Electrochemical properties of tungsten oxysulphide thin films as positive electrodes for lithium microbatteries

Several WOySz tungsten oxysulphide thin films were tested as positive electrodes for lithium microbatteries. The amorphous WO1.05S2 thin film was found very promising. A capacity decrease occurred during the first few cycles, after which the films were able to intercalate reversibly up to 11 lithium ion per formula unit under high regime (75 μA/cm2). They were tested for 250 charge-discharge cycles, between 30 V and 1.2 V. X-ray photoelectron spectroscopy measurements were performed on different compounds in both intercalated (Li1WO1.05S2, Li2.7WO1.05S2 and Li3.8WO1.05S2) and partially deintercalated (Li1WO1.05S2) states in order to understand the redox processes occurring during the first dischargecharge cycle. The analysis of both the W4f and the S2p peaks has shown that the redox processes involve not only the tungsten atoms but also sulphur atoms. At the beginning of the intercalation, W6+ was first partially reduced into W5+, and then into W4+, but the important stage was the reduction of W4+ into W0. In W0, the electron binding energy was very close to that of metallic tungsten. At the same time, S 2 2- ions were partially reduced into S2- ions. But only the reduction process of tungsten atoms appeared to be totally reversible.