Mathematical Modeling of Cation Contamination in a Proton-exchange Membrane

Transport phenomena in an ion-exchange membrane containing both H+ and K+ are described using multicomponent diffusion equations (Stefan-Maxwell). A model is developed for transport through a Nafion 112 membrane in a hydrogen-pump setup. The model results are analyzed to quantify the impact of cation contamination on cell potential. It is shown that limiting current densities can result due to a decrease in proton concentration caused by the build-up of contaminant ions. An average cation concentration of 30 to 40 percent is required for appreciable effects to be noticed under typical steady-state operating conditions

Combining X-ray Nano-CT and XANES Techniques for 3D Operando Monitoring of Lithiation Spatial Composition evolution in NMC Electrode

In this study, we present a well-defined methodology for conducting Operando X-ray absorption near-edge structure spectroscopy (XANES) in conjunction with transmission X-ray nano computed tomography (TXM-nanoCT) experiments on the LiNi$_{0.5}$Mn$_{0.3}$Co$_{0.2}$O$_2$ (NMC) cathode electrode. To minimize radiation-induced damage to the sample during charge and discharge cycles and to gain a comprehensive 3D perspective of the (de)lithiation process of the active material, we propose a novel approach that relies on employing only three energy levels, strategically positioned at pre-edge, edge, and post-edge. By adopting this technique, we successfully track the various (de)lithiation states within the three-dimensional space during partial cycling. Furthermore, we are able to extract the nanoscale lithium distribution within individual secondary particles. Our observations reveal the formation of a core-shell structure during lithiation and we also identify that not all surface areas of the particles exhibit activity during the process. Notably, lithium intercalation exhibits a distinct preference, leading to non-uniform lithiation degrees across different electrode locations. The proposed methodology is not limited to the NMC cathode electrode but can be extended to study realistic dedicated electrodes with high active material (AM) density, facilitating exploration and quantification of heterogeneities and inhomogeneous lithiation within such electrodes. This multi-scale insight into the (de)lithiation process and lithiation heterogeneities within the electrodes is expected to provide valuable knowledge for optimizing electrode design and ultimately enhancing electrode performance in the context of material science and battery materials research.Comment: 6 figures (SI, 3 figures

A systematic review of strategies to recruit and retain primary care doctors

Background There is a workforce crisis in primary care. Previous research has looked at the reasons underlying recruitment and retention problems, but little research has looked at what works to improve recruitment and retention. The aim of this systematic review is to evaluate interventions and strategies used to recruit and retain primary care doctors internationally. Methods A systematic review was undertaken. MEDLINE, EMBASE, CENTRAL and grey literature were searched from inception to January 2015.Articles assessing interventions aimed at recruiting or retaining doctors in high income countries, applicable to primary care doctors were included. No restrictions on language or year of publication. The first author screened all titles and abstracts and a second author screened 20%. Data extraction was carried out by one author and checked by a second. Meta-analysis was not possible due to heterogeneity. Results 51 studies assessing 42 interventions were retrieved. Interventions were categorised into thirteen groups: financial incentives (n=11), recruiting rural students (n=6), international recruitment (n=4), rural or primary care focused undergraduate placements (n=3), rural or underserved postgraduate training (n=3), well-being or peer support initiatives (n=3), marketing (n=2), mixed interventions (n=5), support for professional development or research (n=5), retainer schemes (n=4), re-entry schemes (n=1), specialised recruiters or case managers (n=2) and delayed partnerships (n=2). Studies were of low methodological quality with no RCTs and only 15 studies with a comparison group. Weak evidence supported the use of postgraduate placements in underserved areas, undergraduate rural placements and recruiting students to medical school from rural areas. There was mixed evidence about financial incentives. A marketing campaign was associated with lower recruitment. Conclusions This is the first systematic review of interventions to improve recruitment and retention of primary care doctors. Although the evidence base for recruiting and care doctors is weak and more high quality research is needed, this review found evidence to support undergraduate and postgraduate placements in underserved areas, and selective recruitment of medical students. Other initiatives covered may have potential to improve recruitment and retention of primary care practitioners, but their effectiveness has not been established

Mathematical Modeling of Aging of Li-Ion Batteries

International audienceThe recent interest in full and hybrid electric vehicles powered with Li-ion batteries has prompted for in-depth battery aging characterization and prediction. This topic has become popular both in academia and industry battery research communities. Because it is an interdisciplinary topic, different methods for aging studies are being pursued, ranging from black box types of approaches from the electrical engineering community all the way to physics-based methods mainly brought about by the chemical engineering community. This chapter describes an overall methodology for aging characterization and prediction in Li-ion batteries based on physics-based modeling. In a first section, the typical aging phenomena in LIBs are reviewed along with their effects on the cell internal balancing and performance loss. In a second section, the physics-based models used for aging studies are presented, which includes both the performance models (i. e., aging-free) and aging models. In a third section, the typical aging experiments and characterization methods are introduced, along with their analysis with the physics-based models. Finally, the last section presents an outlook of physics-based aging modeling

Apport de la chimie des solutions à la préparation de phosphates de métaux de transition (influence de la structure et de la morphologie sur le comportement électrochimique dans les accumulateurs au lithium)

AMIENS-BU Sciences (800212103) / SudocSudocFranceF

Experimental and Modeling Analysis of Graphite Electrodes with Various Thicknesses and Porosities for High-Energy-Density Li-Ion Batteries

International audienceThe influence of the negative electrode design on its electrochemical performance with regard to Li insertion/de-insertion is analyzed in this work. A combined experimental/modeling approach is undertaken relying on Newman continuum model. Various designs of industry-grade graphite electrodes (2-6 mAh cm −2) were previously characterized by measuring geometric and physical parameters that are used as input parameters in the present model analysis. The half-cell model is successfully validated against rate-capability experiments without any further parameter fitting. The various polarization contributions are then identified based on the model analysis of rate-capability tests on the various electrodes. It emerges that low-loading electrodes suffer from larger particle-scale limitations (mainly solid-diffusion limitation) than high-loading electrodes because of a lower active surface area per geometric area. However, high-loading electrodes undergo large liquid-phase limitations at medium to high current densities: a large overpotential develops because of the formation of a large salt concentration gradient across the cell. Finally, the graphite electrode model is used into a full-cell model vs. LiNi 0.33 Mn 0.33 Co 0.33 O 2 (NMC) as the positive electrode. Simulations allow for a forecast of the occurrence of Li plating for various cell designs with the constraint of a constant ratio of negative to positive electrode loading. As of today, electric vehicles (EV) are being promoted as a substitute to internal-combustion-engine (ICE) vehicles in an effort to mitigate CO 2 , NO x and particulate matter (PM) emissions from the road transportation sector. Although the effectiveness of EV market penetration toward mitigating air pollution strongly depends upon the source of electricity production (e.g., fossil vs. nuclear or renewable), it may still improve air quality in cities and thereby citizens' health. In fact, the annual cost of air pollution was evaluated to over US$ 1.431 trillion in Europe by the World Health Organization in 2010. 1 Nonetheless, the effectiveness of EV market penetration relies on whether consumers are willing to shift from ICE to electric vehicles. Among factors refraining citizens from shifting to EVs are the high price, the vehicle charging time and the driving range. The most straightforward way to tackle both the high price and limited driving range, with state-of-the-art Lithium-ion technology, is to increase electrode loading. Packing more active material in the electrode increases the cell energy density and decreases the amount of inactive material in a Lithium-ion battery pack. Fewer electrodes per stack are needed in a single cell, hence less current collector is used. However , high-loading electrodes suffer large power limitations, which might preclude fast charging of the EV battery pack. Power limitations mostly arise from lithium-ion transport limitations across the electrode porosity filled with the electrolyte and are known to increase with the electrode thickness and/or with a decrease in porosity. 2-4 Accordingly , an optimization of the porous electrode design is necessary to achieve a high energy density while retaining enough power for the targeted application. Yet, electrode design optimization is not straightforward, as it requires performance analysis of a number of different electrode designs. Moreover, a lithium-ion battery (LIB) is a closed system from which only a small number of operating variables can be set and/or measured, e.g., the voltage, the current, and the surface temperature. Electrochemical techniques such as rate-capability tests (galvanos-tatic charge/discharge at different current densities), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry are regular methods to shed light on cell performance limitations but are unable to give definite insight on any concentration and/or potential gradients forming inside the cell. The experimental investigation of * Electrochemical Society Student Member. z E-mail: [email protected] the influence of electrode loading and density on cell performance is rather scarce in the published literature. Fongy et al. characterized LiFePO 4 (LFP) electrodes with different thicknesses, porosity values and binder content by analyzing rate-capability experiments using Prosini's approach. 5,6 An optimal design was found that balances electronic and ionic limitations that appear at high and low porosity values, respectively. Zheng et al. studied separately the influence of electrode composition, calendering and loading of Li(Ni 1/3 Mn 1/3 Co 1/3)O 2 (NMC 111) cathodes by carrying out rate-capability tests and EIS experiments. 7-9 Ogihara et al. performed EIS on symmetric cells based on graphite electrodes with different loadings, and obtained estimation of charge transfer and ionic resistances. 10 Shim and Striebel observed that an increased electrode density induces a slight reduction in both the reversible and irreversible capacity for the first cycle of natural graphite. 11 Buqa et al. examined electrode loadings (1.5-10 mg cm −2) from different synthetic graphites with relatively high electrode porosity (50-80%). 12 They showed that the limitation at high Crate stems from electrode design and not from the graphite material itself. Singh et al. compared rate-capability performance of cathodes and anodes with various loadings. 13,14 Gallagher et al. also studied cathodes and anodes with various loadings (2.2-6.6 mAh cm −2) and presented a physics-based quantitative relationship to link electrode thickness and rate of operation to performance losses. 15 Beside studies based on electrochemical techniques, imaging techniques of operando and in-situ cells were developed and allow to provide additional information on local state of charge (SoC) and salt concentration gradient across the cell. 16-23 However, the analysis turns out to be tedious when screening a large panel of electrode designs. Mathematical models represent a relevant alternative over experimental time-consuming methods. LIB models are a fast, low-cost and accurate tool to perform electrode design optimization. Providing that model equations represent the underlying physics well enough and that corresponding input parameters are accurate, a LIB model enables to predict what is actually happening inside a cell in terms of, e.g., local SoC and temperature, solid and liquid phase potentials , electronic and ionic current densities and solid/liquid lithium concentration gradients. Moreover, simulated data are easy to handle or display, and power-limitation sources are readily identified by switching on or off the corresponding physical phenomena. Among LIB continuum models, the so-called Newman model offers the best compromise between computation speed and physical significance. It is a pseudo-2D (P2D) model that relies on porous electrode theory and) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 194.57.107.121 Downloaded on 2018-07-16 to I

Investigation of the Passivation Properties of the Solid Electrolyte Interphase Using a Soluble Redox Couple

International audienceThe solid electrolyte interphase (SEI) that forms at carbonaceous anodes makes Li-ion battery a viable technology because it inhibits solvent-decomposition reactions. However, passivation is never complete and SEI ``leakage'' appears as the main contributor to Li-ion battery aging. There has been a great deal of experiments focusing on the chemical analysis of SEIs over the past decades. Still, a direct evaluation of their passive character has not been much regarded. In this work, SEIs formed cathodically on glassy carbon electrodes are characterized using the rotating disk electrode method and ferrocene/ferrocenium as a redox shuttle, as originally proposed in Tang and Newman [M. Tang and J. Newman, This journal, 158(5), A530-A536 (2011)]. A comprehensive model is developed that accounts for transport of soluble redox species across the diffusion layer and the porous SEI as well as charge-transfer kinetics at the modified electrode surface. From a model analysis of electrodes with SEIs formed in various conditions, values of SEI porosity and effective rate constant of ferrocenium reduction are derived and discussed. First attempts are conducted to extend the method to SEIs formed at graphite composite electrodes. Preliminary results suggest SEIs are less passivating than those on glassy carbon. (C) The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved

Determination of Tortuosity Using Impedance Spectra Analysis of Symmetric Cell

International audienceAn approach to increase the autonomy of batteries developed for transportation applications, without changing currently-used positive and negative active materials, is to increase the battery energy density by increasing the active material loading (mg.cm(-2)) of the electrodes. A direct consequence of a higher loading is the increase of mass transport-related issues across the electrode porosity. Therefore, the optimization of the porous electrode structure is mandatory to facilitate the access of lithium ions to the whole electrode volume. In this regard, pore tortuosity is a key parameter whose determination is not so straightforward. Although tomography techniques and corresponding analyses are promising methods to acquire precise geometrical information about porous electrode, they hardly can be used as a routine technique. In this work, a transmission-line-model analysis of the electrochemical impedance diagram of symmetric cells containing porous electrodes in blocking condition, i.e. without any charge transfer reaction, is proposed in order to readily derive pore tortuosity. The method is applied to a set of graphite electrodes composed of anisotropic particles. (C) The Author(s) 2017. Published by ECS. All rights reserved

Guidelines for the Analysis of Data from the Potentiostatic Intermittent Titration Technique on Battery Electrodes

International audienceThe Li-ion battery (LIB) modeling community needs to rely on a vast bank of measured Li-ion cell parameters. Most of them are determined by routine experiments but some can solely be estimated by fitting a model to a well-designed experiment. For instance, the determination of the lithium chemical diffusion coefficient has been the focus of numerous studies. Specific techniques have been proposed for its estimation, e.g., the potentiostatic intermittent titration technique (PITT), the galvanostatic intermittent titration technique (GITT), the cyclic voltammetry (CV), and the electrochemical impedance spectroscopy (EIS). Our study focuses on the PITT so as to provide guidelines for experimental measurements and corresponding data analysis. The validity of underlying assumptions of published analytic solutions used to fit PITT data is assessed using a pseudo-2D (P2D) model. Among the tested assumptions are the drop of the porous-electrode effect, of the particle size distribution and of the finite kinetics of Li insertion/de-insertion. Tests are also conducted to assess the error made on the chemical diffusion coefficient and reaction-rate constant determination by fitting 2-electrode simulated data with a 3-electrode P2D model. An example of the P2D model fitting of PITT data obtained on a thin graphite electrode is provided. (c) The Author(s) 2017. Published by ECS