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,
[FeII2(X2An)2(bpym)] (X
= F, 1; Cl, 2; Br, 3; X2An2– = 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.