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High‐Ni‐content LiNixCoyMn1−x−yO2 is regarded as a feasible cathode material to meet the urgent requirement for high energy density batteries. However, such cathode has a poor safety performance because of reactive oxygen releasing at elevated temperatures. In pursuit of high‐safety lithium‐ion batteries, a heatproof–fireproof bifunctional separator is designed in this study by coating ammonium polyphosphate (APP) particles on a ceramic‐coated separator modified with phenol‐formaldehyde resin (CCS@PFR). The CCS@PFR separator acts as a thermal‐supporting layer to inhibit the shrinkage of the separator at elevated temperatures, whereas the APP‐coated layer functions as a fireproof layer, forming a dense polyphosphoric acid (PPA) layer above 300 °C. The PPA layer not only isolates the combustibles from the highly reactive oxygen released from the cathodes but also converts violent combustion reactions into mild stepwise exothermic reactions by carbonizing the combustibles in the batteries. Enabled with such a heatproof–fireproof bifunctional separator, LiNi0.8Co0.1Mn0.1O2|SiOx−Gr full cells are constructed and these exhibit an excellent safety performance by not catching fire during a 30 s combustion test and surviving the 10 min high‐temperature test above 300 °C. Additionally, an adiabatic rate calorimeter and nail penetration test are conducted with 3 Ah LiNi0.8Co0.1Mn0.1O2|SiOx−Gr pouch cells to further verify the safety performance. [ABSTRACT FROM AUTHOR]