An Electrically Conducting Li-Ion Metal–Organic Framework

Abstract

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.

The Role of Critical Raw Materials for Novel Strategies in Sustainable Secondary Batteries

“…Further steps in the field have included the use of MOFs with both redox-active metals and ligands, [82,83] as well as the use of MOFs with anionic networks and charge-compensating Li ions. [84] Additionally, MOFs can also be used as anode materials. Thus, the nanoporous Fe-MIL-88B nanorods display a reversible capacity of 744.5 mAh g À1 for more than 400 cycles at 60 mA g À1 , which is ascribed to the contribution of both the transition metals and the organic part to the lithiation/delithiation process and the structural characteristics of the MOF.…”

“…More examples of MOFs and their derivatives employed in LIBs can be found in the literature. [84,[90][91][92]…”

“…Further steps in the field have included the use of MOFs with both redox‐active metals and ligands, [ 82,83 ] as well as the use of MOFs with anionic networks and charge‐compensating Li ions. [ 84 ]…”

Roadmap of Solid-State Lithium-Organic Batteries toward 500 Wh kg–1

“…Several redox classes have been explored for this purpose, and some of them have shown excellent electrochemical performances with opportunities to attain simultaneously high energy and power densities combined with good cycling stabilities. In addition, various topologies were studied, including crystalline small molecules, polymers, and covalent/metal–organic frameworks (COFs, , MOFs , ), whose employment demonstrated a significant effect on the electrochemical and battery performances. OBEMs with redox potential values ranging from 0.65 to 4.1 V vs Li + /Li and capacity values varying between 90 and 589 mAh g –1 are ready to be integrated in SSBs.…”

Revealing the Solid-State Electrochemistry of Conjugated Oximates: Towards a New Functionality for Organic Batteries

“…Unlike the large diversity of inorganic cathode materials, the foundation of the state-of-the-art practical Organic Li-Ion Cathodes (OLICs, all developed over the past 4 years as n-type) was laid extensively on enolate/carbonyl redox chemistry. It is encouraging to see the advancements in OLICs through electronwithdrawing substituted quinones 11 , sacrificial metal-mediated charge delocalization 12,13 , and stereoelectronic chameleonic effect 14 , yet they still suffer from low capacity and inefficient redox kinetics. An ideal OLICs must possess ambient stability, reversible multi-electron redox, high theoretical capacity and insolubility in Li-reservoir state, which motivates us to push the limits of organic chemistry in the search and design of new organic Li-ion redox active materials.…”