Source of AcquisitionNASA Johnson Space Center

EVALUATION OF PERFORMANCE AND SAFETY OF ELECTROFUELLITHIUM-ION POLYMER CELLS

JUDITH A. JEEVARAJAN, I BOBBY J. BRAGG 2, WALTER A. TRACINSKI3

'Lockheed Martin Space Operations, 2101, NASA Rd 1, Mail Stop EP5, Houston, TX77058. Ph:(281)483-4528; Fax:(281)483-3096; email.-jjeel)ara@ems.jsc.nasa.gov2NAS<A-Johnson Space Center, 2101, NASA Rd 1, Mail Stop EP5, Houston, TX 77058.Ph: (281)483-9060; Fax: (281) 483-3096; entail: bbragg@enzs jsc.nasa.gov3Applied Power International, 1236, N. Columbus Ave, Suite 41, Glendale, CA 91202-1672.Ph:(818)243-3127; Fax:(818)243-3127; email.-wati-acinski@earthlink.net

AbstractLithium-ion batteries of the conventionaland polymer type are being used widely forcellular phones, cameras, camcorders,personal computers, PDAs and in severalother portable electronic equipment. TheElectrofuel 11-ion polymer battery is one ofthe first available polymer batteries to beused for commercial applications. In ourstudy, the tests carried out on these cellswere aimed at determining if these batteriescan be used in extravehicular activity toolsfor both Shuttle and International SpaceStation.

IntroductionSince the discovery of LI000 2 cathode byGoodenough and coworkers in 1980, thesecondary ]]-ion battery technology hasadvanced greatly.' The demand forlightweight, compact secondary batteries forspace, civilian and military applications isalways increasing. Present development inthe field of lithium polymer cells is directedtowards lithium-ion2 (carbon-intercalatedlithium) and lithium-metal polymerelectrolyte high energy density batteries.Lithium-ion polymer batteries are currentlybeing produced for consumer electronicapplications. Arthur D. Little, Bellcore andUltralife were the first ones to develop cellswith solid polymer or gel electrolytes

containing dielectric organic solvents, whichalso served as separators. In the past fewyears several other commercial cellmanufacturers have presented informationon their polymer li-ion cells .4,5,6

Performance and abuse test results, of onekind of 11-ion polymer cell, was presented atmeetings by our group in the past two years.The Electrofuel lithium-ion polymer cellswere one of the first polymer cells to becommercialized for portable electronicequipment like laptops. The lightweight,flat and compact structure make them a goodcandidate for several weight and volumecritical space applications. In this study, theElectrofuel cells were tested to determinetheir perfonnance and abuse tolerances.Physical, electrochemical and safety testswere performed on these cells.

ExperimentalThe Electrofuel PowerPad 160 Tm batterieswere purchased and the cells removed fromthem for testing. The Maccor 4000 with amultirange software was used for the tests.The environmental tests were performedwith an Associated environmental chamberwith a Watlow controller. Discharge intoreversal tests were carried out with powersupplies and diodes.

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Results and DiscussionThe cells were subjected to the testsdescribed below.

Physical and ElectrochemicalCharacterization.Dimensions and weight.The cells have an average dimension of4.008 in. (height) by 5.269 in. (width) by0.366 in.(thickness). The weights of thecells ranged from 234.8 to 244.7 grams withan average of 240.18 grams and a standarddeviation of 2.59 g.The Open Circuit Voltage (OCV).The OCV of the cells were measured andrecorded. The open circuit voltage wasconsistent with an average voltage of 3.7596V with a standard deviation of 0.0135 V.The range varied between 3.7434 to 3.7815V.Closed Circuit Voltage (CCV)The CCV test was performed with a currentof approximately 16.5 A (1.5 C) for 30seconds. The cells averaged a CCV of 3.152V with a standard deviation of 0.058 V. Therange varied between 3.0513 to 3.2698 V.Capacity MeasurementAll cells were subjected to onecharge/discharge cycle before they weresubjected to further testing. The cells werecharged using the constant current/constantvoltage protocol where the cells werecharged using a C/7 current ofapproximately 1.6 A to 4.1 V and then heldat a constant voltage of 4.1 V until thecurrent dropped to about 100 mA.Discharge was at C/7 rate (1.6 A) to a cutoffvoltage of 3.0 V. The average capacity was9.546 Ah with a standard deviation of 0.528Ah.Performance Tests.Rate Capabilit>>.The performance test protocols involveddifferent rates of charge/ discharge cycling

as given in Table 1 to determine theoptimum charge/discharge rates. The testwas stopped when the cells dropped incapacity to below 4.0 Ah. The charging wasperformed with a given current to 4.2 V andthen changed to constant voltage until thecurrent dropped to 100 mA. Discharge wasperformed to 3.0 V using the appropriatecurrent. Figure 1 gives the cycle lifeperformance data for a C/7 charge and C/4discharge.

Table 1: Charge /Discharge RateProtocols Used in the Cycle Life Test.

Charge Rate Dichar e Rate0.1C (I.l A) 0.1C (1.1 A)

Approx. 0.5C (5.0 A) 0.25 C (2.75 A)Approx 0.5 C (5.0 A) 0.15 C (1.6 A)

0.15C (1.6 A) 0.1C (1.1 A)0.15C(1.6A) 0.25C(2.75A)0.15C(1.6A) 0.15C(1.6A)

• Cycle 1

4 .2 -r, Cycle 2

Cycle 5

4 ­Cycle 25

-X -Cycle 50

X3.8Cycle 75

cycle Cycle 90

Cycle 10003.6

3.4Cycle 100 may,:

3.2f i 1

30 2 4 Ah 6 8 10

Figure 1. Cycle life testing for a typicalElectrofuel cell at C/7 Charge and C/4Discharge.

Performance at different temperatures.Four cells underwent five cycles at fourdifferent temperatures of 0, 25, 45 and 65°C. The cells were all charged at ambienttemperature. The cells were soaked at the

appropriate temperatures for approximatelyone hour before discharge. The cells werethen discharged with 1.6 A to 3.0 V. Somecapacity degradation was evident for alltemperature conditions after the five cycles.The capacity degradation was greatest at thelow temperatures and was least at thehighest temperature (Figure 2). The capacitypeaked at +45 °C although +25 °C wasapproximately the same. At 0 °C thecapacity was 10% lower than the maximumcapacity (+45 °C) on cycle 5.

106

45 deg

102 25 degC

Q 65 degCT'—" 9.8U

QAU

9.40 deg C

9.0

0 1 2 3 - 5 6Cycle Number

Figure 2. Performance of the Electrofuel cellat different temperatures.

Effective Internal Resistance.Two cells were subjected to an effectiveinternal resistance test. After three normalcharge/discharge cycles, one cycle ofdynamic internal resistance measurementsversus the state of charge for the cell wasperformed. For this, the cell was chargedusing the CC/CV protocol and thendischarged at 10 % intervals of state ofcharge using high current pulses (1C). Theinternal cell resistance between 20-90 %SOC was 50-55 mohms for the two cellstested.

Safety Tests.Overcharge.A cell was charged to 4.5 V using a currentof 1.6 A. The voltage was maintained fortwo hours. The peak temperature for this

test was 17.3 °C after 2 hours, an increase of1.5 °C over the ambient. There was noventing or noticeable expansion of the cellduring this test.The same cell was charged to 5.0 V using acurrent of 1.6 A and the voltage maintainedfor twelve hours (Figure 3). The peaktemperature obtained was 22.5 °C, anincrease of 5 °C over room temperature. Nosignificant venting occurred; however, therewas expansion of the cell thickness toapproximately 1 inch. No weight loss wasobserved. The capacity delivered duringdischarge after the overcharge test was 15.55Ali.

Figure 3. Overcharge Test on the Electrofuelli-ion polymer cell.

Overdischwge.Fully charged cells were discharged with 1.6A current to 2.5 V/cell, 2.0 V/cell and 1.0V/cell and held at each voltage for 60minutes. The cells were then discharged to0 V and then further taken into reversal for-150 % of the IC capacity and held at thatvoltage for at least two hours. The first partOf the test to 0 V was uneventful. In thereversal portion of the test, the peaktemperature observed was 29 °C afterapproximately 1 hour. There were noobvious signs of venting and this wasconfirmed by the weight loss of 0.1 g, whichwas within the limit of error for the scaleused.

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35 80

Q 30 70

25 60 U

U 20 50

15

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10 300

5 20

High Temperature Exposure and Heat-to-Vent.Fully charged and weighed cells wereexposed to 150 °F for 2 hours. The test wasuneventful with the exception of a mildether like odor at a temperature of —240 °Cand a significant cell expansion. Scorchingof the paper cell wrapper on the top sideonly was observed with no obvious evidenceof the cell venting other than a weight lossof 10.5g.External Short Circuit. Fully charged cellswere shorted using a 0.03 ohm and 0.05 ohmresistor (one cell each). For the 0.05 ohmtest, the initial momentary current draw wascalculated to be in the order of 38 A (Figure4). The initial current then quickly droppedto approximately 15 A and tapered at aslower rate to 3 A after 30 minutes.

10

40 80 120 160

Time (Minutes)

Figure 4. External Short Test on the

Electrofuel cell with a 0.05 ohm resistor.

The temperature began to rise shortly afterthe resistor was attached and peaked at 83°C after about 20 minutes. The temperaturestabilized at 37 °C after 60 minutes andincreased to 42 °C after 120 minutes. Thetemperature again dropped off slowly untilthe test was discontinued. There was nosignificant venting. The weight loss wasmeasured to be 0.15 g and was within thelimits of error for the balance. For the 0.03ohm test, the initial momentary current draw

captured was 62 A and the maximumtemperature observed was 75 °C which wasafter ten minutes into the test.

Summary.The performance tests on the Electrofuel

11-ion polymer cells show that these cellsperform well at low rates of charge anddischarge. The safety tests show that theyare safe under most abusive conditions asexpected of polymer batteries althoughexpansion of the cells occurs in most cases.

Acknowledgment.We would Iike to thank Electrofuel forgiving us a chance to purchase and test theircells and for their technical guidance.

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Goodenough, J. B., Solid State Ionics;1985, 17, 13.

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4. Papers presented at Power 99, SantaClara, CA, 1999.

5. Roller, D. P. and Slane, S., Proceedingsof the 13`h Annual Battery Conference,Long Beach, CA, 1998

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