Phosphorus-activated carboxyl small molecule positive electrode for high specific capacity and long-life iron-organic batteries

Iron-ion batteries represent a compelling energy storage solution due to the cost-effectiveness, suitable redox potential, and high capacity of Fe negative electrodes. Polyaniline positive electrodes for iron-ion batteries have demonstrated promising electrochemical redox properties, but face limited redox-accessible groups and unstable -NH- sites. Here we show phosphorus redox activity in a carboxyl small molecule electrode. 4,4',4″-phosphanetriyltribenzoic acid and 4,4',4″-nitrilotribenzoic acid are designed via modulating the electron-donating P and tert-N motifs, showing tuned charge distributions and energy levels. With the decrease of the electronegativity and energy barrier (N > P), 4,4',4″-phosphanetriyltribenzoic acid exhibits stronger Fe²⁺ coordination with carboxyl sites, and brings closed CF₃SO₃^- proximity to P centers. This feature ensures high activity of carboxyl/phosphorus sites with low activation energy (0.24 vs. 0.29 eV for 4,4',4″-nitrilotribenzoic acid). 4,4',4″-phosphanetriyltribenzoic acid with P-extended conjugated structure achieves low energy gap (2.28 eV) compared to its individual carboxyl or P-containing counterparts (2.71/3.16 eV), thereby enabling high utilization of carboxyl/P motifs (98.5%) and enhanced redox voltage (0.8 V). A stable 4 e^- Fe²⁺/CF₃SO₃^- storage of 4,4',4″-phosphanetriyltribenzoic acid positive electrode endows Fe battery with high specific capacity (276 mAh g^-1) and cycling stability (60,000 cycles). This work highlights the potential of phosphorus-active organic materials toward iron-ion batteries.