Bi@C Nanospheres with the Unique Petaloid Core–Shell Structure Anchored on Porous Graphene Nanosheets as an Anode for Stable Sodium- and Potassium-Ion Batteries

Bismuth (Bi) has emerged as a prospective candidate as Na-ion and potassium-ion battery anodes because of its unique advantages of low cost, high theoretical gravimetric capacity (386 mAh g-1), and superior volumetric capacity (3800 mAh cm-3). However, the low electronic conductivity and the huge volume expansion of Bi during the alloying/dealloying reactions are extremely detrimental to cycling stability, which seriously hinder its practical application. To overcome these issues, we propose a rational design: Bi@C nanospheres with the unique petaloid core-shell structure are synthesized in one step for the first time and then combined with different contents of graphene (GR) nanosheets to form the composites Bi@C@GR. The Bi@C nanospheres with a core-shell structure are beneficial to shortening the transmission path of electrons/ions and reducing the risk from structural rupture of the particles during cycling. In addition, the combination of Bi@C nanospheres and porous GR could greatly improve the conductivity and prevent the aggregation of particles, which is conducive to better cycling stability and rate performance. Consequently, Bi@C@GR-2 presents a superior reversible capacity for sodium storage (300 mAh g-1 over 80 cycles) and potassium storage (200 mAh g-1 over 70 cycles) at 0.1 A g-1. Furthermore, in situ electrochemical impedance spectroscopy and ex situ transmission electron microscopy are carried out to analyze and reflect the kinetic reaction mechanism and the phase change of the Bi@C@GR-2 electrode during the charge/discharge processes.