Engineering the modulation of the active sites and pores of pristine metal-organic frameworks for high-performance sodium-ion storage

Metal-organic framework materials have numerous significant merits for use as electrodes in sodium-ion batteries, such as multiple active sites and porous structures. However, the masked active sites and pores limit their performances. Herein, we rationally selected Ni-HHTP (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) with a stable structure, and adopted thermal treatment prior to its carbonization to open blocked pores and expose multiple active sites. As expected, after thermal treatment at a low temperature of 250 degrees C, the obtained Ni-HHTP-250 sample still retained the intrinsic crystal skeleton and crystal type, and exposed more actives sites and pores than those of pristine Ni-HHTP. The obtained Ni-HHTP-250 as an electrode material for sodium-ion batteries exhibits a large reversible discharge capacity of 420 mA h g(-1) at 0.1 A g(-1) along with a fine rate capability of 200 mA h g(-1) at 2.0 A g(-1). Mechanism studies show that both the Ni ions and organic ligands in the Ni-HHTP-250 sample are active sites. The H2O molecules and the -OH subgroups in the pores masking the active site can be removed after heat treatment. Hence, the excellent electrochemical performance is largely due to the open porous structure and exposed active sites, resulting in rapid Na+ diffusion and relatively high electronic conductivity. This work offers an innovative idea for preparing high-performance electrodes of metal-ion batteries.