Submicron-sized Li4Ti5O12/Li2TiO3 composites with Li-rich grain boundaries are successfully synthesized by a simple, environmentally benign, mass production preferred solid state process. The crystal phase and morphology are characterized by XRD and SEM. The electrochemical performance is collected by galvanostatic discharge–charge tests, cyclic voltammograms (CV) and electrochemical impedance spectra (EIS) tests. The initial discharge capacity of Li4Ti5O12/Li2TiO3 composite is 155 mAh g−1 at 0.5 C. While the current rate is as high as 10 C, the specific capacity is 113 mAh g−1, and the capacity retention is 98.2% even after 500 cycles. As a reference, the ionic conductivity and the electrochemical performance of Li2TiO3 and Li4Ti5O12/TiO2 composite are also characterized. The extraordinary high rate performance and cycling stability are explained by high ionic conductivity and rich grain boundaries of Li4Ti5O12/Li2TiO3 composite.