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Abstract: Niobium tungsten oxides are gaining attention as anodes for lithium-ion batteries due to their high volumetric energy storage densities obtained at high cycling rates. Two new niobium tungsten bronze structures, NbWO5.5 and β-Nb2WO8, were prepared with microwave-assisted solution-based methods at 800°C. These adopt a simple tetragonal tungsten bronze (TTB) and a √2 × √2 TTB superstructure, respectively. Nb3WO10.5 with a structure closely related to β-Nb2WO8 was formed at higher Nb:W ratios. Nb:W ≥ 4 compositions result in two-phase behavior forming Nb2O5 and Nb3WO10.5, while W-rich bronzes (Nb:W < 1) exhibited local domains of WO3 within the NbWO5.5 lattice. Diffraction and electron microscopy analysis revealed cation ordering in the bronzes at different length scales. The microwave synthesis method produced microporous spheres, with the high-Nb-content phases showing promising high-rate capabilities and long cycle lives, making them suitable for energy-storage applications. The microwave-assisted solution method holds potential for synthesizing complex oxide materials across diverse applications.

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Abstract: The use of solid-state electrolytes (SSE) promises electrochemical cells with high energy densities as they can enable high-voltage cathodes and lithium metal anodes while minimising the risk of dendrite formation and widening the thermal and electrochemical stability window [1]. Among different types of solid-state electrolytes that have been researched until now, cubic Li7La3Zr2O12 (LLZO) has been shown to have good thermal, mechanical, and electrochemical stability [1]. Nonetheless, one of the main challenges of inorganic SSE is the poor interfacial contact with the electrodes, especially with the cathode active material (CAM). The rigidity of the SSE and CAM makes it necessary to co-sinter the SSE/CAM composite at high temperatures (>1000 ºC) to densify the catholyte to guarantee conformal contact between the solid components [2]. However, CAMs usually react at these temperatures, which leads to formation of poorly lithium-ion conducting intermediate secondary phases due to interdiffusion of atoms and chemical reactions between the SSE and CAM [3]. Recent research elucidating decomposition products resulting from processing of LLZO and various cathode materials has mainly focused on LiCoO2 (LCO) given its high thermodynamical stability against LLZO [4-7]. However, the limited practical reversible capacity of LCO, high cost, and ethical issues concerning cobalt-rich cathode materials motivate the optimisation of other CAMs that are equally stable against LLZO [8]. Among them, LiNixCoyMnzO2 (NMCs) are perceived as strong candidates to decrease the Co content while increasing the specific capacity and maintaining sufficient thermal stability for co-sintering with LLZO [8,9]. A few studies have recently attempted to probe the interfacial stability between LLZO/NMC, but the nature of decomposition products, reaction mechanism, and onset temperature are inconsistent between reports and poorly understood [8-13]. Moreover, these reports do not account for the formation of secondary phases containing dopant elements such as Al3+ in cubic LLZO. To develop a fundamental understanding of the effect of sintering conditions on catholyte composition and reaction products, a comprehensive study of composite materials from microscale to atomic level is required [8]. Commonly used techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), or Raman spectroscopy can provide some information about the material’s local composition but cannot accurately detect any amorphous secondary phases formed in the nanometre scale that are buried at interfaces between SSE and CAM. Consequently, decomposition products can be easily overlooked, resulting in a wide range of reported results. On the other hand, magic angle spinning nuclear magnetic resonance (MAS-NMR) is an ideal technique to identify the decomposition products and probe their local structure even if they are present in small amounts and are highly disordered and amorphous in nature [14]. In this work, we present a detailed characterisation of the changes in the crystal structure and composition of Al-doped LLZO (Al-LLZO) and NMC811 composite as a function of temperature, sintering atmosphere, and conductive agent using ex-situ/variable-temperature XRD, thermogravimetric analysis/differential scanning calorimetry coupled with mass spectrometry (TGA/DSC-MS), and MAS-NMR. Ex-situ 27Al MAS NMR spectroscopy provides evidence of the evolution of Al3+ coordination environment upon annealing, providing accurate and high-resolution atomic scale insights on elemental interdiffusion and reaction onset temperature between Al-LLZO and NMC811.

Open Access

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Abstract: Achieving reversible O-redox through the formation of electron–holes on O could hold the key to a new generation of high energy density Na-ion cathodes. However, to date, it has only been demonstrated in a small handful of cathode materials and none of these materials exploit the dual benefit of high voltage transition metal redox and O-redox, instead relying on Mn3+/4+ capacity close to 2 V vs Na+/Na. Here, a new Na-ion cathode exhibiting electron–holes on O is demonstrated, P2-type Na0.67Li0.1Ni0.3Mn0.6O2, which also utilizes the high voltage Ni3+/4+ redox couple to deliver the highest reported energy density among this class of compound. By employing a low Li content and avoiding honeycomb ordering within the transition metal layer, it is possible to stabilize the hole states, and the high voltage plateau is preserved in Na0.67Li0.1Ni0.3Mn0.6O2 over cycling.

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Abstract: Producing green hydrogen in a cost-competitive manner via water electrolysis will make the long-held dream of hydrogen economy a reality. Although platinum-based catalysts show good performance towards hydrogen evolution reaction (HER), the high cost and scarce abundance challenge their economic viability and sustainability. Here, we show a non-platinum, high-performance electrocatalyst for HER achieved by engineering high fractions of stacking fault defects for MoNi4/MoO2 nanosheets (d-MoNi) through a combined chemical and thermal reduction strategy. The d-MoNi catalyst offers ultralow overpotentials of 78 and 121 mV for HER at current densities of 500 and 1000 mA cm−2 in 1 M KOH, respectively. The defect-rich d-MoNi exhibits 4 times higher turnover frequency than the benchmark 20% Pt/C, together with its excellent durability (>100 h), making it one of the best-performing non-platinum catalysts for HER. The experimental and theoretical results reveal that the abundant stacking faults in d-MoNi induce a compressive strain, decreasing the proton adsorption energy and promoting the associated combination of *H into hydrogen and molecular hydrogen desorption, enhancing the HER performance. This work provides a new synthetic route to engineer defective metal and metal alloy electrocatalysts for emerging electrochemical energy conversion and storage applications.

Open Access

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Abstract: An increased electrification of society calls for a revolution of battery technologies to further improve energy densities, safety and reduce dependencies on critical raw materials. Here we present a new type of fast magnesium electrolytes for all solid-state batteries created as solid solutions of two other fast Mg2+ ionic conductors, Mg(BH4)2 ∙ NH3 and Mg(BH4)2 ∙ CH3NH2. However, the different ligands introduce stacking faults in the structures of the solid solutions, which are eliminated upon heating to T > 40 °C. The stacking faults appear to influence ionic conductivity, as the samples are less conductive after heating. Interestingly, the ionic conductivity does not correlate directly with the relative ligand content, as the highest conductivity is observed for the 1:1 molar composition (σ(Mg2+) = 7.3 ∙ 10−6 S cm−1 at 40 °C), which also has the lowest melting point of 60 °C. Thus, this work demonstrates a new approach to increase cationic conductivity using mixed ligand systems to alter conduction pathways and introduce microstructural strain.