Revealing interfacial space charge storage of Li/Na/K by operando magnetometry.

Interfacial space charge storage between ionic and electronic conductor is a promising scheme to further improve energy and power density of alkali metal ion batteries (AMIBs). However, the general behavior of space charge storage in AMIBs has been less investigated experimentally, mostly due to the complicated electrochemical behavior and lack of proper characterization techniques. Here, we use operando magnetometry to verify that in FeSe AMIBs, abundant Li/Na/K (M) can be stored at MSe phase while electrons accumulate at Fe nanoparticles, forming interfacial space charge layers. Magnetic and dynamics tests further demonstrate that with increasing ionic radius from Li, Na to K, the reaction kinetics can be hindered, resulting in limited Fe formation and reduced space charge storage capacity. This work lays solid foundation for studying the complex interfacial effect in electrochemical processes and designing advanced energy storage devices with substantial capacity and considerable power density.