3D battery architectures overcome limitations of traditional planar designs by creating non-planar electrode structures, which increase energy densities within a small areal footprint while maintaining the short ionic transport distances necessary for high power densities. One of the key technological limitations to the development of 3D battery architectures is the synthesis of a nanoscale ion-conducting thin film electrolyte that can uniformly cover complex geometries of 3D electrodes while occupying minimum volume. Shorter ionic diffusion lengths result in orders of magnitude lower ionic diffusion times in these nanoscale films, when compared with bulk or microscale polymeric electrolytes like poly-(ethylene oxide) (PEO). The shorter diffusion times can thus compensate for modest ionic conductivities in these nanoscale films, realizing a new class of electrolytes. Here, we report the development of nanoscale (10-40 nm), conformal thin film electrolytes realized by doping lithium ions (Li+) into poly-(tetravinyltetramethylcyclotetrasiloxane) (PV4D4) films, which were synthesized by initiated chemical vapor deposition (iCVD). This is the first time nanoscale films with siloxane ring moieties, which are excellent electrical insulators, have been demonstrated as room temperature ionic conductors. The films exhibit minimal changes in morphology and thickness during lithiation and are also demonstrated to be easily scalable over large areas. We show that the conformal nature of the iCVD polymerization process realizes complete coverage of nanostructured electrodes like nanowires by a uniform, continuous, and pinhole-free thin film, making the polysiloxane films attractive as a novel class of nanoscale electrolytes for the emerging field of three-dimensional (3D) batteries.