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

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 (ASBs), 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 1500 Wh kg$^{−1}$ (and up to 588 Wh kg$^{−1}$ at the stack level under techno-economic modeling), chloride-ion electrode screening identifies over 1225 anode–cathode combinations with thermodynamic volumetric densities exceeding 2000 Wh L−1, and dual-ion architectures sustain operating voltages of 4.5–5.5 V with near-unity Coulombic efficiency over thousands of cycles. ... mehrYet 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 differenti-
ate between limitations intrinsic to anionic charge-carrier physics and those addressable through rational materials design. ASBs 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.