Effect of the Metal Node and Synthesis Method of Triphenylene Based MOFs on the Performance of Li-Ion Batteries
Authors: Haro Marta , Ciria Ramos Isabel , Tejedor Ines , Urtizberea Ainhoa , Roubeau Olivier , Gascon Ignacio 
Published: February 2024

The development of efficient energy storage systems is considered crucial to achieve carbon-free energy, being the electrochemical energy storage technologies especially relevant for mobile devices and the transportation sector. In particular, lithium ion batteries (LIBs) are currently the most used technology in these applications because of their high energy and power density, long cycle life, and competitive cost 1. Due to these properties, the future lithium-based battery market predictions forecast a dramatic increase in demand of battery energy of more than an order of magnitude 2. However, since LIBs commercialization in 1991 the advances have been incremental and the component electrode and electrolyte materials still remain the same with improved engineering modifications. In this context, the European Commission calls for transformative battery research and setting the development of the competitive battery industry in Europe.
New materials for electrodes can provide new properties and mechanisms of energy storage than commonly used inorganic materials. For example, metal-organic framework (MOF) materials composed by inorganic nodes (clusters or metal ions) connected by organic ligands provide coordination networks extended in either two or three dimensions, and high porosity 3. Owing to their rational design, structural diversity and tunable physical and chemical properties (including surface area, pore size and functionalities, etc.) MOFs have been intensively studied as advanced functional materials for a large variety of applications 4, including energy storage 5.
In the present work we perform a systematic study of MOFs based on a triphenylene ligand, HHTP (2,3,6,7,10,11-hexahydroxytriphenylene) varying the metal node. Triphenylene based MOFs are characterized by high electronic conductivity6 due to the π-d conjugation in the ab plane, behaving as 2D graphene analogues. Besides, space conductivity is observed thanks to the interlayer π-π stacking in the c direction 7. Here two methods of preparation are evaluated, bulk synthesis and electrode preparation by doctor-blade deposition and in-situ synthesis of the 2D MOF at the air-liquid interface and posterior transference to the battery substrate8. The MOF powders were characterized by XRD diffraction and IR and XPS spectroscopies, techniques used when possible to characterize the obtained films. The performance of the fabricated lithium half-cell batteries has been analyzed electrochemically to have basic information and the potential of the films as protective layers in anode-less batteries. The electrochemical characterization includes galvanostatic charging-discharging, platting-striping plots at different charge rate, electrochemical impedance spectroscopy (EIS), and XPS analysis after cycling. The results show that the energy density of the electrode can be duplicated depending on the metal node, presumably by a change of electrochemical energy storage. Also the thin films of these MOFs are promising candidates as protective layers with the formation of a more stable solid electrolyte interface (SEI).

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Acknowledgements

The authors acknowledge the DGA/fondos FEDER (construyendo Europa desde Aragón) for funding the research group Platon (E31_20R) and the project LMP71_21. M.H. acknowledges the funding support from MCIN/AEI/10.13039/501100011033 for the Ramón y Cajal fellowship (RYC-2018-025222-I).