Transition metal dichalcogenide (TMDCs) alloys could provide a wide range of physical and chemical properties, ranging from charge density waves to superconductivity and electrochemical activities. While many exciting behaviors of unary TMDCs have been predicted, the vast compositional space of TMDC alloys has remained largely unexplored due to our lack in understanding of their stability when accommodating different cations or chalcogens in a single-phase. Here, we report a theory-guided synthesis approach to achieve unexplored quasi-binary TMDC alloys through computationally predicted stability maps. We have generated equilibrium temperature-composition phase diagrams using first-principles calculations to identify the stability for 25 quasi-binary TMDC alloys, including those involving non-isovalent cations and verify them experimentally by synthesizing a subset of 12 predicted alloys using a scalable chemical vapor transport method. We demonstrate that the synthesized alloys can be exfoliated into 2D structures, and some of them exhibit: (i) outstanding thermal stability tested up to 1230 K, (ii) exceptionally high electrochemical activity for CO 2 reduction reaction in a kinetically limited regime with near zero overpotential for CO formation, (iii) excellent energy efficiency in a high rate Li-air battery, and (iv) high break-down current density for interconnect applications. This framework can be extended to accelerate the discovery of other TMDC alloys for various applications.As a class of 2D materials, transition metal dichalcogenides (TMDCs) display diverse physical properties, including topological insulator properties, [1,2] superconductivity, [3][4][5][6] valley polarization, [7][8][9][10] and enhanced electrocatalytic activity for various chemical reactions. [11][12][13][14][15][16][17][18] This diversity arises due to the ability of TMDCs to accommodate different transition-metal elements, such as Mo, W, V, Nb, Ta, Re and others, with the three chalcogens (S, Se, and Te) in stable layered structures -that can be exfoliated to a desired number of 2D layers to control quantum confinement. Their properties can be further tuned

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with applications in health monitoring and diagnostics, [1][2][3][4][5] human-machine interface devices, [6,7] cell phones and laptops, [8,9] internet of things, [10] and athletics applications. [11,12] Among various alternative energy storage systems, Li-Oxygen (Li-O 2 ) batteries are promising candidates to meet the requirements of modern flexible electronics with a long-time operation due to their ultrahigh theoretical energy density of ≈3500 Wh kg −1 which is about one order of magnitude higher than that of Li-ion batteries (≈400 Wh kg −1 ). [13][14][15] However, most flexible Li-O 2 batteries operate with a current density in the range of 100-500 mA g −1 , [16][17][18][19][20][21][22][23][24][25] which is far from practical applications of flexible electronics. In addition, the majority of these batteries operate in a pure oxygen environment. Thus, it is imperative that these batteries operate at much higher current rates in an air-like atmosphere since it enables a much higher volumetric energy density compared to its operation in a pure oxygen environment. It also provides a safe and cost-effective approach. Nevertheless, in the presence of all components of air (e.g., nitrogen (N 2 ), carbon dioxide (CO 2 ), and moisture), the Li-O 2 battery operation becomes more complex and serious issues are imposed on the battery including: i) degradation of anode due to its reaction with air compounds, ii) clogging of the cathode due to formation of poorly reversible side products such as lithium hydroxide (LiOH), and iii) degradation of the electrolyte due to sides reactions. [26][27][28] These issues negatively affect the round-trip efficiency and cause other problems, such as parasitic reactions, which lead to poor cyclability and early death of the battery. [29][30][31][32] To resolve these issues, in this study, we designed, fabricated, and tested a new architecture for sheet-type flexible Li-O 2 batteries that operate in ambient air with an open system (flow in and out) where unlike closed systems, no gas storage chamber is needed. In addition, our system is comprised of a Fomblin-based protection layer to filter unwanted air species, such as H 2 O, [33] and an electrolyte blend of 1 m bis(trifluoromethanesulfonyl) imide (LiTFSI) salt, redox mediator (RM) of indium bromide (InBr 3 ) to simultaneously reduce the charge potential and protect the anode from parasitic reactions, [34] and dimethyl sulfoxide (DMSO) and ionic liquid of 1-Ethyl-3-methylimidazolium tetrafluoroborate EMIM-BF4 Lithium-oxygen (Li-O 2 ) batteries possess the highest theoretical energy density (3500 Wh kg −1 ), which makes them attractive candidates for modern electronics and transportation applications. In this work, an inexpensive, flexible, and wearable Li-O 2 battery based on the bifunctional redox mediator of InBr 3 , MoS 2 cathode catalyst, and Fomblin-based oxygen permeable membrane that enable long-cycle-life operation of the battery in pure oxygen, dry air, and ambient air is designed, fabricated, and tested. The battery operates in...