High-nickel ternary cathodes hold a great application prospect in solid-state lithium metal batteries to achieve high-energy density, but they still suffer from structural instability and detrimental side reactions with the solid-state electrolytes. To circumvent these issues, a continuous uniform layer polyacrylonitrile (PAN) was introduced on the surface of LiNi_0.8Mn_0.1Co_0.1O₂ via in situ polymerization of acrylonitrile (AN). Furthermore, the partial-cyclized treatment of PAN (cPAN) coating layer presents high ionic and electron conductivity, which can accelerate interfacial Li⁺ and electron diffusion simultaneously. And the thermodynamically stabilized cPAN coating layer cannot only effectively inhibit detrimental side reactions between cathode and solid-state electrolytes but also provide a homogeneous stress to simultaneously address the problems of bulk structural degradation, which contributes to the exceptional mechanical and electrochemical stabilities of the modified electrode. Besides, the coordination bond interaction between the cPAN and NCM811 can suppress the migration of Ni to elevate the stability of the crystal structure. Benefited from these, the In-cPAN-260@NCM811 shows excellent cycling performance with a retention of 86.8% after 300 cycles and superior rate capability. And endow the solid-state battery with thermal safety stability even at high-temperature extreme environment. This facile and scalable surface engineering represents significant progress in developing high-performance solid-state lithium metal batteries.