Effect of Interfacial Resistance on Determination of Transport Properties of Mixed-Conducting Electrolytes (original) (raw)
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Electrochimica Acta, 1999
Measurements of the apparent cation transference number of a solid polymer electrolyte by a voltammetric technique are compared with results from the steady-state dc polarization method. The combination of measurements made at a microelectrode and at a macroelectrode, performed in the absence of supporting electrolyte, yields both the apparent transference number y, and the salt diusion coecient D. Lithium cationic transference numbers in a series of macroscopically solid gels were determined as a function of salt concentration.
The determination of transference numbers in solid polymer electrolytes using the Hittorf method
Solid State Ionics, 1992
Actual transference numbers of M ÷ and X-ion-constituents in polymer electrolytes have been determined for an amorphous poly (ethylene oxide)-based polymer electrolyte containing lithium perchlorate at 120 ° C by means of the Hittorf technique. The indicated value of cation transference number for an 8:1 electrolyte (0.06_+ 0.05) is significantly lower than those found from other experimental techniques, such as NMR and steady-state polarisation, demonstrating the unsuitability of those methods for estimating this parameter. The Hittorf procedure described should be applicable to other polymer electrolytes.
Journal of The Electrochemical Society, 2018
We compare two experimental approaches for measuring the cation transference number in mixtures of polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt: the well-established currentinterrupt method proposed by Ma et al., 1 and a more recent method based on measuring the steady-state current proposed by Balsara and Newman. 2 In electrolytes comprised of high molecular weight PEO, the data from the two techniques agree, highlighting the equivalence of these two approaches. However, in lower molecular weight PEO electrolytes the values of the two approaches diverge at low salt concentrations. We posit this is because the approach of Ma et al. requires measurements that are sensitive to the nature of the interface between the electrolyte and the electrode. The transference numbers measured by the approach of Balsara and Newman for both low and high molecular weight samples vary from 0.7 to-0.8 and are within experimental error throughout the entire salt concentration window.
Lithium transference numbers, salt diffusion coefficients and effective lithium conductivities are investigated with the steady state polarization method for poly(ethylene oxide) (PEO) based polymer electrolytes containing lithium bis(oxalato)borate (LiBOB) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). We compare the commonly used evaluation technique with our approach. Here, the focus is laid on the potential relaxation of the polarized sample which delivers information about the salt diffusion and the potential distribution in symmetric Li/electrolyte/Li cell. The new approach circumvents the well-known initial current issue and thus increases the reliability of the experiment. The temperature dependence of the lithium transference numbers and the importance of data acquisition time are also discussed. The highest lithium transference numbers are 0.19 for P(EO) 60 ·LiTFSI at 90 • C.
Polymers
Whereas the major potential of the development of lithium-based cells is commonly attributed to the use of solid polymer electrolytes (SPE) to replace liquid ones, the possibilities of the improvement of the applicability of the fuel cell is often attributed to the novel electrolytic materials belonging to various structural families. In both cases, the transport properties of the electrolytes significantly affect the operational parameters of the galvanic and fuel cells incorporating them. Amongst them, the transference number (TN) of the electrochemically active species (usually cations) is, on the one hand, one of the most significant descriptors of the resulting cell operational efficiency while on the other, despite many years of investigation, it remains the worst definable and determinable material parameter. The paper delivers not only an extensive review of the development of the TN determination methodology but as well tries to show the physicochemical nature of the discre...
Transference number measurements on a hybrid polymer electrolyte
Electrochimica Acta, 1995
Two methods were used to measure the transference number of a hybrid polymer electrolyte at different salt concentrations. The hybrid polymer electrolyte was based on a radiation induced cross-linked polyether network with propylene carbonate in an interpenetrating liquid phase and LiAsF6 as electrolyte salt. One method for transference number measurements was based on the Hittorf method, which gave values ranging from 0.15 at 0.1 mole kg−1 to 0.31 at 2.1 mole kg−1. The other method was the DC-polarization method, which gave values from 0.31 at 0.1 mole kg−1 to 0.72 at 2.1 mole kg−1. The difference in the transference numbers between the two methods is explained by contribution from mobile ion-pairs.
Electrochimica Acta, 2015
The performance of a new method for ionic conductivity measurements in physical gels with use of thermal scanning approach is investigated. It has been shown that this new experimental protocol has several advantages over conventional conductometry with temperature stabilization of the sample. Improved sensitivity of "ongoing" changes, in situ observations of the gel phase and measurements during gelation stage showed interesting features in conductivity and temperature dependences undetectable in conventional conductometry. The comparison of the results for both methods showed very good agreement concerning global features and additional important detail accessible only by the new proposed here method. Ultimately, the method can be used for characterization of the conductivity properties of the system in the gel phase as well as during gelation process, together with detection and validation of the gel-to-sol phase transition temperatures.
Characterisation and modelling of the transport properties in lithium battery gel electrolytes
Electrochimica Acta, 2004
A recent development trend for rechargeable lithium batteries is the use of ternary gel electrolytes. The main advantage of the gels is the mechanical rigidity, which improves as the polymer content is increased. However, the transport properties deteriorate with increasing polymer amount. This dualistic optimisation problem has caused an increased interest in understanding the transport processes in gels, however no full characterisation or modelling study could be found in the literature. In this paper, which is the first part of a study of the transport in the ternary gel system PMMA/PC/LiClO 4 , the liquid electrolyte PC/LiClO 4 is characterised and modelled for concentrations between 0.1 and 2 M according to a previously employed methodology, based on electrochemical measurements. A model using concentration dependent interaction parameters proved to describe the results in the whole concentration region well. The cationic transport number and salt diffusivity were determined to be approximately 0.3 and 1e−10 m 2 /s, respectively. The mean ionic activity factor variations prove to be substantial. Furthermore, it was demonstrated that the inter-ionic friction was important to consider at concentrations above 1 M. The fundamental friction parameters determined in this part will be used in the following part of the study to describe the friction between ions and solvent.
Electrochimica Acta, 1998
AbstractÐThis work concerns amorphous poly(ethylene oxide) (PEO) rubbery electrolytes with new lithium salts, LiN(CF 3 CO) 2 and Li[CF 3 SO 2 N(CH 2 ) 3 OCH 3 ] (LiMPSA), which contain carbonyl or ether groups in their anions. Although these salts are markedly less conductive than LiN(CF 3 SO 2 ) 2 or LiClO 4 , one of them (LiMPSA) yields a large fraction of a species that promotes lithium transport through back diusion under polarization in Li/PEO±LiX/Li symmetrical cells. Polarization experiments carried out on similar cells ®lled with PEO±LiX±KX, mixed-alkali electrolytes show that this species is a LiX À 2 complex of lower mobility than Li + in PEO. In turn, these experiments reveal that LiN(CF 3 CO) 2 yields a large fraction of LiX ion pairs more mobile than Li + in PEO. These features, which are founded on theoretical models for polarization of PEO±LiX±KX electrolytes between lithium electrodes, agree with other properties (T g elevation and conductivity) of the present systems. #
Transport properties and microstructure of gel polymer electrolytes
Gel polymer electrolytes based on a copolymer poly(vinylidene fluoride (VdF)/hexafluoropropylene (HFP)) and a solution of ethylene carbonate (EC), diethyl carbonate (DEC) and as a salt LiN(CF 3 SO 2) 2 were prepared by changing the content of the polymer in the range 20–80 wt%. The effects of changing the salt concentration in the solution were also explored. The conductivity was found to vary in the range 10 − 2 ohm − 1 cm − 1 (20 wt% of polymer) to 10 − 8 ohm − 1 cm − 1 (80 wt% of polymer). Pulsed field gradient (PFG)-NMR was used to determine the diffusion coefficient of lithium (D Li+)and fluoride species (D F−)and in consequence the transport number. Both D Li+ and D F− decrease with increasing polymer content. Cation transport number (~ +) values between 0.49 and 0.60 were found, depending on the solution content. Pores having average diameter of about 0.5 mm were observed by scanning electron microscopy (SEM).