Advanced potassium ion batteries anode enhanced by Fe-doping strategy

KVPO₄F (KVPF) is a novel insertion-type anode for potassium ion batteries. For improving its electrochemical performance, the doping strategy was selected and a series of Fe-doped KVPF materials were synthesized by a facile solid-state sintering method to find out the suitable ratio. After comparation, KVPF sample with 5 % Fe-doped ratio (KVPF/Fe-5) delivers the best performance, e.g. rate capacity (94.3 mA h g^-1 at 500 mA g^-1) and long-term discharge capacity (74.1 mA h g^-1 after 1000 cycles at 200 mA g^-1, 0.03 % capacity decay ratio per cycle), which is mainly attributed to its larger K⁺ diffusion rate. The in-situ X-ray diffraction analysis clarify the two-step K⁺ storage mechanism of KVPF/Fe-5 anode, and the density functional theory calculations verify that Fe-doping can reduce diffusion energy barrier of K⁺ and increase electronic conductivity of KVPF. When used as cathode, KVPF/Fe-5 delivers 51.8 mA h g^-1 at 100 mA g^-1 after cycling 200 cycles. In addition, the symmetric cell by employing KVPF/Fe-5 as anode and cathode simultaneously was also assembled successfully, pointing out a new research direction for potassium-ion batteries.