A poly (acrylic acid–tannic acid) (PAA–TA) crosslinked composite is reported on for the first time as an aqueous binder for LiMn_0.6Fe_0.4PO₄ (LMFP) cathodes to suppress manganese dissolution. Benefiting from hydrogen bonding and chemical crosslinking, the PAA-TA binder constructs a robust three-dimensional network that strengthens the interfacial adhesion among active particles, conductive additives, and the current collector, stabilizing the LMFP crystal structure upon cycling. More importantly, this polymeric network effectively restrains manganese leaching, which is the major origin of the capacity fading of LMFP cathodes. Meanwhile, continuous electron and ion transport pathways are well established. The LMFP electrode with an optimized PA5TA1 binder delivers a reversible capacity of 112 mAh g⁻¹ at 5 C and retains 81.3% of its capacity after 300 cycles, outperforming the PVDF/LMFP electrode. Post–mortem analyses of cycled LMFP cathodes further confirm that there was substantially suppressed Mn²⁺ dissolution in the PAA–TA electrode, corroborating the structural integrity of the cathode–electrolyte interface. This crosslinked aqueous polymer provides a sustainable alternative to the PVDF binder for long–life LMFP cathodes.