Determination of Transport Parameters in Liquid Binary Lithium Ion Battery Electrolytes

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128

199

A key for the interpretation of porous lithium ion battery electrode impedance spectra is a meaningful and physically motivated equivalent-circuit model. In this work we present a novel approach, utilizing a general transmission line equivalent-circuit model to exemplarily analyze the impedance of a porous high-voltage LiNi 0.5 Mn 1.5 O 4 (LNMO) cathode. It is based on a LNMO/graphite full-cell setup equipped with a gold wire micro-reference electrode (GWRE) to obtain impedance spectra in both, non-blocking conditions at a potential of 4.4 V cell voltage and in blocking configuration achieved at 4.9 V cell voltage. A simultaneous fitting of both spectra enables the deconvolution of physical effects to quantify over the course of 85 cycles at 40 • C: a) the true charge transfer resistance (R CT ), b) the pore resistance (R Pore ), and c) the contact resistance (R Cont. ). We demonstrate that the charge transfer resistance would be overestimated significantly, if the spectra are fitted with a conventionally used simplified R/Q equivalent-circuit compared to our full transmission line analysis. Advanced analysis techniques for lithium ion batteries are a key requirement to deconvolute the complex interplay between the aging mechanisms occurring at the anode and the cathode. In principle, this can be accomplished by electrochemical impedance spectroscopy (EIS), if the individual contributions of anode and cathode to the overall cell impedance can be determined, and if this EIS response can be fitted unambiguously to physically motivated equivalent-circuit models. In general, the measured cell and/or electrode impedances are usually fitted with a serial connection of an ohmic resistor (R), with a parallel circuit of a resistor and a capacitor (C), commonly referred to as R/C element and often also modified to an R/Q element (Q representing a constant-phase element), as well as with a Warburg element (W). [1][2][3][4][5] Recently, more elaborate equivalentcircuits using a transmission line model are getting more and more attention. [6][7][8] In order to independently obtain the impedance of anode and cathode, there are two possible options: i) the assembly of symmetric cells as shown by Chen et al. 9 or Petibon et al., 10 where coin cells out of two anodes (impedance of the negative electrode) or two cathodes (impedance of the positive electrode) are assembled in a glove box or dry-room from two (aged) full-cells at a specified state-of-charge (SOC); ii) the use of three-electrode setups consisting of a working electrode (WE), a counter electrode (CE) and a reference electrode (RE), which allows to individually determine the impedance of the anode and the cathode of a lithium ion battery full-cell. The latter is a more convenient approach, as individual impedance spectra can be recorded continuously during battery cycling, so that anode and cathode impedance can be monitored during cycle-life studies on a fullcell instead of obtaining only one set of anode and cathode impedance spectra after disassembly of a full-cell...

Section: Discussionmentioning

confidence: 99%

An Analysis Protocol for Three-Electrode Li-Ion Battery Impedance Spectra: Part I. Analysis of a High-Voltage Positive Electrode

Landesfeind

Pritzl

Gasteiger

2017

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128

199

“…1 The porosity ε and the tortuosity τ are parameters related to the morphology of the porous medium and are frequently used, e.g., to obtain the so-called effective binary diffusion coefficient D ±,eff = ε τ −1 D ± or the effective conductivity κ eff = ε τ −1 κ eff . While porosities can be measured gravimetrically the tortuosity of porous media may be determined from impedance measurements in symmetric cells.…”

Section: Theorymentioning

confidence: 99%

“…is usually determined in a polarization cell as described, e.g., in Ehrl et al 1 In a second step, the polarization cell can also be used for a second experiment, in which one can determine an additional factor of the form…”

Section: A2718mentioning

confidence: 99%

Determination of Transport Parameters in Liquid Binary Electrolytes: Part II. Transference Number

Ehrl

Landesfeind

Wall

et al. 2017

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In the literature, various numerical methods for the simulation of ion-transport in concentrated binary electrolytes for lithium ion batteries can be found, whereas the corresponding transport parameters are rarely discussed. In this contribution, a novel method for the determination of the transference number in non-aqueous electrolytes is proposed. The method is based on data from a concentration cell and on the value of the thermodynamic factor obtained from independent measurements based on quantifying the redox potential of ferrocene. The concentration dependent transference numbers obtained by this new method are compared to values obtained by the classical approach, which is based on experiments in a polarization cell and a concentration cell. For the latter, a set of commonly used and some newly proposed analysis methods as well as their theoretical justification are discussed. Using an exemplary electrolyte (lithium perchlorate in a mixture of ethylene carbonate and diethyl carbonate), we will demonstrate that our newly proposed method based on concentration cell experiments and a thermodynamic factor derived from independent measurements is a more accurate approach for obtaining concentration dependent transference numbers. At the end, the experimentally determined concentration dependent transference numbers are compared to data available in the literature.

“…−6 cm 2 s = 1600 s [5] In a commercial cell the distance between anode and cathode is ∼25 μm with an average tortuosity of τ ≈ 4 (see, e.g., J. Landesfeind et al 41 ), yielding an approximate diffusion time constant of: t = (25 μm · 4) 2 …”

mentioning

confidence: 99%

Analysis of Vinylene Carbonate (VC) as Additive in Graphite/LiNi_0.5Mn_1.5O₄Cells

Pritzl

Solchenbach

Wetjen

et al. 2017

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124

Vinylene Carbonate (VC) is an effective electrolyte additive to produce a stable solid electrolyte interphase (SEI) on graphite anodes, increasing the capacity retention of lithium-ion cells. However, in combination with LiNi 0.5 Mn 1.5 O 4 (LNMO) cathodes, VC drastically decreases cell performance. In this study we use on-line electrochemical mass spectrometry (OEMS) and electrochemical impedance spectroscopy (EIS) with a micro-reference electrode to understand the oxidative (in-)stability of VC and its effect on the interfacial resistances of both anode and cathode. We herein compare different VC concentrations corresponding to VC to graphite surface area ratios typically used in commercial-scale cells. At low VC concentrations (0.09 wt%, corresponding to 1 wt% in commercial-scale cells), an impedance increase exclusively on the anode and an improved capacity retention is observed, whereas higher VC concentrations (0.17 wt -2 wt%, corresponding to 2 wt -23 wt% in commercial-scale cells) show an increase in both cathode and anode impedance as well as worse cycling performance and overcharge capacity during the first cycle. By considering the onset potentials for VC reduction and oxidation in graphite/LNMO cells, we demonstrate that low amounts of VC can be reduced before VC oxidation occurs, which is sufficient to effectively passivate the graphite anode. During the first charge of a lithium ion battery (LiB), the so called solid electrolyte interphase (SEI) 1 is formed on the surface of the negative electrode. The standard electrolyte for LiBs consists of a mixture of cyclic and linear carbonates, e.g., ethylene carbonate (EC) and ethyl methyl carbonate (EMC), typically with lithium hexafluorophosphate (LiPF 6 ) as salt. Starting from a potential of ∼0.8 V vs. Li/Li + , EC is reduced electrochemically into ethylene gas and lithium ethylene dicarbonate (LEDC), which is a key component of the SEI.2,3 Vinylene carbonate (VC) is one of the most effective additives to modify the SEI on graphite anodes, as it is reduced at potentials more positive than 1.0 V vs. Li/Li + and hence suppresses the reduction of EC. 4,5Aurbach et al. have used VC as electrolyte additive in an EC/DMC (dimethyl carbonate) based electrolyte and that time reported a reduction of the irreversible capacity in the first cycles and an improved cycling stability at elevated temperatures for graphite anodes. The SEI resulting from the reduction of VC consists mainly of poly (vinylene carbonate) (poly(VC)). 4,6Important studies on the impact of different VC concentrations in graphite/NMC pouch cells have been carried out by the Dahn group. For example, Burns et al. 7 investigated the effect of different concentrations of VC (0, 1 and 2 wt%) on cycle life and impedance growth of full-cells with graphite anodes and either LCO (LiCoO 2 ) or NMC (Li(Ni 0.42 Mn 0.42 Co 0.16 )O 2 ) cathodes, employing galvanostatic cycling experiments coupled with high precision coulombic efficiency and electrochemical impedance spectroscopy (EIS) measurements. For cel...