Metal–organic framework (MOF)-derived electroactive materials have drawn great attention due to their diverse design and high electrochemical activity. Herein, a unique hybrid structure consisting of Ni–Co layered double hydroxides (LDHs) and hollow carbon spheres (HCSs) is first synthesized using HCS@ZIF-67 as a precursor. Particularly, when the ZIF-67 content in the precursor is optimized, a novel hybrid with a hollow-in-hollow structure can be obtained by distributing ZIF-67-derived hollow Ni–Co LDHs on a HCS shell. Owing to their distinctive structural characteristics, the as-prepared architectures deliver a high specific capacity (156.4 mA h g⁻¹ at 5 A g⁻¹) and excellent cycle stability (73.4% capacity retention after 10 000 cycles). Notably, during the charge storage process, the dual hollow system of the composite effectively decreases the ion diffusion path, resulting in drastically improved rate performance (70% retention of its highest capacity at 80 A g⁻¹). Furthermore, an all-solid-state hybrid supercapacitor is assembled using the as-fabricated hybrid as the positive material and mesoporous plasma-reduced graphene oxide as the negative material, and exhibits a maximum energy density of 34.5 W h kg⁻¹ and a peak power density of 55 kW kg⁻¹.

Hollow TiO₂ nanoparticles (HNPs) capped with conducting polymers, such as polythiophene (PT), polypyrrole (PPy), and polyaniline (PANI), have been studied to be used as polarizability-tunable electrorheological (ER) fluids. The hollow shape of TiO₂ nanoparticles, achieved by the removal of the SiO₂ template, offers colloidal dispersion stability in silicone oil owing to the high number density. Conducting polymer shells, introduced on the nanoparticle surface using vapor deposition polymerization method, improve the yield stress of the corresponding ER fluids in the order of PANI < PPy < PT. PT-HNPs exhibited the highest yield stress of ca. 94.2 Pa, which is 5.0-, 1.5-, and 9.6-times higher than that of PANI-, PPy-, and bare HNPs, respectively. The improved ER response upon tuning with polymer shells is attributed to the space charge contribution arising from the movement of the charge carriers trapped by the heterogeneous interface. The ER response of studied ER fluids is consistent with the corresponding polarizability results as indicated by the permittivity and electrophoretic mobility measurements. In conclusion, the synergistic effect of hollow nanostructures and conducting polymer capping effectively enhanced the ER performance.

Lithium-ion batteries (LIBs) are increasingly employed in electric vehicles (EVs) owing to their advantages, such as low weight, and high energy and power densities. However, the uncertainty encountered in the manufacturing of LIB cells increases the failure rate and causes cell-to-cell variations, thereby degrading the battery capacity and lifetime. In this study, the reliability and robustness of LIB cells were improved using the design of experiments (DOE), and the reliability-based robust design optimization (RBRDO) approaches. First, design factors sensitive to the energy density and power density were selected as design variables through sensitivity analysis using the DOE. RBRDO was performed to maximize the energy density while reducing the failure rate and cell-to-cell variations. To verify the superiority of the reliability and robustness offered by RBRDO, the obtained results were compared with those from conventional deterministic design optimization (DDO), and reliability-based design optimization (RBDO). RBRDO increased the mean of the energy density by 33.5% compared to the initial value and reduced the failure rate by 98.9%, due to improved reliability, compared to DDO. Moreover, RBRDO reduced the standard deviation in the energy density (i.e., cell-to-cell variations) by 30.0% due to the improved robustness compared to RBDO.