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In-situ observation on the insulator-to-metal transition and non-equilibrium phase transition for Li1-XCoO2 films with preferred (003) orientation nanorods Yue Chen, Qing Yu, Guigui Xu, Guiying Zhao, Jiaxin Li, Zhensheng Hong, Yingbin Lin, Chung Li Dong, and Zhigao Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11140 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 17, 2019
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In-situ observation on the insulator-to-metal transition and non-equilibrium phase transition for Li1-XCoO2 films with preferred (003) orientation nanorods Yue Chen1,a,b Qing Yu1,a,b Guigui Xu,a,b Guiying Zhao,a,c Jiaxin Li,*a,c Zhensheng Hong,a,b Yingbin Lin,a,b Chung-Li Dong,* d and Zhigao Huang * a,c
a. College of Physics and Energy, Fujian Normal University, Fujian Provincial Key Laboratory of Quantum Manipulation and New Energy Materials, Fuzhou，350117, China. b. Fujian Provincial Engineering Technical Research Centre of Solar-Energy Conversion and Stored Energy, Fuzhou，350117, China. c. Fujian Provincial Collaborative Innovation Center for Optoelectronic Semiconductors and Efficient Devices, Xiamen, 361005, China. d. Department of Physics, Tamkang University, 151 Yingzhuan Road, Tamsui 25137, Taiwan.
Keywords: Lithium batteries; LiCoO2 thin film electrode; in situ Peak Force Tunneling AFM; non-equilibrium phase transition; X-ray Absorption Spectroscopy.
ABSTRACT: The stoichiometric LiCoO2 (LCO) with O3-I structure is notoriously difficult to be distinguished from its lithium defective O3-Ⅱ phase due to their similar crystal symmetry. Interestingly, moreover, the O3-II phase shows metallic conductivity, while the O3-I phase is electronic insulator. How to effectively reveal the intrinsic mechanism of the conductivity difference and non-equilibrium phase transition induced by the lithium deintercalation is of vital importance for its practical application and development. Based on the developed technology of in situ Peak
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Force Tunneling AFM (PF-TUNA) in liquid, the phase transition from O3-I to O3-Ⅱ and consequent insulator-to-metal transition (IMT) of LCO thin film electrodes with preferred (003) orientation nanorods designed and prepared via magnetron sputtering, were observed under organic electrolyte for the first time in this work. Then, studying the post-mortem LCO thin film electrode by using ex-situ time-dependent XRD and conductive AFM (C-AFM), we find the phase relaxation of LCO electrodes after the non-equilibrium deintercalation, and further proving the differences of the electronic conductivity and work function between O3-I and O3-II phases. Moreover, XAS results indicate that the oxidation of Co ions and the increasing of O 2p-Co 3d hybridization in O3-II phase lead to the electrical conductivity improvement in Li1-xCoO2. Simultaneously, it is found that the non-equilibrium deintercalation at high charging rate can result in the phase transition hysteresis and the O3-I/O3-Ⅱ coexistence at the charging end, which is explained well by an ionic blockade model with antiphase boundary. At last, this work strongly suggests that PF-TUNA can be used to reveal the unconventional phenomena on the solid/liquid interfaces.
1. INTRODUCTION Layered transition metal oxide (LTMO) remains the most researched and commonly used cathode material in lithium/sodium-ion battery scientists.1-3 Since all the stoichiometric lithium/sodium-based LTMO can be derived from the prototype structure of LiCoO2,4 the studies on its crystal structure and intrinsic physical properties during deintercalation is indispensable for the optimizing LTMO cathode materials. LiCoO2, which has the similar structure as NaCoO2 with alkali metal
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alternate stacking of layers and two-dimensional CoO2 layers, was firstly reported as a cathode materials for batteries in 1980.5 After the discovery of excellent thermoelectric properties for NaxCoO2,6 there have been intense interests in the insulator-to-metal transition of layered Li1-xCoO2.7-11 By using electrochemical delithiated method, Motohashi et al. obtained a series of bulk samples of Li1-xCoO2 with different Li content.8 The measured results of the resistivity indicated that the Li1-xCoO2 samples exhibit a semiconductivity for x< 0.06, while doing metallic conductivity for 0.25
