Is Li4Ti5O12 a Solid-Electrolyte-Interphase-Free Electrode Material in Li-Ion Batteries? Reactivity Between Li4Ti5O12 Electrode and Electrolyte
Authors: Song Min-Sang , Shon Jeong-Kuk , Kim Ryoung-Hee , Choi Jae-Man , Park Kyusung , Benayad Anass 
Published: February 2020

Li4Ti5O12 with a cubic spinel structure (space group, Fd3(_)m) has a high redox potential at around 1.5 V vs. Li+/Li with a theoretical capacity of 175 mA h g−1.2 The negligible structural difference between pristine Li4Ti5O12 and lithiated Li7Ti5O12 at the two-phase equilibrium junction guarantees an outstanding electrochemical reversibility during the charge/discharge process.3 Furthermore, the high redox potential would prevent not only the lithium metal deposition on the anode at high current conditions but also the formation of the resistive solid electrolyte interphase (SEI) layer, which may lead to an active Li-ion loss and an increase of the cell impedance.4

No SEI formation at the surface of Li4Ti5O12 is a widely accepted argument from the literature point of view. However, in our previous report regarding the electrochemical study of the carbon-free Li4Ti5O12 electrode,5 we noticed the formation and dissolution of the SEI layer through the change in the intensity of Ti 2p XPS core peaks during the charge and discharge process. This fact led us to suspect the stability of Li4Ti5O12
vis-à-vis to the electrolyte in spite of its high redox potential. Despite the interesting properties of Li4Ti5O12, only few literature studies were reported on its reactivity to the electrolyte. Based on a detailed XPS study on the electrolyte/electrode interfaces in LiMn1.6Ni0.4O4/ Li4Ti5O12 system, Dedryvère et al. have reported the formation of organic and inorganic species on the surface of Li4Ti5O12 anode after cycling.6 However, they concluded that those species were first formed at the cathode and then, adsorbed on the surface of Li4Ti5O12 either by diffusion or by migration of organic cationic species. In addition, the lower voltage limit of Li4Ti5O12 anode couldn’t be also guaranteed to be over 1 V in their study because measuring the voltage of Li4Ti5O12 itself is impossible in two-electrode full cell. He et al. also pointed out the formation of SEI film on the Li4Ti5O12 electrode cycled between 2.5 and ~ 0 V vs. Li+/Li., but they mainly focused on the SEI formation occurred below 1 V.7 Moreover, in the aforementioned studies, the results were obtained only at room temperature cycling, and the effects of carbon conducting agent contained in the conventional Li4Ti5O12electrodes were neither considered nor clarified.
In this report, for the first time, the Li4Ti5O12/ electrolyte interface is investigated at room and high temperature using the carbon-free Li4Ti5O12 electrode. The new electrode concept5,8 allows us to examine the reactivity of Li4Ti5O12 to the electrolyte and avoid any kind of parasite reaction which may be induced by the high-surface-area carbon conducting additive. Chemical changes at the surface of Li4Ti5O12 were investigated using a step by step X-ray photoelectron spectroscopy (XPS) analysis during charge/discharge cycling. The time-of-flight secondary ion mass spectroscopy (ToF-SIMS) study and scanning electron microscopy (SEM) observation were carried out to examine a quantitative and qualitative change in the surface chemistry and the electrode morphology after cycling, respectively. The differences between the carbon-free and carbon-containing Li4Ti5O12electrodes in terms of stability and cyclability were also discussed.
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