Promoting Reversible Anionic Redox in Sodium-Ion Cathodes by Doping and Phase Control
Bibliographic record
Abstract
Sodium-ion batteries (SIBs) have emerged as promising energy storage systems due to their reliance on earth-abundant elements, environmental friendliness, and excellent electrochemical performance.[1] However, traditional cathode materials are constrained by the redox activity of transition metals, which limits achievable capacities and makes it challenging to reach high operational voltages. An innovative approach to overcome these limitations is to exploit anionic redox, specifically through oxygen in layered oxides, to access both high voltages and additional capacities.[2] While some progress has been made with Li doping on the TM layer to achieve materials like NaxLi0.25Mn0.75O2 and with model Na-rich oxides like Na2IrO3, the compositional/structural factors that favours the desirable reversible oxygen redox are not fully understood, and predictive design strategies remain elusive.[3] To address this gap, we conducted an extensive high-throughput screening of over 50 dopants across the periodic table [4, 5], examining their effects on oxygen redox activation. Our analysis revealed a significant correlation between bond valence mismatch and oxygen activity.[4] Specifically, we found that a larger bond valence mismatch induces local structural distortions in the layered oxides, destabilizing the non-bonding O-2p orbitals. This destabilization makes oxygen redox accessible at high voltages of approximately 4.2V and 4.5V vs. Na/Na+, with notable features of high reversibility and low overpotential. Based on this screening, we identified five dopants—K, Cu, Rb, Cs, and Tl—as particularly interesting for further study, each exhibiting a high bond valence mismatch. We then incorporated these dopants at a 10% level into Na0.66MnO2. We synthesized two polymorphs for each: one taking the P2 structure and the other the P’2 layered structures. Electrochemical testing revealed stark differences between these two phases: P2 materials demonstrated robust, reversible oxygen redox activity at high voltages, while P’2 materials showed irreversible oxygen redox. To investigate these differences at the atomic level, we employed advanced synchrotron-based techniques, including X-ray Absorption Spectroscopy (XAS), Wavelet Transform Extended X-ray Absorption Fine Structure (WT-EXAFS), and Resonant Inelastic X-ray Scattering (RIXS). These analyses confirmed that doped P2 structures effectively stabilize electron-holes on oxygen at high voltages, thus avoiding the formation of (O-O)n- dimers or trapped O2 species that can lead to irreversible structural changes. We attribute this stability to the large dopants increasing the separation between oxygens thereby preventing their interaction. WT-EXAFS further indicated that dopants play a crucial role in regulating metal migration, with Cu migration in particular stabilizing Mn within the lattice, thereby preventing irreversible structural reordering. This stabilization supports sustained oxygen redox activity and enhances the overall durability of the material. Thus, we establish the key mechanisms involved in inducing stable reversible oxygen redox in the P2 materials and thereby provide new design strategies for this emerging class of cathodes. References [1] Jia, Shipeng, Shinichi Kumakura, and Eric McCalla. "Unravelling air/moisture stability of cathode materials in sodium ion batteries: characterization, rational design, and perspectives." Energy & Environmental Science (2024). [2] McCalla, Eric, et al. "Visualization of OO peroxo-like dimers in high-capacity layered oxides for Li-ion batteries." Science 350.6267 (2015): 1516-1521. [3] Zhang, Xiaoyu, et al. "Manganese‐based Na‐rich materials boost anionic redox in high‐performance layered cathodes for sodium‐ion batteries." Advanced Materials 31.27 (2019): 1807770. [4] Jia, Shipeng, et al. "Chemical speed dating: the impact of 52 dopants in Na–Mn–O cathodes." Chemistry of Materials 34.24 (2022): 11047-11061. [5] Jia, Shipeng, et al. "Stabilization of Na‐Ion Cathode Surfaces: Combinatorial Experiments with Insights from Machine Learning Models." Advanced Energy and Sustainability Research (2024): 2400051.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".