Attachment of redox active molecules on the carbon additive and its effect on the cycling performance of LiFePO4 electrodes

Highlights: • Tris(1,10-phenanthroline) iron (II) was grafted on surface of conductive carbon. • Grafting was done by in-situ generation and reduction of diazonium ions. • The modified carbon showed a redox peak at 4.1 V vs. Li/Li⁺. • Improved rate and cycling performance obtained for LiFePO₄ with modified carbon. -- Abstract: The bis(1,10-phenanthroline)-5-amino-1,10-phenanthroline iron (II) complex (Fe²⁺[Phen]₂[Phen-NH₂]) was synthesized and characterized before covalent attachment to the surface of acetylene black carbon via diazonium chemistry. The metal complex was characterized by UV–visible spectroscopy and elemental analysis. Elemental analysis revealed a low grafting yield of approximately 3 wt%. Cycling voltammetry of modified-carbon electrode showed an apparent redox potential of 4.1 V (vs. Li) characteristic of the iron complex. The voltammetric charge associated to the redox peak corresponded to approximately 3.3 wt% of grafted groups, in agreement with the value estimated by elemental analysis. The electrochemical performance of LiFePO₄ cathodes using modified and unmodified acetylene black carbons as conducting additive were compared through galvanostatic cycling and electrochemical impedance spectroscopy measurements. At a 5C rate, the electrode made with the modified carbon delivered a specific capacity of about 60 mAh.g⁻¹ in comparison to only 25 mAh.g⁻¹ for the composite with unmodified conductive additive. Charge/discharge cycling experiments over 200 cycles at C/2.5 revealed a capacity fade of about 0.2 and 0.14% per cycle for the LiFePO₄ electrodes made with the unmodified and modified carbons, respectively. Furthermore, the charge-transfer resistance of LiFePO₄ electrode using the grafted-carbon is significantly smaller (150 Ω) than the unmodified acetylene black (300 Ω).