Anode-free Na metal batteries developed by nearly fully reversible Na plating on the Zn surface.

The anode-free battery architecture has recently emerged as a promising platform for lithium and sodium metal batteries as it not only offers the highest possible energy density, but also eliminates the need for handling hazardous metal electrodes during cell manufacturing. However, such batteries usually suffer from much faster capacity decay and are much more sensitive to even trace levels of irreversible side reactions on the anode, especially for the more reactive Na metal. This work systematically investigates electrochemical interfaces for Na plating and stripping and describes the use of the Zn surface to develop nearly fully reversible Na anodes with 1.0 M NaPF6 in a diglyme-based electrolyte. The high performance includes consistently higher than 99.9% faradaic efficiencies for a wide range of cycling currents between 0.5 and 10 mA cm-2, much more stable interfacial resistance and nearly no formation of mossy Na after 500 cycles compared with conventional Al and Cu surfaces. This improved reversibility was further confirmed under lean electrolyte conditions with a wide range of electrolyte concentrations and cycling temperatures and can be attributed to the strong interfacial binding and intrinsic sodiophilic properties of the Zn surface with Na, which not only ensured uniform Na plating but also eliminated most side reactions that would otherwise cause electrolyte depletion. As a result, full cells assembled with Na-free Zn foil and a high capacity Na3V2(PO4)3 cathode delivered ∼90% capacity retention for 100 cycles, higher than the 73% retention of Cu foils and much higher than the 39% retention of Al foils. This work provides new approaches to enable stable cycling of anode-free batteries and contribute to their applications in practical devices.

Synergistic Multisites Fe2Mo6S8 Electrocatalysts for Ambient Nitrogen Conversion to Ammonia.

Electrochemical hydrogenation of N2 under ambient conditions is attractive for sustainable and distributable NH3 production but is limited by the lack of selective electrocatalysts. Herein, we describe active site motifs based on the Chevrel phase chalcogenide Fe2Mo6S8 that exhibit intrinsic activities for converting N2 to NH3 in aqueous electrolytes. Despite having a very low specific surface area of ∼2 m2/g, this catalyst exhibited a Faradaic efficiency of 12.5% and an average rate of 70 µg h-1 mgcat-1 for NH3 production at -0.20 V vs RHE. Such activities were attributed to the unique composition and structure of Fe2Mo6S8 that provide synergistic multisites for activating and associating key reaction intermediates. Specifically, Fe/Mo sites assist adsorption and activation of N2, whereas S sites stabilize hydrogen intermediate Had* for N2 hydrogenation. Fe in Fe2Mo6S8 enhances binding of S with Had* and thus inhibits the competing hydrogen evolution reaction. The spatial geometry of Fe, Mo, and S sites in Fe2Mo6S8 promotes conversion of N2-Had* association intermediates, reaching a turnover frequency of ∼0.23 s-1 for NH3 production.

Synergistics of Fe3C and Fe on Mesoporous Fe-N-C Sulfur Host for Nearly Complete and Fast Lithium Polysulfide Conversion.

The practical deployment of advanced Li-S batteries is severely constrained by the uncontrollable lithium polysulfide conversion under realistic conditions. Although a plethora of advanced sulfur hosts and electrocatalysts have been examined, the fundamental mechanisms are still elusive and predictive design approaches have not yet been established. Here, we examined a series of well-defined Fe-N-C sulfur hosts with systematically varied and strongly coupled Fe3C and Fe electrocatalysts, prepared by one-step pyrolysis of a novel Fex[Fe(CN)6]y/polypyrrole composite at different temperatures. We revealed the key roles of Fe3C and metallic Fe on modulating polysulfide conversion, in that the polar Fe3C strongly adsorbs polysulfide whereas the Fe particles catalyze fast polysulfide conversion. We then highlight the superior performance of the rational host with strongly coupled Fe3C and Fe on mesoporous Fe-N-C host on promoting nearly complete polysulfide conversion, especially for the challenging short-chain Li2S4 conversion to Li2S. The electrodeposited Li2S on this host was extremely reactive and can be readily charged back to S with minimal activation overpotential. Overall, Li-S batteries equipped with the novel sulfur host delivered a high specific capacity of 1350 mAh g-1 at 0.1C with a capacity retention of 96% after 200 cycles. This work provides new insights on the functional mechanism of advanced sulfur hosts, which could eventually translate into new design principles for practical Li-S batteries.