X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectroscopy studies of electrodeposited molybdenum oxysulfide cathodes for lithium and lithium-ion microbatteries

Thin-film molybdenum oxysulfide cathodes for lithium and lithium-ion microbatteries were fabricated by a simple electrodeposition method. According to Scanning Electron Microscopy (SEM) data, the deposition parameters affect the morphology of the cathodes. X-ray diffraction (XRD) tests indicated that the sub-micron-thick molybdenum oxysulfide films are amorphous or form too small crystallites to give rise to detectable X-ray diffraction peaks. A variety of poly-ion clusters containing both oxygen and sulfur (like MoOS, MoO ₂S and MoS ₂O and others) detected by TOF-SIMS tests unambiguously indicates the formation of molybdenum oxysulfide compounds, and not a mixture of oxides and sulfides, during electrodeposition. The sulfur-to-oxygen ratio in the bulk of the deposit is about 1.76 and does not depend much on the electrodeposition parameters. XPS studies reveal that electrodeposition in unbuffered solutions produces deposits with high oxygen and low sulfur content, as compared with cathodes deposited in buffered solutions. Potentiostatic, as compared to galvanostatic deposition, is followed by the formation of cathode films with slightly higher sulfur and lower oxygen content at the same pH. An increase in the pH of electrolyte solutions from 8 to 9.5 slightly reduces sulfur content, but appreciably increases oxygen concentration. Charge-discharge overpotential of Li/hybrid polymer electrolyte microbatteries is lower in sulfur-rich MoO _xS _y cathodes.