Y-doped Li8ZrO6: A Li-Ion Battery Cathode Material with High Capacity

Quantifying Effects of Ligand-Metal Bond Covalency on Oxygen-Redox Electrochemistry in Layered Oxide Cathodes

Oxygen-redox electrochemistry is attracting tremendous attention due to its enhanced energy density for layered oxide cathodes. However, quantified effects of ligand-metal bond covalency on the oxygen-redox behaviors are not fully understood, limiting a rational structure design for enhancing the oxygen redox reversibility. Here, using Li₂Ru_1-xMn_xO₃ (0 ≤ x ≤ 0.8) which includes both 3d- and 4d-based cations as model compounds, we provide a quantified relation between the ligand-metal bond covalency and oxygen-redox electrochemistry. Supported by theoretical calculations, we reveal a linear positive correlation between the transition metal (TM)-O bond covalency and the overlap area of TM nd and O 2p orbitals. Furthermore, based on the electrochemical tests on the Li₂Ru_1-xMn_xO₃ systems, we found that the enhanced TM-O bond covalency can increase the reversibility of oxygen-redox electrochemistry. Due to the strong Ru-O bond covalency, the thus designed Ru-doped Li-rich Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode shows an enhanced initial coulombic efficiency, increased capacity retention, and suppressed voltage decay during cycling. This systematic study provides a rational structure design principle for the development of oxygen-redox-based layered oxide cathodes.

Characterizing Grain Boundary Effects on Mg2+ Conduction in Metal-Organic Frameworks

Next-generation materials for fast ion conduction have the potential to revolutionize battery technology. Metal-organic frameworks (MOFs) are promising candidates for achieving this goal. Given their structural diversity, the design of efficient MOF-based conductors can be accelerated by a detailed understanding and accurate prediction of ion conductivity. However, the polycrystalline nature of solid-state materials requires consideration of grain boundary effects, which is complicated by challenges in characterizing grain boundary structures and simulating ensemble transport processes. To address this, we have developed an approach for modeling ion transport at grain boundaries and predicting their contribution to conductivity. Mg²⁺ conduction in the Mg-MOF-74 thin film was studied as a representative system. Using computational techniques and guided by experiments, we investigated the structural details of MOF grain boundary interfaces to determine accessible Mg²⁺ transport pathways. Computed transport kinetics were input into a simplified MOF nanocrystal model, which combines ion transport in the bulk structure and at grain boundaries. The model predicts Mg²⁺ conductivity in the MOF-74 film within chemical accuracy (<1 kcal/mol activation energy difference), validating our approach. Physically, Mg²⁺ conduction in MOF-74 is inhibited by strong Mg²⁺ binding at grain boundaries, such that only a small fraction of grain boundary alignments allow for fast Mg²⁺ transport. This results in a 2-3 order-of-magnitude reduction in conductivity, illustrating the critical impact of the grain boundary contribution. Overall, our work provides a computation-aided platform for molecular-level understanding of grain boundary effects and quantitative prediction of ion conductivity. Combined with experimental measurements, it can serve as a synergistic tool for characterizing the grain boundary composition of MOF-based conductors.

Li8MnO6: A Novel Cathode Material with Only Anionic Redox

In Li-excess transition metal-oxide cathode materials, anionic oxygen redox can offer high capacity and high voltages, although peroxo and superoxo species may cause oxygen loss, poor cycling performance, and capacity fading. Previous work showed that undesirable formation of peroxide and superoxide bonds was controlled to some extent by Mn substitution, and the present work uses density functional calculations to examine the reasons for this by studying the anionic redox mechanism in Li₈MnO₆. This material is obtained by substituting Mn for Sn in Li₈SnO₆ or for Zr in Li₈ZrO₆, and we also compare this to previous work on those materials. The calculations predict that Li₈MnO₆ is stable at room temperature (with a band gap of 3.19 eV as calculated by HSE06 and 1.82 eV as calculated with the less reliable PBE+U), and they elucidate the chemical and structural effects involved in the inhibition of oxygen release in this cathode. Throughout the whole delithiation process, only O^2- ions are oxidized. The directional Mn-O bonds formed from unfilled 3d orbitals effectively inhibit the formation of O-O bonds, and the layered structure is maintained even after removing 3 Li per Li₈MnO₆ formula unit. The calculated average voltage for removal of 3 Li is 3.69 V by HSE06, and the corresponding capacity is 389 mAh/g. The high voltage of oxygen anionic redox and the high capacity result in a high energy density of 1436 Wh/kg. The Li-ion diffusion barrier for the dominant interlayer diffusion path along the c axis is 0.57 eV by PBE+U. These results help us to understand the oxygen redox mechanism in a new lithium-rich Li₈MnO₆ cathode material and contribute to the design of high-energy density lithium-ion battery cathode materials with favorable electrochemical properties based on anionic oxygen redox.

Thermodynamics and phase stability of Li8XO6octalithium ceramic breeder materials (X= Pb, Ce, Ge, Zr, Sn)

Octalithium ceramics with their high stoichiometric concentration of lithium offer exceptionally high tritium breeding ratios in comparison with other candidate breeder materials for tokamak fusion reactors, this is especially true with incorporation of a neutron multiplier into the crystal structures. Although, there are concerns surrounding the stability of these materials at operational temperatures. Therefore in this paper, we explore the thermodynamic properties of a selection of candidate octalithium ceramics in low and high temperature regimes (0-1200 K) using density functional perturbation theory. Enthalpies as well as Gibbs formation energies were used to distinguish candidates which may or may not be susceptible to degradation.

Theoretical study on Y-doped Na2ZrO3 as a high-capacity Na-rich cathode material based on anionic redox

First-principles calculations based on density functional theory were utilized to study the performance of Na₂ZrO₃ (NZO) and yttrium-doped Na₂ZrO₃ (Y-NZO) as cathode materials for sodium ion batteries (SIBs), including the stability of the desodiated structures, desodiation energy, redox mechanism, and the diffusion of Na. When 62.5% sodium is removed from NZO, its structure and volume change little and the layered structure is retained, whereas the structure starts to distort and shift to the ZrO₃ phase with the extraction of more than 62.5% sodium. As desodiation proceeds, oxygen anions act as the only redox center for charge compensation, yielding a high initial voltage of 4.03 eV vs. Na/Na⁺ by PBE + U-D3 functional and 4.82 eV vs. Na/Na⁺ by HSE06-D3 functional. When the desodiation content is less than 31.25%, O₂^3- is formed with an O-O distance of 2.38 Å. At the desodiation content of 31.25%, peroxide dimer O₂^2- starts to form; at the desodiation content of 56.25%, the O-O bond distance is further shortened to 1.3 Å, corresponding to the formation of superoxide O₂^-. However, for Y-NZO, the redox reaction firstly involves O^2-/O^1-, which does not occur in NZO. Peroxides and superoxides appear when the sodium removal concentration is 59.38% and 75%, respectively. This indicates that the O-O dimers appear in Y-NZO at much deeper sodium removal. The calculations of diffusion paths and barriers of Na ions in NZO by PBE + U-D3 predict that the barrier of Na escaping from the mixed layer to the Na layer in NZO is 0.48 eV (the reverse barrier is 0.76 eV), smaller than those of other O3-type layered transition metal compounds, such as Na₂IrO₃ and Na₂RuO₃. After yttrium doping, the diffusion of Na ions becomes easier, indicating that the Y-doping improves the diffusion ability. This investigation interprets the mechanism of oxygen oxidation of NZO as a cathode for SIBs, and provides theoretical support for a better design of Na-rich layered oxide Na₂MO₃ (M represents the transition metal element) in the future research.

Lithium-Cation Conductivity of Solid Solutions in Li6-xZr2-xAxO7 (A = Nb, Ta) Systems

Li6-xZr2-xAxO7 (A = Nb; Ta) system with 0 < x < 0.30 is synthesized by glycine-nitrate method. Boundaries of solid solutions based on monoclinic Li6Zr2O7 are determined; temperature (200-600 °C) and concentration dependences of conductivity are investigated. It is shown that monoclinic Li6Zr2O7 exhibits better transport properties compared to its triclinic modification. Li5.8Zr1.8Nb(Ta)0.2O7 solid solutions have a higher lithium-cation conductivity at 300 °C compared to solid electrolytes based on other lithium zirconates due the "open" structure of monoclinic Li6Zr2O7 and a high solubility of the doping cations.

Iridate Li8IrO6: An Antiferromagnetic Insulator

A polycrystalline iridate Li₈IrO₆ material was prepared via heating Li₂O and IrO₂ starting materials in a sealed quartz tube at 650 °C for 48 h. The structure was determined from Rietveld refinement of room-temperature powder neutron diffraction data. Li₈IrO₆ adopts the nonpolar space group R3̅ with Li atoms occupying the tetrahedral and octahedral sites, which is supported by the electron diffraction and solid-state ⁷Li NMR. This results in a crystal structure consisting of LiO₄ tetrahedral layers alternating with mixed IrO₆ and LiO₆ octahedral layers along the crystallographic c-axis. The +4 oxidation state of Ir⁴⁺ was confirmed by near-edge X-ray absorption spectroscopy. An in situ synchrotron X-ray diffraction study of Li₈IrO₆ indicates that the sample is stable up to 1000 °C and exhibits no structural transitions. Magnetic measurements suggest long-range antiferromagnetic ordering with a Néel temperature (T_N) of 4 K, which is corroborated by heat capacity measurements. The localized effective moment μ_eff (Ir) = 1.73 μ_B and insulating character indicate that Li₈IrO₆ is a correlated insulator. First-principles calculations support the nonpolar crystal structure and reveal the insulating behavior both in paramagnetic and antiferromagnetic states.

Cerium-doped bimetal organic framework as a superhigh capacity cathode for rechargeable alkaline batteries

In this work, cerium (Ce)-doped NiCo-MOF (metal organic framework) was investigated for its application as a cathode material of alkaline batteries. Inert substitution of Ni/Co by Ce in MOF can make Ce to become part of the backbone of the framework and then ensure structure stability during redox reaction, which greatly improved charge and discharge cycle stability. With dopant mole fraction up to 5%, the redox potential of NiCo-MOF increased by 85%. Adequate Ce doping can potentially enhance rate capacity dramatically due to the large ion radius that provided an extra space for electrolyte ion shutting channel. 1% Ce-doped NiCo-MOF, having a capacity of 286 mA h g^-1 at 2 A g^-1 and retaining 93% of its capacity (265 mA h g^-1) at 20 A g^-1, emerged as the best performing material among all the Ce-doped NiCo-MOFs presented in this study. A full cell coupling Ce-doped NiCo-MOF cathode and Fe₂O₃ anode was assembled to verify its practical application. The full cell showed an initial capacity of 280 mA h g^-1 at 2 A g^-1 and retained 92% after 1000 charge and discharge cycles. Therefore, Ce doping emerges as a powerful strategy for the designing of cathode materials used in advanced alkaline battery.

Polyhedral perspectives on the capacity limit of cathode compounds for lithium-ion batteries: a case study for Li6CoO4

Anionic redox revealed reversibility in Li-rich layered oxides Li2MO3, which was strongly dependent on transition metal element M and geometrical structures. This sheds new light on high energy lithium ion batteries, and also inspires the question whether super high capacity is achievable in lithium compounds with stoichiometry close to Li2O. The tetrahedron structured Li6CoO4 is one kind of oxides with extremely high Li stoichiometry. In this study, DFT calculations combined with ex situ experimental stoichiometry detection are performed to investigate the delithiation mechanism during its full range. It reveals that Li6CoO4 undergoes two distinct delithiation reactions. The first process is a topotactic delithiation with conventional oxidation of Co2+ to Co3+ then continuing to Co4+; and the successive one is suggested to be decomposition reaction to Li2O and cobalt oxides. Surprisingly, very dense and uniform cracks are present over the cross-section of the micron-sized particles even at the early stage of charging with a capacity of 320 mA h g-1, the EDS of which suggests that the delithiated phase is homogeneous Li4CoO4. This phenomenon may be attributed to the unusually large discrepancy between ionic and electronic conductivity. CI-NEB calculations show a barrier of ca. 0.31 eV for the two dimensional Li ion migration network, corresponding to an ionic conductivity in the order of 10-6 S cm-1. On the other hand, there is lack of an effective path for electron hopping, because CoO4 tetrahedra are isolated from each other, pointing to electronic conductivity lower than 10-14 S cm-1. This study proposes a strategy to achieve super high capacity by invoking a reversible anionic redox to replace the decomposition reaction in tetrahedron structured lithium compounds. It is also worth pointing out that the geometrical connectivity of MO4 is crucial in the design of a new generation of cathode materials.

Ab initio identification of the Li-rich phase in LiFePO4

This article provides a systematic theoretical study of Li-rich phase Li_1+xFe_1−xPO₄ (x ≤ 12.5%) cathode materials for lithium-ion batteries.

Anionic Redox in Rechargeable Lithium Batteries

The extraordinarily high capacities delivered by lithium-rich oxide cathodes, compared with conventional layered oxide electrodes, are a result of contributions from both cationic and anionic redox processes. This phenomenon has invoked a lot of research exploring new kinds of lithium-rich oxides with multiple-electron redox processes. Though proposed many years ago, anionic redox is now regarded to be crucial in further developing high-capacity electrodes. A basic overview of the previous work on anionic redox is given, and issues related to electronic and geometric structures are discussed, including the principles of activation, reversibility, and the energy barrier of anionic redox. Anionic redox also leads to capacity loss and structural degradation, as well as voltage hysteresis, which shows the importance of controlling anionic redox reactions. Finally, the techniques used for characterizing anionic redox processes are reviewed to aid the rational choice of techniques in future studies. Important perspectives are highlighted, which should instruct future work concerning anionic redox processes.

Nanofabrication strategies for advanced electrode materials

The development of advanced electrode materials for high-performance energy storage devices becomes more and more important for growing demand of portable electronics and electrical vehicles. To speed up this process, rapid screening of exceptional materials among various morphologies, structures and sizes of materials is urgently needed. Benefitting from the advance of nanotechnology, tremendous efforts have been devoted to the development of various nanofabrication strategies for advanced electrode materials. This review focuses on the analysis of novel nanofabrication strategies and progress in the field of fast screening advanced electrode materials. The basic design principles for chemical reaction, crystallization, electrochemical reaction to control the composition and nanostructure of final electrodes are reviewed. Novel fast nanofabrication strategies, such as burning, electrochemical exfoliation, and their basic principles are also summarized. More importantly, colloid system served as one up-front design can skip over the materials synthesis, accelerating the screening rate of highperformance electrode. This work encourages us to create innovative design ideas for rapid screening high-active electrode materials for applications in energy-related fields and beyond.

Polyanion-Type Electrode Materials for Sodium-Ion Batteries

Sodium-ion batteries, representative members of the post-lithium-battery club, are very attractive and promising for large-scale energy storage applications. The increasing technological improvements in sodium-ion batteries (Na-ion batteries) are being driven by the demand for Na-based electrode materials that are resource-abundant, cost-effective, and long lasting. Polyanion-type compounds are among the most promising electrode materials for Na-ion batteries due to their stability, safety, and suitable operating voltages. The most representative polyanion-type electrode materials are Na3V2(PO4)3 and NaTi2(PO4)3 for Na-based cathode and anode materials, respectively. Both show superior electrochemical properties and attractive prospects in terms of their development and application in Na-ion batteries. Carbonophosphate Na3MnCO3PO4 and amorphous FePO4 have also recently emerged and are contributing to further developing the research scope of polyanion-type Na-ion batteries. However, the typical low conductivity and relatively low capacity performance of such materials still restrict their development. This paper presents a brief review of the research progress of polyanion-type electrode materials for Na-ion batteries, summarizing recent accomplishments, highlighting emerging strategies, and discussing the remaining challenges of such systems.