Corrosion of Lithium-Ion Battery Current Collectors

Journal of The ElectrochemicalSociety

Corrosion of Lithium‐Ion Battery Current CollectorsTo cite this article Jeffrey W Braithwaite et al 1999 J Electrochem Soc 146 448

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448 Journal of The Electrochemical Society146 (2) 448-456 (1999)S0013-4651(98)04-099-3 CCC $700 copy The Electrochemical Society Inc

Corrosion of Lithium-Ion Battery Current CollectorsJeffrey W Braithwaite z Angelo Gonzales Ganesan Nagasubramanian Samuel J Lucero

Diane E Peebles James A Ohlhausen and Wendy R Cieslak

Sandia National Laboratories Albuquerque New Mexico 87185-0340 USA

The primary current-collector materials being used in lithium-ion cells are susceptible to environmental degradation aluminum topitting corrosion and copper to environmentally assisted cracking Localized corrosion occurred on bare aluminum electrodes dur-ing simulated ambient-temperature cycling in an excess of electrolyte The highly oxidizing potential associated with the positive-electrode charge condition was the primary factor The corrosion mechanism differed from the pitting typically observed in aque-ous electrolytes because each site was filled with a mixed metalmetal-oxide product forming surface mounds or nodules Elec-trochemical impedance spectroscopy was shown to be an effective analytical tool for characterizing the corrosion behavior of alu-minum under these conditions Based on X-ray photoelectron spectroscopy analyses little difference existed in the composition ofthe surface film on aluminum and copper after immersion or cycling in LiPF6 electrolytes made with two different solvent formu-lations Although Li and P were the predominant adsorbed surface species the corrosion resistance of aluminum may simply bedue to its native oxide Finally copper was shown to be susceptible to environmental cracking at or near the lithium potential whenspecific metallurgical conditions existed (work hardening and large grain size)copy 1999 The Electrochemical Society S0013-4651(98)04-099-3 All rights reserved

Manuscript submitted submitted April 15 1998 revised manuscript received October 12 1998

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Advanced rechargeable lithium-ion batteries are attractive for uconsumer electronic and electric vehicle applications because favorable combination of energy and power density service life cand safety High interest also exists for using this technology in cialized low-volume applications where higher reliability and posslonger service life are required (eg military and aerospace) For latter applications long-term chemical degradation of the cell hardwmaterials is a concern because of its potential for adversely affeelectrical performance capacity life andor safety These effectsbe caused by (i) an increase in cell electrical resistance even to the pthat continuity is lost (ii) production of corrosion products that caattack or passivate the active electrode materials or (iii ) compromise ofcell hermeticity that can permit electrolyte loss or the introductioncontaminants (which can also react with active materials)

Potentially serious corrosion problems have previously beenserved in primary lithium batteries For example environmentassisted cracking (EAC) has occurred at highly stressed portions nickel anode current collector grid in LiSOCl2 cells and in the nickel-plated carbon steel material used for containing LiSO2 cells1-3 Thecracking in the LiSOCl2 cells is postulated to be related to alkali meembrittlement1Additionally unacceptable chemical degradation of glassmetal seal occurred in both of these primary technologies2

This knowledge in combination with the higher potential levand extended service-life capability of the lithium-ion technology pvided the motivation for performing a dedicated study the ultimgoal was to determine if reliability and service life will be compmised by chemical degradation of the materials of constructionobjective of the first phase of this project was to identify and chaterize the possible extent of the degradation processes Becaustudy of such processes is very difficult in functional cells an insout approach was chosen and followed That is the intrinsic behof selected materials was evaluated using simulated electrochecycling conditions that were unencumbered by the more complexnomena associated with the presence of active electrode materilarge excess of electrolyte was used that minimized influences asated with corrosion product concentration or electrolyte decomption but may have also exaggerated any effects of solution impur

This paper contains a description of the results from the instudy of two potential vulnerabilities localized corrosion of the aminum positive current collector and EAC of the copper negacurrent collector The plan for the second phase of this study fabricate functional cells with varying physical configurations

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that the corresponding degradation processes under actual contions can be addressed

ExperimentalCorrosion behaviormdashThe corrosion characteristics of aluminum

alloys 1100 (990 minimum and 012 Cu) and 1145 (9945 minimum) along with copper alloy 110 (999 min) were studied inflooded half-cells that had a standard three-electrode configuratio(Fig 1) These cells contained a relatively large excess of electrolycompared to fully functional lithium-ion cells to help ensure that constant conditions existed over the extended time period of the aginexperiments The cell container was a 10 mL plastic bottle with thremetal feed-through seals placed in the lid that also functioned points of attachment for the electrodes A platinum counter electrodand the aluminum or copper working electrode were each spot-weled to a small strip of nickel which was then subsequently spot-weled to the tip of one of the feed-throughs Similarly a lithium reference electrode was cold welded to a strip of nickel that was also spwelded to a feed-through A more detailed description of the expermental and surface analysis procedures is provided in Ref 4

In addition to alloy composition the other parameters that werstudied included electrolyte (solvent) composition aging (cyclingtime temperature (ambient [22-248C] 358C and 508C) and initialelectrolyte water content (as received1 20 ppm) The two electrolyteformulations that were used nominally represented those originaldeveloped by Sony and Bellcore respectively 1 M LiPF6 in either a11 solvent mixture of propylene carbonate and diethylene carbona(PCDEC) or in a 11 solvent mixture of ethylene carbonate an

Figure 1 Photographs of the experimental flooded half-cell configuration

Journal of The Electrochemical Society146 (2) 448-456 (1999) 449S0013-4651(98)04-099-3 CCC $700 copy The Electrochemical Society Inc

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dimethyl carbonate (ECDMC) The solvents were purchased frMitsubishi Chemical and had a maximum water content of 10 pThe source of the LiPF6 salt was Hoshimoto Chemical Corp Finallythe effect of two forms of a carbon-fluorocarbon-based coating on rosion behavior was also evaluated These types of coatings are considered by some developers as a means to improve adhesiactive materials to the current collectors The as-received precumaterial was applied to the active face of the aluminum electrodeair brushing The first coating was produced by curing the spralayer for 15 min at 1208C A second coating version involved a simlar initial cure followed by polymerization at 2308C for 10 min Thecured coating thickness varied from about 13 to 15 mm

Each cell was aged by continuously applying a simulated eleccal cycle that was based on a low-earth-orbit (LEO) aerospace acation Each 150 min cycle regime consisted of four phasescharge constant current charge to potential cutoff potentiostcharge at the potential cutoff and dwell or rest (Fig 2) This typeaging simulated the electrical conditions the respective current lectors in an aerospace application might undergo with the excepthat the 1 h rest period was added to accelerate degradation dhighly oxidative conditions associated with fully charged cells

The relatively high charge potential of 42 V (vs Li) for the alminum electrodes was also selected as a slight stress conditionthough Co-based cells (eg Sony) can be charged at this level to imize capacity most manufacturers recommend a nominal chpotential of 41 6 005 V or lower for high reliabilitylong cycle lifeuse (eg aerospace) Electrical cycling was performed by conneall cells on test in parallel and controlling the potential on the assblage with a programmable potentiostat

Periodically cells were removed from cycling to characterize corrosion kinetics and passivation behavior of the candidate minum and copper alloys For this purpose electrochemical impance spectroscopy (EIS) was used Frequency was scanned 65 kHz to 10 mHz and the measurements were conducted eithopen circuit (Al and Cu) or 42 V vs the lithium reference electro(Al only) The Al potentials were chosen to measure the responsthe two extremes that an actual current collector could encounte

Environmental cracking of copper and cell validationmdashThe sus-ceptibility of copper to EAC was assessed using constant extenrate testing (CERT) and direct exposure of stressed foils in floocells Photographs of the CERT test apparatus are shown in FiStrain rates varied from 1026 to 1027 cms The CERT experimentswere performed on 63 mm round copper tensile bar samples eat open circuit or near the lithium potential This latter condition wachieved by either (i) cold welding a strip of lithium onto the copperod (cell configuration shown in Fig 3a) or (ii ) using an AardvarkV-2LR potentiostat a concentric platinum mesh counter electroand a lithium reference electrode in the stainless steel cell show

Figure 2Simulated electrical cycle regime based on a low-earth-orbit (LEapplication

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Fig 3b To determine if EAC might occur under more realistic con-ditions a number of half-cells were constructed and cycled withcopper foil electrodes bent into a U-shaped configuration Thisshape was maintained by spot welding the foil end back onto thbody The radius of curvature was relatively tight at about 008 cmto ensure significant plastic deformation

Surface analysismdashThe surface of selected Al 1100 and Cu 110samples was analyzed using X-ray photoelectron spectroscop(XPS) and Auger spectroscopy Analyses were performed on samples obtained directly after initial electrode preparation and cleaningafter static immersion in each electrolyte solution for 64 h and aftecompleting approximately 100-200 cycles in cells In addition aheavily pitted Al 1100 sample was examined after 690 cycles in thePCDEC electrolyte

A PHI 5483027 X-ray photoelectron spectrometer was used inthese analyses at an energy level such that the sampling depths wetypically less than 150 Aring Sample compositions were determined bassuming homogeneous distributions of all observed species acrothe surface and in depth using standard handbook sensitivity factors5 In addition Auger spectra were obtained for the heavily pittedAl 1100 sample using a PHI 3067 scanning Auger spectrometer

Results and DiscussionAluminum corrosionmdashDuring electrical cycling a form of local-

ized corrosion occurred in the aluminum current collector After sev-eral days of cycling in the PCDEC electrolyte (40 cycles) somegeneral attack of the aluminum surface was visible in that an almoselectropolished appearance existed but more importantly somscattered localized corrosion had begun (compare Fig 4 and 5) Wit

)

Figure 3 Photographs of the equipment used to determine susceptibility ocopper to EAC (a) the CERT apparatus (b) stainless steel cell used to coduct constant potential CERT experiments

Figure 4 Photograph of the bare Al surface prior to electrical cycling


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continued cycling the density of the attack increased (Fig 6cycles) The use of the ECDMC electrolyte resulted in an even her density (Fig 7 at only 150 cycles) Curiously what opticaappeared to be pits were actually mounds Cross-sectional scaelectron micrograph examination of these foils showed that mounds were actually filled in pits (Fig 8) Surface analyses uXPS and Auger analyses were then performed to help understhis phenomenon (detailed surface results are presented sepabelow) These characterizations showed that the mounds contboth Alo and Al2O3 Because of the primarily ldquoanodicrdquo conditionimposed on the aluminum during electrical cycling the existencmetallic aluminum practically required that the mounds were sohow electrically isolated from the foil Two possible explanationsthis situation were postulated (i) corrosion and its associated reation products undermined the surface of a developed pit and cathe overlying metal to bulge and break away or (ii ) a soluble alu-minum-ion corrosion product was electrodeposited on poorly cductive solid corrosion products during the latter part of the charge portion of the cycle during which cathodic conditions actly existed The open-circuit potential for Al ranged from 32 a36 V vs Li and this portion of the cycling brought the potendown to 30 V

As noted in the previous section EIS was used as the printechnique to characterize corrosion behavior The simplified equ

Figure 5Photographs of the Al surface after 40 electrical cycles in PCDelectrolyte

Figure 6 Photograph of an Al surface after 690 electrical cycles in PCDelectrolyte

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used

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lent electrical circuit shown in Fig 9 has been successfully usedassess the corrosion resistance of chemically passivated Al allo6

As is shown this same circuit can be applied to the corrosion of bor coated aluminum in lithium-ion environments The value of th

EC

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Figure 7Photographs of the Al surface after 150 electrical cycles in ECDMelectrolyte The light areas are mounds or nodules

Figure 8SEM photographs of Al foil cross section after 150 electrical cyclein ECDMC electrolyte Photograph (b) is an expanded view of one moufrom photograph (a)


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pitting resistance parameter (Rpit) represents a credible figure-ofmerit measure for the susceptibility of aluminum to corrosion (ithe lower the Rpit value the higher the susceptibility to corrosioand was calculated from the measured EIS response

Figures 4-7 showed that the pit density was higher in the ECDelectrolyte than in PCDEC The EIS results confirmed that ECDMC electrolyte was more corrosive (lower Rpit) than the PCDECelectrolyte over the majority of the tested cycle life At cycle 100diameter of the semicircular component in the Nyquist plots (Fig 1that corresponded to Rpit was much smaller and as shown in Fig 10a distinct separation existed in the Rpit parameter values explicitly calculated from the model over the first several hundred cycles Thecreased corrosion current that was observed initially during dc murements (Fig 11a) also supported this finding Importantly the pitresistance appeared to improve with cycling in both electrolytesespecially in the ECDMC solvent (Fig 10b) However as discussemore detail below this observation does not mean that the piprocess was self-limiting over time and stopped in either electroThe improvement in behavior may not represent an intrinsic electroproperty because an associated impurity effect could have beecause (eg scavenging or introduction) In this phase of the workpurities were not analyzed or monitored as a function of cyclingeither electrolyte The surface spectroscopy analyses indicated thECDMC electrolyte used in this study may have had a higher chloconcentration which is known to cause pitting corrosion in orgaelectrolytes7

Using electrochemical response and Rpit as a measure of corrosion susceptibility the general effects of the other parameterswere studied can be summarized as follows

Figure 9 Simplified equivalent electrical circuit representation of the eletrochemical processes occurring at the surface of a coated aluminum trode model

Figure 10The effect of electrolyte composition on corrosion behavior ofalloy 1100 (a) Nyquist plot after 100 electrical cycles (b) the calculatedRpitparameter as a function of cycling

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(i) Cycle aging The general electrochemical behavior was not sustantially affected by cycling in either electrolyte (EIS in Fig 12 adc in Fig 11b) The removal of the secondary peak in the dc polartion response (Fig 11b) that occurred during early cycling was prably due to a one-time irreversible reaction involving an electrolimpurity

(ii ) Charge potential Larger applied anodic potential levels resuled in greater anodic corrosion currents (Fig 11b) a result consiswith a lower Rpit value (smaller semicircle diameter in the Nyquist ploshown in Fig 13)

(iii ) Alloy The resistance to pitting in the PCDEC electrolyte wnot effected by the metallurgical purity of the electrode material (al1145 vs 1100) during the first 100 cycles (Fig 14) However ba

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Figure 11Anodic dc polarization response (a) effect of electrolyte compsition prior to cycling (b) effect of cycling in PCDEC electrolyte

Figure 12The effect of cycling on the electrochemical behavior of Al allo1100 in (a) PCDEC and (b) ECDMC

Figure 13Effect of applied potential on electrochemical behavior of Al allo1100 in PCDEC electrolyte


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on the trend shown the resistance for alloy 1145 may be superior wextended cycling

(iv) Water contamination 20 ppm water was added to the PCDECelectrolyte to assess the effect of using an initially impure electrolyThis addition actually appeared to improve corrosion resistance a1001 cycles (Fig 15a) A similar beneficial effect due to water (athough at much higher levels) was observed in a related work tinvolved a PC-based electrolyte and stainless steel couple The efwas attributed to a stabilizing effect on the passive layer8 However amore detailed analysis is needed with this system before a definitconclusion can be reached Of possible relevance to this understanand confirmed by the dc response obtained during electrical cyclinthe water appeared to be effectively electrolyzed out during the fifew electrical cycles (Fig 15b) Possibly a chemical radical productthe electrolysis acted as an effective passive-layer stabilizer

(v) Temperature The attempt to accelerate aging using higher temperature with either electrolyte formulation was not successful Aftonly a few days of cycling at 358C and a day of cycling at 508C theLi reference electrode was significantly corroded resulting in a blaelectrolyte At this point further cycling had to be terminated Subsquent experimentation was performed that identified the problem LiPF6 attack on the lithium Although contaminated it was possibthat the electrolyte itself was still functional Because actual cells cfunction at these temperatures this problem appears to be uniquthe experimental configuration that was used in this study Importaly at the ambient temperature used in the majority of the experimennot even limited attack of the lithium reference was ever observed

The effect of applying the two carbon-based coatings on the genal electrochemical behavior of Al is captured by the Bode phase resshown in Fig 16 As explained in the Experimental section the movation for potentially using such coatings is to improve active-mater

Figure 14 Effect of alloy composition on the calculated Rpit parameter inPCDEC electrolyte

Figure 15The effect of an initial addition of water on the corrosion behavior of Al alloy 1100 in PCDEC electrolyte (a) calculated Rpit parameter asfunction of cycling (b) anodic polarization response

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adhesion not corrosion resistance The uncoated electrode had a sdominant process (represented with a single capacitiveresistive cbination and thus one pseudotime constant) whereas the two coelectrodes showed two processes the nonpolymerized with two olapping time constants and the polymerized with two distinct time cstants Modeling of the EIS data taken around cycle 100 was comed and an excellent correlation existed between the actual and preed response The results at the low frequency limit indicated that pitresistance increased by close to one order of magnitude for coatedtrodes The higher resistivity values for the coated electrodes impthat less corrosion should have occurred The photograph presentFig 17 confirmed this prediction (compare with Fig 6 for uncoatAl) It should be noted that some interaction occurred (swelling adelamination) between the electrolyte and both coatings Although rosion protection was still evident the ability to maintain it over a loterm was not demonstrated A possible reason that these coatingproved the corrosion resistance of aluminum is simply that much ofactive pit area became physically sealed or blocked

A practical validation of the experimental results presented in tsection was provided by a section of a positive electrode taken frocommercial Sony lithium-ion cell (1991 vintage) that had been sjected to about 4000 LEO cycles A representative photograph ofAl current collector extracted from this cell is shown in Fig 18 Sinificant localized pitting corrosion of the aluminum occurred to t

Figure 16 Electrochemical behavior (Bode phase) for uncoated and coaAl alloy 1100 in the PCDEC electrolyte

Figure 17Photograph of the Al substrate following removal of polymerizecoating after 445 simulated electrical cycles


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point that the foil had a large number of visible perforations Whetor not the holes were at some point similar to the mounds descrabove could not be determined because of the difficult nature ofmoving the active materials without physical disruption of the surfaHowever this observation does prove that the pitting process wasself-limiting An assessment or measurement of the final electricalsistivity of the corroded aluminum interface would provide valuabinformation relative to the any effect that the limited general corroswould have on cell resistance

Copper corrosionmdashAs expected copper was not susceptible general or localized corrosion at the cathodic potentials associwith actual cells That is copper had an open-circuit potential in thelectrolytes that was gt3 V vs Li and the ldquoappliedrdquo potential on telectrode ranged from 0 V to 15 V vs Li Thus copper was effectively cathodically protected The range of conditions that was studwas similar to those described above for aluminum baseline inwater addition of 20 ppm two carbon coatings and temperaturewith aluminum the use of elevated temperature to accelerate awas not successful the lithium reference electrode again corrodethe LiPF6 and turned the electrolyte black Overall no condition wfound that resulted in any measurable quantity of either uniformlocalized attack of the copper The presence of either of the two bon coatings did not change this situation

Surface analysismdashAn important part of this study involved thecharacterization of the chemical films that were present on the minum and copper current collectors and possibly contributed to tpassivity The characterization techniques that were used incluXPS and scanning Auger microscopy A more detailed discussiothese analyses especially that concerned with minor elements istained in Ref 4

Chemical compositionmdashIn general Li was the predominant elemental species observed on the surface of all Al and Cu electroHowever the surface layer was not simply adsorbed electrolabout twice as much F was observed on the surface than woulassociated with the deposition of LiPF6 The existence of these nonstoichiometric chemical compositions suggested that decomposiof the electrolyte had occurred In addition higher pitting and crosion appeared to correlate with increased surface concentratioCl presumably present as a contaminant in the electrolyte

The variation in the surface composition for Al 1100 is shownFig 19 for four conditions (i) after initial electrode preparation and(ii ) cleaning (iii ) after simple immersion for 64 h and (iv) after com-pleting 212 and 690 cycles The latter three exposures were formed in the PCDEC electrolyte Little difference was observbetween immersion in the electrolyte and cycling the cell for a fhundred cycles a finding that also applied to Cu After the 212 cyc

Figure 18 Photograph of Al current collector from commercial Sony ceafter 4000 LEO electrical cycles

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slightly more Li and less F was on the surface relative to immersin the electrolyte while the surface concentrations of other specwere essentially unchanged After 690 cycles the heavily pitted etrode showed considerably less F and Li and more O Cl (B) Pcontaminants (Ni Mn) relative to the unpitted electrode (212 cycleThe source of some of these contaminants is unclear The nickethe pit could possibly have been the nickel feed-through leads buevidence of any corrosion on it was observed

The surface layers on copper remained quite thin well below 150 Aring maximum sampling depth of XPS The thickness of the aminum surface layers could not be determined More aluminuexisted in the air-formed oxide layer than in the samples exposethe electrolytes However because the surface compositions wnot affected by aging in these cell environments either the thickndid not change or the composition of the layer was constant

The effect of electrolyte solvent on the surface compositions Al 1100 electrodes after completion of a limited number of cycl(PCDEC-212 cycles and ECDMC-150 cycles) is shown in Fig 2The surfaces of the Al electrodes were very similar when cycledeither electrolyte suggesting that the common LiPF6 salt had thedominant effect The primary difference observed was an increasCl in the ECDMC solvent This peak could possibly be associawith B instead of Cl because the observed binding energies (BEsthe peak fell between that expected for Cl and B Thus a definitidentification could not be made

Chemical speciationmdashMost of the surface species contained thsame chemical state regardless of the exposure conditions (immevs cycling electrolyte composition) with minor differences observbetween Al and Cu surfaces The exception was the carbon-coatedface which showed multiple peaks and chemical states for most face species some were quite different from those observed onnoncoated surfaces The results for specific elements of interestshown in Fig 21 as regional spectral plots The type of lithium flurine and phosphorus contained on electrodes simply immersed in

ll

Figure 19XPS surface analysis results of uncoated aluminum for four contions cleaned immersed cycled to 212 and 690 cycles All exposures wethe PCDEC electrolyte Composition is in atom

Figure 20XPS results comparing the surface composition of aluminum afcycling in the PCDEC electrolyte (212 cycles) and the ECDMC electroly(150 cycles) Composition is in atom


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PCDEC electrolyte and those after completion of 150-250 cycleeither electrolyte was very similar (Fig 21a b c) The high Li BE sgests that the Li was present in an environment that was extremelytron withdrawing Considerably less lithium was contained in the bon-coated electrode and the pitted 690 cycle electrode The obsBE for fluorine is typical for a -CHF2 species9 and is quite differentfrom that expected for metal fluorides Phosphorus had a BE typicPF6 species9 The pitted electrode showed considerably more P wiBE that was lower than that on the other electrode surfaces suggP was present in a more electron-donating environment

Figure 21 XPS region plots for several elements lithium as a functionexposure conditions for an Al 1100 electrode surface (a) lithium (b) frine (c) phosphorus and (d) aluminum All exposures were in the PCelectrolyte except for the one marked ECDMC The cycle number for are as follows immersed 0 PCDEC 212 ECDMC 150 carbon coatedpitted 690

ing-

elec-ar-rved

l ofh asting

Although not presented an XPS analysis of the native air-forfilm on aluminum was consistent with the presence of a thin layAl2O3 The similar peaks exhibited in Fig 21d by the immersedshort-term cycled electrodes yielded a BE that is high for typicaspecies higher than that expected for either Al2O3 (74 eV) or AlF3(765 eV)9 The higher BE again indicated the presence of an elecwithdrawing environment The pitted electrode surface showed ablet peak with a small peak near 78 eV similar to the other elecsurfaces and a larger peak near 71 eV that is more typical of Alo9

Pit nodule compositionmdashAuger spectroscopy was used to exaine the nodules that formed in the corrosion pits on the Al electsurfaces after long-term cycling As shown in Fig 22 these aexhibited a complex chemistry that varied from nodule to nodPoint-mode Auger spectra taken at selected locations on noduleother general locations of the electrode surface demonstratedgreat inhomogeneity existed on the microscopic spatial level buspecies observed agreed with those identified using XPS In gethe nodules had high surface concentrations of Al Li and F andcontained Al2O3 Metallic Alo and Al31 were readily distinguishedin the Auger spectra based upon differences in the lineshape cby the chemical state although this assignment was complicatthe presence of both electrically conducting and insulating regwithin the nodules Some nodules were high in Cu and P

Copper environmental crackingmdashTo determine the susceptibilitof copper to EAC a series of constant extension rate tests waformed The primary parameters that were studied included appotential grain size degree of work hardening and strain rateunsuccessful attempt also was made to also include grain orient

The conclusion from these activities was that only one combtion of metallurgical and environmental conditions existed unwhich copper was susceptible to intergranular EAC work-hardewith a relatively large grain size and an applied potential of 0 V vsThe elimination of any of these conditions was sufficient to remany susceptibility The existence of this susceptibility to EAC alwith the importance of a large grain size is shown by comparinggrain structure for two different copper specimens is Fig 23 anwith the respective SEM fracture surfaces in Fig 25 Interestinintergranular cracking occurred in the large grained material de

ofo-ECach15

Figure 22Auger maps of distributions ofprimary species identified on an Al 1100current collector that had undergone 690electrical cycles in PCDEC electrolyteThe image size is 086 mm across theedge


Figure 23 Optical photographs of cross-sectioned copper tensile bar that did exhibit brittle behavior (a) transverse (b) longitudinal

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an unfavorable orientation of the grains (longitudinal elongation) ative to internal stress the degree of work hardening was similboth sets of samples large-grained susceptible sample 116 Vihardness number (VHN) small-grained sample 119 VHN The ditions under which susceptibility existed are consistent with thpreviously observed for Ni in LiSOCl2 cells1 This situation impliesthat the Cu EAC phenomenon is mechanistically similar that is ali metal-induced EAC

No cracking of U-bend electrodes aged in flooded cells or acfoil from commercial cells was observed Thus based on the reto date the susceptibility to EAC can be eliminated by proper ation to metallurgical conditions

Conclusions

The intrinsic (bare-metal) corrosion behavior of the primary crent-collector materials used in lithium-ion cells was studied at aent temperature in flooded half-cells that contained a relatively lexcess of electrolyte Two vulnerabilities were addressed aluminulocalized corrosion and copper to environmentally assisted cracLocalized pit-like corrosion of aluminum positive-electrode currcollectors occurred at the highly oxidizing potentials that existed the top-of-charge condition However the corrosion mechanismpeared to be different than that observed in aqueous systems pro

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lts-

i-etog

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because of the organic electrolyte and the imposition of electricacycling Under these conditions each corrosion site was filled with acorrosion product that formed mounds or nodules Electrochemicaimpedance spectroscopy was shown to be an effective analytical tool study corrosion behavior in these systems in that results correlated wivisual observations and expected trends The major EIS findings included (i) over the first few hundred cycles a PCDEC electrolyte for-mulation was less corrosive than one composed from a ECDMC sovent (ii) the general electrochemical behavior was not a function of theextent of electrical cycling or metallurgical purity (alloy 1100 vs1145) (iii ) an increased charge potential decreased the corrosion resisance and (iv) a potentially beneficial effect of a small initial water addi-tion was observed The application of two fluorocarbon-based coatingthat could possibly be used to improve adhesion of the active materiato the current collectors increased the short-term resistance of Al tlocalized pitting Because these coatings were only cycled for a sixmonth period their long-term effectiveness was not established

Detailed XPS and Auger analyses were performed to identifyimportant species on the surface of the electrodes that could direcly influence corrosion behavior There was little difference in thefilms observed after simple immersion in the electrolytes vs thoseresulting after electrical cycling Lithium was the predominant sur-face species that was detected In general about twice as much

Figure 24Optical photographs of cross sectioned copper tensile bar that did not exhibit brittle behavior (a) transverse (b) longitudinal


Figure 25 SEM photographs of fracture surfaces (a) brittle fracture of coarse-grained work-hardened copper (b) ductile fracture of fine-grained work-hard-ened copper

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la oesTh du

rooningua

t oroa

f

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g-

(relative to Li and P) was observed on the surface than would besent from the direct deposition of LiPF6 from the electrolyte Rela-tive to electrolyte composition the Al surfaces were very simiwhether cycled in either PCDEC or ECDMC electrolytes Basedthese analyses the role if any of surface species that were prdue to exposure to the electrolyte could not be determined observed corrosion resistance of the aluminum could simply beto its native oxide layer

The copper negative current collector was susceptible to envimental cracking at or near the lithium potential (charge conditionly when specific metallurgical conditions existed (work hardenand large grain size) Although thin foils can possess conditions sas these proper metallurgical control could be implemented to ely eliminate any problems in practice

AcknowledgmentThis work was supported by the United States Departmen

Energy under contract DE-AC04-94AL85000 Sandia is a multipgram laboratory operated by Sandia Corporation a Lockheed Mtin Company for the United States Department of Energy

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Sandia National Laboratories assisted in meeting the publication costs othis article

References1 R Scully W R Cieslak and F S BovardJ Electrochem Soc 138 2229 (1995)2 R K Quinn and S C Levy24th National SAMPE Symposium Proceedings p 229

(1982)3 A Attewell Metallurgical Examination of Cracking in Buses from Lithium-Sul-

phur Dioxide Cells Royal Aircraft Establishment RAE(F)MT44E1141 (1982) 4 J W Braithwaite A Gonzales S J Lucero D E Peebles J A Ohlhausen W

Cieslak and G Nagasubramanian SAND97-0507 Sandia National Laboratorie(1997) (available through NTIS accession number DE97005178)

5 C D Wagner W M Riggs L E Davis J F Moulder and G E MuilenbergHandbook of X-Ray Photoelectron Spectroscopy G E Muilenberg Editor Perkin-Elmer Corporation Eden Prairie MN (1979)

6 R G Buchheit M Cunningham H Jensen and M W KendigCorrosion 54 61(1998)

7 W B Ebner and W C MerzPower Sources B B Owens and N Margalit Edi-tors PV 80-4 p 265 The Electrochemical Society Proceedings Series Penninton NJ (1980)

8 D A Shifler P J Moran and J KrugerElectrochim Acta 40 897 (1995)9 J F Moulder W F Stickle P E Sobol K D Bomben and J ChastainHandbook

of X-Ray Photoelectron Spectroscopy 2nd ed J Chastain Editor Perkin-ElmerCorporation Eden Prairie MN (1992)

Experimental

Results and Discussion

Conclusions

Acknowledgment

References


Corrosion of Lithium-Ion Battery Current CollectorsJeffrey W Braithwaite z Angelo Gonzales Ganesan Nagasubramanian Samuel J Lucero

Diane E Peebles James A Ohlhausen and Wendy R Cieslak

Sandia National Laboratories Albuquerque New Mexico 87185-0340 USA

The primary current-collector materials being used in lithium-ion cells are susceptible to environmental degradation aluminum topitting corrosion and copper to environmentally assisted cracking Localized corrosion occurred on bare aluminum electrodes dur-ing simulated ambient-temperature cycling in an excess of electrolyte The highly oxidizing potential associated with the positive-electrode charge condition was the primary factor The corrosion mechanism differed from the pitting typically observed in aque-ous electrolytes because each site was filled with a mixed metalmetal-oxide product forming surface mounds or nodules Elec-trochemical impedance spectroscopy was shown to be an effective analytical tool for characterizing the corrosion behavior of alu-minum under these conditions Based on X-ray photoelectron spectroscopy analyses little difference existed in the composition ofthe surface film on aluminum and copper after immersion or cycling in LiPF6 electrolytes made with two different solvent formu-lations Although Li and P were the predominant adsorbed surface species the corrosion resistance of aluminum may simply bedue to its native oxide Finally copper was shown to be susceptible to environmental cracking at or near the lithium potential whenspecific metallurgical conditions existed (work hardening and large grain size)copy 1999 The Electrochemical Society S0013-4651(98)04-099-3 All rights reserved

Manuscript submitted submitted April 15 1998 revised manuscript received October 12 1998

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Advanced rechargeable lithium-ion batteries are attractive for uconsumer electronic and electric vehicle applications because favorable combination of energy and power density service life cand safety High interest also exists for using this technology in cialized low-volume applications where higher reliability and posslonger service life are required (eg military and aerospace) For latter applications long-term chemical degradation of the cell hardwmaterials is a concern because of its potential for adversely affeelectrical performance capacity life andor safety These effectsbe caused by (i) an increase in cell electrical resistance even to the pthat continuity is lost (ii) production of corrosion products that caattack or passivate the active electrode materials or (iii ) compromise ofcell hermeticity that can permit electrolyte loss or the introductioncontaminants (which can also react with active materials)

Potentially serious corrosion problems have previously beenserved in primary lithium batteries For example environmentassisted cracking (EAC) has occurred at highly stressed portions nickel anode current collector grid in LiSOCl2 cells and in the nickel-plated carbon steel material used for containing LiSO2 cells1-3 Thecracking in the LiSOCl2 cells is postulated to be related to alkali meembrittlement1Additionally unacceptable chemical degradation of glassmetal seal occurred in both of these primary technologies2

This knowledge in combination with the higher potential levand extended service-life capability of the lithium-ion technology pvided the motivation for performing a dedicated study the ultimgoal was to determine if reliability and service life will be compmised by chemical degradation of the materials of constructionobjective of the first phase of this project was to identify and chaterize the possible extent of the degradation processes Becaustudy of such processes is very difficult in functional cells an insout approach was chosen and followed That is the intrinsic behof selected materials was evaluated using simulated electrochecycling conditions that were unencumbered by the more complexnomena associated with the presence of active electrode materilarge excess of electrolyte was used that minimized influences asated with corrosion product concentration or electrolyte decomption but may have also exaggerated any effects of solution impur

This paper contains a description of the results from the instudy of two potential vulnerabilities localized corrosion of the aminum positive current collector and EAC of the copper negacurrent collector The plan for the second phase of this study fabricate functional cells with varying physical configurations

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ExperimentalCorrosion behaviormdashThe corrosion characteristics of aluminum

alloys 1100 (990 minimum and 012 Cu) and 1145 (9945 minimum) along with copper alloy 110 (999 min) were studied inflooded half-cells that had a standard three-electrode configuratio(Fig 1) These cells contained a relatively large excess of electrolycompared to fully functional lithium-ion cells to help ensure that constant conditions existed over the extended time period of the aginexperiments The cell container was a 10 mL plastic bottle with thremetal feed-through seals placed in the lid that also functioned points of attachment for the electrodes A platinum counter electrodand the aluminum or copper working electrode were each spot-weled to a small strip of nickel which was then subsequently spot-weled to the tip of one of the feed-throughs Similarly a lithium reference electrode was cold welded to a strip of nickel that was also spwelded to a feed-through A more detailed description of the expermental and surface analysis procedures is provided in Ref 4

In addition to alloy composition the other parameters that werstudied included electrolyte (solvent) composition aging (cyclingtime temperature (ambient [22-248C] 358C and 508C) and initialelectrolyte water content (as received1 20 ppm) The two electrolyteformulations that were used nominally represented those originaldeveloped by Sony and Bellcore respectively 1 M LiPF6 in either a11 solvent mixture of propylene carbonate and diethylene carbona(PCDEC) or in a 11 solvent mixture of ethylene carbonate an

Figure 1 Photographs of the experimental flooded half-cell configuration


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Corrosion of Lithium-Ion Battery Current Collectors

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