Design principles for heterointerfacial alloying kinetics at metallic anodes in rechargeable batteries.

How surface chemistry influences reactions occurring thereupon has been a long-standing question of broad scientific and technological interest. Here, we consider the relation between the surface chemistry at interfaces and the reversibility of electrochemical transformations at rechargeable battery electrodes. Using Zn as a model system, we report that a moderate strength of chemical interaction between the deposit and the substrate-neither too weak nor too strong-enables highest reversibility and stability of the plating/stripping redox processes. Focused ion beam and electron microscopy were used to directly probe the morphology, chemistry, and crystallography of heterointerfaces of distinct natures. Analogous to the empirical Sabatier principle for chemical heterogeneous catalysis, our findings arise from competing interfacial processes. Using full batteries with stringent negative electrode-to-positive electrode capacity (N:P) ratios, we show that such knowledge provides a powerful tool for designing key materials in highly reversible battery systems based on Earth-abundant, low-cost metals such as Zn and Na.

Designing interphases for practical aqueous zinc flow batteries with high power density and high areal capacity.

Aqueous zinc flow batteries (AZFBs) with high power density and high areal capacity are attractive, both in terms of cost and safety. A number of fundamental challenges associated with out-of-plane growth and undesirable side reactions on the anode side, as well as sluggish reaction kinetics and active material loss on the cathode side, limit practical deployment of these batteries. We investigated artificial interphases created using a simple electrospray methodology as a strategy for addressing each of these challenges. The effectiveness of the electrospray interphases in full cell zinc-iodine flow batteries was evaluated and reported; it is possible to simultaneously achieve high power density [115 milliwatts per square centimeter (mW/cm2)] and high areal capacity [25 milliampere hour per square centimeter (mA·hour/cm2)]. Last, we extended it to aqueous zinc-bromine and zinc-vanadium flow batteries of contemporary interest. It is again found that high power density (255 and 260 mW/cm2, respectively) and high areal capacity (20 mA·hour/cm2) can be simultaneously achieved in AZFBs.

Highly Reversible Sodium Metal Battery Anodes via Alloying Heterointerfaces.

As a promising pathway toward low-cost, long-duration energy storage, rechargeable sodium batteries are of increasing interest. Batteries that incorporate metallic sodium as anode promise a high theoretical specific capacity of 1166 mAh g-1 , and low reduction potential of -2.71 V. The high reactivity and poor electrochemical reversibility of sodium anodes render sodium metal anode (SMA) cells among the most challenging for practical implementation. Here, the failure mechanisms of Na anodes are investigated and the authors report that loss of morphological control is not the fundamental cause of failure. Rather, it is the inherently poor anchoring/root structure of electrodeposited Na to the electrode substrate that leads to poor reversibility and cell failure. Poorly anchored Na deposits are prone to break away from the current collector, producing orphaning and poor anode utilization. Thin metallic coatings in a range of chemistries are proposed and evaluated as SMA substrates. Based on thermodynamic and ion transport considerations, such substrates undergo reversible alloying reactions with Na and are hypothesized to promote good root growth-regardless of the morphology. Among the various options, Au stands out for its ability to support long Na anode lifetime and high reversibility (Coulombic Efficiency > 98%), for coating thicknesses in the range of 10-1000 nm. As a first step toward evaluating practical utility of the anodes, their performance in Na||SPAN cells with N:P ratio close to 1:1 is evaluated.

Crystallographically Textured Electrodes for Rechargeable Batteries: Symmetry, Fabrication, and Characterization.

The vast of majority of battery electrode materials of contemporary interest are of a crystalline nature. Crystals are, by definition, anisotropic from an atomic-structure perspective. The inherent structural anisotropy may give rise to favored mesoscale orientations and anisotropic properties whether the material is in a rest state or subjected to an external stimulus. The overall perspective of this review is that intentional manipulation of crystallographic anisotropy of electrochemically active materials constitute an untapped parameter space in energy storage systems and thus provide new opportunities for materials innovations and design. To that end, we contend that crystallographically textured electrodes, as opposed to their textureless poly crystalline or single-crystalline analogs, are promising candidates for next-generation storage of electrical energy in rechargeable batteries relevant to commercial practice. This perspective is underpinned first by the fundamental─to a first approximation─uniaxial, rotation-invariant symmetry of electrochemical cells. On this basis, we show that a crystallographically textured electrode with the preferred orientation aligned out-of-plane toward the counter electrode represents an optimal strategy for utilization of the crystals' anisotropic properties. Detailed analyses of anisotropy of different types lead to a simple, but potentially useful general principle that "Pec//Pc" textures are optimal for metal anodes, and "Pec//Sc" textures are optimal for insertion-type electrodes.

Production of fast-charge Zn-based aqueous batteries via interfacial adsorption of ion-oligomer complexes.

Aqueous zinc batteries are attracting interest because of their potential for cost-effective and safe electricity storage. However, metallic zinc exhibits only moderate reversibility in aqueous electrolytes. To circumvent this issue, we study aqueous Zn batteries able to form nanometric interphases at the Zn metal/liquid electrolyte interface, composed of an ion-oligomer complex. In Zn||Zn symmetric cell studies, we report highly reversible cycling at high current densities and capacities (e.g., 160 mA cm-2; 2.6 mAh cm-2). By means of quartz-crystal microbalance, nuclear magnetic resonance, and voltammetry measurements we show that the interphase film exists in a dynamic equilibrium with oligomers dissolved in the electrolyte. The interphase strategy is applied to aqueous Zn||I2 and Zn||MnO2 cells that are charged/discharged for 12,000 cycles and 1000 cycles, respectively, at a current density of 160 mA cm-2 and capacity of approximately 0.85 mAh cm-2. Finally, we demonstrate that Zn||I2-carbon pouch cells (9 cm2 area) cycle stably and deliver a specific energy of 151 Wh/kg (based on the total mass of active materials in the electrode) at a charge current density of 56 mA cm-2.

Textured Electrodes: Manipulating Built-In Crystallographic Heterogeneity of Metal Electrodes via Severe Plastic Deformation.

Control of crystallography of metal electrodeposit films has recently emerged as a key to achieving long operating lifetimes in next-generation batteries. It is reported that the large crystallographic heterogeneity, e.g., broad orientational distribution, that appears characteristic of commercial metal foils, results in rough morphology upon plating/stripping. On this basis, an accumulative roll bonding (ARB) methodology-a severe plastic deformation process-is developed. Zn metal is used as a first example to interrogate the concept. It is demonstrated that the ARB process is highly effective in achieving uniform crystallographic control on macroscopic materials. After the ARB process, the Zn grains exhibit a strong (002) texture (i.e., [002]Zn //ND). The texture transitions from a classical bipolar pattern to a nonclassical unipolar pattern under large nominal strain eliminate the orientational heterogeneity of the foil. The strongly (002)-textured Zn remarkably improves the plating/stripping performance by nearly two orders of magnitude under practical conditions. The performance improvements are readily scaled to achieve pouch-type full batteries that deliver exceptional reversibility. The ARB process can, in principle, be applied to any metal chemistry to achieve similar crystallographic uniformity, provided the appropriate temperature and accumulated strains are employed. This concept is evaluated using commercial Li and Na foils, which, unlike Zn (HCP), are BCC crystals. The simple process for creating strong textures in both hexagonal and cubic metals and illustrating the critical role such built-in crystallography plays underscores opportunities for developing highly reversible thin metal anodes (e.g., hexagonal Zn, Mg, and cubic Li, Na, Ca, Al).

Engineering Multiscale Coupled Electron/Ion Transport in Battery Electrodes.

Coupled electron/ion transport is a defining characteristic of electrochemical processes, for example, battery charge/discharge. Analytical models that represent the complex transport and electrochemical processes in an electrode in terms of equivalent electrical circuits provide a simple, but successful framework for understanding the kinetics of these coupled transport phenomena. The premise of this review is that the nature of the time-dependent phase transitions in dynamic electrochemical environments serves as an important design parameter, orthogonal to the intrinsic mixed conducting properties of the active materials in battery electrodes. A growing body of literature suggests that such phase transitions can produce divergent extrinsic resistances in a circuit model (e.g., Re, describing electron transport from an active electrode material to the current collector of an electrode, and/or Rion, describing ion transport from a bulk electrolyte to the active material surface). It is found that extrinsic resistances of this type play a determinant role in both the electrochemical performance and long-term stability of most battery electrodes. Additionally, successful suppression of the tendency of extrinsic resistances to accumulate over time is a requirement for practical rechargeable batteries and an important target for rational design. We highlight the need for battery electrode and cell designs, which explicitly address the specific nature of the structural phase transition in active materials, and for advanced fabrication techniques that enable precise manipulations of matter at multiple length scales: (i) meso-to-macroscopic conductive frameworks that provide contiguous electronic/ion pathways; (ii) nanoscale uniform interphases formed on active materials; and (iii) molecular-level structures that promote fast electron and/or ion conduction and mechanical resilience.

On the crystallography and reversibility of lithium electrodeposits at ultrahigh capacity.

Lithium metal is a promising anode for energy-dense batteries but is hindered by poor reversibility caused by continuous chemical and electrochemical degradation. Here we find that by increasing the Li plating capacity to high values (e.g., 10-50 mAh cm-2), Li deposits undergo a morphological transition to produce dense structures, composed of large grains with dominantly (110)Li crystallographic facets. The resultant Li metal electrodes manifest fast kinetics for lithium stripping/plating processes with higher exchange current density, but simultaneously exhibit elevated electrochemical stability towards the electrolyte. Detailed analysis of these findings reveal that parasitic electrochemical reactions are the major reason for poor Li reversibility, and that the degradation rate from parasitic electroreduction of electrolyte components is about an order of magnitude faster than from chemical reactions. The high-capacity Li electrodes provide a straightforward strategy for interrogating the solid electrolyte interphase (SEI) on Li -with unprecedented, high signal to noise. We find that an inorganic rich SEI is formed and is primarily concentrated around the edges of lithium particles. Our findings provide straightforward, but powerful approaches for enhancing the reversibility of Li and for fundamental studies of the interphases formed in liquid and solid-state electrolytes using readily accessible analytical tools.

Stabilizing Zinc Electrodeposition in a Battery Anode by Controlling Crystal Growth.

Reversible electrodeposition of metals at liquid-solid interfaces is a requirement for long cycle life in rechargeable batteries that utilize metals as anodes. The process has been studied extensively from the perspective of the electrochemical transformations that impact reversibility, however, the fundamental challenges associated with maintaining morphological control when a intrinsically crystalline solid metal phase emerges from an electrolyte solution have been less studied, but provide important opportunities for progress. A crystal growth stabilization method to reshape the initial growth and orientation of crystalline metal electrodeposits is proposed here. The method takes advantage of polymer-salt complexes (PEG-Zn2+ -aX- ) (a = 1,2,3) formed spontaneously in aqueous electrolytes containing zinc (Zn2+ ) and halide (X- ) ions to regulate electro-crystallization of Zn. It is shown that when X = Iodine (I), the complexes facilitate electrodeposition of Zn in a hexagonal closest packed morphology with preferential orientation of the (002) plane parallel to the electrode surface. This facilitates exceptional morphological control of Zn electrodeposition at planar substrates and leads to high anode reversibility and unprecedented cycle life. Preliminary studies of the practical benefits of the approach are demonstrated in Zn-I2 full battery cells, designed in both coin cell and single-flow battery cell configurations.

Controlling electrochemical growth of metallic zinc electrodes: Toward affordable rechargeable energy storage systems.

Scalable approaches for precisely manipulating the growth of crystals are of broad-based science and technological interest. New research interests have reemerged in a subgroup of these phenomena-electrochemical growth of metals in battery anodes. In this Review, the geometry of the building blocks and their mode of assembly are defined as key descriptors to categorize deposition morphologies. To control Zn electrodeposit morphology, we consider fundamental electrokinetic principles and the associated critical issues. It is found that the solid-electrolyte interphase (SEI) formed on Zn has a similarly strong influence as for alkali metals at low current regimes, characterized by a moss-like morphology. Another key conclusion is that the unique crystal structure of Zn, featuring high anisotropy facets resulting from the hexagonal close-packed lattice with a c/a ratio of 1.85, imposes predominant influences on its growth. In our view, precisely regulating the SEI and the crystallographic features of the Zn offers exciting opportunities that will drive transformative progress.

Designing electrolytes with polymerlike glass-forming properties and fast ion transport at low temperatures.

In the presence of Lewis acid salts, the cyclic ether, dioxolane (DOL), is known to undergo ring-opening polymerization inside electrochemical cells to form solid-state polymer batteries with good interfacial charge-transport properties. Here we report that LiNO3, which is unable to ring-open DOL, possesses a previously unknown ability to coordinate with and strain DOL molecules in bulk liquids, completely arresting their crystallization. The strained DOL electrolytes exhibit physical properties analogous to amorphous polymers, including a prominent glass transition, elevated moduli, and low activation entropy for ion transport, but manifest unusually high, liquidlike ionic conductivities (e.g., 1 mS/cm) at temperatures as low as -50 °C. Systematic electrochemical studies reveal that the electrolytes also promote reversible cycling of Li metal anodes with high Coulombic efficiency (CE) on both conventional planar substrates (1 mAh/cm2 over 1,000 cycles with 99.1% CE; 3 mAh/cm2 over 300 cycles with 99.2% CE) and unconventional, nonplanar/three-dimensional (3D) substrates (10 mAh/cm2 over 100 cycles with 99.3% CE). Our finding that LiNO3 promotes reversibility of Li metal electrodes in liquid DOL electrolytes by a physical mechanism provides a possible solution to a long-standing puzzle in the field about the versatility of LiNO3 salt additives for enhancing reversibility of Li metal electrodes in essentially any aprotic liquid electrolyte solvent. As a first step toward understanding practical benefits of these findings, we create functional Li||lithium iron phosphate (LFP) batteries in which LFP cathodes with high capacity (5 to 10 mAh/cm2) are paired with thin (50 µm) lithium metal anodes, and investigate their galvanostatic electrochemical cycling behaviors.

Spontaneous and field-induced crystallographic reorientation of metal electrodeposits at battery anodes.

The propensity of metal anodes of contemporary interest (e.g., Li, Al, Na, and Zn) to form non-planar, dendritic morphologies during battery charging is a fundamental barrier to achievement of full reversibility. We experimentally investigate the origins of dendritic electrodeposition of Zn, Cu, and Li in a three-electrode electrochemical cell bounded at one end by a rotating disc electrode. We find that the classical picture of ion depletion-induced growth of dendrites is valid in dilute electrolytes but is essentially irrelevant in the concentrated (≥1 M) electrolytes typically used in rechargeable batteries. Using Zn as an example, we find that ion depletion at the mass transport limit may be overcome by spontaneous reorientation of Zn crystallites from orientations parallel to the electrode surface to dominantly homeotropic orientations, which appear to facilitate contact with cations outside the depletion layer. This chemotaxis-like process causes obvious texturing and increases the porosity of metal electrodeposits.

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.

Rechargeable Lithium Metal Batteries with an In-Built Solid-State Polymer Electrolyte and a High Voltage/Loading Ni-Rich Layered Cathode.

Solid-state batteries enabled by solid-state polymer electrolytes (SPEs) are under active consideration for their promise as cost-effective platforms that simultaneously support high-energy and safe electrochemical energy storage. The limited oxidative stability and poor interfacial charge transport in conventional polymer electrolytes are well known, but difficult challenges must be addressed if high-voltage intercalating cathodes are to be used in such batteries. Here, ether-based electrolytes are in situ polymerized by a ring-opening reaction in the presence of aluminum fluoride (AlF3 ) to create SPEs inside LiNi0.6 Co0.2 Mn0.2 O2 (NCM) || Li batteries that are able to overcome both challenges. AlF3 plays a dual role as a Lewis acid catalyst and for the building of fluoridized cathode-electrolyte interphases, protecting both the electrolyte and aluminum current collector from degradation reactions. The solid-state NCM || Li metal batteries exhibit enhanced specific capacity of 153 mAh g-1 under high areal capacity of 3.0 mAh cm-2 . This work offers an important pathway toward solid-state polymer electrolytes for high-voltage solid-state batteries.

Proton Intercalation/De-Intercalation Dynamics in Vanadium Oxides for Aqueous Aluminum Electrochemical Cells.

Understanding cation (H+ , Li+ , Na+ , Al3+ , etc.) intercalation/de-intercalation chemistry in transition metal compounds is crucial for the design of cathode materials in aqueous electrochemical cells. Here we report that orthorhombic vanadium oxides (V2 O5 ) supports highly reversible proton intercalation/de-intercalation reactions in aqueous media, enabling aluminum electrochemical cells with extended cycle life. Empirical analyses using vibrational and x-ray spectroscopy are complemented with theoretical analysis of the electrostatic potential to establish how and why protons intercalate in V2 O5 in aqueous media. We show further that cathode coatings composed of cation selective membranes provide a straightforward method for enhancing cathode reversibility by preventing anion cross-over in aqueous electrolytes. Our work sheds light on the design of cation transport requirements for high-energy reversible cathodes in aqueous electrochemical cells.

Reversible epitaxial electrodeposition of metals in battery anodes.

The propensity of metals to form irregular and nonplanar electrodeposits at liquid-solid interfaces has emerged as a fundamental barrier to high-energy, rechargeable batteries that use metal anodes. We report an epitaxial mechanism to regulate nucleation, growth, and reversibility of metal anodes. The crystallographic, surface texturing, and electrochemical criteria for reversible epitaxial electrodeposition of metals are defined and their effectiveness demonstrated by using zinc (Zn), a safe, low-cost, and energy-dense battery anode material. Graphene, with a low lattice mismatch for Zn, is shown to be effective in driving deposition of Zn with a locked crystallographic orientation relation. The resultant epitaxial Zn anodes achieve exceptional reversibility over thousands of cycles at moderate and high rates. Reversible electrochemical epitaxy of metals provides a general pathway toward energy-dense batteries with high reversibility.

Solid electrolyte interphases for high-energy aqueous aluminum electrochemical cells.

Electrochemical cells based on aluminum (Al) are of long-standing interest because Al is earth abundant, low cost, and chemically inert. The trivalent Al3+ ions also offer among the highest volume-specific charge storage capacities (8040 mAh cm-3), approximately four times larger than achievable for Li metal anodes. Rapid and irreversible formation of a high-electrical bandgap passivating Al2O3 oxide film on Al have, to date, frustrated all efforts to create aqueous Al-based electrochemical cells with high reversibility. Here, we investigate the interphases formed on metallic Al in contact with ionic liquid (IL)-eutectic electrolytes and find that artificial solid electrolyte interphases (ASEIs) formed spontaneously on the metal permanently transform its interfacial chemistry. The resultant IL-ASEIs are further shown to enable aqueous Al electrochemical cells with unprecedented reversibility. As an illustration of the potential benefits of these interphases, we create simple Al||MnO2 aqueous cells and report that they provide high specific energy (approximately 500 Wh/kg, based on MnO2 mass in the cathode) and intrinsic safety features required for applications.

Nano-Sized Cuboid-Shaped Phase in Mg-Nd-Y Alloy and its Behavior During Isothermal Aging.

In the present study, nano-sized cuboid-shaped particles in Mg-Nd-Y are studied by means of Cs-corrected atomic-scale high-angle annular dark-field scanning transmission electron microscopy. The structure of the cuboid-shaped phase is identified to be yttrium (major component) and neodymium atoms in face-centered cubic arrangement without the participation of Mg. The lattice parameter a=5.15 Å. During isothermal aging at 225°C, Mg3(Nd,Y) precipitates adhere to surface (100) planes of the cuboid-shaped particles with the orientation relationship: $[100]_{{{\rm Mg}_{{\rm 3}} {\rm RE}}} \,/\,\,/\,[100]_{{{\rm Cuboid}}} $ and $[310]_{{{\rm Mg}_{{\rm 3}} {\rm RE}}} \,/\,\,/\,[012]_{{{\rm Cuboid}}} $ . The fully coherent interfaces between the precipitates and the cuboid-shaped phases are reconstructed and categorized into two types: $(400)_{{{\rm Mg}_{{\rm 3}} {\rm RE}}} $ interface and $(200)_{{{\rm Mg}_{{\rm 3}} {\rm RE}}} $ interface.

Unexpected Fe-enriched compounds observed in Mg-Ce alloy: An atomic-scale STEM investigation.

This paper investigates into the unexpected nanosize Fe-enriched particles with defects incorporated, using EDS under STEM mode and atomic-resolution HAADF. The particle that locates between eutectic Mg12 Ce phase and α-Mg phase is identified as CeFe2 Si2 compound. Two types of defects are incorporated: Type-I defect is Ce-Si in a simple cubic lattice; Type-II defect is Ce in a FCC lattice. The interface between the defect-free CeFe2 Si2 and the defects is fully coherent. In addition, another two Ce/Fe/Si-enriched particles are observed, proving that the enrichment of Ce, Fe, and Si is not an accidental phenomenon. The observed formation of Ce/Fe/Si-enriched particles provides a possible explanation for the improvement in corrosion resistance and the purification effect induced by addition of rare earth elements into alloys. In addition, the present study demonstrates an advanced application of Cs-corrected HAADF by successfully directly imaging the dislocation and complex defects incorporated in the compound and by achieving a one-angstrom resolution. SCANNING 38:783-791, 2016. © 2016 Wiley Periodicals, Inc.