N and the substituent group. Specific approaches that include the reduction of aromatic rings and the introduction of functional groups at β sites in n-type PZ derivatives can improve the redox potential to approximately 3 V. In addition, we develop a new index denoted as Ediff to investigate the solubility of n-type PZ derivatives. The most effective way to reduce the dissolution of electrodes in solvents is to improve intermolecular attraction between the electrode molecules by introducing π-π stacking and hydrogen bonds. Such all-around guidelines should promote the application of n-type PZ-based organic cathodes with a high redox potential and low electrode solubility for lithium batteries.