Double-layer carbon coated on the micrometer bismuth by photothermal effect as anode for high-rate sodium-ion batteries

Bismuth (Bi) is considered a promising anode material for high-rate sodium-ion batteries (SIBs) due to its unique layered crystal structure which can easily insert/extract sodium-ion (Na+). However, the cycle stability of micrometer Bi with great commercial potential needs to be improved. Because the large volume change occurs during alloying/dealloying of Bi and sodium (Na) resulting in the crack of Bi, which reduces the stability of the battery. Herein, we design a Bi/Bi2O3@C composite where a double-layer carbon coated commercial micrometer Bi/Bi2O3 particles. The heat generated by Bi/Bi2O3 particles under ultraviolet (UV) exposure (called photothermal effect) accelerates the decomposition rate of photoinitiator, which in turn improves the polymerization rate of the monomer on the surface of Bi/Bi2O3 particles. So that, the surface of Bi/Bi2O3 particles is coated with a dense polymer, while the polymer away from the particle is relatively loose, thus forming a dense-loose double-layer carbon during the annealing process. The dense carbon layer can effectively protect against the volume effect of Bi/Bi2O3, and the loose carbon layer can provide channels for the transport of Na+. As a result, Bi/ Bi2O3@C exhibits excellent cycling stability with a capacity retention rate of 94.5 % after 1600 cycles at 10 A g(-1).