Rational synthesis of uniform yolk-shell Ni-Fe bimetallic sulfide nanoflakes@porous carbon nanospheres as advanced anodes for high-performance potassium-/sodium-ion batteries

The identification of electrode materials suitable for hosting both K+ and Na+ is more challenging than that for Li+ due to the larger ionic radii of K+ and Na+. Thus, the design and fabrication of advanced electrode materials with excellent electrochemical properties for both potassium-ion batteries (KIBs) and sodium-ion batteries (SIBs) is extremely challenging. Herein, a unique yolk-shell-structured Ni-Fe bimetallic sulfide nanoflake@carbon nanosphere (NFS@C) is designed as a high-performance anode for both KIBs and SIBs via a combination of the infiltration method and sulfidation process. During the first infiltration process under vacuum, Ni and Fe precursors can easily penetrate though the shell into the central void of HMCSs with the help of capillary force. The subsequent sulfidation can transform these precursors into their corresponding sulfides, resulting in the formation of NFS@C yolk-shell nanospheres. These rationally engineered architectures provide enough space to buffer the huge volume expansion of Ni-Fe sulfides upon cycling and supply ample channels for the diffusion of ions, thus providing a well-defined conductive network in the entire electrode. Therefore, NFS@C yolk-shell nanospheres exhibit excellent cycling stability (297 mA h g(-1) at 0.1 A g(-1) after 250 cycles) and rate capability (52 mA h g(-1) at 7.0 A g(-1)) for potassium storage. Furthermore, NFS@C nanospheres as anodes for SIBs exhibited high discharge capacity (417 mA h g(-1) at 0.2 A g(-1) after 250 cycles) and stable capacity (140 mA h g(-1)) even at a high current density (10.0 A g(-1)).