Cobalt(II) Tetraaminophthalocyanine-modified Multiwall Carbon Nanotubes as an Efficient Sulfur Redox Catalyst for Lithium-Sulfur Batteries

An efficient Li-S redox catalyst consisting of MWCNTs covalently modified by cobalt(II) tetraaminophthalocyanines (TaPcCo-MWCNTs) is developed. Effective lithium polysulfide (LiPS) capturing is enabled by the lithiophilic N-containing phthalocyanine rings and the sulfiphilic Co central atoms. This adsorption geometry utilizes the Co unoccupied d-orbitals as electron super-exchange highways. Elevated kinetics of LiPSs reactions in the liquid phase as well as liquid-solid transitions were revealed by electrochemical measurements and density functional theory calculations. Uniform deposition of Li₂ S films was also observed, which preserves cathode integrity and sulfur utilization during cell cycling. The catalyzed sulfur redox is also significantly facilitated by the fast electron and Li-ion transport to and from the reaction sites through the conductive MWCNT skeletons and the lithiophilic substituent amino groups on TaPcCo. With 6 wt % addition of TaPcCo-MWCNT in the cathode coatings, high sulfur utilization is achieved with areal sulfur loadings of up to 7 mg cm^-2 . Stable long-term cycling is achieved at 1 C at a sulfur loading of 5 mg cm^-2 , with an initial areal capacity of 4.4 mAh cm^-2 retention of 3.5 mAh cm^-2 after 500 cycles. Considering the high structural diversity of phthalocyanines macromolecules, this study provides opportunities for a new class of Li-S catalysts.