Efficient Elimination of Lithium Carbonate/Carboxylates By NiO for Rechargeable Li–O2 Battery

The non-aqueous lithium–oxygen (Li–O₂) battery has been extensively investigated by virtue of its remarkably high theoretical energy density. However, the Li–O₂ cell has suffered from parasitic side reactions due to superoxide (O₂^•−) attack and lithium peroxide (Li₂O₂) activity to non-aqueous electrolytes and carbon cathodes, which form lithium carbonates (Li₂CO₃) and carboxylates (RCO₂Li, R = H or alkyl). These poor-conducting side products deposited on the cathode increase the recharge potential, which triggers the parasitic side reactions further. Some efforts to employ superoxide-tolerant electrolytes, carbon-free cathodes and cycling with limited depth of discharge have alleviated the side reactions whereas the lithium carbonates/carboxylates inevitably formed are overwhelmingly deposited on the cathode for cycling thus terminating the Li–O₂ cell operation rapidly.

            Here, we present new approach to promote elimination of lithium carbonate/carboxylates using nanoporous nickel oxide (NiO), which enhances cycling performance and round trip efficiency of Li–O₂ battery for cycling. The crystalline nanoporous NiO plates that were homogeneously distributed on the entangled carbon nanotubes (CNT) network via hydrothermal synthesis (NiO/CNT) exhibit striking capacity retention of over 70 cycles at a limited discharge capacity of 1000 mAh g_CNT⁻¹ with a stable recharge potential of 4.1 V vs. Li/Li⁺ in tetraglyme electrolyte. The role of nanoporous NiO is investigated under the cycling at deep depth of discharge. Unlike the first cycle providing predominant Li₂O₂, the increasing extent of lithium carbonates/carboxylates for cycling are observed by ¹H NMR and FTIR spectroscopies. In particular, the lithium carbonates become a main side product over 10 cycles. Interestingly, the NiO/CNT cathodes reveal disappearance of these side products after every recharge and high capacity retention, which are different from NiO-free CNT cathodes providing the accumulated side products and swift capacity fading. Further analysis using anodic potential sweeps of side products in three-electrode cell demonstrate ~300 mV lower oxidation potential on the NiO/CNT than the NiO-free CNT while negligible influence on the electrolyte oxidation, which implies a promoter of nanoporous NiO to eliminate lithium carbonates/carboxylates. TEM images display that the side products seem to transport along the CNT and make a lump close to the NiO, where the decomposition of side products can be carried out actively. The nanoporous NiO has been very stable during deep cycling, where either chemical or structural change is not observed, which indicates that the NiO can contribute toward stable long-term cycling performance of Li–O₂ cell.