Microheterogeneous Electrolytes: From Chemical Composition to Spatial Architecture─A Paradigm Shift in Electrolyte and Interphase Design

Conspectus Electrolytes underpin all electrochemical energy-storage devices by mediating ion transport, interfacial reactions, and electrochemical stability. Conventional electrolyte design has long relied on a homogeneous-solution paradigm, wherein ions are uniformly solvated and electrochemical behavior is primarily dictated by composition and bulk thermodynamic properties. Within this framework, tailoring local solvation environments has enabled important incremental improvements. However, ion transport, interfacial chemistry, and electrochemical stability remain intrinsically coupled, rendering the simultaneous optimization of multiple performance metrics fundamentally challenging. Recent experimental and computational advances have increasingly challenged the assumption of homogeneity in liquid electrolytes. Electrolytes are now recognized as structurally complex soft-matter systems that can spontaneously develop spatial heterogeneity across molecular, nanometric, and mesoscale lengths. Even in macroscopically uniform electrolytes, ions and solvent molecules may self-organize into clusters, micelles or reverse micelles, bicontinuous networks, and microemulsion-like domains. These microscopic heterostructures are not incidental: they play decisive roles in governing ion-transport pathways, interfacial reactivity, and electrochemical stability, yet have largely remained outside traditional electrolyte design frameworks. Drawing on our work over the past five years, together with related advances from the broader community, this Account introduces the concept of microheterogeneous electrolytes (MHEs), which converts electrolyte design from compositional optimization to microstructural regulation. In MHEs, spatially differentiated domains act as functional units that decouple the otherwise conflicting electrochemical requirements. Ion transport can be accelerated along percolating low-energy barrier pathways; solvent reactivity can be suppressed through confinement and topological control, and interfacial reactions are regulated via selective enrichment of active species. We first trace the historical development and thermodynamic origins of microheterogeneity (MH) in liquids, elucidating how competition between energetic and entropic contributions stabilizes nanoscale domains without macroscopic phase separation. Building on this foundation, we establish a structure–function paradigm linking solvation topology, mesoscale connectivity, and electrochemical behavior. This framework clarifies how MHEs enable fast ion transport, broaden electrochemical stability windows, and promote adaptive interphase formation under extreme conditions of temperature, voltage, and current density. To enable the rational design of MHEs, we articulate interaction-level design principles through anion and solvent. Together, these principles transform molecular interactions into programmable electrolyte architectures. We further summarize experimental and computational approaches that render these invisible structures observable and quantifiable. By redefining electrolytes as spatially organized and dynamically adaptive media, this Account establishes MHEs as a general design principle applicable to Li +, Na +, multivalent, and aqueous batteries. More broadly, it builds a conceptual bridge between soft-matter physics and electrochemical engineering, opening new opportunities for designing electrolytes capable of meeting the stringent demands of next-generation energy-storage technologies.