Atomic Cobalt Covalently Engineered Interlayers for Superior Lithium-Ion Storage.

With the unique-layered structure, MXenes show potential as electrodes in energy-storage devices including lithium-ion (Li+ ) capacitors and batteries. However, the low Li+ -storage capacity hinders the application of MXenes in place of commercial carbon materials. Here, the vanadium carbide (V2 C) MXene with engineered interlayer spacing for desirable storage capacity is demonstrated. The interlayer distance of pristine V2 C MXene is controllably tuned to 0.735 nm resulting in improved Li-ion capacity of 686.7 mA h g-1 at 0.1 A g-1 , the best MXene-based Li+ -storage capacity reported so far. Further, cobalt ions are stably intercalated into the interlayer of V2 C MXene to form a new interlayer-expanded structure via strong V-O-Co bonding. The intercalated V2 C MXene electrodes not only exhibit superior capacity up to 1117.3 mA h g-1 at 0.1 A g-1 , but also deliver a significantly ultralong cycling stability over 15 000 cycles. These results clearly suggest that MXene materials with an engineered interlayer distance will be a rational route for realizing them as superstable and high-performance Li+ capacitor electrodes.