Polyanionic-based cathode materials for K-ion batteries

Abstract

The urgent need for sustainable energy storage solutions beyond lithium-ion batteries (LIBs) has propelled K-ion batteries (KIBs) into the spotlight, leveraging potassium's crustal abundance, cost-effectiveness, and favorable ionic mobility. This review critically examines the transformative potential of polyanionic cathode materials in addressing the unique challenges posed by K+ ions notably their large ionic radius (1.38 Å) and structural compatibility while capitalizing on their high-voltage operation and robust cycling stability. We elucidate the structural and electrochemical merits of polyanionic frameworks (e.g., phosphate, fluorophosphate, sulfates, and pyrophosphate), emphasizing their capacity to stabilize high-voltage operation (>4.0 V vs. K/K+) through inductive effects enabled by strong covalent X–O bonds (X = P, S, F). Key material families, including NASICON-type K3V2(PO4)3, fluorosulfate (KFeSO4F), and mixed polyanion systems (K4Fe3(PO4)2P2O7), are systematically analyzed to unravel structure-property-performance relationships. Advanced synthesis strategies such as sol-gel processing, hydrothermal templating, and electrochemical ion exchange are highlighted for their role in optimizing ionic/electronic conductivity and mitigating interfacial instability. Despite progress, challenges persist in balancing energy density (>400 Wh kg-1 calculated based on the cathode mass) with cyclability (>1000 cycles), necessitating synergistic strategies like nanoscale engineering, anion/cation co-doping, and conductive matrix integration. The review underscores the untapped potential of titanium- and manganese-based polyanionics, metastable fluorophosphate derivatives, and hierarchical architectures to overcome kinetic limitations. By bridging fundamental insights with scalable manufacturing considerations, this work provides a roadmap for advancing KIBs toward grid-scale storage and electrified transportation, circumventing lithium's geopolitical constraints while unlocking new frontiers in high-energy, sustainable electrochemistry.