Beyond cations: the rise of anion-shuttle chemistries in the emerging battery landscape

Abstract

Abstract

Abstract The search for electrochemical energy storage beyond lithium-ion batteries has yielded a broad landscape of alternative chemistries, yet most still adhere to the fundamental paradigm of cationic charge carriers. Anion-shuttle batteries, spanning dual-ion, chloride-ion, fluoride-ion, hydride-ion, bromide-ion, and polyiodide systems, represent a mechanistically distinct departure in which anions serve as the primary electroactive charge carriers, functioning as the exclusive shuttling species in fluoride-ion, chloride-ion, hydride-ion, and bromide-ion systems, while in dual-ion batteries anion intercalation at the cathode is coupled to simultaneous cation insertion or deposition at the anode. This Perspective provides a critical and comprehensive account of this emerging field, covering charge-transfer mechanisms, the development of electrode and electrolyte materials, and the failure modes that currently separate laboratory demonstrations from practical relevance. The thermodynamic case is compelling: fluoride-ion conversion chemistries project material-level theoretical energy densities exceeding 1,500 Wh kg-1 (and up to 588 Wh kg-1 at the stack level under techno-economic modeling), chloride-ion electrode screening identifies over 1,225 anode–cathode combinations with thermodynamic volumetric densities exceeding 2,000 Wh L-1; and dual-ion architectures sustain operating voltages of 4.5 to 5.5 V with near-unity coulombic efficiency over thousands of cycles. Yet three material challenges persist across all chemistries, regardless of anion identity: the kinetic penalties of anion solvation and interfacial desolvation, the electrochemical instability of the electrode–electrolyte interphase, and mechanical degradation arising from large volumetric strains during conversion cycling. We differentiate between limitations intrinsic to anionic charge-carrier physics and those addressable through rational materials design. Anion-shuttle batteries will not broadly displace lithium-ion technology, but their natural domain, grid-scale storage, high-temperature operation, and applications free from critical mineral dependencies, represent a strategically significant and scientifically compelling frontier.