Neutron reflectometry analysis of Li 4 Ti 5 O 12 /organic electrolyte interfaces: characterization of surface structure
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Norifumi L. Yamada and Masao Yonemura Neutron Science Division, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tokai, Ibaraki 319-1106, Japan
Kota Suzuki and Ryoji Kanno Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, Midori-ku, Yokohama 226-8502, Japan (Received 8 June 2016; accepted 19 August 2016)
The structure changes and lithium intercalation properties in the surface region of Li4Ti5O12 were investigated using epitaxial Li4Ti5O12(111) film model electrodes. The discharge–charge measurements, which were conducted with 1 mol/dm3 LiPF6-containing propylene carbonate, revealed that a 23.8 nm-thick film exhibited a small capacity of 115 mA h/g compared to the theoretical value of 175 mA h/g. In situ neutron reflectometry and ex situ x-ray diffractometry and reflectometry indicated that an irreversible phase change had occurred in the 10-nm surface region of Li4Ti5O12 during the initial reaction processes. The level of deterioration of the surface structure was significantly reduced by decreasing the LiPF6 concentration; in addition, side reactions of the cell components with the electrolyte species, and their products, may be associated with the deterioration of the Li4Ti5O12 surface. The surface reactions have a significant impact on the capacity of lithium intercalation in nano-sized Li4Ti5O12.
I. INTRODUCTION
Lithium ion batteries are driven by electrochemical intercalation, whereby lithium ions are inserted and extracted within gaps in the host lattice without substantial changes to the lattice itself,1–3 and the performance of lithium ion batteries is intrinsically dependent on the structure and/or composition of the intercalation electrodes.4–6 Li4Ti5O12, which possesses a spinel-type structure, has been advocated as an attractive anode material for use in lithium ion batteries of long calendar life. The deintercalation of lithium ions from the Li4Ti5O12 lattice proceeds with the two-phase coexistence of Li 4Ti 5O12 and Li7 Ti 5O12 domains with similar lattice parameters, 7–9 which leads to a very small variation in volume during lithium intercalation/deintercalation.7 The macroscopic zero-strain behavior contributes to the superior cycle stability of the Li4Ti5O12 electrode during electrochemical cycling.
Contributing Editor: Chongmin Wang a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.320
Nanosizing of Li4Ti5O12 has recently been the focus for improving the rates of lithium deintercalation by reducing the path lengths for electronic and lithium ion transport in the electrode.10–13 As the surface area of the Li4Ti5O12 electrode increases with decreasing particle size, interfacial phenomena at the Li4Ti5O12/organic liquid electrolyte interface influences the electrode properties.12,14 Although interfacial behavioral patterns have been widely reported for oxide cathodes and carbon anodes,15–20 there is insufficient experimental informa
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