Operando hard X-ray photoelectron spectroscopy of LiCoO2 thin film in an all-solid-state lithium ion battery

A novel electrochemical setup for operando hard X-ray photoelectron spectroscopy (HAXPES) was proposed to investigate the bulk-sensitive electronic structure of cations and anions in the active materials of an all-solid-state Li-ion battery. A model all-solid-state thin film cell was prepared by depositing a LiCoO2 positive electrode and an Al current collector on the front of the solid electrolyte substrate, and a Li3PO4 buffer solid electrolyte and a Li negative electrode on the back side, to give a structure of Al (10 nm)/LiCoO2 (35 nm)/Li1+x+yAlx(Ti,Ge)(2-x)SiyP3-yO12 (180 mu m)/Li3PO4 (500 nm)/Li (1000 nm). The reversible changes of the core levels (Al 1s, P 1s, Li 1s, Co 2p(3/2), O 1s) and the valence bands in the charged (4.2 V)/discharged (3.0 V) states were observed without disassembling the LiCoO2 thin film cell inside the vacuum chamber. The operando HAXPES spectra acquired during the charge-discharge process revealed that the reversible changes in the Co 2p(3/2) and O 1s spectra were attributed to the participation of cobalt cations and oxygen anions from LiCoO2 in the redox mechanism.