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high-performance dendrite-free seawater-based batteries

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high-performance dendrite-free seawater-based batteries ( high-performance-dendrite-free-seawater-based-batteries )

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ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-20334-6 The strong safety concerns caused by the decomposition of organic electrolytes are challenging non-aqueous lithium- ion battery (LIB) communities, posing formidable barriers to reliable electric vehicles (EVs) and personal electronics1. Alternatively, emerging metal-anode-based aqueous batteries are attracting increasing attention due to the high-safety of non- flammable electrolytes2–4 and environmental benignity5–7. More importantly, when coupled with earth-abundant elements (e.g., O2 and S) at the cathodes, high-energy-density are possible, leading to cutting-edge technology for the advanced battery sys- tems that exceed the energy density of 500 Wh kg−1 required for the future EVs8. However, inhomogeneous metal plating and electrochemical instability at the liquid-solid (electrolyte/metal anode) interface severely jeopardize the performance and life span of aqueous batteries9–13. The inhomogeneous metal plating incurs uncontrollable dendrite growth on the anode surface during charge/discharge cycling, inevitably leading to low Coulombic efficiency (CE), poor cyclability, and even operating failure caused by short-circuit14–16. In recent years, various strategies have been suggested to resolve the aforementioned interfacial instability issues of metal anodes in aqueous batteries from the perspectives of materials science and surface chemistry, including structural optimization17, surface modification18, arti- ficial solid-electrolyte interphases (SEI)19, understanding the metal-based battery chemistry, and controlling metal plating15,20. Nevertheless, progress in stabilizing metal anodes is still in early infancy, which encourages the aqueous battery communities to explore more efficient and universal strategies for addressing the issues of inhomogeneous metal plating and interfacial instability. On the other hand, from the perspective of electrolyte chem- istry, the solvents and salts used in aqueous electrolytes are among the most important components in aqueous batteries that determine their performance21. In practice, deionized (DI) water and high-purity water are commonly used solvents16,21 in aqu- eous batteries to achieve well-controlled battery chemistry by eliminating the interference of hetero-ions (e.g., Ca2+, Mg2+, Na+, SO4−, Cl−, NO3−, F−, etc.) on the battery stability21. Besides, blended salts have been used in the electrolytes to improve the electrochemical performance of aqueous batteries22,23 by tuning the composition of cations and anions in the electrolyte, thereby achieving high ionic conductivity22,24,25. However, the complexity of the electrolyte components used in those strategies makes them economically less competitive than current rechargeable battery technologies for industrial-level applications. Herein, a three-dimensional (3D) alloy anode has been pro- posed and demonstrated to resolve the interfacial instability issues and improve the electrochemical performance of aqueous batteries using low-cost seawater-based electrolytes. Different from the strategies using surface passivation layers to prevent dendrite growth in non-aqueous lithium electrochemical systems26,27, we propose a strategy that will efficiently minimize and suppress the dendrite formation in aqueous systems by controlling: (1) the surface reaction thermodynamics with the favorable diffusion channel of Zn on the Zn3Mn alloy, and (2) the reaction kinetics through the 3D nanostructures on the electrodes, at the same time. The relatively higher binding energy on the surface of the Zn3Mn alloy could help to guide and regulate Zn nucleation and growth and minimize the dendrite formation at the early stage of the deposition. The porous 3D nanostructure will be favorable for controlling the Zn2+ ions diffusion kinetics, further minimizing the dendrite growth throughout the entire deposition process. We designed an optical in-situ visualization protocol that could exactly mimic the actual electrochemical conditions in the aqu- eous systems. Using this protocol, we observed reversible metal plating and stripping processes within the 3D Zn-Mn anode under different aqueous electrolytes including seawater. Also, theoretical (density functional theory, DFT) and experimental (microscopic and spectroscopic) studies proved that the proposed 3D alloy anode has outstanding interfacial stability achieved by the- favorable diffusion channel of Zn on the alloy surface. As a proof-of-concept, the proposed Zn-Mn alloy anodes were demonstrated to be ultra-stable during the Zn plating and strip- ping processes, leading to durable and dendrite-free electrodes for aqueous battery even under a high current density of 80 mA cm−2. This work presents a big step towards high-performance, high- flexibility, and reliable rechargeable batteries using seawater-based electrolytes. This work also provides a further understanding of aqueous battery chemistry that will advance the use of aqueous batteries in the renewable energy field and beyond. Results Preparation and characterizations of alloy anode. An alloy electrodeposition approach was developed to prepare 3D struc- tured Zn-Mn anodes as proposed in this work. This method can be used as a universal strategy for synthesizing various alloy anodes by adjusting the composition of deposition solution, applied deposition current or voltage, and deposition time. In this work, we focused on validating the proposed concept of 3D alloy anode by studying the electrochemical performance of Zn-Mn anode. Compared with Zn2+/Zn, the standard equilibrium potential of Mn2+/Mn is much lower (Supplementary Table 1), enabling the Zn deposition on the surface of Zn-Mn alloy unfa- vorable for Zn dendrite formation due to the electrostatic shield effect10,28. We also demonstrated the potential extension of this alloy electrodeposition strategy by showcasing another anode − Zn-Cu alloy at the end of this paper. We further suggested other alloys beyond Zn-Mn and Zn-Cu, such as Zn-Ni, Zn-Co, Zn-Fe, Zn-Mg, etc., based on their high corrosion resistance among the typical Zn-based alloys29, which will inspire more follow-up works from the battery and materials science communities. The electrodeposition of 3D Zn-Mn alloy was performed in a two- electrode electrochemical cell by a galvanostatic method (more experimental details in the Methods section). Continuous hydrogen (H2) bubbles were observed during the alloy electro- deposition because of water dissociation incurred by the extre- mely high current density of 0.3 A cm−2 used in this work. We varied the electrodeposition time from 10 min to 40 min and found that the evolved H2 bubbles served as gaseous templates for the 3D structure formation following the Stranski-Krastanov mechanism (Supplementary Fig. 1 and Supplementary Discus- sion 1)30. The morphologies of the Zn-Mn alloy changed from an isolated island-like structure to an interconnected 3D structure with a cauliflower-like surface (Supplementary Fig. 2). Based on the microscopic characterizations, the proposed alloy electro- deposition processes mainly include: (i) co-electrodeposition of various ions (Zn2+ and Mn2+);31 (ii) H2 bubbles evolution at the solid-liquid interface leading to the formation of the 3D structure (Fig. 1a and Supplementary Fig. 3). Meanwhile, the hierarchical pores on the surfaces of the cauliflower-like 3D structures (Sup- plementary Fig. 4) are beneficial for the facilitated mass transfer during charge/discharge cycling32,33. XRD pattern (Fig. 1b) and energy-dispersive X-ray spectroscopy (EDS, Supplementary Fig. 5) elemental mapping confirm the formation of Zn-Mn alloy. The main peaks in the XRD pattern primarily correspond to the phase of P63/mmc(194)-hexagonal Zn3Mn (note: in the following discussion Zn-Mn alloy and Zn3Mn denote the same material). The topography of the Zn-Mn alloy was observed with atomic force microscopy (AFM, Fig. 1c and Supplementary Fig. 6) over a 20 × 20 μm area. The cauliflower-like 3D structures show a hierarchical roughness due to the co-existence of both micro- and 2 NATURE COMMUNICATIONS | (2021)12:237 | https://doi.org/10.1038/s41467-020-20334-6 | www.nature.com/naturecommunications

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