Ultrafine Ni0.85Se nanoparticles encapsulated inside hollow porous carbon spheres and their excellent Na storage performance

As a novel anode material for sodium-ion batteries, ultrafine Ni 0.85 Se nanoparticles encapsulated inside hollow porous carbon spheres (HPCS@Ni 0.85 Se) are prepared by impregnating Ni(NO 3 ) 2 into HPCSs, calcination and selenization, which is based on the strong capillary effect of HPCSs. Material characterization reveals that Ni 0.85 Se nanoparticles with a diameter of 5–15 nm are evenly dispersed and intimately coupled to the internal wall of HPCSs, and the content of Ni 0.85 Se reaches 52.81%. HPCSs as unique reactors and Ni 0.85 Se carriers improve electronic conductivity of Ni 0.85 Se, facilitate electrolyte penetration and storage, buffer volume variation of Ni 0.85 Se, effectively confine Ni 0.85 Se during long-term cycles. Consequently, HPCS@Ni 0.85 Se exhibits excellent cycling durability and extraordinary rate capability. High reversible capacities of 341 mA h g −1  at 1 A g −1 after 1000 cycles, 280 mA h g −1  at 5 A g −1 after 2000 cycles and 305 mA h g −1  at 10 A g −1 after 540 cycles are achieved. The reaction kinetics and Na  +  storage mechanism are further analyzed by galvanostatic intermittent titration technique, cyclic voltammetry and electrochemical impedance spectra measurements. Ex-situ characterizations confirm outstanding structural stability of HPCS@Ni 0.85 Se. It is demonstrated that HPCS@Ni 0.85 Se is an excellent anode material, and this unique HPCS-based nanoencapsulation structure is an effective composite strategy for transition metal selenides .