Mixed Metal‐Organic Frameworks as Efficient Semi‐Solid Electrolytes for Magnesium‐Ion Batteries

Abstract

One of the main issues of metal organic framework (MOF)‐based solid electrolytes (SE) is their high guest solvent content reaching up to >50 wt% of the total mass of SE pellets. The presence of large solvent amounts reduces the SE hardness and the electrochemical stability in presence of a magnesium (Mg) anode. Moreover, this often leads to misleading ionic conductivity values. In the present work, a strategy to minimize the guest solvent in MOF‐based SE from 44–55 wt% to 20–30 wt% of the total SE's mass is presented. Moreover, mixed metal organic frameworks of different structures and crystallinity are demonstrated for the first time to enhance the ionic conductivity of Mg2+ ions inside the MOFs’ structures. The presence of both highly crystalline and amorphous MOFs increases the degree of disorder in the mixture and consequently opens up extra pathways for Mg2+ ion diffusion. The ionic conductivity of mixed MOFs [amorphous Mgbp3dc and crystalline α‐Mg3(HCOO)6] showed an enhanced value of 3.8×10−5 S cm−1 at 30 °C compared to 1.1×10−6 S cm−1 for α‐Mg3(HCOO)6. Mixed MOF‐SEs with a transference number (t+) of 0.335 showed a good stability in the presence of Mg electrodes with an enhanced reversibility upon galvanostatic cycling.

Ionic Liquid‐Incorporated Metal‐Organic Framework with High Magnesium Ion Conductivity for Quasi‐Solid‐State Magnesium Batteries

“…Only very recently, Hassan et al. proposed a strategy to minimize the amount of guest solvent in MOF SEs, and meanwhile successfully assembled a symmetric Mg/Mg cell to realize reversible Mg plating/stripping [19] …”

Effect of Guest Solvents on the Ionic Conductivity and Electrochemical Performance of Metal‐Organic Framework‐Based Magnesium Semi‐Solid Electrolytes

“…That means that 46.6 % of the transported charge of the electrolyte is carried by cations. This value is significantly improved compared to what was previously mentioned in literature for polymer‐based sSEs, [40] while there is no reported t + ‐value for MOF‐based sSEs, except in our previous study [16] . Galvanostatic measurements were carried out at different current densities, and showed a reversible Mg deposition/stripping process as shown in Figure S8a.…”

“…This method has been used to minimize the solvent extent in MOFsSEs as mentioned in our previous work. [16] To investigate any ionic conductivities that may come from MOF, the solvated MOF was dispersed in G4 in the same way without the addition of Mg salts.…”

“…[17][18][19] These properties make α-Mg 3 [HCOO] 6 a well-suited model system for investigat-ing the role of the guest solvent and to avoid any contribution from the open framework provided by the previously reported mixed MOF-sSEs. [16]…”

“…Here, α‐Mg 3 [HCOO] 6 has been chosen as MOF matrix for Mg salts due to its simpler structure, high stability against temperature, pressure, and chemicals, ease of synthesis, and low cost [17–19] . These properties make α‐Mg 3 [HCOO] 6 a well‐suited model system for investigating the role of the guest solvent and to avoid any contribution from the open framework provided by the previously reported mixed MOF‐sSEs [16] …”