A CoSe₂-based 3D conductive network for high-performance potassium storage: Enhancing charge transportation by encapsulation and restriction strategy

Potassium-ion batteries (PIBs) are expected as a supplement for lithium-ion batteries (LIBs) due to the abundant potassium resource and low cost. However, the large radius of the potassium ion hinders the ion transport dynamics and structural stability of the electrode material. Herein, a sandwich-like CoSe2@NC/rGO composite is successfully synthesized by a two step co-precipitation and pyrolysis/selenization method. Taking full advantage of this unique 3D structure, the electronic conductivity of CoSe2@NC/rGO-5 is about 20 times higher than that of CoSe2@NC, and the specific surface area of CoSe2@NC/rGO-5 is about 6 times higher than that of CoSe2@NC, which provides sufficient reactive sites. Moreover, the empty space between graphene layers can effectively alleviate the volume expansion and prevent the peeling of the active material during cycling. As a consequence, the as-prepared CoSe2@NC/rGO-5 anode exhibits high reversible capacity (527.5 mA h g-1 at 0.1 A g-1), good cycle stability (226 mA h g-1 at 0.5 A g-1 after 400 cycles) and enhanced rate capability (206 and 157 mA h g-1 at 5 and 10 A g-1, respectively). A two-step reaction process from CoSe2 to K2CoSe2 to K2Se during discharging is confirmed by ex situ TEM and ex situ XPS. This work provides a facile approach to prepare high-performance electrodes with a 3D conductive network, and further deepens the understanding of the evolution of CoSe2 in the potassium storage process.