Li4Ti5O12 Degradation Kinetics during Galvanostatic Cycling
Authors: Chiu Hsien-Chieh , Lu Xia , Demopoulos George P. 
Published: February 2020

Spinel lithium titanate  Li4Ti5O12 (LTO) has been one of the most prominent electrode materials that are able to exhibit extremely long-term stability1 because of its negligible volume change (0.2%)2 during Li insertion / extraction. During Li insertion, external Li ions enter into the empty octahedral 16c sites in spinel structure, while in the meantime self-diffusion of originally existing Li+ at 8a sites occurs towards the neighboring 16c sites3 to form the rock-salt structure. As a result of this behavior the two-phase partially lithiated LTO, Li4+x
Ti5O12 exhibits a very flat voltage plateau around 1.55 V vs. Li/Li+ over a wide range of Li content (e.g., 0.09 < x < 2.91). Of the two phases3,4, one is insulator Li4Ti5O12 (10-9 S/cm) and the other is conductor Li7Ti5O12 (8 S/cm)
5. Besides these intrinsic physicochemical properties of LTO, its nanosizing to enhance its power capability can influence its Li storage properties; e.g., the orientation-dependent 8a – 16c co-intercalation in the near surface region of LTO nanoparticles results in considerable irreversible capacity loss due the formation of overlithiated phase. Furthermore the high surface area of LTO nanoparticles leads to higher reactivity with the electrolyte aggravating the notorious gas generation issue, which may be accompanied structural relaxation in the near surface region6-11.

            Thanks to many efforts devoting to the size-effect on LTO for the last decade, the mystery of the size-effect mechanism has been largely revealed even though there are still several unanswered questions. One of these is the formation kinetics of the overlithiated phase near surface region, which is particularly critical from an industrial perspective as it can lead to optimized formation protocols and electrolyte recipes for LTO batteries. In this context, the universal Johnson-Mehl-Avrami-Kolmogorov (JMAK) kinetic model was employed to elucidate the correlation between overlithiated phase growth and the resistivity variation of LTO electrodes during standard galvanostatic charge / discharge cycle tests at room temperature. Based on this theoretical model, 8a-16c co-intercalated overlithated phase isotropically grows along the 3D Li diffusion pathway in the spinel framework, until the transition from 8a / 16c occupancy to 8a – 16c co-occupancy gets saturated in the near surface region.

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