Electric vehicles (EVs) have attracted attention all over the world because of their advantages, such as energy conservation and carbon emission reduction. Li-ion batteries (LIBs) for energy storage in EVs are used because of their high energy density. LiFePO4 is one of the most preferred cathode materials for EVs because of several advantages, such as high specific energy, long life cycle, environmental safety, and low cost. However, at high temperatures, LiFePO4 devices exhibit iron dissolution, which limits the use of LiFePO4 in EVs. This study fabricates an in operando, self-growing, solid electrolyte interphase (SEI) on the LiFePO4 electrode by using an atmospheric pressure plasma jet (APPJ) to prevent iron dissolution and extend its life cycle. The APPJ treatment results in the plasma-induced grafting of hydrophilic functional groups on the LiFePO4 surface, which are used for the in operando synthesis of an amorphous a-FePO4·H2O thin layer on the surface and subsequent electrochemical reaction with ethylene carbonate to form a unique core-shell SEI layer. Contact angle measurements and optical emission and Raman spectroscopy analyses reveal the surface characteristics of LiFePO4 after the APPJ treatment. TEM images confirm that the SEI was formed by an amorphous layer with a thickness of 2.73 nm. High-temperature (60 °C) testing reveals that the SEI considerably improved the cycle performance by suppressing iron dissolution. The present study fabricates an in operando, self-growing, core-shell SEI on a LiFePO4 electrode to improve the high-temperature performance of LIBs.