Graphene acid/MXenes heterostructures for lithium- and sodium-ion batteries. (Acronym GRAPhMAX)

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The aim of GRAPhMAX is the development of a high-performance organic anode for Li and Na-ion batteries, exploiting graphene-acid as the active electrode and explore its operation as a free-standing film at high areal masses, by developing covalent and non-covalent heterostructures with MXenes. MXenes and graphene acid both can, when combined carefully, greatly benefit from mutually synergetic properties.MXenes’ properties are strongly influenced by their surface termination and, in theory, may provide a metal-like conductivity. As electrodes, they can reach a conductivity of 15,100 S/cm, while graphene acid only reaches 0.25 S/cm. They have also demonstrated their potential for high areal mass electrodes. However, capacity-wise, MXenes show low values; Ti2C, for example, stands below 200 mAh/g at 0.03 A/g. Ti3C2, the most widely studied MXene, shows capacities around 100 mAh/g at 0.03 A/g. Only after strict selection of monolayers for electrode preparation, the capacity may reach 410 mAh/g at 0.41 A/g, due to layer expansion, larger surface area, and maximum exposure of MXene’s redox-active centers to the electrolyte ions. On the contrary, the previously reported capacities for graphene acid were obtained without any layer-thickness selection during the whole synthetic procedure, but free-standing films and thickness-persistent properties have been so far unattainable. On this basis, we hypothesize that graphene acid /MXene heterostructures could benefit from their complementary assets, leading to profound synergism: the high conductivity of MXenes and their film-forming properties may enhance charge transfer and film stability in their heterostructures with graphene acid, enabling free-standing high-performance graphene acid electrodes with increased areal mass. To achieve this objective, an efficient hybridization methodology must be developed involving covalent and/or non-covalent approaches for preparing graphene acid/MXene free-standing heterostructures with optimum mass ratio with respect to the final required properties screened by structural characterization and electrochemical testing.