Kinetics enhanced hierarchical Ni2P1−xSx/Ni@carbon/graphene yolk–shell microspheres boosting advanced sodium/potassium storage

Abstract

The large radius of Na⁺/K⁺ makes it difficult to maintain the structural stability of electrode materials during repeated Na⁺/K⁺ insertion/deintercalation, especially for anode materials with high-rate and long-life cycles. Herein, the yolk–shell hetero-structured Ni₂P_1−xS_x/Ni with a porous carbon/graphene coating (YS Ni₂P_1−xS_x/Ni@C/G) is synthesized and successfully applied in Na/K storage. The YS Ni₂P_1−xS_x/Ni with the interior void space can alleviate the volume expansion and facilitate Na⁺/K⁺ diffusion during the charge/discharge cycles, while the embedded metallic Ni and wrapped conductive C/G layer can accelerate the fast charge transfer. Thus, the optimized anode (YS Ni₂P_0.75S_0.25/Ni@C/G) delivers high reversible capacities of 553 and 435 mA h g⁻¹ after 1000 cycles at 2000 and 4000 mA g⁻¹ for Na⁺-half-cells, respectively, and a high energy density of 131 W h kg⁻¹ at a power density of 221 W kg⁻¹ for Na-ion capacitor full-cells. For the K⁺-half-cells, it exhibits an excellent rate capability (238 mA h g⁻¹ at 3200 mA g⁻¹) and cycle life. Density functional theory (DFT) calculations reveal that the Ni₂P decreases the ion diffusion energy barrier, the Ni₉S₈ and Ni₂P–Ni₉S₈ interface have improved adsorption capacity for ions, and the metallic Ni optimizes the electronic structure, thus leading to excellent electrochemical performances.