Metal-Organic Framework Based Nanomaterials Applied in Battery Cathodes

Metal-Organic Framework Based Nanomaterials Applied in Battery Cathodes: Comparison

Please note this is a comparison between Version 1 by Antonis A. Zorpas and Version 2 by Vivi Li.

Metal-Organic Frameworks have attracted profound attention the latest years for use in environmental applications. They can offer a broad variety of functions due to their tunable porosity, high surface area and metal activity centers. Not more than ten years ago, they have been applied experimentally for the first time in energy storage devices, such as batteries. Specifically, MOFs have been investigated thoroughly as potential materials hosting the oxidizing agent in the cathode electrode of several battery systems such as Lithium Batteries, Metal-Ion Batteries and Metal-Air Batteries.

metal-organic frameworks
battery
cathodes
electrocatalyst
lithium battery
nanomaterials
nanocomposites

1. Introduction

Continuously expanding energy demands constitute one of the most crucial worldwide concerns. Batteries offer a viable solution to store power deriving from renewable energy sources, however, there are still challenges regarding the environmental footprint and the abundance of resources of batteries when taking into account the production route and their disposal.

Primary and rechargeable batteries’ performance is closely related to the efficiency and durability of the oxidizing material, contained in the cathode. In general, cathodes need to present ionic and electronic conductivity, mechanical and chemical endurance, rechargeability and of course nontoxicity and safety, like all battery electrodes. For all those requirements to be met while enhancing the battery’s performance, researchers have been developing over the years more and more delicate solutions which offered highly functional materials. However, current worldwide material and energy restrictions demand dramatic reductions of complexity and value of the solutions proposed for future upscale. In other words, synthesis procedures need to be simplified, materials used have to be widely accessible and cost-effective and all the processes of cathode manufacture should meet current requirements of environmental friendliness ^{[1][2][3][4][5]}[1,2,3,4,5].

Metal-Organic Frameworks (MOFs) have attracted much interest in energy applications due to the ample unique geometries and porosity they can acquire which lead to distinct electrochemical properties ^[1][6][1,6]. Their synthesis routes have been identified as scalable, while they have proved to be highly versatile when forming hybrids. MOFs include a considerable variety of different linkers and metals that can be combined, with a significant part of them being receptive to rational modifications in order to bring novel functionalities to the table ^[7][8][7,8]. Nevertheless, one of the main drawbacks of MOFs for electrochemical applications is the low conductivity caused by the organic linkers’ inherent insulation and poor overlap between their π orbitals and metal centers d orbital ^[9][10][9,10].

When combined with other nanostructures, MOFs are able to form multifunctional hybrids with abundant active sites exhibiting enhanced electronic and ionic conductivity and electrochemical properties. In such ways, the intrinsic capacity of the electrode can be expanded while the kinetics of redox reactions can be accelerated. MOFs have also been considered as self-sacrifice templates of carbon-based architectures that act as a support for catalytic molecules or even precursors of oxides. Calcination in the air has been very popular for the preparation of metal oxides, whereas pyrolysis in an inert atmosphere can produce hierarchical carbons. The development of methods in order to tailor the morphology and porosity of such materials has been a significant obstacle over the years, thus MOFs can alleviate this with fine-tuning during the initial synthesis. The atomically homogeneous structure ensures a uniform distribution between the carbon and the metal-based part, which can also be removed completely via acid treatments. The generation of such intriguing designs can ameliorate the cycling stability and overall battery performance [11].

2. Lithium Batteries

When referring to Lithium Batteries reswearchers describe batteries with Li anodes and porous cathodes accommodating Sulfur (or Selenium) as an active material. Their fundamental operation lies in the transfer of lithium ions towards the confined Sulfur in the cathode, in order to form lithium sulfide via several redox reactions and intermediates [12]. The overall electrochemical reaction can be summarized below:

16 Li + + 16 e - + S 8 \leftrightarrow 8 Li 2 S

Their theoretical capacity reaches relatively high numbers (1C = 1672 mAh/g) however they suffer from a destructive phenomenon known as the shuttle effect. Shuttle Effect describes the corrosive mobility of intermediate polysulfides towards the anode and back, leading to self-discharge and early stability fade. These short-chain polysulfides can be mitigated by more efficient encapsulation of sulfur in the cathode pores. MOFs have been thoroughly explored as potential hosts due to their extended surface area and pore structure, including carbon derivatives which possess higher electrical conductivity and mechanical flexibility ^{[13][14][15][16][17]}[13,14,15,16,17]. The first attempt to insert a MOF in a Lithium- Sulfur (Li-S) battery cathode goes back to 2011 and MIL-100 (Cr). MIL-100 (Cr) is based on trimesic acid linker and chromium octahedra, possessing mesoporous cages of 25–29 Å and pore volume of ~1 cm³/g. For the preparation of the cathode at least 25 wt% of carbon was needed to increase the conductivity while the integration of Sulfur took place at 155 °C (melting point of 115 °C). The cathode capacity did not exceed 500 mAh/g after 50 cycles probably due to the fragile binding of polysulfide anions to the framework or blockage of sulfur from the carbon matrix [18]. The corresponding vanadium variant was also applied for 200 cycles at 0.1C, delivering a reversible capacity of ~550 mAh/g, along with a reduced graphene oxide (RGO) composite, which demonstrated a slightly enhanced efficiency with 650 mAh/g at 0.1C after 75 cycles and 450 mAh/g over 300 cycles at 0.5C, albeit not directly comparative results [19]. Particle size proved to acquire a principal role among other parameters concerning the capacity of the cathode. Baumann et al. [20] modified the particle size of HKUST-1 and studied the effect on battery performance. The samples were of particle sizes 0.16, 1.6, and 5.9 μm samples and delivered correspondingly 679, 540 and 480 mAh/g and capacity retentions of 64, 60 and 54% (end of 20 cycles at 0.1C). It is highlighted that an autoclave-free “precooling” procedure was followed in order to synthesize the nanosized MOFs (0.16 nm) which showed the most favorable electrochemical properties. Furthermore, the features of ZIF-8 were extensively investigated when nanosizing [20]. Zhou et al. [21] prepared particles of 2 μm, 800 nm, 200 nm, 70 nm and 15 nm via a facile solution synthesis route and reported their long-term stability for 250 cycles at 0.5 C. The cathode comprised of the MOF NPs of 15 nm delivered the highest maximum capacity 968 mAh/g. One year before, the same team conducted similar research also for HKUST-1, MIL-53 (Al) and NH₂-MIL-53 (Al). Although the capacities noted for ZIF-8 were significantly lower, the results verified that downsizing particles benefit sulfur utilization due to the larger total external surface area and shorter electron diffusion pathway for sulfur species reduction in the framework. Among the above MOFs, MIL-53 exhibited the best performance, reaching a discharge capacity of 793 mAh/g and maintaining 44% of the capacity over 300 cycles at 0.5C. Geng et al. [22] recently conducted an investigation on which morphological factors have the strongest effect on MIL-96-Al crystals’ performance in a Li-S battery application. Another strategy reported to enhance sulfur confinement by modifying active surface area is boosting macro and microporosity of ZIF-8 by exploiting PMMA nanospheres as templates and ZnO as a precursor. The 3DOM ZIF-8 presented remarkable stability over 500 galvanostatic cycles at 2C (capacity retention 84%) when applied as a cathode in a Li-S battery. The prohibition of early degradation was attributed to the powerful interaction between 3DOM ZIF and polysulfides, as verified by UV-Vis and XPS analysis [23]. A similar method was followed recently by Wang et al. [24] who utilized a PS template to form a 3DOM ZIF67 matrix to confine Sulfur molecules. The resulted cathode delivered an initial capacity of 1079.8 mAh/g at 0.2C and endured 500 cycles at 1C. Beyond their role in downsizing, creating heterostructures with carbon conductive substances has been a very popular method to enhance the activity of electrode material. MOFs’ weaknesses can be alleviated by integrating carbon materials, possessing numerous benefits, such as different forms (graphene, nanotubes), high conductivity, mechanical strength and chemical robustness [25]. Regarding GO, it is mainly used in its reduced form. Wang and co-workers studied ZIF-8@rGO as a cathode by firstly preparing the zinc carbonate hydroxide/rGO composite, which was used as a precursor for the final sample [26]. The nanocomposite’s micropores can provide better dispersion of S₈ crowns and the abundant nitrogen sites can immobilize the polysulfides by forming Li-N bonds and enhance cycling stability. At 200 mA/g the cathode supplied a high initial discharge capacity of 1544 mAh/g, retaining at 523 mAh/g at the end of 200 cycles, compared to pure ZIF-8, which delivered 1206 and 346 mAh/g, respectively. On the other hand, setting higher current densities (1 A/g) leads to much lower initial capacities; 678 mAh/g for the composite, and 426 mAh/g for the pristine ZIF-8, after 300 cycles [26]. Carbon nanotubes (CNTs) have also been popular due to their high tensile strength, flexibility, conductivity, and aspect ratio. A hollow ZIF-8 (HZIF) with CNTs was reported by Wu et al. by using tannic acid as a modulator [27]. SEM pictures reveal the hollow structure with a shell thickness of 55 nm. Through the electrostatic force, negative charged CNTs are attached to the positive charged HZIF (functionalized with PDDA), to receive the hybrid sulfur host. The polysulfides adsorption was studied with XPS analysis; after the Li₂S₆ adsorption, weaker C-O, C=O and C-N peaks were observed, indicating the strong capture capabilities of O and N functional groups. The cathode (75 wt.% sulfur content) managed to deliver good cycling stability, with a capacity of 625 mAh/g after 500 cycles at 0.5C) and promising efficiency at the increased rate of 3C (696 mAh/g) [27].

Tannic acid was also utilized as a modulator by Ge et al. [28] in an attempt to form a more functional ZIF-67 in order to mitigate the shuttle effect. The idea is that the hydroxyl groups of tannic acid can modify polysulfides into being insoluble. Tannic acid also tends to deform Co-N bonds, promotes the formation of N-H bonds, activates polarity and restricts the dissolution of polysulfide intermediates in the electrolyte [13]. The pristine ZIF-67 delivered a specific capacity of 422 mAh/g for 100 cycles at 100 mA/g, while 5 min of treatment with tannic acid attributed to the battery and raise the capacity to 757 mAh/g.

Turning back to nanocomposites’ investigation, Bao et al. prepared mesoporous MIL-101@rGO, which displayed a capacity of 650 mAh/g after 50 cycles at 0.2C with a capacity retention of 66.6% [29]. The mesopores can significantly slow down the shuttle effect, due to the strong adsorption of polysulfides [30]. Additionally, micropores can also prevent the above issue, while promoting the entrapment of elemental sulfur for increased discharge capabilities.

An rGO coating layer can offer a better sulfur utilization rate along with increased conductivity while suppressing the volume expansion throughout the redox reaction. Such an example has been investigated with an iron MOF based on azobenzenetetracarboxylic acid ligand [31]. A faster capacity fade is observed during the first 80 cycles dropping from 1643 to 865 mAh/g and reaching 639 mAh/g after a total of 200 cycles at 0.5C. The high initial capacity of the composite is induced by the layer conductivity and the reduction of the interface contact resistance [32].

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 Li₂S₄ 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 LiMO₂, LiM₂O₄ and LiMPO₄ (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 ^[48][77]. 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 LiCoO₂ cathode offers a theoretical capacity of 274 mAh/g ^[49][78] while a LiFePO₄ cathode until 160 mAh/g ^[50][79]. The first report of a MOF applied in a Li-ion battery can be found back in 2005 with MOF-177 ^[51][80]. A few years later Fe²⁺ and Fe³⁺ started being explored as redox centers in MOFS such as MIL-53 ^[52][53][81,82] and MIL-68 ^[54][83]. 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 ^[55][56][84,85] 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 ^[56][85]. A higher initial specific capacity of 172 mAh/g at 50 mA/g was reached by another iron-containing MOF based on ferrocenedicarboxylate (Fe₂(DFc)₃). Moreover, the cathode could attain 10,000 cycles delivering a specific capacity of 70 mAh/g at 2000 mA/g ^[57][86]. 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 ^[58][87]. Later, Nagatomi et al. ^[59][88] 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 m²g⁻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 ^[60][89] and a MnCo-MOF ^[61][90] 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 ^[62][91]. 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/cm³ and a power density of 1.46 W/cm³. 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_1/3O₂) is one of the most well-studied commercial applications of Li-ion batteries ^[63][92]. 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 ^[64][93]. 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 Li₃V₂(PO₄)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 ^[65][66][67][94,95,96]. 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 Li₃V₂(PO₄)₃ by flakes and nanorods under thermal treatment in order to form a sandwich-like geometry ^[65][94]. 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) ^[66][95] and a V-MOF ^[67][96] can be considered a precursor for both Li₃V₂(PO₄)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 ^[67][96]. In the same direction, MIL-47(V) was used as a precursor to synthesize both active material particles (V₂O₅) and the carbon-coated layer via a controlled thermal decomposition process ^[68][97]. The electrochemical performance of the material proved to surpass bare V₂O₅. V₂O₅ has been also investigated as a cathode material for Li-ion batteries when stabilized on ZIF-67-derived carbon dodecahedra ^[69][98]. 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 ^[69][98]. 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_0.13O₂ ^[70][71][99,100]. 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 ^[72][101]. 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 ^[73][102]. The cathode material denoted as PAQS@3D-C could attain a specific capacity above 200 mAh/g for 500 cycles at 0.2C.