Polymer electrolytes based on crosslinked silylated poly-vinyl-ether and lithium perchlorate

Solid State lonics 72 (1994) 140-146 North-Holland

SOLID STATE IOIIICS

Polymer electrolytes based on crosslinked silylated poly-vinyl-ether and lithium perchlorate

Maria Andrei *, Luca Marchese, Arnaldo Roggero Eniricerche S.p.A., Via Maritano 26, 20097 S. Donato Milanese, Milano, Italy

Paola Prosperi Dipartimento di Chimica, Universita "La Sapienza'" P.le A. Moro 5, 00185 Roma, Italy

Poly-vinylethers (PVE) containing triethoxy-silyl groups in the side-chains were synthesised and crosslinked during membrane processing in the presence of LiCIO4. The electrolytic membranes had excellent dimensional stability and showed promising ionic conductivity, approaching the value of 10-5 S/cm at 25 ° C. The effect of the side chain length, in term of EO units, and of the salt concentration on conductivity was also investigated.

1. Introduction

Polymer electrolytes have attracted considerable interest in the last few years because of their poten- tial applications as electrolytic membranes in pri- mary and secondary high energy density lithium batteries. Poly-ethyleneoxide (PEO) and lithium salt complexes were extensively studied in high temperature applications but their low ionic conductivity and poor mechanical properties below 65°C (cor- responding to crystalline-amorphous phase transition) have hindered their use in room temperature applications.

Different strategies were adopted to enhance the conductivity at room temperature; frequently ma- cromolecules with oligo-oxyethylene units in the main or side-chains were explored [ 1 ]. Examples of this type of materials are comb-like polysiloxanes [2], polyphosphazenes [ 3 ], polymethacrylates [ 4 ], poly- itaconates [5], leading ethylene-oxide side chains.

Our recent studies on electrolytical membranes based on comb-like poly-vinylethers (PVE) and lithium salts have shown the promising ionic conductivity of these systems [6,7 ]. Using these fully amorphous polymeric matrices conductivity (~) up to

* To whom correspondence should be addressed.

0167-2738/94/$ 07.00 © 1994 Elsevier Science B.V. All rights reserved.

10 -s S/cm at room temperature were reached, but the dimensional stability of the membranes was poor.

Improvements of the mechanical properties, without compromising the ionic conductivity and the thermal properties of the material, were achieved in chain-extended PVE, by copolymerizing mono and divinyl-ethers, whose employment allowed to increase the molecular weight. The product was very sensitive to monomer's ratio, so that the polymer could be actually crosslinked, and then insoluble, if the amount of chain-extender exceeded a certain value.

To overcome processing problems due to insoluble PVE, copolymers containing triethoxy-silyl groups were synthesised, activated for crosslinking during the electrolytical membrane processing [8 ]; the introduction of flexible siloxane knots affected posi- tively the thermal properties of the final membranes, without compromising their ionic conductivity.

The method reported here provides a straightfor- ward pathway for obtaining electrolytic membranes without employing casting procedures.

M. Andrei et al. /Polymer electrolytes 141

2. Experimental

2.1. Synthesis of vinyl-ethers (I)

CI-I2 = CHO (CH 2 CH20 ) n-CH3

n = 2 , 3 , 4 , 5. (I)

The procedure followed for the synthesis of the vinylether with n = 2 is reported as an example. Ethyl vinyl ether ( 1.8 mole), diethyleneglycolmonomethyl ether (0.6 mole) and mercuric acetate (0.0057 mole) were mixed together, heated to reflux temperature and maintained for 10 h. The reaction was extin- guished by adding potassium carbonate and the mixture was distilled to separate the vinylether from the unreacted diethyleneglycol monomethylether. The purity of vinyl ether was greater than 99% and the yield was about 80% on the initial glycol. The structure was confirmed by NMR, FTIR and mass spectroscopy.

1H NMR (CDCls): d 3.5-3.85 (m, CH2CH20), 3.9-4.2 (m, CH=CH2), 6.5 (q, OCH=CH2) ppm;

FTIR (neat): 1636 and 1620 (C=C) cm -~.

2.2. Synthesis o f allylvinylether (II)

CH2 = CHO (CH2 CH2 O)2-CH2 CH = CH2 . (II)

A mixture of DMSO (250 ml), ground KOH (0.97 mole) and distilled allyl alcohol (0.5 mole) was stirred for one hour at room temperature, then ethylene chlorohydrin vinyl ether (0.415 mole) was dripped in slowly. The mixture was heated to 80 ° C, and maintained for two hours until completeness, checked by GC analysis. The mixture was then poured into cold water (500 ml) and extracted with methylene chloride. The allyl-vinylether was re- covered as colourless liquid, by distillation (70% yield, 99% purity). The structure was confirmed by mass, NMR and FTIR spectrometry.

IH NMR (CDC13): c~ 3.5-3.85 (m, CH2CHzO), 3.91-3.95 (m, OCH2CH=CH2), 4.0-4.2 (m, OCH=CH2), 5.1-5.95 (m, CH2CH=CH2), 6.5 (OCH = CH2) ppm.

FTIR (neat): 3082 (CH=CH2), 2927 and 1422 (OCH2CH=CH2), 927 (=CH2) cm-l .

2. 3. Copolymerisation of vinyl ether (I) and allyl vinyl ether (II)

The functionalised copolymer was obtained by cationic polymerization according to the pathway reported in scheme 1.

The vinyl ether (I) (58 mmoles) and the allyl vi-

100-ct CH3~.O CH2~CH2~ ? CH:CH 2

n=2,3,4,5

+

(I)

o~ CH 2 CH CH2[~O~CH2 CH2~ ? CH=CH 2

-78°C [ BF3Et20 (H) ! CH2CI2

......... ~ CH2- C H ~ C H 2 CH'~ . . . . . . . . . . . . .

100 ~ a

CH3~O CH 2 CH2In

O O

[CH 2 CH 2 0 ] C H 2 CH=CH 2

Toluene HSi(OEt)3 II0°C H2PtCI 6

100-c~ : ct

O O

CH3~ O CH2 CH2] n [CH2-CH2-O~CH 2 C H 2 ~ " 2 Si (OEt)3

Scheme 1. Synthetic pathway of PVE-silylated polymers.

142 M. Andrei et al. / Polymer electrolytes

nyl ether (II) (1.16 mmoles), dissolved in anhydrous methylene chloride (20 ml), were introduced into a reactor equipped with a spiral mechanical stir- rer and reagent feed inlets. After cooling to - 7 8 ° C , the catalyst BF3Et20 (0.63 mmole), dissolved in 2 ml of solvent, was added under stirring. The viscos- ity was observed to increase immediately after catalyst addition and after two hours turbidity was noticed. The polymerisation was quenched adding an excess of methanol and the product, taken up in methylene chloride, was poured into 100 ml water- bicarbonate mixture; the organic phase was then sep- arated, washed with water and finally dried at 60 °C under vacuum, obtaining a colourless sticky product. IH and ~3C NMR spectra evidenced the presence of the allylic groups in the copolymer and the disap- pearance of vinyl bonds; moreover the ratio between the aUylic double bonds and the methoxy pendant groups of vinylether (I) gave the relative percentage of the two comonomers in the polymer. DSC analysis confirmed that the polymer was amorphous (Tg= - 7 1 °C).

~H NMR (CDC13): a 1.52 (m, CH-CHzCH polymer backbone), 3.32 (s, CH30), 3.49 (m CHzCH20), 5.00 (m, CH2CH=CH2), 5.05-5.77 (m, CH2CH=CH2) ppm;

FTIR (neat): 1643 (C=C), 949 (=CH2) cm -I .

2.4. Hydrosilylation o f the copolymer

A copolymer solution, 5 g in 20 ml of anhydrous toluene, was introduced into a tailed test tube in a nitrogen atmosphere, then an excess of triethoxy silane (2.5 mmoles) (4/1 over the molar double bond content) was added; finally 40 tll of a 3.3% hexa- chloroplatinic acid solution (isopropanol) were in- jected. The reactor was hermetically sealed and the reaction performed at 100°C for seven hours; the solvent and the excess triethoxy silane were removed by heating at 45 ° C under vacuum [9 ]. The polymer preserved the same appearance as the starting material and it was only slightly yellow coloured.

FTIR and NMR analysis evidenced the disap- pearance of allylic double bonds, the presence of the bonded triethoxy-silyl group and the absence of Si- ll terminations. DSC analysis, performed on copolymer before and after silylation, indicated the absence of crystallization peaks in the range of tem-

perature investigated ( 143-373 K); besides silylated copolymers and chain extended-PVE had comparable Tgs.

IH NMR (CDC13): ~ 0.4-0.7 (m, CHzSiO), 1.2 (t, CH3CH2OSi), 1.75 (m, CH2CHzSi), 3.9 (q, CH3CH2OSi ) ppm;

FTIR (neat): 1377 (SiCH2) cm ~.

2.5. Membrane processing

The electrolytic membranes were prepared in a dry- box dissolving the functionalised copolymer in acetonitrile (0.5 g in 10 ml of solvent) already containing LiCIO4. A small amount of slightly acidified diethyleneglycol ( 10-20 ~1) was added and the mixture was left to homogenise at room temperature. Fi- nally the solution was poured into a Teflon mould and the volatile components were removed in an ar- gon forced circulation chamber. The crosslinked membrane was heated to 40°C under vacuum and left overnight. The absence of O - H stretching was checked by FTIR.

Following this procedure transparent, homogene- ous and easily handled membranes, about 100 IJm thick, could be obtained. The occurrance of crosslinking was confirmed by the evident improvement in the mechanical consistence of the membrane and by its insolubility in acetonitrile.

2.6. Conductivity measurements

The ionic conductivity of the electrolytic membranes was determined by impedance measurements employing a frequency response analyser (FRA by Solartron model 1250) in the range of temperature 8 0 - ( - 20) ° C, using cells with stainless steel block- ing electrodes for ionic transport and a teflon spacer to control the thickness.

3. Results and discussion

Crosslinkable PVEs were obtained according to the pathway reported in scheme 1; the triethoxy-silyl groups were introduced in the final step and they be- came the flexible knots of the resulting PVE network during the membrane processing, by reacting with a diol.

M. Andrei et aL /Polymer electrolytes 143

- 3 . 5

log a [ E O / L i - 10 ] (S / cm)

- 4

[

[

- 4 . 5

I 5

. . n -3 c h a i n ex t . | L_

I n -2

_ 6 [ , 2 .8 2.9 3 3.1 3.2 3 .3 3 .4 3 .5

I O 0 0 / T (K- l )

Fig. 1. C o m p a r i s o n o f log a o f P V E - s i l y l a t e d a n d P V E - c h a i n ex-

t e n d e d / L i C l 0 4 complexes .

The Tg values of the lithium complexes and their precursors are reported in table 1. Tgs were comparable to those measured in chain-extended PVE,

even though the dimensional stability of silylated system was notably improved: these membranes could be easily handled whereas those tended to be sticky.

In fig. 1 the ionic conductivity (log a) of the electrolytical membranes based on silylated-PVE and chain-extended-PVE is reported, comparing systems having the same number of EO units in the side chains (n = 2 and n = 3) and equal salt concentration. The conductivities of analogous systems were comparable in all the range of temperatures: the introduction of siloxane bonds has then a positive effect as the dimensional stability of the membranes is improved without compromising thermal and con- ductive properties.

3.1. Influence of the side chain length

The influence of the side-chain length on conductivity was investigated by varying the number of ethylene-oxide (EO) units in the main monomer (I). Different materials having 2, 3, 4 and 5 EO units in the side-chains were synthesized, as in chain-extended PVE the conductivity was observed to increase continuously by lengthening the side-chain from 2 to 5 EO units [6 ]. The same trend was also observed in the silylated systems as the polymer hav-

T a b l e 1

G l a s s t r a n s i t i o n t e m p e r a t u r e s o f P V E - s i l y l a t e d l i t h i u m c o m p l e x e s a n d the i r c o r r e s p o n d i n g p recu r so r s

n a S y s t e m type Tg ( ° C ) E O / L i C I O 4 Tg ( ° C )

2 a l l y l - t e r m i n a t e d - 71 - -

2 s i l y l - t e r m i n a t e d - 62 - -

2 l i t h i u m c o m p l e x - 10 - 48



3 l i t h i u m c o m p l e x - I 0 - 36

3 l i t h i u m c o m p l e x - 16 - 49

3 l i t h i u m c o m p l e x 24 - 55



4 l i t h i u m c o m p l e x 10 - 45



5 l i t h i u m c o m p l e x - 8 - 35

5 l i t h i u m c o m p l e x 10 - 43

5 l i t h i u m c o m p l e x 16 - 51

5 l i t h i u m c o m p l e x 24 - 62

a n n u m b e r o f E O u n i t s in m o n o m e r ( I ) . R e m a r k s : all the l i t h i u m c o m p l e x e s a n d t h e i r c o r r e s p o n d i n g p recu r so r s are a m o r p h o u s .


- 3 .5

log a

(S /cm)

- 4

-4 .5 [

- 5

t

- 5 . 5 ' ~ a-2 I i - - rl-3

I r l-4

~ 11,5

- 6 l

2.8 3

L

3.2

IO00 /T (K "~)

3.4 3.6

Fig. 2. Temperature dependence of log a of PVE-silylated/Li- 004 complexes at different side chain length ( n ).

ing 5 EO units showed the better performances in term of a, in all the range of temperature: conductivity exceeding the value of 10 -s S/cm at 25°C was reached in this system (fig. 2).

The lengthening of the side-chain has, then, a considerable and positive effect on conductivity: this can be reasonably explained considering that longer chains are probably more flexible than shorter ones and this should facilitate the ion mobility through the polymer matrix.

3.2. Influence q f the salt concentration

The ionic conductivity can be further improved by choosing the salt concentration representing the best compromise between the ionic mobility and the number of charge carriers, factors that both contrib- ute to the conductivity.

To this purpose a screening of the conductivity, changing the EO/Li ratio, was carried out selecting the 3 EO and the 5 EO polymeric matrices.

The variation of cr versus 1 / T is shown in figs. 3 and 4 for the 3 EO and the 5 EO systems respec-

~3

log o

(S /cm) - 4

I

- 6

- 7

- 8

- - EO/Li=8

EO/Li-IO - 9

- ~ EO/Li=16 i I i -~-- EO/L,.2,

- 1 0 ' : 2.8 3 3.2 3.4

IO00 /T

\ \

\ \

3.6 3.8 4 4.2

(K "I)

Fig. 3. Temperature dependence of the log (7 of PVE-sily]ated (5 EO units in side chains)/LiC104 complexes at different salt concentration.

tively. Improvement in conductivity by lowering the salt concentration was observed and correspond- ingly a decrease in the T~ values was noticed for both systems, confirming the correlation between mobility of charge carriers and flexibility of the polymer backbone assisting their movement.

The decrease in temperature has a remarkable effect on the most concentrated system, that showed the highest conductivity at 60°C and the lowest at - 2 0 ° C ; moreover the decrease in conductivity with temperature was more marked in the 3 EO system than in the 5 EO one. Even though at high temperature the conductivity was almost the same in both materials, the more flexible system, having an op- timum side-chain length and showing the lowest T~, was less sensible to decreases in temperature: at - 2 0 ° C an improvement in conductivity of one or- der of magnitude was detected by passing from the 3 EO system to the 5 EO one.

The contribution of charge carrier's number of the conductivity seems then to prevail at high temperature, when the system is highly flexible, while the decrease in polymer chains flexibility and conse-

M. Andrei et al. ~Polymer electrolytes 145

- 3 -

log O i

i - 5 i

-6

-7 I i

- 9 ~ EO/Li-IO I EO/Li-16

I 1+-_ EO/Li-24

- 10 ~ ~ ~ 1 ~ 2.8 3 3.2 3.4 3.6 3.8 4 4.2

IO00/T (K "1)

Fig. 4. Temperature dependence of the log a of PVE-silylated ( 3 EO in side chains)/LiC104 complexes at different salt concentration.

log o (S/cm)

-5

- 6

-7

-8

o log o a t ~

l ogo a t ~

250

Tg (K)

240

230

j220

210

-9 -- ~ L ~ L ~ J 200 0 4 8 12 16 20 24 28 32

EO/Li

Fig. 5. Isothermal plots o f log a and Tz's trend against E O / L i rat io fo r 5EO-PVE-s i l y la ted /L iC l04 complexes.

quently in ionic species mobility becomes increasingly consistent by approaching the glass transition temperature.

Finally the isothermal behaviour of the conductivity at two different temperatures (20°C and - 2 0 °C) and the Tgs trend of lithium complexes, as a function of EO/Li ratio, are evidenced in fig. 5, considering the 5 EO system. At - 2 0 ° C conductivity and Tgs followed the known relationship: the lower the glass transition the higher the conductivity; at 20°C the trend was still the same but the differences were less pronounced and at higher temperatures they tended to disappear.

The flexibility of the system seems, then, to play an increasingly important role by lowering the temperature; nevertheless a different contribution of salt dissociation by decreasing the temperature cannot be completely neglected.

4. Conc lus ions

Improvement in the mechanical consistence of PVE-based electrolytic membrane was obtained by introducing in the side-chains silyl groups suitable to be crosslinked during the membrane processing. This method provided consistent and easy to handle membranes.

The introduction of siloxane bonds in the network, because of its high flexibility, contributed to maintain a low glass transition temperature, without depressing the conductivity with respect to analogous PVE unfunctionalised systems.

The influence of side-chain length on conductivity was clearly confirmed by the continuous increase of a by lengthening the side-chain up to 5 EO units. This effect demonstrates the importance of polymer chemical structure which modification can provide more flexible systems with low Tg and improved conductivity.

Electrochemical characterisation (stability to- wards lithium, stability window, lithium transfer- ence number) and the possibility of application of these systems as electrolyte membranes in lithium rechargeable batteries will be further investigated.


References

[1]J.R. MacCallum and C.A. Vincent, eds., in: Polymer Electrolytes Reviews-I and 2 (Elsevier Applied Science, London, 1987, 1989).

[2] R. Spindler and D.F. Shriver, Macromol. 21 (1988) 648; P.G. Hall, G.R. Davies, J.E. McIntyre, I.M. Ward, D.J. Bannister and K.M.F. Le Brocq, Polym. Commun. 27 ( 1986 ) 98; D. Fish, I.M. Khan and J. Smid, Makromol. Chem. Rapid Commun. 7 (1986) 115.

[ 3 ] P.M. Blonsky and D.F. Shriver, J.A.C.S. 106 (1984) 6854; H.R. AIIcock, P.E. Austin, J.X. Weenan, J.T. Sisko, P.M. Blonsky and D.F. Shriver, Macromol. 19 (1986) 1508.

[4]N. Kobayashi, T. Hamada, H. Ohno and E. Tsuchida, PolymerJ. 18 (1986)661.

[ 5 ] J.M.G. Cowie, M.Y. Pedram and R. Ferguson, Eur. Polymer J. 21 (1985)227; J.M.G. Cowie and A.C.S. Martin, Polymer 28 ( 1987 ) 627.

[6] S. Pantaloni, S. Passerini, F. Croce, B. Scrosati, M. Andrei and A. Roggero, Electrochim. Acta 34 (1989) 635; M. Andrei, L. Marchese, A. Roggero, S. Passerini and B. Scrosati, in: Second Intern. Symp. on Polymer Electrolytes, ed. B. Scrosati (Elsevier Applied Science, London, 1990) p. 107.

[ 7 ] J.M.G. Cowie, A.C.S. Martin and A. Firth, Br. Polymer J. 20 (1988) 247: J.M.G. Cowie and K. Sadaghianizadeh, Solid State lonics 42 (1990) 243.

[8] U.S. Patent 5.173.205 (1992). [9 ] K. Honda, M. Fujita, H. Ishida, R. Yamamoto and K. Ohgaki,

J. Electrochem. Soc. 135 (1988) 3151.

Documents

Polymer electrolytes based on crosslinked silylated poly-vinyl-ether and lithium perchlorate