Solid-liquid interfaces are important in a range of chemical, physical and biological processes, but are often not fully understood owing to the lack of high-resolution characterization methods that are compatible with both solid and liquid components. For example, the related processes of dendritic deposition of lithium metal and the formation of solid-electrolyte interphase layers are known to be key determinants of battery safety and performance in high-energy-density lithium-metal batteries. But exactly what is involved in these two processes, which occur at a solid-liquid interface, has long been debated because of the challenges of observing such interfaces directly. Here we adapt a technique that has enabled cryo-transmission electron microscopy (cryo-TEM) of hydrated specimens in biology-immobilization of liquids by rapid freezing, that is, vitrification. By vitrifying the liquid electrolyte we preserve it and the structures at solid-liquid interfaces in lithium-metal batteries in their native state, and thus enable structural and chemical mapping of these interfaces by cryo-scanning transmission electron microscopy (cryo-STEM). We identify two dendrite types coexisting on the lithium anode, each with distinct structure and composition. One family of dendrites has an extended solid-electrolyte interphase layer, whereas the other unexpectedly consists of lithium hydride instead of lithium metal and may contribute disproportionately to loss of battery capacity. The insights into the formation of lithium dendrites that our work provides demonstrate the potential of cryogenic electron microscopy for probing nanoscale processes at intact solid-liquid interfaces in functional devices such as rechargeable batteries.

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Secondary batteries based on earth-abundant sodium metal anodes are desirable for both stationary and portable electrical energy storage. Room-temperature sodium metal batteries are impractical today because morphological instability during recharge drives rough, dendritic electrodeposition. Chemical instability of liquid electrolytes also leads to premature cell failure as a result of parasitic reactions with the anode. Here we use joint density-functional theoretical analysis to show that the surface diffusion barrier for sodium ion transport is a sensitive function of the chemistry of solid–electrolyte interphase. In particular, we find that a sodium bromide interphase presents an exceptionally low energy barrier to ion transport, comparable to that of metallic magnesium. We evaluate this prediction by means of electrochemical measurements and direct visualization studies. These experiments reveal an approximately three-fold reduction in activation energy for ion transport at a sodium bromide interphase. Direct visualization of sodium electrodeposition confirms large improvements in stability of sodium deposition at sodium bromide-rich interphases.

An artificial solid electrolyte interphase on aluminum enables aqueous batteries with high specific energy and good reversibility.

Substrates able to rectify transport of ions based on charge and/or size are ubiquitous in biological systems. Electrolytes and interphases that selectively transport electrochemically active ions are likewise of broad interest in all electrical energy storage technologies. In lithium-ion batteries, electrolytes with singleor near-single-ion conductivity reduce losses caused by ion polarization. In emergent lithium or sodium metal batteries, they maintain high conductivity at the anode and stabilize metal deposition by fundamental mechanisms. We report that 20-to 300-nm-thick, single-ion-conducting membranes deposited at the anode enable electrolytes with the highest combination of cation transference number, ionic conductivity, and electrochemical stability reported. By means of direct visualization we find that single-ion membranes also reduce dendritic deposition of Li in liquids. Galvanostatic measurements further show that the electrolytes facilitate long (3 mAh) recharge of full Li/LiNi 0.8-Co 0.15 Al 0.05 O 2 (NCA) cells with high cathode loadings (3 mAh cm À2 /19.9 mg cm À2) and at high current densities (3 mA cm À2).

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rechargeable batteries has emerged as a requirement for large-scale electrification of transportation. [1] It is now known, for example, that replacing the carbon-based anode in today's lithium-ion batteries with metallic lithium would lead to a tenfold increase in the amount of charge stored (from 360 to 3860 mA h g −1 ) per unit mass of the battery anode. Such lithium metal batteries (LMBs) are also promising for a variety of other reasons. The most important is that they enable the use of high-energy unlithiated materials, such as sulfur, oxygen, and carbon dioxide [2] as the active species in the cathode. This raises the prospect of multiple battery platforms that offer large improvements in specific energy (SE) on either a mass or volumetric basis considering the electrode materials only (e.g., SE Li-S = 2.5 kW h kg −1 or 2.8 kW h L −1 ; SE Li-O2 = 12 kW h kg −1 ; SE Li-O2/CO2 = 10.5 kW h kg −1 ), relative to today's state-of-the-art Li-ion technology (SE Li-ion = 0.5 kW h kg −1 ).Unregulated, rough/dendritic lithium electrodeposition during charging is now understood to be the main hurdle to practical LMBs that can be operated stably and safely over the thousands of charge-discharge cycles required for applications in transportation. [3] Several recent studies, including a few excellent reviews, [4,5] summarize the physicochemical factors that produce unstable Li deposition and discuss possible strategies to prevent Li dendrite formation and stabilize LMBs during cell recharge. Previously, we reported that a family of nanoporous γ-Al 2 O 3 /polymer laminate membranes able to imbibe large amounts of liquid electrolyte in their pores break the conventional modulus-ionic conductivity tradeoff that had previously prohibited solutions based on solid electrolytes. The membranes were also reported to exhibit impressive ability to retard Li dendrite proliferation in Li/Li symmetric as well as Li/Li 4 Ti 5 O 12 half cells. [6,7] A recent theoretical study of Li electrodeposition in elastic media suggests that aside from their high mechanical modulus and high ionic conductivity, there are at least two fundamental reasons why nanoporous ceramic membranes may stabilize the anode in a LMB. First, the channels constrain dendrite nucleate sizes below critical dimensions where surface tension alone can completely stabilize electrodeposition at the Li-metal/electrolyte interface. [8] Second, nanochannels with charged walls can effectively regulate fluid flow or rectify ion transport in the Successful strategies for stabilizing electrodeposition of reactive metals, including lithium, sodium, and aluminum are a requirement for safe, highenergy electrochemical storage technologies that utilize these metals as anodes. Unstable deposition produces high-surface area dendritic structures at the anode/electrolyte interface, which causes premature cell failure by complex physical and chemical processes that have presented formidable barriers to progress. Here, it is reported that hybrid electrolytes created by infusing convention...