Reducing cathode porosity is essential to balancing the electrolyte distribution in lithium–sulfur (Li–S) cells, conserving more pore-filling electrolyte to extend cell cycle life. However, low-porosity electrodes built with nanosized sulfur/carbon (S/C) materials suffer from high tortuosity that significantly deteriorates electrode wetting and hence sulfur utilization. Enabling operation of high-loading sulfur electrodes under both low-porosity and lean-electrolyte conditions is still a challenge and is seldom discussed. In this study, we demonstrated a facile strategy for constructing low-tortuosity through-pores across both vertical and planar directions of electrodes by casting large particles into single-particle-layer electrodes. Through multi-scale characterizations and simulations, correlations between material/electrode structures, electrolyte permeability, polysulfide migration, and sulfur reactions were elucidated. The high-loading and dense sulfur cathode fabricated by this method delivers a high specific capacity (>1000 mA h g⁻¹) at a very low electrolyte/sulfur (E/S) ratio of 4 μL mg⁻¹. This study provides a practical approach to reducing the tortuosity of dense sulfur electrodes by manipulating the porosity distribution, which would be also applicable to improving the rate capability of other high-energy electrodes.

The migration of a capsule enclosed by an elastic membrane in a wall-bounded linear shear is investigated using a front-tracking method. A detailed comparison with the migration of a viscous drop is presented varying the capillary number (in the case of a capsule, the elastic capillary number) and the viscosity ratio. In both cases, the deformation breaks the flow reversal symmetry and makes them migrate away from the wall. They quickly go through a transient evolution to eventually reach a quasi-steady state where the dynamics becomes independent of the initial position and only depends on the wall distance. Previous analytical theories predicted that for a viscous drop, in the quasi-steady state, the migration and slip velocities scale approximately with the square of the inverse of the drop–wall separation, whereas the drop deformation scales as the inverse cube of the separation. These power law relations are shown to hold for a capsule as well. The deformation and inclination angle of the capsule and the drop at the same wall separation show a crossover in their variation with the capillary number: the capsule shows a steeper variation than that of the drop for smaller capillary numbers and slower variation than the drop for larger capillary numbers. Using the Green’s function of Stokes flow, a semi-analytic theory is presented to show that the far-field stresslet that causes the migration has two distinct contributions from the interfacial stresses and the viscosity ratio, with competing effects between the two defining the dynamics. It predicts the scaling of the migration velocity with the capsule–wall separation, however, matching with the simulated result very well only away from the wall. A phenomenological correlation for the migration velocity as a function of elastic capillary number, wall distance and viscosity ratio is developed using the simulation results. The effects of different membrane hyperelastic constitutive equations – neo-Hookean, Evans–Skalak, and Skalak – are briefly investigated to show that the behaviour remains similar for different equations.

Interactions between a pair of equal-size viscous drops in shear are numerically investigated at finite Reynolds number $\left(\text{Re}=0.1–10\right)$ . At low Reynolds number the simulation compares well with a previous experimental observation. Apart from the usual pairwise motion where drops driven by shear pass over each other (type I trajectory), finite inertia introduces a new type (type II) of reversed trajectory where drops approaching each other reverse their initial trajectories. The new trajectory is explained by a reversed streamline pattern observed around a single drop in an imposed shear, and is similar to what is also observed for rigid spheres at finite inertia. However, drop deformability introduces a nonuniform transition from one to the other type of trajectory—drops display type I trajectory for high and low capillary numbers and type II for intermediate capillary numbers. The phenomenon is explained by noting that increasing capillary number gives rise to competing effects—while it increases drop deformation and therefore increases resistance to sliding motion, it also increases drop flexibility, decreases inclination angle, and overall effect of the drop’s presence is reduced, all helping them to slide by. The nonuniform behavior—type II trajectory for an intermediate range of capillary numbers—occurs only at Reynolds number above a critical value. Further increase in Reynolds number increases the range of capillary numbers for type II trajectory. For type I trajectory, terminal cross-stream separation increases linearly with increasing inertia indicating an enhanced shear induced diffusion. Increasing initial streamwise separation aids in reversed (type II) trajectory due to increased overlap with the reversed streamline zone. Increasing cross-stream distance expectedly facilitates (type I) sliding motion. For passing drops (type I trajectory), terminal cross-stream separation is not appreciably affected by capillary number and initial drop separation.