Rechargeable aqueous zinc anodes have gained tremendous attention because of their merits of intrinsic safety, low cost, and high theoretical volumetric capacity (5854 mAh cm −3 for Zn metal). In aqueous electrolytes, zinc anodes suffer from severe dendritic metal deposition. The regulation of Zn by inducing Zn-alloying metals has been reported. However, the underlying mechanisms have remained elusive. Here, for the first time, we did a comprehensive analysis to elucidate the mechanisms for the seeded and nondendritic growth of Zn on alloy anodes. We achieved uniform Zn deposition by introducing a Zn-alloying and soluble metal, Ag, on Zn anodes. Due to a shift of thermodynamic potential and the spatial confinement, the Ag-modified Zn anode exhibited improved overall cycling performance compared with previous deep-cycle Zn anodes. Furthermore, the seeded Zn deposition was visualized in operando for the first time using an optical microscope. The alloy-seeding design principle here can potentially be applied to improve the rechargeability of other metal anodes.

COMMUNICATION (1 of 7)between safety and performance has been a challenge due to the use of intrinsically flammable organic electrolyte materials. In addition to the high level of recent interest in all-solid-state LIBs, [5][6][7][8] an alternative is to develop battery technologies that use aqueous electrolytes which are intrinsically safe. [9][10][11] Among candidate anode materials for aqueous batteries, Zn is the most active metal that is stable in water and also has one of the highest specific capacities. As an anode Zn has roughly three times the volumetric capacity (5854 mAh cm −3 ) compared to Li (2062 mAh cm −3 ). [12,13] When paired with an oxygen cathode, the theoretical volumetric energy density of a Zn-air battery (4400 Wh L −1 ) approaches that of a Li-S battery (5200 Wh L −1 ). Additional advantages of the Zn-air cell compared to the Li-S cell are that Zn is much more economical than Li [14][15][16] and the battery is safer due to absence of flammable organic liquid, making Zn-based batteries attractive candidates for electric vehicles and large-scale energy storage. There has been recent progress on rechargeable Zn anode materials in neutral or mildly acidic conditions that eliminate concerns of ZnO passivating the Zn surface. [17][18][19] In order for Zn-based aqueous batteries to have higher specific energy than state-of-the-art LIBs, however, an oxygen cathode must be used, [16] which favors alkaline electrolytes (e.g., KOH) to facilitate the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Although developing efficient ORR and OER electrocatalysts could lower the polarization and improve the round trip energy efficiency of Zn-air batteries, their reversibility is mainly limited by the Zn anode, which has received far less attention. [20][21][22][23][24][25] A deeply rechargeable zinc anode in lean alkaline electrolyte (a cell utilizing the minimum amount of electrolyte) is a critical step toward zinc air battery that has not been achieved yet. A few attempts have been made before. [26][27][28] However, to the best of our knowledge, all of the electrochemical data in past reports were obtained in beaker cells (Figure 1) with ZnO saturated electrolyte or a low depth of discharge (DOD), which raises several problems: (1) the amount of electrolyte exceeds the amount of electrode materials by ≈1000 times, which lowers the overall energy density and covers the problem of As an alternative to lithium-ion batteries, Zn-based aqueous batteries feature nonflammable electrolytes, high theoretical energy density, and abundant materials. However, a deeply rechargeable Zn anode in lean electrolyte configuration is still lacking. Different from the solid-to-solid reaction mechanism in lithium-ion batteries, Zn anodes in alkaline electrolytes go through a solid-solute-solid mechanism (Zn-Zn (OH) 4 2− -ZnO), which introduces two problems. First, discharge product ZnO on the surface prevents further reaction of Zn underneath, which leads to low utilization of active material and poor rec...

Chemical doping of semiconducting polymers predominantly takes place via integer charge transfer (ICT), where an electron is entirely removed from the host conjugated polymer and transferred to reside on the dopant guest species. In contrast, chemical doping of small conjugated molecules and oligomers often leads to the formation of charge transfer complexes (CTCs), which have significant orbital overlap and shared electron density between the host and guest species. To date, the observation of fractional charge transfer in doped conjugated polymers is relatively rare, occurring only under extreme processing conditions that can be difficult to achieve, which is fortunate given that CTC formation generally yields fewer mobile carriers per dopant. In this work, we use the classic conjugated polymer/dopant pair of P3HT and F4TCNQ to demonstrate how simply adjusting the casting solvent for the dopant in sequential processing can fundamentally alter the nature of doping in this well-studied system, leading to tunable production of CTCs. Using solvent blends of dichloromethane and chloroform, selected for their low and high solubility toward P3HT, respectively, we show that the relative amount of polymer-dopant CTCs can be readily controlled over an order of magnitude. Increasing the amount of chloroform in the dopant solvent blend favors the creation of CTCs, while increasing the dichloromethane content results in doping by the more standard ICT; the results allow us to explain why CTC formation is common in charge-transfer salts but generally less so in doped conjugated polymers. We also explore the role of the doping method and the crystallinity of P3HT films in controlling the relative amounts of ICT and CTC formation. We find that the use of evaporation doping and higher-crystallinity material discourages CTC formation, but that even in the most favorable case of evaporation doping with high polymer crystallinity, fractional charge transfer always occurs to some extent. Finally, we show that brief thermal annealing can convert CTCs to integer charge transfer species, indicating that ICT is the thermodynamically preferred doping mechanism in conjugated polymers, and that fractional charge transfer is the result of kinetic trapping. With this understanding, we offer guidelines for limiting the occurrence of charge transfer complexes during sequential doping of conjugated polymers, thus avoiding the deleterious effects of CTCs on charge transport.

Based on traditional electronic, X-ray, and optical microscopies, novel tools and methodologies have been developed to monitor batteries in situ or in operando. This review surveys the recent development of several types of in situ and in operando microscopy and their utilization in studying batteries. The article is organized mainly by the in situ and in operando techniques used (X-ray microscopy, electron microscopy, scanning probe microscopy, etc.) rather than the chemical reactions that govern battery behavior. We compare different methodologies to illustrate their advantages and disadvantages in the study of different systems and problems and, at the end of the article, discuss generalized principles for the selection of methodologies and possible future directions.

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