Electrokinetics in Viscoelastic Liquid Electrolytes above the Diffusion Limit

Electroconvective Flow in Liquid Electrolytes Containing Oligomer Additives

Metal electrodeposition in batteries is fundamentally unstable and affected by different instabilities depending on operating conditions and electrolyte chemistry. Particularly, at high charging rates, a hydrodynamic instability loosely termed electroconvection sets in, which complicates all electrochemical processes by creating a nonuniform ion flux and preferential deposition at the electrode. Here, we isolate and study electroconvection by experimentally investigating how oligomer additives in liquid electrolytes interact with the hydrodynamic instability at a cation selective interface. From electrochemical measurements and direct visualization experiments, we find that electroconvection is delayed and suppressed at all voltages in the presence of oligomers. The underlying mechanism is revealed to involve formation of an oligomer ad-layer at the interface, which in response to perturbation is believed to exert an opposing body force on the surrounding fluid to preserve the ad-layer structure and in so doing suppresses electroconvection. Our results therefore reveal that in battery electrolytes without obvious sources of bulk elasticity, surface forces produced by adsorbed polymers can be used to advantage for suppressing instability.

Ionic Conductivity Enhancement in UHMW PEO Gel Electrolytes Based on Room-Temperature Ionic Liquids and Deep Eutectic Solvents

Physical gels made of poly(ethylene oxide) (PEO) and deep eutectic solvents urea-Li bis(trifluoromethanesulfonyl)imide (TFSI) and ethylene glycol/LiTFSI, or pyrrolidinium ionic liquid solutions PYR13TFSI-LiTFSI and PYR14TFSI-LiTFSI, are prepared by a fast, single-step process, which involves no auxiliary solvents or intermediates and is reproducible and scalable. The properties of these gels are studied as a function of the PEO content and its molecular weight and the nature of the liquid electrolyte. The gels prepared with a low concentration (1-5 wt %) of ultrahigh molecular weight (UHMW) PEO are tough, stretchable materials which resemble soft elastomers and are also self-healing and transparent. Their rheology shows the conventional behavior of physical polymer gels, so that the higher the molecular weight of PEO, the lower the polymer concentration needed to produce the gel. However, the ion conductivities and diffusivities of the gels are striking, in many cases being equal to or significantly higher than those of pure liquid electrolytes. This ion conductivity enhancement is the highest for the lowest PEO concentration with the highest molecular weight. This unprecedented molecular weight dependence of conductivity and diffusivity is the result of two combined effects: the liquid electrolyte chemical structure modification as a consequence of the addition of PEO and the development of elastic networks, where ion mobility and rheology are uncoupled when the PEO added is of UHMW.

Addressing Manufacturability and Processability in Polymer Gel Electrolytes for Li/Na Batteries

Gel electrolytes are prepared with Ultra High Molecular Weight (UHMW) polyethylene oxide (PEO) in a concentration ranging from 5 to 30 wt.% and Li- and Na-doped 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (PYR14-TFSI) by a simple procedure consisting of dissolving PEO by melting it directly in the liquid electrolyte while stirring the blend. This procedure is fast, reproducible and needs no auxiliary solvents, which makes it sustainable and potentially easy to scale up for mass production. The viability of the up-scaling by extrusion has been studied. Extrusion has been chosen because it is a processing method commonly employed in the plastics industry. The structure and morphology of the gel electrolytes prepared by both methods have been studied by DSC and FTIR, showing small differences among the two methods. Composite gels incorporation high concentrations of surface modified sepiolite fibers have been successfully prepared by extrusion. The rheological behavior and ionic conductivity of the gels have been characterized, and very similar performance of the extruded and manually mixed gels is detected. Ionic conductivity of all the gels, including the composites, are at or over 0.4 mS cm^-1 at 25 °C, being at the same time thermoreversible and self-healing gels, tough, sticky, transparent and stretchable. This combination of properties, together with the viability of their industrial up-scaling, makes these gel electrolyte families very attractive for their application in energy storage devices.

Structure and Dynamics of Electric-Field-Driven Convective Flows at the Interface between Liquid Electrolytes and Ion-Selective Membranes

At voltages above a certain critical value, V_c ≈ 20 kT/e, a space charge layer forms near ion-selective interfaces in liquid electrolytes. Interactions between the space charge and an imposed electric field drives a hydrodynamic instability known as electroconvection. Through particle tracking velocimetry we experimentally study the structure and dynamics of the resultant electroconvective flow. Consistent with previous numerical simulations, we report that, following imposition of a sufficiently large voltage, electroconvection develops gradually as pairs of counter-rotating vortices, which nucleate at the interface between an ion-selective substrate and a liquid electrolyte. Depending on the imposed voltage and cell geometry, the vorticies grow to length scales of hundreds of micrometers. Electroconvective flows are also reported to be structured and multiscale, with the size ratio of the largest to the smallest observable vortices inversely proportional to the Debye screening length.

Structure, Rheology, and Electrokinetics of Soft Colloidal Suspension Electrolytes

When ion transport in a binary liquid electrolyte is driven at potentials above the thermal voltage, an extended space charge region forms at the electrolyte/electrode interface and triggers the hydrodynamic instability termed electroconvection. We experimentally show that this instability can be completely arrested in soft colloidal suspension electrolytes composed of low concentrations of polymer-grafted nanoparticles in a liquid host. The mechanism is revealed by means of X-ray scattering, Brownian dynamics calculations, and linear stability analysis to involve overlap of the soft particles at low particle fractions to create a jammed, nanoporous medium that resists convective flow by a Darcy-Brinkman like drag on the electrolyte solvent.

Spontaneous and field-induced crystallographic reorientation of metal electrodeposits at battery anodes

The propensity of metal anodes of contemporary interest (e.g., Li, Al, Na, and Zn) to form non-planar, dendritic morphologies during battery charging is a fundamental barrier to achievement of full reversibility. We experimentally investigate the origins of dendritic electrodeposition of Zn, Cu, and Li in a three-electrode electrochemical cell bounded at one end by a rotating disc electrode. We find that the classical picture of ion depletion-induced growth of dendrites is valid in dilute electrolytes but is essentially irrelevant in the concentrated (≥1 M) electrolytes typically used in rechargeable batteries. Using Zn as an example, we find that ion depletion at the mass transport limit may be overcome by spontaneous reorientation of Zn crystallites from orientations parallel to the electrode surface to dominantly homeotropic orientations, which appear to facilitate contact with cations outside the depletion layer. This chemotaxis-like process causes obvious texturing and increases the porosity of metal electrodeposits.

Regulating electrodeposition morphology of lithium: towards commercially relevant secondary Li metal batteries

Rational approaches for achieving fine control of the electrodeposition morphology of Li are required to create commercially-relevant rechargeable Li metal batteries.