Tuning the Electronic Energy Level of Covalent Organic Framework for Crafting High-rate Na-ion Battery Anode
Crystalline Covalent Organic Frameworks (COFs) possess ordered accessible nano-channels. When these channels are decorated with redox-active functional groups, they can serve as the anode in metal ion battery (LIB and SIB etc). Though sodium's superior relative abundance makes it a better choice over lithium, the energetically unfavourable intercalation of larger sodium ion makes it incompatible with the commercial Graphite anodes used in Li-ion battery. Also, their sluggish movement inside the electrodes restricts the fast sodiation of SIB. Creating an electronic driving force at the electrodes via chemical manipulation can be a versatile approach to overcome this issue. Here we present anodes for SIB drawn on three isostructural COFs with nearly the same Highest Occupied Molecular Orbitals (HOMO) levels but with varying Lowest Unoccupied Molecular Orbitals (LUMO) energy levels. This variation in LUMO levels has been deliberately performed by the inclusion of electron-deficient centers (phenyl vs. tetrazine vs. bispyridine-tetrazine) substituents into the modules that make up the COF. With the reduction of cell- potential electrons accumulate in the anti-bonding LUMO. Now, these electron-dosed LUMO levels become efficient anodes in attracting the otherwise sluggish sodium ions from the electrolyte. While the intrinsic porosity of the COF favors the lodging and diffusion of the Na+ ions. Cells made with these COFs achieve a high specific capacity (energy density) and rate-performance (rapid charging-discharging), something that is not as easy for a Na+ compared to the much smaller sized Li+. The bispyridine-tetrazine COF with the lowest LUMO energy shows a specific capacity of 340 mAh/g at 1 A/g and 128 mAh/g at high current density of 15 A/g. Only a 24% drop appears upon increasing the current density from 0.1 to 1 A/g, which is the lowest among all the top-performing COF derived Na-ion battery anodes.