Design of Heterostructures of MXene/Two-Dimensional Organic Frameworks for Na-O2 Batteries with a New Mechanism and a New Descriptor.

Na-O2 batteries are promising candidates to replace Li-O2 batteries for their excellent performance. However, the charge overpotential of Na-O2 batteries is usually too high. In this work, we designed combinations of MXene and a two-dimensional organic framework for Na-O2 batteries. The results show that the Ti2CO2/Cu-BHT has low OER and ORR overpotentials of 0.24 and 0.32 V, respectively. Besides this, the conductivity and the adsorption energy to Na+ (Eads(Na+)) are promoted due to the charge transfer between layers. We also found that the OER and ORR overpotentials are negatively and positively correlated with Eads(Na+), respectively, where Ti2CO2/Cu-BHT has a moderate Eads(Na+) (-2.20 eV) and, therefore, has good performance. Moreover, a new mechanism called the Na encapsulation mechanism was proposed on a two-dimensional organic framework surface. Through least absolute shrinkage and selection operator (LASSO) regression, we found a new descriptor that consists of inherent properties that could help us screen better heterostructures for Na-O2 batteries.