Tongchao Liu

Elevated-voltage operation in layered Na-ion cathodes often increases capacity but accelerates degradation, yet the structural origin of this trade-off remains unclear. Here, we investigate the coupled evolution of charged-state structure and redox partitioning in O3-type NaNi1/3Fe1/3Mn1/3O2. Charging to 4.2 V delivers greater capacity but promotes O/P intergrowth with deeply desodiated O-type slabs and accelerates capacity decay, whereas a 4.0 V cutoff stabilizes a reversible P3 framework. Synchrotron X-ray diffraction, transition-metal X-ray absorption spectroscopy, Na K-edge spectroscopy, and electrochemical impedance spectroscopy combined with distribution of relaxation times analysis (EIS-DRT) reveal distinct charge-compensation pathways associated with these structures. The P3 framework is associated with a progressive decrease in the Ni contribution and a comparatively more reversible local Fe environment. In contrast, stacking evolution at 4.2 V is associated with sustained Ni-dominant redox, greater Fe local-environment reconstruction, and persistent kinetic limitations. These results identify the charged-state stacking as a key structural descriptor linking redox partitioning and Na-ion transport kinetics.

Increasing the energy density of Li‐ion batteries requires cathodes that can store more than one electron per transition‐metal center. Iron‐based conversion cathodes such as FeF ₃ are attractive due to their high theoretical capacity and low cost; however, their practical performance is limited by large voltage hysteresis, sluggish kinetics, and capacity loss during cycling. Here, we report a ball‐milled FeF ₃ /FePO ₄ composite cathode that combines the high capacity of FeF ₃ with the superior reversibility of FePO ₄ . Compared to pure FeF ₃ , the composite retains high capacity while exhibiting improved rate capability, lower polarization, and markedly enhanced cycling stability. After prelithiation, the composite delivers a reversible discharge capacity of 572 mAh g ⁻¹ , corresponding to a specific energy of 1175 Wh kg ⁻¹ . These results show that integration of FeF ₃ and FePO ₄ offers an effective route to cathodes that combine high capacity with high reversibility.

Increasing the energy density of Li‐ion batteries requires cathodes that can store more than one electron per transition‐metal center. Iron‐based conversion cathodes such as FeF ₃ are attractive due to their high theoretical capacity and low cost; however, their practical performance is limited by large voltage hysteresis, sluggish kinetics, and capacity loss during cycling. Here, we report a ball‐milled FeF ₃ /FePO ₄ composite cathode that combines the high capacity of FeF ₃ with the superior reversibility of FePO ₄ . Compared to pure FeF ₃ , the composite retains high capacity while exhibiting improved rate capability, lower polarization, and markedly enhanced cycling stability. After prelithiation, the composite delivers a reversible discharge capacity of 572 mAh g ⁻¹ , corresponding to a specific energy of 1175 Wh kg ⁻¹ . These results show that integration of FeF ₃ and FePO ₄ offers an effective route to cathodes that combine high capacity with high reversibility.

The use of iron metal powder directly as the cathode active mass is a transformative opportunity to increase energy density and reduce costs for rechargeable batteries. Realizing this potential, however, requires creating a chemical environment that can activate multiple‐electron transfer of iron redox chemistry. Here, we report that the composite synthesized by ball milling Na ₂ SO ₄ , Fe metal, and CuS can be directly desodiated as a sodium‐ion battery cathode, which delivers a reversible discharge capacity of 149 mAh g ⁻¹ at ~2.6 V with stable cycling performance. Comprehensive spectroscopic characterization reveals that Fe serves as the primary redox center, while CuS functions as a redox mediator. The results demonstrate that the heterointerfaces formed by ball milling are critical for charge transfer and high iron utilization. This work establishes iron metal sodium‐salt composites as viable sodium‐ion battery cathodes by unlocking multi‐electron Fe redox within a composite architecture.

Singe-crystal (SC) nickel-rich layered oxide cathodes, composed of boundary-free particles with high tap density, offer significant advantages in volumetric energy density and mechanical strength compared with polycrystalline (PC) cathode materials. However, as the nickel content increases (≥80%), SC Ni-rich cathodes often suffer from faster performance degradation than PC cathodes of the same composition, and the underlying causes of this discrepancy remain poorly understood. Herein, we reveal the distinct Ni redox behaviors that govern the electrochemical performance of SC and PC Ni-rich cathodes using multiscale and operando characterization techniques. Our results indicate that the increasingly heterogeneous Ni oxidation process in SC cathodes leads to the additional irreversible oxygen redox activity that deteriorates both the mechanical and chemical structures. In contrast, PC cathodes, despite with more pronounced surface reconstruction, exhibit greater chemomechanical stability due to homogeneous redox reactions during charging. Consequently, we find that bulk degradation, more than surface reactions, ultimately leads to fast capacity decay of SC Ni-rich cathodes during cycling. This work offers a comprehensive view on the impact of Ni redox evolutions on the chemomechanical stability in Ni-rich layered oxide cathodes, providing new insights into the longstanding performance gap between SC and PC cathodes, and guiding the rational design of Ni-rich cathode architectures.

The transmission X-ray microscopy (TXM) based X-ray absorption near-edge structure (XANES) technique provides three-dimensional mapping of element-specific chemical states at nanometer-scale spatial resolution and micrometer-scale fields of view. However, compared to conventional volume-averaged XANES (VA-XANES) measurements, the inherently small voxel size in TXM-XANES leads to a lower signal-to-noise ratio, making full-spectrum analysis computationally demanding and less robust. Here, we present the structural and compositional conditions for a statistical white-line analysis framework under which chemical state information can be directly extracted from the white-line peak position in voxel spectra without the need for voxel-wise background subtraction or normalization, under well-defined structural and compositional conditions. The method is validated on layered oxide cathode materials, where low-order polynomial fitting accurately reproduces white-line features, and the extracted energy distributions correlate strongly with VA-XANES results. This statistical approach enables high-throughput, dose-efficient, and noise-robust chemical state quantification in TXM-XANES, offering broad applicability to functional materials requiring nanoscale oxidation-state mapping.

The quest for high-energy-density and cost-effective cathode materials has revitalized lithium-rich iron-based oxides, where Fe³⁺ ions with their stable half-filled d⁵ electronic configuration demonstrate unique potentials in regulating oxygen redox activity through ligand-to-metal charge transfer. Using operando ⁵⁷Fe Mössbauer spectroscopy, synchrotron X-ray techniques, and density functional theory calculations, we unravel the enhanced Fe-O redox activity through the LMCT process and the concomitant evolution of local structure distortion in the Li-rich layered system Li_1.2MO₂ (M = Ni, Mn, Fe). The metastable Fe⁴⁺ intermediates facilitate early activation of oxygen redox through LMCT along with increased delithiation at a lowered potential (below 4.5 V). This process inherently suppresses Jahn-Teller distortion of Fe⁴⁺ and exhibits Fe-O redox dominance over conventional Fe³⁺/Fe⁴⁺ redox in Li-rich oxides. The Fe-O coupling redox exhibits improved initial Coulombic efficiency and 95% capacity retention over 200 cycles at 4.4 V. However, the LMCT process is accompanied by aggravated cation migration and voltage fade at low cutoff potentials, which requires further mitigation for the utilization of Fe-O redox. Overall, these findings demonstrate the regulating mechanism of iron for oxygen redox through LMCT and provide fundamental insights into the design principles for high-performance, cost-effective iron-based cathode materials.

Single-crystal Ni-rich layered oxides (SC-NMC) with a grain-boundary-free configuration have effectively addressed the long-standing cracking issue of conventional polycrystalline Ni-rich materials (PC-NMC) in lithium-ion batteries, prompting a shift in optimization strategies. However, continued reliance on anisotropic lattice volume change—a well-established failure indicator in PC-NMC—as a metric for understanding strain and guiding compositional design for SC-NMC becomes controversial. Here, by leveraging multiscale diagnostic techniques, we unravelled the distinct nanoscopic strain evolution in SC-NMC during battery operation, challenging the conventional composition-driven strategies and mechanical degradation indicators used for PC-NMC. Through particle-level chemomechanical analysis, we reveal a decoupling between mechanical stability and lattice volume change in SC-NMC, identifying that structural instability in SC materials is primarily driven by multidimensional lattice distortions induced by kinetics-driven reaction heterogeneity and progressively deactivating chemical phases. Using this mechanical failure mode, we redefine the roles of cobalt and manganese in maintaining mechanical stability. Unlike cobalt’s detrimental role in PC-NMC, we find cobalt to be critical in enhancing the longevity of SC-NMC by mitigating localized strain along the extended diffusion pathway, whereas manganese exacerbates mechanical degradation.