© 2019 American Chemical Society. In this work, we show that the well-known lithium-ion anode material, Li4Ti5O12, exhibits exceptionally high initial capacity of 310 mAh g-1 when it is discharged to 0.01 V. It maintains a reversible capacity of 230 mAh g-1, far exceeding the "theoretical" capacity of 175 mAh g-1 when this anode is lithiated to the composition Li7Ti5O12. Neutron diffraction analyses identify that additional Li reversibly enters into the Li7Ti5O12 to form Li8Ti5O12. density functional theory (DFT) calculations reveal the average potentials of the Li4Ti5O12 to Li7Ti5O12 step and the Li7Ti5O12 to Li8Ti5O12 step are 1.57 and 0.19 V, respectively, which are in excellent agreement with experimental results. Transmission electron microscopy (TEM) studies confirm that the irreversible capacity of Li4Ti5O12 during its first cycle originates from the formation of a solid electrolyte interface (SEI) layer. This work clarifies the fundamental lithiation mechanism of the Li4Ti5O12, when lithiated to 0.01 V vs Li.