リチウムイオン電池の正極界面反応の機構解明

平成24年度 京都大学化学研究所 スーパーコンピュータシステム 利用報告

Multiscale and hierarchical reaction mechanism in a lithium-ion battery

The key to improving the performance of lithium-ion batteries is to precisely elucidate the temporal and spatial hierarchical structure of the battery. Lithium-ion batteries consist of cathodes and anodes and a separator containing an electrolyte. The cathodes and anodes of lithium-ion batteries are made of a composite material consisting of an active material, a conductive material, and a binder to form a complex three-dimensional structure. The reaction proceeds as lithium ions are repeatedly inserted into and removed from the active material. Therefore, the lattice of the active material is restructured due to ion diffusion, which results in phase change. At the active material–electrolyte interface, the insertion and de-insertion of lithium ions proceed with the charge transfer reaction. The charge–discharge reaction of a lithium-ion battery is a nonequilibrium state due to the interplay of multiple phenomena. Analysis after disassembling a battery, which is performed in conventional battery research, does not provide an accurate understanding of the dominant factors of the reaction rate and the degradation mechanism, in some cases. This review introduces the results of research on the temporal and spatial hierarchical structure of lithium-ion batteries, focusing on operando measurements taken during charge–discharge reactions. Chapter 1 provides an overview of the hierarchical reaction mechanism of lithium-ion batteries. Chapter 2 introduces the operando measurement technique, which is useful for analysis. Chapter 3 describes the reaction at the electrode–electrolyte interface, which is the reaction field, and Chapter 4 discusses the nonequilibrium structural change caused by the two-phase reaction in the active material. Chapter 5 introduces the study of the unique reaction heterogeneity of a composite electrode, which enables practical energy storage. Understanding the hierarchical reaction mechanism will provide useful information for the design of lithium-ion batteries and next-generation batteries

Phase Evolution of Trirutile Li₀.₅FeF₃ for Lithium-Ion Batteries

Extensive studies on trirutile Li₀.₅FeF₃ phase have been commissioned in the context of the Li–Fe–F system for Li-ion batteries. However, progress in electrochemical and structural studies has been greatly encumbered by the low electrochemical reactivity of this material. In order to advance this class of materials, a comprehensive study into the mechanisms of this phase is necessary. Therefore, herein, we report for the first time overall reaction mechanisms of ordered trirutile Li₀.₅FeF₃ at elevated temperatures of 90 °C with the aid of a thermally stable ionic liquid electrolyte. Ordered trirutile Li₀.₅FeF₃ is prepared by high-energy ball milling combined with heat treatment followed by electrochemical tests, X-ray diffraction, and X-ray absorption spectroscopic analyses. Our results reveal that a reversible topotactic Li⁺ extraction/insertion from/into the trirutile structure occurs in a two-phase reaction with a minor volume change (1.09% between Li₀.₅FeF₃ and Li₀.₁₁FeF₃) in the voltage range of 3.2–4.3 V. The extension of the lower cutoff voltage to 2.5 V results in a conversion reaction to LiF and rutile FeF₂ during discharging. The subsequent charge triggers the formation of the disordered trirutile structure at 4.3 V without showing the reconversion from LiF and rutile FeF₂ to ordered trirutile Li₀.₅FeF₃ or FeF₃

Crystalline maricite NaFePO₄ as a positive electrode material for sodium secondary batteries operating at intermediate temperature

Maricite NaFePO₄ (m-NaFePO₄) was investigated as a positive electrode material for intermediate-temperature operation of sodium secondary batteries using ionic liquid electrolytes. Powdered m-NaFePO₄ was prepared by a conventional solid-state method at 873 K and subsequently fabricated in two different conditions; one is ball-milled in acetone and the other is re-calcined at 873 K after the ball-milling. Electrochemical properties of the electrodes prepared with the as-synthesized m-NaFePO₄, the ball-milled m-NaFePO₄, and the re-calcined m-NaFePO₄ were investigated in Na[FSA]-[C₂C₁im][FSA] (C₂C₁im⁺ = 1-ethyl-3-methylimidazolium, FSA⁻ = bis(fluorosulfonyl)amide) ionic liquid electrolytes at 298 K and 363 K to assess the effects of temperature and particle size on their electrochemical properties. A reversible charge-discharge capacity of 107 mAh g⁻¹ was achieved with a coulombic efficiency >98% from the 2nd cycle using the ball-milled m-NaFePO₄ electrode at a C–rate of 0.1 C and 363 K. Electrochemical impedance spectroscopy using m-NaFePO₄/m-NaFePO₄ symmetric cells indicated that inactive m-NaFePO₄ becomes an active material through ball-milling treatment and elevation of operating temperature. X-ray diffraction analysis of crystalline m-NaFePO₄ confirmed the lattice contraction and expansion upon charging and discharging, respectively. These results indicate that the desodiation-sodiation process in m-NaFePO₄ is reversible in the intermediate-temperature range

Honeycomb Layered Frameworks with Metallophilic Bilayers

Honeycomb layered frameworks with metallophilic bilayers have garnered traction in various disciplines due to their unique configuration and numerous physicochemical and topological properties, such as fast ionic conduction, coordination chemistry, and structural defects. These properties make them attractive for energy storage applications, leading to increased attention towards their metallophilic bilayer arrangements. This Review focuses on recent advancements in this field, including characterisation techniques like X-ray absorption spectroscopy and high-resolution transmission electron microscopy, particularly for silver-based oxides. It also highlights strategies related to cationic-deficient phases induced by topology or temperature, expanding the compositional space of honeycomb layered frameworks with a focus on cationic bilayer architectures. The Review further discusses theoretical approaches for understanding the bilayered structure, especially concerning critical phenomena at the monolayer-bilayer phase transition. Honeycomb layered frameworks are described as optimised lattices within the congruent sphere packing problem, equivalent to a specific two-dimensional conformal field theory. The monolayer-bilayer phase transition involves a 2D-to-3D crossover. Overall, this Review aims to provide a panoramic view of honeycomb layered frameworks with metallophilic bilayers and their potential applications in the emerging field of quantum matter. It is valuable for recent graduates and experts alike across diverse fields, extending beyond materials science and chemistry.Comment: 68 pages, 24 figure

Cation-Disordered Li3VO4: Reversible Li Insertion/Deinsertion Mechanism for Quasi Li-Rich Layered Li1+x[V1/2Li1/2]O2 (x = 0–1)

The reversible lithiation/delithiation mechanism of the cation-disordered Li3VO4 material was elucidated, including the understanding of structural and electrochemical signature changes during cycling. The initial exchange of two Li induces a progressive and irreversible migration of Li and V ions from tetrahedral to octahedral sites, confirmed by the combination of in situ/operando X-ray diffraction and X-ray absorption fine structure analyses. The resulting cation-disordered Li3VO4 can smoothly and reversibly accommodate two Li and shows a Li+ diffusion coefficient larger by 2 orders of magnitude than the one of pristine Li3VO4, leading to improved electrochemical performance. This cation-disordered Li3VO4 negative electrode offers new opportunities for designing high-energy and high-power supercapacitors. Furthermore, it opens new paths for preparing disordered compounds with the general hexagonal close-packing structure, including most polyanionic compounds, whose electrochemical performance can be easily improved by simple cation mixing

A reversible oxygen redox reaction in bulk-type all-solid-state batteries

An all-solid-state lithium battery using inorganic solid electrolytes requires safety assurance and improved energy density, both of which are issues in large-scale applications of lithium-ion batteries. Utilization of high-capacity lithium-excess electrode materials is effective for the further increase in energy density. However, they have never been applied to all-solid-state batteries. Operational difficulty of all-solid-state batteries using them generally lies in the construction of the electrode-electrolyte interface. By the amorphization of Li₂RuO₃ as a lithium-excess model material with Li₂SO₄, here, we have first demonstrated a reversible oxygen redox reaction in all-solid-state batteries. Amorphous nature of the Li₂RuO₃-Li₂SO₄ matrix enables inclusion of active material with high conductivity and ductility for achieving favorable interfaces with charge transfer capabilities, leading to the stable operation of all-solid-state batteries

Ultrafast charge–discharge characteristics of a nanosized core–shell structured LiFePO4 material for hybrid supercapacitor applications

Highly dispersed crystalline/amorphous LiFePO4 (LFP) nanoparticles encapsulated within hollow-structured graphitic carbon were synthesized using an in situ ultracentrifugation process. Ultracentrifugation triggered an in situ sol–gel reaction that led to the formation of core–shell LFP simultaneously hybridized with fractured graphitic carbon. The structure has double cores that contain a crystalline LFP (core 1) covered by an amorphous LFP containing Fe3+ defects (core 2), which are encapsulated by graphitic carbon (shell). These core–shell LFP nanocomposites show improved Li+ diffusivity thanks to the presence of an amorphous LFP phase. This material enables ultrafast discharge rates (60 mA h g-1 at 100C and 36 mA h g-1 at 300C) as well as ultrafast charge rates (60 mA h g-1 at 100C and 36 mA h g-1 at 300C). The synthesized core–shell nanocomposites overcome the inherent one-dimensional diffusion limitation in LFP and yet deliver/store high electrochemical capacity in both ways symmetrically up to 480C. Such a high rate symmetric capacity for both charge and discharge has never been reported so far for LFP cathode materials. This offers new opportunities for designing high-energy and high-power hybrid supercapacitors