It is still a great challenge at present to combine the high rate capability of the electrochemical capacitor with the high electrochemical capacity feature of rechargeable battery in energy storage and transport devices. By studying the lithiation mechanism of Li4Ti5O12 (LTO) using in-situ electron holography, we find that double charge layers are formed at the interface of the insulating Li4Ti5O12 (Li4) phase and the semiconducting Li7Ti5O12 (Li7) phase, and can greatly boost the lithiation kinetics. The electron wave phase of the LTO particle is found to gradually shrink with the interface movement, leaving a positive electric field from Li7 to Li4 phase. Once the capacitive interface charges are formed, the lithiation of the core/shell particle could be established within 10 s. The ultrafast kinetics is attributed to the built-in interface potential and the mixed Ti3+/Ti4+ sites at the interface that could be maximally lowering the thermodynamic barrier for Li ion migration.