Electrochemistry Communications 10 (2008) 1912–1915

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Electrochemistry Communications

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Plastic–polymer composite electrolytes: Novel soft matter electrolytes forrechargeable lithium batteries

Monalisa Patel, Aninda J. Bhattacharyya *

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, Karnataka 560012, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 September 2008Received in revised form 3 October 2008Accepted 7 October 2008Available online 14 October 2008

Keywords:Solid composite electrolytesPlastic crystalsIonic conductivityMechanical propertyRechargeable lithium battery

1388-2481/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.elecom.2008.10.009

* Corresponding author. Tel.: +91 80 2293 2616; faE-mail address: aninda_jb@sscu.iisc.ernet.in (A.J. B

We present here a soft matter solid composite electrolyte obtained by inclusion of a polymer in a semi-solid organic plastic lithium salt electrolyte. Compared to lithium bis-trifluoromethanesulfonimide-suc-cinonitrile (LiTFSI-SN), the (100 � x)%-[LiTFSI-SN]: x%-P (P: polyacrylonitrile (PAN), polyethylene oxide(PEO), polyethylene glycol dimethyl ether (PEG)) composites possess higher ambient temperature ionicconductivity, higher mechanical strength and wider electrochemical window. At 25 �C, ionic conductivityof 95%-[0.4 M LiTFSI-SN]: 5%-PAN was 1.3 � 10�3 X�1 cm�1 which was twice that of LiTFSI-SN. TheYoung’s modulus (Y) increased from Y ? 0 for LiTFSI-SN to a maximum �1.0 MPa for (100 � x)%-[0.4 M LiTFSI-SN]: x%-PAN samples. The electrochemical voltage window for composites was approxi-mately 5 V (Li/Li+). Excellent galvanostatic charge/discharge cycling performance was obtained withcomposite electrolytes in Li|LiFePO4 cells without any separator.

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1. Introduction

Soft matter electrolytes [1–7] are very promising for electro-chemical applications such as rechargeable lithium batteries[1,8]. Polymer electrolytes [1,2], the most widely studied soft mat-ter electrolyte system, generally have very low ambient tempera-ture ionic conductivity (610�6 X�1 cm�1). Two of the mostwidely employed approaches for improving ionic conductivity ofpolymer electrolytes have been (a) heterogeneous doping [9] ofpolymer electrolytes with functional oxide materials [10] and (b)addition of non-aqueous molecular liquid solvents to form gel elec-trolytes [11]. However, both approaches fail to optimize materialsproperties requisite for electrochemical applications. In the light ofsuch shortcomings, new soft matter electrolytes based on plasticcrystalline [4–6] and ionic liquids [7] have received considerableattention. While these electrolytes exhibit high ambient tempera-ture ionic conductivity and wide electrochemical voltage window,they have poor mechanical properties. Succinonitrile (N�C�CH2�CH2�C�N), a highly polar and non-ionic solid organic molec-ular plastic material [4,5] forms a promising matrix for generationof solid ionic conductors with ambient temperature ionic conduc-tivity �10�2 X�1 cm�1. SN-salt electrolytes conduct only in theplastic phase of SN (approximate range: �30 to 60 �C) due to pres-ence of trans-gauche isomerism involving rotation of moleculesabout the central C–C bond [4,12]. The trans isomer has been pro-posed as an impurity phase which results in enhancement of lat-

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tice defects and lowering of activation energy for ionicconduction. In this contribution we demonstrate a procedurewhere improvement in ionic conductivity, mechanical propertyand electrochemical stability of LiTFSI-SN, a prototype plastic elec-trolyte is obtained via dispersion of a polymer (PAN, PEO, PEG).

2. Experimental

PAN, PEO, PEG all from Aldrich and LiTFSI were heat treated un-der vacuum and SN (Aldrich) was sublimated twice prior to elec-trolyte preparation. The composites were prepared by castingmolten mixture of polymer (5–15% weight of SN) and 0.4 M LiT-FSI-SN in teflon moulds and subsequently dried and stored undervacuum at 25 �C. Due to lack of mechanical strength 0.4 M LiT-FSI-SN sample was stored in glass vial. Ionic conductivity versustemperature of plastic polymer films (film thickness = 0.2 cm)was estimated using ac-impedance spectroscopy (Novocontrol).Tensile stress versus strain measurements (Gabo) were performedat 25 �C. Various phase transitions were obtained from differentialscanning calorimetry (Mettler Toledo) and microstructure from X-ray diffraction (Philips, Cu � Ka radiation). Cyclic voltammetry (CHinstruments) was done with stainless steel as working electrodeand lithium metal as reference and counter electrode with a scanrate of 1 mV/s in the voltage range of �0.5–5 V. Galvanostaticcharge/discharge cycling was done at various current rate valuesof 7–31 mA g�1 with composite electrolytes (film thick-ness = 0.08 cm; 80 mg cm�2) in Li|LiFePO4 [13]. LiFePO4 comprised85% of the total composite cathode weight (carbon black (Alfa Ae-sar): 8%, polyvinylidene fluoride (Kynarflex 2800): 7%). All cell

M. Patel, A.J. Bhattacharyya / Electrochemistry Communications 10 (2008) 1912–1915 1913

assembly was done at 25 �C in a glove box (MBraun) under argon(water: <0.1 ppm).

3. Results and discussion

Fig. 1 shows ionic conductivity (r) versus temperature for(100 � x)%-[0.4 M LiTFSI-SN]: x%-P (P = PAN, PEO, PEG) samples.The error bars for r are approximately ±10%. At 25 �C, in general,r shows an initial increase with increase in x, reaches a maximumand then decreases after the maximum (Fig. 1 inset). At 25 �C con-ductivity of 95%-[0.4 M LiTFSI-SN]: 5%-PAN was 1.3 � 10�3 X�1 cm�1

which was 2 � r0.4M LiTFSI-SN Conductivity of 97.5%-[0.4 M LiT-FSI-SN]: 2.5%-PEG and 97.5%-[0.4 M LiTFSI-SN]: 2.5%-PEO at 25 �Crecorded a value of 9.8 � 10�4 X�1 cm�1 (1.4 � r 0.4 M LiTFSI-SN)and 3 � 10�4 X�1 cm�1 (0.9 � r0.4M LiTFSI-SN), respectively. Thecomposite conductivities are 2–5 orders higher compared to re-cently reported solid glyme (CH3O(CH2CH2O)nCH3, n = 3,4) electro-lytes [1,2]. Ionic conductivity at low temperatures (such as at�10 �C) �1.2 � 10�5�5 � 10�5 X�1 cm�1 also promises a possibleapplication of the composite electrolytes in low temperature bat-tery operations [14]. Whereas addition of PAN and PEO lead tosolid electrolyte for all values of x, free standing films for LiTFSI-SN-PEG composites could not be obtained using the drying proce-dure described in Section 2. The appearance of the PEG compositeswas similar to LiTFSI-SN (or SN). Further, among PAN and PEOcomposites, PAN composites were more elastic than PEO. In lightof the conductivity results and visual observations regardingmechanical strength, we discuss only the (100 � x)%-[0.4 M LiT-FSI-SN]: x%-PAN composites. Increase in (100 � x)%-[0.4 M LiTFSI-SN]: x%-PAN ionic conductivity over that of LiTFSI-SN is attributedto decrease in crystallinity (Fig. 2A) which probably enhances thetrans concentration. The trans phase being a high defect densitystate [4,12] results in increase in ionic mobility. Further, presenceof CN functional groups on PAN (–CH2–CH–C�N–)n and SN in thecomposite samples leads to a competition between the two to bindLi+. As a result there is a high probability that the composites alsohave higher free Li+ ion concentration compared to LiTFSI-SN. Low-er activation energy, Ea (obtained using Arrhenius equation) in theplastic phase for composites (Ea � 0.5 eV especially x 6 7.5%) com-pared to LiTFSI-SN (Ea = 0.6 eV) serve as additional evidence for our

Fig. 1. Ionic conductivity versus temperature and at 25 �C (inset) for (100 � x)%-[0.4 M LiTFSI-SN]: x%-P (P: PAN, PEO, PEG) samples.

Fig. 2. (A) X-ray diffraction at 25 �C and (B) differential scanning calorimetry(heating rate = 10 �C/min) for various samples.

proposition of increase in ion mobility and charge carrier concen-tration. The subsequent fall in ionic conductivity after the maxi-mum is due to decrease in effective charge carrier concentration(increase in x effectively reduces salt content in composites andblocking of percolation pathways by the polymer. In case of PEO((–CH2CH2O)n) and PEG no appreciable enhancement in ionic con-ductivity is observed probably due to the fact that O� being moreelectronegative than N� (on SN) leads to Li+ trapping.

The XRD pattern (Fig. 2A) for LiTFSI-SN appears to be more crys-talline than SN. This is attributed to formation of LiTFSI-SN com-plexes [15]. Inclusion of PAN in LiTFSI-SN leads to completelyamorphous samples. Peaks pertaining to SN, LiTFSI-SN and PANare absent in composites. Competitive interaction between thecomposite constituents viz. SN and PAN having the same CN func-tional group leads to lowering in crystallinity. Evidence of interac-tion between CN moieties on PAN and SN can also be observedfrom SN: 7.5%/15%–PAN samples which clearly show shift in peakpositions with regard to SN. Endothermic peaks (Fig. 2B) at�30.8 �C (DHnp = 75 J g�1, DSnp = 24 J mol�1 K�1) and 58 �C(DHm = �35 J g�1, DSm = 8.5 J mol�1 K�1) correspond respectivelyto normal to plastic crystalline (Tnp) and melting (Tm) transitions

Fig. 4. (A) Galvanostatic cycling and cyclic voltammograms (inset) at 25 �C and (B)plot of specific charge/discharge capacity versus cycle number and rate capability(inset) at 25 �C.

1914 M. Patel, A.J. Bhattacharyya / Electrochemistry Communications 10 (2008) 1912–1915

for SN. For LiTFSI-SN Tnp shifts by approximately 5 �C to �34.5 �C(DHnp = 80 J g�1) whereas for composites (especially for x 6 7.5%)Tnp shifts to lower temperatures by 1–2 �C (DHnp � �76 J g�1; perSN weight). For x > 7.5% no appreciable shift in Tnp with regard toSN was observed. The observed changes in Tnp are directly relatedto trans-gauche isomerism. In general, presence of impurity re-duces the trans-gauche energy barrier [12] compared to SN andhence higher probability for SN to exist in trans phase at T < Tnp

of SN. Higher disorder is further confirmed from slightly higherDSnp values for x 6 7.5% samples (�27 J mol�1 K�1; per SN weight).LiTFSI-SN and (100 � x)%-[0.4 M LiTFSI-SN]: x%-PAN are moreamorphous than SN. Melting endotherm appear at lower tempera-tures (Tm = 29–44 �C) and are shallower ((85–95%)-[0.4 M LiTFSI-SN]: 5–15%-PAN: D Hm = 16–25 J g�1) compared to pure SN(DHm = 27.4 J g�1). Thus higher amorphicity and trans conformerconcentration accounts for the enhancement in ionic conductivityobtained for composites with x = 5–7.5%.

Although suitable mechanical property is a much desired fea-ture for electrolytes in electrochemical applications, but very rarelyefforts are made to report electrolyte mechanical properties. Ten-sile testing (Fig. 3) of LiTFSI-SN was not possible due to its verypoor mechanical properties. As mentioned earlier, LiTFSI-SN wasdifficult to cast and was liquid-like (Young’s modulus (Y), Y ? 0dashed line in Fig 3). With inclusion of PAN drastic changes wereobserved in the mechanical property of the LiTFSI-SN plastic elec-trolyte. Stress versus strain plot for composites show that the filmsare partially elastic, with degree of elasticity depending on x. Com-posites with x 6 10% have a higher elongation at break (300–480%of initial length = 1 cm) than with x > 10% (70–190%). Young’s mod-ulus for (100 � x)%-[0.4 M LiTFSI-SN]: x%-PAN range from 0.01 to1.0 MPa.

Fig. 4 shows lithium battery feasibility studies of the plastic–polymer composites in SwagelokTM cells without separators. Thecomposite has a wider electrochemical voltage window comparedto LiTFSI-SN (Fig. 4A inset). 92.5%-[0.4 M LiTFSI-SN]: 7.5%-PAN wasstable approximately up to 5 V versus Li/Li+ whereas an irrevers-ible oxidation in LiTFSI-SN was observed at 4.5 V versus Li/Li+

[16]. Galvanostatic cycling of 92.5%-[0.4 M LiTFSI-SN]: 7.5%-PANwas done in a Li|LiFePO4 cell with current rates equal to 7–31 mA g�1 (voltage range: 2–3.8 V). The plastic polymer electro-lytes showed excellent charge/discharge cycling (Fig. 4A) andcapacity retention over several cycles (Fig. 4B). This suggests thatthe plastic–polymer composites do not have any detrimental effect

Fig. 3. Stress versus strain% of (100 � x)%-[0.4 M LiTFSI-SN]: x%-PAN samples.

towards lithium. The Li|92.5%-[0.4 M LiTFSI-SN]: 7.5%-PAN|LiFePO4

cell delivered an average capacity of 163 mA hg �1 (1st cycle:173 mA hg�1) and 153 mA hg�1 (1st cycle: 151 mA hg�1) respec-tively for charge and discharge cycles. The composite electrolytesalso showed satisfactory rate performance (Fig. 4B inset). The dis-charge capacity changes from 153 mA hg�1 (7 mA g�1) to101 mA hg�1 (31 mA g�1).

4. Conclusions

We have successfully demonstrated here a soft matter solidcomposite electrolyte for prospective application in rechargeablelithium battery. The (100 � x)%-[LiTFSI-SN]: x%-PAN system is aprototype of a new class of soft matter electrolytes which mightmake polymer separators for lithium battery systems redundantand also lead to compact cell design. Further optimization with re-gard to materials processing and choice of salt and polymer, wouldlead to possible application of the present plastic–polymer com-posites with a wide range of lithium battery electrode materialsincluding high-voltage cathode materials. The method employedhere to convert a liquid to a solid electrolyte under suitable optimi-zation, may also be extended to other soft matter systems such asionic liquids.

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Acknowledgements

The authors acknowledge S. Pandit, I.S. Jarali for technical sup-port, Morita Chemical Company for LiTFSI, R. Dominko for C-LiFe-PO4. MP and AJB thank CSIR and DST (SR/S1/PC-07/2007) for JRFand funding, respectively.

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