Synthesis and spectroscopic characterization of Ni^II coordination network: Poly-[tris(µ₄-Benzene-1,4-dicarboxylato)-tetrakis(µ₁-dimethylformamide-κ¹O)-trinickel(II)] as material for lithium ion batteries

The compound Ni₃(C₈H₄O₄)₃(C₃H₇NO)₃, poly-[tris(µ₄-Benzene-1,4-dicarboxylato)-tetrakis(µ₁-dimethylformamide-κ¹O)-trinickel(II)], was synthesized by the solvothermal method prepared via reaction between NiCl₂•6H₂O and terephthalic acid using N,N-dimethylformamide (DMF) as solvent. The structure was characterized by powder X-ray diffraction and infrared spectroscopy analyses. The electrochemical properties as a potential active material in lithium-ion batteries were characterized by electrochemical impedance spectroscopy and galvanostatic charge-discharge curves in a battery half-cell. The characterization results show that the coordination network contains one independent structure in the asymmetric unit. It is constructed from Ni²⁺ ions, terephthalate bridges and in-situ-generated DMF ligands, forming two similar two-dimensional (2D) layer structures. These similar 2D layers are in an alternating arrangement and are linked with each other by dense H—H interactions (45%) to generate a three-dimensional (3D) supramolecular framework with ordered and disordered DMF molecules. The electrochemical measurements, conducted in the potential range of 0.5–3.5 V vs Li/Li⁺, show that Ni₃(C₈H₄O₄)₃(C₃H₇NO)₄ has good electrochemical properties and can work as anode in lithium-ion batteries. The material presents an initial specific capacity of ∼420 mAh g⁻¹, which drops during consecutive scans but stabilizes at ∼50 mAh g⁻¹. However, due to the wide potential range there are indications of a gradual collapse of the structure. The electrochemical impedance spectroscopy shows an increase of charge transfer resistance from 24 to 1190 Ohms after cycling likely due to this collapse.