The nature of protective surface layers formed on lithium in propylene carbonate solutions of LiClO{sub 4} and LiAsF{sub 6} at open circuit has been investigated by electrochemical pulse measurements. The results are consistent with the fastformation of a compact thin layer resulting from the reaction with residual water. This layer acts as a solid ionicconductor. Slow corrosion or decomposition processes produce a thicker porous overlayer

Geronov, Y.

Muller, R.H.

Schwager, F.

The nature of protective surface layers formed on lithium in propylene carbonate solutions of LiClO/sub 4/ and LiAsF/sub 6/ at open circuit has been investigated by electrochemical pulse measurements and other techniques. The results are consistent with the fast formation of a compact thin layer of Li/sub 2/O by reaction with residual water. This layer acts as a solid ionic conductor. Slow corrosion processes produce a thicker porous overlayer

Muller, R. H.

UNT Digital Library

Electrochemical studies of the film formation on lithium in propylene carbonate solutions under open circuit conditions

Y. Geronov

F. Schwager

R.H. Muller

Crossref

LBL-11126 f'Preprint 'Submitted to the Journal of the El ectrochemi ca1 Soci etyFILM FORMATION ON LITHIUM IN PROPYLENE CARBONATESOLUTIONS UNDER OPEN CIRCUIT CONDITIONSY. Geronov, F. Schwager and R. H. MullerJune 1980TWO-WEEK LOAN PYThis is a Library Circulating Copywhich may be borrowed for two weeks.For a personal retention copy, callTech. Info. [)iuision, Ext. 6782Prepared for the U.S. Department of Energy under Contract W-7405-ENG-48DISCLAIMERThis document was prepared as an account of work sponsored by the United StatesGovernment. While this document is believed to contain correct information, neither theUnited States Government nor any agency thereof, nor the Regents of the University ofCalifornia, nor any of their employees, makes any warranty, express or implied, orassumes any legal responsibility for the accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, or represents that its use would notinfringe privately owned rights. Reference herein to any specific commercial product,process, or service by its trade name, trademark, manufacturer, or otherwise, does notnecessarily constitute or imply its endorsement, recommendation, or favoring by theUnited States Government or any agency thereof, or the Regents of the University ofCalifornia. The views and opinions of authors expressed herein do not necessarily state orreflect those of the United States Government or any agency thereof or the Regents of theUniversity of California.Film Formation on Lithium 1nPropylene Carbonate Solutions undcr Opcn CircuitConditionsY. Geronov*, F. Schwager and R. H. MullerMaterials and Molecular Research DivisionLawrence Berkeley LaboratoryBerkeley, California 94720Lithium is thermodynamically unstable in contact with mOs,t non-aqueous battery electrolytes and can be used only because of the forma-tion of protective films on the metal. In a review paper(l) Dey haspointed out the importance of surface layers for battery performance andshown SEM pictures of anode surfaces after reaction under different con-ditions. In more recent papers by Peled(2,3) the effect of passivatingfilms on electrode kinetic parameters has been shown. It is now gener-ally assumed(2-5) that in most cases the rate determining step (rds) ofthe dissolution-deposition process of alkali metals in nonaqueo~s solu-tions is not electron charge transfer but the migration of cation latticedefects through the surface layer.up to now the main effort of investigation of passivity of lith-ium has dealt with Li/SOCI2 batteries where the Ilvoltage delayllproblemis very significant, Studies of the formation of passivating films on Liin PC solutions have been initiated more recently. Their properties havebeen shown to affect the cycling efficiency of secondary Li elcctrodes(6,7).Alloying with an aluminum substrate has been found beneficial(8). Attemptshave been made by Dey(l) to examine the morphology of Li films in PC using* Permanent address: Central Laboratory of Electrochemical Power Sources.Bulgarian Academy of Sciences, Sofia, BulgariaSEM. His conclusion was that a 1m of to be extremelythin ll • The same author, as well as peled(3). have used the lOllSdata'9 1Scan l } deal with the dual Tafel behavior oftions, in order to their ion of the existence of a surface layer and its influence on the electrode Jdnetics. Searl' has ana-lyzed his experimental results by means of the Butler-Volmer equation.The aims of the present s are: (1) to investi e theof film tion on lithium in PC/LiCI04 solutions at ambientunder open circuit conditions and to determine some filmcharacteristics such as resistance, conduct and thickness; (2) tocompare film thicknesses, derived from electrochemical capacitance mea-surementstalthose calculated from simultaneous ellipsometric measure-An electrochemical cell was built to make simultaneous electro-chemical and elllPsomc\:ric measurements in situ. Optically smooth 1ith-ium electrodes were and inserted into the cell in inert atmos-by press lithium discs (previously with a blad on apoJ e foil in a special jig. The cross-section of a 3.2 mm diam.extruded Li wire, positioned 1 ~n from the periphery of the testelectrode, served as reference electrode. 'l'he electrolyte solution(1 ~·l LiCI04 in PC) was prejlilrcd El'om freshly distilled PC ;llld LiCI04(AHa p.a.) previously dried under vacuum (4-5 )J!Il Hg) at 220°C for atleast 24 hrs. In order to avoid any possible reaction of PC vapor withthe lithium surface before contact with the liquid, different glove boxeswere used for the preparation of electrodes and solutions.After the ellipsometric measurements on the bare lithium surfacewere taken, the cell was filled with electrolyte. The delay in electro-chemical measurements from the moment of electrode scraping was up to,30-45 minutes; the delay after electrolyte filling was 15-30 seconds.Film formation under open circuit conditions was followed by periodic de-terminations of electrode capacitance and micropolarization resi~tanceR = (an/ai) -0 by means of the galvanostatic pulse polarization tech-p n·- .nique. The capacitance is estimated from the initial slope of shortn-t transients (5-10 jJs) (Fig. la) .. The steady-state IR-free overpoten-tial values for the determination of R were obtained from oscilloscopicptraces with a duration of up to 20 ms. (Fig. Ib). Precautions were takento avoid damaging the film by applying pulses with the smallest possibleelectric charge.Theoretical ConsiderationsThe basic assumption of this study is that lithium in PC is cover-ed by a surface layer. It had been suggested before(3) that this layeris formed instantly by the contact of the metal with the solution. Itcontinues to grow with time. The layer consists of some insoluble pro-ducts of the reaction of the lit]lium with propylene carbonate, such asLi2C03, which may form(l) according to the reaction:CH3··,CH~/ Io 0\ ICITo+ 21,i+ ~CH + LiIt acts as an Li conduct solid electro (6) with no electronic con-111"I,<:b IldSl'dE'j" It It c (c:r..·.I)O).aru • _ een ca. e 01 'Aocero yen erpllase ~L Thecapacitance across this film is assumed to be electrically connected inseries with that of the electrol ic double layer.The electrochemical behavior of SEI electrodes will be governedby the properties of the SEI. When the SEI IS thick enough the migrationof ions it may be the rate step. In this case it ispossible to use the basic ion of the classical theory of ionic con-duction in solids developed by Frenkel, Vanvey, Cabrera and Mott, andYoungei '" 42, v exp (··w/RT) sinh (aZFE/RT), (1)which at electric elds (E > lOS V/cm) can be simplified to:i io(BE) ::: i expon(By) , (2)where B is the field coefficient, n is the potential difference in thefilm, E is the electrical field and Y is the film thickness, and a Tafel-like polarization dependence is expected.From this equation Gl TGlfel slope, b, which increases with thefilm thickness is obtained:by2 ..3 B (3)-5-For 101'1 field conditions, Eq, (1) reduces to Ohm's law:i=X!l (4)Ywhere X is the conductivity of the SEI, The reaction resistance of anelectrode is defined as:(5)(6)From Eq, (2), for the limiting case, when the field strengthtends to be zero:( i) = i BEE=O 0At low fields, as has been shown, Ohm's law is applicable and hence:where i is the extrapolated zero field ionic current density.oResults and DiscussionThe thickness Y of the film formed during the contact of the(7)(8)lithium electrode with the PC-l M LiCI04 solution has been derived fromthe capacitance measurements by use of the formula for two capacitors con-nected in series(3,4),0S 0Y = - Y'c/.s', AO,113C (9)where E: 4.9 is d lectric constant of the lithium carbonate fiIm CI ),65 that of PC, C the itance of the electrode in ~ , andC5 is the v,,,,,,,u.,,,,,,s factor, as a first approximation is asummed tobe unity. The thickness Y' of the Helmholz layer in concentrated elec-tro es can be approximated by the 1oof the dipole (c.a. 5 A forPC); the second right hand term in Eq. (9) can therefore be neglected.The steady-state iR-free overpotential values were calculatedfrom the transients in the range of current densities between 0.100-20mAl (see . Ib). At higher c.d., i.e. at electric fields,S(E > 2.10' V/cm), a Tafel pol zation dependence is expected (F 2 presents a series of Tafel plots, obtained by the pulse tech-nique for different film thicknesses. As required for a field-assistedion current across an insulat film, the Tafel slope, b, increaseswith film thickness. 1be v1'Y""'''lated 1ines intersect at zero overpo·-tential at the zero field c.d.,i '" 3 x 1oThe value of the field coefficient B CEq. 3) may be obtainedfrom the slope of the plot b-Y, presented in Figure 3. This value,B 1.24 x 10- 6 em/V, is very close to that of typical barrier films (10 ).The same value was used to calculate the specific conductivity of thefilm the low field approximation (8) :x "·3··6B = 3.0110 x 1.2410-7-For an electric field lower than 2xlOS V/cm, as has to be expec-ted from Eq. (6), the relationship between the polarization resistanceR and film thickness is linear. The slope of this relationship yieldspthe specific conductivity X of the film. However, in our case the plotR - Y does not pass through the origin (Fig. 3, curve 2). Figure 4 pre-psents the decrease of the film conductivity X calculated at c.d. = 0.1mA/cm2 with the growth of the film thicness. After a steep decrease upoto 60-70 A thickness, a steady-state value of the conductivityX ~ 3.4 - 3.7xIO~9 ohm- l em-I, which is very close to that calculatedfrom Eq. (8), was obtained.oThe higher conductivity values of thinner films (y < 60 A) couldbe attributed to a number of factors among which the most important are:pin-hole porosity, tunnelling effect, the decrease of impurity leveland Li+ vacancy concentration with the film growth. The same figure(curve 1) shows film growth with time. The shape of the time depen-dence of the thickness is not the same as that expected for insulatingfilms growing through ionic transport across the film. Deviations fromthe parabolic growth law and difficulties of making quantitative analyses(1-3)from such data also have been found by other authors .The experimentally found values of the Tafel slope, b, at differ-ent film thicknesses are compared with theoretical values in Fig. 2. Ashas been seen from Eqs. (1-3), the Tafel slope is equal to:b =2.3 RT Y (10)For Z 1 and a °3 A this formula is simplified to:b(mv) 20 Y (11 )A comparison between the experImental and theoretical values ofb is given in Table I. If one considers that other combinations of die-lectric constant c and hal jump distance a of the film could be used,the of measured and computed Tafel slopes (for E = 4.9 and°a = 3 A) is satisEll MeasurementThe elllPsomecric measurements of film were conductedsimultaneous with the electrochemical measurements using a self-compen-automatic ell Relative phase 6 and relative amplitudetan ~ were derived from polarizer and analyzer azimuths, respectiveThe angle of incidence of the 1 beam was 75.0°; a mercury lamp was°used as light source with a wave length of 5461 A.The optical properties of bare lithium have been measured beforeaddition of solution for every experiment. The substrate refractiveindex proved to be quite reproducible [0.23 ± 0.04 .- (2.30 ~t 0.03)i].The thickness of the film was determined by comparing the mea-surements with calculations based on the experimental substrate refrac-tlva index and litcr~lture values for rcfrnctiveindices of differentfilm materials. In 5 the experimental polarizer read arepresented as a function of tim6. The indicated thicknesses are calcula-ted for a film material with n - 1.5, the refractive index of Li 2C03 ,-9-using a single-film model. As compared to the film thicknesses obtainedfrom the electrochemical measurements (see Fig. 2) these values are 4 to5 times higher. A possible reason for this discrepancy is a multiple-layer or inhomogeneous film structure. According to such a model, an in-ner nonporous insulating region would be accessible by capacitance mea-surement while the total film, including porous outer regions are ob-served by ellipsometry.ConclusionThe experimental results support to the notion that the passi-vity of Li in PC-LiCl04 solutions at open circuit conditions and ambienttemperature can be attributed to the formation of a thin, nonporous, andelectronically insulating film (possibly Li 2C03). Differences betweenthe film thickness, calculated by electrochemical and ellipsometric mea-surements, indicate the presence of a porous, thicker outer region offilm material.AcknowledgementThis work was supported by the Division of Energy Storage Systems,Office of Conservation and Solar Energy, U. S. Department of Energy,under Contract No. W-740S-Eng-48,REFERENCES1. A. N. Dey, 1bin Solid Films, 43 131 (1977).2. E. Peled and fL Yamin, 28th Power Sources Symposium, p. 237 (1978).3. E. Peled, J. Electrochem. Soc., 126, 2047 (1979).4. rL V. Moshtev, Y. Geronov, B. Puresheva and A. Nasalevska, 1SE,28th Meet , Varna, Extend. Abstr. #153 (1977).5. A. Leef, A. Gilmour, J. 1. Electrochem., 9, 663 (1979)6. R. D. Rauh and S. B. Brummer, Electrochim. Acta, J:l, 75 (1977).7. M. Garreau, J. Thevenin and D. Warin,JEC Press, Cleveland, Ohio (1979).U~'{,n~'~cs in Batter v. 28. 1. Epelboin, ~{L Froment, M. Garreau, J. Thevenin and D. Warin, inPO\ver Sources for Biomedical Implantable lications and AmbientTemperature Lithium Batteries, B. B. Owens and N. Margalit, eds.,Proc. Vol. 80~4J Electrochem. Soc. (1980).9. R. F. Scarr, J. Electrochem. Soc., 117, 295 (1970).1L American Institu te of Physics Handbook J McGraw-Hill Book Company,Inc. (1963).TABLE IComparison betweenExperimental and TheoreticalTafel SlopesY b b =20Yexp theor.0A mV mV25 460 50037 670 74065 1150 1300103 1840 2060130 2400 2600138 2650 2760162 3155 3240FIGURE CAPTIONSFigure 1 Galvanostatic Transients on Li Electrode in 1 M LiCI04 PropyleneCarbonate Solution(a) Short current pulses for capacitance measurement. Traces for(b tt t) 0 1 0 2 l '~ "3 A/ ,,2o - om to 'op: .. , . anc U. j m cmCorresponding scales: 2, 5 and 10 mY/clivi 2 microseconds/div.(b) Long current pulses for overpotential measurement. Traces for-= '}(bottom to top): 4, 6, 8, 10, 11.5, 13.3, IS, 16.7, 20 mA/cm "Scales: 0.5 ; 1 ms/div.Figure 2 - Tafel Plots at Constant Film Thickness:4-103 X; 5-130 X; 6-138 X; 7-162 X.0001-25 A; 2-37 A; 3-65 A;Figure 3 - Relationsh Between Film Thickness and:( 2)Tafel slope bPolarization resistance RpFigure 4 - Time-denpendence of:(1) Film conductivity X(2) Film thickness YFigure 5 - Change of Polarizer Azimuth Reading with Time for a LithiumElectrode in PC/l M LiCI04 "Refractive indices for: Li = 0.2694-2.4776 i; Li 2C03 1.5;C3H6C03 = 1.429a)b)XBB 806-7739Figure 1t,A·c-210Figure 2-15-N CDEN"'"0U .-<~ . IC. 0lD00:: 0CD-'[QxoIJJII.. X~ 0E E 0u u.. 0<:[> >-k>((1lv bN >(-~CIII \f'() \IICO \ 0f'()0 \ IJJN \II II \.c 0ID X \\ 0\\\\> IJJ IJJ 0 IJJN - dFigure 3o"N~ro.-<,'0CJco0 -1w.~ 0'0-• ~ Vt4-17-I 0~ it)...J u en<l wcr: NIJ..W - (\Jcr:::J!AllFigure 5This report was done with support from theDepartment of Energy. Any conclusions or opinionsexpressed in this report represent solely those of theauthor(s) and not necessarily those of The Regents ofthe University of California, the Lawrence BerkeleyLaboratory or the Department of Energy.Reference to a company or product name doesnot imply approval or recommendation of theproduct by the University of California or the U.S.Department of Energy to the exclusion of others thatmay be suitable.

Film Formation on Lithium in Propylene Carbonate Solutions Under Open Circuit Conditions

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