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.

z

E-mail:ms គwu@

Journal of The Electrochemical Society,152͑6͒A1041-A1046͑2005͒

0013-4651/2005/152͑6͒/A1041/6/$7.00©The Electrochemical Society,Inc.

A1041