Cycle Life Characterisation of Large Format Lithium-ion...
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EVS27Barcelona, Spain, November 17 - 20, 2013
Cycle Life Characterisation of Large Format Lithium-ionCells
Raghavendra Arunachala1, Kamyar Makinejad1,Satyajit Athlekar2, Andreas Jossen2, Juergen Garche3
1TUM CREATE, Singapore, [email protected] Universität München, Munich, Germany; 3FCBAT Ulm, Germany
To predict the remaining useful life (RUL), commercially available lithium-ion cells of 63 Ah with nickelmanganese cobalt oxide (NMC) cathode material underwent cycle life ageing tests at different currentrates (C) and temperature. The key performance parameters such cell capacity, discharge capability andimpedance were measured for fresh and aged cells to evaluate the RUL of the cells. The results ofageing tests showed, cell capacity was not influenced by impedance increase slightly regardless of thecycling conditions, rather power fade significant in determining the RUL of the cells. Detailed studyof impedance characteristics revealed series resistance (Rser) and solid electrolyte interface resistance(Rsei) as main contributors to power fade.
Keywords: large format cells, power fade, fast charge, remaining useful life
The lithium-ion battery is characterised by su-perior energy density, power density, low self-discharge and long cycle life . Despite theseadvantages, high cost, safety and ageing are thechallenges to be addressed to make it a promis-ing candidate for electric vehicle (EV) applica-tions. Substantial research had been carried outin the past two decades to overcome these chal-lenges [2, 3]. For EV applications the energydensity is responsible for the distance range andthe power density for high acceleration, highrecuperation power and fast charging capabil-ity. Unfortunately, capacity fade and power fadeare the main drawbacks which affect the perfor-mance of the battery over its lifetime . Struc-tural and chemical changes in the active ma-terial, electrolyte decomposition and formationof solid electrolyte interface (SEI) layer, currentcollector corrosion [5, 6] and separator degrada-tion contribute to ageing [7, 8]. Recent studiesshow that non-uniform spatial distribution of cur-rent causes non-uniform temperature and state ofcharge (SOC) distribution in large format cells[9, 10]. Multi scale multidimensional modelled
by Kim et al.  shows the impact of the celldesign on current, temperature and SOC distri-bution in large prismatic cells. As a result, elec-trical and thermal non-uniformity causes inho-mogeneous ageing within the cell. Large formatcells are becoming more popular in EVs due toreduced number of cells, connections and bettercooling strategy for the battery pack. But limitedstudies have been reported on their ageing mech-anisms and lifetime prediction.In this paper we focus on accelerated ageing testsperformed on large format lithium-ion cells of 63Ah capacity to be used in a battery pack for aprototype electric taxi (E-taxi) for Singapore andevaluate its cycle life ageing characteristics. Asthe ageing tests using real operating conditionsare time consuming and cost ineffective, acceler-ated ageing at high temperature and high currentrate (C) were chosen to be a powerful method.The aim of this work was to study the capacityfade, power fade and fast charging capability ofthe cells. The cycling involving fast charging at3C was aimed to charge the battery pack of anEV in 20 min. In a real application, repeatedfast charging could be needed and sufficient cool-ing is required to keep the battery pack temper-
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ature from exceeding 28 ◦C. When the batteryages due to capacity fade and power fade, the av-erage temperature of the battery pack increases,therefore more cooling power is required to keepit from exceeding the target temperature. Thispaper summarises which factors influence the re-maining useful life (RUL) of a battery and meth-ods to evaluate RUL.
2.1 Test matrixExperimental studies were performed on lithium-ion high power pouch cells with the nominal ca-pacity of 63 Ah and a nominal voltage of 3.7 V.The active material consists of graphite as an-ode material, LiNi1/3Mn1/3Co1/3O2 (NMC) ascathode material and LiPF6 blended with ethy-lene carbonate (EC) and ethyl methyl carbonate(ECM) as electrolyte material.
Table 1: Test matrix for cycle ageing
Cell C TemperatureCharge Discharge ◦C
1 1C 1C 402 3C 1C 253 1C 3C 254 1C 2C 25
Figure 1: Snapshot the cell tested inside MemmertTTC 256 temperature chamber
Cycle ageing tests are being performed at differ-ent temperatures and C. The test matrix is shownin Tab.1. Three cells were used in each testcondition to get statistically reliable data. Thus,the total number of cells tested during this studyamounts to 12.The tests will be concluded when the cells fall be-low of 80% of its nominal capacity (CN ). All ex-periments were conducted using Digatron (MCT200-06-6 ME S) cell testers. Each channel has
Figure 2: Cell mounted on a specially designed cellholder
the voltage range of 0 − 6 V and current range of0.2 − 200 A. The current and voltage parameterswere controlled by a linear unit CAN-LC. Allexperiments were conducted inside a controlledtemperature environment as shown in Fig.1. Thetemperature chambers (Memmert TTC 256 andEspec PU-3J) were used for the purpose of main-taining high ambient conditions (40 ◦C ) and tocool down the cells at faster rate when operat-ing at high currents (up to 189 A). Self-made cellholders were used to mount the cells. The powerand voltage sense connections were isolated. Thecontact screws were locked with a torque meterto provide identical surface contacts for all thecells as shown in Fig.2. The surface temperatureof the cells was measured using AD 590 sensors.
2.2 Ageing and characterisation testThe cells were cycled between 2.7 V and 4.2V. The charging was done in CCCV mode withthe cut-off current 0.05C (3 A) at 4.2 V. Dur-ing the discharge phase, the cells were contin-uously discharged to 2.7 V. Additional safetylimits were used for high C cycling. If thecell temperature exceeded the safety limit duringcharge/discharge, it was allowed to cool down inthe pause step until the temperature reduced toa safe operating condition. The safety limit forcells operating at 25 ◦C and 50 ◦C was set to 50◦C and 60 ◦C respectively. The tests were con-tinued with the previous charge/discharge step.Prior to the ageing tests, initial checkup was per-formed on each cell by running a characterisa-tion test at 25 ◦C: (1) discharge capacity, (2) hy-brid pulse power characterisation (HPPC) and (3)electrochemical impedance spectroscopy (EIS).The details of these tests are described here:(1) For the discharge capacity measurement, the
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cells were completely discharged to remove theresidual capacity. Then they were completelycharged in CCCV regime. 1 h pause time wasprovided between successive charge/dischargesteps to attain temperature and voltage equilib-rium. The fully charged cells were completelydischarged at 1C to measure the cell capacity(C1). (2) HPPC test was performed at every 10%DOD step. Prior to pulse sequence, the fullycharged cells were discharged to 10% DOD at1C and then rested for 1 h. The pulse power se-quence consisted of 3C discharge for 10 s, rest atOCV for 3 min, 3C charge for 10 s and rest atOCV for 3 min . After the pulse sequence,the cells were discharged by 10% DOD at 1Cand rested for 1 h before starting the new pulsesequence. This procedure was repeated for ev-ery 10% DOD increment until the cells reachedthe cut-off voltage 2.7 V. (3) EIS characterisationwas performed at every 20% DOD step startingfrom 0 to 100% DOD, on fully charged cells. Themeasurements were carried out between 700 Hzto 10 mHz at 8 points per decade and an AC am-plitude of 10 mV.After the characterisation test, the cells were cy-cled from a fully discharged state. To evaluatethe state of health (SOH) of the cells, the age-ing tests were interrupted to perform intermedi-ate checkups after every 100 cycle step. The pro-cedure followed during these checkups was sim-ilar to initial checkups. There was a rest periodbetween successive characterisation and ageingtests. A rest period of 8 h and 24 h was providedto cells aged at 25 ◦C and 40 ◦C respectively. Thework flow for the cycle life ageing test is shownin Fig.3.
3 Results and Discussion
3.1 Cell CapacityAt the beginning of ageing test, the capacity ofeach cell was measured. The average capacityfor all 12 cells in the beginning was 63 ± 1.5Ah. For this reason, normalised cell capacity wascalculated for each cell by dividing by its initialvalue. The ageing tests were stopped periodicallyafter every 100 cycles to monitor 1C dischargecapacity of the cells at 25 ◦C. Fig.4 shows thenormalised discharge capacity of cells tested un-der different cycling condition. The cells showedno capacity fade irrespective of the test condi-tions. Even after 400 cycles, the cell capacitieswere nearly constant.Cell capacity is determined by the amount oflithium-ions available to insert into the electrodeby considering the upper and lower cut-off volt-age criteria. In an ideal cell all the lithium-ions should be intercalated into negative elec-trode at charge and back to positive electrodeat discharge. But in reality not all ions are in-tercalated back to positive electrode as some ofthem are trapped and consumed to form SEI layermainly on the anode. Hence the positive to nega-tive electrode mass ratio designed for optimumutilization of the two electrodes is kept higherthan theoretical value . In the first few cycles
Figure 3: Flow chart for cycle life ageing test
negative electrode becomes the limitation for in-tercalation and at the same time formation andgrowth of SEI layer takes place. During this pe-riod the cell capacity is nearly equal to its ini-tial capacity. Once the lithium-ion concentrationfalls below the critical value, due to trapping ofmore ions in the SEI layer, capacity fade willdominate the ageing .
3.2 Power capabilityThe peak power capability (PPC) of cells wasevaluated by performing HPPC test as describedin section 2.2 at different SOC intervals by ap-plying a 3C current pulse for 10 seconds andmeasuring the voltage response. The PPC re-duces as the DOD increases. For EV applica-tion, PPC at 80% and 20% interval are criticalto meet both charge and discharge power de-mands required for the traction. The procedureto calculate PPC is explained in . Figs.5a& 5b shows the normalised power capability ofcells calculated from HPPC test. The cells un-
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Figure 4: Normalised discharge capacity measured at1C current, 25 ◦C for different cycling conditions
derwent rapid power fade in the first 100 cycles.This phenomenon occurs due to the consump-tion of lithium-ions during the first few cyclesto form SEI layer, which increases the internalimpedance of the cells. At later stages of cy-cling, the SEI layer continues to grow and sta-bilise, therefore the rate of power fade decreaseswith subsequent cycles. It can be also observedthat power fade increased with the increase inDOD, which limited the PPC at 20% SOC. Thecells cycled at 3C charge were the maximum af-fected. During fast charge the polarization volt-age is high which causes the cells to reach thecut off voltage sooner and simultaneously tem-
perature increases rapidly due to internal heat-ing of the cells. Since the cells were charged inCCCV, the charging mode switches to CV at 4.2V and continues to charge until the minimum cutoff current is attained. High cell voltage and hightemperature increases the rate of growth of SEIlayer. The cells cycled at 40 ◦C showed signif-icant power fade, as high temperature increasesthe rate of chemical reaction leading to thickerSEI layers.In contrast the high discharge rate showed lowpower fade. Impedance rise is primarily con-tributed by the growth of SEI layer . SEIlayer has linear relationship with square root oftime function [17, 18].At high discharge rateslower average voltage impeded the growth of SEIlayer. The impedance rise has also resulted insteady increase in the average temperature of thecells during cycling.This test is applicable to measure the power capa-bility of the battery. To understand the dynamicbehaviour of a battery, a more detailed frequencybased EIS test is necessary. The details of thistest are described in section 3.3.
3.3 Impedance spectroscopyEIS is a common analysis method used inelectrochemistry to investigate detailed cellimpedance at wide range of frequencies. In thismethod a small sinusoidal current signal is im-posed on the cell to measure its sinusoidal volt-age response. The complex cell impedance at agiven frequency is calculated from the input cur-rent signal and the measured voltage response byassuming the cell behaviour is linear.The theory and measurement procedure for EIStest is described in Waag et al. . The re-sults of the EIS test are shown in Figs.6(a)−(f). In general the cell impedances were relativelyhigher at 20% SOC in comparison to 80% SOCunder all cycling conditions. After 200 cyclesthere was a significant change in the impedancespectra of the cells. The series ohmic resis-tance (Rser) increased, which is attributed toright shift in the x-intercept of impedance spec-tra. In the mid frequency range, a superimpo-sition of two distinct semicircles was observed,indicating the growth of SEI layer. The mid fre-quency impedance is the contribution of chargetransfer resistance (Rct) and SEI resistance (Rsei)in parallel combination with the respective ca-pacitances. Both Rsei and Rser were depen-dent on SOC, which can be observed in Figs.6(b)& (e). Therefore the contribution to the powerfade comes from these resistances. At 400 cy-cles (Figs.6(c) & (f)) Rser and mid frequencysection contributes to the impedance rise. TheSOC dependency diminished in Rser, whereasRsei increased with decrease in SOC. Thereforethe power capability was limited by Rsei.Comparison of individual cycling conditionsshowed that Rsei and Rser significantly limitedthe power capability of the battery under 3C fastcharge condition, with Rsei being the major con-tributor to ageing. At 40 ◦C cycling conditions,Rsei and Rser increased at a steady rate, with Rseidominating at later cycles. The high discharge
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Figure 5: Normalised power capability at 25 ◦C for cells aged under different cycling conditions. (a) at 80% SOC(b) at 20% SOC, calculated from HPPC test
rate (3C discharge) has resulted in marginallylow Rsei in comparison to 2C discharge. Thiseffect was reflected in power capability compari-son
3.4 Remaining useful lifeBattery Management System (BMS) is an insep-arable part of any EV battery, which is respon-sible for maintaining the battery pack within itsnormal operating range by predicting the batterysinternal states such as State of Charge (SOC),State of Health (SOH), State of Life (SOL1 ),State of Function (SOF), State of Safety (SOS),Remaining Useful life (RUL) and other internalstates .As these states are required by BMS to per-form different tasks such as, cell balancing whichis dependent on accurate SOC estimation, RULwhich is dependent on accurate SOH estimation.The initial capacity of cell, also known as beginof life (BOL) capacity is an important parameterin estimating SOH. For a healthy cell, the accept-able capacity degradation range is between 0 to20%. If the capacity of the cell falls below 80%of its BOL capacity, the end of life (EOL) of thecell is reached as described in Eq.1.
CEOL ∼= 0.8 ∗ CBOL (1)SOH of a cell depends on number of factors suchas, number of cycles, DOD per cycle, tempera-ture, magnitude of current, time and abuse oper-ating conditions . As shown in Eq.1, whenthe battery capacity reaches CEOL, its SOH be-comes zero.
SOHC(i) = (1 −C0 − C(i)
0.2) ∗ 100 (2)
1SOH is used to describe the health status of the batterywhereas SOL denotes the remaining life of the battery (howmany cycles remain, usable charge, etc.)
C0 is the capacity of the cell at the BOL andC(i) is the capacity of the cell measured at ithcycle. C0 and C(i) can be measured by cell ca-pacity test as describe in section 3.1. Other cellsparameters like impedance rise or power capabil-ity (Figs. 5 & 6) contribute to EOL as shown inEq.3. It is important to note that when the in-ternal impedance is twice its BOL value, EOL isreached.
REOL ∼= 2 ∗R0 (3)
SOHR(i) = (1 −R(i) −R0
0.2) ∗ 100 (4)
R0 refers to the discharge resistance of the cellat its BOL and R(i) is the impedance of the cellmeasured at ith cycles. R0 and R(i) can be mea-sured by HPPC tests as described in section 3.2.RUL as a part of the BMS task informs the enduser about the remaining life in the battery untilits functionality is lost. In real-time applicationlike BMS of EVs, RUL is predicted through bat-tery model developed for the BMS, this model isbased on the electrical equivalent circuit whichwas previously developed by authors in . Itis noteworthy to remember that RUL predictionis affected by multiple sources of errors like bat-tery modelling inconsistencies, system noise anddegraded sensor . One method to estimatethe RUL of the battery is to use the calculationof the capacity fade and the power fade as shownin Fig.7. The expression for capacity fade andpower fade is described in Eqs.5 & 6.
pf(i) = 1 − power(i)power0
= 1 − R0R(i)
pf(i) = 1 − capacity(i)capacity0
= 1 − C0C(i)
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Figure 6: Nyquist plots at 25 ◦C for the cells aged in different cycling conditions (Fig. (a)−(c) at 80% SOC at 0,200 and 400 cycles respectively, Fig. (d)−(f) at 20% SOC at 0, 200 and 400 cycles respectively)
The cell capacity, HPPC and EIS test results wereanalysed using MATLAB scripts to parameterisethe cell. The reference RUL was calculated fromEq.7 as a function of power fade and capacityfade.
RUL(i) = f(cf(i), pf(i)) (7)
The prediction of RUL of battery will be donein future work. The methodology to predict the
RUL is described in the flowchart in Fig.8. Theparticle filter algorithm and support vector ma-chine (SVM) will be used to predict the RUL ofthe battery. The sensors will read the necessaryoutput data from cell capacity, EIS and HPPCtests. Results of the cell impedance and capac-itance after i cycles will be mapped based on theoutput of the combined algorithm. Finally thepredicted RUL will be evaluated with the refer-
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Figure 7: Flow chart of cell parameter extraction,characterisation and reference data for RUL
ence RUL to find the reliability of the method forEV application.
It was shown that capacities of 63 Ah cellsremained nearly constant till 400 cycles underall cycling conditions (different C and temper-atures). The PPC of cells reduced with the in-crease in number of cycles. Cells cycled under3C fast charge and at 40 ◦C temperature showedsignificant power fade. The impedance charac-teristics of the aged cells showed, Rser and Rseiresistance as dominant contributors to ageing.This work also presented the method to estimateRUL of a battery. The main findings of this work:The cell impedance has continuously increasedcaused by the growth of the anode SEI whichleads to a decrease of the PPC, however, not ofthe C1 value. The latter result contradicts withthe some of the contemporary works. An expla-nation for our capacity result could be that onlya slight increase of the Rser (0.085 mΩ) till 400cycles did not affect the C1 discharge curve andtherefore the C1 value has not change too much.Further studies such as post-mortem analyses,conducting similar tests on short format cells ofsame chemistry could help the understanding ofcomplex ageing mechanism in large format cells.
Figure 8: Flow chart of RUL prediction method
This work has been done in the framework ofCREATE research programme funded by theSingapore National Research Foundation (NRF).
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Raghavendra Arunachala earnedhis B.Eng. in electrical andelectronic engineering fromVisveswaraya Technological Uni-versity, India. He completed hisM.Sc from Rheinische WestflischeTechnische Hochschule (RWTH)Aachen University, Germany in2011. He is currently a doctoralstudent in TUM CREATE, Sin-gapore, working in the area ofelectrical energy storage systems.
Kamyar Makinejad received hisB.Sc. in electrical engineeringin 2007 from Shiraz University,Iran. He earned his M.Sc de-gree from Leibniz University ofHanover, Germany in 2011 respec-tively. He is currently a ResearchAssociate in TUM CREATE inSingapore and pursuing his PhDwith the Technical university ofMunich. His research interest isbattery diagnostic and state deter-mination in EV applications.
Satyajit Athlekar earned his B.Eng.in mechanical engineering fromPune University, India. He isenrolled as a Masters student ofPower Engineering at TechnischeUniversität München, Germany.Currently, he is doing his masterthesis at TUM CREATE, Singa-pore.
Andreas Jossen earned his doctor-ate, dealing with “Management ofphoto-voltaic plants using energystorage systems”at University ofStuttgart. From 1994 he was groupleader for different battery relatedtopics with ZSW, Ulm. Since 2010he is full professor at the Insti-tute for Electrical Energy StorageTechnology, TUM.
Juergen Garche received his PhDand habiltation (post-doctoral lec-turing qualification) in theoreti-cal and applied electrochemistry atDresden University of Technology.He is dealing since >40 years withbatteries and fuel cells. He build upthe ZSW Ulm as director. Now hemanaged the Fuel Cell and Batteryconsulting office FCBAT.
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IntroductionExperimentalTest matrixAgeing and characterisation test
Results and DiscussionCell CapacityPower capabilityImpedance spectroscopy Remaining useful life