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Ball milling modification endows the expanded graphite with smaller grain size and smaller specific surface area. The increase of defects is beneficial for the adsorption of potassium ions on the surface of expanded graphite. In situ XRD demonstrated that defects promote the staging reaction, resulting in higher potassium ion intercalation voltage, slower diffusion kinetics, and more thorough stage mechanism. [Display omitted] • Ball milled samples indicate that defects are beneficial for the insertion of potassium ions between graphite layers, promoting the progress of stage reactions. • The thorough stage reactions provide the sample with superior capacity and rate performance. • The diluted stage reacts at a relative higher voltage promoted by defects, resulting in higher average voltages for defect-rich sample. Expanded graphite (EG) stands out as a promising material for the negative electrode in potassium-ion batteries. However, its full potential is hindered by the limited diffusion pathway and storage sites for potassium ions, restricting the improvement of its electrochemical performance. To overcome this challenge, defect engineering emerges as a highly effective strategy to enhance the adsorption and reaction kinetics of potassium ions on electrode materials. This study delves into the specific effectiveness of defects in facilitating potassium storage, exploring the impact of defect-rich structures on dynamic processes. Employing ball milling, we introduce surface defects in EG, uncovering unique effects on its electrochemical behavior. These defects exhibit a remarkable ability to adsorb a significant quantity of potassium ions, facilitating the subsequent intercalation of potassium ions into the graphite structure. Consequently, this process leads to a higher potassium voltage. Furthermore, the generation of a diluted stage compound is more pronounced under high voltage conditions, promoting the progression of multiple stage reactions. Consequently, the EG sample post-ball milling demonstrates a notable capacity of 286.2 mAh g-1 at a current density of 25 mA g−1, showcasing an outstanding rate capability that surpasses that of pristine EG. This research not only highlights the efficacy of defect engineering in carbon materials but also provides unique insights into the specific manifestations of defects on dynamic processes, contributing to the advancement of potassium-ion battery technology. [ABSTRACT FROM AUTHOR]

Potassium-ion batteries (PIBs) have emerged as promising candidates for cost-effective and sustainable energy-storage systems. Nevertheless, limited by the large K+ radius, PIBs have great difficulty in figuring out and designing suitable host materials. Herein, a suitable cathode material K3V2(PO4)2O2F (KVPOF) for PIBs has been carefully prepared. It exhibits a high specific capacity close to the theoretical value, 116.3 mAh/g at 20 mA/g within the voltage window of 2.0–4.5 V vs K+/K, corresponding to a de-/intercalation process of ∼2 mol K+ per formula unit. In addition, it presents an average operating voltage plateau of about 3.5 V, resulting in an energy density of about 410 Wh/kg. The crystal structure and phase transition are revealed by in situ x-ray diffraction, and the structure is found to be fully reversible during the de-/intercalation of K+. Furthermore, the potential of KVPOF cathode for applications at low temperatures was explored, and the full cell matched with graphite anode demonstrated fair electrochemical performance. The experimental results suggest the feasibility of using KVPOF as cathode material for rechargeable PIBs. [ABSTRACT FROM AUTHOR]

• This paper reviews history and evolution of layered oxides. • The challenges and prospects are included. • This review paper also offers some guidance for further designing layer oxides. Rapid exploitation of renewable energy sources for replacing the conventional fossil fuels drives the development of electrical energy storage (EES) systems. Sodium-ion batteries (NIBs) and potassium-ion batteries (KIBs) are considered as the promising low-cost candidates for the application in large-scale energy storage by virtue of the abundant reserves of sodium and potassium resources. In NIBs and KIBs, cathode plays a critical role in the electrochemical performances, and hence searching for appropriate cathode materials becomes the key point. Particularly, layered oxide cathodes with superior specific capacity and appropriate operating voltage are the most fascinating electrode materials for NIBs and KIBs. In light of performances, the fundamental and recent researches of layered oxide cathodes are reviewed for NIBs and KIBs. However, several major challenges including irreversible phase transition, low energy density, poor air stability, oxygen redox chemistry and inferior cycling stability need to be overcome. All in all, a comprehensive review for the layered oxide cathodes is present accompanied with solutions for these problems, especially mentioning the different functions of different elements and ionic potential (Ф) for guiding the design of layer oxides with exceptional performances. The challenges and prospects are also included with the hope of these materials applying in the next-generation energy storage devices. [ABSTRACT FROM AUTHOR]