Redesign of Li₂MP₂O₇ (M = Fe or Mn) by Tuning the Li Diffusion in Rechargeable Battery Electrodes

Defects in crystals such as antisites generally lead to the deterioration of the ionic conductivity of solid-state ionic conductors. Herein, using first-principles calculations, we demonstrate that the Li diffusion in Li₂MP₂O₇ (M = Fe or Mn), a promising battery material, is sensitively affected by the presence of Li/M antisites; however, unexpectedly, the antisites significantly promote Li diffusion. The calculations reveal that the presence of antisites reduces the barrier of Li hopping and opens new paths for Li diffusion in the Li₂MP₂O₇ crystal. In our experimental verification, we succeeded in synthesizing crystalline Li₂MnP₂O₇ with varying Li/Mn antisite contents and demonstrated that the inclusion of antisites results in improved power capability with faster Li diffusion for Li-ion battery electrodes. We believe that this unexpected finding of increasing the ionic conductivity by introducing antisite defects broadens our understanding of solid-state ionic conductors and provides a new strategy for improving Li diffusion in conventional electrode materials for Li rechargeable batteries

High Energy Organic Cathode for Sodium Rechargeable Batteries

Organic electrodes have attracted significant attention as alternatives to conventional inorganic electrodes in terms of sustainability and universal availability in natural systems. However, low working voltages and low energy densities are inherent limitations in cathode applications. Here, we propose a high-energy organic cathode using a quinone-derivative, C₆Cl₄O₂, for use in sodium-ion batteries, which boasts one of the highest average voltages among organic electrodes in sodium batteries (∼2.72 V vs Na/Na⁺). It also utilizes a two-electron transfer to provide an energy of 580 Wh kg^–1. Density functional theory (DFT) calculations reveal that the introduction of electronegative elements into the quinone structure significantly increased the sodium storage potential and thus enhanced the energy density of the electrode, the latter being substantially higher than previously known quinone-derived cathodes. The cycle stability of C₆Cl₄O₂ was enhanced by incorporating the C₆Cl₄O₂ into a nanocomposite with a porous carbon template. This prevented the dissolution of active molecules into the surrounding electrolyte

Theoretical Evidence for Low Charging Overpotentials of Superoxide Discharge Products in Metal–Oxygen Batteries

Li–oxygen and Na–oxygen batteries are some of the most promising next-generation battery systems because of their high energy densities. Despite the chemical similarity of Li and Na, the two systems exhibit distinct characteristics, especially the typically higher charging overpotential observed in Li–oxygen batteries. In previous theoretical and experimental studies, this higher charging overpotential was attributed to factors such as the sluggish oxygen evolution or poor transport property of the discharge product of the Li–oxygen cell; however, a general understanding of the interplay between the discharge products and overpotential remains elusive. Here, we investigated the charging mechanisms with respect to the oxygen evolution reaction (OER) kinetics, charge-carrier conductivity, and dissolution property of various discharge products reported in Li–oxygen and Na–oxygen cells. The OER kinetics were generally faster for superoxides (i.e., LiO₂ and NaO₂) than for peroxides (i.e., Li₂O₂ and Na₂O₂). The electronic and ionic conductivities were also predicted to be significantly higher in superoxide phases than in peroxide phases. Moreover, systematic calculations of the dissolution energy of the discharge products in the electrolyte, which mediate a solution-based OER reaction, revealed that the superoxide phases, particularly NaO₂, exhibited markedly low dissolution energy compared with the peroxide phases. These results imply that the formation of superoxides instead of peroxides during discharge may be the key to improving the energy efficiency of metal–oxygen batteries in general

RIS-empowered LEO satellite networks for 6G: promising usage scenarios and future directions

Low-Earth orbit (LEO) satellite systems have been deemed a promising key enabler for current 5G and the forthcoming 6G wireless networks. Such LEO satellite constellations can provide worldwide three-dimensional coverage, high data rate, and scalability, thus enabling truly ubiquitous connectivity. On the other hand, another promising technology, reconfigurable intelligent surfaces (RISs), has emerged with favorable features, such as flexible deployment, cost & power efficiency, less transmission delay, noise-free nature, and in-band full-duplex structure. LEO satellite networks have many practical imperfections and limitations; however, exploiting RISs has been shown to be a potential solution to overcome these challenges. Particularly, RISs can enhance link quality, reduce the Doppler shift effect, and mitigate inter-/intra beam interference. In this article, we delve into exploiting RISs in LEO satellite networks. First, we present a holistic overview of LEO satellite communication and RIS technology, highlighting potential benefits and challenges. Second, we describe promising usage scenarios and applications in detail. Finally, we discuss potential future directions and challenges on RIS-empowered LEO networks, offering futuristic visions of the upcoming 6G era.</p

RIS-empowered LEO satellite networks for 6G: promising usage scenarios and future directions

Low-Earth orbit (LEO) satellite systems have been deemed a promising key enabler for current 5G and the forthcoming 6G wireless networks. Such LEO satellite constellations can provide worldwide three-dimensional coverage, high data rate, and scalability, thus enabling truly ubiquitous connectivity. On the other hand, another promising technology, reconfigurable intelligent surfaces (RISs), has emerged with favorable features, such as flexible deployment, cost & power efficiency, less transmission delay, noise-free nature, and in-band full-duplex structure. LEO satellite networks have many practical imperfections and limitations; however, exploiting RISs has been shown to be a potential solution to overcome these challenges. Particularly, RISs can enhance link quality, reduce the Doppler shift effect, and mitigate inter-/intra beam interference. In this article, we delve into exploiting RISs in LEO satellite networks. First, we present a holistic overview of LEO satellite communication and RIS technology, highlighting potential benefits and challenges. Second, we describe promising usage scenarios and applications in detail. Finally, we discuss potential future directions and challenges on RIS-empowered LEO networks, offering futuristic visions of the upcoming 6G era.</p

First-Principles Study of the Reaction Mechanism in Sodium–Oxygen Batteries

Li/O₂ battery has the highest theoretical energy density among any battery systems reported to date. However, its poor cycle life and unacceptable energy efficiency from a high charging overpotential have been major limitations. Recently, much higher energy efficiency with low overpotential was reported for a new metal/oxygen system, Na/O₂ battery. This finding was unexpected since the general battery mechanism of the Na/O₂ system was assumed to be analogous to that of the Li/O₂ cell. Furthermore, it implies that fundamentally different kinetics are at work in the two systems. Here, we investigated the reaction mechanisms in the Na/O₂ cell using first-principles calculations. In comparative study with the Li/O₂ cell, we constructed the phase stability maps of the reaction products of Na/O₂ and Li/O₂ batteries based on the oxygen partial pressure, which explained why certain phases should be the main discharge products under different operating conditions. From surface calculations of NaO₂, Na₂O₂, and Li₂O₂ during the oxygen evolution reaction, we also found that the minimum energy barrier for the NaO₂ decomposition was substantially lower than that of Li₂O₂ decomposition on major surfaces providing a hint for low charging overpotential of Na/O₂ battery

Native Defects in Li₁₀GeP₂S₁₂ and Their Effect on Lithium Diffusion

Defects in crystals alter the intrinsic nature of pristine materials including their electronic/crystalline structure and charge-transport characteristics. The ionic transport properties of solid-state ionic conductors, in particular, are profoundly affected by their defect structure. Nevertheless, a fundamental understanding of the defect structure of one of the most extensively studied lithium superionic conductors, Li₁₀GeP₂S₁₂, remains elusive because of the complexity of the structure; the effects of defects on lithium diffusion and the potential to control defects by varying synthetic conditions also remain unknown. Herein, we report, for the first time, a comprehensive first-principles study on native defects in Li₁₀GeP₂S₁₂ and their effect on lithium diffusion. We provide the complete defect profile of Li₁₀GeP₂S₁₂ and identify major defects that are easily formed regardless of the chemical environment while the presence of path-blocking defects is sensitively dependent on the synthetic conditions. Moreover, using <i>ab initio</i> molecular dynamics simulation, it is demonstrated that the major defects in Li₁₀GeP₂S₁₂ significantly alter the diffusion process. The defects generally facilitate lithium diffusion in Li₁₀GeP₂S₁₂ by enhancing the charge carrier concentration and flattening the site energy landscape. This work delivers a comprehensive picture of the defect chemistry and structural insights for fast lithium diffusion of Li₁₀GeP₂S₁₂-type conductors