Hydrogenated Core–Shell MAX@K2Ti8O17 Pseudocapacitance with Ultrafast Sodium Storage and Long-Term Cycling

Sodium-ion batteries are considered alternatives to lithium-ion batteries for energy storage devices due to their competitive cost and source abundance. However, the development of electrode materials with long-term stability and high capacity remains a great challenge. Here, this paper describes for the first time the synthesis of a new class of core–shell MAX@K 2 Ti 8 O 17 by alkaline hydrothermal reaction and hydrogenation of MAX, which grants high sodium ion-intercalation pseudocapacitance. This composite electrode displays extraordinary reversible capacities of 190 mA h g −1 at 200 mA g −1 (0.9 C, theoretical value of ≈219 mA h g −1 ) and 150 mA h g −1 at 1000 mA g −1 (4.6 C). More importantly, a reversible capacity of 75 mA h g −1 at 10 000 mA g −1 (46 C) is retained without any apparent capacity decay even after more than 10 000 cycles. Experimental tests and first-principle calculations confirm that the increase in Ti 3+ on the surface layers of MAX@K 2 Ti 8 O 17 by hydrogenation increases its conductivity in addition to enhancing the sodium-ion intercalation pseudocapacitive process. Furthermore, the distorted dodecahedrons between Ti and O layers not only provide abundant sites for sodium-ion accommodation but also act as wide tunnels for sodium-ion transport.