In situ atomic-scale observation of size-dependent (de) potassiation and reversible phase transformation in tetragonal FeSe anodes

Potassium-ion batteries(PIBs)are considered promising alternatives to lithium-ion batteries owing to cost-effective potassium resources and a suitable redox potential of-2.93 V(vs.-3.04 V for Li+/Li).However,the exploration of appro-priate electrode materials with the correct size for reversibly accommodating large K+ions presents a significant challenge.In addition,the reaction mecha-nisms and origins of enhanced performance remain elusive.Here,tetragonal FeSe nanoflakes of different sizes are designed to serve as an anode for PIBs,and their live and atomic-scale potassiation/depotassiation mechanisms are revealed for the first time through in situ high-resolution transmission electron micros-copy.We found that FeSe undergoes two distinct structural evolutions,sequen-tially characterized by intercalation and conversion reactions,and the initial intercalation behavior is size-dependent.Apparent expansion induced by the intercalation of K+ions is observed in small-sized FeSe nanoflakes,whereas unexpected cracks are formed along the direction of ionic diffusion in large-sized nanoflakes.The significant stress generation and crack extension originating from the combined effect of mechanical and electrochemical interactions are elucidated by geometric phase analysis and finite-element analysis.Despite the different intercalation behaviors,the formed products of Fe and K_(2)Se after full potassiation can be converted back into the original FeSe phase upon depotassiation.In particular,small-sized nanoflakes exhibit better cycling perfor-mance with well-maintained structural integrity.This article presents the first successful demonstration of atomic-scale visualization that can reveal size-dependent potassiation dynamics.Moreover,it provides valuable guidelines for optimizing the dimensions of electrode materials for advanced PIBs.

Recent progresses on alloy-based anodes for potassium-ion batteries

Potassium-ion batteries(KIBs)are one of the most promising large-scale electric energy storage systems due to the high abundance and low redox potential of K.As the key component,anode determines their energy density and safety.Alloy-based anodes,such as P,Sn,Sb,and Bi,have attracted extensive attention due to their abundant resources,suitable working potentials,and large theoretical capacities.However,the dramatic volume variation upon(de)potassiation results in pulverization of particles and their detaching from the current collector accompanied with performance decay.Various strategies,including designing micro-/nanostructures,introducing carbon substrates,and optimizing electrode/electrolyte interface,have been demonstrated to effectively alleviate these issues.Herein,we summarize the recent research progresses on alloy-based materials in KIBs.The synthesis methods,electrochemical performance,reaction mechanisms,and structure-activity relationships of these materials are considered,and challenges and perspectives are provided.This review provides new insight into designing of high-activity electrode materials for KIBs and beyond.