Co3 Se4 Quantum Dots as an Ultrastable Host Material for Potassium-Ion Intercalation.

Potassium-ion batteries (KIBs) are receiving increased attention due to their cost-effective and similar energy-storage mechanism to lithium-ion batteries. However, the lack of appropriate electrode materials is still hampered for their development, which is mainly caused by the large size of the potassium ions (1.38 Å) including low structural stability and poor electrochemical redox reaction kinetics. Herein, Co3 Se4 quantum dots (QD) encapsulated by N-doped carbon (CSC) are reported as an anode material for KIBs, in which a morphology change process occurs. Benefiting from the unique uniform nanostructure reducing the ion-diffusion length, the improved electronic conductivity, and the enhanced protective effect of N-doped carbon (NC) alleviating volume fluctuation, the CSC demonstrates excellent electrochemical performance. The core-shell-like CSC composite demonstrates remarkable discharge capacity (410 mA h g-1 at 0.1 A g-1 after 550 cycles, 360 mA h g-1 at 0.5 A g-1 after 3200 cycles) and excellent cyclic performance over 10 000 cycles at 1 A g-1 . Density functional theory calculations show a larger reaction energy of Co3 Se4 QD than bulk Co3 Se4 , a lower barrier of K atom migration in Co3 Se4 QD than bulk Co3 Se4 , and also favor the intercalation reaction rather than replacement reaction. In situ X-ray diffraction and ex situ transmission electron microscopy are further used to evaluate potassiation/depotassiation phenomena.