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

Na₃V(PO₄)₂: A New Layered-Type Cathode Material with High Water Stability and Power Capability for Na-Ion Batteries

We introduce Na₃V­(PO₄)₂ as a new cathode material for Na-ion batteries for the first time. The structure of Na₃V­(PO₄)₂ was determined using X-ray diffraction and Rietveld refinement, and its high water stability was clearly demonstrated. The redox potential of Na₃V­(PO₄)₂ (∼3.5 V vs Na/Na⁺) was shown to be sufficiently high to prevent the side reaction with water (Na extraction and water insertion), ensuring its water stability in ambient air. Na₃V­(PO₄)₂ also exhibited outstanding power capability, with ∼79% of the theoretical capacity being delivered at 15C. First-principles calculation combined with electrochemical experiments linked this high power capability to the low activation barrier (∼433 meV) for the well-interconnected two-dimensional Na diffusion pathway. Moreover, outstanding cyclability of Na₃V­(PO₄)₂ (∼70% retention of the initial capacity after 200 cycles) was achieved at a reasonably fast current rate of 1C

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

Simple and Effective Gas-Phase Doping for Lithium Metal Protection in Lithium Metal Batteries

Increasing demands for advanced lithium batteries with higher energy density have resurrected the use of lithium metal as an anode, whose practical implementation has still been restricted, because of its intrinsic problems originating from the high reactivity of elemental lithium metal. Herein, we explore a facile strategy of doping gas phase into electrolyte to stabilize lithium metal and suppress the selective lithium growth through the formation of stable and homogeneous solid electrolyte interphase (SEI) layer. We find that the sulfur dioxide gas additive doped in electrolyte significantly improves both chemical and electrochemical stability of lithium metal electrodes. It is demonstrated that the cycle stability of the lithium cells can be remarkably prolonged, because of the compact and homogeneous SEI layers consisting of Li–S–O reduction products formed on the lithium metal surface. Simulations on the lithium metal growth process suggested the homogeneity of the protective layer induced by the gas-phase doping is attributable for the effective prevention of the selective growth of lithium metal. This study introduces a new simple approach to stabilize the lithium metal electrode with gas-phase doping, where the SEI layer can be rationally tunable by the composition of gas phase

Tailored Oxygen Framework of Li₄Ti₅O₁₂ Nanorods for High-Power Li Ion Battery

Here we designed the kinetically favored Li₄Ti₅O₁₂ by modifying its crystal structure to improve intrinsic Li diffusivity for high power density. Our first-principles calculations revealed that the substituted Na expanded the oxygen framework of Li₄Ti₅O₁₂ and facilitated Li ion diffusion in Li₄Ti₅O₁₂ through 3-D high-rate diffusion pathway secured by Na ions. Accordingly, we synthesized sodium-substituted Li₄Ti₅O₁₂ nanorods having not only a morphological merit from 1-D nanostructure engineering but also sodium substitution-induced open framework to attain ultrafast Li diffusion. The new material exhibited an outstanding cycling stability and capacity retention even at 200 times higher current density (20 C) compared with the initial condition (0.1 C)

Bifunctional MnO₂‑Coated Co₃O₄ Hetero-structured Catalysts for Reversible Li‑O₂ Batteries

The structural design and synthesis of effective cathode catalysts are important concerns for achieving rechargeable Li-O₂ batteries. In this study, hexagonal Co₃O₄ nanoplatelets coated with MnO₂ were synthesized as bifunctional catalysts for Li-O₂ batteries. The oxygen reduction reaction catalyst (MnO₂) was closely integrated on the surface of the oxygen evolution reaction catalyst (hexagonal Co₃O₄) so that this hetero-structured catalyst (HSC) hybrid would show bifunctional catalytic activity in Li-O₂ batteries. A facile synthesis route was developed to form a unique HSC structure, with {111} facet-exposed Co₃O₄ decorated with perpendicularly arranged MnO₂ flakes. The catalytic activity of the HSCs was controlled by tuning the ratio of Co to Mn (the ratio of OER to ORR catalysts) in the hybrids. With the optimized Co₃O₄-to-MnO₂ ratio of 5:3, a Li-O₂ cell containing the HSC showed remarkably enhanced electrochemical performance, including discharge capacity, energy efficiency, and especially cycle performance, compared to cells with a monofunctional catalyst and a powder mixture of Co₃O₄ and MnO₂. The results demonstrate the feasibility of reversible Li-O₂ batteries with bifunctional catalyst hybrids

Hollow Nanostructured Metal Silicates with Tunable Properties for Lithium Ion Battery Anodes

Hollow nanostructured materials have attracted considerable interest as lithium ion battery electrodes because of their good electrochemical properties. In this study, we developed a general procedure for the synthesis of hollow nanostructured metal silicates via a hydrothermal process using silica nanoparticles as templates. The morphology and composition of hollow nanostructured metal silicates could be controlled by changing the metal precursor. The as-prepared hierarchical hollow nanostructures with diameters of ∼100–200 nm were composed of variously shaped primary particles such as hollow nanospheres, solid nanoparticles, and thin nanosheets. Furthermore, different primary nanoparticles could be combined to form hybrid hierarchical hollow nanostructures. When hollow nanostructured metal silicates were applied as anode materials for lithium ion batteries, all samples exhibited good cyclic stability during 300 cycles, as well as tunable electrochemical properties

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

High-Rate and High-Areal-Capacity Air Cathodes with Enhanced Cycle Life Based on RuO₂/MnO₂ Bifunctional Electrocatalysts Supported on CNT for Pragmatic Li–O₂ Batteries

Despite their potential to provide high energy densities, lithium–oxygen (Li–O₂) batteries are not yet widely used in ultrahigh energy density devices like electric vehicles, owing to various challenges, including poor cyclability, low efficiency, and poor rate capability, especially at high areal mass loading. Even the most promising Li–O₂ cells are unsuitable for practical applications, owing to a limited areal mass loading below 1 mg cm^–2, resulting in low areal capacity. Here, we demonstrate air cathodes of unprecedentedly high areal capacity at a high rate with sufficient cycle life for pragmatic operation of Li–O₂ batteries. A separator-carbon nanotube (CNT) monolith-type cathode of massive loading is prepared to achieve high areal capacity, but the cycle life and round-trip efficiency of CNT-only separator monolith cathodes are limited. The reversible and energy-efficient operation at high areal capacity and a high rate is enabled by adopting RuO₂/MnO₂ solid catalysts on the CNT (RMCNT). RMCNTs show a bifunctional catalytic effect in both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) and also completely decompose LiOH and Li₂CO₃ byproducts that may exist in discharged electrodes. This separator-RMCNT monolith offers beneficial features such as high mass loading, binder-free, intimate contact with the separator, and most importantly, catalysts for reversibility. Together, these features provide a remarkably long cycle life at unprecedentedly high capacity and high rate: 315, 45, and 40 cycles, with areal capacity limits of 1.5, 3.0, and 4.5 mAh cm^–2, respectively, at a rate of 1.5 mA cm^–2. Cycling is possible even at the curtailing capacity of 10 mAh cm^–2

Efficient Method of Designing Stable Layered Cathode Material for Sodium Ion Batteries Using Aluminum Doping

Despite their high specific capacity, sodium layered oxides suffer from severe capacity fading when cycled at higher voltages. This key issue must be addressed in order to develop high-performance cathodes for sodium ion batteries (SIBs). Herein, we present a comprehensive study on the influence of Al doping of Mn sites on the structural and electrochemical properties of a P2–Na_0.5Mn_{0.5–<i>x</i>}Al_<i>x</i>Co_0.5O₂ (<i>x</i> = 0, 0.02, or 0.05) cathode for SIBs. Detailed structural, morphological, and electrochemical investigations were carried out using X-ray diffraction, cyclic voltammetry, and galvanostatic charge–discharge measurements, and some new insights are proposed. Rietveld refinement confirmed that Al doping caused TMO₆ octahedra (TM = transition metal) shrinkage, resulting in wider interlayer spacing. After optimizing the aluminum concentration, the cathode exhibited remarkable electrochemical performance, with better stability and improved rate performance. Electrochemical impedance spectroscopy (EIS) measurements were performed at various states of charge to probe the surface and bulk effects of Al doping. The material presented here exhibits exceptional stability over 100 cycles within a 1.5–4.3 V window and outperforms several other Mn–Co-based cathodes for SIBs. This study presents a facile method for designing structurally stable cathodes for SIBs