Pillared-Layer Metal–Organic Frameworks for Improved Lithium-Ion Storage Performance

Recently, more and more metal–organic
frameworks (MOFs)
have been directly used as anodic materials in lithium-ion batteries,
but judicious design or choice of MOFs is still challenging for lack
of structural–property knowledge. In this article we propose
a pillared-layer strategy to achieve improved Li-storage performance.
Four Mn­(II) and Co­(II) MOFs with mixed azide and carboxylate ligands
were studied to illustrate the strategy. In these 3D MOFs, layers
(1, 3, and 4) or chains (2) with short bridges are linked by long organic spacers.
All the MOFs show very high lithiation capacity (1170–1400
mA h g–1 at 100 mA g–1) in the
first cycle owing to the rich insertion sites arising from the azide
ion and the aromatic ligands. After the formation cycles, the reversible
capacities of the anodes from 1, 3, and 4 are kept at a high level (580–595 mA h g–1) with good rate and cycling performance, while the anode from 2 undergoes a dramatic drop in capacity. All the MOFs lose
the crystallinity after the first cycle. While the amorphization of
the chain-based framework of 2 leads to major irreversible
deposit of Li ions, the amorphous phases derived from the pillared-layer
frameworks of 1, 3, and 4 still
retain rich accessible space for reversible insertion and diffusion
of active Li ions. Consistent with the analysis, electrochemical impedance
spectra revealed that the pillared-layer MOFs led to significantly
smaller charge-transfer resistances than 2. Soft X-ray
absorption spectroscopy suggested that no metal conversion is involved
in the lithiation process, consistent with the fact that the isomorphous
Co­(II) (3) and Mn­(II) (4) MOFs are quite
similar in anodic performance.