Unlocking the Ligand-Dominated Redox Activity in π-d Conjugated Coordination Polymers for High-Capacity and Stable Potassium Storage.

Abstract

Potassium-ion batteries (KIBs) offer a cost-effective, resource-abundant alternative to lithium-ion systems, yet the development of high-performance anodes with adequate capacity, stability, and rate capability remains a major challenge. Here, an electronic structure engineering strategy is introduced via d-orbital configuration optimization in a novel class of π-d conjugated coordination polymers (TM-BTA, TM = Ni, Co, Mn). Orbital-level and charge density analyses reveal that the metal center's electronic configuration governs metal-ligand interaction strength, thereby modulating charge delocalization and ligand redox behavior. Among the series, Ni²⁺ exhibits the strongest π-d conjugation with nitrogen donor atoms, stabilizing C═N bonds and enabling highly reversible C═N/C─N transformations as the dominant redox process. This optimized coordination lowers the K⁺ adsorption energy barrier by 44% compared to Co²⁺ and Mn²⁺, markedly improving kinetics. As a result, Ni-BTA delivers a high reversible capacity of 452 mAh g^-1 with 99.2% retention over 500 cycles at 100 mA g^-1, and maintains 292 mAh g^-1 after 4,000 cycles at 1,000 mA g^-1. In situ spectroscopy and DFT calculations reveal a ligand-centered three-electron redox mechanism, where nitrogen heterocycles dominate K⁺ storage and electrochemically inert Ni centers maintain structural integrity. This work establishes a general design principle for KIB anodes via d-orbital engineering in coordination polymers.