Jelena Popovic

Ultrafast lithium diffusion in bilayer graphene

Solids that simultaneously conduct electrons and ions are key elements for the mass transfer and storage required in battery electrodes. Single-phase materials with a high electronic and high ionic conductivity at room temperature are hard to come by, and therefore multiphase systems with separate ion and electron channels have been put forward instead. Here we report on bilayer graphene as a single-phase mixed conductor that demonstrates Li diffusion faster than in graphite and even surpassing the diffusion of sodium chloride in liquid water. To measure Li diffusion, we have developed an on-chip electrochemical cell architecture in which the redox reaction that forces Li intercalation is localized only at a protrusion of the device so that the graphene bilayer remains unperturbed from the electrolyte during operation. We performed time-dependent Hall measurements across spatially displaced Hall probes to monitor the in-plane Li diffusion kinetics within the graphene bilayer and measured a diffusion coefficient as high as 7 × 10-5 cm2 s-1.

Interfacial Effects in Solid-Liquid Electrolytes for Improved Stability and Performance of Dye-Sensitized Solar Cells

With the purpose of achieving stable dye-sensitized solar cells (DSSCs) with high efficiency, a new type of soft matter electrolyte is tested in which specific amounts of nanosized silica particles are finely dispersed in short-chained polyethylene glycol dimethylether encompassing an iodide/triiodide redox mediator. This results in a solid-liquid composite having synergistic electrical and favorable mechanical properties. The combination of interfacial effects and particle network formation promotes enhanced ion transport, which directly impacts the short-circuit photocurrent density. Thorough analysis reveals that this newly elaborated class of electrolytes is able to improve, at the same time, the thermal and long-term stability of DSSCs, as well as power conversion efficiency under standard and lower irradiation intensities. Lab-scale devices with champion efficiency exceeding 11% under attenuated sunlight (20 mW cm-2, with a compact TiO2 blocking layer) are obtained, along with impressively stable performance under both thermal stress and light soaking in an indoor environment (>96% performance retention after 2500 h of accelerated aging under full sun alternated with thermal ramps), matching the durability criteria applied to silicon solar cells for outdoor applications. The new findings might foster widespread practical application of DSSCs.

Lithium Potential Variations for Metastable Materials: Case Study of Nanocrystalline and Amorphous LiFePO4

Much attention has been paid to metastable materials in the lithium battery field, especially to nanocrystalline and amorphous materials. Nonetheless, fundamental issues such as lithium potential variations have not been pertinently addressed. Using LiFePO4 as a model system, we inspect such lithium potential variations for various lithium storage modes and evaluate them thermodynamically. The conclusions of this work are essential for an adequate understanding of the behavior of electrode materials and even helpful in the search for new energy materials.

Soggy-sand effects in liquid composite electrolytes with mesoporous materials as fillers

Mesoporous silica particles are used as fillers for soggy-sand electrolytes. Favorable surface chemistry that leads to anion adsorption (as reflected by zeta potential) and conductivity enhancement effects is observed for small volume fractions. More remarkable than the absolute conductivities are the significant transference number increase and the noticeable stationarity of the results for a given preparation. Internally connected pores were shown to contribute to the overall conductivity if not extremely small.

Infiltrated porous oxide monoliths as high lithium transference number electrolytes

We show for the first time that liquid–solid lithium electrolytes can exhibit both a very high lithium transference number (up to 0.89) and high overall ionic conductivity (up to 0.48 mS cm−1) when the solid contains a large number of mesopores covered by a high density of –OH groups enabling anionic adsorption.

Interfacial Layering and Screening Behavior of Glyme-Based Lithium Electrolytes

Understanding of electrical double layers is essential to all electrochemical devices, particularly at high charge carrier concentrations. Using a combined approach (surface force apparatus, zeta potential, infrared spectroscopy), we propose a model for the interfacial structure of triglyme electrolytes on muscovite mica. In contact with the pure triglyme, a brush-like polymeric structure grows on the mica surface. When lithium triflate is present in the triglyme, this structure is suppressed by anion adsorption and an extended double layer is formed. A surprising result of great fundamental significance is that the effective screening length measured by surface force apparatus at considerable lithium triflate concentrations (above 0.2 M) is substantially higher than expected from the Debye-Hückel theory. This suggests a high degree of complex salt association as a novel characteristic feature of salt-containing electrolytes.

Correction: Infiltrated porous oxide monoliths as high lithium transference number electrolytes

Correction for ‘Infiltrated porous oxide monoliths as high lithium transference number electrolytes’ by Jelena Popovic et al., J. Mater. Chem. A, 2016, 4, 7135–7140.