Anatomy of a SPAN Particle: A Four‐Level Structural Framework for Lithium–Sulfur Batteries

ABSTRACT Sulfurized polyacrylonitrile (SPAN) has re‐emerged as one of the most promising organosulfur cathodes for lithium–sulfur batteries, delivering substantial reversible capacity while largely suppressing the classical dissolved‐polysulfide shuttle. Yet after more than two decades of study, the field still needs consensus on a deceptively basic question: what, structurally, is SPAN? Existing descriptions routinely conflate molecular bonding, mesoscale organization, and particle architecture; we argue that the central obstacle is not a shortage of high‐quality studies but the absence of a shared structural language connecting observations across length scales. To close this gap, we propose a four‐level hierarchical framework for dissecting a single SPAN particle: Primary Structure (Level I), the conjugated carbon–nitrogen backbone setting the electronic and insolubility foundation; Secondary Structure (Level II), sulfur‐bearing covalent groups governing reversible and irreversible capacity; Tertiary Structure (Level III), noncovalent interchain organization defining a mesoscale reaction field; and Quaternary Structure (Level IV), particle‐level hierarchy and intraparticle porosity. Rather than advocating a single “correct” structural formula, we frame SPAN as a coupled, thermal‐history‐dependent ensemble whose electrochemistry emerges from the interplay among levels. We further argue that the underdeveloped mesoscale (the interchain organization and soft confinement of Level III, and the intraparticle porosity of Level IV) may supply the missing structural basis for short‐chain sulfur retention, partial sulfur dissolution–recovery, and capacities beyond classical stoichiometric expectations, all without conventional long‐chain polysulfide signatures. By organizing the field around level‐specific descriptors and explicit cross‐level coupling, this perspective aims to provide an organizing, forward‐looking basis for reproducible, high‐loading, and scalable SPAN design.