Yunju Lee

Effect of Al2O3 Coatings Prepared By RF Sputtering on Polyethylene Separators for High Power Lithium-Ion Batteries

In this study, we demonstrated the effects of aluminum oxide (Al2O3)-based ceramic coatings deposited by radio-frequency (RF) magnetron sputtering on commercial polyethylene (PE) microporous separators. Due to the superb thermal stability of the ceramic materials themselves, the Al2O3 coatings solved the chronic thermal shrinkage problem of PE separators. Separators with sputtered Al2O3 coatings maintained their initial dimensions even after high temperature exposure at 140 °C for 30 min. The sputtered Al2O3 layer effectively changed the surface of a PE separator from being hydrophobic to hydrophilic too, improving its wettability with liquid electrolyte. Additionally, a sputtered Al2O3 coating can improve the rate capability (~130%) compared with a bare PE separator under a high current density (7.75 mA cm-2, 5 C rate) because the layer does not require additional use of polymeric binder materials, which usually inhibit the formation of pore structures in microporous membranes.

Polysulfide rejection layer from alpha-lipoic acid for high performance lithium–sulfur battery

The polysulfide shuttle has been an impediment to the development of lithium–sulfur batteries with high capacity and cycling stability. Here, we report a new strategy to remedy the problem that uses alpha-lipoic acid (ALA) as an electrolyte additive to form a polysulfide rejection layer on the cathode surface via the electrochemical and chemical polymerization of ALA and a stable solid electrolyte interface (SEI) layer on the Li metal anode during the first discharge. The poly(ALA) layer formed in situ effectively prevents the polysulfide shuttle and consequently enhances the discharge capacity and cycling stability, owing to the Donnan potential developed between the polysulfide-concentrated cathode and the fixed negative charge-concentrated poly(ALA) layer. Also, the SEI layer additionally prevents the chemical reaction of the polysulfide and Li metal anode. The approach, based on the double effect, encompasses a new scientific strategy and provides a practical methodology for high performance lithium–sulfur batteries.

A facile approach to prepare biomimetic composite separators toward safety-enhanced lithium secondary batteries

A mussel-inspired polydopamine (PDA) coating turns radio-frequency (RF) Al2O3 sputtering – which thus far has not been appropriate for the surface treatment of porous polyolefin-based separators – into a damage-free, reliable, and cost-efficient process. Due to the thermally resistive PDA layers, polyethylene (PE) separators can sustain high-power Al2O3 sputtering conditions over 75 W, which significantly reduces processing time. Furthermore, compared to the as-prepared separators, PDA/Al2O3-coated PE separators also reveal improved thermal stability and cycle performance for lithium secondary batteries. PDA/Al2O3-coated PE separators retained their original size when exposed to temperatures of 145 °C over 30 min, while the bare PE separators shrank to 9% of their original size. At a temperature of 25 °C, the unit cell (LiMn2O4/separator/Li metal) employing the PDA/Al2O3-coated PE separators maintained 94.8% (103.4 mA h g−1) of the initial discharge capacity after 500 cycles at C/2 rate and 51.7% (56.7 mA h g−1) at 25 C rate, while the corresponding values for the bare PE separators were 89% (98.6 mA h g−1) at C/2 rate and 24.5% (27.2 mA h g−1) at 25 C rate.

Influence of grain size on the electronic and the magnetic properties of La0.7Ca0.3MnO3−δ

Polycrystalline La0.7Ca0.3MnO3−δ (LCMO), prepared by a solid-state reaction, was annealed at different temperatures TA. The variation of TA was responsible for observed changes in the physical properties, including a shift in the metal-insulator transition temperature, which is explained by variation in the oxygen content. At low temperatures the magnetization of LCMO samples was also observed to be dependent on TA. A reduction of the grain boundaries leads to enhancement of both the magnetization and the sensitivity for low-field magnetoresistance. By using Mn L3-edge x-ray absorption, it was also found that the Mn3+∕Mn4+ ratio increases with the grain size, and that the oxygen content in LCMO is governed by the grain boundaries.