Regulating electrodeposition morphology of lithium: towards commercially relevant secondary Li metal batteries.

Lithium, the lightest and most electronegative metallic element, has long been considered the ultimate choice as a battery anode for mobile, as well as in some stationary applications. The high electronegativity of Li is, however, a double-edged sword-it facilitates a large operating voltage when paired with essentially any cathode, promising a high cell-level energy density. It is also synonymous with a high chemical reactivity and low reduction potential. The interfaces a Li metal anode forms with any other material (liquid or solid) in an electrochemical cell are therefore always mediated by one or more products of its chemical or electrochemical reactions with that material. The physical, crystallographic, mechanical, electrochemical, and transport properties of the resultant new material phases (interphases) regulate all interfacial processes at a Li metal anode, including electrodeposition during battery recharge. This Review takes recent efforts aimed at manipulating the structure, composition, and physical properties of the solid electrolyte interphase (SEI) formed on an Li anode as a point of departure to discuss the structural, electrokinetic, and electrochemical requirements for achieving high anode reversibility. An important conclusion is that while recent reports showing significant advances in the achievement of highly reversible Li anodes, e.g. as measured by the coulombic efficiency (CE), raise prospects for as significant progress towards commercially relevant Li metal batteries, the plateauing of achievable CE values to around 99 ± 0.5% apparent from a comprehensive analysis of the literature is problematic because CE values of at least 99.7%, and preferably >99.9% are required for Li metal cells to live up to the potential for higher energy density batteries offered by the Li metal anode. On this basis, we discuss promising approaches for creating purpose-built interphases on Li, as well as for fabricating advanced Li electrode architectures for regulating Li electrodeposition morphology and crystallinity. Considering the large number of physical and chemical factors involved in achieving fine control of Li electrodeposition, we believe that achievement of the remaining ∼0.5% in anode reversibility will require fresh approaches, perhaps borrowed from other fields. We offer perspectives on both current and new strategies for achieving such Li anodes with the specific aim of engaging established contributors and newcomers to the field in the search for scalable solutions.

Ultrahigh-strain ferroelasticity in two-dimensional honeycomb monolayers: from covalent to metallic bonding.

We propose a possible ferroelastic switching pathway of two-dimensional (2D) honeycomb lattice (including graphene, BN, stanene, etc.) that may swap its armchair and zigzag direction, reversing an unprecedented strain of 73.2%. Our ab initio calculations reveal that such pathway cannot work in covalent systems like graphene and BN; for monolayer with metallic bonds like stanene, stanane and InBi that have all been synthesized, however, such pathway can be feasible with a low switching barrier (<0.15 eV) and stress (

Stabilizing polymer electrolytes in high-voltage lithium batteries.

Electrochemical cells that utilize lithium and sodium anodes are under active study for their potential to enable high-energy batteries. Liquid and solid polymer electrolytes based on ether chemistry are among the most promising choices for rechargeable lithium and sodium batteries. However, uncontrolled anionic polymerization of these electrolytes at low anode potentials and oxidative degradation at working potentials of the most interesting cathode chemistries have led to a quite concession in the field that solid-state or flexible batteries based on polymer electrolytes can only be achieved in cells based on low- or moderate-voltage cathodes. Here, we show that cationic chain transfer agents can prevent degradation of ether electrolytes by arresting uncontrolled polymer growth at the anode. We also report that cathode electrolyte interphases composed of preformed anionic polymers and supramolecules provide a fundamental strategy for extending the high voltage stability of ether-based electrolytes to potentials well above conventionally accepted limits.

Design of Single-Molecule Multiferroics for Efficient Ultrahigh-Density Nonvolatile Memories.

It is known that an isolated single-molecule magnet tends to become super-paramagnetic even at an ultralow temperature of a few Kelvin due to the low spin switching barrier. Herein, single-molecule ferroelectrics/multiferroics is proposed, as the ultimate size limit of memory, such that every molecule can store 1 bit data. The primary strategy is to identify polar molecules that possess bistable states, moderate switching barriers, and polarizations fixed along the vertical direction for high-density perpendicular recording. First-principles computation shows that several selected magnetic metal porphyrin molecules possess buckled structures with switchable vertical polarizations that are robust at ambient conditions. When intercalated within a bilayer of 2D materials such as bilayer MoS2 or CrI3, the magnetization can alter the spin distribution or can be even switched by 180° upon ferroelectric switching, rendering efficient electric writing and magnetic reading. It is found that the upper limit of areal storage density can be enhanced by four orders of magnitude, from the previous super-paramagnetic limit of ≈40 to ≈106 GB in.-2, on the basis of the design of cross-point multiferroic tunneling junction array and multiferroic hard drive.

Cryo-STEM mapping of solid-liquid interfaces and dendrites in lithium-metal batteries.

Solid-liquid interfaces are important in a range of chemical, physical and biological processes1-4, but are often not fully understood owing to the lack of high-resolution characterization methods that are compatible with both solid and liquid components5. For example, the related processes of dendritic deposition of lithium metal and the formation of solid-electrolyte interphase layers6,7 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 debated8-11 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, vitrification12. 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.

Confining electrodeposition of metals in structured electrolytes.

Electrochemical cells based on alkali metal (Li, Na) anodes have attracted significant recent attention because of their promise for producing large increases in gravimetric energy density for energy storage in batteries. To facilitate stable, long-term operation of such cells a variety of structured electrolytes have been designed in different physical forms, ranging from soft polymer gels to hard ceramics, including nanoporous versions of these ceramics that host a liquid or molten polymer in their pores. In almost every case, the electrolytes are reported to be substantially more effective than anticipated by early theories in improving uniformity of deposition and lifetime of the metal anode. These observations have been speculated to reflect the effect of electrolyte structure in regulating ion transport to the metal electrolyte interface, thereby stabilizing metal electrodeposition processes at the anode. Here we create and study model structured electrolytes composed of covalently linked polymer grafted nanoparticles that host a liquid electrolyte in the pores. The electrolytes exist as freestanding membranes with effective pore size that can be systematically manipulated through straightforward control of the volume fraction of the nanoparticles. By means of physical analysis and direct visualization experiments we report that at current densities approaching the diffusion limit, there is a clear transition from unstable to stable electrodeposition at Li metal electrodes in membranes with average pore sizes below 500 nm. We show that this transition is consistent with expectations from a recent theoretical analysis that takes into account local coupling between stress and ion transport at metal-electrolyte interfaces.

Design Principles of Functional Polymer Separators for High-Energy, Metal-Based Batteries.

Next-generation rechargeable batteries that offer high energy density, efficiency, and reversibility rely on cell configurations that enable synergistic operations of individual components. They must also address multiple emerging challenges,which include electrochemical stability, transport efficiency, safety, and active material loss. The perspective of this Review is that rational design of the polymeric separator, which is used widely in rechargeable batteries, provides a rich set of opportunities for new innovations that should enable batteries to meet many of these needs. This perspective is different from the conventional view of the polymer separator as an inert/passive unit in a battery, which has the sole function to prevent direct contact between electrically conductivecomponents that form the battery anode and cathode. Polymer separators, which serve as the core component in a battery, bridge the electrodes and the electrolyte with a large surface contact that can be utilized to apply desirable functions. This Review focuses specifically on recent advances in polymer separator systems, with a detailed analysis of several embedded functional agents that are incorporated to improve mechanical robustness, regulate ion and mass transport, and retard flammability. The discussion is also extended to new composite separator concepts that are designated traditionally as polymer/gel electrolytes.

Building Organic/Inorganic Hybrid Interphases for Fast Interfacial Transport in Rechargeable Metal Batteries.

We report a facile in situ synthesis that utilizes readily accessible SiCl4 cross-linking chemistry to create durable hybrid solid-electrolyte interphases (SEIs) on metal anodes. Such hybrid SEIs composed of Si-interlinked OOCOR molecules that host LiCl salt exhibit fast charge-transfer kinetics and as much as five-times higher exchange current densities, in comparison to their spontaneously formed analogues. Electrochemical analysis and direct optical visualization of Li and Na deposition in symmetric Li/Li and Na/Na cells show that the hybrid SEI provides excellent morphological control at high current densities (3-5 mA cm-2 ) for Li and even for notoriously unstable Na metal anodes. The fast interfacial transport attributes of the SEI are also found to be beneficial for Li-S cells and stable electrochemical cycling was achieved in galvanostatic studies at rates as high as 2 C. Our work therefore provides a promising approach towards rational design of multifunctional, elastic SEIs that overcome the most serious limitations of spontaneously formed interphases on high-capacity metal anodes.

Electrochemical Interphases for High-Energy Storage Using Reactive Metal Anodes.

Stable electrochemical interphases play a critical role in regulating transport of mass and charge in all electrochemical energy storage (EES) systems. In state-of-the-art rechargeable lithium ion batteries, they are rarely formed by design but instead spontaneously emerge from electrochemical degradation of electrolyte and electrode components. High-energy secondary batteries that utilize reactive metal anodes (e.g., Li, Na, Si, Sn, Al) to store large amounts of charge by alloying and/or electrodeposition reactions introduce fundamental challenges that require rational design in order to stabilize the interphases. Chemical instability of the electrodes in contact with electrolytes, morphological instability of the metal-electrolyte interface upon plating and stripping, and hydrodynamic-instability-induced electroconvection of the electrolyte at high currents are all known to cause metal electrode-electrolyte interfaces to continuously evolve in morphology, uniformity, and composition. Additionally, metal anodes undergo large changes in volume during lithiation and delithiation, which means that even in the rare cases where spontaneously formed solid electrode-electrolyte interphases (SEIs) are in thermodynamic equilibrium with the electrode, the SEI is under dynamic strain, which inevitably leads to cracking and/or rupture during extended battery cycling. There is an urgent need for interphases that are able to overcome each of these sources of instability with minimal losses of electrolyte and electrode components. Complementary chemical synthesis strategies are likewise urgently needed to create self-limited and mechanically durable SEIs that are able to flex and shrink to accommodate volume change. These needs are acute for practically relevant cells that cannot utilize large excesses of anode and electrolyte as employed in proof-of-concept-type experiments reported in the scientific literature. This disconnect between practical needs and research practices makes it difficult to translate promising literature results, underscoring the importance of research designed to reveal principles for good interphase design. This Account considers the fundamental processes involved in interphase formation, stability, and failure and on that basis identifies design principles, synthesis procedures, and characterization methods for enabling stable metal anode-electrolyte interfaces for EES. We first review results from experimental, continuum theoretical, and computational analyses of interfacial transport to identify fundamental connections between the composition of the SEI at metal-electrolyte interfaces and stability. Design principles and tools for creating stable artificial solid-electrolyte interphases (ASEIs) based on polymers, ionic liquids, ceramics, nanoparticles, salts, and their combinations are subsequently discussed. Interphases composed of a second electrochemically active material that stores charge by different processes from the underlying metal electrode emerge as particularly attractive routes toward so-called hybrid electrodes that enable facile scale-up of ASEI designs for commercially relevant EES.

Designing solid-liquid interphases for sodium batteries.

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.The chemistry at the interface between electrolyte and electrode plays a critical role in determining battery performance. Here, the authors show that a NaBr enriched solid-electrolyte interphase can lower the surface diffusion barrier for sodium ions, enabling stable electrodeposition.

Electroless Formation of Hybrid Lithium Anodes for Fast Interfacial Ion Transport.

Rechargeable batteries based on metallic anodes are of interest for fundamental and application-focused studies of chemical and physical kinetics of liquids at solid interfaces. Approaches that allow facile creation of uniform coatings on these metals to prevent physical contact with liquid electrolytes, while enabling fast ion transport, are essential to address chemical instability of the anodes. Here, we report a simple electroless ion-exchange chemistry for creating coatings of indium on lithium. By means of joint density functional theory and interfacial characterization experiments, we show that In coatings stabilize Li by multiple processes, including exceptionally fast surface diffusion of lithium ions and high chemical resistance to liquid electrolytes. Indium coatings also undergo reversible alloying reactions with lithium ions, facilitating design of high-capacity hybrid In-Li anodes that use both alloying and plating approaches for charge storage. By means of direct visualization, we further show that the coatings enable remarkably compact and uniform electrodeposition. The resultant In-Li anodes are shown to exhibit minimal capacity fade in extended galvanostatic cycling when paired with commercial-grade cathodes.

Designer interphases for the lithium-oxygen electrochemical cell.

An electrochemical cell based on the reversible oxygen reduction reaction: 2Li+ + 2e - + O2↔ Li2O2, provides among the most energy dense platforms for portable electrical energy storage. Such Lithium-Oxygen (Li-O2) cells offer specific energies competitive with fossil fuels and are considered promising for electrified transportation. Multiple, fundamental challenges with the cathode, anode, and electrolyte have limited practical interest in Li-O2 cells because these problems lead to as many practical shortcomings, including poor rechargeability, high overpotentials, and specific energies well below theoretical expectations. We create and study in-situ formation of solid-electrolyte interphases (SEIs) based on bromide ionomers tethered to a Li anode that take advantage of three powerful processes for overcoming the most stubborn of these challenges. The ionomer SEIs are shown to protect the Li anode against parasitic reactions and also stabilize Li electrodeposition during cell recharge. Bromine species liberated during the anchoring reaction also function as redox mediators at the cathode, reducing the charge overpotential. Finally, the ionomer SEI forms a stable interphase with Li, which protects the metal in high Gutmann donor number liquid electrolytes. Such electrolytes have been reported to exhibit rare stability against nucleophilic attack by Li2O2 and other cathode reaction intermediates, but also react spontaneously with Li metal anodes. We conclude that rationally designed SEIs able to regulate transport of matter and ions at the electrolyte/anode interface provide a promising platform for addressing three major technical barriers to practical Li-O2 cells.

Two-Dimensional Metal-Free Organic Multiferroic Material for Design of Multifunctional Integrated Circuits.

Coexistence of ferromagnetism and ferroelectricity in a single 2D material is highly desirable for integration of multifunctional units in 2D material-based circuits. We report theoretical evidence of C6N8H organic network as being the first 2D organic multiferroic material with coexisting ferromagnetic and ferroelectric properties. The ferroelectricity stems from multimode proton-transfer within the 2D C6N8H network, in which a long-range proton-transfer mode is enabled by the facilitation of oxygen molecule when the network is exposed to the air. Such oxygen-assisted ferroelectricity also leads to a high Curie temperature and coupling between ferroelectricity and ferromagnetism. We also find that hydrogenation and carbon doping can transform the 2D g-C3N4 network from an insulator to an n-type/p-type magnetic semiconductor with modest bandgap. Akin to the dopant induced n/p channels in silicon wafer, a variety of dopant created functional units can be integrated into the g-C3N4 wafer by design for nanoelectronic applications.

Highly Stable Sodium Batteries Enabled by Functional Ionic Polymer Membranes.

A sodium metal anode protected by an ion-rich polymeric membrane exhibits enhanced stability and high-Columbic efficiency cycling. Formed in situ via electropolymerization of functional imidazolium-type ionic liquid monomers, the polymer membrane protects the metal against parasitic reactions with electrolyte and, for fundamental reasons, inhibits dendrite formation and growth. The effectiveness of the membrane is demonstrated using direct visualization of sodium electrodeposition.

A stable room-temperature sodium-sulfur battery.

High-energy rechargeable batteries based on earth-abundant materials are important for mobile and stationary storage technologies. Rechargeable sodium-sulfur batteries able to operate stably at room temperature are among the most sought-after platforms because such cells take advantage of a two-electron-redox process to achieve high storage capacity from inexpensive electrode materials. Here we report a room-temperature sodium-sulfur battery that uses a microporous carbon-sulfur composite cathode, and a liquid carbonate electrolyte containing the ionic liquid 1-methyl-3-propylimidazolium-chlorate tethered to SiO2 nanoparticles. We show that these cells can cycle stably at a rate of 0.5 C (1 C=1675, mAh g(-1)) with 600 mAh g(-1) reversible capacity and nearly 100% Coulombic efficiency. By means of spectroscopic and electrochemical analysis, we find that the particles form a sodium-ion conductive film on the anode, which stabilizes deposition of sodium. We also find that sulfur remains interred in the carbon pores and undergo solid-state electrochemical reactions with sodium ions.

Nanostructured electrolytes for stable lithium electrodeposition in secondary batteries.

Secondary batteries based on lithium are the most important energy storage technology for contemporary portable devices. The lithium ion battery (LIB) in widespread commercial use today is a compromise technology. It compromises high energy, high power, and design flexibility for long cell operating lifetimes and safety. Materials science, transport phenomena, and electrochemistry in the electrodes and electrolyte that constitute such batteries are areas of active study worldwide because significant improvements in storage capacity and cell lifetime are required to meet new demands, including the electrification of transportation and for powering emerging autonomous aircraft and robotics technologies. By replacing the carbonaceous host material used as the anode in an LIB with metallic lithium, rechargeable lithium metal batteries (LMBs) with higher storage capacity and compatibility with low-cost, high-energy, unlithiated cathodes such as sulfur, manganese dioxide, carbon dioxide, and oxygen become possible. Large-scale, commercial deployment of LMBs are today limited by safety concerns associated with unstable electrodeposition and lithium dendrite formation during cell recharge. LMBs are also limited by low cell operating lifetimes due to parasitic chemical reactions between the electrode and electrolyte. These concerns are greater in rechargeable batteries that utilize other, more earth abundant metals such as sodium and to some extent even aluminum. Inspired by early theoretical works, various strategies have been proposed for alleviating dendrite proliferation in LMBs. A commonly held view among these early studies is that a high modulus, solid-state electrolyte that facilitates fast ion transport, is nonflammable, and presents a strong-enough physical barrier to dendrite growth is a requirement for any commercial LMB. Unfortunately, poor room-temperature ionic conductivity, challenging processing, and the high cost of ceramic electrolytes that meet the modulus and stability requirements have to date proven to be insurmountable obstacles to progress. In this Account, we first review recent advances in continuum theory for dendrite growth and proliferation during metal electrodeposition. We show that the range of options for designing electrolytes and separators that stabilize electrodeposition is now substantially broader than one might imagine from previous literature accounts. In particular, separators designed at the nanoscale to constrain ion transport on length scales below a theory-defined cutoff, and structured electrolytes in which a fraction of anions are permanently immobilized to nanoparticles, to a polymer network or ceramic membrane are considered particularly promising for their ability to stabilize electrodeposition of lithium metal without compromising ionic conductivity or room temperature battery operation. We also review recent progress in designing surface passivation films for metallic lithium that facilitate fast deposition of lithium at the electrolyte/electrode interface and at the same time protect the lithium from parasitic side reactions with liquid electrolytes. A promising finding from both theory and experiment is that simple film-forming halide salt additives in a conventional liquid electrolyte can substantially extend the lifetime and safety of LMBs.

Metal-Sulfur Battery Cathodes Based on PAN-Sulfur Composites.

Sulfur/polyacrylonitrile composites provide a promising route toward cathode materials that overcome multiple, stubborn technical barriers to high-energy, rechargeable lithium-sulfur (Li-S) cells. Using a facile thermal synthesis procedure in which sulfur and polyacrylonitrile (PAN) are the only reactants, we create a family of sulfur/PAN (SPAN) nanocomposites in which sulfur is maintained as S3/S2 during all stages of the redox process. By entrapping these smaller molecular sulfur species in the cathode through covalent bonding to and physical confinement in a conductive host, these materials are shown to completely eliminate polysulfide dissolution and shuttling between lithium anode and sulfur cathode. We also show that, in the absence of any of the usual salt additives required to stabilize the anode in traditional Li-S cells, Li-SPAN cells cycle trouble free and at high Coulombic efficiencies in simple carbonate electrolytes. Electrochemical and spectroscopic analysis of the SPAN cathodes at various stages of charge and discharge further show a full and reversible reduction and oxidation between elemental sulfur and Li-ions in the electrolyte to produce Li2S as the only discharge product over hundreds of cycles of charge and discharge at fixed current densities.

A Dendrite-Free Lithium Metal Battery Model Based on Nanoporous Polymer/Ceramic Composite Electrolytes and High-Energy Electrodes.

Nanoporous polymer/ceramic composite electrolytes that suppress dendrite growth in full-cell, high-energy secondary lithium metal batteries are reported. The battery cathode design used in the study is energetically balanced with the metallic lithium anode. The results reported show that such batteries can stably operate for over 1000 h without signs of short circuit.

Stable lithium electrodeposition in liquid and nanoporous solid electrolytes.

Rechargeable lithium, sodium and aluminium metal-based batteries are among the most versatile platforms for high-energy, cost-effective electrochemical energy storage. Non-uniform metal deposition and dendrite formation on the negative electrode during repeated cycles of charge and discharge are major hurdles to commercialization of energy-storage devices based on each of these chemistries. A long-held view is that unstable electrodeposition is a consequence of inherent characteristics of these metals and their inability to form uniform electrodeposits on surfaces with inevitable defects. We report on electrodeposition of lithium in simple liquid electrolytes and in nanoporous solids infused with liquid electrolytes. We find that simple liquid electrolytes reinforced with halogenated salt blends exhibit stable long-term cycling at room temperature, often with no signs of deposition instabilities over hundreds of cycles of charge and discharge and thousands of operating hours. We rationalize these observations with the help of surface energy data for the electrolyte/lithium interface and impedance analysis of the interface during different stages of cell operation. Our findings provide support for an important recent theoretical prediction that the surface mobility of lithium is significantly enhanced in the presence of lithium halide salts. Our results also show that a high electrolyte modulus is unnecessary for stable electrodeposition of lithium.

Ionic-liquid-nanoparticle hybrid electrolytes: applications in lithium metal batteries.

Development of rechargeable lithium metal battery (LMB) remains a challenge because of uneven lithium deposition during repeated cycles of charge and discharge. Ionic liquids have received intensive scientific interest as electrolytes because of their exceptional thermal and electrochemical stabilities. Ionic liquid and ionic-liquid-nanoparticle hybrid electrolytes based on 1-methy-3-propylimidazolium (IM) and 1-methy-3-propylpiperidinium (PP) have been synthesized and their ionic conductivity, electrochemical stability, mechanical properties, and ability to promote stable Li electrodeposition investigated. PP-based electrolytes were found to be more conductive and substantially more efficient in suppressing dendrite formation on cycled lithium anodes; as little as 11 wt % PP-IL in a PC-LiTFSI host produces more than a ten-fold increase in cell lifetime. Both PP- and IM-based nanoparticle hybrid electrolytes provide up to 10 000-fold improvements in cell lifetime than anticipated based on their mechanical modulus alone. Galvanostatic cycling measurements in Li/Li4 Ti5 O12 half cells using IL-nanoparticle hybrid electrolytes reveal more than 500 cycles of trouble-free operation and enhanced rate capability.