consisting of a compound in which R1 is a branched chain alkyl consisting of 10 to 31 carbons； R2 is a linear alkyl, alkenyl, or alkynyl consisting of 2 to 20 carbons； L1 and L2 are the same or different, each a linear alkylene or alkenylene consisting of 2 to 20 carbons； X1 is S or O； R3 is a linear or branched alkylene consisting of 1 to 6 carbons； and R4 and R5 are the same or different, each a hydrogen or a linear or branched alkyl consisting of 1 to 6 carbons； or a pharmaceutically acceptable salt thereof.
