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HighlightsThe flower-like δ-MnO2 nanostructures with controlled oxygen vacancies as an extraordinary ZIBs cathode are innovatively developed.The cathode can present large capacity and high-rate zinc-ion storage.DFT analysis substantially unveils the effects of various vacancy concentrations on their electrochemical performances.In recent years, manganese-based oxides as an advanced class of cathode materials for zinc-ion batteries (ZIBs) have attracted a great deal of attentions from numerous researchers. However, their slow reaction kinetics, limited active sites and poor electrical conductivity inevitably give rise to the severe performance degradation. To solve these problems, herein, we introduce abundant oxygen vacancies into the flower-like δ-MnO2 nanostructure and effectively modulate the vacancy defects to reach the optimal level (δ-MnO2−x−2.0). The smart design intrinsically tunes the electronic structure, guarantees ion chemisorption–desorption equilibrium and increases the electroactive sites, which not only effectively accelerates charge transfer rate during reaction processes, but also endows more redox reactions, as verified by first-principle calculations. These merits can help the fabricated δ-MnO2−x−2.0 cathode to present a large specific capacity of 551.8 mAh g−1 at 0.5 A g−1, high-rate capability of 262.2 mAh g−1 at 10 A g−1 and an excellent cycle lifespan (83% of capacity retention after 1500 cycles), which is far superior to those of the other metal compound cathodes. In addition, the charge/discharge mechanism of the δ-MnO2−x−2.0 cathode has also been elaborated through ex situ techniques. This work opens up a new pathway for constructing the next-generation high-performance ZIBs cathode materials.

The ever‐growing demand for high‐energy lithium‐ion batteries in portable electronics and electric vehicles has triggered intensive research efforts over the past decade. An efficient strategy to boost the energy and power density of lithium‐ion batteries is to increase the Ni content in the cathode materials. However, a higher Ni content in the cathode materials gives rise to safety issues. Herein, thermal expansion and oxygen vacancies are proposed as new critical factors that affect the thermal stability of charged Ni‐rich cathode materials based on a systematic synchrotron‐based X‐ray study of Li0.33Ni0.5+xCo0.2Mn0.3‐xO2 (x = 0, 0.1, 0.2) cathode materials during a heating process. Charged cathode materials with higher Ni contents show larger thermal expansion, which accelerates transition metal migration to the Li layers. Oxygen vacancies are formed and accumulate mainly around Ni ions until the layered‐to‐spinel phase transition begins. The oxygen vacancies also facilitate transition metal migration to the Li layers. Thermal expansion and the presence of oxygen vacancies decrease the energy barrier for cation migration and facilitate the phase transitions in charged cathode materials during the heating process. These results provide valuable guidance for developing new cathode materials with improved safety characteristics. Close relationship between Ni content and inherent thermal instability of Ni‐rich cathode material has been widely investigated. However, the underlying cause of thermal instability of Ni‐rich cathode material involving Ni has not been fully studied. The thermal expansion and oxygen vacancy are newly proposed as critical factors to affect the inherent thermal instability of Ni‐rich cathode materials.

Transition metal fluorides (TMFs) cathode materials have shown extraordinary promises for electrochemical energy storage, but the understanding of their electrochemical reaction mechanisms is still a matter of debate due to the complicated and continuous changing in the battery internal environment. Here, we design a novel iron fluoride (FeF2) aggregate assembled with cylindrical nanoparticles as cathode material to build FeF2 lithium‐ion batteries (LIBs) and employ advanced in situ magnetometry to detect their intrinsic electronic structure during cycling in real time. The results show that FeF2 cannot be involved in complete conversion reactions when the FeF2 LIBs operate between the conventional voltage range of 1.0–4.0 V, and that the corresponding conversion ratio of FeF2 can be further estimated. Importantly, we first demonstrate that the spin‐polarized surface capacitance exists in the FeF2 cathode by monitoring the magnetic responses over various voltage ranges. The research presents an original and insightful method to examine the conversion mechanism of TMFs and significantly provides an important reference for the future artificial design of energy systems based on spin‐polarized surface capacitance. In situ magnetometry was first employed to investigate the charge storage mechanism of FeF2 cathode, which clearly reveals that spin‐polarized surface capacitance and incomplete conversion occur when the battery operates between 1.0 and 4.0 V and the corresponding conversion ratio of FeF2 can be further estimated.

Sodium‐ion batteries (SIBs) have attracted considerable research interest for large‐scale energy storage due to the natural abundance and wide geographic distribution of sodium. Among various cathode materials, conversion‐type cathodes have garnered particular attention due to their element richness, high specific capacity, and enhanced safety and reliability. However, their practical application faces challenges, including huge volume expansion, incomplete reversible conversion reactions, and severe side reactions. This review summarizes the unique advantages and critical issues of conversion‐type cathode materials, along with recent advances in various cathodes for SIBs. First, the reaction mechanisms of different conversion‐type cathode materials are analyzed and summarized to provide theoretical foundations for practical implementation. Particularly, we propose novel countermeasures addressing common cathode challenges, offering new perspectives for future research on these materials. Notably, composite conversion‐type cathodes demonstrate substantial potential as evidenced by their remarkable energy density. Future research should focus on in‐depth investigations of reaction mechanisms, modification strategies, and characterization techniques for conversion‐type cathodes, thereby advancing the development of high‐energy‐density cathode materials. Focusing on high‐energy‐density sodium‐ion batteries, this review highlights the advantages offered by conversion‐type cathode materials. The currently studied cathode materials are systematically introduced. By analyzing sulfur, oxygen, and transition metal halides and other conversion‐type cathodes, the key challenges associated with these cathodes are identified, and common strategies are proposed. The promising development prospects of conversion‐type cathode materials are emphasized.

Lithium manganese oxide (LiMn2O4) is an effective cathode material for high-capacity lithium-ion (Li-ion) batteries. Therefore, to optimize battery efficiency, it is essential to understand how sputtering deposition conditions affect the quality and performance of LiMn2O4. This research examines how argon deposition pressure affects the stoichiometric characteristics and electrochemical performance of LiMn2O4. The study finds that changing argon deposition pressures, from a low of 5 mTorr to a high of 30 mTorr, results in the formation of different coating stoichiometries. At low argon deposition pressures, stoichiometric LiMn2O4 cathode coatings formed, exhibiting the highest discharge capacity of 115 mAh/g. Conversely, at high argon deposition pressures, non-stoichiometric LiMn2O4 with lithium deficiency was produced. These coatings exhibited diminished electrochemical behavior, achieving a discharge capacity of only 70 mAh/g at 5 mTorr. The lack of lithium resulted in a significant reduction in electrochemical performance, indicated by a high surface charge transfer resistance (R2 = 48,529 Ω), which led to a low discharge capacity of 40 mAh/g.

Single‐crystal high‐nickel cathode (SC‐HN) materials are promising candidates for advanced lithium‐ion batteries due to their exceptional volumetric and gravimetric energy densities. However, SC‐HN materials face air instability, causing distinct storage failure mechanisms compared to polycrystalline high‐nickel cathode (PC‐HN) materials. The characteristics of SC‐HN, such as their lower specific surface area and reduced grain boundaries, make their failure mechanisms distinct and not directly applicable to PC‐HN materials. To address these unique degradation pathways, this study systematically investigated the storage failure mechanisms of SC‐HN material under ambient air exposure. Using advanced characterization techniques including soft X‐ray absorption spectra (sXAS), wide‐angle X‐ray scattering (WAXS), aberration‐corrected scanning transmission electron microscopy (STEM), and etching‐based X‐ray photoelectron spectroscopy (XPS), we conducted comprehensive multi‐dimensional analyses over 6 months to track the evolution of chemical and structural changes. The results reveal that SC‐HN materials experience a nonlinear progression of structural and surface composition degradation, and surface structural transformations are found to be the main cause of performance decline. The findings deepen understanding of SC‐HN air instability and provide a basis for targeted strategies to enhance storage stability. Single‐crystal high‐nickel cathodes exhibit unique air instability, with surface structural transformations identified as the primary cause of performance decline. Using advanced characterization, this study reveals nonlinear chemical and structural degradation under air exposure, offering critical insights into the storage failure mechanisms and strategies to enhance their storage stability for next‐generation batteries.

Research on the regeneration of cathode materials of spent lithium-ion batteries for resource reclamation and environmental protection is attracting more and more attention today. However, the majority of studies on recycling lithium-ion batteries (LIBs) placed the emphasis only on recovering target metals, such as Co, Ni, and Li, from the cathode materials, or how to recycle spent LIBs by conventional means. Effective reclamation strategies (e.g., pyrometallurgical technologies, hydrometallurgy techniques, and biological strategies) have been used in research on recycling used LIBs. Nevertheless, none of the existing reviews of regenerating cathode materials from waste LIBs elucidated the strategies to regenerate lithium nickel manganese cobalt oxide (NCM or LiNixCoyMnzO2) cathode materials directly from spent LIBs containing other than NCM cathodes but, at the same time, frequently used commercial cathode materials such as LiCoO2 (LCO), LiFePO4 (LFP), LiMn2O4 (LMO), etc. or from spent mixed cathode materials. This review showcases the strategies and techniques for regenerating LiNixCoyMnzO2 cathode active materials directly from some commonly used and different types of mixed-cathode materials. The article summarizes the various technologies and processes of regenerating LiNixCoyMnzO2 cathode active materials directly from some individual cathode materials and the mixed-cathode scraps of spent LIBs without their preliminary separation. In the meantime, the economic benefits and diverse synthetic routes of regenerating LiNixCoyMnzO2 cathode materials reported in the literature are analyzed systematically. This minireview can lay guidance and a theoretical basis for restoring LiNixCoyMnzO2 cathode materials.

Polycrystalline Ni‐rich layered oxides are promising cathodes for Li‐ion batteries of high‐power density and long cycle life. However, their practical application is still hindered by the sluggish Li+ diffusion rate and reaction inhomogeneity during redox cycles. In this work, LiNi0.9Co0.05Mn0.05O2 (NCM9055) cathode with a desired internal radial structure was designed and successfully synthesized using Nb2O5 as a dual‐functional structural and interfacial modulator. During calcination, the Nb2O5 reacts to form an intergranular LiNbO3 phase at grain boundaries. This phase, forming before high‐temperature grain growth, acts as a structural modulator to preserve the desirable radial alignment of primary particles by impeding random grain growth. It also functions as an interfacial conductor, creating fast Li+ diffusion pathways along the grain boundaries. These structural and interfacial modifications synergistically mitigate chemical inhomogeneity and relieve accumulated strain during cycling. Consequently, the Nb‐modified NCM9055 exhibits superior electrochemical performance, delivering an excellent rate capacity (152.4 mA h g−1 at 10 C) and robust cycling stability under high‐rate conditions (83.0% capacity retention after 500 cycles at 5C). These findings clarify the mechanism of Nb modulation and demonstrate a robust strategy for preserving desirable microstructures in high‐rate, Ni‐rich cathode materials. A dual‐functional Nb2O5 modulation strategy is reported to overcome kinetic limitations in Ni‐rich cathodes. The formation of LiNbO3 phase at the grain boundary of the polycrystalline NCM9055 cathode preserves a radially aligned microstructure and establishes fast lithium‐ion pathways. These synergistic effects alleviate internal strain and mitigate chemical inhomogeneity, delivering an exceptional fast‐charging performance and a robust cycling stability for next‐generation batteries.

Co‐sintering cathode materials with Li7La3Zr2O12 (LLZ) is a promising strategy for fabricating bulk‐type all‐solid‐state batteries (ASSBs). However, preventing reactions between different materials, which is difficult with high‐capacity cathode materials such as LiNi0.6Mn0.2Co0.2O2 (NMC622), is a pre‐requisite for applying this strategy. To overcome this issue, Li1+xNi0.6Mn0.2Co0.2O2 (x = 0.01–0.2), which intentionally deviates from the stoichiometric NMC622 composition, is synthesized here. The formation of impurity phases in the co‐sintering process can be controlled by adjusting the co‐sintering temperature and x. Impurity phases are not formed on co‐sintering with x = 0.075 at 800 °C because reduced cation mixing in NMC622 and the presence of a self‐formed Li2CO3 layer on the particle surface, ensured by adjusting x, effectively suppresses reactions. Furthermore, good results are observed at sintering temperatures where the proportions of Ni2+ and Co2+, which promote cation mixing, are low. This study clarifies relevant reaction mechanisms using various analytical methods (such as temperature‐rise X‐ray absorption fine structure analysis and scanning transmission electron microscopy‐electron energy loss spectroscopy), and confirms the repetitive operation of bulk‐type ASSBs assembled using co‐sintered Li1+xNi0.6Mn0.2Co0.2O2 (x = 0.075)/LLZ electrolyte systems. The method reported herein can be potentially adopted for cost‐effective and high‐energy‐capacity ASSB production. A bulk‐type all‐solid‐state battery is produced by co‐sintering the proprietary NMC622 and LLZ. This proprietary NMC622 can avoid the formation of impurity phases under co‐sintering. The reduced Ni2+ fraction in NMC622 and the presence of a self‐formed Li2CO3 surface layer result in this outcome. It is possible to realize a rechargeable all‐solid‐state battery.

Cobalt‐free LiNi0.5Mn1.5O4 (LNMO) has recently emerged as a highly promising cathode material owing to its benefits of a high operating voltage platform (≈4.7 V vs Li), high theoretical energy density (≈650 Wh kg−1), eco‐friendliness, and resource abundance. However, it has also demonstrated low cycle and poor rate performances. Researchers have hitherto identified multiple LNMO failure and degradation mechanisms, including the Jahn‐Teller effect, Transition Metal (TM) dissolution, electrolyte decomposition, and Oxygen Vacancies (OVs). The Jahn‐Teller effect causes structural material degradation, while TM dissolution could lead to the loss of reactive species and interfacial side reactions. On the other hand, OVs and electrolyte decomposition accelerate capacity decay. Notably, deeply understanding LNMO structural failure mechanisms and the targeting of corresponding modifications presents a vital avenue for modulating its surface‐interface structure and improving its electrochemical performance. Although researchers have extensively investigated the failure mechanisms of LNMO to elucidate its modification strategies, a comprehensive and detailed summary of the latest research advancements has yet to be provided. In this work, the research background, encompassing the advantages and disadvantages of LNMO cathode materials, is first introduced. The crystal structure and discharge mechanisms, among other fundamental principles of LNMO, are subsequently analyzed. Finally, recent research findings on the aforementioned failure mechanisms in high‐voltage spinel LNMO are synthesized. Subsequently, a critical assessment of recent advancements in modification strategies targeting the failure mechanisms of LNMO is performed, encompassing the tools employed (e.g., doping modification, surface coating, morphology and size management, and surface orientation management) as well as their synergistic effects. Finally, potential future research directions to guide the rational design of high‐performance LNMO, particularly manganese‐based spinel cathode material, are proposed. This review provides an overview of the development history, working mechanisms, and scientific challenges of LiNi0.5Mn1.5O4 cathode materials for Lithium‐ion battery and discusses the strategies to improve the electrochemical performance of LiNi0.5Mn1.5O4.