High voltage Li-ion cathodes push the limits of stability for both cathode and electrolyte. Here sub-nanometer coatings of an amorphous thin-film electrolyte (Lipon) improved the room temperature and 60°C cycling stability of a LiMn1.5Ni0.5O4 spinel cathode when charged to 4.9 V with a standard LiPF6 carbonate electrolyte. The cathodes delivered superior C-rate performances up to a 5C discharge, when compared to the uncoated cathodes. Enhanced performance extended for at least 100 cycles. Electrochemical impedance spectroscopy indicates that Lipon slows the increase of interface resistance. Thicker 1-3 nm Lipon coatings are sufficiently insulating as to block electronic transport to the cathode particles. Thick coatings also slow Mn dissolution. Results suggest that Lipon may act to scavenge impurities or block active sites that promote electrolyte decomposition. While greatly improved by the Lipon coating, cycling is not yet sufficiently stable for applications requiring thousands of cycles or prolonged operation at higher temperatures. Further work is needed to assess what surface properties of coatings will ultimately stabilize the high voltage cathodes in carbonate liquid electrolyte cells. Comments include insight from other studies of Lipon-coated cathodes and the focus for future research. © 2013 The Electrochemical Society.