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Abstract: Layered perovskite oxides continue to be the subject of intense research efforts due to their highly tunable crystal structures, which often arise from the competition between various lattice, spin, charge and orbital degrees of freedom. In particular, a number of recent works have focused on the mechanisms through which polar phases (those with globally broken inversion symmetry) emerge through the coupling of different structural distortions. The so-called hybrid improper mechanism, in which nonpolar structural distortions couple to break inversion symmetry, has been invoked to explain the appearance of polar structures in many different layered perovskite oxides. We use a combined experimental and computational approach to investigate the pseudo-Ruddlesden–Popper system Li2SrxCa1–xTa2O7 (0 < x < 1), which exhibits multiple competing polar phases that arise through distinct mechanisms. We untangle the complex interactions between various structural modes and find that, in contrast with previous work, the hybrid improper mechanism cannot by itself account for the observed polar phases. Our work demonstrates that there are significant differences in the mechanisms through which polar phases emerge in even nominally the same family of layered perovskites, suggesting a rich playground for further exploration and functional materials design.

Open Access

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Abstract: NaNiO2 is a promising cathode material for sodium-ion batteries due to its high theoretical capacity of 235.8 mAh.g–1. However, as with many Na-ion cathode materials, a series of poorly understood phase transitions occur on electrochemical cycling, inducing volume mismatch-based stress/strain, resulting in particle cracking, electrochemically disconnected particles and, therefore, irreversible capacity loss. This behavior is one key obstacle to developing long-lasting, high-performance Na-ion batteries. Although the series of phases that form as NaxNiO2 is electrochemically cycled have been previously identified, their structures remained unsolved, limiting our ability to understand and control the phase transition behavior. Here, we report structural solutions based on Rietveld refinement against high-resolution synchrotron x-ray diffraction (SXRD) and neutron powder diffraction (NPD) for the phases obtained on desodiation: P″3-Na1/2NiO2, O″3-Na2/5NiO2, and O‴3-Na1/3NiO2. Each phase contains a unique Na+/vacancy ordering, minimizing intralayer electrostatic repulsions between Na+ ions, and Nix+-charge ordering decreasing interlayer repulsions through the location of lower valence Nix+ nearer to vacancies. Using these structures, we conduct sequential Rietveld refinement against operando SXRD data, which supports prior identification of a transient P‴3-Na1/2<x<2/3NiO2 phase, not isolable ex situ. Operando data also identify the presence of a solid-solution phase O″3δ-Na1/3<x<2/5NiO2 and second-order behavior of the O″3-Na2/5NiO2 → O‴3-Na1/3NiO2 phase transition at the top of charge. This work provides unprecedented insight into structural evolution during electrochemical cycling in Ni-rich Na cathodes (and likely Li analogues), paving the way toward rational doping regimes designed to disrupt degradation-inducing phase transitions, increasing capacity and cycle lifetime, thus improving the performance of Co-free Na and Li cathodes.

Open Access

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Abstract: Polymorphism plays a critical role in determining the performance of bio-derived phase-change materials (PCMs). In this work, we investigate the polymorphism of methyl behenate, a fatty acid ester with potential for sustainable phase-change material for thermal energy storage. Three previously unknown polymorphs were identified, and their thermophysical properties were characterised by differential scanning calorimetry and hot-stage microscopy, highlighting the strong influence of polymorphism on the material properties. The structure of two polymorphs were determined using laboratory and synchrotron powder X-ray diffraction. The comparison with a related fatty acid ester reveals common structural trends, suggesting emerging structure–property relationships within this class of materials. These results provide new insights into ester-based PCMs and support the rational design of sustainable thermal energy storage materials.

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Abstract: Thick electrodes are a promising route to increase battery energy density by increasing the fraction of active material relative to inactive components. However, at high cycling rates, greater mass transport limitations in thick electrodes can lead to poor capacity utilisation and reduced power density. Although advanced electrode structuring strategies have been explored, many require expensive manufacturing changes or complex post-processing. An alternative approach uses standard battery manufacturing methods to sequentially coat active materials with different particle sizes or compositions. In this work, polycrystalline LiNi1/3Mn1/3Co1/3O2 (NMC111) particles of two sizes were used to fabricate particle size-graded bilayer electrodes. An impedance-based finite element model was developed to evaluate transport properties in the graded structures and was validated using electrochemical impedance spectroscopy (EIS) and rate tests. Operando synchrotron diffraction revealed a more homogeneous state of charge when smaller particles were positioned near the separator and larger particles near the current collector. Together, the modelling and experimental results show that simple particle size grading improves ion transport and reaction uniformity, enhancing capacity utilisation. This approach offers a practical pathway to improve the power performance of next-generation battery electrodes.

Open Access

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Abstract: Developing earth-abundant electrocatalysts that rival the commercial platinum/carbon catalyst for the hydrogen evolution reaction (HER) remains a central challenge in renewable-energy conversion. Here, we reveal an electrochemically induced, in situ phase transformation in a Ru-MgO catalyst that leads to true active material during operation. Under acidic HER conditions, nominal 20 wt.% Ru nanoparticles supported on polar MgO(111) nanocrystals undergo a topotactic hydrolysis to Ru-Mg(OH)2(001), generating an ordered hydroxide layer that serves as a highly conductive proton-hopping network. After activation, the catalyst delivers performance comparable to commercial Pt/C under identical conditions, matching the current density of −1.1 V and surpassing it by approximately 10% at −2.3 V. Operando synchrotron X-ray diffraction combined with ex situ characterization techniques directly captures this transformation, while density-functional theory calculations reveal that water-assisted Grotthuss proton transfer across the hydroxide requires only a 0.10 eV energy barrier. These findings establish electrochemically driven oxide-to-hydroxide conversion as a new design principle for creating low-Pt or Pt-free HER electrocatalysts with intrinsically fast proton transport.