Fast-charging and Long-Lasting Mg-Na Hybrid Batteries Based on Extremely Pseudocapacitive Bronze TiO2 Nanosheet Cathodes

Despite of their inexpensive and sustainable characteristics, practical application of Mg-Na hybrid batteries are limited due to the lack of high performance dual-ion compatible cathode materials. This is mainly due to the increased size of Na-ions and improved electrostatic repulsion resulting from the high charge density of Mg-ions. Herein, we report for the first time a fast charging and ultralong-life Mg-Na hybrid battery based on an extremely pseudocapacitive hierarchical bronze TiO2 (TiO2-B) nanosheet cathode. This two dimensional cathode exhibited outstanding pseudocapacitance (up to 94%), specific capacities (195 mAh/g @ 25 mA/g), rate performance (140 mAh/g @ 1A/g), cycling stability (similar to 76% after 6000 cycles @ 1A/g), coulombic efficiency (similar to 100%) and fastcharging (similar to 8 min). These performances are vastly superior to the previously reported metal oxide type MgNa hybrid battery cathodes. Mechanistic investigations revealed Mg-Na dual-ion intercalation pseudocapacitance with no significant structural changes. Exceptional electrochemical performance of the TiO2-B nanosheet cathode is credited to the dominant pseudocapacitive Mg-Na dual-ion diffusion through the nanointerfaces resulting from the hierarchical microstructure of TiO2-B nanosheets. High surface area, ultrathin nature and mesoporous structure are also contributed as secondary factors by facilitating superior contact with the electrolyte solution. The demonstrated method of nanointerfaces induced pseudocapacitive Mg-Na dual-ion intercalation provides new opportunities for the development of high-performance Mg-Na hybrid batteries.