Conventional Li-ion batteries are severely limited in high temperature applications mainly due to the poor thermal stability of the separator membrane (shrinkage/shutdown) and electrolyte decomposition in the full-cell. We have demonstrated a high temperature Li-ion system capable of good rate performance from 20 to 120 °C, well beyond the typical 60 °C limit of traditional Li-ion batteries. We have developed a printable and highly flexible Al2O3-poly(vinylidene fluoride) nanoporous separator membrane (Pyrolux™) infiltrated with a carefully designed high boiling point lithium bis(oxoalato)borate-based (LiBOB) liquid electrolyte. This unique electrolyte/membrane combination can offer a substantial safety advantage over conventional polyolefin separators embedded with liquid electrolyte, such as reduced flammability and enhanced dimensional stability at high temperature. Li//LiFePO4 and Li//graphite half-cells were tested at 120 °C and achieved reversible capacities of 155 and 340 mA h/g, respectively, with Coulombic efficiencies (CEs) > 99.4%. To date, reports on high temperature Li-ion full-cells have been limited mainly due to a combination of poor thermal stability of the separator membrane, accelerated side reactions, and solid-electrolyte interphase reformation. By utilizing our membrane and electrolyte, LiFePO4//graphite full-cells were successfully demonstrated for the first time at 120 °C with excellent cyclability, representing a major step forward in the development of high temperature Li-ion batteries.