Fucoidan Cross-Linking Polyacrylamide as Multifunctional Aqueous Binder Stabilizes LiCoO2 to 4.6 V

Raising the cutoff voltage can efficiently increase the energy density of lithium cobalt oxide (LCO). However, upon charging over 4.55 V the LCO undergoes irreversible phase transition from the pristine O3 phase to the metastable H1-3 phases, causing serious side reactions, which results in poor cycling stability. Herein, a multifunctional aqueous composite binder derived from the cross-linking of fucoidan (FUC) and polyacrylamide (PAM) is developed to enhance the stability of LCO cathode at 4.6 V. The cross-linking interaction of FUC and PAM provides a uniform coating on the surface of LCO and ensures a high peel strength for the electrode, effectively mitigating irreversible phase transition and detrimental interface side reactions. More importantly, the sulfur ester and amide groups of FUC-PAM favorably function as surface charge compensators to the high valent Co upon charging under high voltages, thus stabilizes the surface lattice of LCO and suppresses the detrimental oxygen release. As expected, the LCO with a cutoff voltage of 4.6 V exhibits a high capacity retention of 90% after 100 cycles at a current density of 110 mA g⁻¹. The interfacial coordination effect of composite binders offers a novel strategy to enhance the stability of high-voltage LCO for high-energy lithium-ion batteries.