TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Component: The thermal protective film covers the material of the cathode plate, which comprises lithium mixed metal oxide, conductive additive, and binder. The lithium mixed metal oxide comprises lithium manganese oxide (LiMnO2), LiMn2O4, lithium cobalt oxide (LiCoO2), lithium chromium oxide (Li2Cr2O7), Li2CrO4, lithium nickel oxide (LiNiO2), lithium iron oxide (LiFeO2), LiNixCO1-xO2, LiMPO4, LiMn0.5Ni0.5O2, LiNixCOyMnzO2, lithium nickel carbon oxide aluminum oxide (LiNixCOyAlzO2), and/or LiMc0.5Mn1.5O4.x=greater than 0, less than 1;M=transition metal;orx+y+z=1;andMc=bivalent metal.    TECHNOLOGY FOCUS - METALLURGY - Preferred Component: The conductive additive comprises carbon black, graphite, acetylene black, nickel powder, aluminum powder, titanium powder, and/or stainless steel powder.    TECHNOLOGY FOCUS - POLYMERS - Preferred Component: The binder comprises polyvinylidene fluoride, styrene-butadiene rubber, polyamide, and/or melamine resin. The thermal protective film comprises nitrogen-containing polymer, which is a reaction product of amines, amides, imides, maleimides, or imines, with diones comprising barbituric acid, barbituric acid derivatives, acetylacetone, or acetylacetone derivatives. The nitrogen-containing polymer is a reaction product of barbituric acid and bismaleimide monomer. The onset temperature is 100-220, preferably 130-200 degrees C. The thermal protective film shows N-1s or S-1s signals when analyzed by x-ray photoelectron spectroscopy (XPS). It has a thickness of not less than 25 nm. A ratio of peak areas of transition metal 2p signal of the thermal protective film at 5 minutes to 0 minute is greater than 6 according to XPS analysis. Preferred Method: Modifying comprises modifying the lithium mixed metal oxide powder with a solution of nitrogen-containing polymer having a solid content of 1-20%. A weight ratio of the modified electrode material powder, conductive additive, and binder in the modified electrode plate is 80-93:0.1-5:0.1-5.