Metal-Metal Bonding as an Electrode Design Principle in the Low-Strain Cluster Compound LiScMo3O8

Redox Mechanisms upon the Lithiation of Wadsley-Roth Phases

The Wadsley-Roth family of transition metal oxide phases are a promising class of anode materials for Li-ion batteries due to their open crystal structures and their ability to intercalate Li at high rates. Unfortunately, most early transition metal oxides that adopt a Wadsley-Roth crystal structure intercalate Li at voltages that are too high for most battery applications. First-principles electronic structure calculations are performed to elucidate redox mechanisms in Wadsley-Roth phases with the aim of determining how they depend on crystal structure. A comparative study of two very distinct polymorphs of Nb2O5 reveal two redox mechanisms: (i) an atom-centered redox mechanism at early stages of Li intercalation and (ii) a redox mechanism at intermediate to high Li concentrations involving the bonding orbitals of metal-metal dimers formed by edge-sharing Nb cations. Our study motivates several design principles to guide the development of new Wadsley-Roth phases with superior electrochemical properties.

Crystal Engineering of Silica Anode Achieving Intrinsic Zero-Strain

Si-based anodes have large intrinsic volume expansion, which hinders their practicality and commercialization. To address this challenge, the design principle of intrinsic zero-strain anodes (① large intracrystalline cavities and ② strong bonds) is proposed, and silica with large intracrystalline cavities (SLIC) established by strong Si─O bonds ([SiO4 ] coordinate structures) is obtained and acts as an anode, achieving the intrinsic zero-strain feature first in silicon-based anodes. The phase structure of SLIC is maintained and the [SiO4 ] coordinate structure merely shows slight disorder during cycling. The feature stems from lithiation taking place by the solid-solution insertion reaction rather than the conventional conversion/alloying addition reactions, because the solid-solution insertion reaction for the SLIC has the lowest change in the Gibbs free energy. The SLIC anode demonstrates excellent cycling stability and high initial Coulombic efficiency (≈85%). Moreover, owing to the low working voltage (≈0.28 V) and relatively high specific capacity, the SLIC anode presents the highest gravimetric energy density among reported zero-/quasi-zero-strain anodes and high volumetric energy density (around twice as much as graphite). The universality of the designing principle is also validated. This work provides design guidelines for zero-strain anodes in next-generation batteries.

Redox Mechanisms, Structural Changes, and Electrochemistry of the Wadsley-Roth LixTiNb2O7 Electrode Material

The TiNb2O7 Wadsley-Roth phase is a promising anode material for Li-ion batteries, enabling fast cycling and high capacities. While already used in commercial batteries, many fundamental electronic and thermodynamic properties of LixTiNb2O7 remain poorly understood. We report on an in-depth first-principles study of the redox mechanisms, structural changes, and electrochemical properties of LixTiNb2O7 as a function of Li concentration. First-principles electronic structure calculations reveal an unconventional redox mechanism upon Li insertion that results in the formation of metal-metal bonds. This metal dimer redox mechanism has important structural consequences as it results in a shortening of cation-pair distances, which in turn affects lattice parameters of the host and thereby alters Li site preferences as the Li concentration is varied. The new insights about redox mechanisms in TiNb2O7 and their effect on the structure and Li site preferences provide guidance on how the electrochemical properties of a promising class of anode materials can be tailored by exploiting the tremendous structural and chemical diversity of Wadsley-Roth phases.

Lattice-Strain Engineering for Heterogenous Electrocatalytic Oxygen Evolution Reaction

The energy efficiency of metal-air batteries and water-splitting techniques is severely constrained by multiple electronic transfers in the heterogenous oxygen evolution reaction (OER), and the high overpotential induced by the sluggish kinetics has become an uppermost scientific challenge. Numerous attempts are devoted to enabling high activity, selectivity, and stability via tailoring the surface physicochemical properties of nanocatalysts. Lattice-strain engineering as a cutting-edge method for tuning the electronic and geometric configuration of metal sites plays a pivotal role in regulating the interaction of catalytic surfaces with adsorbate molecules. By defining the d-band center as a descriptor of the structure-activity relationship, the individual contribution of strain effects within state-of-the-art electrocatalysts can be systematically elucidated in the OER optimization mechanism. In this review, the fundamentals of the OER and the advancements of strain-catalysts are showcased and the innovative trigger strategies are enumerated, with particular emphasis on the feedback mechanism between the precise regulation of lattice-strain and optimal activity. Subsequently, the modulation of electrocatalysts with various attributes is categorized and the impediments encountered in the practicalization of strained effect are discussed, ending with an outlook on future research directions for this burgeoning field.

2D TaSe2 as a zero-strain and high-performance anode material for Li+ storage

The zero-strain property of anode materials plays a determining role in their safety and cycling performance. However, the zero-strain anode materials currently developed are very limited and show significant performance trade-offs. Herein, novel highly conductive two-dimensional (2D) TaSe₂ is first explored as an anode material. Due to the reaction mechanism of Li⁺ solid-solution, the 2D TaSe₂ anode shows the zero-strain feature (0.042%) at full lithiation, resulting in excellent cycling stability. Owing to the dual active sites (Ta and Se atoms) and the double-sided Li⁺ storage mechanism, the 2D TaSe₂ anode exhibits the highest specific capacity of zero-strain anode materials, and it is the only zero-strain anode material that exceeds the graphite anode in energy density. Additionally, because of its remarkable Li⁺/electronic conductivity and high mass density, the anode is insensitive to the loading mass (3.78-12.66 mg cm^-2), resulting in high areal (9.94 mA h cm^-2), volumetric (∼5 times greater than the graphite anode), and gravimetric specific capacities (the only zero-strain anode higher than the graphite anode). When matched with the high-loading LiFePO₄ cathode (11.4 mg cm^-2), the full cell also presents overall good performance. Experiment results combined with density functional theory (DFT) calculations are adopted together to reveal the mechanisms of Li⁺ storage and transfer. This work provides a good paradigm to design zero-strain and high-performance alkali-metal-ion anode materials.