<p>The formation and evolution process of the solid electrolyte interphase (SEI) is critical for stable cycling of the lithium metal anode (LMA). The concept of regulating SEI components with additives is widely incorporated into electrolyte design, as these additives can alter the lithium ion (Li⁺) deposition behavior on the surface of LMA. However, conventional additives are limited in their ability to produce only loose and porous SEI. In this study, we propose an organic additive of methyl methacrylate (MMA) that facilitates in-situ polymerization on the surface of LMA by generating anions or free radicals from LiTFSI. The MMA and LiNO₃ work in tandem to produce a polymer/inorganic SEI (PI–SEI) characterized by an outer layer enriched with PMMA–Li short–chain polymers and an inner layer enriched with Li₂O and Li₃N inorganics. Unlike the SEI formed by conventional additives, this PI–SEI exhibits higher stability and better Li⁺ transfer properties. The presence of short–chain polymers in PI–SEI alters the transport uniformity of Li⁺, facilitating stable cycling of Li || Li cell for over 2000 cycles with a capacity of 1&#xa0;mAh cm⁻². Furthermore, these PMMA–Li can chemically adsorb lithium polysulfides (LiPSs), thereby inhibiting Li corrosion by LiPSs, and enabling the capacity of lithium–sulfur batteries to achieve 474.3&#xa0;mAh g⁻¹ after 500 cycles at 0.5C. This study presents a strategy for generating SEI through the in-situ polymerization, which supports the commercial development of LMA in future liquid/solid Li metal batteries.</p> Graphical abstract <p></p>