Metal-organic framework based electrode materials for lithium-ion batteries: a review

MOFite: A High-Density Lithiophilic and Scalable Metal-Organic Framework Anode for Rechargeable Lithium-Ion Battery

Developing an anode material that has better performance efficiency than commercial graphite while keeping the features of economic scalability and environmental safety is highly desirable yet challenging. MOFs are a promising addition to the ongoing efforts, however, the relatively poor performance, chemical instability, and large-scale economic production of efficiency-proven pristine MOFs restrict their utility in real-life energy storage applications. Furthermore, hierarchical porosity for lucid mass diffusion, high-density lithiophilic sites are some of the structural parameters for improving the electrode performance. Herein, we have demonstrated the potential of economically scalable salicylaldehydate 3D-conjugated-MOF (Fe-Tp) as a high-performance anode in Li-ion batteries: the anode-specific capacity achieved up to 1447 mAh g-1 at 0.1 A g-1 and 89 % of cyclic stability after 500 cycles at 1.0 A g-1 for pristine MOF. More importantly, incorporating 10 % Fe-Tp doping in commercial graphite (MOFite) significantly enhanced lithium storage, doubling capacity after 400 cycles. It signifies the potential practical utility of Fe-Tp as a performance booster for commercial anode material.

Mn-Fe dual metal-organic framework based on trimesic acid as a high-performance electrode for lithium metal batteries

A novel Mn-Fe dual metal-organic framework (Mn-Fe-BTC DMOF) was synthesized via a one-step hydrothermal method and employed as a cathode material in lithium metal batteries. The Mn-Fe-BTC DMOF exhibited a high initial capacity (1385 mA h g-1) and after 100 cycles (687 mA h g-1), demonstrating its potential for high-performance energy storage devices.

Synthesis and Characterization of Vanadium Nitride/Carbon Nanocomposites

The present work focuses on the synthesis of a vanadium nitride (VN)/carbon nanocomposite material via the thermal decomposition of vanadyl phthalocyanine (VOPC). The morphology and chemical structure of the synthesized compounds were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), Fourier transformed infrared spectroscopy (FTIR), X-ray diffraction (XRD), and X-ray photoemission spectroscopy (XPS). The successful syntheses of the VOPC and non-metalated phthalocyanine (H2PC) precursors were confirmed using FTIR and XRD. The VN particles present a needle-like morphology in the VN synthesized by the sol-gel method. The morphology of the VN/C composite material exhibited small clusters of VN particles. The XRD analysis of the thermally decomposed VOPC indicated a mixture of amorphous carbon and VN nanoparticles (VN(TD)) with a cubic structure in the space group FM-3M consistent with that of VN. The XPS results confirmed the presence of V(III)-N bonds in the resultant material, indicating the formation of a VN/C nanocomposite. The VN/C nanocomposite synthesized through thermal decomposition exhibited a high carbon content and a cluster-like distribution of VN particles. The VN/C nanocomposite was used as an anode material in LIBs, which delivered a specific capacity of 307 mAh g-1 after 100 cycles and an excellent Coulombic efficiency of 99.8 at the 100th cycle.

Green Synthesis of CoZn-Based Metal-Organic Framework (CoZn-MOF) from Waste Polyethylene Terephthalate Plastic As a High-Performance Anode for Lithium-Ion Battery Applications

The recycling of discarded polyethylene terephthalate (PET) plastics produced metal-organic frameworks can effectively minimize environmental pollution and promote sustainable economic development. In this study, we developed a method using NaOH in alcohol and ether solvent environments to degrade PET plastics for synthesizing terephthalic acid. The method achieved a 97.5% degradation rate of PET plastics under a reaction temperature of 80 °C for 60 min. We used terephthalic acid as a ligand from the degradation products to successfully synthesize two types of monometallic and bimetallic CoZn-MOF materials. We investigated the impact of different metal centers and solvents on the electrochemical performance of the MOF materials. The result showed that the MOF-DMF/H2O material maintained a specific capacity of 1485.5 mAh g-1 after 100 cycles at a current density of 500 mA g-1, demonstrating excellent rate capability and cycling stability. In addition, our finding showed that the performance difference might be attributed to the synergistic effect of bimetallic Co2+ and Zn2+ in MOF-DMF/H2O, rapid lithium-ion diffusion and electron transfer rates, and the absence of coordinating solvents. Additionally, the non-in situ X-ray powder diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy analysis results showed that lithium storage in the MOF-DMF/H2O electrode mainly depended on the aromatic C6 ring and carboxylate portions of the organic ligands in different charge and discharge states. Lithium ions can be reversibly inserted/removed into/from the electrode material. The physical adsorption on the MOF surface through electrostatic interactions enhanced both capacity and cycling stability. This research provides valuable insight for mitigating solid waste pollution, promoting sustainable economic development, and advancing the extensive applications of MOF materials in lithium-ion batteries.

Metal-Organic Frameworks for the Enhancement of Lithium-Based Batteries: A Mini Review on Emerging Functional Designs

Metal-organic frameworks (MOFs) have played a crucial role in recent advancements in developing lithium-based battery electrolytes, electrodes, and separators. Although many MOF-based battery components rely on their well-defined porosity and controllable functionality, they also boast a myriad of other significant properties relevant to battery applications. In this mini-review, the distinct advantages of MOFs in battery applications are discussed, including using MOFs to 1) scavenge impurities to increase cycling stability, 2) widen the operation temperature range of conventional electrolytes, 3) widen the operation voltage range of common electrolytes, and 4) employ as artificial solid-electrolyte interphases to prevent lithium dendrite growth. Furthermore, subsisting challenges of developing these emerging MOF-based battery technologies are discussed and guidance for shaping the future of this field is given.

Two-Dimensional Electrically Conductive Metal-Organic Frameworks as Chemiresistive Sensors

Metal-organic frameworks (MOFs) have emerged as attractive chemical sensing materials due to their exceptionally high porosity and chemical diversity. Nevertheless, the utilization of MOFs in chemiresistive type sensors has been hindered by their inherent limitation in electrical conductivity. The recent emergence of two-dimensional conductive MOFs (2D c-MOFs) has addressed this limitation by offering enhanced electrical conductivity, while still retaining the advantageous properties of MOFs. In particular, c-MOFs have shown promising advantages for the fabrication of sensors capable of operating at room temperature. Thus, active research on gas sensors utilizing c-MOFs is currently underway, focusing on enhancing sensitivity and selectivity. To comprehend the potential of MOFs as chemiresistive sensors for future applications, it is crucial to understand not only the fundamental properties of conductive MOFs but also the state-of-the-art works that contribute to improving their performance. This comprehensive review delves into the distinctive characteristics of 2D c-MOFs as a new class of chemiresistors, providing in-depth insights into their unique sensing properties. Furthermore, we discuss the proposed sensing mechanisms associated with 2D c-MOFs and provide a concise summary of the strategies employed to enhance the sensing performance of 2D c-MOFs. These strategies encompass a range of approaches, including the design of metal nodes and linkers, morphology control, and the synergistic use of composite materials. In addition, the review thoroughly explores the prospects of 2D c-MOFs as chemiresistors and elucidates their remarkable potential for further advancements. The insights presented in this review shed light on future directions and offer valuable opportunities in the chemical sensing research field.

Biochar-Derived Hierarchical Porous Carbon as Tellurium Host for High-Performance Potassium-Tellurium Batteries

Potassium-ion battery is promising for its high abundance and low redox potential. As a conversion cathode, Te possesses high conductivity and theoretical volumetric capacity to couple with potassium. The stubborn issues of K-Te battery focus on the large volume change and rapid structure degradation of Te. Herein, we produce biomass carbon from mangosteen shell in a facile method, and obtain a hierarchical porous host with abundance of micropores and mesopores, which is obviously beneficial for hosting Te during K+ storage in K-Te battery. The specific capacity reach to 560 mAh g-1 in the initial cycle at 0.1 A g-1 , and remained 83.8 % after 200 cycles. Impressively, at a high current density of 2.0 A g-1 , the specific capacity still remained 62.6 % after 5000 cycle. These results endow such strategy an efficient way for the development of K-Te batteries.

Construction of Well-Defined Two-Dimensional Architectures of Trimetallic Metal-Organic Frameworks for High-Performance Symmetric Supercapacitors

The high surface-to-volume ratio and extraordinarily large-surface area of two-dimensional (2D) metal-organic framework (MOF) architectures have drawn particular interest for use in supercapacitors. To achieve an excellent electrode material for supercapacitors, well-defined 2D nanostructures of novel trimetallic MOFs were developed for supercapacitor applications. Multivariate MOFs (terephthalate and trimesate MOF) with distinctive nanobrick and nanoplate-like structures were successfully synthesized using a straightforward one-step reflux condensation method by combining Ni, Co, and Zn metal species in equimolar ratios with two different ligands. Furthermore, the effects of the tricarboxylic and dicarboxylic ligands on cyclic voltammetry, charge-discharge cycling, and electrochemical impedance spectroscopy were studied. The derived terephthalate and trimesate MOFs are supported with stainless-steel mesh and provide a suitable electrolyte environment for rapid faradaic reactions with an elevated specific capacity, excellent rate capability, and exceptional cycling stability. It shows a specific capacitance of 582.8 F g-1, a good energy density of 40.47 W h kg-1, and a power density of 687.5 W kg-1 at 5 mA cm-2 with an excellent cyclic stability of 92.44% for 3000 charge-discharge cycles. A symmetric BDC-MOF//BDC-MOF supercapacitor device shows a specific capacitance of 95.22 F g-1 with low capacitance decay, high energy, and power densities which is used for electronic applications. These brand-new trimetallic MOFs display outstanding electrochemical performance and provide a novel strategy for systematically developing high-efficiency energy storage systems.

A zeolitic imidazolate framework (ZIF-67) and graphitic carbon nitride (g-C3N4) composite based efficient electrocatalyst for overall water-splitting reaction

Designing of non-noble, cost-effective, sustainable catalysts for water splitting is essential for hydrogen production. In this research work, ZIF-67, g-C3N4, and their composite (1, 3, 5, 6, 8 wt% g-C3N4@ZIF-67) are synthesized, and various techniques, XRD, FTIR, SEM, EDX and BET are used to examine their morphological properties for electrochemical water-splitting. The linkage of ZIF-67 with g-C3N4 synergistically improves the electrochemical kinetics. An appropriate integration of g-C3N4 in ZIF-67 MOF improves the charge transfer between the electrode and electrolyte and makes it a suitable option for electrochemical applications. In alkaline media, the composite of ZIF-67 MOF with g-C3N4 over a Ni-foam exhibits a superior catalyst activity for water splitting application. Significantly, the 3 wt% g-C3N4@ZIF67 composite material reveals remarkable results with low overpotential values of -176 mV@10 mA cm-2, 152 mV@10 mA cm-2 for HER and OER. The catalyst remained stable for 24 h without distortion. The 3 wt% composite also shows a commendable performance for overall water-splitting with a voltage yield of 1.34 v@10 mA cm-2. The low contact angle (54.4°) proves the electrocatalyst's hydrophilic nature. The results of electrochemical water splitting illustrated that 3 wt% g-C3N4@ZIF-67 is an electrically conductive, stable, and hydrophilic-nature catalyst and is suggested to be a promising candidate for electrochemical water-splitting application.

Formulation and mechanism of copper tartrate - a novel anode material for lithium-ion batteries

Batteries play an increasingly critical role in the functioning of contemporary society. To ensure future proofing of battery technology, new materials and methods that overcome the current shortcomings need to be developed. Here we report the use of the inexpensive and off the shelf metal-carboxylate, copper tartrate, as a high-capacity anode material for lithium-ion batteries, providing a specific capacity of 744 mA h g-1 when cycled at 50 mA g-1. Additionally, an unusual capacity gain with cycling is investigated using advanced techniques including X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and small and ultra-small angle neutron scattering (SANS and USANS), providing insight into the structure-performance relationship of the electrode. Subsequently, a novel method of in situ generation of the active material is demonstrated using the reaction between the parent acid, tartaric acid, and the copper current collector during electrode formulation. This serves to increase and stabilise the electrode performance, as well as to make use of a cheaper feedstock (tartaric acid), and reduce some of the "dead mass" of the copper current collector.

Synthesis of Sb-pyromellitic acid metal-organic framework material and its sodium storage properties

Developing electrode materials with high capacity and low cost is crucial for promoting the application of sodium-ion batteries. In this paper, a new Sb-PMA-300 metal-organic framework (MOF) material is synthesized by chelation of Sb3+ and pyromellitic acid (PMA) followed by a heat treatment at 300 °C. As anodes for sodium-ion batteries, the Sb-PMA-300 composite exhibits a stable capacity of 443 mA h g-1 at a current density of 0.1 A g-1. At a current density of 1 A g-1, the discharge capacity is maintained at 326.4 mA h g-1 after 200 cycles. The electrode process dynamics of this material are mainly controlled by diffusion. The values of the diffusion coefficient of Na+ are between 10-12 and 3.0 × 10-10 cm2 s-1 during discharging, while they are between 10-12 and 5.0 × 10-11 cm2 s-1 during charging. The excellent cycle stability is attributed to the special structure of the MOF material, where the organic ligand prevents the aggregation of Sb alloy particles and buffers the tension resulting from volume variation.

Mitigating Concentration Polarization through Acid-Base Interaction Effects for Long-Cycling Lithium Metal Anodes

Lithium (Li) metal has attracted great attention as a promising high-capacity anode material for next-generation high-energy-density rechargeable batteries. Nonuniform Li⁺ transport and uneven Li plating/stripping behavior are two key factors that deteriorate the electrochemical performance. In this work, we propose an interphase acid-base interaction effect that could regulate Li plating/stripping behavior and stabilize the Li metal anode. ZSM-5, a class of zeolites with ordered nanochannels and abundant acid sites, was employed as a functional interface layer to facilitate Li⁺ transport and mitigate the cell concentration polarization. As a demonstration, a pouch cell with a high-areal-capacity LiNi_0.95Co_0.02Mn_0.03O₂ cathode (3.7 mAh cm^-2) and a ZSM-5 modified thin lithium anode (50 μm) delivered impressive electrochemical performance, showing 92% capacity retention in 100 cycles (375.7 mAh). This work reveals the effect of acid-base interaction on regulating lithium plating/stripping behaviors, which could be extended to developing other high-performance alkali metal anodes.

Ta4C3-Modulated MOF-Derived 3D Crosslinking Network of VO2(B)@Ta4C3 for High-Performance Aqueous Zinc Ion Batteries

A two-dimensional MXene (Ta₄C₃) was innovatively used herein to modulate the space group and electronic properties of vanadium oxides, and the MXene/metal-organic framework (MOF) derivative VO₂(B)@Ta₄C₃ with 3D network cross-linking was prepared, which was then employed as a cathode to improve the performance of aqueous zinc ion batteries (ZIBs). A novel method combining HCl/LiF and hydrothermal treatments was used to etch Ta₄AlC₃ to obtain a large amount of accordion-like Ta₄C₃, and the V-MOF was then hydrothermally grown on the surface of the stripped Ta₄C₃ MXene. During the annealing process of V-MOF@Ta₄C₃, the addition of Ta₄C₃ MXene liberates the V-MOF from agglomerative stacking, allowing it to show additional active sites. More significantly, Ta₄C₃ prevents the V-MOF in the composite structure from converting into V₂O₅ of space group Pmmn but into VO₂(B) of space group C2/m after annealing. A considerable advantage of VO₂(B) for Zn²⁺ intercalation is provided by the negligible structural transformation during the intercalation process and the special tunnel transport channels, which have an enormous area (0.82 nm² along the b axis). According to first-principles calculations, there is a strong interfacial interaction between VO₂(B) and Ta₄C₃, which deliver remarkable electrochemical activity and kinetic performances for the storage of Zn²⁺. Therefore, the ZIBs prepared with the VO₂(B)@Ta₄C₃ cathode material exhibit an ultra-high capacity of 437 mA h·g^-1 at 0.1 A·g^-1 while showing good cycle performance and dynamic performance. This study will offer a fresh approach and a reference for creating metal oxide/MXene composite structures.

In situ synthesis of Cu(II) dicarboxylate metal organic frameworks (MOFs) and their application as battery materials

New materials for battery electrodes are paramount to ensuring future battery supply can meet the ever-increasing demand for energy storage. Furthermore, detailed investigation on the various physical and chemical aspects of these materials is required to allow the same level of nuanced microstructural and electrochemical tuning that is available for conventional electrode materials. Here a comprehensive investigation is undertaken on the poorly understood in situ reaction between dicarboxylic acids and the copper current collector that occurs during electrode formulation, using a series of simple dicarboxylic acids. Specifically, we focus on the relationship between the extent of the reaction and the properties of the acid. Additionally, the extent of the reaction was demonstrated to affect both the electrode microstructure and the electrochemical performance. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and small and ultra-small angle neutron scattering (U/SANS) are used to provide unprecedented detail on the microstructure ultimately leading to a deeper understanding of formulation based performance enhancing techniques. Ultimately, it was determined that the copper-carboxylates are the active material, not the parent acid, and in some cases i.e., copper malate, capacities as high as 828 mA h g^-1 were achieved. This work lays the foundation for future studies that use the current collector as an "active" component in electrode formulation and function rather than simply an inactive component of a battery.

Ionic-Liquid-Assisted One-Step Construction of Mesoporous Metal-Organic Frameworks

The synthesis of ionic-mesoporous-metal-organic frameworks (ionic-meso-MOFs) has received considerable interest in the fields of macromolecular adsorption, acid-base catalysis, ionic conductivity, etc.; yet, their synthesis still presents significant difficulties. In this study, functionalized mesoporous MIL-101-ILs (Cr) was facilely constructed via an in situ self-assembly method by using aromatic-anion-functionalized ionic liquids (ILs) as competitive ligands. It has been demonstrated that the inclusion of an aromatic moiety into an IL improves the coordination ability and is advantageous for the anchoring of ILs on Cr³⁺ via amino-metal coordination. Thus, ionic-meso-MOFs with a specific surface area of 441.9-624.9 cm²/g and an average pore diameter of 5.5 to 8.4 nm were successfully synthesized. Because of the presence of open Lewis acidic metal sites on the MOFs and basic active sites on the ILs, the resulting ionic-meso-MOFs demonstrated both an acid-base cooperative effect and a mesoporous structure, indicating a high potential for acid-base catalysis. This in situ synthesis procedure for ionic mesoporous MOFs offers a simple method for developing and fabricating multifunctional mesoporous materials.

A Strategy for the Determination of Alkaline Phosphatase Based on the Self-Triggered Degradation of Metal-Organic Frameworks by Phosphate

Alkaline phosphatase (ALP) is widely present in the human body and is an important biomarker. Numerous ALP detection studies have been carried out, and ascorbic acid (AA) is often used as the reducing component in the sensors to monitor ALP levels since it can be produced from ascorbic acid 2-phosphate (AA2P) hydrolysis in the presence of ALP. However, it is well-known that AA is a strong reducing agent and can be easily oxidized. The disproportion between oxidized AA and reduced AA reactions results in the generation of AA free radicals with single electrons that may lead to inaccurate results in assays. To solve this problem, we synthesized a core-shell metal-organic framework sensor (PATP-Au@ZIF-8 NP) and used it as a sensitive and accurate ALP detection sensor with self-triggered control of phosphate ions (Pi) to avoid the potential inaccuracy of the method that uses AA as the reducing component. By establishing a physical shell on the surface of the gold nanoparticles (Au NPs), the sensor not only can eliminate the random assembly of metal nanoparticles caused by plasma exposure but also can generate self-triggering of Pi caused by ALP. Pi can decompose ZIF-8 through coordination with Zn²⁺ and thus can destroy the ZIF-8 shell structure of the prepared PAZ NPs. Au NPs are released and then become aggregated, in turn causing the SERS "hot spot" area to increase. The enhancement of the SERS signals was found to be directly associated with the level of Pi released from ALP-triggered hydrolysis. The response of the strategy was linear at ALP concentrations ranging from 0.1 to 150 mU/mL (r = 0.996) with a detection limit of 0.03 mU/mL. Lastly, the developed strategy was employed in the evaluation of ALP inhibitors, and the possibility to implement the developed SERS strategy for rapid and selective analysis of ALP in human serum was demonstrated.

Graphene-Based Metal-Organic Framework Hybrids for Applications in Catalysis, Environmental, and Energy Technologies

Current energy and environmental challenges demand the development and design of multifunctional porous materials with tunable properties for catalysis, water purification, and energy conversion and storage. Because of their amenability to de novo reticular chemistry, metal-organic frameworks (MOFs) have become key materials in this area. However, their usefulness is often limited by low chemical stability, conductivity and inappropriate pore sizes. Conductive two-dimensional (2D) materials with robust structural skeletons and/or functionalized surfaces can form stabilizing interactions with MOF components, enabling the fabrication of MOF nanocomposites with tunable pore characteristics. Graphene and its functional derivatives are the largest class of 2D materials and possess remarkable compositional versatility, structural diversity, and controllable surface chemistry. Here, we critically review current knowledge concerning the growth, structure, and properties of graphene derivatives, MOFs, and their graphene@MOF composites as well as the associated structure-property-performance relationships. Synthetic strategies for preparing graphene@MOF composites and tuning their properties are also comprehensively reviewed together with their applications in gas storage/separation, water purification, catalysis (organo-, electro-, and photocatalysis), and electrochemical energy storage and conversion. Current challenges in the development of graphene@MOF hybrids and their practical applications are addressed, revealing areas for future investigation. We hope that this review will inspire further exploration of new graphene@MOF hybrids for energy, electronic, biomedical, and photocatalysis applications as well as studies on previously unreported properties of known hybrids to reveal potential "diamonds in the rough".

Exploring ionic liquid-laden metal-organic framework composite materials as hybrid electrolytes in metal (ion) batteries

This review focuses on the combination of metal-organic frameworks (MOFs) and ionic liquids (ILs) to obtain composite materials to be used as solid electrolytes in metal-ion battery applications. Benefiting from the controllable chemical composition, tunable pore structure and surface functionality, MOFs offer great opportunities for synthesizing high-performance electrolytes. Moreover, the encapsulation of ILs into porous materials can provide environmentally benign solid-state electrolytes for electrochemical devices. Due to the versatility of MOF-based materials, in this review we also explore their use as anodes and cathodes in Li- and Na-ion batteries. Finally, solid IL@MOF electrolytes and their implementation into Li and Na batteries have been analyzed, as well as the design and advanced manufacturing of solid IL@MOF electrolytes embedded on polymeric matrices.

Economic synthesis of sub-micron brick-like Al-MOF with designed pore distribution for lithium-ion battery anodes with high initial Coulombic efficiency and cycle stability

Metal-organic frameworks (MOFs) have exhibited great potential for lithium-ion batteries (LIBs). However, to date, it is difficult to fabricate MOF electrode materials with regular shape and rational pore distribution by an economic approach, and the currently achieved MOF electrode materials usually have a relatively low initial Coulombic efficiency and poor cycle stability, which is not satisfactory for practical application. In this study, by using the recycled AlCl₃ solution after dealloying treatment of Al-Si alloy, an evenly distributed brick-like Al-MOF with sub-micron size and rational pore distribution was synthesized for the first time. Because of the larger size and more macropores, the as-prepared Al-MOF electrode exhibits superior initial Coulombic efficiency as high as 96.6% for LIB anodes. Moreover, on account of the irregular crystal defects at the edge of the designed macropores, which result from unstable connection between the inorganic nodes (AlO₆ octahedral cluster) and the organic linkers (PTA) and result in the formation of spherical nano-sized particles with better structural stability, the electrode materials show excellent cycle stability with discharge attenuation rate of 0.051%. The electrochemical performance considerably outperforms that of reported Al-MOF anodes and some representative MOF anodes in other studies. The robust realization of high initial Coulombic efficiency and cycle stability defines a critical step to capturing the full potential of MOF electrode materials in practical LIBs.

Metal-organic frameworks and their derivatives as electrode materials for Li-ion batteries: a mini review

In recent decades, in order to obtain more excellent performance and wider application of rechargeable lithium-ion batteries (LIBs), researchers have been exploring potential electrode materials. MOFs possess attractive features, such...

Insights into the thermal conductivity of MOF-5 from first principles

Metal-organic frameworks (MOFs) have been extensively studied in many fields due to their abundant porous structures. The mechanism underlying the thermal conduction properties of MOFs, which plays an essential role in a wide variety of applications such as adsorbents and thermoelectric devices, remains elusive. It is also highly desirable to achieve the efficient modulation of thermal conductivity in MOFs via experimentally accessible methods such as metal substitution and strain engineering. In this work, we perform first-principles calculations to investigate the thermal transport properties of MOF-5, a representative prototype of MOFs. We find an ultralow thermal conductivity (κ) of 0.33 W m-1 K-1 at room temperature, in excellent agreement with the experimental measurement. Such ultralow κ is attributed to the strong phonon-phonon scattering that arises from the dense and intertwined low-frequency phonons. The phonon dispersion leads to unusual tuning strategies of κ, since conventional designing guidelines (e.g. substitution of heavier atoms or application of tensile strain is preferred in pursuit of lower thermal conductivity) are not fully obeyed in MOF-5. We find that isovalent substitutions of Zn atoms with (lighter) Mg and (heavier) Cd atoms both result in significant reduction of κ, due to the enhanced phonon scattering rates that are associated with the stronger bond strength and the larger atomic mass, respectively. We further demonstrate that the so-called "guitar string" vibrations are responsible for the anomalous non-monotonic variation of κ in MOF-5 under tensile strain. This work provides fundamental insights into the thermal transport mechanisms in MOF-5, which may have some important implications for the thermal management applications utilizing MOFs.