Garnet electrolytes having a room-temperature Li+ conductivity of 10−3 S cm−1 suffer from adsorbents that prevent wetting by a lithium anode; formation and growth of lithium-anode dendrites into grain boundaries and a huge interfacial resistance to anode plating and stripping result. A thin coating of a garnet surface by a Li+-conducting polymer with a transfer number of 0.9 is shown to suppress dendrite formation and to reduce greatly the interfacial resistance. A coulombic efficiency near 100% with a single thin polymer coat can provide Li/garnet/LiFePO4 all-solid-state cell with a long cycle life.