TYC Symposium: Batteries

The Mirage of Anionic Redox for High-Energy Batteries – Marie-Liesse Doublet, University of Montpelier

Our growing reliance on lithium-ion batteries for energy storage demands continuous advancements in the performance of their positive electrodes. Traditionally, these electrodes have relied exclusively on the cationic redox activity of transition-metal ions to drive electrochemical reactions. In recent years, however, the discovery of anionic redox has transformed strategies for
designing advanced cathode materials. This phenomenon is most prominently observed in Li-rich transition-metal oxides (Li-rich TMOs), with Li₁.₂Ni₀.₁₃Co₀.₁₃Mn₀.₅₄O₂ (Li-rich NMC) serving as the archetypal example. [1–3]

Unlike conventional LiMO₂ oxides, Li-rich TMOs can access an additional electron reservoir through anionic redox which enables theoretical capacities approaching 300 mAh/g, [3] therefore offering the potential for improving energy density. Yet, despite these advantages, anionic redox introduces several critical challenges—including voltage fade, O₂ release, and voltage hysteresis—that severely compromise cycling stability and battery lifetime. [4] These limitations remain major obstacles to the commercialization of Li-rich cathodes.

To elucidate the origin and consequences of anionic redox, we developed a theoretical framework based on chemical bonding concepts. [5] When integrated with electronic-structure DFT calculations and molecular dynamics simulations, this framework revealed several key parameters governing both the onset and reversibility of the anionic reaction—most notably the material’s
electronic ground state and the number of holes generated on the oxygen sublattice during charging. [5,6] These parameters enable the reliable prediction of anionic redox behavior, [7] providing critical insight for the rational design of Li-rich cathodes. Overall, our results reveal that anionic redox is far from fulfilling its initial promise of enhancing battery energy density, as its intrinsic limitations continue to undermine the practical viability of Li-rich materials.

[1] Lu, Z. et al. Layered cathode materials Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 for lithium-ion batteries. Electrochemical
Solid-State Letters 4, A191–A194 (2001).
[2] Koga, H. et al. Reversible oxygen participation to the redox processes revealed for Li1.20Mn0.54Co0.13Ni0.13O2
Journal of the Electrochemical Society 160, A786–A792 (2013).
[3] Sathiya, M. et al. Reversible anionic redox chemistry in high-capacity layered-oxide electrodes. Nature
Materials 12, 827–835 (2013).
[4] Assat, G. et al. Fundamental understanding and practical challenges of anionic redox activity in Li-ion
batteries. Nature Energy 3, 373–386 (2018).
[5] Ben Yahia, M. et al. Unified picture of anionic redox in Li/Na-ion batteries. Nature Materials 18, 496–502
(2019).
[6] Xie, Y. et al. Requirements for reversible extra-capacity in Li-rich layered oxides for Li-ion batteries. Energy
Environmental Science 10, 266–274 (2017).
[7] Gao et al. Clarifying the origin of molecular O2 in cathode oxides Nature Materials, 24, 743–752 (2025).

Mixed-anion NaTaOxCl6-2x oxychlorides: From crystalline to amorphous networks for high Na+ conductivity – Alexander Squires, University of Birmingham

As the demand for efficient and sustainable energy storage solutions grows, sodium-ion batteries have emerged as a promising alternative to lithium-ion batteries. With sodium’s abundance and wide geographical distribution, they offer advantages in cost, sustainability, and economic viability. Solid-state sodium batteries, in particular, hold potential for enhanced safety, higher energy density, and longer lifetimes through the use of solid electrolytes.

Halide-based electrolytes such as NaTaCl6 provide a useful starting point but exhibit limited ionic conductivities in their ordered form. Improvements have been achieved through disordering strategies, while related chemistries such as NaTaOCl4 have recently shown much higher ionic conductivities and promising catholyte behaviour. Building on these advances, the mixed-anion oxychloride series NaTaOxCl6–2x (x = 0.5, 1) offers a platform to investigate how oxygen incorporation modifies structure and Na+ transport.

Here, we establish a computational workflow to probe this series, combining density functional theory with ab initio random structure searching to identify low-energy configurations and the dominant local motifs. The calculations show that oxygen incorporation drives amorphization through the formation of corner-sharing TaCl5 dimers and ultimately trans-linked TaCl4O2 chains. These structural motifs generate percolating Na+ diffusion pathways, rationalising the enhanced transport behaviour observed experimentally in compositions such as NaTaO0.5Cl5.

This work was carried out in close collaboration with experimental partners, whose diffraction, spectroscopy, and electrochemical measurements provide critical validation of the structural and transport mechanisms identified in our simulations. By bridging computational predictions with experimental insights, we establish a framework for understanding the atomistic origins of fast-ion conduction in amorphous oxyhalides.

Abigail Parsons, Alexander G. Squires*, Justin Leifeld, Alexandra Morscher, Xabier Martinez de Irujo-Labalde, Marvin A. Kraft, Bibek Samantha, Wiebke Zielasko, Niina Jalarvo, Michael Ryan Hansen, David O. Scanlon, Wolfgang G. Zeier*

Modelling Nanoscale Structural Changes in Layered Li-rich Mn Oxide Cathode Materials – Benjamin Morgan, University of Bath

Lithium-rich manganese-based layered oxides are promising cathode materials for next-generation lithium-ion batteries, offering exceptionally high energy densities through combined transition metal and oxygen redox. However, this high energy density presents a critical limitation: these materials suffer progressive loss of energy density upon cycling, due to progressive decrease in average voltage; a phenomenon termed ‘voltage fade’ [1–4]. Understanding and controlling the underlying mechanisms of voltage fade are essential to realise the full potential of these high-capacity cathode materials.

Voltage fade has been linked to the formation and growth of nanoscale voids within the cathode bulk [1], but the atomic-scale mechanisms of this process are not well understood. The conventional approach for modelling battery cathode materials at the atomic scale is density functional theory (DFT). However, DFT cannot be used to directly investigate nanoscale void formation and growth, because the necessary system sizes are too large to be computed.

To investigate void formation over extended cycling, we have developed a novel computational approach combining DFT calculations, cluster expansion models, and Monte Carlo simulations. By applying this methodology to Li-rich Mn-based cathodes across the Li2MnO3–LiMnO2 compositional space, we find that nanoscale voids form through two concurrent processes: formation of O2 molecules within the bulk and extensive transition metal migration that forms transition-metal-deficient regions via phase segregation. Under extended cycling, these voids coalesce, driven by surface energy minimisation, in a process analogous to Ostwald ripening.

We further find that void coalescence—and thus voltage fade—depends strongly on the initial Mn/Li configuration in the Mn-rich layer, suggesting that targeting specific initial structures can inhibit deleterious structural evolution during cycling. By establishing the direct link between void growth and voltage loss, we show that preventing coalescence offers a route to maintaining electrochemical performance. Through systematic mapping of voltage fade across the Li2MnO3–LiMnO2 compositional space, we identify optimal structures and compositions that minimise degradation whilst retaining high energy density. These findings establish clear structural and compositional design principles for developing Li-rich cathodes with sustained performance over extended cycling.

[1] McColl, K.; Coles, S. W.; Zarabadi-Poor, P.; Morgan, B. J.; Islam, M. S. Phase Segregation and Nanoconfined Fluid O2 in a Lithium-Rich Oxide Cathode. Nat. Mater. 2024, 23, 826−833.

[2] Csernica, P. M.; McColl, K.; Busse, G. M.; Lim, K.; Rivera, D. F.; Shapiro, D. A.; Islam, M. S.; Chueh, W. C. Substantial Oxygen Loss and Chemical Expansion in Lithium-Rich Layered Oxides at Moderate Delithiation. Nat. Mater. 2025, 24, 92−100.

[3] House, R. A.; Rees, G. J.; McColl, K.; Marie, J. J.; Garcia-Fernandez, M.; Nag, A.; Zhou, K.-J.; Cassidy, S.; Morgan, B. J.; Islam, M. S.; Bruce, P. G. Delocalized Electron Holes on Oxygen in a Battery Cathode. Nat. Energy 2023, 8, 351−360.

[4] McColl, K.; House, R. A.; Rees, G. J.; Squires, A. G.; Coles, S. W.; Bruce, P. G.; Morgan, B. J.; Islam, M. S. Transition Metal Migration and O2 Formation Underpin Voltage Hysteresis in Oxygen-Redox Disordered Rocksalt Cathodes. Nat. Commun. 2022, 13, 5275.