锂电池高温存放后的电化学容量衰减

Electrochemical Investigations on Capacity Fading of Advanced Lithium-Ion Batteries after Storing at Elevated Temperature
Mao-Sung Wu,*,z Pin-Chi Julia Chiang,and Jung-Cheng Lin
Industrial Technology Research Institute,Materials Research Laboratories,Hsinchu 310,Taiwan
Capacity fading of advanced lithium-ion batteries after elevated temperature storage was investigated by three-electrode measure-ments.Capacity fading of a battery increases by increasing the state-of-charge ͑SOC ͒during storage,especially at elevated temperatures.The reversible capacity of a battery ͑SOC =100%͒at 60°C decreases from 820to 650mAh ͑79.3%capacity retention ͒after 60days.At room temperature,a battery SOC influences the capacity fading only slightly;after 65days of storage,the reversible capacity decreases from 820to 805mAh ͑98.2%capacity retention ͒.Individual effects by the anode,cathode,and electrolyte on capacity fading are analyzed with three-electrode electrochemical ac impedance.The major contribution,from X-ray photoelectron spectroscopy ͑XPS ͒and energy-dispersive spectroscopy results,comes from cathode degradation as a result of cobalt dissolution at the LiCoO 2surface layer.A minor contribution comes from the continuous reactions between lithiated mesocarbon microbead ͑MCMB ͒electrode and electrolyte components,which in turn thicken the SEI film and consume available lithium ions.From X-ray diffraction and XPS results,high-temperature storage influences only the surface properties of MCMB and LiCoO 2electrodes;bulk properties remain unchanged.
©2005The Electrochemical Society.͓DOI:10.1149/1.1896325͔All rights reserved.
Manuscript submitted August 17,2004;revised manuscript received December 15,2004.Available electronically April 21,2005.
In recent years,a new type of lithium-ion battery,the advanced lithium-ion battery ͑ALB,with laminated aluminum foil exterior ͒,has emerged because of its high energy density,long cycle life,and low self-discharge properties.ALB offers similar energy character-istics as the traditional lithium-ion battery but with a higher flexibil-ity on the wide variety of sizes and shapes in design.1,2
In practical application,batteries are operated and stored at vari-ous conditions ͑temperature and humidity ͒.Temperature is a crucial factor in the performance of lithium-ion batteries.Detriments may result from high temperature because it significantly affects capacity fading.3-5Amatucci et al.3report that LiMn 2O 4-based lithium-ion rechargeable batteries suffer from poor storage and cycling perfor-mance at elevated temperatures.A LiMn 1.7Al 0.3O 4-hard carbon bat-tery is deteriorated because of anode film formation between 50and 75°C.The film is generated from the decomposed products of LiPF 6,polymerized ethylene carbonate ͑EC ͒,and Mn ions dissoci-ated from the positive active materials.4Wang et al.5propose a mechanism for irreversible capacity loss of lithium-ion spinel cells ͑coin cell ͒in high-temperature storage.Loss of cyclable lithium ions to the carbonaceous anode because of cathode acid generation is the reason.Another effect of the acid is that spinels form from Mn dissolution,but the formation cannot be accounted for capacity loss,nor does it cause degradation of the SEI layer on the carbonaceous anode.
Capacity fading of the commercially available LiCoO 2-based lithium-ion batteries cycled at room temperature has been investi-gated by means of electrochemical impedance spectroscopy.Results show that cycled positive electrode contributes more to the fading because of continuous electrolyte oxidation.6Capacity fading of Sony 18650cells cycled at elevated temperatures has been investi-gated by Ramadass et al.,7concluding that the fading was due to a repeated film formation and dissolution over the surface of anode.This repetition increases the rate of lithium loss and increases the anode resistance.In both cases,6,7the external metallic cans are opened for electrode retrieval,and new half-cells are made in glove boxes filled with ultrapure argon to test for the electrodes’separate properties.Reassembly is inconvenient and may cause damage to the electrodes.
As mentioned earlier,capacity fading of lithium-ion batteries may result from the anode,the cathode,and the electrolyte.It is difficult to analyze the phenomena with a two-electrode system.If a
reference electrode may be added,then more mechanisms may be studied and phenomena understood.Therefore,this paper is to in-vestigate the capacity fading of commercial ALB after high-temperature storage using a three-electrode system.Three-electrode electrochemical impedance is used to analyze the individual effects by the anode,the cathode,and the electrolyte.Structural changes in the electrode materials after storage are also studied.
Experimental
Composition of the lithium-cobalt-oxide electrode was 90wt %LiCoO 2͑10␮m diam,Nippon Chemical ͒,7wt %KS6͑Timcal SA ͒,and 3wt %polyvinylidene fluoride ͑PVDF,Kuraha Chemical ͒binder.Powder was mixed in a solvent of N -methyl-2-pyrrolidone ͑NMP,Mitsubishi Chemical ͒to form slurry.The slurry was coated onto aluminum foil ͑20␮m in thickness ͒and dried at 140°C.The electrode ͑200␮m in thickness ͒was then pressed to a resultant thickness of 150␮m.The mesocarbon microbead ͑MCMB ͒elec-trode,composed of 92wt %MCMB ͑Osaka Gas,25␮m diam ͒with 8wt %PVDF binder and NMP,was subjected to the same processing steps as the lithium-cobalt-oxide electrode,except that it was coated onto copper foil ͑15␮m thick ͒.Resultant thickness of the MCMB electrode was 135␮m ͑before pressing the thickness was 180␮m ͒.
Batteries were assembled in a dry room.The manufacturing pro-cess was as follows:Both electrodes were dried at 120°C for 3h in vacuum and then cut into appropriate sizes for winding with sepa-rator ͑Celgard 2320,20␮m in thickness ͒.The roll of electrodes and separator was inserted into an aluminum-plastic laminated film case.3.2g of electrolyte was injected and then the case was sealed off at a reduced pressure.Electrolyte was 1M lithium hexafluorophos-phate ͑LiPF 6,Tomiyama Pure Chemical ͒in a mixture of 25%EC ͑Merck ͒,25%propylene carbonate ͑PC,Merck ͒,and 50%diethyl-ene carbonate ͑DEC,Merck ͒by volume.Water content of the elec-trolyte measured via Carl Fischer titration in an argon-filled glove box was less than 10ppm.The fresh battery had external dimen-sions of 3.8ϫ35ϫ70mm.The capacity was about 820mAh and weighed 17.5g.
To monitor changes in voltage and impedance of the anode or cathode,a reference electrode was placed in the center of the battery between the two electrodes.A lithium chip was pressed onto one end of a fine copper wire to make the reference electrode.Before stor-age,the three-electrode batteries were cycled between 4.2and 2.75V for three times with a charge/discharge unit ͑Maccor model series 4000͒.The procedure consisted of constant current at 82mA followed by constant voltage at 4.2V until the current tapered down
*Electrochemical Society Active Member.
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E-mail:ms គwu@
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to 20mA.Discharge current was 82mA.The batteries were charged to different SOCs ͑40,70,and 100%͒and stored open-circuited at room temperature and at 60°C for 1-65days.During storage,in order to determine the reversible capacity,batteries were charged/discharged occasionally for two cycles at 82mA ͑about 0.1C ͒at room temperature.Then the batteries were charged again to the desired SOC ͑s ͒and the storage process continued.
Maccor facilitated simultaneous and independent recordings of the total cell voltage and the half-cell voltage for both positive and negative electrodes vs.the reference electrode.Three-electrode im-pedance measurements were taken by means of a potentiostat/galvanostat ͑Schlumberger SI 1286͒and a frequency response ana-lyzer ͑Schlumberger SI 1255͒.Scanning frequencies ranged from 50kHz to 0.01Hz,perturbation amplitude 10mV.
Scanning electron microscopy ͑SEM ͒and energy-dispersive spectrometry ͑EDS ͒were done with a field emission SEM ͑FE-SEM,LEO-1530at an accelerating voltage of 15keV and coupled with an EDS ͑LEO-1550͒.Crystal structures of the MCMB and LiCoO 2were identified by X-ray diffraction ͑XRD,XD-5͒with a Cu K ␣target ͑wavelength 1.54056Å͒.Diffraction data were col-lected for 1s at each 0.04°step width over 2␪,ranging from 10to 90°.Surface properties of the cathode after storage were confirmed by X-ray photoelectron spectroscopy ͑XPS;Perkin Elmer,PHI Quantera SXM ͒with a focused monochromatic Al K ␣radiation ͑1486.6eV ͒.Before any experiment,batteries were fully charged,disassembled in a glove box,washed with DEC,and dried in vacuum at 100°C for 5h.Sample powders of anode and cathode were scraped off the electrodes’current collector.
Results and Discussion
Capacity variations of ALB during storage .—Figure 1shows the capacity variation of ALBs during storage at different SOCs and temperatures.Capacity decay at room-temperature storage ͑Fig.1a ͒is negligible as compared with 60°C ͑Fig.1b ͒.At room temperature,a battery’s SOC influences the fading trend slightly.The original capacity of a battery SOC =100%is 820mAh;after 65days of room-temperature storage it decreased to 805mAh ͑98.2%capacity retention ͒.Lowering a battery’s SOC hinders its capacity decay,as one can see from Fig.1a that the capacity remains unchanged for a battery SOC =40%.Therefore,in addition to the storage tempera-ture,a battery’s SOC is a factor in capacity fading.
Batteries stored at 60°C show a steeper capacity fading trend,and the decrease is most significant in the first few days ͑Fig.1b ͒.The fading depends strongly on a battery’s SOC;the higher the SOC,the more the fading.Capacity of a fully charged battery ͑SOC =100%͒decreases from 820to 650mAh after 60days at
60°C ͑79.3%capacity retention ͒.The fade of a battery SOC =40%is relatively less,from 820to 750mAh ͑91.5%capacity re-tention ͒.The influence of SOC on capacity fading becomes more pronounced with elevating the storage temperature.
Three-electrode electrochemical impedance analysis .—Early re-searchers believed that impedance of a battery is contributed by different factors,such as the electrolyte,the passivation film,charge transfer,lithium-ion diffusion in electrodes,etc.8-13Three-electrode electrochemical impedance spectroscopy ͑EIS ͒has been developed to analyze the individual effects of each component on capacity paring the Nyquist plot with an equivalent circuit model identifies the sources of impedance.
Figure 2a shows both the measured and the simulated impedance spectra of a full battery before and after 15days of storage at 60°C ͑batteries are fully charged,SOC =100%͒.With respect to the ref-erence lithium electrode,impedance of the anode ͑Fig.2b ͒,and cathode ͑Fig.2c ͒are also shown in the figure.Before any measure-ment,the spectra of individual anode and cathode are summed up to check the method validity by ensuring that the resultant combined spectra are to be equal to the full battery spectra ͑Fig.2a ͒.The nearly overlapping curves prove its applicability and reliability.Cor-responding equivalent circuits of the anode and cathode are pre-sented in Fig.3.R e resembles the ohmic electrolyte resistance.R 1
,
Figure 1.Capacity variations of ALB with different SOCs during storage at ͑a ͒room temperature and ͑b ͒
60°C.
Figure 2.Measured and simulated impedance spectra of the ͑a ͒full battery,͑b ͒anode,and ͑c ͒cathode before and after 15days storage at 60°C.͑Bat-teries are fully charged,SOC =100%.
͒
Figure 3.The corresponding equivalent circuit used for the analysis of the impedance spectra of ͑a ͒anode and ͑b ͒cathode.11
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R 2,and R 3are the different-layer SEI-film resistances.C 1,C 2,and C 3are the corresponding capacitance to R 1,R 2,and R 3.R CT is the charge-transfer resistance and C DL is the double-layer capacitance.W is the Warburg impedance.The semicircle in the high-frequency range,corresponding to the surface film resistance,is composed of smaller semicircles.Each contributes resistance and capacitance from different layers of the SEI.In the low-frequency range,the semicircle resembles the charge-transfer resistance,and the linear section resembles the solid-state lithium-ion diffusion.11In general,the presence of such a linear portion implies that diffusion of lithium ions is the semi-infinite diffusion condition.Semi-infinite diffusion in host materials is slower than in the electrolyte solution;therefore,the linear portion is assumed to be the semi-infinite diffusion in solid materials.Literature has shown many different corresponding cir-cuits to simulate the anode and cathode precisely.10-13In order to have a higher precision in modeling,different circuits have been simulated,shown in Fig.3.There are three R-C combinations in parallel to resemble anode SEI,but only one in the cathode circuit.Simulation results are identical to the experimental measurements.From Fig.2,changes in the cathode spectra are quite different from that of the anode after storage.The two electrodes have different resistance and surface chemistry,and therefore are affected differ-ently by high temperature.
Individual contribution from each of the electrolyte resistance,film resistance,and charge-transfer resistance in anode and cathode are presented,respectively,in Fig.4.During high-temperature stor-age,changes in the anode resistance are smaller than that of the cathode.In the anode ͑Fig.4a ͒,resistance changes are larger in the surface film ͑the sum of R1,R2,and R3͒and charge-transfer than in the electrolyte.Both resistances are increased with time,especially the film resistance.Electrolyte is believed to decompose partially and continuously on the MCMB surface to thicken the SEI film,and this process is accelerated above room temperature.14As the SEI film thickens,lithium-ion migration in the film may be delayed and results in increased film resistance.The thickened film covers the active sites on the MCMB surface and blocks lithium ions from intercalating/deintercalating into the layer structure and charge-transfer resistance increases.Resistance of the electrolyte does not change because the decomposition amount is small ͑compared with the total electrolyte amount in a test battery ͒and therefore has little effect.
In the cathode ͑Fig.4b ͒,resistance change patterns are similar;electrolyte resistance remains unchanged,and both the resistance of surface film and charge transfer are increased.The increase in film resistance comes from the formation of SEI on the surface of the
LiCoO 2electrode.However,unlike the anode,the major contribu-tion to cathode impedance is the charge-transfer resistance,which increases most significantly with storage at 60°C.Charge-transfer resistance generally depends strongly on the surface properties of electrode materials.Therefore,a possible source for the increased charge-transfer resistance is the structural collapse of the LiCoO 2electrode surface during high-temperature storage.The deteriorated surface may block lithium ions from intercalating/deintercalating into the layer structure and increases the charge-transfer resistance.When the anode and cathode componential resistances are com-pared,the resistance that is most high-temperature-storage affected and controlled is the charge-transfer resistance of the cathode.With the three-electrode system,individual resistances of the ALB com-ponents may be studied separately,so improvements on the electro-chemical performance of each and even of the whole battery are possible.
As the cathode resistance contributes primarily to the total cell resistance after high-temperature storage,it is also interesting to investigate the changes in the diffusion resistance of cathode after storage.In general,at low frequencies,the electrochemical interca-lation process is controlled by the semi-infinite diffusion.Ideally Z Јvs.Z Љis a 45°straight line ͑Warburg region ͒.15-17The slope of the straight line in the Warburg region yields the Warburg prefactor ͑␴͒.Apparent diffusion coefficients of lithium intercalation can be cal-culated according to the following equation 16,17
␴=
RT
n 2F 2A ͱ2ͩ
1
C Li
D Li
0.5
ͪ
͓1͔
where C Li is the concentration of Li ion incorporated inside a com-posite electrode,D Li is the apparent diffusion coefficient,A the geo-metrical area of the composite electrode,n the number of electrons transferred,F is Faraday’s constant,R the ideal gas constant,T absolute temperature,and ␻is the angular frequency.The real part of the complex impedance ͑Z Ј͒obtained from the cathode before and after 15days storage at 60°C plotted vs.␻−1/2is shown in Fig.5͑batteries are fully charged,SOC =100%͒.When comparing these two plots,lithium-ion concentration is assumed to be the same be-cause the electrodes are charged to the same state;composition of the materials,geometrical area,and the density are the same too.The change in Warburg prefactor value is only attributed to the diffusion coefficient.Warburg prefactors for the fresh and high-temperature-aged cathode obtained from the slopes are 0.00072⍀s −1/2and 0.0011⍀s −1/2,respectively.A small Warburg prefactor value may lead to high utilization of the electrode
under
Figure 4.Individual contribution from each of the electrolyte resistance,film resistance,and charge-transfer resistance in the ͑a ͒anode and ͑b ͒cath-ode.͑Batteries are fully charged,SOC =100%.
͒
Figure 5.Real part of the complex impedance ͑Z Ј͒obtained from the cath-ode ͑a ͒before and ͑b ͒after 15days of storage at 60°C plotted vs.␻−1/2.͑Batteries are fully charged,SOC =100%.͒
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high-rate discharge conditions ͑diffusion control ͒.After high-temperature storage,the cathode surface layer structure has changed and the destructed layers may reduce the amount of diffusion path-ways,decreasing the utilization of active materials.
In order to show the significant changes in charge-transfer resis-tance,impedance spectra of the cathode are measured while passing through with a C/10current.Generally,discharging a battery during impedance scanning,the battery’s charge-transfer resistance de-creases with increasing the current value,because of the increasing driving force in the electrode kinetics.Figure 6shows the EIS after different storage periods at 60°C by passing a C/10current through.The EIS spectrum of a battery after 3day storage ͑Fig.6a ͒shows significant changes.Charge-transfer resistance changes from 0.03to 0.0245⍀with the addition of C/10current.The decrease in charge-transfer resistance shows that an imposing current acceler-ates electrochemical reactions on the electrode surface.However,the battery after 30day storage at 60°C ͑Fig.6b ͒shows no signifi-cant changes.Charge-transfer resistance changes only from 0.1365to 0.1339⍀with the addition of C/10current.When a bat-tery with an original higher charge-transfer resistance is applied with a C/10current density,electrochemical reactions are not enhanced significantly.Electrochemical reactions are improved by increasing the driving force,i.e.,increased the implied currents,so to decrease the charge-transfer resistance.It may be concluded that the charge-transfer resistance of a LiCoO 2electrode after high-temperature storage for a period of time has increased significantly.
Surface properties and bulk structures of the MCMB and LiCo O 2electrodes .—It has been reported that LiCoO 2is unstable at an open-circuit potential ͑OCP ͒higher than 4.2V vs.Li/Li +due to a possibility of cobalt dissolution from LiCoO 2.18Dissolved cobalt ions therefore should be deposited onto MCMB surfaces in fully charged batteries because the reduction potential of cobalt is much higher than the potential for lithium ions to intercalate into MCMB during charging.19EDS,XPS,and XRD are used to observe the changes in surface properties and bulk structure of the electrodes after storage.Figure 7shows the EDS patterns of fully charged MCMB electrodes during storage at different temperatures after 25days.The battery stored at 60°C shows a cobalt peak,indicating the presence of cobalt on the MCMB electrode surface.After high-temperature storage,the LiCoO 2surface structure has deteriorated,and cobalt is dissociated and deposited onto the MCMB surface during charging.Cathode surface deterioration is responsible for the increased charge-transfer resistance ͑Fig.4b ͒.High storage tempera-
ture and high SOC of a battery may be the reasons for the acceler-ated dissolution rate of cobalt.
The XPS technique was chosen to observe the changes on sur-face properties after high-temperature storage.The Co 2p XPS spec-tra of the fully charged cathode electrodes at different storing tem-peratures after 25days is shown in Fig.8.There are two main peaks of binding energies,corresponding to Co 2p 1/2͑around 795eV ͒and Co 2p 3/2͑around 780eV ͒.20The two XPS spectra have a similar shape except a shift in their binding energy.The binding energy of cathode after high-temperature storage shifts to a higher value and has a shoulder on the high-energy side of the Co 2p 1/2component.This difference indicates that the oxidation state of cobalt in the cathode after storage is higher than that of a fresh cathode.Previous publications show that when the amount of Co 4+ions increases in a redox system of lithium-cobalt-oxide ͑Co 4+/Co 3+͒,the XPS peaks of Co shift toward the high-energy side.20Accordingly,as both the lithium and oxygen contents are kept constant in the lithium-cobalt-oxide electrode,an increase of the cobalt oxidation state increases the amount of cobalt dissolution,suitably explaining the XPS
data.
Figure 6.EIS of cathode after ͑a ͒3-day and ͑b ͒30-day storage at 60°C.EIS is measured while passing a current ͑C/10͒through.͑Batteries are fully charged,SOC =100%.
͒
Figure 7.EDS pattern of MCMB electrodes at fully charged states after 25days of storage at ͑a ͒room temperature,and ͑b ͒
60°C.
Figure 8.Co 2p XPS peaks of cathodes with different storage temperature at fully charged state after 25days of storage at ͑a ͒room temperature,and ͑b ͒60°C.
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According to Amatucci et al.,18LiCoO 2starts structural deterio-ration when the voltage charged is higher than 4.20V ͑vs.Li/Li +͒.Lithium ions may not be able to intercalate/deintercalate into the cathode,leading to a decrease in battery capacity.The OCP of com-mercial lithium-ion batteries in fully charged state ͑SOC =100%͒is around 4.2V,referring to an OCP of LiCoO 2electrode higher than 4.2V vs.Li/Li +.Therefore,in practical usages,capacity fading of high-temperature storage lithium-ion batteries is inevitable.
Figure 9shows the XRD pattern of MCMB and LiCoO 2elec-trodes ͑SOC =100%͒after storing at different temperatures.Changes in the patterns of MCMB are small ͑Fig.9a ͒,referring to a very little changed MCMB in its bulk structure.The only noticeable change occurs on the surface film,according to the ac impedance data ͑Fig.4a ͒.Similar results are found in the LiCoO 2electrode:high-temperature storage has little effect on the bulk structure,with the only difference being in its surface chemistry.From XPS data,cobalt dissolution occurs at the cathode.From EDS,these dissoci-ated cobalt ions are deposited on the anode.Cobalt dissolution af-fects the surface structure of lithium-cobalt-oxide electrodes,and affects the surface film of MCMB electrodes.A generality may be concluded that capacity fading of ALBs after high-temperature stor-age is not caused by structural changes of the materials but the surface phenomena on both the MCMB and LiCoO 2electrodes.OCP and charge/discharge curve of ALB after high temperature storage .—During storage,batteries are charged/discharged occa-sionally for two cycles at 82mA ͑about 0.1C ͒to determine their reversible capacity.After two cycles,batteries are charged to their original SOCs and the storage process continues.During the two-cycle capacity-determining step,batteries are charged at room tem-perature and OCP measured.Figure 10shows the OCP variations of the MCMB and LiCoO 2electrodes after 60°C storage at their fully charged states ͑SOC =100%͒.OCP of the MCMB electrode in-creases with the storage time,from 0.01to 0.06V after 25days.A fully charged MCMB electrode self-discharges and loses lithium ions during storage,leading to an increase in its OCP ͑OCP is de-pendent on the lithium-ion concentration in a particular electrode ͒.The loss is mainly attributed by the SEI.As partial dissolution and decomposition of the SEI film possibly thins itself,slowly becoming more porous and less protective,the film becomes incapable of pre-venting electrons from tunneling through anymore.14Intercalated lithium ions may continuously diffuse out from the interior of the MCMB electrode through the damaged SEI to react with the elec-trolyte;consequently,a decrease in the lithium-ion concentration in the MCMB electrode ͑higher OCP ͒has resulted.
In LiCoO 2,OCP increases from 4.20to 4.25V after 25days of storage at 60°C.Increase in OCP indicates a decreased lithium-ion
concentration in LiCoO 2electrode,and the decrease results from the consumption of lithiated lithium ions with electrolyte.In both elec-trodes,lithium-ion concentration decreases because reversible lithium ions from LiCoO 2are decreased after high-temperature stor-age;in which anode SEI and surface structural deterioration of LiCoO 2͑cobalt dissolution ͒are the two major sources.Generally,high temperature accelerates the continuous decomposition-formation process of the SEI and accelerates the surface structure deterioration.When a battery is fully charged ͑SOC =100%͒,MCMB has high reactivity with the electrolyte,and LiCoO 2has a low structural stability which favors the cobalt dissolution.
Figure 11shows the charge/discharge curves of a MCMB elec-trode before and after 25days of storage.Below 0.2V ͑vs.Li/Li +͒,there are three significant oxidation-reduction plateaus ͑marked 1,2,3͒,each representing the formation and decomposition of lithiated carbons.According to previous studies on lithiation of carbon fiber and graphite,these oxidation-reduction plateaus correspond to the potentials of two-phase coexistence.21-23Charge/discharge curves before and after storage are almost identical,different only in poten-tial plateau 3.Due to a shortage in the reversible lithium ions,con-sumed by anode SEI and cobalt dissolution from cathode,the bat-tery after storage can never be fully charged back to its original capacity,and the difference in plateau yer structures
of
Figure 9.XRD patterns of ͑a ͒MCMB and ͑b ͒LiCoO 2electrodes after storage at room temperature and 60°C ͑SOC =100%͒
.
Figure 10.OCP variations of MCMB and LiCoO 2electrodes after 60°C storage at fully charged state ͑SOC =100%͒
.
Figure 11.Charge/discharge curves of the MCMB electrode before and after 25days of storage.
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MCMB remain unchanged after storage͑Fig.11͒,corresponding to the XRD pattern͑Fig.9͒.The only difference is in the surface char-acteristics.
Conclusions
Capacity fading of an ALB after storage depends on the battery’s SOC and its storage temperature.The relationships are directly pro-portional.Capacity of the battery SOC=100%decreases from 820to650mAh after60days of storage at60°C.Three-electrode electrochemical ac impedance technique is used to analyze the indi-vidual effects by the anode,cathode,and electrolyte on capacity fading.After storage,changes in the anode resistance are smaller than that of the cathode.In anode,changes in electrolyte resistance are small.Both thefilm and charge-transfer resistance increase slightly with storage time.But a different resistance result has been obtained for the cathode.After high-temperature storage,the surface layer structure has changed.The binding energy of cathode after high-temperature storage shifts to a higher value and has a shoulder on the high-energy side of the Co2p1/2component,indicating the cobalt dissolution.The destructed cathode layers therefore reduce the amount of diffusion pathways for lithium ions and decrease the utilization of the active material.A major contribution to capacity fading is the cathode degradation due to cobalt dissolution from the surface layer.Lithium-ion concentration decrease in both the MCMB and LiCoO2electrodes after storage suggests less reversible lithium ions,mainly due to the continual SEI formation/ decomposition on MCMB electrode,and to the surface structural deterioration of LiCoO2electrode͑cobalt dissolution͒.From the XRD results,high-temperature storage affects only the surface prop-erties of electrodes,the original bulk properties remain unchanged. Charge/discharge curves of MCMB electrodes demonstrate a short-age of reversible lithium ions,and most importantly,an undamaged internal structure.
Acknowledgments
This work was supported by the Ministry of Economic Affairs of Taiwan under Contract no.93-EC-17-A-08-R7-0312.The authors also thank Dr.J.T.Lee for assistance with sample preparation and XPS analysis.
The Industrial Technology Research Institute assisted in meeting the pub-lication costs of this article.
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