Gabin Yoon

Sodium intercalation chemistry in graphite

The insertion of guest species in graphite is the key feature utilized in applications ranging from energy storage and liquid purification to the synthesis of graphene. Recently, it was discovered that solvated-Na-ion intercalation can occur in graphite even though the insertion of Na ions alone is thermodynamically impossible; this phenomenon enables graphite to function as a promising anode for Na-ion batteries. In an effort to understand this unusual behavior, we investigate the solvated-Na-ion intercalation mechanism using in operando X-ray diffraction analysis, electrochemical titration, real-time optical observation, and density functional theory (DFT) calculations. The ultrafast intercalation is demonstrated in real time using millimeter-sized highly ordered pyrolytic graphite, in which instantaneous insertion of solvated-Na-ions occurs (in less than 2 s). The formation of various stagings with solvated-Na-ions in graphite is observed and precisely quantified for the first time. The atomistic configuration of the solvated-Na-ions in graphite is proposed based on the experimental results and DFT calculations. The correlation between the properties of various solvents and the Na ion co-intercalation further suggests a strategy to tune the electrochemical performance of graphite electrodes in Na rechargeable batteries.

Large-Scale Synthesis of Carbon-Shell-Coated FeP Nanoparticles for Robust Hydrogen Evolution Reaction Electrocatalyst

A highly active and stable non-Pt electrocatalyst for hydrogen production has been pursued for a long time as an inexpensive alternative to Pt-based catalysts. Herein, we report a simple and effective approach to prepare high-performance iron phosphide (FeP) nanoparticle electrocatalysts using iron oxide nanoparticles as a precursor. A single-step heating procedure of polydopamine-coated iron oxide nanoparticles leads to both carbonization of polydopamine coating to the carbon shell and phosphidation of iron oxide to FeP, simultaneously. Carbon-shell-coated FeP nanoparticles show a low overpotential of 71 mV at 10 mA cm-2, which is comparable to that of a commercial Pt catalyst, and remarkable long-term durability under acidic conditions for up to 10 000 cycles with negligible activity loss. The effect of carbon shell protection was investigated both theoretically and experimentally. A density functional theory reveals that deterioration of catalytic activity of FeP is caused by surface oxidation. Extended X-ray absorption fine structure analysis combined with electrochemical test shows that carbon shell coating prevents FeP nanoparticles from oxidation, making them highly stable under hydrogen evolution reaction operation conditions. Furthermore, we demonstrate that our synthetic method is suitable for mass production, which is highly desirable for large-scale hydrogen production.

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.

Hierarchical Surface Atomic Structure of a Manganese-Based Spinel Cathode for Lithium-Ion Batteries†

The increasing use of lithium-ion batteries (LIBs) in high-power applications requires improvement of their high-temperature electrochemical performance, including their cyclability and rate capability. Spinel lithium manganese oxide (LiMn2O4) is a promising cathode material because of its high stability and abundance. However, it exhibits poor cycling performance at high temperatures owing to Mn dissolution. Herein we show that when stoichiometric lithium manganese oxide is coated with highly doped spinels, the resulting epitaxial coating has a hierarchical atomic structure consisting of cubic-spinel, tetragonal-spinel, and layered structures, and no interfacial phase is formed. In a practical application of the coating to doped spinel, the material retained 90% of its capacity after 800 cycles at 60 °C. Thus, the formation of an epitaxial coating with a hierarchical atomic structure could enhance the electrochemical performance of LIB cathode materials while preventing large losses in capacity.

The Reaction Mechanism and Capacity Degradation Model in Lithium Insertion Organic Cathodes, Li2C6O6, Using Combined Experimental and First Principle Studies.

Herein, we explore the capacity degradation of dilithium rhodizonate salt (Li2C6O6) in lithium rechargeable batteries based on detailed investigations of the lithium de/insertion mechanism in Li2C6O6 using both electrochemical and structural ex situ analyses combined with first-principles calculations. The experimental observations indicate that the LixC6O6 electrode undergoes multiple two-phase reactions in the composition range of 2 ≤ x ≤ 6; however, the transformations in the range 2 ≤ x ≤ 4 involve a major morphological change that eventually leads to particle exfoliation and the isolation of active material. Through first-principles analysis of LixC6O6 during de/lithiation, it was revealed that particle exfoliation is closely related to the crystal structural changes with lithium deinsertion from C6O6 interlayers of the LixC6O6. Among the lithium ions found at various sites, the extraction of lithium from C6O6 interlayers at 2 ≤ x ≤ 4 decreases the binding force between the C6O6 layers, promoting the exfoliation of C6O6 layers and pulverization at the electrode, which degrades capacity retention.

A comparative study of graphite electrodes using the co-intercalation phenomenon for rechargeable Li, Na and K batteries

Here, we demonstrate that graphite can serve as a versatile electrode for various rechargeable battery types by reversibly accommodating solvated alkali ions (such as K, Na, and Li) through co-intercalation in its galleries. The co-intercalation of alkali ions is observed to occur via staging reactions. Notably, their insertion behaviors, including their specific capacity, are remarkably similar regardless of the alkali ion species despite the different solubility limits of K, Na, and Li ions in graphite. Nevertheless, the insertion potentials of the solvated alkali ions differ from each other and are observed to be correlated with the interlayer distance in the intercalated graphite gallery.

Moisture Barrier Composites Made of Non-Oxidized Graphene Flakes

Graphene flakes (GFs) with minimized defects and oxidation ratios are incorporated into polyethylene (PE) to enhance the moisture barrier. GFs produced involving solvothermal intercalation show extremely low oxidation rates (3.17%), and are noncovalently functionalized in situ, inducing strong hydrophobicity. The fabricated composite possesses the best moisture barrier performance reported for a polymer-graphene composite.

Lithium-excess olivine electrode for lithium rechargeable batteries

Lithium iron phosphate (LFP) has attracted tremendous attention as an electrode material for next-generation lithium-rechargeable battery systems due to the use of low-cost iron and its electrochemical stability. While the lithium diffusion in LFP, the essential property in battery operation, is relatively fast due to the one-dimensional tunnel present in the olivine crystal, the tunnel is inherently vulnerable to the presence of FeLi anti-site defects (Fe ions in Li ion sites), if any, that block the lithium diffusion and lead to inferior performance. Herein, we demonstrate that the kinetic issue arising from the FeLi defects in LFP can be completely eliminated in lithium-excess olivine LFP. The presence of an excess amount of lithium in the Fe ion sites (LiFe) energetically destabilizes the FeLi-related defects, resulting in reducing the amount of Fe defects in the tunnel. Moreover, we observe that the spinodal decomposition barrier is notably reduced in lithium-excess olivine LFP. The presence of LiFe and the absence of FeLi in lithium-excess olivine LFP additionally induce faster kinetics, resulting in an enhanced rate capability and a significantly reduced memory effect. The lithium-excess concept in the electrode crystal brings up unexpected properties for the pristine crystal and offers a novel and interesting approach to enhance the diffusivity and open up additional diffusion paths in solid-state ionic conductors.

Amorphous Cobalt Phyllosilicate with Layered Crystalline Motifs as Water Oxidation Catalyst

The development of a high-performance oxygen evolution reaction (OER) catalyst is pivotal for the practical realization of a water-splitting system. Although an extensive search for OER catalysts has been performed in the past decades, cost-effective catalysts remain elusive. Herein, an amorphous cobalt phyllosilicate (ACP) with layered crystalline motif prepared by a room-temperature precipitation is introduced as a new OER catalyst; this material exhibits a remarkably low overpotential (η ≈ 367 mV for a current density of 10 mA cm-2 ). A structural investigation using X-ray absorption spectroscopy reveals that the amorphous structure contains layered motifs similar to the structure of CoOOH, which is demonstrated to be responsible for the OER catalysis based on density functional theory calculations. However, the calculations also reveal that the local environment of the active site in the layered crystalline motif in the ACP is significantly modulated by the silicate, leading to a substantial reduction of η of the OER compared with that of CoOOH. This work proposes amorphous phyllosilicates as a new group of efficient OER catalysts and suggests that tuning of the catalytic activity by introducing redox-inert groups may be a new unexplored avenue for the design of novel high-performance catalysts.

In Situ Tracking Kinetic Pathways of Li+/Na+ Substitution during Ion-Exchange Synthesis of LixNa1.5–xVOPO4F0.5

Ion exchange is a ubiquitous phenomenon central to wide industrial applications, ranging from traditional (bio)chemical separation to the emerging chimie douce synthesis of materials with metastable structure for batteries and other energy applications. The exchange process is complex, involving substitution and transport of different ions under non-equilibrium conditions, and thus difficult to probe, leaving a gap in mechanistic understanding of kinetic exchange pathways toward final products. Herein, we report in situ tracking kinetic pathways of Li+/Na+ substitution during solvothermal ion-exchange synthesis of LixNa1.5-xVOPO4F0.5 (0 ≤ x ≤ 1.5), a promising multi-Li polyanionic cathode for batteries. The real-time observation, corroborated by first-principles calculations, reveals a selective replacement of Na+ by Li+, leading to peculiar Na+/Li+/vacancy orderings in the intermediates. Contradicting the traditional belief of facile topotactic substitution via solid solution reaction, an abrupt two-phase transformation occurs and predominantly governs the kinetics of ion exchange and transport in the 1D polyanionic framework, consequently leading to significant difference of Li stoichiometry and electrochemical properties in the exchanged products. The findings may help to pave the way for rational design of ion exchange synthesis for making new materials.