Principal Investigator Yang Shao-Horn
Lithium-oxygen (Li-O2) batteries are promising as next-generation energy storage devices because they offer a theoretical 3-5 times higher gravimetric energy density than existing Li-ion batteries utilizing e.g. LiCoO2 for the positive electrode. This significant advantage in gravimetric energy was recently demonstrated experimentally for the first time in a Li-O2 cell utilizing aligned carbon nanofibers as the positive electrode, where high porosity (>90% void volume) enabled dense filling of the electrode with the solid discharge product Li2Ox (lithium (per)oxide) that forms and grows during discharge via reduction of O2. By normalizing the performance to the weight of the discharged electrode, when the electrode is in its heaviest state, we demonstrated an energy enhancement of ~4 times compared to LiCoO2, indicating that design of electrode structure is a key consideration for approaching the full potential of Li-O2 batteries.
Perhaps even more interestingly, the unique structure of carbon nanofiber electrodes allowed for clear observation of Li2O2 formation and dissolution during discharge and charge using ex-situ SEM and TEM studies. Using this approach, we reported a unique Li2O2 particle toroidal structure and showed its detailed evolution during discharge, and its disappearance following charge. This work raises intriguing questions about how Li2O2 nucleation and growth may influence the discharge capacity, round-trip efficiency and cycling capability of Li-O2 batteries, which is not currently understood, and opens up exciting opportunities to develop fundamental understanding of factors currently limiting practical applications of Li-O2 batteries.