Charge Transfer Kinetics of Redox-Active Microgels.

Polymer microgel particles decorated with redox-active functional groups are a new and promising object for electrochemical applications. However, the process of charge exchange between an electrode and a microgel particle carrying numerous redox-active centers differs fundamentally from charge exchange involving only molecular species. A single act of contact between the microgel and the electrode surface may not be enough to fully discharge the microgel, and partial charge states are to be expected. Understanding the specifics of this process is crucial for the correct analysis of the data obtained from electrochemical experiments with redox-active microgel solutions. In this study, we employed coarse-grained molecular dynamics to investigate in detail the act of charge transfer from a microgel particle to a flat electrode. The simulations take into account both the mobility of functional groups carrying the charge, which depend on the microgel architecture and the charge exchange between the groups, which can accelerate the propagation of charge within the microgel volume. A set of different microgel systems were simulated in order to reveal the impact of their characteristics: fraction of redox-active groups, microgel molecular mass, cross-linker content, cross-linking topology, and solvent quality. We have found trends in microgel composition leading to the most efficient charge transfer kinetics. The obtained results would be useful for understanding experimental results and for optimizing the design of redox-active microgel particles aimed at faster discharge rates.

Homogeneous nucleation of Li2O2 under Li-O2 battery discharge.

The development of high-energy lithium-oxygen batteries has significantly slowed by numerous challenges including capacity limitations due to electrode surface passivation by the discharge product Li2O2. Since the passivation rate and intensity are dependent on the deposit morphology, herein, we focus on the mechanisms governing Li2O2 formation within the porous cathode. We report evidence of homogeneous nucleation of Li2O2 crystallites and their further assembly in bulk of the electrolyte solution in DMSO, which possesses a high donor number. After careful estimation of the superoxide ion concentration distribution within a phenomenological model, it was found that the high stability of superoxide ions formed during the ORR towards disproportionation and sufficient diffusivity of (0.5-1.2) × 10-6 cm2 s-1 enabled Li2O2 nucleation and crystallization not only at the surface but also in the electrolyte, and the reaction zone spread throughout the internal space of the porous electrode. High initial supersaturation promoted the homogeneous nucleation of Li2O2 nanoplates, which instantly assembled into mesocrystals also in the solution bulk. These results were supported by operando SAXS/WAXS and morphology observations. Thus, although homogeneous nucleation is not dominant, it is important for achieving a high capacity in Li-O2 batteries.

Free-standing Li+-conductive films based on PEO-PVDF blends.

Solid electrolytes are of high interest for the development of advanced electrochemical energy storage devices with all-solid-state architectures. Here, we report the fabrication of the electrolyte membranes based on LiTFSI (LiN(CF3SO2)2) and PEO-PVDF blends with improved properties. We show that addition of PVDF enables preparation of free-standing films of the compositions within the so called "crystallinity gap" of the LiTFSI-PEO system known to provide high ion conductivity. We show that optimal PVDF content enables preparation of the films with reasonable elastic modulus and high ionic conductivity of about 0.3 mS cm-1 at 60 °C and about 0.1 mS cm-1 at room-temperature. Combining FTIR spectroscopy, XRD and DSC measurements we show that a noticeable fraction of PVDF remains crystalline and enhances the mechanical properties of the material, and at the same time it additionally promotes LiTFSI dissociation and disordering. Density functional theory calculations showed that the Li+-PEO-PVDF complexation energy magnitude is almost as high as that of Li-PEO complexes, thus the salt dissociation ability can be retained in spite of the introduction of the substantial amounts of PVDF required for mechanical stability.

Small-angle neutron scattering studies of pore filling in carbon electrodes: mechanisms limiting lithium-air battery capacity.

Many obstacles impede the development of Li-air batteries for practical applications. In particular, there is lack of understanding of the dynamics of processes occurring in porous air electrodes during discharge, including oxygen transport limitations, pore clogging and electrode passivation by both insulating discharge and parasitic reaction products. Here, using small-angle neutron scattering, which provides information on the whole electrode adequate to electrochemical data, we uncover the mechanisms limiting the Li-O2 porous carbon electrode capacity by analysis of the cathode pore filling in highly and poorly solvating media - dimethyl sulfoxide and acetonitrile. The results obtained allowed us suppose that in both cases the cell death is mainly triggered by blocking of oxygen transport pathways inside carbon black particle agglomerates. Total discharge capacities are, indeed, higher in highly solvating solutions due to a higher discharge intermediate lifetime and longer diffusion distance, which enable Li2O2 formation outside the carbon black agglomerates, which are, as we demonstrated, in fact mesocrystals that are confirmed by the appearance of a diffraction peak in scattering curves.