Metal–organic frameworks (MOFs) have emerged as an important, yet highly challenging class of electrochemical energy storage materials. The chemical principles for electroactive MOFs remain, however, poorly explored because precise chemical and structural control is mandatory. For instance, no anionic MOF with a lithium cation reservoir and reversible redox (like a conventional Li-ion cathode) has been synthesized to date. Herein, we report on electrically conducting Li-ion MOF cathodes with the generic formula Li2-M-DOBDC (wherein M = Mg2+ or Mn2+; DOBDC4– = 2,5-dioxido-1,4-benzenedicarboxylate), by rational control of the ligand to transition metal stoichiometry and secondary building unit (SBU) topology in the archetypal CPO-27. The accurate chemical and structural changes not only enable reversible redox but also induce a million-fold electrical conductivity increase by virtue of efficient electronic self-exchange facilitated by mix-in redox: 10–7 S/cm for Li2-Mn-DOBDC vs 10–13 S/cm for the isoreticular H2-Mn-DOBDC and Li2-Mg-DOBDC, or the Mn-CPO-27 compositional analogues. This particular SBU topology also considerably augments the redox potential of the DOBDC4– linker (from 2.4 V up to 3.2 V, vs Li+/Li0), a highly practical feature for Li-ion battery assembly and energy evaluation. As a particular cathode material, Li2-Mn-DOBDC displays an average discharge potential of 3.2 V vs Li+/Li0, demonstrates excellent capacity retention over 100 cycles, while also handling fast cycling rates, inherent to the intrinsic electronic conductivity. The Li2-M-DOBDC material validates the concept of reversible redox activity and electronic conductivity in MOFs by accommodating the ligand’s noncoordinating redox center through composition and SBU design.
Coordination polymers (CPs) made of redox-active organic moieties and metal ions emerge as an important class of electroactive materials for battery applications.
The organic metal-ion battery field remains challenged by the lack of high-voltage alkali-cation reservoir cathode materials. Whereas a few recent breakthroughs provided valuable solutions for Li-ion storage, Na-ion and K-ion organic reservoirs with high voltage and ambient stability remain elusive. Herein, we show that the versatile benzene-1,2,4,5-tetrayltetrakis methylsulfonyl-amide (PTtSA) tetra-anionic framework displays universal performance for alkali cation storage. The new synthesized Na4-PTtSA and K4-PTtSA phases reversibly exchange two Na+ or K+ equivalents per formula unit at redox potentials of 2.5 V vs Na+/Na and 2.6 V vs K+/K, respectively. A singular comparative analysis of Li-, Na-, and K-ion phases discloses the impact of the alkali cation on the physicochemical properties, with direct impact on the electrochemistry of the materials. This work not only offers guidance and principles to tune the redox properties of organic redox materials via spectator cations but also highlights the versatility of organic materials for alkali cation storage.
Metal-free and metal(II) all-endo-tetraalkoxy-phthalocyanines of C4h symmetry are synthesised regiospecifically from 3-(2-butyloctyloxy)phthalonitrile with lithium octanolate and subsequent metal ion exchange. The voluminous, yet not overly large, and racemically disordered alkoxy substituent not only renders the cyclotetramerisation regiospecific, but also leads to liquid crystalline self-assembly with attainable clearing temperatures and persisting columnar organisation at room temperature. A rare hexagonal mesophase with twelve columns per hexagonal unit cell is found in the metal-free homologue, whereas the metal complexes show rectangular mesophases. The clearing temperature increases with increasing axial component of the metal ion coordination sphere. At low temperature, significant antiferromagnetic exchange between magnetic centres is observed for the Co(II) homologue, whereas the magnetic centres are magnetically independent in the Cu(II) derivative, in line with the observed higher clearing temperature in the Co(II) case that testifies of stronger interdisk interactions.
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