Charles W. Monroe

The Impact of Elastic Deformation on Deposition Kinetics at Lithium/Polymer Interfaces

Department of Chemical Engineering, University of California, Berkeley, California 94720-1462, USAPast theories of electrode stability assume that the surface tension resists the amplification of surface roughness at cathodes andshow that instability at lithium/liquid interfaces cannot be prevented by surface forces alone @Electrochim. Acta, 40, 599 ~1995!#.This work treats interfacial stability in lithium/polymer systems where the electrolyte is solid. Linear elasticity theory is employedto compute the additional effect of bulk mechanical forces on electrode stability. The lithium and polymer are treated as Hookeanelastic materials, characterized by their shear moduli and Poisson’s ratios. Two-dimensional displacement distributions that satisfyforce balances across a periodically deforming interface are derived; these allow computation of the stress and surface-tensionforces. The incorporation of elastic effects into a kinetic model demonstrates regimes of electrolyte mechanical properties whereamplification of surface roughness can be inhibited. For a polymer material with Poisson’s ratio similar to poly~ethylene oxide!,interfacial roughening is mechanically suppressed when the separator shear modulus is about twice that of lithium.© 2005 The Electrochemical Society. @DOI: 10.1149/1.1850854# All rights reserved.Manuscript submitted January 16, 2004; revised manuscript received July 29, 2004. Available electronically January 11, 2005.

Dendrite Growth in Lithium/Polymer Systems A Propagation Model for Liquid Electrolytes under Galvanostatic Conditions

Dendrite growth in a parallel-electrode lithium/polymer cell during galvanostatic charging has been modeled. The growth model is surface-energy controlled, incorporating the effect of dendrite tip curvature into its dendrite growth kinetics. Using data representative of the oxymethylene-linked polylethylene oxide)/LiTFSI system, it is shown that dendrites accelerate across cells under all conditions, and that growth is always slowed by lowering the current density. Cell shorting occurs during typical charges at current densities above 75% of the limiting current. Increased interelectrode distance slows failure, but the advantages decrease as distance lengthens. A factor of 1000 increase in surface forces delays cell failure by only 6%. While larger diffusion coefficients usually extend the time to cell failure, this trend is not consistent at high transference numbers.

The Effect of Interfacial Deformation on Electrodeposition Kinetics

Mullins-Sekerka linear stability analysis and the Barton and Bockris dendrite-propagation model are popular methods used to describe cathodic roughening and dendritic growth. These commonly cited theories employ kinetic relationships that differ in mathematical form, but both contain the effects of surface tension and local concentration deviations induced by surface roughening. Here, a kinetic model is developed which additionally includes mechanical forces such as elasticity, viscous drag, and pressure, showing their effect on exchange current densities and potentials at roughening interfaces. The proposed expression describes the current density in terms of applied overpotential at deformed interfaces with arbitrary three-dimensional interfacial geometry. Both the Mullins-Sekerka and the Barton-Bockris kinetics can be derived as special cases of the general expression, thereby validating the proposed model and elucidating the fundamental assumptions on which the two previous theories rely.

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.

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.

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.

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.

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.

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.

Principles of electrowetting with two immiscible electrolytic solutions

This paper gives a theoretical background for a new electrowetting system based on interfaces between two immiscible electrolytic solutions. It presents a linear-response Poisson–Boltzmann theory to describe an electrolytic droplet on a charged flat electrode, bounded by another electrolytic solution. Immiscibility of the two solutions causes back-to-back double layers to form at the liquid–liquid interface, which dramatically change the polarization response. Useful approximations are developed that apply to droplets with typical experimental volumes. Under the derived approximations, minimization of the free-energy functional proves that polarized droplets take the shape of truncated spheres and reveals a law of contact-angle variation with applied potential. This dependence is determined by the interfacial tensions and the electrolyte concentrations in and dielectric constants of the liquid phases. The study of contact-angle variation with electrode potential may be used, among other applications, as a new tool to investigate the effect of solution properties on liquid–liquid surface tensions.