Thiourea-based polyimide/RGO composite cathode: A comprehensive study of storage mechanism with alkali metal ions

Although organic electrode materials have merits of abundant resources, diverse structures and environmental friendliness, their performance for electrochemical energy storage is far insufficient. In this work, a thiourea-based polyimide/reduced graphene oxide (PNTCSA/RGO) composite was synthesized via a condensation polymerization method. As a cathode material in lithium-ion batteries, excellent performance is demonstrated with high reversible capacity (144.2 mA h g), high discharge voltage (∼2.5 V), and long cycling life (over 2000 cycles at 500 mA g), which are comparable to those of other well documented in organic electrodes. Encouraging electrochemical performance is also demonstrated for sodium ion batteries (a cycling life of 800 cycles at 500 mA g), while poor performance is delivered in potassium ion batteries. Theoretical studies reveal that the active sites are carbonyl groups for all alkali ions but one inserted alkali metal ion is shared by two carbonyl groups from the two neighbor units. More importantly, K ions have stronger interaction with S atoms than Li/Na ions, which may lead to poor structure reversibility and account for the poor cycling performance. Our findings provide a fundamental understanding of polyimide based polymer electrodes and help to design and develop high performance organic electrode materials for alkali metal ion batteries.