β-FeOOH: a new anode for potassium-ion batteries.

Potassium-ion batteries (PIBs) hold great potential in large-scale energy storage systems. Here, beta-FeOOH is employed as an anode for PIBs, exhibiting high capacity and good cycling stability. The study of potassium storage mechanism discloses that beta-FeOOH turns into an amorphous structure in the first discharge process, and remains stable in the amorphous state in the subsequent cycles.