Polypropylene separator grafted with hydrophilic monomers for lithium batteries

j o u r n a l of MEMBRANE SCIENCE ELSEVIER

Journal of Membrane Science 107 (1995) 155-164

Polypropylene separator grafted with hydrophilic monomers for lithium batteries Jean Luc Gineste, Grrald Pourcelly * Laboratoire de Mat~riaux et Procdd~s Membranaires, UMR 9987 CNRS, BP 5051, 34033 Montpellier Cgdex, France Received 5 October 1994; accepted 7 April 1995

Abstract Separator materials play an important part in lithium nonaqueous rechargeable battery operation. Their wettability limits the performances of batteries. To avoid wetting agents or undesirable substances to the electrode system, grafted separators are used. Acrylic acid (AA) and diethyleneglycol-dimethacrylate (DEGDM) are grafted onto 50 /xm polypropylene films. The physicochemical properties of the polymer films obtained are studied versus the characteristics of grafting. The influence of temperature and monomer content on grafting kinetics is pointed out. Cycling performances of secondary lithium batteries including these grafted films as separators are also presented. Keywords: Grafted separator; Kinetics of grafting; Hydrophilic monomer; Lithium battery; Cycling performances

1. Introduction The rapid increase in technology directed toward consumer use and the emphasis on portability and light weight in consumer electronic equipment have increased the demand for light weight high energy power sources. Lithium nonaqueous storage batteries meet this demand. The range of applications of such a rechargeable lithium nonaqueous cell is considerable, including, for instance, power sources for cordless telephones, portable computers, etc. Moreover, they can store up more energy than classical Pb or Ni-Cd batteries. Their cycling performances depend not only on the element composition (electrodes, electrolyte, separator), but also on the technology used. The cathodic material is usually a transition-metal oxide ( C r 3 0 7 , CoOz, MnO2, NiOz, V2Os., etc.) [ 1,2]. These oxides present a good insertion-extraction reversibility toward * Corresponding author. 0376-7388/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10376-7388(95)001 12-3

lithium. The separator is a microporous olefinic polymer film (polyethylene or polypropylene) [3]. The electrolytes are lithium salts (LiBF4, LiAsF6, LiCIO4) solubilized in mixed solutions of esters (polypropylene carbonate, ethylene carbonate, etc.) and low viscosity solvents (dimethoxyethane, dioxolane) [4,5 ]. The main problem encountered in the development of secondary Li batteries is the Li instability and its high reactivity toward the other element components, specially the electrolyte [6]. From a thermodynamic standpoint, Li will react with all materials, inorganic as well as organic and the stability of Li in Li-batteries is conferred by the passivation film formed on the Li surface [ 7 ]. Esters, such as propylene carbonate and ethylene carbonate, are known to be high dielectrical solvents. Electrolytes composed of esters, ethers and lithium salts as the solute, exhibit high Li cycling efficiency on V205 cathode materials. For a high concentration of Li salt, the ether content, which acts as wetting

J.L. Gineste, G. Pourcelly/Journal of Membrane Science 107 (1995) 155-164

156

agent, must be important (more than 50 vol%) to obtain a good wettability of the separator toward this kind of electrolyte. However, ethers are inflammable, volatile and, moreover, they crystallize at low temperature [7]. It is therefore important to optimize their concentration to keep a good wettability of the separator. One of the solutions consists in making the surface of the separator pores hydrophilic by means of a radiochemical grafting of vinylic monomers [8,9]. In the present study, we present on the one hand the radiochemical grafting of a monofunctional monomer (acrylic acid, AA), in the presence of a difunctional crosslinking agent (diethyleneglycol dimethacrylate, DEGDM) on a polypropylene (PP) microporous matrix, and, on the other hand, the characteristics of grafted separators. The performances of secondary Li batteries including these separators are also presented.

2. Experimental 2.1. Synthesis of grafted separators

Polypropylene film (Celgard 2502 from Celanese Co.) of 50/zm thickness was employed as trunk polymer. It was previously irradiated in air by electron beams with a dose ranging from 0.5 to 4 Mrad. The irradiated film was then immersed in a grafting solution composed of monomers (AA or/and DEGDM), metallic salt (CuSO4) and solvent ( water or/and methanol) [ 10-12]. Nitrogen was used to eliminate oxygen which acts as a grafting inhibitor. The temperature of the grafting solution was regulated from 45 to 75°C according to the experiment. Grafted films were then removed from the grafting solution and washed with water and methanol to eliminate homopolymer. After drying under vacuum, the film was weighed and its grafting yield GY was calculated from: GY

M-M°x M

100

2.2. Characterization of grafted films Exchange capacity The principle of the method has already been described [ 13 ]. As DEGDM cannot produce exchanging groups, the determination of the exchange capacity enables the determination of the AA content in a film grafted with AA + DEGDM. Electrical resistance The device consisted of a Teflon cell and a variable frequency conductimeter (CD810 from Solea Tacussel Co.). The experimental cell was composed of two symmetrical half-cells between which the grafted film in contact with mercury was clamped, as illustrated in Fig. 1 [14]. Cycling performances The aim was to study the influence of the different electrolyte components on the cycling reversibility. Li charge-discharge tests were carried out galvanostatically with the cell shown schematically in Fig. 2. The separator was placed in a test-cell having two electrodes, the first one in lithium, the second one in nickel on which a known amount of lithium, Qi, was deposited. The number of cycles obtained enables the calculation of the lithium cycling efficiency ELi through: ELi = ( 1

Qc) x 100 NQc

(2)

Qi-

L

JL -~i"["'. "." •f

"

"

":

-""

,,o,

.

.

[] .... :'f .... "

"'-~"

~ ~

"v ...." I

° . ""

.

(1)

where Mo and M are the mass of the dried film before and after grafting, respectively.

Fig. 1. Measurementcell of electrical resistance: C, PTFE cell; M, membrane;E, platineelectrodes;Hg, mercury.

J.L. Gineste, G. Pourcelly / Journal of Membrane Science 107 (1995) 155-164

£

157

into a cylindrical container which was filled with electrolyte. The cycling conditions were the following: 1. The charge was achieved with a current of 100mA, corresponding to a charge density of 0.254).30 m A / c m 2, until a 3.8 V-voltage was reached. This charge density depends on the Li surface in front of

I

V205. 2. The discharge was achieved with a current of 200 mA (0.54).6 m A / c m 2) until a 2.8 V-voltage was reached. The Li cycling efficiency ELi' is given by:

D

fiLl Fig. 2. Schematic representation of lithium cycling cell. 1, Electrolytic solution; 2, metallic support; 3, nickel electrode; 4, lithium counter-electrode; 5, glass fiber felt.

where Qo, is the amount of Li exchanged during each cycle and N is the number of cycles.

Cycling in rolled cylindrical cell structure The characterization of the grafted film performances was achieved using rolled cylindrical cell structure from Saft Co. [ 15]. This technology was used for cylindrical Ni/Cd batteries or for primary Li batteries. The electrodes and the separator were put together and rolled into a cylindrical shape. The roll was introduced

QTOT,d -- QLi X 100

(3)

QTOT,d

where QTOT,d ( = "~Qdi) is the total discharged capacity, which is the sum of the capacities discharged at each cycle, QL~ is the initial capacity corresponding to the mass of Li initially introduced in the generator: (QLi = m × 3.86 A h/g). The performances of a cycling element are usually expressed by the figure of merit (FOM), defined as the ratio of the total discharged capacity on the Li initial capacity:

QTOT,d

FOM = - -

(4)

QLi

3. Results and discussion A/

100

3.1. Synthesis of grafted separators

90 80 o~ v -o 70

/

>" 60

so

(.9

40 30 20 10 :

0

2

4

~

I

I

i

~

I

i

I

6 8 10 12 Reaction time (hours)

i

I

14

I

I

16

Fig. 3. AA grafting kinetics. Grafting solution: AA 22 vol%, solvent, water/methanol; 78 vol%, copper sulfate, 6 10 -4 g/l, Grafting temperature: 65°C; (a) 45°C; (b) 55°C; (c) 65°C; (d) 75°C.

The grafting mechanism ot AA onto a microporous PP film in the presence or not of DEGDM, is similar to that obtained onto an olefinic or fluorinated dense film [ 10-12 ]. Grafting begins at the film-solution interface. Once the surface has been swollen by the grafting solution, the accessibility of peroxidic sites located more deeply in the film becomes possible. So, grafting goes on step by step inside the film. The only difference between grafting onto dense or microporous film consists in the grafting propagation mode. As grafting is a reaction essentially controlled by the diffusion mechanism of monomers through the polymer matrix, this diffusion will be facilitated by the porosity of the matrix. Fig. 3, Fig. 4 and Fig. 5 show the influence of temperature on grafting kinetics for AA, DEGDM and

158

J.L. Gineste, G. Pourcelly / Journal of Membrane Science 107 (1995) 155-164

220 200

a •

180 160

•

~120 .~_ 100 o"~ 80

~ f ~ J ~ K

o~O

60 40

/

/x_/

/t

/

b _i

20 i i ~ " A - - l ~ k ]

. . . . . . .

0 ,,~'-'---', '~ I i ~ i ! i I 0 5 10 15 20 25 Reaction time (hours)

i

[] I 30

i

I 35

Fig. 4. DEGDM grafting kinetics. Grafting solution: DEGDM 22 vol%, solvent, water/methanol; 78 vol%, Mohr' salt, 0.2 g/l; Methylene Blue, 0.09 g/1.Graftingtemperature 65°C; (a) 45°C; (b) 55°C; (c) 65°C; (d) 75°C. 200 175

b.__A

.i j 150

4

~125

~

/

~100

_/

•

d

o--.lt~/--,~

X~tl/ 0 ~ ll~i Ii I I

•

2s lt/i 0

5

q I I I 10 15 20 Reaction time (hours)

~

i 25

I

I 30

Fig. 5. AA + DEGDM grafting kinetics. Grafting solution: AA, 18 vol%, DEGDM,4 vol%, solvent, water/methanol; 78 vol%, copper sulfate, 6 10-2 g/l. Graftingtemperature 65°C; (a) 45°C; (b) 55°C; (c) 65°C; (d) 75°(2. AA + DEGDM, respectively. The grafting yield curves versus the reaction time present three regions according to the temperature or the monomer composition of the grafting solution:

1. The first region located near the origin corresponds to the initiation and to a more or less fast grafting rate. This grafting rate depends on the composition of the grafting solution (monomer and homopolymerization inhibitor concentrations, nature of the solvent, etc.), on the density of peroxidic functions onto the irradiated film, but also on temperature. 2. In the second region, the grafting rate decreases progressively till a constant value of the grafting ratio is reached. This phenomenon is attributed to a decrease of the amount of peroxidic functions and to a decrease of the monomer diffusion rate through the grafted layers. 3. The third region corresponds to a plateau where all the radical sites have been used for the grafting reaction. From this time, an increase of the grafting time no more affects the value of the grafting yield. An increase of temperature is accompanied by an increase of the grafting rate, regardless of the composition of the grafting solution. This fact may be explain by a double effect of temperature on the grafting reaction. The grafting reaction follows a radical mechanism with initiation, propagation and termination steps. An increase of temperature therefore increases initiation, i.e. peroxide decomposition, but also propagation through an easier diffusion due to a decrease of the solution viscosity. On the contrary, the evolution of the final grafting yield versus temperature is more complex and varies according to the composition of the grafting solution. In the case of AA-grafting (Fig. 3), the grafting yield reaches a plateau regardless of the temperature. However, this plateau is reached more quickly when the temperature increases. This time is close to 40, 25, 8 and 3 h for temperatures of 45, 55, 65 and 75°C, respectively. For similar experimental conditions, the maximum grafting yield was obtained for a temperature close to 55°C. In the case of DEGDM grafting (Fig. 4), the shape of grafting kinetics is different. The three regions cannot be distinguished so easily. Grafting goes on and never reaches a real plateau. An increase of temperature leads to an increase of both the grafting rate and the final grafting yield (FGY). To explain the variations of FGY versus temperature, the parameters which control this final grafting yield must be taken into account. FGY depends, on the one hand, on the number of reactive sites able to initiate the

J.L. Gineste, G. Pourcelly / Journal of Membrane Science 107 (1995) 155-164

grafting reaction (i.e. to the radiation dose), and also, on the chain length of the grafted polymer. For AAgrafting, an increase of temperature up to 55°C favours monomer diffusion by a decrease in viscosity. Beyond this temperature, this decrease of viscosity leads to a higher mobility of molecular chains, favouring their recombination. This effect acts against monomer diffusion. The differences observed between DEGDM and AA grafting may be explained by: 1. The double reactivity of DEGDM, which is a difunctional crosslinking agent. If, as for AA-grafting, an increase of temperature leads to decrease of viscosity and recombination reactions, in this case, polymerization may go on with the second reactive function as illustrated in Fig. 6. 2. The difference between the grafting mechanisms; the grafting of DEGDM only occurs on the pore surface ofpolypropylene film, whereas, those of AA occurs as well on the surface as within the polymer matrix. For instance, on dense polyethylene or poly(ethylene-tetrafluoroethylene) copolymer, the grafting yield never exceeds 20 wt% regardless of the experimental conditions [ 10-12]. As DEGDM does not bear ionizable functional groups, the surface of the film does not swell in the grafting solution and DEGDM cannot reach peroxidic sites located more deeply within the film. In the case of AA + DEGDM grafting (Fig. 5), an intermediate behavior is observed. The values of the overall activation energy Ea of the grafting reaction are reported in Table 1. Ea increases with DEGDM content in the grafting solution. E~ can be decomposed into partial energies of initiation, propagation, termination, viscosity and diffusion [ 16]. Considering the values of Ea obtained with an olefinic matrix or with a fluorinated matrix [ 12], the main parameter concerns the partial energy of diffusion. The monomer diffusion is Table I Overall activationenergyof grafting reaction versus monomercomposition of grafting solution Grafting solution composition Overall activation energy of Vol% Aa/Vol% DEGDM graftingreaction (kJ/mol) 22/0 18/4 0/22

47 65 102

FI"

+

CH2=C

159

CIH3

CH3 C~-~CH 2 I I COO~'%~ OOC

% [] %

FI-- CH2--C.

C=cH9

-4/

FI_(.DEGDMIm

Fig. 6. Partof DEGDMgraftingmechanism(R represents a polymer radical produced during peroxide decomposition). controlled by the swelling of the polymer matrix in the grafting solution composed mainly of water and methanol. COOH groups from AA are only able to swell the polymer film and in this way increase the diffusion and grafting propagation.

3.2. Secondary Li battery function Before presenting the physicochemical characteristics of these grafted separators and their performances in secondary Li batteries, it is necessary to precisely define the functioning conditions of a secondary Li battery. Cycling of Li/V205 element corresponds to a succession of charge and discharge steps. Previous studies [ 16] have shown that a better reversibility of the cycle was observed in a cycling voltage domain ranging from 2.8 ( minimum discharge voltage) to 3.8 V ( maximum charge voltage). The reversible reaction is: la

V205 + xLi ÷ + x e - ¢* LixV205

(5)

lb

During the discharge phase ( l a ) , the electrochemical reduction of vanadium and the insertion of Li in the vanadium oxide network occur simultaneously. During the charge phase ( 1b), the reverse reaction is observed. For an intercalation ratio x < 1, a very good reversibility is observed. In this case, the charged and discharged capacities are equal. The discharge of more than 1 moi of Li per mol of V205 is possible (Fig. 7), but, in this case, the insertion of Li leads to irreversible structural modification of the V205 network. This modification decreases the reversibility of cycling. For instance, for x = 2, i.e. a terminal discharge voltage of 1.75 V, even

160

J.L. Gineste, G. Pourcelly/ Journal of Membrane Science 107 (1995) 155-164 v ,I

3.25

2.4

-

..........:..i

•

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2

I

I

I

I

0.5

1

t .5

2

~ X

Fig. 7. TypicalV205 dischargecurve:potentialdifference (V) versus intercalationyield (x). if the capacity discharged for each cycle is high (220 A h / k g instead of 130 A h / k g for x = 1), the number of cycles is lower and, consequently, the overall discharged capacity too. It is therefore important to limit the discharge to 1 tool of Li per mol of V205. The most widely used films in secondary Li batteries are microporous polypropylene or polyethylene films. They must have specific characteristics: 1. To be chemically or electrochemically stable for the other components of the element. 2. To permit high ionic conductivity between positive and negative electrodes. 3. To be wet without adding undesirable material to the electrolyte system. 4. To present mechanical behavior in order to resist mechanical constraints occurring during cycles. 3.3. Mechanical resistance o f separators For grafting ratio higher than 50%, the separators exhibit poor mechanical properties. They are easily torn

when they are folded in the same direction as the pores are oriented. Radiochemical grafting is composed of two steps, the irradiation step, which considerably weakens the mechanical behavior of the film, and the grafting step. A film irradiated with a 4 Mrad dose cannot be used. In this case, the film decomposition is due to the important crosslinking of the macromolecular network which is directly proportional to the irradiation dose. 3.4. Electrical resistance o f separators and conductivity o f electrolytes Electrolyte is composed of a propylenecarbonate ( P C ) , ethylenecarbonate ( E C ) and dimethoxyethane ( D M E ) mixture in which LiAsF6 is dissolved. Electrical resistance measurements were achieved on separators having a grafting yield lower than 50%. The values are collected in Table 2. Grafting decreases the electrical resistance of the polymer film. But, for grafting yields ranging from 10 to 50% and for the same electrolyte, the electrical resistances of separators are of the same order of magnitude. Therefore, when the separator is wetted by the electrolyte, its electrical resistance does not depend on its grafting yield but on the electrolyte composition. The weak variations of resistance versus grafting yield may be explained by the microporosity of the polymer matrix. In ion exchange membranes, ion transport is achieved through two conduction pathways. The first one consists of a hopping mechanism from one exchanging site to an other, and the second one, in a classical electrolyte

Table 2 Separator electrical resistance versus electrolyte composition: R~ ~ PC/EC/DME 1/ 1/2, LiAsF6 1 M; R2 ~ PC/EC/DME 1/ I/3, LiAsF6 1.5 M R~ ~ PC/EC/DME 5/1/4, LiAsF6 1.5 M; R4 ~ PC/EC 1/ l, LiAsF61 M Film

Grafting yield (%)

RI (J2cm 2)

R2 (Ocm 2)

R3 (g2cm2)

R4 (Ocm ~)

Polypropylene PDA 1 PDA2 PDA3 PDA4 PDA5 PDA6 PD 1 PAl

0 4 9 14 19 28 37 25 23

3.9 3.3 2.9 2.7 2.8 2.5 2.7

3.7 3.4 2.9 2.7 2.8 2.6 2.7

NWa 18 4.7

NW NW 18.5

aNW, non-wettable.

4.5 4.4 4.5 4.3 5

18.2 15 19.4

161

J.L. Gineste, G. Pourcelly /Journal of Membrane Science 107 (1995) 155-164

12 10 E 0

/

\Fl

/

o 6 '

/ /'

b

~{

/"

0

o

/"

~ ' ~

I 0.5

I 1

I 1.5

2

LiAsF6 concentration (M)

Fig. 8. Temperature influence on electrolyte conductivity (PC/EC/ DME, 5/1/4): (a) -40°C; (b) -20°C; (c) 0°C; (d) +20°C. conduction through the interstitial sorbed solution [ 17]. In dense ion exchange membranes, the conductivity strongly depends on the exchange capacity, which is directly related to the grafting yield. In microporous ion exchange membranes, the interstitial conductivity plays the main role. For dense olefinic or fluorinated films grafted with AA and DEGDM, the electrical resistance depends on both the grafting yield and the monomer proportion in the grafted film. In this case, the electrical resistance was related to the swelling (or to AA-content), creating micropores in the macromolecular network [ 11,12]. Electrolyte conductivity depends on its composition but also on its temperature. In Fig. 8, the conductivity of P C / E C / D M E electrolyte is plotted for different LiAsF6 concentrations. A maximum conductivity is observed for 1.25 M LiAsF6. This maximum is less and less pronounced as the temperature decreases. For temperatures lower than - 20°C, the viscosity of the electrolyte increases, leading to a general decrease of conductivity and, consequently, to a decrease in its variations. 3.5. Li/Li cycling tests

The aim is to test the cycling of Li across the separator. The influence of both the nature of the grafted monomer and the grafting yield on the cycling effi-

ciency is illustrated in Table 3. Results show the improvement of cycling for propylene separators grafted with the association AA + D E G D M (PDA). The number of cycles is two times more than for polypropylene separators grafted only by AA (PA) or D E G D M (PD). The dispersion of cycling results is because of the difficulty in reproducing identical experimental conditions for tightening of the element. Tightening conditions are one of factors acting on the cycling performances of elements [ 18,19 ]. Separators obtained from polypropylene grafted with AA + D E G D M present cycling characteristics different from non-grafted separators (Table 4). In the case of P C / E C electrolyte, which does not wet the nongrafted separator, increasing of LiAsF6 concentration improves Li cycling efficiency. For instance, when this concentration passes from 1 to 2 M, the number of cycles is doubled. In the case of a non-grafted film and P C / E C / D M E electrolyte, an increasing of LiAsF6 concentration decreases Li cycling efficiency. To the contrary, no influence of the concentration is observed for grafted films. The Li electrodeposition efficiency depends on both electrolyte composition and diffusion through separator pores (wettability by electrolyte). With a nongrafted separator, wettability is due to DME present in the electrolyte. Increasing of LiAsF6 concentration increases electrolyte viscosity and decreases separator wettability. This phenomenon decreases Li electrodeposition efficiency. With a grafted separator, wettability is due to carboxylic, ester and ether functions fixed on the surface of separator pore. The increase of LiAsF6 Table 3 Influence of nature of grafted monomer on cycling efficiency. Electrolyte is PC/EC, 1/ 1, LiAsF6 1 M Separator Grafting yield (%)

Number of cycles Cycling efficiency (%)

PAl

25

PD1

25

13 15 16

76.9 79.4 81.3

19

84.2

PDA3

14

PDA5

28

PDA6

37

26 30 32 35 37 33

88.5 90 90.6 91.4 91.9 90.9

J.L. Gineste, G. Pourcelly/ Journal of Membrane Science 107 (1995) 155-164

162

Table 4 Influence of electrolyte nature on cycling efficiency (EL0 for non-grafted and grafted separators (N is the number of cycles) Separator (grafting yield)

LiAsF6 concentration (M)

N (ELi) PC/EC 1/1

N (ELi) PC / EC / DME 1/ 1/ 3

Non-grafted polypropylene

0.5

NWa

1

NW

1.5

NW 24 (87.5%) 30 (90%) 27 (88.9%) 35 (91.4%) 44 (93.2%) 60 (95%) 26 (88.5%) 30 (90%) 39 (92%) 40 (92.5%) 43 (93%) 56 (94.6%) 50 (94%) 56 (94.6%)

44 (93.2%) 52 (94.2%) 24 (87.5%) 28 (89.3%) NW 19 (84.2%) 22 (86.4%) 26 (88.5%) 19 (84.2%)

PDA 2 (9%)

1 1.5 1.75

PDA 3 (14%)

1 1.5 1.75

2

concentration in P C / E C electrolyte i m p r o v e s Li electrodeposition efficiency. In this case, D M E does not i m p r o v e s Li cycling through a grafted separator.

19 (84.2%) 22 (86.4%) 26 (88.5%) 19 (84.2%)

The initial characteristics of rolled cylindrical cells and cycling results are reported in Table 5. C y c l i n g results in such cells do not permit confirmation o f the g o o d influence o f low wetting agent concentration ( D M E ) on Li cycling efficiency. The best results are obtained for an electrolyte with a high D M E content. With respect to previous L i / L i cycling tests, the cathodic material wettability must be taken into

3.6. Li cycling in rolled cylindrical cell T w o grafted separators h a v e been used (9 and 14% grafting yield) in two electrolytes ( P C / E C / D M E : 1/ 1/3, 1.5 M LiAsF6 and P C / E C : 1/1, 1.5 M LiAsF6).

account. This cathode is c o m p o s e d o f graphite, V2Os

Table 5 Initial characteristics and cycling results of rolled cylindrical cells elaborated with grafted separators Series of runs

Separator (grafting yield)

Electrolyte

First series

PDA3 (14%)

PC/EC/DME 1/1/ 2.9 3 LiAsF6 1.5 M 4.0 3.9 PC/EC 1/1 LiAsF6 3.9 1.5 M 3.1 3.8 PC/EC 1/1 LiAsF6 2.5 1.5 M 2.8

Second series

PDA3 (14%)

Third series PDA2 (9%)

Elements impedance (12)

Initial tension (V)

Qm (A h/kg)

Cycles number: N

QTOT,d

E~LI

FOM

(Ah)

(%)

3.22

135.8

52

62

88.5

8.7

3.18 3.20 3.18

136.5 136.8 135

55 65 40

66.2 79.5 48.4

89 91 85

9 11.1 6.6

3.19 3.19 3.18

136.5 133 130.9

47 38 46

58.1 45.7 51.5

88 85 87

8.2 6.7 7.5

3.19

130.7

35

40.7

83

5.8

J.L. Gineste, G. Pourcelly / Journal of Membrane Science 107 (1995) 155-164

and binding agent [ 15]. It is poorly wettable by the PC/EC electrolyte. This can explain the better results obtained with PC/EC / DME electrolyte. The lifetime of an element can end either by electrochemical polarization, i.e. by electrolyte exhaustion (minor constituent of the element) or by short-circuit. This last phenomenon is due to formation of non-uniform Li deposition at the Li electrode surface during the charge phase. Li dendrites can perforate a separator. In our case, all elements stopped by short-circuit. This can be due either to imperfect wettability of the cathode for the PC/EC electrolyte, or to high water content of the element. Once grafted, separators can capture water through carboxylic groups of AA. PDA3 separator (14% grafting yield) and PDA2 separator (9% grafting yield) have high water content with respect to the non-grafted separator. Measurements carfled out with a Karl-Fisher apparatus give water contents of 10.3 and 9 m g / g of film for PDA3 and PDA2 separators, respectively. The last results obtained in a rolled cylindrical cell with a PDA5 separator (28% grafting yield) and an electrolyte which wet the separator and the cathodic material at the same time ( P C / E C / D M E 5 / 1 / 4 or 3/ 1/2, 1.5 M LiAsF6 electrolyte) show an improvement on Li cycling. As a matter of fact, the Li cycling efficiency and the FOM are greater than 94% and 20, respectively. If we combine the last results, it seems that the most important parameter acting on Li cycling is the wettability of the different components of the element toward the electrolyte. The element performance probably also depends on the water content, but this effect is not so important.

163

The performances of secondary lithium batteries including classical commercial olefinic separators are generally limited by the wettability of the separator to the electrolyte. The cycling results obtained with new grafted separators show the importance of this surface modification, and the possibility to work with high Li salt concentrations. Now, the optimization of the different components of secondary lithium batteries will no more be limited by the separator wettability but will only depend on both the nature and the wettability of the cathodic material.

5. List of abbreviations and symbols

Products AA EC DEGDM DME PC PE PP Grafted separators PD PDA

acrylic acid ethylene carbonate diethyleneglycoldimethacrylate dimethoxyethane propylene carbonate polyethylene polypropylene polypropylene grafted with AA polypropylene grafted with AA + DEGDM

Abbreviations and symbols 4. Conclusions Grafting of hydrophilic monomers onto polyethylene or polypropylene separators will allow the creation of a new generation of secondary lithium batteries with a lower content of wetting agents but Li salt concentrations above 1 M. The optimization of grafting conditions such as irradiation dose, reaction time or composition of grafting solution (proportion of AA, DEGDM monomers, inhibitor content) enables the obtention of grafted separators with good mechanical and wettability properties.

ELi or E'Li

GY FGY FOM M N a

R x

Li cycling efficiency (%) grafting yield (%) final grafting yield (%) figure of merit mass of the dried film (g) number of cycles amount of Li deposited or exchanged dui'ing cycling tests (g) surface resistance ( ~ cm 2) intercalation ratio of Li in the cathodic material

164

J.L. Gineste, G. Pourcelly / Journal of Membrane Science 107 (1995) 155-164

Acknowledgements T h e a u t h o r s w i s h to t h a n k Dr. B r o u s s e l y ( S a f t C o . ) f o r h e l p f u l d i s c u s s i o n s , S a f t Co. f o r f i n a n c i a l s u p p o r t a n d M o r g a n e Co. f o r p r o v i d i n g i r r a d i a t e d films.

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