Compounds are provided having one of the following structures (I) or (II): or (I) (II) or a pharmaceutically acceptable salt, tautomer or stereoisomer thereof, wherein R3a, R3b, L1, L2, G1a, G1b, G2a, G2b and G3 are as defined herein. Use of the compounds as a component of lipid nanoparticle formulations for delivery of a therapeutic agent, compositions comprising the compounds and methods for their use and preparation are also provided.