Linker-free 3D assembly of nanocrystals with tunable unit size for reversible lithium ion storage

A simple and scalable procedure combining hydrothermal synthesis with post-synthesis calcination was developed to produce a linker-free, thermally stable, mesoscale 3D ordered assembly of spinel-type ZnCo(2)O(4) nanocrystals. The mesoscale assembly with distinctively sharp edges was formed by close-packing the ZnCo(2)O(4) nanocrystal building blocks with a unit size changeable by the synthesis temperature. A self-templating mechanism based on the topotactic transformation of an oxalato-bridged precursor coordination compound was proposed for the assembly. The packaging of crystalline ZnCo(2)O(4) nanoparticles, an active lithium ion storage compound, into a dense organized structure is an effective way to increase the volumetric capacity of ZnCo(2)O(4) nanoparticles for reversible lithium ion storage. The highly ordered 3D assembly of ZnCo(2)O(4) demonstrated excellent reversible lithium ion storage properties and a specific capacity (～800 mAh g(-1)) much higher than that of carbon (typically ～ 350 mAh g(-1)).