Key Takeaways & Executive Findings
- •• • OAR activation occurs at a moderate voltage of 4.1 V in amorphous Li-V-O-F, contrasting with typical high-voltage (>4.5 V) activation in crystalline layered oxides, enabling lower operational stress and improved safety. • • The fluorinated crystalline LiVO2.98F0.02 (F2) cathode undergoes complete amorphization after the initial cycle, with only a small fraction of rock-salt nanodomains remaining, which fully convert in subsequent cycles, indicating a structural transformation that stabilizes reversible OAR. • • RIXS and XAS analyses confirm the absence of oxidized oxygen species during initial charging, instead revealing O-O formal redox with electron holes accommodated via O-O interactions, leading to reversible O-O dimerization in the amorphous phase. • • The OAR mechanism in tetrahedrally coordinated VO4 units, rather than octahedral TM-O configurations, expands the design space for cathode materials, potentially enabling higher energy densities through disordered or amorphous structures.
Abstract
Oxygen anionic redox (OAR) is pivotal for achieving extra lithium storage in high-energy-density Li-ion batteries, yet its activation and stabilization remain challenging. Traditionally, OAR is studied in crystalline layered oxides with ordered frameworks and transition metal (TM)-centered octahedral coordination, where the Li-O-Li configuration is considered a prerequisite for creating unhybridized O 2p states. However, recent findings indicate that the presence of unhybridized O 2p states, rather than a specific configuration, is essential for oxygen activation. This study reports a novel OAR mechanism in an amorphous Li-V-O-F cathode, operating at a moderate voltage of 4.1 V, distinct from conventional Li-O-Li configurations. The cathode, initially crystalline LiVO2.98F0.02 (F2), undergoes amorphization after the first charge-discharge cycle, as evidenced by ex situ XRD, HRTEM, and EXAFS. Resonant inelastic X-ray scattering (RIXS) and X-ray absorption spectroscopy (XAS) reveal that the initial charge involves O-O formal redox without oxidized oxygen features, indicating electron holes are accommodated via O-O interactions. Reversible OAR activity emerges in the second cycle, confirming O-O dimerization in the amorphous phase. Ab initio molecular dynamics (AIMD) simulations further elucidate the mechanism. This work challenges the conventional Li-O-Li paradigm and opens new avenues for designing high-capacity cathode materials through amorphization and tetrahedral coordination.
1. Introduction
Conventional lithium-ion battery cathodes rely on transition metal redox to store charge, but the theoretical capacity limits of layered oxides such as LiCoO2 and LiNi1-x-yCoxMnyO2 have driven exploration of oxygen anionic redox (OAR) to access extra capacity. However, OAR activation typically requires high voltages (>4.5 V) and deep delithiation, leading to irreversible oxygen loss, structural degradation, and voltage fade. The Li-O-Li configuration has been widely accepted as the structural motif enabling OAR by creating unhybridized O 2p states, but this paradigm restricts material design to crystalline, cation-ordered frameworks. Recent discoveries of alternative configurations, such as Na-O-Mg and LiMn6 rings, suggest that unhybridized O 2p states, not the specific Li-O-Li arrangement, are the true prerequisite. Yet, achieving controllable OAR in disordered or amorphous systems remains a formidable challenge.
This study breaks new ground by demonstrating OAR in an amorphous Li-V-O-F cathode, where tetrahedrally coordinated VO4 units trigger oxygen redox at a moderate 4.1 V. The initial charge-discharge cycle induces amorphization, converting the crystalline LiVO2.98F0.02 into a disordered phase that hosts reversible O-O dimerization. This mechanism bypasses the need for Li-O-Li configurations and octahedral TM coordination, offering a new pathway to design high-capacity cathodes with improved structural stability and lower operational voltages. The findings challenge conventional wisdom and suggest that amorphization can be harnessed to stabilize OAR, potentially overcoming the bottlenecks of voltage fade and oxygen loss that plague crystalline OAR cathodes.
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LIU Shiqi, ZHANG Xu, YU Haijun (2026). Beyond Li-O-Li Configuration in Oxygen Anionic Redox: Amorphization-Driven O-O Dimerization in Fluorinated Li-V-O Cathodes. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3635-6
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Frequently Asked Questions
What is the specific voltage at which OAR is activated in the amorphous Li-V-O-F cathode, and how does it compare to conventional crystalline OAR cathodes?
OAR is activated at a moderate voltage of 4.1 V in the amorphous Li-V-O-F cathode, significantly lower than the typical >4.5 V required for crystalline layered oxides like Li-rich NMC. This lower voltage reduces electrolyte decomposition and improves safety, while still providing extra capacity.
How does the amorphization process affect the reversibility of OAR over multiple cycles?
The initial cycle induces amorphization, converting the crystalline LiVO2.98F0.02 into an amorphous phase. Reversible OAR activity is observed from the second cycle onward, indicating that the amorphous structure stabilizes O-O dimerization and prevents irreversible oxygen loss, leading to improved cycling stability.
What experimental evidence confirms the absence of oxidized oxygen species during initial charging, and what does this imply about the OAR mechanism?
Resonant inelastic X-ray scattering (RIXS) and X-ray absorption spectroscopy (XAS) show no feature corresponding to oxidized oxygen (e.g., O2- → O-) during initial charging. Instead, electron holes are accommodated via O-O interactions, forming peroxo-like species. This indicates a formal O-O redox mechanism, distinct from the conventional formation of localized oxygen holes.
How does the tetrahedral coordination of VO4 in the amorphous cathode differ from the octahedral TM-O coordination in crystalline OAR cathodes, and why is this significant?
In crystalline layered oxides, TM ions are octahedrally coordinated, and OAR is associated with unhybridized O 2p states in Li-O-Li configurations. In the amorphous Li-V-O-F, vanadium is tetrahedrally coordinated (VO4), which alters the electronic structure and enables OAR at lower voltage. This demonstrates that OAR is not limited to octahedral coordination, expanding the design space for cathode materials.
What are the potential scalability and practical implications of using amorphous cathodes for commercial Li-ion batteries?
Amorphous cathodes can be synthesized via simple fluorination and amorphization processes, potentially reducing manufacturing costs. The lower operating voltage (4.1 V) may allow the use of conventional electrolytes without high-voltage additives, simplifying cell design. However, the initial irreversible capacity loss due to amorphization and the need for further optimization of cycling stability and rate capability remain challenges for commercial adoption.
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