Impact of Layer Stacking Manner on the Lithium-Ion-Battery Performance in Electrically Neutral Tetraoxolene-Bridged Iron(II) Hexagonal Layer Metal–Organic Frameworks

Cathode materials for lithium-ion battery (LIB) cells present a fascinating target for applications in metal–organic frameworks (MOFs). While the framework moiety of MOFs acts as an electron container involving redox reactions, the pores store Li⁺ ions in MOF-incorporating LIBs. Thus, in addition to the redox potential and electron-conjugating nature of the framework, the path features for Li⁺-ion migration between the frameworks are closely associated with the LIB performance. Herein, we demonstrate the impact of porosity on the LIB performance using a series of charge-neutral layered MOFs, [Fe^II₂(X₂An)₂(bpym)] (X = F, 1; Cl, 2; Br, 3; X₂An^2– = 2,5-dihalogeno-3,6-dihydroxy-1,4-benzoquinonate; bpym = 2,2′-bipyrimidyl), as cathode materials for LIBs. All compounds have a similar layered structure with the same electronic state; however, 1 has an eclipsed layer-stacking, whereas the isostructural 2 and 3 have polymorphic staggered structures, which results in one-dimensional channel paths in 1 and isolated pores in 2 and 3. The battery capacity was not dependent on the compound at low current densities but was largely affected by the stacking manner at high current densities and overpotentials: owing to the 1D channel that offers a good diffusion path for lithium ions, 1 exhibited desirable characteristics for LIBs.