Polymer electrolytes from plasticized polymobs and their gel forms

Transcription

1 Electrochimica Acta 48 (2003) 2029/ Polymer electrolytes from plasticized polymobs and their gel forms Wu Xu, C. Austen Angell * Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ , USA Received 19 May 2002; accepted 8 October 2002 Abstract Plasticized and crosslinked versions of poly(lithium oligoetherato mono-oxalato borate), called lithium polymobs, have been studied. In heavily plasticized forms of both polymob, and a LiBH 4 -crosslinked polymob, the ionic conductivity reaches 10 3 S cm 1 at room temperature while single ion conductivity is automatically retained. The electrochemical stability window is up to 5 V, for both stainless steel (SS) electrodes. The plasticized forms are fluid, not a gel, due to the low molecular weight of the polyanions. Freestanding gel electrolytes with high single ionic conductivity of 10 4 S cm 1 at ambient can be obtained by incorporation of high molecular weight poly(methyl methacrylate) (PMMA) into the solution. Electrochemical cells using these electrolytes will not suffer from concentration polarization. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Polyanionic electrolytes; PolyMOBs; Gel electrolytes 1. Introduction * Corresponding author. address: caa@asu.edu (C.A. Angell). Solvent-free polymer electrolytes have ionic conductivities in the range 10 8 /10 4 S cm 1 at ambient temperatures*/generally too low for use in ambient temperature electrochemical devices. One simple way to improve the conductivity is to add molecular solvents into the polymer electrolytes. Such gel type electrolytes have now found general application, particularly in lithium ion rechargeable batteries [1 /7]. All such electrolytes to date have had conductivities dominated by anions and the lithium ion transference number (t Li ) has proved to be far below 0.5 [2,3]. Recently we reported a new polyanionic electrolyte, poly(lithium oligoetherato mono-oxalatoborate) named polymob, i.e. P(LiOEG n B) where n represents the number of repeating oxyethylene units between the inchain anionic moieties. The anionic moieties are very weakly coordinating [8,9]. These polyanionic materials showed both high ionic conductivity (for a polymer electrolyte), 10 5 S cm 1 at 25 8C, and high electrochemical oxidation potential, up to 4.5 V versus Li /Li. PolyMOBs were synthesized via simple two-step reactions from very cheap materials and are benign to the environment. Since the anions are fixed onto the polymer chains they naturally have t Li /1. In the continuation of this work, we have studied the plasticized versions of these polyanions and their crosslinked versions. The plasticizers used are common solvents such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC) and 1,2- dimethoxyethane (DME). The effect of plasticizers on the ionic conductivity will be discussed. Since no other lithium salts have been added into these plasticized polyelectrolytes, t Li must remain at unity, hence cells using these electrolytes will not suffer from concentration polarization. 2. Experimental section 2.1. LiBH 4 crosslinked polyanion electrolytes P(LiOEG n B) with different n values was dissolved in anhydrous THF and cooled in acetone-dry ice bath (/78 8C). An amount of LiBH 4 in THF solution, estimated to react with all residual chain ends, was dropwise added into the above solution with vigorous stirring. After addition, the solution was stirred at /03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi: /s (03)

2 2030 W. Xu, C.A. Angell / Electrochimica Acta 48 (2003) 2029/2035 /78 8C for 2 h and then at room temperature overnight. The solvent was then evaporated at reduced pressure and the residual polymer was dried in a vacuum oven at ca. 70 8C for 48 h Preparation of plasticized electrolytes Different amount of P(LiOEG n B) or crosslinked P(LiOEG n B) with different n values was weighed into a 2 ml volumetric tube in a dry glove box filled with purified nitrogen. Different plasticizers such as pure EC and PC, or plasticizer mixtures such as EC/PC (1:1 by mol, 1:1 by wt. and 4:1 by wt.), EC/DMC (1:2 by wt.) and PC/DME (1:1 by wt.) were added to dissolve the polymob. The compositions are recorded as weight percent plasticizer in some cases, and as molar concentration, mol l 1, in others as indicated in the figure captions. The solutions were stored in sealed containers for conductivity and electrochemical measurements Preparation of PMMA gel electrolytes The gel electrolytes were prepared in a dry glove box filled with purified nitrogen. The macromolecular salt polymob was dissolved in a certain amount of EC/PC (1:1 by wt.) mixture in a vial. A quantity of poly(methyl methacrylate) (PMMA) was added. The vial was sealed and heated to about 130 8C with occasional shaking till completely homogenized to a clear viscous mass. The solution was cooled to room temperature and the fluidity of the solution was evaluated visually. If the mixture did not flow, even when the vial was left inverted for some time, the solution was re-heated to about 130 8C and the hot viscous liquid was pressed between two stainless steel (SS) plates covered with Teflon films. After cooling, the self-standing membrane formed was pealed off. 3. Results and discussion 3.1. Plasticization of polymobs by EC /PC mixture solvents Fig. 1 shows the temperature dependence of ionic conductivity of P(LiOEG 3 B) plasticized by 1:1 (mol) EC /PC mixture. All exhibit super-arrhenius behavior as expected for system with T/T g. The same is found for other plasticized polymobs with n :/5, 9 and 14. The isothermal conductivities at room temperature for this series are plotted in Fig. 2 against lithium ion concentration. Comparison is made with the conductivity of simple lithium bis(oxalato)borate (LiBOB) [10] solutions in PC. The important thing to note is that when EC /PC is used as plasticizer, the overall conductivity is highest for the polymer with the shortest spacing, n/3. s 25 8C / S cm 1 was obtained for 90 wt.% EC/PC plasticized P(LiOEG 3 B). However, in the vicinity of the maximum there is little difference between n/3 and n :/5 spacer cases. The maximum conductivity at room temperature is located at [Li ]/ 0.4 mol l 1. A factor of ten separates the highest 25 8C conductivity from that obtainable with free salt solution. Considering that the polyanion mass is large enough so to render it immobile in the solution, resulting in the conductivity being due entirely to Li cations, these results are considered encouraging. The effect of different EC /PC plasticizer compositions on the conductivity of P(LiOEG 5 B) was also studied (Fig. 3). As for the solutions, it is seen that for T/25 8C the pure EC plasticized polymob has the highest conductivity, while the pure PC plasticized 2.4. Measurements All the conductivities of the viscous mixtures and the gel electrolytes in this work were measured by complex impedance spectroscopy during slow cooling from 120 to /50 8C, using dip-type conductance cells with cell constants between 0.3 and 0.7, calibrated with a 0.1 m KCl aqueous solution. The automated impedance spectroscopy system has been described in previous papers [10,11]. The electrochemical stability of the plasticized electrolytes was measured by cyclic voltammetry, using a three-electrode dip type cell, as described previously [10]. Fig. 1. Temperature dependence of ionic conductivity of EC/PC (1:1 by mol) plasticized P(LiOEG 3 B) with different EC/PC content which is shown in the legend as weight percentage.

3 W. Xu, C.A. Angell / Electrochimica Acta 48 (2003) 2029/ moves to lower temperatures as PC is added, but only at the cost of conductivity decreases Plasticization of polymobs by other lower viscosity solvents Fig. 2. The room temperature conductivity of P(LiOEG n B)s with different anionic spacings n/3, 5, 9 and 14, dissolved in EC/PC (1:1 by mol) plasticizer as a function of lithium concentration. Comparison is made with the conductivity of simple LiBOB solutions in PC. Note that the lowest point of the conductivity /concentration curve for each n value is corresponding to the polymob without any plasticization. P(LiOEG 5 B) has the lowest conductivity. The sharp drop of ionic conductivity of EC plasticized P(LiOEG 5 B) is due to the crystallization of EC. This The conductivity can be increased above the maximum value in Fig. 2 especially by use of non-carbonate plasticizers, e.g. DME. Fig. 4 shows the isothermal conductivity at 25 8C of PC/DME (1:1 by wt.) plasticized P(LiOEG n B) with n/3, 5 and 9. It is found that ionic conductivity as high as 10 3 S cm 1 at room temperature can be obtained for polymobs with short spacing of n/3 and n :/5. Such conductivity is high enough for the polymobs to be applied in practical electrochemical devices such as lithium rechargeable batteries. Cells using these electrolytes will not suffer from concentration polarization, hence should serve well for higher power applications that are usually limited by concentration gradients as the cathode/ electrolyte interface. EC/DMC (1:2 by wt.) and pure PC were also used to plasticize the n/3 polymob, i.e. P(LiOEG 3 B). Fig. 5 compares the concentration dependence of 25 8C conductivity for plasticization by the above-mentioned four plasticizers or plasticizer mixtures for this case. It is seen that the EC /DMC mixture is also a favorable plastici- Fig. 3. Temperature dependence of ionic conductivity of plasticized P(LiOEG 5 B) by different EC/PC compositions. The plasticizer content is 80 wt.% in all plasticized electrolytes.

4 2032 W. Xu, C.A. Angell / Electrochimica Acta 48 (2003) 2029/2035 conclude that polymob dissociation remains high in DME- and DMC-containing solutions although either DME or DMC has a rather low dielectric constant (o 25 8C is 6.99 for DME and 3.1 for DMC) compared with EC (o 40 8C is 89.6) and PC (o 25 8C is 64.4). Evidently the decreased viscosity is the dominant effect on the high conductivity of the DME- and DMC-containing solutions plasticized systems (h 25 8C is 0.42 for DME, 0.59 for DMC, 2.51 for PC and h 40 8C of EC is 1.85). On the other hand, the interesting variation of the conductivity with salt concentration for the EC /DMC plasticized system, which we have not seen before, must be due to the competition between the dissociation and viscosity effects, coupled to the non-ideal mixing of EC with DMC (due to their very different dielectric constants). Usually s versus c plots are usually smooth curves passing through a maximum (as for n :/9 in Fig. 4). Fig. 4. The room temperature conductivity of P(LiOEG n B)s with different anionic spacings n/3, 5 and 9, dissolved in PC/DME (1:1 by wt.) plasticizer mixture with the variation of lithium concentrations. Comparison is also made with the conductivity of simple LiBOB solutions in PC. Note that the lowest point of the conductivity/ concentration curve for each n value corresponds to the plasticizerfree polymob Electrochemical properties The electrochemical properties of EC /PC plasticized polymob were investigated using cyclic voltammetry. Figs. 6 and 7 show lithium deposition-stripping process and electrochemical oxidation of EC /PC (1:1 wt.) plasticized P(LiOEG 3 B) on the different working electrodes platinum (Pt), SS, aluminum (Al) and copper (Cu), respectively. Except for Pt, all these electrodes show very good lithium deposition-stripping characteristics. The cyclic voltammogram of P(LiOEG 3 B)/EC/ PC on Pt shows two oxidation peaks between 0 and 1 V, presumably due to the usual Li /Pt alloy formation. The electrochemical oxidation voltages of the plasticized polymob is 4.50 V versus Li /Li for Pt, 4.95 V for SS, 4.30 V for Al and 3.67 V for Cu, respectively, according to Fig. 7. It is seen that Al and Cu could be used as the collector electrodes for anode and cathode, respectively Crosslinked polymobs Fig. 5. The room temperature conductivity of P(LiOEG 3 B) plasticized by different solvents and solvent mixtures with the variation of lithium concentrations. Comparison is also made with the conductivity of simple LiBOB solutions in PC. Note that the common low point of the conductivity/concentration curve for each plasticizer corresponds to the polymob without any plasticization. zer as well as the PC/DME mixture, yielding the room temperature conductivity very close to 10 3 S cm 1. Optimization of the EC /DMC ratio may well yield s 25 8C /10 3 S cm 1. From the fact that the ionic conductivity of PC / DME and EC/DMC plasticized P(LiOEG 3 B) is higher than that of EC /PC and PC plasticized electrolyte, we As mentioned in our previous report [9], polymobs have molecular weight below 5000 Da due to the exponentially decreasing probability of chain-end with chain-end reaction. The residual terminal /OH s can, however, react with metallic Li when the polymer is used in lithium ion batteries, causing poor cell performance. In this work, we have taken advantages of the chain ends to crosslink the original polymob chains and convert sticky liquids into stiff rubbers. This is achieved by using a reaction with a stronger driving force than that of the original polymerization reaction. This is the reaction between the /OH chain ends and the BH 4 anion which joins four chain-ends at a network center, as shown in the following reaction. The crosslinked versions of P(LiOEG n B)s have much larger molecular

5 W. Xu, C.A. Angell / Electrochimica Acta 48 (2003) 2029/ Fig. 6. Lithium deposition-stripping process of 80% EC/PC (1:1 by wt.) plasticized P(LiOEG 3 B) on platinum, SS, aluminum and copper electrodes. weights and accordingly different rheological properties. 4OHBH 4 0 B(O) 4 4H 2 On the other hand, due to the reducing effect of BH 4 anion, part of the C/O groups on the mono-oxalato borate rings may be reduced to C/OH groups, even at very low temperature (/78 8C). However the ring structure has not been broken and the C/OH groups formed in the above process will immediately react with the BH 4 anion to form new chains. The decrease in the number of C /O groups on the mono-oxalato borate rings decreases the polarity of the ring structure which in turn decreases the number of free cations. Thus, the conductivity goes down. Fig. 7. Electrochemical oxidation voltage of 80% EC/PC (1:1 by wt.) plasticized P(LiOEG 3 B) on platinum, SS, aluminum and copper electrodes.

6 2034 W. Xu, C.A. Angell / Electrochimica Acta 48 (2003) 2029/2035 Fig. 8. Temperature dependence of ionic conductivity of LiBH 4 - crosslinked P(LiOEG n B) with different length of ethyleneoxy repeating units, compared with that of non-crosslinked P(LiOEG n B). Extensive crosslinking also severely decreases the ionic conductivity of the polyelectrolytes, as shown in Fig. 8, due to the decrease in segmental mobility caused by the decrease in configurational entropy. However, such decrease in conductivity can be compensated by plasticization. The plasticized crosslinked polymers are gel-like materials at very low plasticization (less than 10 /20 wt.% plasticizer content depending on the oligoether spacer length), viscous liquids at moderate plasticization (20 /50 wt.% plasticizer content) and again fluid in heavily plasticized form (above 50 wt.% plasticizer). If the crosslinking is carried further by addition of excess LiBH 4, even highly plasticized polymers (up to 50 wt.% plasticizer) remain solid-like (gels). The ionic conductivity of the plasticized crosslinked polymobs P(LiOEG 5 B) was studied as a function of temperature and the results are shown in Fig. 9, where they are compared with the conductivity for polymers before crosslinking. It is seen that the gelled crosslinked polymob should have very low ambient conductivity, less than 10 6 S cm 1. However, a high conductivity of S cm 1 at room temperature has been achieved in a 90 wt.% EC /PC (all carbonates) plasticized crosslinked electrolyte, which is twice the maximum in plasticized non-crosslinked cases. These show good potential for applications in lithium ion rechargeable batteries. For heavily plasticized polyanionic electrolytes, the LiBH 4 -crosslinked electrolytes have slightly higher conductivities than non-crosslinked cases. This may be due to their higher lithium ion concentration, which causes from both polymob and the crosslinker LiBH 4. Fig. 9. Temperature dependence of ionic conductivity of EC/PC (1:1 by wt.) plasticized LiBH 4 -crosslinked P(LiOEG 5 B) with different content of plasticizer. The 30 wt.% EC/PC plasticized electrolyte is a very viscous liquid at room temperature and when the EC/PC content is above 50 wt.% the electrolyte is fluid Gel electrolytes The liquid state of the heavily plasticized (non-crosslinked) polyanionic electrolytes can be transformed into gel electrolytes by adding certain amount of high molecular weight polymers. In this work, PMMA with molecular weight of (from Aldrich) and high boiling point plasticizer mixture EC /PC (1:1 by wt.) were employed. It is known from the above results that the maximum conductivity at room temperature is , i.e. 4.0/10 4 S cm 1, for 90 wt.% EC/PC plasticized P(LiOEG 3 B). Therefore, the gel formation Fig. 10. Temperature dependence of ionic conductivities of n% PMMA/(100/n)% [10% P(LiOEG 3 B)/90% EC/PC] gel electrolytes, compared with that of 10% P(LiOEG 3 B)/90% EC/PC (1:1 by wt).

7 W. Xu, C.A. Angell / Electrochimica Acta 48 (2003) 2029/ Conclusions Fig. 11. Effect of PMMA content on isothermal conductivities of PMMA/P(LiOEG 3 B)/EC/PC system, which was formed by addition of PMMA into 90% EC/PC plasticized P(LiOEG 3 B) solution. and conductivity of the P(LiOEG 3 B)/EC/PC system were studied by adding different PMMA content into the 90 wt.% EC/PC plasticized P(LiOEG 3 B) solution. It is found that when PMMA content is less than 13 wt.%, such as 2, 4, 7 and 10 wt.% studied, the system is not a homogeneous but a phase separation mixture with the polymer coagulation floating in the solution. When the PMMA content is higher than 13 wt.%, a gel is formed. Fig. 10 shows the temperature dependence of the PMMA/P(LiOEG 3 B)/EC/PC gels and Fig. 11 shows the isothermal conductivity of the gel system with the content of PMMA. When 13 wt.% PMMA is added, the room temperature conductivity decreases from , i.e. 4.0/10 4 S cm 1 to , i.e. 2.2/10 4 S cm 1. It should be noted that this gel electrolyte is actually a single lithium ionic conductor. Such a conductivity value is relatively high for single ion conductors. Therefore, it has been shown that this kind of gel electrolytes can be used in lithium and lithium ion batteries requiring single cation conduction. In heavily plasticized forms of polymob and LiBH 4 - crosslinked polymob, the ionic conductivity reaches 10 3 S cm 1 at room temperature. The electrochemical stability window is up to 5 V on SS electrodes. However, these plasticized electrolytes are not a gel, but fluid because the polyanions are of low molecular weight. This may be compensated by incorporation of high MW PMMA into the solution. Then freestanding gel electrolytes can be obtained and the single ionic conductivity is still as high as 2/10 4 S cm 1 at room temperature. By choosing different plasticizers or plasticizer mixtures and different kind of high molecular weight polymers, and optimizing the composition of electrolyte systems, the gel electrolytes with high single ionic conductivity and good cell performance can be obtained. Acknowledgements This work was supported by Mitsubishi Chemical Corporation of Japan. References [1] G.B. Appetecchi, F. Croce, B. Scrosati, Electrochim. Acta 40 (1995) 991. [2] H.S. Choe, B.G. Carroll, D.M. Pasquariello, K.M. Abraham, Chem. Mater. 9 (1997) 369. [3] K.M. Abraham, Z. Jiang, B. Carroll, Chem. Mater. 9 (1997) [4] K.M. Abraham, H.S. Choe, D.M. Pasqyariello, Electrochim. Acta 43 (1998) [5] G.B. Appetecchi, F. Croce, B. Scrosati, Prog. Batteries Battery Mater. 17 (1998) 68. [6] D.-W. Kim, B.-K. Oh, Y.-M. Choi, Solid State Ionics 123 (1999) 243. [7] D.-W. Kim, B.-K. Oh, J.-H. Park, Y.-K. Sun, Solid State Ionics 138 (2000) 41. [8] W. Xu, C.A. Angell, Solid State Ionics 147 (2002) 295. [9] W. Xu, M.D. Williams, C.A. Angell, Chem. Mater. 14 (2002) 401. [10] W. Xu, C.A. Angell, Electrochem. Solid State Lett. 4 (2001) E1. [11] W. Xu, J.-P. Belieres, C.A. Angell, Chem. Mater. 13 (2001) 575.