Swelling and Softening of Lithium-Ion Battery Separators in Electrolyte Solvents
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Journal of Power Sources 294 (2015) 167e172 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour Swelling and softening of lithium-ion battery separators in electrolyte solvents * Gennady Y. Gor a, , John Cannarella b, Collen Z. Leng b, Aleksey Vishnyakov c, Craig B. Arnold b, 1 a NRC Research Associate, Resident at Naval Research Laboratory, 4555 Overlook Ave., SW Washington, DC 20375, United States b Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, United States c Department of Chemical and Biochemical Engineering, Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway, NJ 08854, United States highlights graphical abstract We perform compressive tests of polypropylene battery separator in different solvents. Some electrolyte solvents (DMC, DEC, Ethyl Acetate) cause softening and swelling of the separator. The separator mechanical properties reduction is due to the polymer- solvent interactions. We show that Hildebrand and FH parameters can be used to identify stable separator/solvent pairs. article info abstract Article history: The mechanical stability of inactive polymeric components (e.g. separator and binder) can play an Received 11 April 2015 important role in the long term performance of lithium-ion batteries. Here we investigate the effects of Received in revised form electrolyte solvents on the mechanical properties of a polypropylene battery separator through experi- 16 May 2015 mental measurements of thickness and elastic modulus of separator samples immersed in different Accepted 7 June 2015 solvent environments. We find that certain electrolyte solvents such as dimethyl carbonate, diethyl Available online xxx carbonate, and ethyl acetate cause noticeable softening of the separator. However, in other solvent en- vironments such as propylene carbonate and acetonitrile, the separator retains the mechanical proper- Keywords: Polymer separator ties of a dry material. We show that the mechanical property reduction can be attributed to polymer e Lithium ion battery swelling and explain these observations in the context of the Hildebrand solubility and Flory Huggins Mechanical tests interaction parameters. The solubility/interaction parameter analysis provides a straightforward method Porous polypropylene for predicting the in situ mechanical behavior of polymer separators in solvent environments. The re- Celgard lationships discussed herein can be used to screen and identify mechanically-stable polymer and elec- Polymer swelling trolyte solvent pairs for use in lithium-ion batteries designed for long life. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The separator is a porous polymer membrane that electronically * Corresponding author. isolates a battery cell's positive and negative electrodes while E-mail addresses: [email protected] (G.Y. Gor), [email protected] e (C.B. Arnold). allowing ion transport between them [1 3]. The mechanical sta- 1 URL: www.princeton.edu/~spikelab/. bility of this membrane is critical to battery cell operation, as http://dx.doi.org/10.1016/j.jpowsour.2015.06.028 0378-7753/© 2015 Elsevier B.V. All rights reserved. 168 G.Y. Gor et al. / Journal of Power Sources 294 (2015) 167e172 mechanical failure of the membrane can cause catastrophic failure All tests are carried out at room temperature in either air (“dry”) of the battery through an internal short circuit [4e7]. More or immersed in a liquid solvent (“wet”). Following Ref. [20] for the recently, mechanical stability of the separator has been shown to be wet tests, samples are immersed in solvent for at least 20 min prior important in the context of battery aging, whereby deformation to testing to ensure complete wetting. It is verified that longer and subsequent pore-closure in the separator leads to accelerated wetting times do not alter the measured mechanical properties. degradation [8e11]. This deformation proceeds due to compressive The solvents used for testing are summarized in Table 1. In addition mechanical stresses that accumulate as a result of the swelling of to battery electrolyte solvents DMC, diethyl carbonate (DEC), pro- battery electrodes that occurs during battery operation [10,12,13], pylene carbonate (PC), acetonitrile, and ethyl acetate, we test a and is expected to be more severe in next generation systems number of other organic solvents of different quality with respect employing high expansion, high capacity electrode materials to PP. The quality of solvent is quantified via the single-component [14e16]. Consequently, there has been much work in developing Hildebrand solubility parameter d [26] and FloryeHuggins new separator membranes with enhanced properties [17,18] as well parameter c [27,28] for PP-solvent interactions. The d values are as in characterizing and understanding the mechanical properties obtained from Refs. [35,36], while the c values are calculated as of separator membranes [19e25]. described below. Much of the recent work on separator mechanical properties To calculate the FloryeHuggins interaction parameters for the shows substantial weakening upon immersion in typical electrolyte polymer-solvent pairs, we use the UNIFAC-FV model [37] for cal- environments compared with the mechanical properties of a dry culations of activities. This model is based on representation of both separator membrane [20e24]. These studies show reductions in solvent and polymer molecules as a set of relatively simple func- elastic modulus, creep modulus, and yield strength, both in tension tional groups with well-determined parameters. The free-volume and compression. Despite the importance of the electrolyte envi- (FV) correction is essential for polymers, and it is taken into ac- ronment in dictating in situ mechanical properties, the only investi- count as an additional correction term in the free energy. gation into the physical origins of this softening phenomenon is a First we calculate the solvent activity a1 for different values of modeling treatment by Yan et al. [24]. In Ref. [24], Yan et al. attribute mass fraction w1 of solvent in the polymer-solvent mixture. We use À À the reduction in mechanical properties to softening resulting from the range 10 5 to 10 1, which is relevant to the polymer swelling as solvent penetration into the polymer host based on the results of opposed to total dissolution. We present the calculated data for a1 molecular dynamics simulations of polypropylene in dimethyl car- as a function of w1 as a dependence of ln(a1/w1) on ln(w1), so that it bonate (DMC). In this present work, we experimentally investigate can be easily fit to the FloryeHuggins equation [27] using the these phenomena by measuring the thickness and mechanical standard least squares algorithm. The result of the fitting gives the properties of the separator in a wide range of solvent environments to FloryeHuggins interaction parameter c. determine swelling and softening. We then use these experimental The UNIFAC-FV calculations are done assuming an amorphous measurements to show that the reduction in mechanical properties PP density of 0.866 g/cm3 and using standard UNIFAC parameters can be explained in the context of the commonly-used Hildebrand for most of the functional groups [38]. More recent data is used for solubility and FloryeHuggins interaction parameters, which provides the carbonate group parameters [39]; the propylene carbonate a straightforward means of quantifying and predicting the solvent- molecule is treated as a single group following Ref. [40]. We vali- polymer interactions associated with polymer swelling [26e28]. date our procedure for calculation of the FloryeHuggins parameter The ability to predict the in situ mechanical behavior of these on several PP-solvent systems, for which the values of c are given in separator membranes is useful for both understanding and simu- the literature [35], namely benzene, acetone, and 1-pentanone lating the mechanical behavior of current lithium-ion batteries as dissolved in PP at room and elevated temperatures. The calculated well as for developing advanced materials for future lithium-ion values of c show good agreement with established literature values batteries. In particular, there is much recent interest in exploring [35]. Note that hypothetically it is also possible to calculate c based the use of new electrolyte solvents (e.g. ethyl acetate and acetoni- on d, but this approach often gives poor agreement with the trile [29e31]), new separator materials [17], and new binders experiment for polymer solutions [41], which we find to be the case [32e34]. The results described herein contribute to these devel- for separator system in this work as well. opment efforts by providing an easy method for identifying promising pairs of electrolyte solvents and polymer components with respect to mechanical stability. It is anticipated that the Table 1 consideration of both chemical and mechanical compatibility of the Solvents discussed in the current work, their Hildebrand parameters, and Flor- electrolyte, separator, and binders in the design of advanced bat- yeHuggins parameters for PP-solvent pairs. All d values are taken from Ref. [35], teries will result in better batteries with greater durability. except for the value for perfluorooctane, which was estimated based on the values for perfluoroheptane and perfluorononane [36]. All c values are calculated by UNIFAC-FV at the temperature 293.15 K as described in the text.