Jang-Kyo Kim

Manganese‐based layered oxides are among the most promising cathode materials for potassium‐ion batteries. Yet their application is hindered by issues like poor structural stability and multiple phase transitions. Herein, the occupancy ratio (K _t /K _h ) of the conventional triangular prism center site (K _t ) to the previously unknown KO ₁₄ hexagonal prism center site (K _h ) is finely tuned to stabilize the layered oxide framework for P3‐type K _x MnO ₂ cathodes. The first‐principles calculations along with ex‐situ X‐ray diffraction, aberration‐corrected scanning transmission electron microscopy, and K L‐edge soft X‐ray absorption spectroscopy reveal that the K ⁺ at the K _h site are locally pinned as self‐pillars, where moderate K _h pillars stabilize the oxide framework by suppressing phase transitions and facilitating a solid‐solution K ⁺ storage mechanism, while excessive K _h immobilizes ions and blocks diffusion pathways. The K content and thermal treatment are functionally interconnected and determine the K _h fraction and phase evolution. The K _0.5 MnO ₂ cathode with an optimal (K _t /K _h ) ratio delivers excellent high‐rate performance of 46.1 mAh g ⁻¹ at 2 A g ⁻¹ and maintains 76.7% of its initial capacity after 2000 cycles at 1 A g ⁻¹ . This work demonstrates the potential benefits of alkali‐ion occupancy engineering as a versatile strategy for stable layered oxide cathodes in high‐performance alkali‐ion batteries.

We design an anion-dominated, dual-electrode-compatible, fire-resistant cosolvent electrolyte (CSE) composed of 1.5 M KFSI in TFP/DEE. K|CSE|K _0.5 MnO ₂ cells operate steadily up to 4.1 V for 2500 cycles at 1 A g ⁻¹ .

Sn has been considered among the most promising metallic anode materials for lithium-ion batteries (LIBs) thanks to its high specific capacity. Herein, we report a novel amorphous tin-titanium-ethylene glycol (Sn-Ti-EG) bimetal organic compound as anode for LIBs. The Sn-Ti-EG electrode exhibits exceptional cyclic stability with high Li-ion storage capacities. Even after 700 cycles at a current density of 1.0 A g^-1, the anode maintains a capacity of 345 mAh g^-1. The unique bimetal organic structure of the Sn-Ti-EG anode and the strong coordination interaction between Sn/Ti and O within the framework effectively suppress the aggregation of Sn atoms, eliminating the usual pulverization of bulk Sn through volume expansion. Further, the Sn M-edge of X-ray absorption near edge structure spectra obtained from the soft X-ray absorption spectroscopy signifies the conversion of Sn²⁺ ions to Sn⁰ during the initial lithiation process, which is reversible to Sn²⁺ upon de-lithiation. These findings manifest Sn being among the most active components that account for the excellent electrochemical performance of the Sn-Ti-EG electrode, while Ti has no practical contribution to the electrode’s capacity. The reversible formation of organic functional groups on the solid electrolyte interphase is also partly responsible for its cyclic stability.

Potassium-ion batteries are considered a low-cost alternative to lithium-ion batteries thanks to the abundant potassium resources. However, their electrochemical performance remains rather unsatisfactory arising from the sluggish kinetics of potassium ions and the large ionic radius. Here, NiCo₂Se₄ nanotube spheres are synthesized as efficient potassium storage hosts via a facile two-step hydrothermal process. The rationally designed electrode has many ameliorating morphological and functional features, including (i) the hollow structure to relieve the volume expansion, while offering an excellent electrochemical reactivity to accelerate the conversion kinetics, (ii) a high electrical conductivity for enhanced electron transfer, and (iii) myriad vacancies to supply active sites for electrochemical reactions. As such, the electrode delivers an initial reversible capacity of 458.1 mAh g^-1, and retains 346.6 mAh g^-1 after 300 cycles at 0.03 A g^-1. It sustains a high capacity of 101.4 mAh g^-1 even at a high current density of 5 A g^-1, and outperforms the majority of state-of-the-art anodes in terms of both cyclic capacity and rate capability especially at above 1.0 A g^-1. This work not only proves bimetallic selenides as a promising candidate for potassium storage devices, but also offers a new insight into the rational design of electrode materials for high-rate potassium-ion batteries.

A Zn anode can offset the low energy density of a flow battery for a balanced approach toward electricity storage. Yet, when targeting inexpensive, long‐duration storage, the battery demands a thick Zn deposit in a porous framework, whose heterogeneity triggers frequent dendrite formation and jeopardizes the stability of the battery. Here, Cu foam is transferred into a hierarchical nanoporous electrode to homogenize the deposition. It begins with alloying the foam with Zn to form Cu₅Zn₈, whose depth is controlled to retain the large pores for a hydraulic permeability ≈10⁻¹¹ m². Dealloying follows to create nanoscale pores and abundant fine pits below 10 nm, where Zn can nucleate preferentially due to the Gibbs–Thomson effect, as supported by a density functional theory simulation. Morphological evolution monitored by in situ microscopy confirms uniform Zn deposition. The electrode delivers 200 h of stable cycles in a Zn–I₂ flow battery at 60 mAh cm⁻² and 60 mA cm⁻², performance that meets practical demands.

Following earlier research efforts dedicated to the realization of multifunctional sensing, recent developments of artificial skins endeavor to go beyond human sensory functions by integrating interactive visualization of strain and pressure stimuli. Inspired by the microcracked structure of spider slit organs and the mechanochromic mechanism of chameleons, this work aims to design a flexible optical/electrical skin (OE-skin) capable of responding to complex stimuli with interactive feedback of human-readable structural colors. The OE-skin consists of an ionic electrode combined with an elastomer dielectric layer, a chromotropic layer containing photonic crystals and a conductive carbon nanotube/MXene layer. The electrode/dielectric layers function as a capacitive pressure sensor. The mechanochromic photonic crystals of ferroferric oxide-carbon magnetic arrays embedded in the gelatin/polyacrylamide stretchable hydrogel film perceive strain and pressure stimuli with bright color switching outputs in the full visible spectrum. The underlying microcracked conductive layer is devoted to ultrasensitive strain sensing with a gauge factor of 191.8. The multilayered OE-skin delivers an ultrafast, accurate response for capacitive pressure sensing with a detection limit of 75 Pa and long-term stability of 5000 cycles, while visualizing complex deformations in the form of high-resolution spatial colors. These findings offer deep insights into the rational design of OE-skins as multifunctional sensing devices.

Aerogel materials with an anisotropic nature have attracted increasing interest because of fascinating, potential applications arising from their novel functional capabilities. However, the strategy to achieve a highly anisotropic structure of aerogel materials for thermal superinsulation has not been fully exploited. In this work, the microstructure and thermal conductivity (k) of waterborne polyurethane (WPU) aerogels are tailored by using different freezing temperatures ranging from −196 to −20 °C in unidirectional freeze-casting. A more anisotropic porous aerogel is obtained at a lower freezing temperature, yielding highly different k in two orthogonal directions. The anisotropy of WPU aerogels is further intensified with the addition of two-dimensional boron nitride nanosheets (BNNSs) possessing anisotropic k values and a high reflectance. Surprisingly, an ultralow density of 20.2 mg cm⁻³ is achieved by the composite aerogel freeze-cast at −196 °C, much lower than the WPU matrix acting alone, by creating more pores of smaller sizes. The unique thermo-optical properties of BNNSs in highly aligned cell walls of the BNNS/WPU composite aerogels provide fast heat dissipation in the alignment direction while largely diminishing heat transfer through the thickness direction, achieving an ultralow k of 16.2 mW m⁻¹ K⁻¹ for thermal superinsulation. A new theoretical approach is also proposed to estimate the anisotropic k of porous materials, verifying the positive roles played by BNNSs in efficient thermal management for directional insulation. In addition, the presence of abundant pores and reflective BNNSs is responsible for the excellent solar reflectance (∼97%) of the aerogels. The coupling effect of highly anisotropic k and high sunlight reflectance offers better thermal management under direct sunlight with up to 6 °C lower internal temperature than a commercial SiO₂ blanket and expanded polystyrene foam coated with commercial reflective paint in the outdoor test. This work can provide general guidelines for designing and producing highly anisotropic aerogels for more energy efficient cooling applications.

The increasing demand for high energy storage devices calls for concurrently enhanced dielectric constants and reduced dielectric losses of polymer dielectrics. In this work, we rationally design dielectric composites comprising aligned 2D nanofillers of reduced graphene oxide (rGO) and boron nitride nanosheets (BNNS) in a polyvinylidene fluoride (PVDF) matrix through a novel press-and-fold technique. Both nanofillers play different yet complementary roles: while rGO is designed to enhance the dielectric constant through charge accumulation at the interfaces with polymer, BNNS suppress the dielectric loss by preventing the mobility of free electrons. The microlaminate containing eight layers each of rGO/PVDF and BNNS/PVDF films exhibits remarkable dielectric performance with a dielectric constant of 147 and an ultralow dielectric loss of 0.075, due to the synergistic effect arising from the alternatingly electrically conductive and insulating films. Consequently, a maximum energy density of 3.5 J/cm³—about 18 times the bilayer composite counterpart—is realized. The high thermal conductivities of both nanofillers and their alignment endow the microlaminate with an excellent in-plane thermal conductivity of 6.53 Wm⁻¹K⁻¹, potentially useful for multifunctional applications. This work offers a simple but effective approach to fabricating a composite for high dielectric energy storage using two different 2D nanofillers.

Vanadium diselenide (VSe₂) exhibits versatile electronic and magnetic properties in the trigonal prismatic (H-) and octahedral (T-) phases. Compared to the metallic T-phase, the H-phase with a tunable semiconductor property is predicted to be a ferrovalley material with spontaneous valley polarization. Herein we report an epitaxial growth of the monolayer 2D VSe₂ on a mica substrate via the chemical vapor deposition (CVD) method by introducing salt in the precursor. Our first-principles calculations suggest that the monolayer H-phase VSe₂ with a large lateral size is thermodynamically favorable. The honeycomb-like structure and the broken symmetry are directly observed by spherical aberration-corrected scanning transmission electron microscopy (STEM) and confirmed by giant second harmonic generation (SHG) intensity. The p-type transport behavior is further evidenced by the temperature-dependent resistance and field-effect device study. The present work introduces a new phase-stable 2D transition metal dichalcogenide, opening the prospect of novel electronic and spintronics device design.