Investigation On The Phase Transformation And Degradation Mechanism Of The Active Interface Of WO₃

Tungsten trioxide（WO₃）as a transition metal oxide has been considered as the most promising cathodic electrochromic material,owing to low power consumption,high optical contrast and long-term optical memory effect.However,the phase transformation and electrochromic degradation mechanism on the active interface of WO₃ are still ambiguous,and it suffers from performance decline and structural corrosion during electrochemical cycles which limits practical application in electrochromic devices.In this paper,WO₃ nanowires and thin films are prepared by hydrothermal method and electron beam evaporation,respectively.By taking advantage of in situ transmission electron microscopy（TEM）and electrochemical quartz crystal microbalance technology（EQCM）,the phase transformation and electrochromic degradation mechanism on the active interface of WO₃are studied systematically,respectively.Herein,the lithium ions were pre-introduced into the active interface,and the mechanism of action of dry prelithiation is investigated.The main research contents of this paper can be summarized as follows:1.Investigation on the structure transformation of the active interface of WO₃.The WO₃nanowires are synthesized by hydrothermal method,and a single WO₃ nanowire based in situ TEM cell is assembled.Investigation on the real-time phase transformation,dynamic morphology evolution,electrochemical reaction kinetics and microscopic mechanism of WO₃ during the lithiation process is performed by in situ TEM technology.When an external driving potential of-2.0 V is applied,the lithiation reaction of WO₃ is occurred,forming a lithiated layer on the surface and causing an average expansion in ab-plane direction by 24.7%.The migration of lithium ions along the ab-plane（radial）and c-axis（longitudinal）directions is anisotropic.Most importantly,our unique experimental design directly demonstrates the irreversible phase transformation from h-WO₃to Li₂WO₄ during deep lithiation.Generally,WO₃ is reversibly transformed into Li_xWO₃ phase under the action of voltage.However,when the structural transformations of the WO₃ active interface occur during deep lithiation reaction,the WO₃/electrolyte interface properties should change abruptly,which suggests that the active interface properties of the WO₃ electrodes should depend on all the stable Li₂WO₄ and metastable Li_xWO₃ structures and their transformations.2.Study on the degradation mechanism of the WO₃ thin films.The WO₃ thin films are deposited on the Au-coated quartz crystal oscillator by electron beam evaporation technology.The real-time mass change of the WO₃ films during the electrochemical process is recorded by electrochemical quartz crystal microbalance technology in an open three-electrode electrochemical cell composed of WO₃ films/quartz crystal working electrode,Au counter electrode and Ag/Ag Cl reference electrode.Based on electrochemical quartz crystal microbalance analysis,it can be concluded that three kinds of mass carriers（Li⁺,Cl O₄^-and C₄H₆O₃）are participated in the charge transfer process,establishing the migration mechanism of mass carriers at the electrode/electrolyte interface.The mass carrier intercalation/extraction are mainly depended on the applied voltage and scan rates.The irreversible interfacial electrochemical reactions and the collapse of the oxide network structure co-contributes to the electrochromic degradation of WO₃ thin films.3.Research on the dry prelithiation of the WO₃ electrochromic films and devices.A novel dry prelithiation of the WO₃ thin films is carried out by in situ continuous preparation process of electron beam evaporation to modify the active interface.The electrochromic and electrochemical properties of prelithiated and unlithiated WO₃ films and devices is systematically investigated.After dry prelithiation,the initial color of the WO₃ films is changed from transparent to light blue,accompanying transmittance variation from 77.8%to 27.6%atλ₆₃₃ nm.Chronoamperometry test and analysis show that the intercalated lithium can be divided into 47.8%of‘live lithium’and 52.2%of‘dead lithium’.On one hand,the enhancement of the electrochemical cycling stability and charge density can be ascribed to the preactivation of‘active lithium’,which enlarges the surface area for ion transporting.On the other hand,the irreversible formation of‘dead lithium zone’in the prelithiated WO₃ films originates from‘dead lithium seed’,leading to the detriment of optical properties.