Sung-Kyun Jung

Unexpected discovery of low-cost maricite NaFePO4 as a high-performance electrode for Na-ion batteries

Battery chemistry based on earth-abundant elements has great potential for the development of cost-effective, large-scale energy storage systems. Herein, we report, for the first time, that maricite NaFePO4 can function as an excellent cathode material for Na ion batteries, an unexpected result since it has been regarded as an electrochemically inactive electrode for rechargeable batteries. Our investigation of the Na re-(de)intercalation mechanism reveals that all Na ions can be deintercalated from the nano-sized maricite NaFePO4 with simultaneous transformation into amorphous FePO4. Our quantum mechanics calculations show that the underlying reason for the remarkable electrochemical activity of NaFePO4 is the significantly enhanced Na mobility in the transformed phase, which is ∼one fourth of the hopping activation barrier. Maricite NaFePO4, fully sodiated amorphous FePO4, delivered a capacity of 142 mA h g−1 (92% of the theoretical value) at the first cycle, and showed outstanding cyclability with a negligible capacity fade after 200 cycles (95% retention of the initial cycle).

Anomalous Jahn–Teller behavior in a manganese-based mixed-phosphate cathode for sodium ion batteries

We report a 3.8 V manganese-based mixed-phosphate cathode material for applications in sodium rechargeable batteries; i.e., Na4Mn3(PO4)2(P2O7). This material exhibits a largest Mn2+/Mn3+ redox potential of 3.84 V vs. Na+/Na yet reported for a manganese-based cathode, together with the largest energy density of 416 W h kg−1. We describe first-principles calculations and experimental results which show that three-dimensional Na diffusion pathways with low-activation-energy barriers enable the rapid sodium insertion and extraction at various states of charge of the Na4−xMn3(PO4)2(P2O7) electrode (where x = 0, 1, 3). Furthermore, we show that the sodium ion mobility in this crystal structure is not decreased by the structural changes induced by Jahn–Teller distortion (Mn3+), in contrast to most manganese-based electrodes, rather it is increased due to distortion, which opens up sodium diffusion channels. This feature stabilizes the material, providing high cycle stability and high power performance for sodium rechargeable batteries. The high voltage, large energy density, cycle stability and the use of low-cost Mn give Na4Mn3(PO4)2(P2O7) significant potential for applications as a cathode material for large-scale Na-ion batteries.

Phase transition of neodymium yttrium aluminum borate with composition

The NYAB powder with Nd composition was synthesized by solid-state reaction. Space groups for YAB and NAB were confirmed to be R32 and C2/c, respectively. Lattice parameters and decomposition temperature of NYAB, according to Nd concentration, were measured by X-ray diffraction and DTA. As the Nd content in NYAB was increased, the lattice constants were increased as well, but the decomposition temperature of NYAB was lowered. The space group of NYAB is R32 in the composition region of x = 0.0 ∼ 0.8 and C2/c in the region of x = 0.81 ∼ 1.0.

Recent Progress in Organic Electrodes for Li and Na Rechargeable Batteries

Organic rechargeable batteries, which use organics as electrodes, are excellent candidates for next-generation energy storage systems because they offer design flexibility due to the rich chemistry of organics while being eco-friendly and potentially cost efficient. However, their widespread usage is limited by intrinsic problems such as poor electronic conductivity, easy dissolution into liquid electrolytes, and low volumetric energy density. New types of organic electrode materials with various redox centers or molecular structures have been developed over the past few decades. Moreover, research aimed at enhancing electrochemical properties via chemical tuning has been at the forefront of organic rechargeable batteries research in recent years, leading to significant progress in their performance. Here, an overview of the current developments of organic rechargeable batteries is presented, with a brief history of research in this field. Various strategies for improving organic electrode materials are discussed with respect to tuning intrinsic properties of organics using molecular modification and optimizing their properties at the electrode level. A comprehensive understanding of the progress in organic electrode materials is provided along with the fundamental science governing their performance in rechargeable batteries thus a guide is presented to the optimal design strategies to improve the electrochemical performance for next-generation battery systems.

Crystal growth of neodymium yttrium aluminum borate with several neodymium contents

Abstract NdxY1-xAl3(BO3)4 (x ≤ 0.1) single crystals were grown from high temperature solution. Because the flux was volatile, the solvent evaporation as well as slow cooling method was used simultaneously. Inclusion-free crystals were grown below the cooling rate of 2 °C/day and in the seed orientation. The Keff of Nd ion was measured as 0.4∼0.6 when CNd,sol. was 5∼15%. The solution with 7∼14% of CNd,sol. was suitable for the growth of NYAB crystals for a green laser generator. Several absorption peaks were confirmed by UV and FTIR spectroscopy.

Crystal growth of NdAl3(BO3)4 from K2O/MoO3/Nd2O3/B2O3/KF flux

NdAl3(BO3)4 single crystals were grown by the flux method and the TSSG technique using a K2O/3MoO3/B2O3/0.5Nd2O3/KF flux system. Light-violet clear crystals could be obtained. The effects of fluoride on the growth of NAB crystals were investigated. As the content of KF was gradually increased, the growth form of NAB was changed from the equant to the columnar and the primary crystalline region of NAB was shrinked. At the ratio of KFK2O = 0.75, NAB crystals could not be grown.

NaF–FeF 2 nanocomposite: New type of Na-ion battery cathode material

Na-ion batteries (NIBs) are considered one of the most attractive alternatives for Li-ion batteries (LIBs) because of the natural abundance of Na and the similarities between the NIB technology and the well-established LIB technology. However, the discovery of high-performance electrode materials remains a key factor in the success of NIBs. Herein, we propose a new type of cathode material for NIBs based on a nanocomposite of an alkali metal fluoride (NaF) and a transition metal fluoride (FeF2). Although neither of these components is electrochemically active with Na, the nanoscale mixture of the two can deliver a reversible capacity of ∼125 mAh/g in the voltage range of 1.2–4.8 V vs. Na/Na+ via an Fe2+/Fe3+ redox couple. X-ray absorption spectroscopy reveals that the reversible Na storage is aided by the F–ions due to the decomposition of NaF, which are absorbed on the surface of FeF2, promoting the redox reaction of Fe and triggering the gradual transformation of the mother structure (FeF2) into a new (FeF3-like) host structure for the Na ions. This unique Na-ion storage phenomenon, which is reported for the first time, will introduce an avenue for designing novel cathode materials for NIBs.