A current focus in battery research, particularly for electric vehicles and stationary energy storage applications, is identifying new chemistries that could enable a higher energy density and lower cost battery than currently available Li-ion batteries. Li-ion electrode materials, which dictate attainable battery energy densities, used in these batteries have changed little in the past decades, and therefore physical limits in Li-ion battery pack energy density are being approached given the enormous development efforts in the field.  In this work, we take a close look at molten salt lithium batteries and in particular alkali metal nitrate/nitrite molten salt electrolytes which exhibit low melting point temperature, high ionic conductivity, high thermal stability and good interfacial properties against lithium metal anode. More specifically, we investigated the O₂ electrode electrochemical behavior as well as the reduction of nitrates on metal and metal oxide nanoparticles to form lithium oxide and nitrites. The results reported herein provide a potentially transformative approach to enable a rechargeable Li-O₂ battery chemistry through the use of inorganic molten salts as electrolytes. Building on our previous foundational research, in which we identified that all organic electrolytes are incompatible with the reversible Li-O₂ electrochemistry, the electrolyte compositions reported here do not contain unstable organic solvents. As a result, the reversibility of the Li-O₂ electrochemistry reported here is unprecedented.  Another approach developed at Liox Power, Inc. is the use of nitrates as electroactive material, or in other words, as both the electrolyte and the cathode material. In this case the battery discharge reaction proceeds as the electrochemical reduction of nitrate anions NO₃^- present in the electrolyte phase. High Li⁺ and NO₃^-concentration and ionic conductivity allow extremely fast kinetics at 150 °C resulting in very low discharge overpotentials. Furthermore, we demonstrate that such electrochemical reaction is reversible and lithium oxide particles (octahedron equilibrium Wulff shape) can be recharged with energy efficiencies greater than 97%.

Figure 1. Top, voltage profile (first 2 cycles) of a cell employing a Li metal anode, a LiNO₃-KNO₃ eutectic and a Ni cathode, cycling at 150 °C at 0.32 mA/cm², under Ar gas. Bottom, voltage profiles of (black) organic electrolyte Li-O₂ cell and (red) LiNO₃-KNO₃ molten salt Li-O₂ cell; both cells used a Li metal anode, a Super P carbon black cathode (5% PTFE binder) and cycled at 0.32 mA/cm² under pure O₂ gas.