Metallic Graphene-Like VSe2 Ultrathin Nanosheets: Superior Potassium-Ion Storage and Their Working Mechanism.

Potassium-ion batteries (KIBs) are receiving increasing interest in grid-scale energy storage owing to the earth abundant and low cost of potassium resources. However, their development still stays at the infancy stage due to the lack of suitable electrode materials with reversible depotassiation/potassiation behavior, resulting in poor rate performance, low capacity, and cycling stability. Herein, the first example of synthesizing single-crystalline metallic graphene-like VSe2 nanosheets for greatly boosting the performance of KIBs in term of capacity, rate capability, and cycling stability is reported. Benefiting from the unique 2D nanostructure, high electron/K+ -ion conductivity, and outstanding pseudocapacitance effects, ultrathin VSe2 nanosheets show a very high reversible capacity of 366 mAh g-1 at 100 mA g-1 , a high rate capability of 169 mAh g-1 at 2000 mA g-1 , and a very low decay of 0.025% per cycle over 500 cycles, which are the best in all the reported anode materials in KIBs. The first-principles calculations reveal that VSe2 nanosheets have large adsorption energy and low diffusion barriers for the intercalation of K+ -ion. Ex situ X-ray diffraction analysis indicates that VSe2 nanosheets undertake a reversible phase evolution by initially proceeding with the K+ -ion insertion within VSe2 layers, followed by the conversion reaction mechanism.