Bimetallic conductive molecular 2D metal-organic framework @ 2D MXene nanosheet heterostructure for lithium storage

Two-dimensional metal-organic frameworks (2D MOFs) have been considered to be one type of potential cathodes for lithium storage due to their regular channels and devisable redox-active sites. However, the relatively low electrical conductivity and weak long-term cyclability hindered their development. Herein, we report an optimized bimetallic conductive 2D MOF (Co_1.5Ni_1.5(HHTP)₂) for enhanced lithium-ion storage. Specifically, the optimal 2D MOF electrode demonstrates a reversible capacity of 615.2 mAh/g at 0.2 A/g and the specific energy density up to 826 Wh Kg^-1 after 100 cycles. Moreover, a heterostructure (Co_1.5Ni_1.5(HHTP)₂@MXene) was constructed by stacking Co_1.5Ni_1.5(HHTP)₂ MOF on the 2D Ti₃C₂ MXene nanosheets to improve the rate performance and cycling stability. The heterostructure electrode kept 74.1 % of the initial capacity with nearly 100 % coulombic efficiency even at 2000 cycles. Ex situ characterizations and density functional theory calculations were carried out to reveal the redox mechanism of the 2D conductive MOF. Electronic states study unveils that the quinoid oxygens and the metal centers are responsible for the redox reaction during lithiation/delithiation process, the composited 2D MXene can promote the electron transfer. The development of bimetallic mixed conductive 2D MOF @ 2D MXene heterogeneity offers valuable insights into the design of electrode materials for lithium storage.