Hybrid carboxylate-Schiff-bases as electroactive anode materials for potassium-ion batteries

Three new crystalline potassium salts of hybrid carboxylate/Schiff-base oligomers are reported as electrochemically active anode materials for potassium ion batteries. One of them is constituted by the phenyl “dimer” KOOC-ϕ-C=N-ϕ-COOK (BSK-1) and the other two by the isomer “trimers” KOOC-ϕ-C=N-ϕ-N=C-ϕ-COOK (BSK-2 A) and KOOC-ϕ-N=C-ϕ-C=N-ϕ-COOK (BSK-2B). A structural model refined from powder X-ray diffraction data is discussed for the short (dimeric) salt. A pseudo-monoclinic unit cell, with space group P1¯ and lattice parameters a = 15.88(1) Å, b = 5.772(2) Å, c = 3.887(1) Å and β = 98.8(1)⁰, contains a single molecule. Crystal packing of the aromatic molecules can be described by a slipped π-stacking along the c-axis. All three oligomers are electrochemically active at voltages low enough to be used as anode in potassium ion batteries, at average voltages of 1.2 V vs K⁺/K for the dimer and 1.05 V vs K⁺/K for the trimers. Combined with 3.9 M KFSI in DME electrolyte, they show reversible capacities of 115 mAh/g (dimer), 233 mAh/g (BSK-2 A) and 104 mAh/g (BSK-2B) at C/10, with capacity retentions at 1C of 74% and 65% for the dimer and the trimers respectively. Based on the dQ/dV plots, the trimers react through two solid solution mechanisms, forming a stable radical intermediate, whereas the dimer changes from a single solid solution to a solid-solution+biphasic reaction. Future in-situ x-ray diffraction studies based on the solved crystal packing should enable to confirm these mechanisms.