Xu et al. synthesized a Ni-MOF-74/CNT composite via a typical solvothermal method
[33]. The MOF structure pores could trap the polysulfides, whereas MWCNTs provided the electric path and necessary channels for the electrolyte transfer. The cathode maintained 503 mAh/g after 400 cycles at 2C, with retention of 65.1%. The CNTs concentration is also integral to the final properties and its effect has been tested during the preparation of UiO-66/CNT
[34]. Additionally, since an ideal UiO-66 does not have available Lewis acidic sites to bind the polysulfides, benzoic acid was used as a modulator, competing with terephthalic acid and creating linker-missing defects in order to introduce more active sites and receive a more open framework
[35,36,37][35][36][37]. The interaction of coordination defects and polysulfides entrapment was studied with DFT
[34]. In short, structures with 1–3 linker defects were simulated, showing not only a stronger affinity to Li
2S
4 than an ideal UiO-66 but an enlargement of inner cavities
[36]. However, more defects lead to the opposite effect weakening the binding energy
[34]. The composite achieved better cycling stability with 34% CNT content, instead of 23 or 56%, both over 300 cycles at 0.5 A/g (765 mAh/g), and over 800 cycles at 1 A/g (~500 mAh/g).
Further thermal treatment of MOFs can form nanocarbons, which adopt the same morphology as their precursor. This provides researchers with the opportunity to explore a wide variety of unique architectures to confine and stabilize sulfur molecules. These architectures exhibit a conductivity appropriate for utilization in a battery system.
ZIF-8 derived carbons have been an attractive option recently to enclose sulfur, forming morphologies such as typical rhombic dodecahedrons
[38,39,40][38][39][40], spheres
[41] and nanosheets
[42] or nanocomposites with graphene
[43,44,45][43][44][45] or grown on 1D structures such as CNTs
[46] or nanofibers
[47] in order to be applied in Li-S cathodes. A successful example has been the production of N-doped hierarchically porous carbon 2D nanosheets by Jiang et al. in order to immobilize sulfur molecules. The fabricated cathode could attain 300 cycles, while in the 100th cycle it delivered capacity retention of 65.2% at 0.2C. The overall battery system supplied an initial capacity of 1226 mAh/g
[42].
3. Metal Ion Batteries
The widespread use of wearable electronics has brought the need for flexible types of energy storage devices, with many efforts taking place in the field of metal-ion batteries. In Metal -Ion batteries like Li-ion, the cathode is the Li-containing electrode and is usually composed of transition metal oxides, such as LiMO
2, LiM
2O
4 and LiMPO
4 (M = Mn, Co, Ni, etc.). On the other side of the battery cell, the anode is still dominated even at the research level by graphitic carbon materials
[77][48]. The specific capacity of the battery depends on the molecular weight of the material used in the cathode, unlike Li-S and Li-Se batteries where Lithium dominates the theoretical capacity of the battery. For instance, a LiCoO
2 cathode offers a theoretical capacity of 274 mAh/g
[78][49] while a LiFePO
4 cathode until 160 mAh/g
[79][50].
The first report of a MOF applied in a Li-ion battery can be found back in 2005 with MOF-177
[80][51]. A few years later Fe
2+ and Fe
3+ started being explored as redox centers in MOFS such as MIL-53
[81,82][52][53] and MIL-68
[83][54]. Other than their common linker, terephthalic acid, these MOFs displayed alternative structures depending on the different synthesis conditions. In fact, MIL-53 comprised of triangular shaped pores while MIL-68 hexagonal shaped pores which restricted the Li-containing electrolyte from fully diffusing into the material.
Further attention was given later to the MIL family, with Fe containing MIL-101
[84,85][55][56] which is considered to demand inexpensive materials and simple solvothermal methods to be synthesized, while the incorporation of Li atoms in the MOF is possible. The MOF was found to secure the battery from oxidation reactions of the electrolyte and attribute to the battery’s thermal stability, while also being notably stable after extensive galvanostatic cycling. However, the initial discharge capacity was still relatively low when compared to conventional cathodes
[85][56]. A higher initial specific capacity of 172 mAh/g at 50 mA/g was reached by another iron-containing MOF based on ferrocenedicarboxylate (Fe
2(DFc)
3). Moreover, the cathode could attain 10,000 cycles delivering a specific capacity of 70 mAh/g at 2000 mA/g
[86][57].
Moving from iron-based MOFs to copper-based MOFs, a Cu-TCA (tricarboxytriphenyl amine linker) was applied in a Li-ion cell and delivered capacity retention of only 39% over 200 cycles possibly due to redox activity loss of Cu ions
[87][58]. Later, Nagatomi et al.
[88][59] prepared a MOF denoted as 2D Cu-CuPC based on a phthalocyanine linker and combines successfully a network of micropores (1.4 nm) and mesopores (11 nm) contributing to a BET surface area of 358 m
2g
−1. The MOF delivers promising initial specific capacity, even comparable to conventional cathodes, however, it needs an electrical conductivity enhancement in order to be fully applicable in a LIB. Tian et al. proposed that a 2D MOF layered structure would be more favorable for Li diffusion kinetics. They synthesized a series of naphthalenediimide-based MOFs containing cadmium or cobalt metal centers via a facile room-temperature anion and thermodynamic control. Afterward, the MOFs were evaluated regarding their electrochemical properties in Li-ion coin cell type batteries. Very recently, a 3-D Co-MOF
[89][60] and a MnCo-MOF
[90][61] were synthesized and applied in a Li-ion cell. The Mn-CO bimetallic MOF specifically exhibited superior catalytic activity as revealed by 600 galvanostatic cycles at 1000 mA/g and the discharge capacity of 337 mAh/g.
He et al. prepared a vanadium MOF (MIL-47) onto CNT fibers via a solvothermal method
[91][62]. The SEM analysis revealed the critical role of reaction time, considering that decreased durations resulted in more dense/compact morphology (12 h), while increasing the reaction time to 24, 48 or even 60 h led to a fluffier framework. The anode was fabricated by electrodepositing zinc onto CNT fibers and the two electrodes were immersed into gel electrolyte (based on zinc chloride and PVA) and then twisted together. The assembled device presented a maximum energy density of 30.7 mWh/cm
3 and a power density of 1.46 W/cm
3. The flexibility was verified under different bending angles and the stability was adequate (81.5% retention after 300 cycles) to enable future research for such materials.
Among the NCM (or NMC) materials (Nickel, Cobalt and Manganese), NCM-333 (LiNi
1/3Co
1/3Mn
1/3O
2) is one of the most well-studied commercial applications of Li-ion batteries
[92][63]. In order to increase the rate capabilities and cycling stability, Li and co-workers modified this cathode by in situ coating the surface with ZIF-8
[93][64]. The capacity retention was doubled to 81.4% over 300 cycles at 800 mA/g, whereas the rate performance was excellent with stable specific capacities. EIS also verified that the ZIF-8 layer could stabilize the growth of solid-electrolyte interface film and increase the charge transfer due to its high ionic conductivity (3.16 × 10
−4 S/cm). Moreover, the group investigated the battery behavior at an elevated temperature of 55 °C, discovering that the ZIF could prevent/limit the NCM erosion by the decomposition products of the electrolyte.
Monocline Li
3V
2(PO
4)
3 has been an attractive option in recent research for cathode materials applied in Li-Ion batteries, due to its increased potential and theoretical capacity of Li ions
[94,95,96][65][66][67]. Nevertheless, this compound suffers from intrinsically low electronic conductivity, thus several composites with carbon matrixes have been explored to form more efficient cathodes. MOFs constitute accessible precursors to form various carbon architectures. ZIF-8 (Zn(MeIM)
2) has been used as a carbon precursor, to be combined with Li
3V
2(PO
4)
3 by flakes and nanorods under thermal treatment in order to form a sandwich-like geometry
[94][65]. The effect of the carbonaceous foundation attributed promising features to the constructed electrode, regarding specific capacity retention (83.2% after 2500 cycles), however, the material was not tested in an assembled Li-ion battery. According to recent research however, MIL-101(V)
[95][66] and a V-MOF
[96][67] can be considered a precursor for both Li
3V
2(PO
4)
3 and carbon frameworks. Particularly, Wang et al. proposed the use of a cathode material denoted as LVP/P-C nanocomposites that presented promising stability over 1100 cycles at a current of 10C
[96][67].
In the same direction, MIL-47(V) was used as a precursor to synthesize both active material particles (V
2O
5) and the carbon-coated layer via a controlled thermal decomposition process
[97][68]. The electrochemical performance of the material proved to surpass bare V
2O
5. V
2O
5 has been also investigated as a cathode material for Li-ion batteries when stabilized on ZIF-67-derived carbon dodecahedra
[98][69]. The aforementioned composite provides a number of advantageous features such as increased electronic and ionic conductivity, extended active surface area and homogeneous dispersion of active sites, while when synthesized at optimum conditions it can deliver an excellent specific capacity of 117.7 mAh/g at an extremely demanding current of 64C
[98][69].
In alternative cases, MOFs can be conceived as precursors for materials, which are intended to be used as coatings around the cathode material particles, such as for Li
1.2Mn
0.54Ni
0.13Co
0.13O
2 [99,100][70][71]. A similar approach has been proposed in order to enhance the efficiency of anthraquinone (AQ) as a cathode material for LIBs by confining molecules inside ZIF-8-derived 3D carbon frameworks
[101][72]. One year later the same group developed a strategy to tackle fading and instability phenomena of the cathode, by polymerization of the active material inside the carbon polyhedral
[102][73]. The cathode material denoted as PAQS@3D-C could attain a specific capacity above 200 mAh/g for 500 cycles at 0.2C.