Electrochemical characterization of a composite polymer electrolyte with improved lithium metal electrode interfacial properties

Ionics 5 (1999)

59

Electrochemical Characterization of a Composite Polymer Electrolyte with Improved Lithium Metal Electrode Interfacial Properties G.B.

A p p e t e c c h i l, F. C r o c e 1, F. R o n c i I , B. S c r o s a t i 1,

F. A l e s s a n d r i n i 2, M. C a r e w s k a 2 and P.P. P r o s i n i 2 1Department of Chemistry, University "La Sapienza", 1-00185 Rome, Italy 2ENEA C.R. Casaccia, Dipartimento di Energia, Divisione Tecnologie Energetiche Avanzate, 1-00060 Rome, Italy

In the development of rechargeable lithium polymer batteries it is of paramount importance to control the passivation phenomena occurring at the lithium electrode interface. It is well estabilished that the type and the growth of the lithium passivation layer is unpredictably influenced by the presence of liquid components and/or impurities in the electrolyte. Therefore, one approach to improve the stability of the lithium interface is the use of liquid-free, highly pure electrolytes. The electrochemical properties of a composite polymer electrolyte obtained by hot pressing a mixture of polyethylene oxide (PEO), a lithium salt (lithium tetrafluoroborate, LiBF4) and a powdered ceramic additive (y-LiA102), will be presented and discussed. The electrochemical characterization included the determination of the ionic conductivity, the anodic break-down voltage and, most importantly, the stability of the lithium metal electrode interface and the lithium stripping-plating process efficiency. The main feature of this dry, true solid-state electrolyte is a very good compatibility with the lithium metal electrode, demonstrated by a very high lithium cycling efficiency, which approaches a value of 99%. Abstract.

1. I n t r o d u c t i o n In previous work [1] we have reported the characteristics of a new family of poly(ethylene oxide), PEO-based composite polymer electrolytes. In particular, we have shown that by assuring a true solid (i.e., liquid-free) state to the electrolyte membranes, a consistent improvement in the stability of the lithium metal electrode interface could be achieved. Such an important property results in a renewed interest in these composite polymer electrolytes of for the development of rechargeable lithium batteries. Indeed, the results [2] confirm this expectation. Considering this relevant technological prospect, we have extended the study of the family of these electrolytes with the aim of further improving the stability of the lithium interface in order to achieve longer battery cyclability. In this work we report the characteristics and the property of a new

member obtained by hot pressing a mixture of poly(ethylene oxide), PEO, a lithium salt (LiBF4) and a micrometric particle size ceramic powder additive (7LiA102). 2. Experimental Particular care was taken to assure a high purity to the composite polymer electrolyte samples. Poly(ethylene oxide) PEO (Aldrich Chemical Co., 4,000,000 average molecular weight) was dried at 50 ~ under vacuum for 48 hours and sieved in order to use only the fraction having particle size less than 125 ~tm. Lithium tetrafluoborate, LiBF4 (Merck "Selectipur", battery grade) was used as received. Low particle size 7-LiA102 (Cyprus Foote Mineral Company, HSA-10) was dried at 300 ~ under vacuum for 24 hours. The three powder components were

60 weighed and mixed inside an environmentally controlled

Ionics 5 (1999)

dry-box (MBraun 150B-G with humidity and oxygen con-

open circuit conditions. The fitting of the response allowed to determine the interfacial resistance and to

tent less than 1 ppm). After homogenization by milling, the mixture was placed between two Mylar sheets and put

monitor its change with time. The interfacial stability and the associated cyclability

in a stainless-steel holder. The entire assembly was sealed within an aluminum envelope (to avoid moisture contamination), brought out of the dry box and placed in a

of the lithium electrode were further studied and measured by imposing galvanostatic pulses on a two-electrode cell using a finely polished nickel working (substrate) elec-

hydraulic press, whose plates had been previously heated to 90 ~ and finally pressed at 300 bar for 30 minutes.

trode, a lithium counter electrode, and the membrane sample as the electrolyte. First, a known amount of

Once the hot-pressing was completed, the envelope was

charge (deposition charge, QD) was passed through the cell

brought back into the dry box and unsealed under the

in order to promote lithium deposition on the nickel sub-

controlled atmosphere. This procedure yielded dry, homogeneous membranes of average 200 g m thickness and

strate. Then a fraction of this charge (cycling charge, Qc) was alternately cycled across the cell to promote lithium

having the PEO20LiBF4 + 20w/o "yLiA102 (w/o = weight

stripping-deposition cycles and the stripping overvoltage was monitored upon cycling. The process was considered

percent) composition. These membranes were characterized both calorimetri-

to be completed when the anodic overvoltage exceeded

cally and electrochemically. The differential scanning ca-

100 mV. The mean value of the electrode cycling effi-

lorimetry, DSC, was performed using a TA Instruments DSC Mod. 2910 in the -5 ~ - 120 ~ temperature range

ciency rl was finally calculated by using the equation:

and at a 10 ~ rate. The electrochemical characterization included the determination of the ionic conductivity, the anodic break-down

[1]

I] = (nQc)/(nQc + QD)

where n is the total number of cycles the cell had

voltage and, most importantly, the stability of the

undergone. The experiment was run and controlled using a

lithium metal electrode interface and the electrode cycla-

multichannel cycler MACCOR 4000.

bility. The ionic conductivity was measured by placing the given membrane sample in a two stainless-steel electrode

3.

cell. The measurements were carried out using a Solartron Frequency Response Analyser (Mod. 1260). The data were analyzed by using an appropriate fitting program [3]. The

PEO20LiBF4 + 20w/o yLiA10 2 electrolyte samples. Figure 1 shows a typical DSC trace of the membrane. Only a major peak, due to the well-known PEO crystalline-

conductivity was measured at various temperatures and

amorphous transition, is observed at 65 ~

Results

The first concern was to control the purity of the

reported in both heating and cooling scans. The anodic break-down voltage was determined by running a scan (0.1 mVs "1) sweep voltammetry in a three-

0.0 -0.2

electrode cell using nickel as the "blocking" working

-0.4

electrode, lithium metal as both the counter and the reference electrode, and the membrane sample as the electrolyte. The scan was stopped as the current density

-0.6

reached 50 ~tAcm2 and the anodic decomposition limitof the electrolyte was obtained by linear extrapolation. A Solartron Schlumberger 1287 Electrochemical Interface was used to run the voltammetry. The data were acquired and treated by using a specifically developed software and run on a PC. The stabilit3/ of the composite polymer electrolyte/lithium metal electrode interface was evaluated by monitoring the time dependence of the impedance response of symmetrical Li/electrolyte/Li cells kept under

-0,8 -1.0 o .d

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-1.2 -l.4 i -1,6 -

q.8 -2.o -' -2.z _

.... . ......... ;........... : : - i ..... ; .......... ; : i 20

40

60

80

..... '.... 100

120

Temperaturel~

Fig. 1. DSC trace of the PEOzoLiBF4 + 20w/o "/LiA1Ozcomposite polymer electrolyte. Scan rate: 10 ~ min-1.

Ionics 5 (1999)

61

50

.

:

i

in the composite polymer electrolyte is indeed very low.

40

Figure 3 shows a typical Arrhenius plot of the PEO2oLiBF4 + 20w/o yLiAlO2 electrolyte. The curve reveals a break at around 65 ~ separating the amorphous and the crystalline temperature domains.

i

20

The values of the activation energy for the two domains are also reported in the figure. Once having ascertained its purity and con-

.....

ductivity, the electrolyte was tested in terms of compatibility with the lithium metal electrode. The

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lithium/electrolyte interface resistance was determined by impedance spectroscopy. By the use of a

'

3.8

4.0

proper fitting program it was possible to separate

Potential vs. Li+/Li*[V]

the contribution of the lithium passivation layer from that of the charge transfer. Figure 4 reports the

Fig. 2. Sweep voltammetry of a nickel electrode in a PEO2oLiBF4 + 20w/o 7LiA102 composite polymer electrolyte cell with Li reference and Li counter electrode. Temperature: 90 "C. Scan rate: 0.1 mVs ~.

value of these two interfacial resistances monitored versus time. The trends are clear in demonstrating that both resistances change moderately upon time

of the lithium/electrolyte contact and, most importantly, they drop dramatically as current flows through the

The purity of the PEO20LiBF4 + 20W/o ]'LiA102 electrolyte was further demonstrated by running a sweep

cell and return to their initial values as the current flow is

voltammetry of a nickel electrode in this electrolyte. The

stopped. This last property is very promising because it

results are shown in Fig. 2. The absence of any detectable current onset prior to the electrolyte anodic decomposition

guarantees passivation at open circuit conditions (necessary for a long shelf life) and a low internal resistance

(which occurs at about 3.7 V vs. Li§

at operative conditions.

~ confirms the

absence of any detectable oxidizable species within the measured voltage range and thu~, that the impurity level

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97.24

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39.37

29.90

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400

Crystalllned~mam

:~ ,,

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* = after g a ] v a n o s t a d c c y c l i n g w i t h i=70 BAcm-: ~* = a f t e r ga]vanostatlc c y c l l n g w i ~ i=180 B A c m "2

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Amorphou~ domain "N

The lithium cycling efficiency was evaluated by monitoring the overvoltage of the lithium stripping process

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300

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= o 106

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scan

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Fig. 3. Arrhenius plot of the PEO2oLiBF4 + 20w/o 7LiA102 composite polymer electrolyte. Data were obtained by impedance spectroscopy.

....

, .... 10

, .... 20

j .... 30

, .... 40

, ....

50

, ....

613

i ....

70

, ....

80

90

Time [days] Fig. 4. Charge transfer resistance and passivation layer resistance of the lithiurrdPEOzoLiBF4 + 20w/o 7LiA102 composite polymer electrolyte interface monitored upon time at 90 ~ Data obtained by impedance spectroscopy.

62

Ionics 5 (1999)

is replaced by the less expensive LiBF 4. The results of Fig. 5 are quite convincing in this respect: they [ ~ ' ~ ~" r' ~, I--~ ' -.............. ! ................. [............... : .............. :.......... /'"" demonstrate a lithium cycling efficiency as high as 98.6%, i.e., to our knowledge the highest value so 60 10 . i ! i i i / o , i i ~ ~ ...kJ far ever reported for PEO-based polymer electrolyte k ~ol . . . . . . . . . . . . . i ~ ~ : - :-.-= ;:..-_:.'.:_=:~ :z .=z.-cells. It may be assumed that in these dry true solid........................... ! .................................. i e~ state polymer electrolytes the passivation process i i i s may only involve a reaction between the lithium -20 . ~--ff.-.~-.--_-~" . - . . - . - - ' - - - - - .. ,. _ ~ . ....... metal and the lithium salt since no other sources L 7 5 ! : .... - .... ,ooo, o,o ~40 ...... i ..... i ' . ~ (e.g. impurities, solvent) are available. This im[ i i i ~ i ...... 400 ~:.~Io ~" ........... i................. '.............................. ~................ ~............... ~ ......................500 ~ cycl ........ plies that the passivation film basically consists of i ~ h -60[. I ,'! .i ': 6 0 0 ~ e,.cle a thin, compact inorganic layer (possibly, in the -80 ............ "".............. {............................. i ................. { ................ * ................ . .... 7oo" ~.~t~ ......... Icase of the electrolyte discussed in this work, a LiF -100 I I , i , , , i I , , I i I , layer). Literature work carried out in liquid organic 0 200 400 600 800 1000 1200 1400 1600 1800 2000 electrolyte cells has demonstrated that this type of Time lsl layer is indeed the most favorable in assuring high Fig. 5. Voltage profile of some plating-stripping cycles of a lithium cyclability [5], and this may concur to lithium metal electrode in a PEOz0LiBF4 + 20w/o gLiA102 composite polymer electrolyte cell with Ni substrate. QD=lc; Qc=0.1C. account for the high efficiency value here obtained. Temperature: 90 ~ Current density: 0.1 mAcm-2. In addition to the compatibility with the lithium electrode, the PEO2oLiBF4 + 20w/o ~'LiA102 composite electrolyte has a high ionic conductivity at in the PEO~0LiBF 4 + 20w/o TLiA102 electrolyte. Figure moderately high temperatures (Fig. 3), a sufficiently high 5 shows the voltage profile of some plating-stripping anodic break-down voltage (Fig. 2), an expected low cost cycles. It can be clearly seen that the increase in the and in addition its lithium salt component is active in anodic overvoltage is limited within 100 mV up to the 100

L

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. . . . . . . . . . .

1"

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700th cycle. The lithium cycling efficiency, calculated by using equation [1], is of the order of 98.6%.

4.

Discussion

In the development of rechargeable lithium polymer batteries it is of paramount importance to control the passivation phenomena occurring at the lithium electrode interface. It is well established that the type and the growth of the lithium passivation layer is unpredictably influenced by the presence of liquid components and/or impurities in the electrolyte [4]. Therefore, one approach to improve the stability of the lithium interface is the use of liquid-free, highly pure electrolytes. Thus, it is expected that changing from a standard casting technique to a preparation procedure whereby the presence of any liquid impurity is eliminated throughout the process, composite PEO-LiX polymer electrolytes having an improved compatibility with the lithium electrode can be obtained. This prevision has been in fact demonstrated in previous works [1,2] using LiCF3SO 3 as the lithium salt and it is further confirmed in this work for an electrolyte where LiCF3SO3

preventing the corrosion of aluminum, i.e. one of the most common substrates for lithium batteries [6]. Given these unique features, the PEO20LiBF4 + 20w/o ~/LiA102 composite electrolyte is expected to behave as improved separator in rechargeable lithium polymer batteries operating in the 3V range. Tests are in progress in our laboratories to confirm this expectations.

5.

Acknowledgement

This work is part of an Italian National Project funded by ENEA which has financially supported the universities of Rome and Bologna (contracts n. 2814 and n. 1221). Two of us (G.B.A. and F.R.) are grateful to Arcotronics Italia S.p.A. for Research Fellowships.

.

[1]

References

G. B. Appetecchi, F. Croce, G. Dautzenberg, M. Mastragostino, F. Ronci, B. Scrosati, F. Soavi, A. Zanelli, F. Alessandrini and P. P. Prosini, J. Electrochem. Soc. 145, 4126 (1998).

Ionics 5 (1999) [2]

[3] [4]

[5]

G.B. Appetecchi, F. Croce, M. Mastragostino, B. Scrosati, F. Soavi, A. Zanell, J. Electrochem. Soc. 145, 4133 (1998). B.A. Boukamp, Solid State Ionics 20, 31 (1986). I.M. Ismail, U. Kadiroglu, N.D. Gray, J.R. Owen, "Fall Meeting Electrochem. Soc.", San Antonio, Texas, Oct. 1996, abstr. No. 63. K. Kanamura, S. Shirashi and. Z. Takehara, J.

63 Electrochem Soc. 141, LI08 (1994); Z. Takehara, J. Power Sources 68, 82 (1997). [6] W.K. Behl and E.J. Plichta, J. Power Sources 72 132 (1997). Paper presented at the 5th Euroconference on Solid State lonics, Benalmddena, Spain, Sept. 13-20, 1998. Manuscript rec. March 4, 1999; acc. Apr. 15, 1999.