Hyungjun Noh

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.

Enhancing the Cycling Stability of Sodium Metal Electrodes by Building an Inorganic–Organic Composite Protective Layer

Owing to the natural abundance of sodium resources and their low price, next-generation batteries employing an Na metal anode, such as Na-O2 and Na-S systems, have attracted a great deal of interest. However, the poor reversibility of an Na metal electrode during repeated electrochemical plating and stripping is a major obstacle to realizing rechargeable sodium metal batteries. It mainly originates from Na dendrite formation and exhaustive electrolyte decomposition due to the high reactivity of Na metal. Herein, we report a free-standing composite protective layer (FCPL) for enhancing the reversibility of an Na metal electrode by mechanically suppressing Na dendritic growth and mitigating the electrolyte decomposition. A systematic variation of the liquid electrolyte uptake of FCPL verifies the existence of a critical shear modulus for suppressing Na dendrite growth, being in good agreement with a linear elastic theory, and emphasizes the importance of the ionic conductivity of FCPL for attaining uniform Na plating and stripping. The Na-Na symmetric cell with an optimized FCPL exhibits a cycle life two times longer than that of a bare Na electrode.

Perfluorinated Ionomer-Enveloped Sulfur Cathodes for Lithium–Sulfur Batteries

Nafion is known to suppress the polysulfide (PS) shuttle effect, a major obstacle to achieving high capacity and long cycle life for lithium-sulfur batteries. However, elaborate control of the layers configuration is required for high performance. In this regard, we designed a Nafion-enveloped sulfur cathode, where the Nafion layer is formed on the skin of the cathode, covering its surface and edge while not restricting the porosity. Discharge capacity and efficiency were enhanced with the enveloping configuration, demonstrating suppression of shuttle. The edge protection exhibited better cycling stability than an edge-open configuration. In the absence of the Nafion envelope, charged sulfur concentrated on the top region of the cathode because of the relatively lower PS concentration at the cathode surface. Surprisingly, for the Nafion-enveloped cathode, sulfur was evenly distributed along the cathode, indicating that the configuration imparts a uniform PS concentration within the cathode.

Li2S/Carbon Nanocomposite Strips from a Low-Temperature Conversion of Li2SO4 as High- Performance Lithium-Sulfur Cathodes

The carbothermal conversion of Li2SO4 provides a cost-effective strategy to fabricate high-capacity Li2S cathodes; however, Li2S cathodes derived from Li2SO4 at high temperatures (>800 °C), having high crystallinity and large crystal size, result in a low utilization of Li2S. Here, we report Li2SO4/poly(vinyl alcohol)-derived Li2S/carbon nanocomposite (Li2S@C) strips at a record low temperature of 635 °C. These Li2S@C nanocomposite strips as a cathode show a low initial activation potential (2.63 V), a high initial discharge capacity (805 mA h g−1 Li2S) and a high cycling stability (0.2C and 1C). These improved results could be ascribed to the nano-sized Li2S particles as well as their low crystallinity due to the PVA-induced carbon network and the low conversion temperature, respectively. An XPS analysis reveals that the CC and CO bonds derived from the carbonization of PVA can promote the conversion of Li2SO4 at such a low temperature.

A novel deep guard-ring InGaAs PIN photodiode structure reducing a crosstalk in SWIR imaging detection

A low crosstalk planar-type InGaAs/InP photodiode structure is demonstrated using a deep guard-ring technique. The fabricated 5×5 photodiode array with the deep guard-ring structure shows the good crosstalk performance characteristic of 2.57 % at the nearest pixel. The obtained crosstalk characteristic is one of the best values among the planar-type InGaAs PIN photodiodes reported so far.

An Ultrahigh Capacity Graphite/Li2S Battery with Holey-Li2S Nanoarchitectures

Abstract The pairing of high‐capacity Li2S cathode (1166 mAh g−1) and lithium‐free anode (LFA) provides an unparalleled potential in developing safe and energy‐dense next‐generation secondary batteries. However, the low utilization of the Li2S cathode and the lack of electrolytes compatible to both electrodes are impeding the development. Here, a novel graphite/Li2S battery system, which features a self‐assembled, holey‐Li2S nanoarchitecture and a stable solid electrolyte interface (SEI) on the graphite electrode, is reported. The holey structure on Li2S is beneficial in decomposing Li2S at the first charging process due to the enhanced Li ion extraction and transfer from the Li2S to the electrolyte. In addition, the concentrated dioxolane (DOL)‐rich electrolyte designed lowers the irreversible capacity loss for SEI formation. By using the combined strategies, the graphite/holey‐Li2S battery delivers an ultrahigh discharge capacity of 810 mAh g−1 at 0.1 C (based on the mass of Li2S) and of 714 mAh g−1 at 0.2 C. Moreover, it exhibits a reversible capacity of 300 mAh g−1 after a record lifecycle of 600 cycles at 1 C. These results suggest the great potential of the designed LFA/holey‐Li2S batteries for practical use.

A low dark current planar-type InGaAs guard-ring PIN photodiode using an ALD-Al 2 O 3 passivation for short-wave infrared imaging applications

This paper presents a low dark current planar-type InGaAs guard-ring PIN photodiode using an atomic layer deposited (ALD) Al₂O₃ passivation. The fabricated guard-ring PIN photodiode with the Al₂O₃ passivation shows a reduced dark current density of 1.4×10^-6 A/cm² and an increased R₀·A product of 1.4×10⁵ Ω·cm². We found that the dark current density was reduced by 60 % simultaneously using the guard-ring device structure and Al₂O₃ passivation, compared to the non-guard-ring PIN photodiode passivated with the conventional SiO₂.