Expanded MoSe2 Nanosheets Vertically Bonded on Reduced Graphene Oxide for Sodium and Potassium-Ion Storage.

The cost-efficient and plentiful Na and K resources motivate the research on ideal electrodes for sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs). Here, MoSe2 nanosheets perpendicularly anchored on reduced graphene oxide (rGO) are studied as an electrode for SIBs and PIBs. Not only does the graphene network serves as a nucleation substrate for suppressing the agglomeration of MoSe2 nanosheets to eliminate the electrode fracture but also facilitates the electrochemical kinetics process and provides a buffer zone to tolerate the large strain. An expanded interplanar spacing of 7.9 Å is conducive to fast alkaline ion diffusion, and the formed chemical bondings (C-Mo and C-O-Mo) promote the structure integrity and the charge transfer kinetics. Consequently, MoSe2@5%rGO exhibits a reversible specific capacity of 458.3 mAh·g-1 at 100 mA·g-1, great cyclability with a retention of 383.6 mAh·g-1 over 50 cycles, and excellent rate capability (251.3 mAh·g-1 at 5 A·g-1) for SIBs. For PIBs, a high first specific capacity of 365.5 mAh·g-1 at 100 mA·g-1 with a low capacity fading of 51.5 mAh·g-1 upon 50 cycles and satisfactory rate property are acquired for MoSe2@10%rGO composite. Ex situ measurements validate that the discharge products are Na2Se for SIBs and K5Se3 for PIBs, and robust chemical bonds boost the structure stability for Na- and K-ion storage. The full batteries are successfully fabricated to verify the practical feasibility of MoSe2@5%rGO composite.