Transition metal ion dissolution due to hydrofluoric acid attack is a long-standing issue in the Mn-based spinel cathode materials of lithium-ion batteries (LIBs). Numerous strategies have been proposed to address this issue, but only a fragmentary solution has been established. In this study, reported is a seaweed-extracted multitalented material, namely, agar, for high-performance LIBs comprising Mn-based cathode materials at a practical loading density (23.1 mg cm−2 for LiMn2O4 and 10.9 mg cm−2 for LiNi0.5Mn1.5O4, respectively). As a surface modifier, 3-glycidoxypropyl trimethoxysilane (GPTMS) is employed to enable the agar to have different phase separation behaviors during the nonsolvent-induced phase separation process, thus eventually leading to the fabrication of an outstanding separator membrane that features a well-defined porous structure, superior mechanical robustness, high ionic conductivity, and good thermal stability. The GPTMS-modified agar separator membrane coupled with a pure agar binder to the LiNi0.5Mn1.5O4/graphite full cell leads to exceptional improvement in electrochemical performance outperforming binders and separator membrane in current commercial products even at 55 °C; this improvement is due to beneficial features such as Mn2+ chelation and PF5 stabilizing capabilities. This study is believed to provide insights into the potential energy applications of natural seaweeds.