Lithium Batteries with Small-Molecule Quinone Cathode Enabled by Lithium Garnet Separators

The lithium garnet solid electrolyte (LLZO) provides an opportunity to consider cathode active materials which are not compatible with the conventional cell architecture based on porous separators and liquid electrolytes. For example, the class of organic materials known as quinones offer high specific capacities (up to 496 mAh/g) but are soluble in liquid electrolytes, leading to rapid capacity degradation. This work demonstrates solid electrolyte/liquid catholyte hybrid battery cells with metallic Li alloy as the anode and lawsone, a biologically derived quinone, as the cathode. LLZO is used as a separator and effectively prevents dissolved cathode material from meeting the anode. Lawsone LLZO hybrid cells reached cathode utilization of up to 67% and had little self-discharge compared to liquid cells. Side reactions at voltages below 2.0 V limited their long-term cycling stability, however. Utilizing a lower cutoff voltage of 2.0 V or higher dramatically reduces capacity fade but prevents complete lithiation of the second lawsone carbonyl group, curtailing capacity to only ca. 120 mAh/g. LLZO cells using 1,4-naphthoquinone, which is structurally identical to lawsone except for the absence of the irreversible hydroxyl lithiation site, were more stable at low voltages. These results suggest that achieving full theoretical capacity in a lawsone LLZO cell will require elimination of the low-voltage side reactions to enable full utilization of both carbonyl groups and mitigation of issues stemming from irreversible lithiation of the hydroxyl group. Nevertheless, use of a solid electrolyte is shown to be a promising approach to enable use of soluble organic cathode active materials.