Structure Control And Lithium Storage Characteristics Of WO₃-based Electrochromic Thin Film And Integration Of Dual-functional Device

As an electrochromic material,WO₃ is well-known for advantages including low cost,fast response,and high coloration efficiency.And it can also serve as the anode in lithium ion batteries.However,applying WO₃ into practical applications widely is still confronted with some problems,for example,poor cycling stability and the problem of energy supply in portable electrochromic devices.In this paper,targeting on solving the problem of poor cycling stability of WO₃ made by magnetron sputtering method in electrochromic and charge-discharge cycling process,MoO₃ thin layer was introduced to prepare double-layered thin film,MoO₃/WO₃.Further,the optimal thickness of MoO₃ layer had also been studyed.When it comes to energy supply of WO₃-based portable electrochromic devices,WO₃ was assembled into full lithium ion battery to drivie the color-changing process of WO₃ thin film,which had verified the feasibility of their integration.The following are the main conclusions.Firstly,by magnetron sputtering method,WO₃ thin films had been made under the sputtering pressure of 1.5,2.0 and 2.5 Pa(shorted as W_1.5,W_2.0 and W_2.5).Statistics showed that the biggest transmittance modulation range(74.87 and 79.07%at 630 and 550 nm relatively),relatively quick color-changing response(coloring and bleaching response time:11.95 and 5.65 s)and higher coloration efficiency manifested in W_2.0.Besides,after the bleaching-coloring cycling test,both CV and CA curves said that W_2.0 had the smallest intrinsic attenuation and the best cycling performance.When it was used in half lithium ion battery,the capacity was the biggest.Specifically,after 450 charge-discharge cycles,its capacity retention was the highest,79.3%.Further,full lithium ion batteries had been assembled with W_2.0 thin film as its anode and then drove the color-changing process of W_2.0electrochromic thin film,which indicated the success of their simple integration.Secondly,in order to strengthen the performance of WO₃,MoO₃,which is also a cathodic electrochromic material and anode material in lithium ion battery,had been chosen to compound with WO₃.Relevant tests had been conducted on these films.When the MoO₃layer is located above the WO₃ layer(WO₃/MoO₃),the micropores on the surface of the film enlarged and the overall porosity increased,doing service to the release of Li⁺from the film,leading to fast bleaching response.But at the same time,it also caused drastic deterioration during cycling.After cycling,the area enclosed by the CV curve was only 45.72%of the area before cycling,and the capacity after 440 charge-discharge cycles when the film was assembled into a half cell was only 0.078 m A h.When the MoO₃ layer was beneath the WO₃layer(MoO₃/WO₃),the cycling stability of the composite film was effectively strengthened.After cycling test,the transmittance modulation range of the film increased slightly(transmittance modulation range at 630 and 550 nm were 71.03 and 70.11%relatively).And the half cell with MoO₃/WO₃ thin layer as anode still had discharge capacity of 0.219 m A h.Although MoO₃/WO₃/MoO₃composite film had better performance than WO₃/MoO₃,the impact of the MoO₃ layer on its surface on cycling performance was greater,so the cycling performance of the sandwich structure film was inferior to that of the MoO₃/WO₃double-layered film.Finally,in order to explore the influence of thickness of MoO₃ layer on the performance of MoO₃/WO₃ films,MoO₃/WO₃composite films with different MoO₃ thicknesses were prepared.Results showed that when the MoO₃ layer was very thin,it has limited positive effect.And when sputtering time was 4.5 min,with thickness of MoO₃ layer was about5.85?6.75 nm,the improvements were most significant.However,if the sputtering time was further prolonged to 10 min and MoO₃ layer was thicker,a large amount of Li⁺were trapped in MoO₃ film.After cycling,the transmittance of bleached state decreased greatly.As for the charge-discharge cycling tests,MoO₃-4.5 min-WO₃ assembled lithium ion half cell also had the highest discharge capacity after 400 cycles.Finally,full battery assembled with MoO₃-4.5min-WO₃ film was used to drive its electrochromic process.And it showed that using only one full battery(charged to 3 V)to drive the coloring behavior of the MoO₃-4.5 min-WO₃electrochromic composite film was possible.After that,to study the effect of increased voltage and increased current(two full batteries in parallel and in series respectively)on the electrochromic behavior of the composite film were conducted.Results showed that both cases can reduce the transmittance of the film in colored state,that is,the film colored more deeply.On the whole,when the voltage was increased to 6 V,the electrochromic performance of the electrochromic film was relatively better.