Gihan Kwon

Extracting uranyl ions (UO22+) from uranium mining leachates can enhance national uranium security and mitigate the environmental impact of nuclear energy. Conventional techniques such as ion exchange and iron-based reductive precipitation have notable limitations. This study reports that the uvanite (U2V6O21) electrode, a layered uranium-vanadium oxide, enables potentiodynamic decoupling of UO22+ and proton intercalation/deintercalation, as well as proton-induced UO22+/UO3 precipitation and dissolution reactions. Building on a comprehensive understanding of charge-transfer and transport mechanisms, we found that potentiodynamic modulation enables selective capture of uranium species using a uvanite electrode. By allowing competing cations to deintercalate at lower potentials and preventing proton deintercalation, UO3 dissolution is inhibited, achieving a high UO3 recovery capacity of 515 mg/g at 10 times the competing cation concentration. The new electrochemistry of the uvanite electrode helps develop cost-effective, sustainable battery electrodes for critical ion extraction from water.

Amorphous magnesium carbonate (AMC) is recognized as a precursor material during the formation of crystalline magnesium carbonates, which are important materials, e.g., for next generation cements and CO2 sorbents, but its amorphous-to-crystalline transition is poorly understood. In this study, we combined in situ X-ray and neutron diffraction and total scattering to determine the activation energy of the amorphous-to-crystalline transition for AMC in water and identify the reaction mechanism. The calculated activation energy Ea of 41 ± 13 kJ/mol for the AMC to nesquehonite (MgCO3·3H2O) transformation matches previous studies indicating a dissolution–reprecipitation mechanism. Ea for AMC to hydromagnesite (Mg5(CO3)4(OH)2·4H2O) of 116 ± 19 kJ/mol is considerably higher and combined with our peak area analysis of total scattering data, we conclude that a second-order solid state transition is likely. Remarkably, we observed 2 orders of magnitude longer lifetime of deuterated AMC compared to protiated AMC in liquid.

Valence dynamics of transition metals are central to oxygen evolution reaction (OER) mechanisms, yet in situ spectroscopic studies capturing these dynamics within amorphous catalysts remain limited. Here, we present in situ X-ray spectroscopy of amorphous catalysts, where CoBi exhibits higher OER performance compared to CoPi. While our data shows successive valence changes toward high-valent states in both catalysts, distinct initial electro-structural states dictate the intermediate evolution. Co(III) dominates in both catalysts at open-circuit potential, yet the Co(III)–O–Co(III) bridge structure observed in CoBi is absent in CoPi. Under active OER, Co(IV) species are identified as distinguishable spectral intensities; these are assigned to highly hybridized low-spin (LS) Co(IV) with reduced electron–electron repulsion, as validated by resonant inelastic X-ray spectroscopy (RIXS) analysis of Li0.75CoO2. Density functional theory supports the conversion from tetrahedral (Td) Co(II) to octahedral (Oh) Co(III) and suggests that factors other than thermodynamic pathways are likely responsible for the performance gap. The lower formal oxidation, larger Co–O length, smaller Co–O–Co angle, prominent pre-edge dipole character and absent oxo-bridging in CoPi suggest a kinetically inefficient Co(IV) state. These findings demonstrate that the initial electro-structure strongly influences the formation of productive high-valent active species and the resulting OER activity.

The design of aqueous battery cathode materials that can store divalent ions with high capacity and satisfactory reversibility is of great technical importance and challenge. Here, we report that divalent Mg²⁺ storage is facilitated by an intercalation-induced amorphization of vanadate electrode materials. Electrokinetic analyses and in situ synchrotron X-ray diffraction and absorption spectroscopy collectively demonstrate that vanadate layered materials (Li–V₃O₈) undergo a structural transformation to amorphization induced by Mg²⁺ intercalation, and a reversible restoration of crystalline structure upon Mg²⁺ deintercalation. Debye scattering simulations suggest that intercalation-induced turbostratic disorder, especially random rotations, translational shifts, oscillatory motions, or varied interlayer spacing of adjacent V–O molecular layers, could be responsible for the observed amorphization. The highly distorted local structure, in turn, facilitates Mg²⁺ intercalation across the vanadate electrode materials, responsible for nearly 3/7 of the total Mg²⁺ ions intercalated. The study presented reveals an intriguing relationship between ion transport and the reversible amorphization-to-crystallization dynamics it induces, opening a paradigm for designing advanced aqueous battery electrodes.

Sulfide-containing alkaline electrolytes fundamentally alter the Fe ²⁺ /Fe ³⁺ redox system by enabling multiple ion-transfer pathways, involving hydrosulfide, protons, and hydroxyl ions.

Energy-efficient and low-temperature iron electrolysis in alkaline solutions is a low-cost and sustainable ironmaking process with zero-carbon emissions when renewable electrical sources are involved. However, its implementation is hindered by electrochemically inert Fe₃O₄ and parasitic H₂ gas formation during the electrochemical reduction process, resulting in the low energy efficiency of iron electrolysis. Here, we further explore the potential of electrochemical reduction of goethite (FeOOH) by employing a low concentration of silicate additive in an alkaline solution to mitigate Fe₃O₄ accumulation and H₂ generation. Electrochemical measurements coupled with operando X-ray diffraction and X-ray absorption spectroscopy suggested FeOOH → Fe₃O₄ → Fe­(OH)₂ → Fe reduction pathways. Interestingly, a poorly crystalline or amorphous Fe­(OH)₂ phase formed in the NaOH/silicate mixed electrolyte, possibly due to the inhibitive effect of silicate on water and ion transport, which eventually contributed to the improved reduction of Fe₃O₄, also supported by atomistic simulations. This work demonstrates the potential for silicate as a low-cost and effective electrolyte additive to improve room-temperature green iron formation via electrolysis.

The advancement of battery technology has been impeded by the voltage constraints of electrolytes. Here, we present a high-energy all-solid-state battery (ASSB) design that achieves both a >5 V operation and an ultrahigh areal capacity of 35.3 mAh cm-2; these attributes were enabled by a new, highly conductive and ultrahigh voltage stable fluoride solid electrolyte (SE), LiCl–4Li2TiF6 (1.7 x 10-5 S cm-1 at 30 °C). This material provides over 100 times higher Li+ conductivity than Li2TiF6, attributable to the enlarged Li+ channels and increased Li+ concentration at the nanocrystallite interfaces, as proved by combined experimental and theoretical investigations. LiCl–4Li2TiF6 shields high-voltage spinel oxide cathodes, achieving excellent performance (e.g., 106 mAh g-1 at 2C and 30 °C with 75.2% retention after 500 cycles for LiNi0.5Mn1.5O4), sharply contrasting with the conventional LiNbO3 counterpart, which decomposes and fails to prevent detrimental interfacial degradation. The efficacy of LiCl–4Li2TiF6 is validated across various systems, including higher-voltage LiCoMnO4, cost-effective LiFe0.5Mn1.5O4, pouch-type LiNi0.5Mn1.5O4//Li ASSBs, operability down to 2.3 V with 258 mAh g-1, and 1.8 mm ultrathick electrodes. This newly proposed SE with over 5 V stability introduces a new design paradigm by revisiting the previously forbidden high-voltage cathodes for high-energy ASSBs.

O3-type layered transition metal (TM) oxides are widely used as cathode materials for Na-ion batteries due to their high energy density potential, enabled by the state of charge (SoC)-dependent transition from octahedral (O-type) to prismatic (P-type) structures during Na-ion (de)sodiation. However, the O-P transition is often criticized for compromising the Na-ion mobility and limiting the cycle life. Herein, we reveal the intrinsic correlation between O-P transitions, oxygen behaviors, and Na-ion kinetics. We demonstrate that a compositionally versatile, entropy-tailored approach can promote preferred transitions (characterized by large lattice parameter deviations in the O-type region and rapid O-P biphasic reactions), enhancing Na-ion migration, as revealed by in situ high-energy synchrotron X-ray diffraction (HEXRD). Additionally, irreversible oxygen loss at high SoC is effectively mitigated, while TM migration and surface reconstruction are greatly suppressed, further accelerating Na-ion transport and stabilizing the structure, as confirmed by X-ray absorption spectroscopy (XAS) and theoretical analyses. The result is an exceptionally high rate capability of 88.7 mAh g^-1 at 20 C (2.4 A g^-1) with a superior normalized retention of 72.6%, accompanied by a prolonged lifetime with 74.3% retention after 1000 cycles. This work advances the understanding of the chemistry-property relationships in O3-type layered cathodes and broadens the prospects for fabricating high-power-density electrodes.