Dendrite initiation and propagation in lithium metal solid-state batteries.

All-solid-state batteries with a Li anode and ceramic electrolyte have the potential to deliver a step change in performance compared with today's Li-ion batteries1,2. However, Li dendrites (filaments) form on charging at practical rates and penetrate the ceramic electrolyte, leading to short circuit and cell failure3,4. Previous models of dendrite penetration have generally focused on a single process for dendrite initiation and propagation, with Li driving the crack at its tip5-9. Here we show that initiation and propagation are separate processes. Initiation arises from Li deposition into subsurface pores, by means of microcracks that connect the pores to the surface. Once filled, further charging builds pressure in the pores owing to the slow extrusion of Li (viscoplastic flow) back to the surface, leading to cracking. By contrast, dendrite propagation occurs by wedge opening, with Li driving the dry crack from the rear, not the tip. Whereas initiation is determined by the local (microscopic) fracture strength at the grain boundaries, the pore size, pore population density and current density, propagation depends on the (macroscopic) fracture toughness of the ceramic, the length of the Li dendrite (filament) that partially occupies the dry crack, current density, stack pressure and the charge capacity accessed during each cycle. Lower stack pressures suppress propagation, markedly extending the number of cycles before short circuit in cells in which dendrites have initiated.

Compressibility of Lithium Hexafluorophosphate Solutions in Two Carbonate Solvents.

Speed-of-sound measurements are performed to establish how the isentropic bulk modulus K s of the electrolyte system comprising lithium hexafluorophospate (LiPF6) in blends of propylene carbonate (PC) and ethyl methyl carbonate (EMC) varies with salt molality m, mass fraction of PC in the PC:EMC cosolvent f, and temperature T. Bulk moduli are calculated by combining acoustic time-of-flight data between parallel walls of a liquid-filled cuvette with densitometric data for a sequence of binary and ternary salt solutions. Correlations are presented to yield K s (m, f, T) accurately for nine compositions spanning the range m = 0-2 mol kg-1 and f = 0-1, at temperatures T ranging from 283.15 to 313.15 K. Electrolyte compressibility varies most with solvent ratio, followed by salt content and temperature, with K s ranging from 1 to 3 GPa. Composition-dependent acoustical properties elucidate the nature of speciation and solvation states in bulk electrolytes, and could be useful to identify the features of individual phases within solution-permeated porous electrodes.

Visualizing plating-induced cracking in lithium-anode solid-electrolyte cells.

Lithium dendrite (filament) propagation through ceramic electrolytes, leading to short circuits at high rates of charge, is one of the greatest barriers to realizing high-energy-density all-solid-state lithium-anode batteries. Utilizing in situ X-ray computed tomography coupled with spatially mapped X-ray diffraction, the propagation of cracks and the propagation of lithium dendrites through the solid electrolyte have been tracked in a Li/Li6PS5Cl/Li cell as a function of the charge passed. On plating, cracking initiates with spallation, conical 'pothole'-like cracks that form in the ceramic electrolyte near the surface with the plated electrode. The spallations form predominantly at the lithium electrode edges where local fields are high. Transverse cracks then propagate from the spallations across the electrolyte from the plated to the stripped electrode. Lithium ingress drives the propagation of the spallation and transverse cracks by widening the crack from the rear; that is, the crack front propagates ahead of the Li. As a result, cracks traverse the entire electrolyte before the Li arrives at the other electrode, and therefore before a short circuit occurs.

Dendrite nucleation in lithium-conductive ceramics.

Solid-state lithium batteries cannot achieve reasonable power densities because of dendrites, whose formation mechanisms remain uncertain. This paper applies principles of chemomechanics to investigate the critical current above which dendrites form in lithium-conductive ceramics. Applied voltage induces stress in solid electrolytes; dendrites appear to nucleate in the exemplary garnet-oxide material Li7La3Zr2O12 (LLZO) when the interfacial pressure exceeds a particular value. The critical pressure of polycrystalline LLZO correlates well with the surface-energy changes incurred by lithium plating in its grain boundaries. A derived formula, validated by experiments, predicts quantitatively how critical current varies with properties including interfacial impedance, bulk permittivity, and grain size. As well as suggesting novel strategies to create more resilient ion-conductive ceramics, the proposed mechanism rationalizes experimental observations of bulk lithium plating and explains how LLZO exhibits an electrically activated transition from stable low-current cyclability to high-current dendrite nucleation.

Multifunctional Water Sensors for pH, ORP, and Conductivity Using Only Microfabricated Platinum Electrodes.

Monitoring of the pH, oxidation-reduction-potential (ORP), and conductivity of aqueous samples is typically performed using multiple sensors. To minimize the size and cost of these sensors for practical applications, we have investigated the use of a single sensor constructed with only bare platinum electrodes deposited on a glass substrate. The sensor can measure pH from 4 to 10 while simultaneously measuring ORP from 150 to 800 mV. The device can also measure conductivity up to 8000 µS/cm in the range of 10 °C to 50 °C, and all these measurements can be made even if the water samples contain common ions found in residential water. The sensor is inexpensive (i.e., ~$0.10/unit) and has a sensing area below 1 mm², suggesting that the unit is cost-efficient, robust, and widely applicable, including in microfluidic systems.

Capacity-limiting mechanisms in Li/O2 batteries.

A continuum model of an aprotic lithium/oxygen battery is validated against experimental first-discharge data and used to examine how the apparent cell capacity is affected by macroscopic multicomponent mass transfer, interfacial kinetics, and electronic conduction or tunneling through the discharge product. The model accounts for the three-phase nature of the positive electrode in detail, including an explicit discharge-product layer whose properties and volume distribution generally depend on the local discharge depth. Several hypothetical positive-electrode reaction mechanisms involving different product morphologies and electron-transfer sites are explored within the theoretical framework. To match experimental discharge-voltage vs. capacity and capacity vs. discharge-current trends qualitatively, the discharge-product layer must be assumed to have electronic resistivity several orders of magnitude lower than typical insulators, supporting the notion that the presence of lithium peroxide does not wholly prevent electrons from reaching dissolved reactants. The discharge product also appears to allow charge transport over length scales longer than electron tunneling permits. 'Sudden death' of voltage in lithium/oxygen cells is explained by macroscopic oxygen-diffusion limitations in the positive electrode at high rates, and by pore clogging associated with discharge-product formation at low rates.

Towards a safe lithium-sulfur battery with a flame-inhibiting electrolyte and a sulfur-based composite cathode.

Of the various beyond-lithium-ion batteries, lithium-sulfur (Li-S) batteries were recently reported as possibly being the closest to market. However, its theoretically high energy density makes it potentially hazardous under conditions of abuse. Therefore, addressing the safety issues of Li-S cells is necessary before they can be used in practical applications. Here, we report a concept to build a safe and highly efficient Li-S battery with a flame-inhibiting electrolyte and a sulfur-based composite cathode. The flame retardant not only makes the carbonates nonflammable but also dramatically enhances the electrochemical performance of the sulfur-based composite cathode, without an apparent capacity decline over 750 cycles, and with a capacity greater than 800 mA h(-1) g(-1) (sulfur) at a rate of 10 C.

Overpotential from Cosolvent Imbalance in Battery Electrolytes: LiPF6 in EMC:EC.

Most liquid lithium-ion-battery electrolytes incorporate cosolvent blends, but the dominant electrochemical transport models adopt a single-solvent approximation, which assumes in part that nonuniform cosolvent ratios do not affect cell voltage. For the popular electrolyte formulation based on ethyl-methyl carbonate (EMC), ethylene carbonate (EC), and LiPF6, we perform measurements with fixed-reference concentration cells, finding appreciable liquid-junction potentials when only the cosolvent ratio is polarized. A previously reported junction-potential correlation for EMC:LiPF6 is extended to cover much of the ternary composition space. We propose a transport model for EMC:EC:LiPF6 solutions grounded in irreversible thermodynamics. Thermodynamic factors and transference numbers are entwined in liquid-junction potentials, but concentration-cell measurements determine observable material properties we call junction coefficients, which appear in the extended form of Ohm's law that accounts for how composition changes induce voltage drops. Junction coefficients of EC and LiPF6 are reported and illustrate the extent to which ionic current induces solvent migration.

Liquid crystal order in colloidal suspensions of spheroidal particles by direct current electric field assembly.

DC electric fields are used to produce colloidal assemblies with orientational and layered positional order from a dilute suspension of spheroidal particles. These 3D assemblies, which can be visualized in situ by confocal microscopy, are achieved in short time spans (t < 1 h) by the application of a constant voltage across the capacitor-like device. This method yields denser and more ordered assemblies than had been previously reported with other assembly methods. Structures with a high degree of orientational order as well as layered positional order normal to the electrode surface are observed. These colloidal structures are explained as a consequence of electrophoretic deposition and field-assisted assembly. The interplay between the deposition rate and the rotational Brownian motion is found to be critical for the optimal ordering, which occurs when these rates, as quantified by the Peclet number, are of order one. The results suggest that the mechanism leading to ordering is equilibrium self-assembly but with kinetics dramatically accelerated by the application of the DC electric field. Finally, the crystalline symmetry of the densest structure formed is determined and compared with previously studied spheroidal assemblies.

Potentiometric MRI of a Superconcentrated Lithium Electrolyte: Testing the Irreversible Thermodynamics Approach.

Superconcentrated electrolytes, being highly thermodynamically nonideal, provide a stringent proving ground for continuum transport theories. Herein, we test an ostensibly complete model of LiPF6 in ethyl-methyl carbonate (EMC) based on the Onsager-Stefan-Maxwell theory from irreversible thermodynamics. We perform synchronous magnetic resonance imaging (MRI) and chronopotentiometry to examine how superconcentrated LiPF6:EMC responds to galvanostatic polarization and open-circuit relaxation. We simulate this experiment using an independently parametrized model with six composition-dependent electrolyte properties, quantified up to saturation. Spectroscopy reveals increasing ion association and solvent coordination with salt concentration. The potentiometric MRI data agree closely with the predicted ion distributions and overpotentials, providing a completely independent validation of the theory. Superconcentrated electrolytes exhibit strong cation-anion interactions and extreme solute-volume effects that mimic elevated lithium transference. Our simulations allow surface overpotentials to be extracted from cell-voltage data to track lithium interfaces. Potentiometric MRI is a powerful tool to illuminate electrolytic transport phenomena.

Multiscale Lithium-Battery Modeling from Materials to Cells.

New experimental technology and theoretical approaches have advanced battery research across length scales ranging from the molecular to the macroscopic. Direct observations of nanoscale phenomena and atomistic simulations have enhanced the understanding of the fundamental electrochemical processes that occur in battery materials. This vast and ever-growing pool of microscopic data brings with it the challenge of isolating crucial performance-decisive physical parameters, an effort that often requires the consideration of intricate interactions across very different length scales and timescales. Effective physics-based battery modeling emphasizes the cross-scale perspective, with the aim of showing how nanoscale physicochemical phenomena affect device performance. This review surveys the methods researchers have used to bridge the gap between the nanoscale and the macroscale. We highlight the modeling of properties or phenomena that have direct and considerable impact on battery performance metrics, such as open-circuit voltage and charge/discharge overpotentials. Particular emphasis is given to thermodynamically rigorous multiphysics models that incorporate coupling between materials' mechanical and electrochemical states.

Impact of Space-Charge Layers on Sudden Death in Li/O2 Batteries.

The performance of Li/O2 batteries is thought to be limited by charge transport through the solid Li2O2 discharge product. Prior studies suggest that electron tunneling is the main transport mechanism through thin, compact Li2O2 deposits. The present study employs a new continuum transport model to explore an alternative scenario, in which charge transport is mediated by polaron hopping. Unlike earlier models, which assume a uniform carrier concentration or local electroneutrality, the possibility of nonuniform space charge is accounted for at the Li2O2/electrolyte and Li2O2/electrode interfaces, providing a more realistic picture of transport in Li2O2 films. The temperature and current-density dependences of the discharge curves predicted by the model are in good agreement with flat-electrode experiments over a wide range of rates, supporting the hypothesis that polaron hopping contributes significantly to charge transport. Exercising the model suggests that this mechanism could explain the observed enhancement in cell performance at elevated temperature and that performance could be further improved by tuning the interfacial orientation of Li2O2 crystallites.

Correlating Li/O2 cell capacity and product morphology with discharge current.

The discharge rate is critical to the performance of lithium/oxygen batteries: it impacts both cell capacity and discharge-phase morphology, and in so doing may also affect the efficiency of the oxygen-evolution reaction during recharging. First-discharge data from tens of Li/O2 cells discharged across four rates are analyzed statistically to inform these connections. In the practically significant superficial current-density range of 0.1 to 1 mA cm(-2), capacity is found to fall as a power law, with a Peukert's-law exponent of 1.6 ± 0.1. X-ray diffractometry confirms the dominant presence of crystalline Li2O2 in the discharged electrodes. A completely air-free sample-transfer technique was developed to implement scanning electron microscopy (SEM) of the discharge product. SEM imaging of electrodes with near-average capacities provides statistically significant measures of the shape and size variation of electrodeposited Li2O2 particles with respect to discharge current. At lower rates, typical "toroidal" particles are observed that are well approximated as cylindrical structures, whose average radii remain relatively constant as discharge rate increases, whereas their average heights decrease. At the highest rate studied, air-free SEM shows that particles take needle-like shapes rather than forming the nanosheets or compact films described elsewhere. Average particle volumes decrease with current while particle surface-to-volume ratios increase dramatically, supporting the notion that Li2O2 grows by a locally mass-transfer-limited nucleation and growth mechanism.

Electrochemistry of magnesium electrolytes in ionic liquids for secondary batteries.

The electrochemistry of Mg salts in room-temperature ionic liquids (ILs) was studied using plating/stripping voltammetry to assess the viability of IL solvents for applications in secondary Mg batteries. Borohydride (BH4(-)), trifluoromethanesulfonate (TfO(-)), and bis(trifluoromethanesulfonyl)imide (Tf2N(-)) salts of Mg were investigated. Three ILs were considered: l-n-butyl-3-methylimidazolium (BMIM)-Tf2N, N-methyl-N-propylpiperidinium (PP13)-Tf2N, and N,N-diethyl-N-methyl(2-methoxyethyl)ammonium (DEME(+)) tetrafluoroborate (BF4(-)). Salts and ILs were combined to produce binary solutions in which the anions were structurally similar or identical, if possible. Contrary to some prior reports, no salt/IL combination appeared to facilitate reversible Mg plating. In solutions containing BMIM(+), oxidative activity near 0.8 V vs Mg/Mg(2+) is likely associated with the BMIM cation, rather than Mg stripping. The absence of voltammetric signatures of Mg plating from ILs with Tf2N(-) and BF4(-) suggests that strong Mg/anion Coulombic attraction inhibits electrodeposition. Cosolvent additions to Mg(Tf2N)2/PP13-Tf2N were explored but did not result in enhanced plating/stripping activity. The results highlight the need for IL solvents or cosolvent systems that promote Mg(2+) dissociation.

Hierarchical sulfur-based cathode materials with long cycle life for rechargeable lithium batteries.

Composite materials of porous pyrolyzed polyacrylonitrile-sulfur@graphene nanosheet (pPAN-S@GNS) are fabricated through a bottom-up strategy. Microspherical particles are formed by spray drying of a mixed aqueous colloid of PAN nanoparticles and graphene nanosheets, followed by a simple heat treatment with elemental sulfur. The pPAN-S primary nanoparticles are wrapped homogeneously and loosely within a three-dimensional network of graphene nanosheets (GNS). The hierarchical pPAN-S@GNS composite shows a high reversible capacity of 1449.3 mAh g(-1) sulfur or 681.2 mAh g(-1) composite in the second cycle; after 300 cycles at a 0.2 C charge/discharge rate the capacity retention is 88.8 % of its initial reversible value. Additionally, the coulombic efficiency (CE) during cycling is near 100 %, apart from in the first cycle, in which CE is 81.1 %. A remarkable capacity of near 700 mAh g(-1) sulfur is obtained, even at a high discharge rate of 10 C. The superior performance of pPAN-S@GNS is ascribed to the spherical secondary GNS structure that creates an electronically conductive 3D framework and also reinforces structural stability.

TPPi as a flame retardant for rechargeable lithium batteries with sulfur composite cathodes.

Triphenyl phosphite (TPPi) is adopted as a flame retardant to improve the safety of rechargeable lithium batteries with sulfur composite cathodes. The thermal stability of the electrolyte is greatly enhanced after the addition of TPPi, which also has a positive impact on the electrochemical performance of the Li-S batteries. TPPi facilitates the formation of SEI, resulting in a smaller interfacial impedance and a better rate performance. The addition of about 5 wt% TPPi greatly reduces the polarization voltage, stabilizing the cycle performance of the battery. This indicates that an optimized addition of TPPi is a favorable additive in conventional liquid electrolytes for rechargeable Li-S batteries with high performances and good safety.

Dual-mode sulfur-based cathode materials for rechargeable Li-S batteries.

A novel dual-mode sulfur-based cathode material is prepared for the first time, in which sulfur is embedded in both the pyrolyzed PAN nanoparticles (pPAN) and mildly reduced graphene oxide nanosheets (mGO). The pPAN-S/mGO-S composite demonstrates outstanding electrochemical performances in the rechargeable Li-S batteries.

Electrowetting with electrolytes.

A theory of electrowetting is developed for systems containing an interface between two immiscible electrolytic solutions. Laws for the dependence of contact angle on electrode potential are presented. Ionic impermeability of the liquid-liquid interface and nonlinear double-layer responses rationalize observed phenomena such as contact-angle saturation and droplet contraction or detachment. The theoretical results can be applied to design new, precisely controllable microfluidic devices.