From salt-in-water to water-in-salt: the emergence of glass-like dynamics and the transference number controversy

The transition from dilute salt-in-water (SiW) electrolytes to highly concentrated lithium bis(trifluoro methylsulfonyl)imide (LiTFSI) water-in-salt (WiS) systems represents a fundamental shift in the physics of ion transport. We propose that this transition is best understood using concepts from glass physics. In the SiW regime (dilute solutions), ions diffuse largely independently within a water-rich environment, and classical electrolyte theories remain valid. Beyond concentrations ranging from 10 to 15 m (mol kg-1 of water), the liquid undergoes a qualitative reorganization in which the bulky TFSI⁻ anions form a percolating, dynamically crowded matrix that exhibits hallmark signatures of glass-like dynamics, including dynamic heterogeneity and cooperative relaxation, without requiring thermodynamic vitrification. Using molecular-level analysis, we demonstrate that LiTFSI-based water-in-salt electrolytes develop a species-dependent cooperativity hierarchy in which TFSI- becomes increasingly glass-like, while Li⁺ retains comparatively liquid-like mobility. This differential cooperativity offers a mechanistic explanation for the long-standing transference number controversy, reconciling the divergence between nuclear magnetic resonance tracer-frame values and electrochemical solvent-fixed measurements. More broadly, we propose that the SiW-to-WiS crossover can be mapped onto glass-physics order parameters, offering a new research direction for electrolyte design centred on tuning anion-matrix fragility rather than invoking mesoscale phase separation. This article is part of the theme issue 'Electrolytes within the domain of electrochemistry and electrochemical energy storage'.