High‐entropy oxides (HEOs) have emerged as promising anode materials for next‐generation lithium‐ion batteries (LIBs). Among them, spinel HEOs with vacant lattice sites allowing for lithium insertion and diffusion seem particularly attractive. In this work, electrospun oxygen‐deficient (Mn,Fe,Co,Ni,Zn) HEO nanofibers are produced under environmentally friendly calcination conditions and evaluated as anode active material in LIBs. A thorough investigation of the material properties and Li⁺ storage mechanism is carried out by several analytical techniques, including ex situ synchrotron X‐ray absorption spectroscopy. The lithiation process is elucidated in terms of lithium insertion, cation migration, and metal‐forming conversion reaction. The process is not fully reversible and the reduction of cations to the metallic form is not complete. In particular, iron, cobalt, and nickel, initially present mainly as Fe³⁺, Co³⁺/Co²⁺, and Ni²⁺, undergo reduction to Fe⁰, Co⁰, and Ni⁰ to different extent (Fe < Co < Ni). Manganese undergoes partial reduction to Mn³⁺/Mn²⁺ and, upon re‐oxidation, does not revert to the pristine oxidation state (+4). Zn²⁺ cations do not electrochemically participate in the conversion reaction, but migrating from tetrahedral to octahedral positions, they facilitate Li‐ion transport within lattice channels opened by their migration. Partially reversible crystal phase transitions are observed.

Aqueous zinc‐ion batteries (AZIBs), have been identified as one of the most promising aqueous rechargeable metal‐ion batteries (ARMIBs) for grid scale electrochemical energy storage application, and as such have received much attention by virtue of their unique properties including the high abundance of metallic zinc resources, their intrinsic safety, and cost effectiveness. Among the cathode materials used in AZIBs, Prussian Blue Analogues (PBAs) represent the most promising active materials in view of their improved cyclability and rate performance. Here, we provide a comprehensive review on recent progress on AZIBs obtained using PBAs cathodes with a particular focus on the present understanding of the structure‐property correlation of different PBAs, which in turn guides further developments of improved cathode materials and optimization strategies to ensure an extended cyclability and capacity retention.

High-entropy oxides with spinel structure (SHEOs) are promising anode materials for next-generation lithium-ion batteries (LIBs). In this work, electrospun (Mn,Fe,Co,Ni,Zn) SHEO nanofibers produced under different conditions are evaluated as anode materials in LIBs and thoroughly characterised by a combination of analytical techniques. The variation of metal load (19.23 or 38.46 wt% relative to the polymer) in the precursor solution and of calcination conditions (700 °C/0.5 h, or 700 °C/2 h followed by 900 °C/2 h) affects the morphology, microstructure, crystalline phase, and surface composition of the pristine SHEO nanofibers and the resulting electrochemical performance, whereas mechanism of Li⁺ storage does not substantially change. Causes of long-term (≥650 cycles) capacity fading are elucidated via ex situ synchrotron X-ray absorption spectroscopy. The results evidence that the larger amounts of Fe, Co, and Ni cations irreversibly reduced to the metallic form during cycling are responsible for faster capacity fading in nanofibers calcined under milder conditions. The microstructure of the active material plays a key role. Nanofibers composed by larger and better-crystallized grains, where a stable solid/electrolyte interphase forms, exhibit superior long-term stability (453 mAh g⁻¹ after 550 cycles at 0.5 A g⁻¹) and rate-capability (210 mAh g⁻¹ at 2 A g⁻¹).

Among different Prussian Blue Analogues (PBAs), manganese hexacyanoferrate (MnHCF), with open framework and two abundant electroactive metal sites, exhibits high potential for the grid‐scale aqueous rechargeable zinc‐ion batteries (ARZIBs) application. Until now, the intercalation mechanism of Zn ²⁺ into MnHCF has not been clearly illustrated. In this work, combining different synchrotron X‐ray techniques, the structural and microscopic evolution of MnHCF in 3 m ZnSO ₄ electrolyte is comprehensively studied, and a thorough understanding of the intercalation/release dynamic, in terms of local and long‐range domain, is provided. The elemental distribution and structural information of Fe, Mn, Zn inside MnHCF electrodes is obtained from the X‐ray fluorescence (XRF) elemental maps and X‐ray absorption spectroscopy (XAS). The in‐depth analysis of extended X‐ray absorption fine structure (EXAFS) signals confirm that the rearrangement of Mn site, evidencing the cleavage of the Mn ─ N bond with the formation of a Mn ─ O bond, in an octahedral environment. The phase transformation of MnHCF takes place exclusively during the 1st cycle, and a mixture of rhombohedral and cubic zinc hexacynoferrate (ZnHCF) phases are formed during the first charge process. Thereafter, the newly formed cubic ZnHCF phase becomes the only stable one, existing in the subsequent cycles and exhibiting excellent electrochemical stability.

Symmetric batteries, in which the same active material is used for the positive and the negative electrode, simplifying the manufacture process and reducing the fabrication cost, have attracted extensive interest for large-scale stationary energy storage. In this paper, we propose a symmetric battery based on titanium hexacyanoferrate (TiHCF) with two well-separated redox peaks of Fe³⁺/Fe²⁺ and Ti⁴⁺/Ti³⁺ and tested it in aqueous Na-ion/ K-ion/Mg-ion electrolytes. The result shows that all the symmetric batteries exhibit a voltage plateau centered at around 0.6 V, with discharge capacity around 30 mAhg⁻¹ at C/5. Compared to a Mg-ion electrolyte, the TiHCF symmetric batteries in Na-ion and K-ion electrolytes have better stability. The calculated diffusion coefficient of Na⁺, K⁺, and Mg²⁺ are in the same order of magnitude, which indicates that the three-dimensional ionic channels and interstices in the lattice of TiHCF are large enough for an efficient Na⁺, K⁺ and Mg²⁺ insertion and extraction.

Manganese hexacyanoferrate is a promising cathode material for lithium and sodium ion batteries, however, it suffers of capacity fading during the cycling process. To access the structural and functional characteristics at the nanometer scale, fresh and cycled electrodes are extracted and investigated by transmission soft X‐ray microscopy, which allows chemical characterization with spatial resolution from position‐dependent x‐ray spectra at the Mn L‐, Fe L‐ and N K‐edges. Furthermore, soft X‐rays prove to show superior sensitivity toward Fe, compare to hard X‐rays. Inhomogeneities within the samples are identified, increasing in the aged electrodes, more dramatically in the Li‐ion system, which explains the poorer cycle life as Li‐ion cathode material. Local spectra, revealing different oxidation states over the sample with strong correlation between the Fe L‐edge, Mn L‐edge, and N K‐edge, imply a coupling between redox centers and an electron delocalization over the host framework.