Boosting Magnesium Storage Performance of π‐Conjugated Polyimide Cathodes Through Synergistic Molecular and Electrolyte Engineering

Magnesium metal batteries (MMBs) offer the promise of low cost, intrinsic safety, and high volumetric energy density, but their development is hindered by the scarcity of cathodes capable of reversible Mg²⁺ storage and by cathode-electrolyte incompatibilities. Here, we demonstrate that coupling molecularly engineered polyimide (PI) cathodes with tailored electrolyte speciation enables fast and durable Mg storage. Two PIs, poly(naphthalene tetracarboxylic dianhydride-urea imide) (NUPI) and poly(perylene tetracarboxylic dianhydride-urea imide) (PUPI), with analogous backbones but distinct degrees of π-conjugation were systematically evaluated in both chloride-containing and chloride-free electrolytes. Systematic studies indicate that chloride-free electrolytes, characterized by weakly coordinating anions, enable reversible enolization (C═O ⇌ C─O^-/[C─O^-]₂Mg²⁺) while suppressing side reactions. Additionally, NUPI, featuring stronger π─π stacking and more ordered layered structures, facilitates Mg²⁺ transport and interfacial charge transfer. When combined with a graphene oxide-modified separator, the NUPI cathode delivers 175 mAh g^-1 at 50 mA g^-1 and exhibits ultralow capacity fading (≈0.05% per cycle over 1000 cycles at 500 mA g^-1). Operando/ex situ spectroscopy analyses and theoretical calculations further confirm the enolization-dominated redox mechanism. This work establishes a molecular-electrolyte co-design paradigm for high-rate, durable MMBs based on carbonyl polymer chemistry_.