Thermally Transformed ZnC8H4O4 Nanocrystals for Enhanced Sodium-Ion Storage

Metal-organic terephthalates composed of earth-abundant elements are promising low-cost and structurally tuneable anodes for sodium-ion batteries, yet their electrochemical behavior, particularly in newly developed compounds, remains insufficiently understood. We report monoclinic nanocrystalline ZnC8H4O4 (ZnTP-2), prepared through the controlled thermal transformation of a hydrated zinc terephthalate precursor, ZnC8H6O5 (ZnTP-1), containing residual terephthalic acid (ZnTP-1@TPA). This transformation involves TPA sublimation and framework dehydration, producing ZnTP-2 with fine crystals of approximately 50-150 nm, increased surface area, and a higher concentration of oxygen-related defect sites compared with ZnTP-1@TPA. ZnTP-2 delivers a reversible Na-ion storage capacity of 222.7 mAh g-1 after 80 cycles at 50 mA g-1, approaching its theoretical capacity and indicating highly efficient utilization of the terephthalate redox framework. It also retains 110 mAh g-1 after 500 cycles at 1000 mA g-1, with a relatively low charge-transfer resistance of 11.5 Ω, demonstrating improved electrochemical stability and rate performance compared with conventional metal-organic terephthalate anodes. Electrochemical kinetic analyses reveal Na-ion diffusion coefficients between 10-14 and 10-12 cm2 s-1, with the differences discussed in terms of apparent, Warburg-related and bulk/chemical diffusion processes. UV-vis analysis reveals a lower optical band gap for ZnTP-2 (2.74 eV) than for ZnTP-1@TPA (3.36 eV), with DFT calculations on idealized molecular models of hydrated and dehydrated zinc terephthalates supporting a similar trend. Full-cell examination provides proof of concept for its applicability.