Ionic Conductivity Mechanisms in Concentrated Electrolyte Systems

Ionic conductivity in electrolyte systems is governed by the motion of charged species under an applied potential and is strongly influenced by ion–ion and ion–solvent interactions. In dilute solutions, ions migrate largely independently within a continuous solvent medium, displaying vehicular transport that scales with simple diffusion laws. As concentration increases into the high and superconcentrated regimes, solvent molecules become coordinatively saturated and free solvent activity diminishes. Ions form contact pairs, triplets and extended clusters, leading to pronounced ion correlations that can either enhance or impede net charge transport. Concerted hopping mechanisms emerge, in which cations jump between coordination sites, assisted by fast ligand exchange. At the same time, increased viscosity and mesoscopic structuring impose kinetic constraints that compete with enhanced electrochemical stability. Understanding the balance between correlated ion motion, cluster dynamics and solvent coordination is critical for the design of advanced lithium‐ion, lithium‐metal and sodium‐ion batteries, as well as supercapacitors and fuel cells. Optimising these parameters can improve transference numbers, suppress dendrite growth and widen electrochemical windows, thereby enabling higher energy densities and safer operation under lean‐electrolyte conditions.

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Recent work on high concentration electrolytes for lithium‐ion batteries has revealed that concerted cation hopping dominates charge transport. Measurements combining electrophoretic NMR and electrochemical analyses demonstrate an increase in lithium transference number in highly concentrated formulations compared to conventional 1 M solutions. Ligand exchange between cations and anions underpins this mechanism, whereas diluent addition disrupts ion‐ion coordination, reducing transference.

Studies of highly concentrated lithium bis(trifluoromethylsulfonyl)imide in acetonitrile have uncovered mesoscopic ordering as salt content increases. Small-angle X-ray scattering and quasi-elastic neutron spectroscopy show that anions progressively enter the solvation shell and that ion transport follows a jump diffusion process. Jump lengths remain largely constant, but residence times grow with concentration, reflecting stronger coordination and slower exchange dynamics.

A broader review of dynamic ion correlations in solid and liquid electrolytes highlights how cation–cation, anion–anion and cation–anion correlations affect key transport coefficients. Onsager transport theory links these correlations to macroscopic observables such as the Haven ratio and salt diffusion coefficient. In solid single‐ion conductors, lattice interactions compete with cation self‐correlations, while in liquid media ion pairing and transient clusters modulate overall conductivity and transference numbers, sometimes even yielding negative cation mobility under polymer‐rich conditions.

Ionic Conductivity Mechanisms in Concentrated Electrolyte Systems publication trend

The graph below shows the total number of articles in ionic conductivity mechanisms in concentrated electrolyte systems across all publications each year (not limited to Nature Index journals).

Technical terms

Ionic conductivity: A measure of an electrolyte’s ability to conduct charge via ion movement under an electric field.

Transference number: The fraction of total current carried by a particular ionic species in an electrolyte.

Ion correlation: The non‐independent, cooperative motions of ions arising from electrostatic and coordination interactions.

Contact ion pair: A cation and anion held in direct electrostatic association without intervening solvent molecules.

High concentration electrolyte: An electrolyte in which salt concentrations exceed conventional values (often >3 mol L⁻¹), leading to solvent‐depleted coordination environments.

Jump diffusion: A transport mechanism in which ions migrate by discrete jumps between coordination sites rather than continuous Brownian motion.

References

1. Ion Transport in (Localized) High Concentration Electrolytes for Li-Based Batteries. ACS Energy Letters (2024).
2. Dynamic Ion Correlations in Solid and Liquid Electrolytes: How Do They Affect Charge and Mass Transport?. ChemElectroChem (2019).
3. Structure and dynamics of highly concentrated LiTFSI/acetonitrile electrolytes. Physical Chemistry Chemical Physics (2021).