MOF-74(M) (M = Mg(II), Fe(II), Ni(II)) frameworks to enable accelerated redox kinetics for Li–S batteries

Energy storage technology is essential for the sustainable improvement of human society, especially in the present period when various electric vehicles are coming to the fore. Lithium-ion (Li-ion) batteries have been in increasing demand in recent decades; however, their energy density is reaching an upper limit, pushing researchers to look for next-generation alternatives1,2. Considering the high abundance on the Earth’s crust, low cost, environmental friendliness, high theoretical capacity (1675 mAh g−1) and energy density (2600 Wh kg−1), lithium-sulphur (Li-S) batteries are a promising alternative for next-generation batteries3,4,5,6. However, its commercial production is hampered by a series of challenging drawbacks. The capacity of Li-S batteries depends on the redox reaction of sulphur during the charging and discharging processes, leading to the production of large amounts of polysulphides7, and their dissolution leads to the shuttle effect8. Other drawbacks are the volumetric expansion of sulphur (~ 30%), poor electronic conductivity of sulphur and discharge product (Li2S), and low utilization of active material9. These issues cause unsatisfactory cycle performance, rapid capacity loss, and a reduction in the amount of active material10.

Different solutions have been proposed in terms of cathode materials to overcome these issues. Porous carbon materials are preferred due to their high conductivity, such as carbon nanotubes11, microporous12/mesoporous13/macroporous carbons14, and graphene15. Carbon materials can also be used as substrates for compounding other materials to improve the electrochemical performance of the electrode material16. However, due to the weak van der Waals interaction, non-polar carbonaceous materials can only mitigate the polysulfide shift to a certain extent. Consequently, chemical confinement has been proposed to remove the weaknesses of carbonaceous materials. Therefore, various polar materials such as metal oxides17, heteroatom doped-carbon18 and polymers19 have been studied to capture polysulphides and mitigate their shuttling. Nevertheless, the effect of this strategy is still unsatisfactory under conditions of high sulphur loading and lean electrolyte due to the adsorption sites of polar materials being finite and the slow conversion kinetics of polysulphides being unchanged. In this regard, the research attention shifts from physical limitations by carbon materials to chemical entrapments using polar materials, which may improve the kinetics of polysulphide conversion via a catalytic effect20. The key to improving the overall performance of Li-S batteries is the design of catalytic materials that synchronously combine efficient polysulphide binding and fast conversion along with smooth adsorption, diffusion and conversion of polysulphides.

A class of porous coordination polymers called metal-organic frameworks (MOFs), which have a large surface area, adjustable pore size and a unique porous structure, have proven to be effective catalysts for Li-S batteries21. They can facilitate the transfer of electrons and ions, reduce the activation energy and help to maintain the structural integrity which can lead to speeding up the reactions, improving the battery’s performance and efficiency. MOFs are hybrid inorganic-organic materials consisting of metal ions or clusters bridged by organic linkers to form robust frameworks with permanent porosity22. Due to their unique structure, they find applications in many research areas, such as drug delivery23,24, gas adsorption and separation25,26,27, heterogeneous catalysis28,29, ion exchange30,31, sensing32,33,34 or as additives in cathode materials for Li-S batteries35,36. Zeolitic imidazolate frameworks (ZIFs) are among the most widely used MOFs in Li-S batteries. Zhou et al. fabricated a sulphur cathode containing carbonized ZIF-67, which exhibited a discharge capacity of 702 mAh g−1 at 0.2 C37. Yang et al. described ZIF-8 deposited on carbon cloth as a sulphur host; the capacity of 1036 mAh g−1 at 0.2 C was achieved38. MOF-74, which has a high number of active sites (coordinatively unsaturated sites (CUSs)), is commonly used as a catalyst in Li-air batteries39,40. Furthermore, using density functional theory (DFT) calculations, it was confirmed that MOF-74 can chemically adsorb polysulphides to the active sites41. The MOF-74 showed a good performance in solid-state Li-ion42 or as an interlayer in Li-S43. The high number of active sites in synergy with the catalytic effect of metal could effectively enhance the performance of Li-S batteries.

The frameworks of MOF-74(M) (where M represents a divalent metal cation) display an isostructural topology with the formula {[M2(DOBDC)(H2O)2]·G}n (DOBDC4− = 2,5-dioxido-1,4-benzene-dicarboxylate, G = guest molecules), where the metal entities serve as nodes linked by organic linkers to form honeycomb skeleton containing hexagonal pores with the size dimension of 11 × 11 Å44, which is sufficient to accumulate S8 molecules (see Fig. 1a). Currently, many MOF-74(M) have been prepared, which contain Mg(II), Ca(II), Sr(II), Ba(II), Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) cations as monometallic nodes45, or combinations thereof leading to the formation of multimetallic compounds e.g. Ni(II)/Zn(II), Mg(II)/Zn(II), Ca(II)/Zn(II), Mg(II)/Co(II) representing bimetallic compounds46,47, Mg(II)/Co(II)/Ni(II)/Zn(II) as tetrametallic, Mg(II)/Sr(II)/Mn(II)/Co(II)/Ni(II)/Zn(II) as hexametallic, Mg(II)/Ca(II)/Sr(II)/Mn(II)/Fe(II)/Co(II)/Ni(II)/Zn(II) as octametallic or Mg(II)/Ca(II)/Sr(II)/Ba(II)/Mn(II)/Fe(II)/Co(II)/Ni(II)/Zn(II)/Cd(II) as decametallic MOF-7448. The central atom is pentacoordinated by four oxygen atoms of the DOBDC4− ligands, utilizing all of their oxygen atoms in coordination bonds with other metal ions, and the fifth coordination site is occupied by water molecule to form a distorted tetragonal pyramid geometry around the central atom (see Fig. 1b). The removal of coordinated water molecules results in the formation of coordinatively unsaturated sites (CUSs), representing Lewis acidic sites, which play an important role as primary adsorption or catalytic active sites. MOF-74-type compounds feature a high density of CUSs lying in the corners of 1D hexagonal channels along the c crystallographic axis, as depicted in Fig. 1a. CUSs or Lewis acidic sites facilitate strong binding of CO2, H2, CH4, Sx and other molecules, acting as Lewis bases, to these sites44,45,49,50. Furthermore, the solvothermal synthesis of MOF-74(M) is relatively straightforward and can be achieved under mild conditions, which also enables facile alteration of the central metal within the same framework. A practical limitation, however, arises from the use of DMF as a reaction solvent, since it remains strongly trapped in the pores, requires high-temperature activation for its removal, and may influence the framework geometry/stability. In the present study, this issue was resolved by a solvent-exchange process, in which DMF was replaced with methanol. Owing to its lower boiling point and weaker interaction with the framework, methanol can be removed more readily, thereby facilitating subsequent activation and preserving structural integrity. Even with this necessary post-synthetic treatment, the solvothermal approach to MOF-74 remains comparatively simple, relying mainly on mixing, heating, and cooling of solutions. In this context, the term ‘cost-effective’ refers to the use of inexpensive and widely available precursors, common solvents, and standard solvothermal conditions, rather than to a quantitative economic analysis. For completeness, other synthesis strategies have also been reported, including solvent-free mechanochemical synthesis51, one-pot52, microwave53, vapour-assisted54, and dry gel synthesis55.

Herein, a composite material of MOF-74, carbon black, and sulphur was designed as a long-lasting electrode material for Li-S batteries, S/MOF-74(M). MOF-74 based on Ni(II), Mg(II), and Fe(II) metal ions were synthesized using the conventional solvothermal method, and cathode material based on sulphur was prepared. MOF-74 has a large number of micropores, which is beneficial for the physical confinement and capture of sulphur. Moreover, as shown in Fig. 1a, the pore size is sufficient to store sulphur molecules. The main objective was to study the influence of different central atoms in MOF-74 on the conductivity of electrode material and the chemical confinement of sulphur. Fe(II) ions, together with Mg(II) cations, were chosen because of their low toxicity and cost, relatively high availability and abundance in nature (fourth and seventh, respectively). In addition, Mg(II) ions have the advantage of high gravimetric/specific capacity as they have the lowest atomic weight of all the selected metal ions. Although Ni(II) ions lag behind in the above properties compared to Mg(II) and Fe(II) cations, the high catalytic properties of Ni have been proven by application in several types of batteries. Ni has been widely used in batteries since the 1980s in nickel-cadmium (Ni-Cd) and nickel metal hydride (NiMH) devices58,59. Two of the most commonly used types of Li-ion batteries, i.e. Nickel Cobalt Aluminium (NCA) and Nickel Manganese Cobalt (NMC), use at least 33% nickel with a tendency to increase its amount60. On the other hand, the advantage of Fe(II) and Ni(II) ions compared to Mg(II) cations is their variability of oxidation states (II/III), which can also help to achieve stability in the kinetics of electrochemical processes. Moreover, the overall porous network may mitigate the volumetric expansion of sulphur during cycling. The prepared S/MOF-74(Ni) composite reached an extremely stable cycle performance which is discussed in detail and compared with Mg(II) and Fe(II) MOF-74 analogues and other sulphur cathodes containing MOF in Li-S batteries.