Kinetic Pathways of Ionic Transport in Fast Charging Lithium Titanate

Fast-charging batteries typically employ electrodes capable of accommodating lithium continuously via solid-solution transformation because they have few kinetic barriers apart from Li + diffusion.[1, 2] One exception is lithium titanate, an anode that can cycle without the Li plating observed in graphite [3] and exhibits extraordinary rate capability seemingly inconsistent with its two-phase reaction and the sluggish diffusion within its endmembers [4]. Through real-time tracking of Li + local environments during migration using operando electron energy-loss spectroscopy in combination with first-principles calculations, we reveal that the kinetic pathway that enables facile ionic transport in lithium titanate consists of distorted face-sharing Li polyhedra in metastable intermediate states. This study highlights that the rate capability of fast-charging electrodes may be controlled not solely by the intrinsic ionic diffusivity of macroscopic phases, but also by the transport pathways available via kinetically accessible low-energy states. Findings from this study, particularly on the improved kinetics originating from the face-sharing Li motifs in intermediate lithium titanate configurations, may open new directions for designing electrode materials for fast-charging batteries. References: [1] H. Liu, F. C. Strobridge, O. J. Borkiewicz, K. M. Wiaderek, K. W. Chapman, P. J. Chupas, C. P. Grey, Capturing metastable structures during high-rate cycling of LiFePO 4 nanoparticle electrodes. Science 344 , 1252817 (2014). [2] R. Malik, F. Zhou, G. Ceder, Kinetics of non-equilibrium lithium incorporation in LiFePO 4 . Nat. Mater. 10 , 587-590 (2011). [3] T.-F. Yi, S.-Y. Yang, Y. Xie, Recent advances of Li 4 Ti 5 O 12 as a promising next generation anode material for high power lithium-ion batteries. J. Mater. Chem. A 3 , 5750-5777 (2015).