Stability and Microheterogeneity in Concentrated Nonaqueous Electrolyte Solutions

Molecular dynamics of redox-active organic small molecules in concentrated solutions depend on the electrolyte structure. The two catholytes studied have very different dynamics, with one system preferring crystalline order and the other favoring supercooled, amorphous states (top). Models fit to neutron scattering data reveal dynamics and instabilities arising from the supercooled solutions (bottom).

Scientific Achievement

We compared the molecular dynamics of two different but structurally-related electrolytes in highly concentrated solutions. One catholyte is compact and symmetrical; the other is bulkier and less symmetrical. Incoherent elastic and quasi-elastic neutron scattering measurements showed significant differences  in crystallization, supercooling, and sudden microheterogeneity.

Significance and Impact

Catholytes of high solubility also disfavor crystallization; this intuitive behavior needs better understanding to realize improved molecular design of high energy-dense flow battery materials. The need is being addressed by structure and dynamics studies of supercooled, highly-concentrated nonaqueous electrolyte solutions. Our findings suggest that solvation inhomogeneities and molecular crowding are important.

Research Detail

  • Synthesis of deuterated redox-active organic molecules to visualize only protiated (hydrogen-containing) electrolyte components.
  • Variable-temperature neutron scattering of two catholyte solutions show that the symmetrical catholyte prefers crystallization while the bulky one exhibits supercooling behavior.
  • Isothermal quasi-elastic neutron scattering from solutions of the supercooled bulky catholyte reveal thermal history-dependent dynamics. The data were fit to models of molecular dynamics and provide details of solution metastability.

DOI: 10.1021/jacs.9b02323

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