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Electrochimica Acta 108 (2013) 191– 195

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

acile preparation and electrochemical characterization of poly4-methoxytriphenylamine)-modified separator as a self-activatedotential switch for lithium ion batteries

aiyan Zhanga, Yuliang Caob, Hanxi Yangb, Shigang Lua, Xinping Aib,∗

General Research Institute for Nonferrous Metals, Beijing 100088, PR ChinaCollege of Chemistry & Molecular Science, Wuhan University, Wuhan, Hubei 430072, PR China

a r t i c l e i n f o

rticle history:eceived 2 January 2013eceived in revised form 11 June 2013ccepted 23 June 2013vailable online xxx

a b s t r a c t

A potential-sensitive separator is prepared by incorporating an electroactive poly (4-methoxytriphenylamine) (PMOTPA) into the micropores of a commercial porous polyolefin filmand tested as an internal voltage control device for overcharge protection of LiFePO4/Li4Ti5O12 lithiumion batteries. The experimental results demonstrate that the PMOTPA polymer embedded in theseparator can be electrochemically p-doped at overcharged voltages into an electrically conductivestate, producing an internal conducting bypass for shunting the charge current to maintain the charge

eywords:ithium ion batteriesafetyvercharge protectionotential-sensitive separatoroly (4-methoxytriphenylamine)

voltage of LiFePO4/Li4Ti5O12 cells at a safety value less than 2.6 V, thus protecting the cell from voltagerunaway. Since this type of the separators works reversibly and has no any discernable impact on thebattery performances, it may offer a self-protection mechanism for development of safer lithium ionbatteries.

© 2013 Elsevier Ltd. All rights reserved.

PMOTPA)

. Introduction

Lithium ion batteries (LIBs) have long been the most popularower sources for various portable electronic devices due to their

ntrinsic advantages of high energy density and excellent cycleife. However, scaling-up LIBs for electric vehicle applications iseverely frustrated by their uncertain safety at abusive conditions1,2]. Apart from the use of highly oxidative cathode and reductivenode, LIBs also use flammable organic electrolytes. These organiclectrolyte solvents are not capable of being reversibly decomposednd recombined, so that the LIBs cannot tolerate strong overcharges the aqueous secondary batteries that can spontaneously controlheir charging voltage at the decomposition voltage of water sol-ent. When overcharged, the voltage of LIB would be monotonouslylimb up to a high value that triggers a number of exothermic sideeactions such as decomposition of the cathode material, organiclectrolyte and the solid electrolyte interphase (SEI) [3–5]. These

eactions produce a large quantity of heat and flammable gas toause rapid increase both in the internal temperature and pressuref the cell, possibly leading to cell cracking, firing or even explosion.o avoid the overcharge abuse in practical applications, LIBs often

∗ Corresponding author. Tel.: +86 27 68754526; fax: +86 27 87884476.E-mail address: xpai@whu.edu.cn (X. Ai).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.06.116

need to be equipped with an electronic control device for monitor-ing their charging voltages. This method is effective, but not alwaysreliable, especially for the power battery systems that are usuallycombined by hundreds of individual cells in parallel or in seriesconnections.

In fact, the most reliable method for voltage control of LIBs isto establish an internal and self-actuating safety mechanism toavoid the voltage runaway at overcharge. Based this consideration,various types of redox shuttles [6–12], as well as polymerizablemonomers [13–16], have been proposed as safety electrolyte addi-tives to prevent LIBs from overcharging. The former can protectthe cell from overcharge by locking the potential of the posi-tive electrode at the redox potentials of the shuttle molecules.However, limited either by their insufficient stability in long-termoperation or by their poor solubility in commonly used organicelectrolytes, the most of shuttle molecules reported so far cannotyet meet the requirement for practical battery applications. Thelater can prevent the overcharged battery from voltage runawayby the electro-oxidative polymerization of the monomer additive athighly oxidized cathode, which produces a dense polymer layer toclose down the cathode surface. Many aromatic compounds, suchas biphenyl [13], xylene [14] and cyclo-hexylbenzene [15,16], have

been reported to be capable of acting as polymerizable safety addi-tives to improve the overcharge tolerance of LIBs. Unfortunately,the use of these additives is indeed not a preferable choice becausethis type of overcharge protection can work only once. Once the

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athode surface is closed down by the electropolymerized film, theell will be dead permanently.

As is well-known, some electroactive redox polymers, such asolypyrrole [17,18], polythiophene [19] and polytriphenylamine20], can be electrochemically p-doped at their oxidation potentialsnto electronically conductive phase and reversibly de-doped atheir reduction potentials into the neutral isolating state. Based onhese properties, we successfully developed the potential-sensitiveeparators by incorporating a conductive polymer into the poresf commercially used polyolefin separator [21,22]. Under normalharging conditions, the electroactive polymers in the separatorxists in their undoped states (neutral insulating states) so that theeparator works normally to allow the electrolyte passing through.nce the cell is overcharged to raise the cathode potential above

he oxidation potential of the polymers, the incorporated polymersould be electrochemically p-doped to convert into a conducting

tate, producing a current bypass for the charging reaction throughhe conductive polymer bridge in-between both the electrodes,o as to prevent the battery from overcharge. Our previous stud-es revealed that this type of separators can provide not only aeversible electrochemical switch for the battery reactions, but also

sufficiently high shunting current for the overcharged LIBs.In this paper, we report a new potential-sensitive separator,

poly (4-methoxytriphenylamine) (PMOTPA)-modified polyolefineparator with a strong overcharge tolerance for LiFePO4-basedIBs. The electrochemical behaviors of this separator and itsnfluences on the normal charge–discharge performances of theiFePO4/Li4Ti5O12 cells are also described.

. Experimental

4-Methoxytriphenylamine (4-MOTPA) (purity ≥97%,igma–Aldrich) was used as received without further purifi-ation. The electrolyte used in this study was 1 M LiPF6 in a 1:1:1by vol.) mixture of ethylene carbonate (EC), dimethyl carbonateDMC) and ethyl-methyl carbonate (EMC) purchased from Guotai-uarong New Chemical Materials Co., Ltd (Zhangjiagang, China).he test batteries used in this study were LiFePO4–Li4Ti5O12 coinells. The cathode electrode comprised 80 wt.% LiFePO4, 12 wt.%cetylene black, and 8 wt.% polyvinylidene fluoride (PVDF) asinder. The anode electrode was composed of 80 wt.% Li4Ti5O12,2 wt.% acetylene black, and 8 wt.% PTFE as binder. The separatoras a PMOTPA-modified polyolefin film. The cells were assembled

n an argon-filled glove box.PMOTPA polymer used in this study was synthesized by chemi-

al oxidative polymerization of 4-MOPTA monomer in chloroformolution using FeCl3 as an oxidant [23]. The mole ratio of FeCl3xidant to monomer was designed as 4:1. A typical experimentrocedure is as follows: 1.38 g 4-MOTPA (5 mmol) was added into

two-necked 50 ml flask containing 10 ml chloroform under nitro-en atmosphere and magnetic stirring. 3.25 g FeCl3 (20 mmol) wasivided into four equivalent portions and then added respectively

nto the reaction solution at an interval of 1 h. The reaction mix-ure was stirred under nitrogen atmosphere at 45 ◦C for 20 h andhen poured into methanol containing small amount of hydrochlo-ic acid for precipitating out the polymer product. The polymerrecipitate was collected and washed several times with methanolnd then dried at 60 ◦C for 12 h to obtain a light yellow product. Allhe chemicals used in this synthesis process were obtained com-

ercially and used as received without further purification.The PMOTPA-modified separator was prepared firstly by dis-

olving the electro-active PMOTPA polymer into chloroform to

orm a homogeneous polymer solution with a concentration of

wt.%, and then immersing a commercial porous polyolefin mem-rane (UBE UP3074, 20 �m thick, 50% porosity) into the polymerolution for ∼1 min, and finally, evaporating the solvents off to

cta 108 (2013) 191– 195

leave a modified separator with a thickness of 24 �m. The load-ing weight of PMOTPA polymer on the separator is ∼0.1 mg cm2,which was calculated by the mass change of the separator beforeand after modification. The porosity and the pore size distributionof the separator samples were measured with a mercury porosime-ter (Autopore IV 9500, Micromeritics). In these measurements, atleast three specimens were tested, and the average value was taken.The tensile strength of the separators was measured at room tem-perature by a tensile machine (UTM-Zwick/roell-Z010, Germany).The size of the specimens was 5 mm × 20 mm, and the head speedwas 10 mm/min.

The surface morphologies of the separators before and afterPMOTPA modification were examined using a Quanta 200 Scan-ning Electron Microscope (FEI Company, Netherlands). The effectof PMOTPA modification on the structure of the separator wasanalyzed using a Nexus 670 FTIR Spectrometer (Thermo-NicoletCorp.). The spectra were measured in reflection mode in a wavenumber range of 4000–650 cm−1. The electrochemical behaviorof the PMOTPA polymer synthesized in this study was charac-terized by cyclic voltammetry (CV) at a powder microelectrodewith a diameter of 100 �m. The preparation method for pow-der microelectrode has been described in detail in Ref. [24]. Thevoltammograms were recorded on a two-electrode cell with a largelithium sheet as both counter electrode and reference electrodeon a CHI 600c electrochemical workstation (Shanghai, China). Theeffects of PMOTPA-modified separator on the overcharge behaviorsof the LiFePO4–Li4Ti5O12 coin cells were investigated by galvano-static charge–discharge at a voltage interval of 1.0–3.0 V. The chargeand discharge measurements were carried out on a programmablecomputer-controlled battery charger (Land Battery Testing System,Wuhan, China).

3. Results and discussion

As for the potential-sensitive separator, the electroactive poly-mer must be required to have a very high p-doping/de-dopingreversibility in the commonly used organic electrolytes and anappropriate p-doping potential, which is slightly higher than thecut-off potential of the charged cathode and much lower thanthe decomposition potential of the organic electrolyte. In LiFePO4-based LIBs, the full charge of the cathode completes at ∼3.6 V(vs. Li+/Li). Therefore, the oxidation potential of the electroactivepolymer chosen for the potential-sensitive separator must be onlyslightly higher than 3.6 V, so as to ensure an effective overchargeprotection as well as the full charging of the protected cathodesimultaneously.

Fig. 1 compares the CV curves of LiFePO4 and PMOTPA in 1 MLiPF6/EC + DMC + EMC electrolyte. As shown in Fig. 1, the PMOTPApolymer gives a pair of well-defined redox peaks between 3.0 and4.2 V, the anodic and cathodic branches of which are very sym-metric in peak shape and peak areas, indicating a highly reversibleand very fast electrochemical redox behavior of the polymer.Based on the electrochemical reaction mechanism of conductingpolymers, the electro-oxidation of PMOTPA should represent thep-doping process of the PF6

− anion from the organic electrolyteinto the polymer chains for charge counterbalancing. Reversely,the electro-reduction of the oxidized PMOTPA should correspondto the de-doping process with the simultaneous extraction of thePF6

− anions from the polymer chains. It can be found by comparingthe CV curves in Fig. 1 that the anodic current of LiFePO4 represent-ing its charge reaction starts from 3.3 V and reaches its maximum at3.45 V, whereas the oxidation peak for PMOTPA, corresponding to

its p-doping process, arises from a higher potential of 3.55 V, at least100 mV more positive than the anodic peak potential of LiFePO4,implying that the electrochemical oxidation of the PMOTPA poly-mer occurs only after the complete charge of the LiFePO4 electrode.

H. Zhang et al. / Electrochimica Acta 108 (2013) 191– 195 193

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As shown in Fig. 4, the spectrum of the blank separator shows thecharacteristic IR absorption bands of polyolefin at 2835–2960 cm−1

and 1376–1460 cm−1, corresponding to the C–H stretching vibra-tion and the CH2 rocking vibration of polypropylene (PP) or

ig. 1. A comparison of the CV curves of PMOTPA and LiFePO4 obtained both fromowder microelectrodes in 1 M LiPF6/EC + DMC + EMC electrolyte at a scan rate of0 mV s−1.

n the reversed cathodic scan, the electrochemical reduction ofhe polymer, corresponding to the de-doping process of the oxi-ized PMOTPA, almost completes at 3.4 V, where the reductionf the oxidized LiFePO4 just starts to take place, indicating thathe electrochemical reduction of PMOTPA polymer occurs beforehe cathodic discharge of LiFePO4 electrode. In addition, it alsoan be seen from Fig. 1 that the anodic and cathodic branches ofMOPTA are very close to each other in the peak positions andeak areas, indicating the very rapid and reversible electrochemical-doping and de-doping behaviors of the polymer. These CV fea-ures indicate that the PMOTPA-modified separator can be suitablysed as an electroactive switch for controlling the charge voltage ofhe LiFePO4-based LIBs without much interference for the normalharge–discharge of the cells.

Fig. 2 compares the SEM images of the polyolefin separatorefore and after PMOTPA modification. As can be seen, the commer-ial blank separator has a clear surface and uniformly distributedore structure as shown in the inset of Fig. 2a. After modification,he separator surface was overlaid uniformly by a thin layer ofolymer network. Furthermore, from the locally magnified SEM

mage of the modified separator (the inset in Fig. 2b), it also cane found that the porous structure of the separator also becomesartially blocked. These phenomena demonstrate that the PMOTPAolymer was not only coated simply on the separator surface, butlso successfully penetrated into the pores of the separator. Fromhe inset in Fig. 2b, it also can be found that only some parts ofhe separator are filled by the polymer and there still remain aarge quantity of void pores in the modified separator. Therefore,t can be expected that the PMOTPA-modified separator can notnly function as normal separator to pass through electrolyte byhe remaining pores, but also serve as a potential switch to provideeversible self-actuating current shunt for overcharge protection ofIBs through the reversible p-doping/de-doping of the pore-filledMOTPA.

In order to ascertain the effect of PMOTPA modification onhe mechanical properties of the separator, we measured the ten-ile strength of the separator before and after modification. TheD tensile strength is ∼1740 Kg cm−2 for the blank separator and1650 Kg cm−2 for the modified separator, indicating that the mod-

fication of PMOTPA polymer only caused a slight decrease in theensile strength of the separator, which should not interfere withhe normal use of the separator in LIBs. Fig. 3 compares the poros-

ty of the separator before and after modification. As shown in thegure, the commercial blank separator has a high porosity of 50%.fter modification, the porosity of the separator was decreased to4%. In addition, from the pore size distribution of the separator

Fig. 2. SEM images for the separator (a) before and (b) after PMOTPA modification.The insets in (a) and (b) are the magnified images of the separators.

(the inset in Fig. 3), it also can be found that the most probablepore size of the blank separator is ∼0.1 �m, while that of the mod-ified separator lies in a smaller value of ∼0.08 �m. These resultsfurther demonstrated that PMOTPA polymer has successfully filledinto the pores of the separator and the modified separator has apartially blocked porous structure.

The effect of PMOTPA modification on the structure of the sepa-rator was also analyzed by FTIR spectroscopy. Fig. 4 shows the FTIRspectra of the polyolefin separator before and after modification.

Fig. 3. The porosity distribution curves of the separators before and after PMOPTAmodification. The inset is the pore size distribution curves of the separators.

194 H. Zhang et al. / Electrochimica Acta 108 (2013) 191– 195

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ig. 4. FTIR spectra of the separator (a) before and (b) after PMOTPA modification.

olyethylene (PE) main chains respectively. However, in thepectrum of the PMOTPA-modified separator, the characteris-ic absorption bands of polyolefin disappear. Instead, new bandsharacterizing the IR absorption of PMOTPA polymer appear at460–1600 cm−1, 1280–1318 cm−1 and 1037–1240 cm−1, whichan be assigned to the C C in-plane stretching vibration of theenzene ring, the C N stretching vibration and the C O C bandtretching vibration. The FTIR analysis results further confirmedhat PMOTPA polymer has been successfully modified onto theurface of the polyolefin separator.

Fig. 5 shows the charge–discharge curves of the Li/LiFePO4oin cells using PMOTPA-modified separator at a current rate of.382 mA cm−2. As shown in Fig. 5, the charging potential profilef the LiFePO4 cathode displays a typical charging plateau at about.5 V at normal charge region, which is identical to that obtainedrom the cells using conventional separator, indicating an indis-ernible impact of the PMOTPA-modified separator on the normalharge behavior of the LiFePO4 cathode. Once the cell was over-harged, the charging potential of the cathode climbs up steeplyt first and then produces a steady plateau at ∼3.72 V (vs. Li+/Li),howing an effective control of the charging potential. Since theharging plateau is in good agreement with the p-doping potentialf the PMOTPA, the overcharge plateau in Fig. 5 should be origi-

ated from the oxidized p-doping of the PMOTPA polymer, whichransforms from its original neutral insulating state to an electron-cally conductive state, thus leading to an internal current bypass.n the following discharge, the LiFePO4 cathode still delivers a high

ig. 5. Charge–discharge curves of the Li/LiFePO4 cell using a PMOTPA-modifiedeparator at a constant current of 0.382 mA cm−2.

Fig. 6. Charge–discharge curves of the LiFePO4/Li4Ti5O12 coin cells using a PMOTPA-modified separator at a current density of 0.382 mA cm−2.

capacity of ∼128 mAh g−1 even subjected to a 50% overcharge of itsnominal capacity, indicating no negative impact of the PMOTPA-modified separator on the discharge of the LiFePO4 cathode.

To further evaluate the feasibility of the PMOTPA-modifiedseparator for practical battery use, we assembled theLiFePO4/Li4Ti5O12 coin cells using PMOTPA-modified separa-tor and tested their overcharge behaviors. It is well-known that theworking voltage of a cell corresponds to the potential differenceof its cathode and anode. Since the charge plateau of the LiFePO4cathode is at 3.45 V (vs. Li+/Li) and the discharge plateau of theLi4Ti5O12 anode is at 1.5 V (vs. Li+/Li), the charge voltage of theLiFePO4/Li4Ti5O12 cells should be 1.95 V. Fig. 6 displays the typicalcharge–discharge curves of the LiFePO4/Li4Ti5O12 coin cells at acurrent density of 0.382 mA cm−2. As shown in the figure, the testcell displays a voltage plateau at ∼1.93 V in the normal chargingprofiles, which is very close to the calculated charge voltage of theLiFePO4/Li4Ti5O12 cells. When the cell is subjected to overcharge,the charging voltage of the cell rises up steeply at the onsetof overcharge and then levels off to produce a voltage plateauat ∼2.27 V, suggesting an effective self-locking of the chargingvoltage. After 80 cycles of overcharging test, the overchargingvoltage plateau remains almost unchanged even the cell wasovercharged for a quite long time of 50% of its normal capacity percycle, indicating a highly reversible and effective overcharge pro-tection of the PMOTPA-modified separator for LiFePO4/Li4Ti5O12cells. Nevertheless, it should be pointed out that, as the cyclingnumber increases, the normal charge plateaus of the cathode riseup gradually and its discharge plateaus correspondingly lowerdown, implying a gradually increased polarization at chargeand discharge. This polarization is most likely given rise by thefurther oxidative polymerization of the PMOTPA polymer at theovercharged cathode, leading to a decrease in the porosity of themodified separator. In addition, it also can be seen from Fig. 6that the normal charge and discharge plateaus remain almostunchanged after 60 cycles, indicating the completion of theelectrochemical polymerization of PMOTPA.

An important factor determining the suitability of the modifiedseparator for applications in practical LIBs is its sustainable currentdensity at charge and discharge. Fig. 7 shows the charge–dischargecurves of the LiFePO4–Li4Ti5O12 coin cells using a PMOTPA-modified separator at various current densities. It can be seen that,as the current density increases from 0.4 to 1.8 mA cm−2, the over-

charge voltage plateaus of the test cells are only slightly elevatedand can all be well controlled at the values below 2.60 V. Mean-while, the discharge capacities of the LiFePO4 cathode are onlydecreased inconspicuously from 140 mAh g−1 at 0.4 mA cm−2 to

H. Zhang et al. / Electrochimica A

Fig. 7. Charge–discharge curves of the LiFePO4–Li4Ti5O12 coin cells using a PMOTPA-modified separator at various current densities.

Fig. 8. Cycling performance of the LiFePO4/Li4Ti5O12 coin cells using conven-to

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[23] J.H. Sim, K. Yamada, S.H. Lee, S. Yokokura, H. Sato, Synthesis and character-ization of triphenylamine derivatives by oxidative polymerization, Synthetic

ional bare separator and PMOTPA-modified separator at normal voltage intervalf 1.0–2.15 V at 0.382 mA cm−2.

15 mAh g−1 at 1.8 mA cm−2, showing a strong high rate charge andischarge capability.

As an applicable separator, it is very required not only to beapable of providing an effective voltage control at overcharge, butlso to have no negative impacts on the normal charge–dischargeerformances of cells. To evaluate the influence of the PMOTPA-odified separator on the cycling performance of the cells, we

ycled the LiFePO4/Li4Ti5O12 coin cells using both conventionalare separator and PMOTPA-modified separator at normal volt-ge interval of 1.0–2.15 V at a current rate of 0.382 mA cm−2. Ashown in Fig. 8, these two types of cells exhibit no any discern-ble difference in capacity during 100 cycles of galvanostatic chargend discharge test, demonstrating a practical applicability of theMOTPA-modified separator.

. Conclusion

In summary, we prepared a PMOTPA-modified separatornd tested it as a potential-sensitive separator for overchargerotection of LiFePO4–Li4Ti5O12 cells. The experimental resultsemonstrated that this separator could transform reversibly fromn electronically isolating state to a conductive state at overcharged

oltages, producing an internal current bypass to prevent the cellsrom voltage runaway. In addition, this separator had no significantmpact on the normal charge–discharge performance of the cells,

[

cta 108 (2013) 191– 195 195

showing a great prospect for practical application in LiFePO4-basedlithium ion batteries

Acknowledgments

This work was financially supported by the National High-techR&D Program of China (2011AA11A256 and 2012AA110102), theNational Basic Research Program of China (2009CB220100), andthe Fundamental Research Funds for the Central University.

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