Rechargeable, thin film all-solid-state Li-ion batteries (TFSSLIBs) with high specific power and energy density are highly desirable to energize an emerging class of miniature, autonomous microsystems that operate without a hardwire for power or communications. TFSSLIBs are also attractive for fundamental studies aimed at understanding how battery geometry, dimensions, composition and the resulting interfaces affect performance. For example, thin film fabrication methods enable precise control over electrode and electrolyte thickness, morphology, geometry and interface area (i.e. 1D, 2D or 3D type electrode). Furthermore, TFSSLIBs are vacuum compatible, meaning that techniques that generally require vacuum such as SEM, TEM, auger electron spectroscopy, secondary ion mass spectroscopy, and Kelvin probe force microscopy can be readily applied to characterize TFSSLIB, often in operando mode. In my presentation, I will discuss recent experiments and modeling efforts to understanding the factors that limit TFSSLIB performance, including the role of interfaces, and which take advantage of thin film fabrication techniques and vacuum based characterization methods.