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Electron Transfer Governed Crystal Transformation of Tungsten Trioxide upon Li Ions Intercalation Zhiguo Wang, Yang He, Meng Gu, Yingge Du, Scott X. Mao, and Chongmin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06581 • Publication Date (Web): 30 Aug 2016 Downloaded from http://pubs.acs.org on September 2, 2016
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ACS Applied Materials & Interfaces
Electron Transfer Governed Crystal Transformation of Tungsten Trioxide upon Li Ions Intercalation *
Zhiguo Wang1 , Yang He2, Meng Gu3, Yingge Du4, Scott X. Mao2, Chongmin Wang3* 1
School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu, 610054, P.R. China 2
Department of Mechanical Engineering & Materials Science, University of Pittsburgh, Pittsburgh, PA 15261, USA 3
Environmental Molecular Sciences Laboratories, Pacific Northwest National Laboratory, Richland, WA 99352, USA 4
Physical Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352, USA
*
Corresponding authors. E-mail:
[email protected] (Z.W);
[email protected] (C.M.W)
ABSTRACT Reversible insertion/extraction of foreign ions into/from a host lattice constitutes the fundamental operating principle of rechargeable battery and electrochromic materials. The insertion of foreign ions is a far more commonly observed structural evolution of the host lattice, and for the most cases such a lattice evolution is subtle. However, it has not been clear what factors control such a lattice structural evolution. Based on the tungsten trioxide (WO3) model crystal, we use in situ transmission electron microscopy (TEM) combined with density functional theory calculation to explore the nature of Li ions intercalation induced crystal symmetry evolution of WO3. We discovered that Li insertion into the octahedral cavity of the WO3 lattice will lead to a low to high symmetry transition, featuring a sequential monoclinic→tetragonal→cubic phase transition. The density functional theory results reveal that the phase transition is essentially governed by the electron transfer from Li to the WO6 octahedrons, which effectively leads to the weakening the WO bond and modifying system band structure, resulting in an insulator-to-metal transition. The observation of the electronic effect on crystal symmetry and conductivity is significant, providing
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deep insights on the intercalation reactions in secondary rechargeable ion batteries and the approach for tailoring the functionalities of material based on insertion of ions in the lattice.
Keywords: Tungsten trioxide; Ion intercalation, Phase transformation; in situ TEM; First principles calculation; Electron transfer; Insulator-to-metal transition
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ACS Applied Materials & Interfaces
Introduction Intercalation reaction is the basic electrochemical process of rechargeable ion batteries, which relies on simultaneous intercalation of cations and injection of electrons into the electrode materials. Phase transformation is often induced by the intercalation of cations into the host lattice, and often involves an order-disorder transition and crystal structure changes. With insertion of ions, some electrode materials instantaneously become amorphous at room temperature, which is termed as “electrochemical driven solid-state amorphization”.1-4 whereas new crystalline phases formed as a result of the insertion of foreign ions into the electrode materials.5,
6
Structural
transformation accompanied with electronic properties transformation has also been observed. For example, semiconducting 2H-MoS2 7 will transform into the metallic 1T-MoS2 phase upon alkali metals intercalation.8-11 Ion insertion induced crystal symmetry evolution appears to be a general phenomenon, and a mechanistic understanding of this phenomenon has not been established. In previous work, based on first principles simulation, we have revealed that solid-state amorphization of Li-Si alloys is governed by the local electron-rich environment.12 Tungsten trioxide (WO3) is an important functional material with widespread applications in sustainable technologies, such as electrochromic devices, photocatalysis, gas sensing, and secondary ion batteries.13-16 As an anode electrode for lithium ion batteries, WO3 has a high theoretical capacity of 693 mAh/g through the conversion reaction mechanism involving formation of lithium oxide and tungsten metal.17 The crystal structure of WO3 is constructed by corner sharing WO6 octahedra. Low symmetric structure (monoclinic, triclinic, tetragonal) and high symmetric structure (orthorhomic, cubic, and hexagonal) can be formed by the distortion of each WO6 octahedron.
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The large vacant center sites in WO3 offer ample space for the
intercalation of small ions (e.g., H+, Li+, and Na+). The concurrent flow of electrons and ions from
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one electrode to the other is the distinguishing feature of an electrochemical reaction. Electrochromic materials exhibit reversible and persistent change in the optical properties upon synchronous injection/removal of ions and compensating electrons. Both electrochromic and electrochemical processes share the same fundamental mechanism: the electronic properties of the host materials changed with the insertion of foreign ions, while the crystal structure of the host materials changed at the same time. The evolution of the symmetry of crystalline structure upon H, Li, Na and Ca insertion has been extensively investigated20-23. It has been reported that the phase sequence of the HxWO3 compound is the following: monoclinic phase → orthorhombic phase → tetragonal phase → cubic phase with the increase of x.20 The intercalation of lithium into monoclinic WO3 to form the LixWO3 compound was studied using in situ x-ray diffraction. Earlier researchers found that the monoclinic structure of WO3 exists in the range of 0
