Solid State Ionics 175 (2004) 781 – 783 www.elsevier.com/locate/ssi
Diffusion coefficients in trimethyleneoxide containing comb branch polymer electrolytes Gao Liu, Craig L. Reeder, Xiaoguang Sun, John B. Kerr* Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., MS 62R0203, Berkeley, CA, 94720, USA Received 1 August 2003; received in revised form 4 November 2003; accepted 4 November 2003
Abstract This paper reports on a new comb branch polymer based on trimethylene oxide (TMO) side chains as a polymer electrolyte for potential application in lithium metal rechargeable batteries. The trimethylene oxide (TMO) units are attached to the side chains of a polyepoxide ether to maximize the segmental motion. Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) salt was used to formulate the polymer electrolyte with the new TMO containing polymers. The new polymer electrolytes show improved salt diffusion coefficients (D s) and conductivity at ambient and subambient temperature compare to the ethylene oxide (EO) counterpart, whereas performance at high temperature (85 8C) remains the same or is actually worse for salt diffusivity. D 2004 Elsevier B.V. All rights reserved. Keywords: Lithium battery; Polymer electrolytes; Comb branch polyethers; Trimethylene oxide; Conductivity; Salt diffusion coefficient
1. Introduction Polyethers are the most extensively studied polymers for lithium metal rechargeable battery applications due to their relatively high stability towards the lithium metal anode. Polyethyleneoxide (PEO) has been in the center of the focus due to its high lithium salt solubility, commercial availability, and ease of use. However, the limited lithium ion transport properties of PEO lithium salt electrolytes is a major obstacle for wide applications [1,2]. Numerous approaches have been undertaken to improve the conductivity of the PEO-based electrolytes system, including adding filler [3–5], introducing side chain structure [2], and developing block copolymers [6,7]. These PEO-based electrolyte systems all seem to reach an unsatisfactory transport limitation at most temperature ranges, which is important for electric vehicle and mobile electronic applications [2]. We recently developed a new comb branch trimethylene oxide (TMO) side chain polymer system as
* Corresponding author. Tel.: +1 510 486 6279; fax: +1 510 486 4995. E-mail address:
[email protected] (J.B. Kerr). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2003.11.041
host to achieve a towards room temperature performance for lithium battery applications [8]. We report here some more properties of these materials and correct previously reported values of the salt diffusion coefficients (D s) at lower salt concentrations.
2. Experimental Synthesis of polyepoxide ethers (Fig. 1) and the TMO and EO chains have been described previously [2,9]. Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) salt was a gift from 3M. Conductivity measurements were carried out using Swagelok cells that have been previously described [1]. A Solartronk SI 1254 four-channel frequency response analyzer and a 1286 electrochemical interface were used to measure the impedance of electrolyte films of known thickness in constant-volume cells with stainless steel blocking electrodes. The methods of preparing the films have been described [1]. Salt diffusion coefficients were measured in the same cells with lithium metal electrodes using the method described by Ma et al. [1]. Cell cycling, polarization, and diffusion measurements
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Fig. 1. Structure of PETMO3 and PEEO3 comb branch polymers.
were carried out using an Arbin (College Station, TX) BT 4020 multichannel cycler. Glass transition temperatures were measured using a Perkin-Elmer DSC7 differential scanning calorimeter. Polymer samples were prepared in a glove box and transferred to the calorimeter without exposure to atmosphere.
ductivity performance of PEEO3 and PETMO3 electrolytes is similar. The conductivity behavior of the new TMO containing polymer electrolyte is consistent with the D s measurements of the PETMO3 electrolytes in Fig. 3. Our initially reported D s values at low lithium ion concentration were incorrect [8]. At 60 8C, the D s values of PETMO3 and PEEO3 are almost identical at the measured oxygen to lithium concentrations. This coincides with the identical conductivity region of the two electrolytes in Fig. 2. However, PETMO3 electrolytes give consistently lower values of D s at 85 8C than the D s value of PEEO3. The improved low-temperature conductivity and D s values can be explained by the increased segmental motion of new PETMO3 polymer system. The segmental motion of a polymer system is gauged by glass transition temperature (T g). The lower the T g of given polymer system, the more mobile its segments are at a given temperature above the T g. Fig. 4 shows the T g plotted as a function of the concentration of LiTFSI salt for the same set of polymers. The points for no added salt are plotted as 100:1 for convenience and labeled as pure polymer. Both pure polymers have very
3. Results and discussion Fig. 1 shows the structure and acronym of the new TMO containing polymer and the PEEO3 counterpart. The new polymer electrolytes were formulated using PETMO3 and LiTFSI salt. The electrolyte performance was compared with the EO-based comb branch polymer PEEO3. Fig. 2 shows the conductivities of these comb branch polymer electrolytes with LiTFSI salt, which is present in a ratio of 10 oxygens to 1 lithium ion. The PETMO3 polymer electrolytes exhibit distinctly better conductivity performance at subambient temperature than that of PEEO3 electrolytes, whereas at high-temperature regime, the con-
Fig. 2. Conductivity vs. temperature of PETMO3 and PEEO3 with LiTFSI polymer electrolyte at [O/Li=10:1] ratio.
Fig. 3. D s of PEEO3 and PETMO3 with LiTFSI polymer electrolyte at different O/Li ratio and temperatures.
G. Liu et al. / Solid State Ionics 175 (2004) 781–783
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of the D s values due to the larger steric bulk and lower dielectric constant of the TMO chains. However, we note that the linear PolyTMO which contains no polypropylene units exhibits much higher diffusion coefficients and slightly lower conductivities at 85 8C. These results will be reported in detail elsewhere. Approaching room temperature, the PETMO3 systems show increasingly higher D s values than the PEEO3 systems. This correlates with the improved low-temperature conductivity performance for the PETMO3 systems in Fig. 2 and is consistent with the path of the lithium ion proceeding via the side chains which are more flexible for the TMO polymer.
4. Conclusions Fig. 4. T g of pure PETMO3 and PEEO3 polymers and different LiTFSI salt concentrations.
similar T g values consistent with their similar polyether backbone structures. The T g values all increase with increasing concentration of lithium salt due to the lithium– oxygen binding interaction. However, the salt dependence of these two polymers is very different. The T g of the PETMO3 electrolyte system increased by less than 20 8C, whereas the PEEO3 increased by more than 50 8C at a ratio of 10 oxygens to 1 lithium. This huge difference in T g values at high salt concentrations accounts for the improved performance of the new TMO-containing electrolyte. At subambient temperature, the PEEO3 electrolyte is approaching the glassy condition which hinders the segmental motion to effectively transport lithium ions. Because of the lower T g, the PETMO3 system is much more mobile for better lithium ion transport. To better understand the improved performance, several theoretical modeling studies have been carried out on the binding energies between lithium ions and EO, PPO or TMO polymer hosts and upon the energy barriers to movement of the lithium ions [8,10]. The improved performance of TMO system is apparently due to increased flexibility of the carbon chain around the coordinated lithium ion that leads to a lower barrier for the lithium ion transport in the TMO containing polymer electrolyte. This is quite consistent with the observed T g behavior. It is possible that the oxygen lithium ion binding is weaker at elevated temperature in the PETMO3 system than in the PEEO3 system due to the steric factors. However, the effect of entropy at higher temperatures is likely to favor binding by a single polymer chain. The lithium ions will tend to move more along the backbone at elevated temperatures rather than to form interchain links out in the side chains. Because the backbone is the same for both PETMO3 and PEEO3, the ion transport by this route will be similar for both polymers. Although no salt precipitation was observed in the DSC measurements, an increase of ion pairing of the LiTFSI salt at elevated temperature may also account for the depression
The new PETMO3 polymer gives better ambient and subambient temperature conductivity than that of the conventional PEEO3 type of electrolyte systems. This improved performance is attributed to a direct contribution from the increased lithium ion mobility in the PETMO3 electrolyte system as demonstrated by the D s measurements. DSC analysis shows that T g values of the PETMO3 electrolyte have weaker salt dependence than that of the PEEO3 system.
Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Freedom CAR and Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098 and by the NASA Glenn PERS Program.
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