Durability and rejuvenation of electrochromic tungsten oxide thin films in LiClO4-propylene carbonate viscous electrolyte: Effect of Ti doping of the film and polyethylene oxide addition to the electrolyte

Tungsten oxide and titanium doped tungsten oxide thin films, deposited by sputtering, were immersed in a viscous electrolyte comprised of LiClO4 in propylene carbonate and 2.0 wt% of polyethylene oxide (PEO). Electrochromic properties of the films were investigated by electrochemical techniques and in situ transmittance measurements. Cyclic voltammetry data were taken in the voltage ranges 2.0-4.0 and 1.5-4.0 V vs Li/Li+ for up to 500 cycles. A potentiostatic rejuvenation treatment was then performed on the degraded electrochromic films, at 6.0 V for 20 h, which was subsequently followed by another cyclic voltammetry measurement. Titanium incorporation into tungsten oxide resulted in a small cyclic stability improvement in the 2.0-4.0-V range, whereas less pronounced effects were observed for cycling in the 1.5-4.0-V range. Combining the results of the present study with our previous work, we are able to assess the relative merits of titanium incorporation and PEO addition to the electrolyte for the durability of electrochromic tungsten oxide thin films. Titanium addition was found advantageous for electrochemical durability in the 2.0-4.0-V range, but no clear benefits of PEO in the electrolyte were seen. On the other hand, in the wider 1.5-4.0-V range, tungsten oxide exhibited better durability than titanium-containing films, and this was especially so after rejuvenation in the PEO-containing electrolyte.