Nanoclustering in doped-lithium titanate and its effects on interfacial electrochemical kinetics

Doping has been widely applied to improve electrochemical kinetics of electrode materials in lithium-ion batteries. However, the complex behaviors of doping, e.g., dopant occupation in the lattice, clustering, and alteration of surface condition, were not well understood, but they would significantly affect the overall electrochemical kinetics. Herein, with the Cr-doped lithium titanate (Li4Ti5O12, LTO) thin-film electrodes as a model system, we demonstrate an atomic clustering of Cr dopants in the octahedral sites of LTO, that cause lattice distortion and charge redistribution. The structural and electronic changes at surface resulted in very low charge-transfer resistance (5.52 Ω from Cr-doped-LTO versus 632.98 Ω from pristine-LTO at 30 ˚C with EC/DEC electrolyte), but a higher and electrolyte-independent activation energy (73-78 kJ•mol-1). The dopant cluster fundamentally changed the rate determining step of charge-transfer processes from the conventional desolvation to Li-ion insertion. Although the high activation energy impeded Li-ion-insertion/charge-transfer, the overall kinetics was improved after Cr doping, through increased electronic (~10-6 S cm-1 which is around 100 times increase) and ionic conductivities and reactive sites on the electrode surface, which increases the reaction frequency. This paper opens the door for refining doping design for fast charging/discharging electrode materials for electrochemical applications.