The research team, led by Seungmin Lee and Hyung-Seok Kim, focused on a model compound known as mesh-type Na2Mn3O7, a layered sodium manganese oxide whose structure contains ordered manganese vacancies and intrinsic stacking faults. Both features have previously been linked to exceptionally reversible oxygen redox, with voltage hysteresis below 50 millivolts. What remained poorly understood was where these beneficial structural motifs actually come from during synthesis. Earlier work suggested that manganese vacancies form when the material absorbs oxygen from the surrounding atmosphere as it cools, a process that is thermodynamically favorable near or below 600 degrees Celsius. The excess oxygen oxidizes Mn3+ ions to Mn4+, and charge balance is maintained by removing manganese atoms from the layers. This view implies that vacancy ordering is not an intrinsic property of the hot parent phase at all, but the product of a cooling-driven, oxygen-mediated reconstruction, and that the cooling rate itself might be the decisive lever controlling the final defect architecture.

To test this hypothesis, the researchers prepared two batches of the cathode material from identical precursors of sodium nitrate and manganese carbonate, heated both to 600 degrees Celsius for four hours, and then diverged only in the cooling step. One sample was quenched in liquid nitrogen, freezing the high-temperature structure almost instantly; the other was cooled slowly at 1.8 degrees Celsius per minute. Synchrotron X-ray diffraction revealed that both materials were predominantly the target P’3-type phase, but the differences were telling. The slowly cooled sample contained 98.3 percent of the P’3 phase, while the quenched sample retained 9.9 percent of an unreconstructed P3-type impurity phase, a signature of incomplete structural transformation. Because both powders had comparable particle sizes of roughly 50 to 100 nanometers, the researchers could attribute the electrochemical consequences entirely to these cooling-dependent structural differences rather than to morphology.

The electrochemical consequences were dramatic. In full voltage windows spanning both cationic and oxygen redox processes, the slowly cooled material delivered a first discharge capacity of 198 mAh per gram, compared with just 127 mAh per gram for the quenched sample, and retained 64 percent of its capacity after 30 cycles versus only 37 percent for the quenched material. When the window was narrowed to isolate oxygen redox between 4.5 and 3.5 volts, the contrast sharpened further. The slowly cooled cathode showed voltage hysteresis of just 29 millivolts against 74 millivolts for the quenched sample, and a first-cycle Coulombic efficiency of 52.9 percent versus 34.3 percent. Most strikingly, in situ differential electrochemical mass spectrometry detected oxygen gas evolution from the quenched cathode during the first charge, while the slowly cooled material released no gas at all, a direct demonstration that its structure suppresses the irreversible oxygen loss that plagues oxygen-redox chemistry.

The durability gap widened with cycling. After 30 cycles in the oxygen-redox window, the slowly cooled cathode retained 87.2 percent of its oxygen-redox charge capacity, with hysteresis of only 45.2 millivolts, while the quenched sample retained 79.7 percent with hysteresis ballooning to 207 millivolts. Over 60 cycles in the wider window, the slowly cooled material held 68 percent of its capacity against 46 percent for the quenched one. Even rate performance favored the slow-cooled framework, which kept 71 percent of its discharge capacity when the current was raised from 7.5 to 150 mA per gram, compared with 54 percent for the quenched sample. Combined galvanostatic intermittent titration, impedance spectroscopy, and distribution of relaxation times analysis suggested the advantage stemmed mainly from reduced diffusion limitations at low states of charge, where the quenched material’s residual P3-derived framework appears prone to local rearrangement.

To find out exactly when the beneficial structure forms, the team turned to time-resolved X-ray diffraction inside a real-time analytical micro-furnace, tracking the synthesis live while thermogravimetric analysis recorded weight changes. During heating and the four-hour hold at 600 degrees Celsius, a layered P3-type sodium manganese oxide emerged around 350 degrees Celsius through topotactic sodiation, complete with a superstructure reflection indicating coupled cation ordering, likely periodic Mn3+/Mn4+ ordering. Crucially, however, the superstructure reflections associated with the characteristic in-plane vacancy ordering of the final P’3 phase never appeared during heating or the isothermal hold. The high-temperature parent framework, in other words, was still missing the very defects that make the cathode work. The target oxygen-redox-active structure was established only afterward, during cooling.

The cooling stage told the real story. As the temperature fell from 600 to 520 degrees Celsius, the P3 reflections shifted to higher angles, signaling contraction of the lattice as Mn3+ was oxidized to Mn4+, and the thermogravimetric data recorded a substantial weight gain of 5.05 percent from oxygen uptake above 500 degrees Celsius. Below 520 degrees, the P3 reflections broadened and split, the high-temperature superstructure peak faded, and new reflections marking in-plane manganese vacancy ordering emerged, all hallmarks of the monoclinically distorted P’3 phase. X-ray absorption spectroscopy confirmed the picture: the slowly cooled sample showed a higher average manganese oxidation state of 3.89, with a Mn4+/Mn3+ ratio of 8.10 versus 3.35 for the quenched material, and reduced manganese-manganese correlation in the extended fine structure, indicating a higher vacancy concentration. An intermediate cooling rate of 9 degrees per minute produced intermediate oxygen uptake of 2.72 percent and compromised reversibility, confirming that the degree of reconstruction scales directly with cooling rate.

High-resolution transmission electron microscopy provided real-space confirmation. The quenched sample contained only locally confined stacking faults within otherwise well-ordered layers, with sharp diffraction spots and faint streaking in its Fourier transforms. The slowly cooled sample, by contrast, displayed a uniformly stacking-fault-rich framework throughout each particle, with pronounced streaks parallel to the stacking direction. Density functional theory calculations then explained why this matters: climbing-image nudged elastic band calculations showed that manganese ions hopping between octahedral sites face a barrier of 2.49 electron volts in bulk-like regions but 3.84 electron volts near stacking faults, an increase of roughly 1.35 electron volts. Since in-plane manganese migration drives vacancy clustering and brings oxidized oxygen ions close enough to form irreversible O-O dimers, stacking faults act as kinetic roadblocks against the very degradation pathway that kills oxygen redox. Soft X-ray O K-edge spectra and in situ diffraction during cycling both showed that the slowly cooled material’s oxygen electronic structure and lattice recover far more completely after discharge.

From these findings the authors distill a coherent mechanism and a practical design principle. Oxygen uptake during cooling above roughly 500 degrees Celsius oxidizes manganese and contracts the MnO6 framework, generating compressive stress that, together with reduced manganese-oxygen connectivity from vacancy formation, promotes layer gliding and stacking-fault development during slow cooling. Rapid cooling arrests this process, freezing an incomplete, fault-poor structure vulnerable to migration and oxygen loss. Remarkably, although the difference in P’3 phase fraction between the two samples was only about 8.2 percent, the improvement in oxygen-redox stability was substantial, underscoring the disproportionate stabilizing power of the fault-rich framework. The principle even carried over into full cells with pre-sodiated hard carbon anodes, where the slowly cooled cathode delivered 188 mAh per gram in the first cycle versus 148 for the quenched one. As sodium-ion batteries push toward commercial deployment, the message is clear: the recipe for a stable high-capacity cathode may be written as much in the cooling ramp as in the chemistry itself.

Subject of Research: Cooling-rate-controlled defect engineering for stable oxygen-redox sodium manganese oxide battery cathodes

Article Title: Regulating Oxygen‐Mediated Reconstruction during Cooling: A New Design Principle for High‐Capacity Sodium Manganese Oxide Cathodes

Article References: Lee, S., Han, D., Park, J.-H., Kwon, E., Barruna, E., Gong, S. H., Kim, M., Yoo, Y., Yu, S., Chung, K. Y., Nam, K.-W., Kim, S. Y., & Kim, H.-S. (2026). Regulating Oxygen‐Mediated Reconstruction during Cooling: A New Design Principle for High‐Capacity Sodium Manganese Oxide Cathodes. Advanced Science, Article e78064. https://doi.org/10.1002/advs.78064

Image Credits: AI Generated

DOI: 10.1002/advs.78064

Keywords: sodium-ion batteries, oxygen redox, cathode materials, manganese oxide, stacking faults, cooling rate, solid-state synthesis, manganese vacancies, energy storage, DFT calculations, voltage hysteresis, defect engineering

Potential hysteresis is the gap between the voltage at which a battery charges and the lower voltage at which it discharges. Every rechargeable cell shows some of it, but in lithium-rich disordered rocksalt materials the gap can be pronounced, meaning that a substantial fraction of the electrical energy pumped into the battery during charging is never recovered during discharge. That lost energy is dissipated as heat, lowering round-trip energy efficiency and complicating thermal management. For a technology whose appeal rests on packing more energy into the same volume, a large and poorly understood hysteresis is a serious obstacle to commercialisation.

Cation-disordered rocksalt compounds, often abbreviated DRX, break with the ordered crystal structures that dominate today’s lithium-ion cathodes. Instead of arranging lithium and transition metal ions on neat, alternating planes, DRX materials scatter them almost randomly across the same lattice sites. The disorder is a feature, not a bug: it frees designers from the strict compositional rules of layered oxides and allows lithium-rich formulations that store charge both in transition metal redox reactions and in the oxidation of lattice oxygen itself, the so-called anionic redox that underpins their exceptional capacities.

Yet the same structural freedom that unlocks capacity also makes DRX cathodes notoriously difficult to understand. During battery operation the material evolves in complicated ways, and previous work has implicated transition metal migration, oxygen-oxygen bond formation, trapped molecular oxygen and ligand-to-metal charge transfer in the voltage hysteresis. Because so many processes overlap, the field has struggled to separate the thermodynamic contributions, which reflect genuine differences in the energy landscapes of charged and discharged states, from the kinetic contributions, which arise when lithium transport and structural relaxation lag behind the applied current.

The new study tackles this ambiguity by comparing two closely related model compounds, one containing niobium and one that does not, through an unusually broad suite of characterisation techniques. The team combined synchrotron X-ray absorption spectroscopy, X-ray pair distribution function analysis and neutron pair distribution function measurements to probe structure from the longest length scales down to individual atomic bonds, supplemented by scanning transmission electron microscopy to image nanoscale domains directly and by density functional theory calculations to interpret what the experiments revealed.

The central finding concerns the stereochemistry of d0 transition metals, ions such as niobium in its highest oxidation state whose d electron shells are empty. Decades of solid-state chemistry have shown that such ions tend to shift away from the centre of their octahedral oxygen coordination, an out-of-centre distortion driven by the pseudo Jahn-Teller effect, a vibronic instability in which low-lying electronic states couple to atomic displacements. In the niobium-containing compound, the researchers found that these distorted octahedra disrupt the cation short-range order that would otherwise develop in the nominally disordered lattice.

That disruption turns out to matter enormously for performance. Short-range order in DRX materials is a double-edged sword: it can stabilise the lattice, but it can also block the percolating network of face-sharing tetrahedral sites through which lithium ions must hop to move through the crystal. By breaking up ordered domains, the out-of-centre distortion of the niobium octahedra opens a more connected lithium percolation pathway, allowing charge and discharge to proceed more reversibly at high rates. In effect, a purely geometric quirk of empty d-shell ions acts as a kinetic lever on the whole electrode.

The pseudo Jahn-Teller effect also operates on the thermodynamic side of the hysteresis ledger. The team’s spectroscopic and computational results indicate that the distortion weakens the ligand-to-metal charge transfer process associated with the nickel-oxygen bonds, the electronic event that couples nickel redox to oxygen redox and has previously been linked to large voltage gaps. With that coupling softened, the oxidation and reduction of lattice oxygen become more reversible, so the energy stored in charged oxygen species is more faithfully returned on discharge. In the niobium-containing DRX electrode, both the kinetic and thermodynamic components of the potential hysteresis in lithium metal cells were effectively mitigated.

The significance of the work lies in its multiscale framing. Rather than attributing hysteresis to a single culprit, the researchers demonstrate that phenomena at very different length scales are entangled: an atomic displacement of a fraction of an angstrom remodels nanoscale chemical ordering, which in turn reshapes both the thermodynamics of oxygen redox and the kinetics of lithium transport. Disentangling these contributions required correlating electrochemical measurements with structural probes across scales, an approach the authors suggest should become standard practice for evaluating DRX chemistry.

For battery designers, the message is actionable. The choice of redox-inactive, d0 transition metals is not merely a way to tune capacity or stability; it is a stereochemical tool that can be used deliberately to sculpt local structure. Compounds engineered so that distorted octahedra suppress deleterious short-range order could deliver the high capacities of lithium-rich disordered rocksalt cathodes without the energy efficiency penalty that has held them back. As lithium metal anodes mature, pairing them with hysteresis-minimised DRX cathodes brings the industry a step closer to batteries that store more energy and waste less of it on every cycle.

Subject of Research: Stereochemical origins of voltage hysteresis in lithium-rich cation-disordered rocksalt battery cathodes

Article Title: Stereochemical origin of potential hysteresis in lithium metal batteries with lithium-rich cation-disordered rocksalt positive electrodes

Article References: Mei, Y., Xia, F., Chen, H., Li, Y., Gao, J., Zhu, K., He, L., Wu, S., Li, H.-F., Fan, L., Zhan, C., Ni, L., Wang, H., Jian, W., Wang, K., Silvester, D. S., Banks, C. E., Wu, J., Deng, W., … Ji, X. (2026). Stereochemical origin of potential hysteresis in lithium metal batteries with lithium-rich cation-disordered rocksalt positive electrodes. Nature Nanotechnology. https://doi.org/10.1038/s41565-026-02301-2

Image Credits: AI Generated

DOI: 10.1038/s41565-026-02301-2

Keywords: lithium metal batteries, cation-disordered rocksalt, voltage hysteresis, anionic redox, pseudo Jahn-Teller effect, short-range order, d0 transition metals, niobium, lithium percolation, cathode materials, energy efficiency, Nature Nanotechnology