A comparative study of lithium and sodium salts in PAN-based ion conducting polymer electrolytes

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1 Ionics (2010) 16: DOI /s ORIGINAL PAPERS A comprtive study of lithium nd sodium slts in PAN-sed ion conducting polymer electrolytes Zurin Osmn & Khirul Bhiyh Md Is & Azhr Ahmd & Lisni Othmn Received: 20 My 2009 /Revised: 23 Octoer 2009 /Accepted: 23 Novemer 2009 /Pulished online: 13 Jnury 2010 # Springer-Verlg 2010 Astrct The conducting polymer electrolyte films consisting of polycrylonitrile (PAN) s the host polymer, lithium triflte (LiCF 3 SO 3 ) nd sodium triflte (NCF 3 SO 3 ) s inorgnic slts were prepred y the solution-cst technique. The pure PAN film ws prepred s reference. The ionic conductivity for the films is chrcterized using impednce spectroscopy. The room temperture conductivity for the PAN+26 wt.% LiCF 3 SO 3 film nd the PAN+24 wt.% NCF 3 SO 3 film is Scm 1 nd Scm 1, respectively. XRD studies show tht the complextion tht hs occurred in the PAN contining slt films nd complexes formed re morphous. The FTIR spectr results confirmed the complextion hs tken plce etween the slt nd the polymer. These results correspond with surfce morphology imges otined from SEM nlysis. The conductivity temperture dependence of the highest conducting film from PAN + LiCF 3 SO 3 nd PAN + NCF 3 SO 3 systems follows Arrhenius eqution in the temperture rnge of 303 to 353 K. The PAN contining 24 wt.% LiCF 3 SO 3 film hs higher ionic conductivity nd lower ctivtion energy compred to the PAN contining 26 wt.% LiCF 3 SO 3 film. These results cn e explined sed on the Lewis cidity of the lkli ions, i.e., the interction etween Li + ion nd the nitrogen tom of PAN is stronger thn tht of N + ion. Keywords Conducting polymer. PAN. XRD. FTIR. Conductivity. SEM. Polymer electrolytes. Ionic conductivities. Electron microscopies Z. Osmn (*) : K. B. Md Is : A. Ahmd : L. Othmn Physics Deprtment, University of Mly, Kul Lumpur, Mlysi e-mil: zurinosmn@um.edu.my Introduction Polymer electrolytes hve received much ttention due to their potentil pplictions in rechrgele tteries nd other electrochemicl devices [1 7]. Most reserch ctivities on polymer electrolytes hve een concentrted on PEO-sed electrolyte systems during the lst few decdes following the initil discovery of Wright [8] tht PEO formed crystlline complexes with lkli metl slts. The pioneering work of Armnd nd his collortors [9] then led to the development of polymer-sed electrolyte for ttery pplictions. Compred to PEO-sed electrolytes, the PAN-sed lithium slt complexes possess mny dvntges, such s higher conductivity nd good mechnicl properties t room temperture. Although severl reports on PEO-sed polymer electrolytes cn e found in the literture [10 12], to our knowledge, no systemtic conductivity study on PEO contining sodium triflte hs een reported. Ionic moility nd chrge crrier concentrtion re two importnt fctors which influence the conductivity of the electrolyte. It hs een oserved y Vondrák et l. [13] tht the moility of smller ions Li + nd/or Mg 2+ is lower thn tht of ctions with lrger ions N + nd/or Zn 2+ in the gel polymer electrolytes. They noted tht the resistivity of gel with smller ction is higher thn tht of gel contining lrger ction. Hence, the moility of Li + is lower thn tht of N + nd the sme reltion is found etween slts contining Mg 2+ nd Zn 2+, respectively. The conductivity dt otined y them lso supported the hypothesis tht smller ctions re emedded or cptured y the polymeric network, nd their moility is lowered. The conductivity of chrge crriers present in n electrolyte generlly depends upon the concentrtion of slt contining the moile species s well s on the extent up to which the slt is dissocited.

2 432 Ionics (2010) 16: The im of this work is to investigte the effect of lithium triflte (LiCF 3 SO 3 ) nd sodium triflte (NCF 3 SO 3 ) on ionic conductivity, surfce morphology, complextion nd the interctions with the polymer PAN y using impednce spectroscopy, X-ry diffrction (XRD), Fourier-trnsform infrred spectroscopy (FTIR), nd scnning electron microscopy (SEM). Experimentl methods Smple preprtion Polycrylonitrile (PAN), with moleculr weight of 150,000 g/mol, lithium triflte (LiCF 3 SO 3 ), sodium triflte (NCF 3 SO 3 ), nd dimethylformmide (DMF) were otined from Aldrich. PAN ws dissolved in DMF nd the mixture ws stirred t 60 C until the solution turned into cler nd homogeneous. LiCF 3 SO 3 nd NCF 3 SO 3 slts were dded ccordingly. The mixtures were continuously stirred for severl hours. After complete dissolution, the solutions were cst in petri dishes nd left to dry under vcuum t 50 C for 48 h until the films were formed. Further drying ws chieved for 24 h t 80 C to remove the residul solvent. The DMF residue in the films estimted from thermogrvimetric nlysis (TGA) ws less thn 5 wt.%. The films were then kept in desicctor for further drying until the chrcteriztions re to e crried out. Chrcteriztion techniques Impednce spectroscopy mesurements were used to determine the conductivity of the films. The films were cut into round shpe tht fit the size of the electrodes. The films were then sndwiched etween the two stinless steel locking electrodes with dimeter of 2 cm. A HIOKI 3532 LCR ridge tht hs een interfced with computer ws used to perform the impednce mesurement for ech polymer electrolyte film in the frequency rnge of 50 Hz to 1 MHz. From the impednce plots otined, the ulk resistnce, R of ech smple ws determined nd hence the conductivity (σ) of the smples ws then clculted using σ=t/r A; where t is the smple thickness (cm), A the effective contct re of the electrode nd the smple (cm 2 ), nd R is the ulk resistnce (Ω). The conductivity temperture dependence studies were crried out in the temperture rnge from 303 to 353 K. To study the phse structure nd complextion of the conducting polymer electrolyte films, XRD mesurement ws crried out using PAN Anlyticl Expert Pro MPD in the rnge of 2θ from 10 to 80. Fourier-trnsform infrred (FTIR) spectr exhiited in this work were tken using MAGNA-IR550 Spectrophotometer- Series II in the wvenumer region etween 400 nd 4,000 cm 1. The films used in this work were cut into suitle sizes nd plced in the specimen holder of the spectrophotometer. In the present work, the FTIR spectrum of pure PAN film ws lso tken to serve s reference. The resolution of the spectrophotometer ws 1 cm 1. The surfce morphology of the films ws oserved y SEM using SEC Mini-SEM-Bruker EDS System. Results nd discussion The ionic conductivity versus slt content nd the conductivity versus frequency plots for the PAN + NCF 3 SO 3 system nd the PAN + LiCF 3 SO 3 system t room temperture re presented in Fig. 1 nd, respectively. It (Scm -1 ) (Scm -1 ) 7.00E E E E E E E E E E E E E E E+00 (h) (i) (j) (k) Log f (Hz) (f) (g) (h) (i) Log f (Hz) Fig. 1 The ionic conductivity versus slt content for the PAN + NCF 3 SO 3 system t room temperture. The inset figure shows the conductivity versus frequency plot for the film contining 24 wt.% 28 wt.% 30 wt.% 26 wt.% 22 wt.% (f) 20 wt.% (g) 18 wt.% (h) 16 wt.% (i) 14 wt.% (j) 10 wt.% nd (k) 6 wt.% slt The ionic conductivity versus slt content for the PAN + LiCF 3 SO 3 system t room temperture. The inset figure shows the conductivity versus frequency plot for the film contining 26 wt.% 28 wt.% 30 wt.% 24 wt.% 22 wt.% (f) 20 wt.% (g) 18 wt.% (h) 6 wt.% nd (i) 14 wt.% slt (f) (g)

3 Ionics (2010) 16: log (S cm -1 ) R 2 = R 2 = /T (K -1 ) Fig. 2 Arrhenius plot for PAN+26 wt.%licf 3 SO 3 film nd PAN +24 wt.%ncf 3 SO 3 film cn e oserved tht the conductivity of pure PAN film increses two orders of mgnitude when 6 wt.% of NCF 3 SO 3 slt ws dded nd continues to do so until it reched the highest conductivity, i.e., Scm 1 t 24 wt.% NCF 3 SO 3. Therefore, 24 wt.% NCF 3 SO 3 is the conductivity optimizing concentrtion. The conductivity for PAN + LiCF 3 SO 3 system is ruptly increses when more thn 10 wt.% of LiCF 3 SO 3 ws dded. The highest ionic conductivity ws chieved t 26 wt.% LiCF 3 SO 3 content nd its vlue ws Scm 1. The conductivity of this film is further incresed to 10 3 S cm 1 with the ddition of plsticizer, ethylene cronte (EC). This work hs een pulished elsewhere [14]. Arhm et l. [15] hve prepred PAN polymer electrolyte films with EC nd PC s plsticizers contining LiPF 6 /AsF 6 /LiN(SO 2 CF 3 ) 2 nd they otined the conductivity vlue in order of Scm 1 t room temperture. These results re in good greement with the works tht re descried nd reported y other reserchers [16 18]. Generlly, ionic conductivity of polymer electrolytes depends on the chrge crrier concentrtion, n nd crrier moility, μ s descried y reltion σ = nqμ, where q representing the chrge of moile crrier. The increse in the ionic conductivity with incresing slt concentrtions cn e relted to the increse in the numer of moile ion in the conducting polymer electrolyte films. The decrese in conductivity vlue t higher slt concentrtion is oserved nd cn e explined y ggregtion of the ions, leding to the formtion of ion cluster, thus decresing the numer of moile ions nd hence the moility [19]. The inset figures in Fig. 1 show the vrition in conductivity with frequency for oth polymer electrolyte systems. The ulk conductivity is very low t low frequencies, <1 khz. This cn e ttriuted to the ccumultion of moile ions t the electrode electrolyte interfce t low frequencies nd hence there re less moile ions in the ulk mteril contriute to conductivity. To nlyze the mechnism of ionic conduction, the conductivity temperture dependence of the highest conducting films from the PAN + NCF 3 SO 3 system nd the PAN + LiCF 3 SO 3 system were investigted t temperture rnge from 303 K to temperture elow the glss trnsition temperture, T g of pure PAN film, 353 K. From therml nlysis, pure PAN film exhiited distinct T g t 85 C (308 K). The conductivity temperture dependence plots of these films re represented in Fig. 2 nd, respectively. The plots show tht the conductivity increses s the temperture increses nd oeys Arrhenius rule, sðtþ ¼ s o expð E =RTÞ, where σ o is the conductivity pre-exponentil fctor nd E is the ctivtion energy for conduction [20]. It is lso understood tht the increse in Fig. 3 XRD ptterns of pure PAN film, LiCF 3 SO 3 slt, c NCF 3 SO 3 slt, d PAN+ 26 wt.% LiCF 3 SO 3, film nd e PAN+24 wt.%ncf 3 SO 3 film Intensity (.u) θ (deg.)

434 Ionics (2010) 16:431 435 Fig. 4 FTIR spectr of pure PAN PAN+10 wt.%licf 3 SO 3 c PAN+26 wt.%licf 3 SO 3 d PAN+30 wt.%licf 3 SO 3 e PAN+10 wt.%ncf 3 SO 3 f PAN+24 wt.%ncf 3 SO 3 g PAN+30 wt.

flexiility [21]. The ctivtion energy, E, cn e evluted from the slope of the plots [21]. The E for the PAN + 24 wt.% NCF 3 SO 3 film nd PAN + 26 wt.% LiCF 3 SO 3 film hve een clculted to e 0.

This result cn e explined sed on the Lewis cidity of the lkli ions, i.e., the strength of the interction of ctions with the Lewis se of the polymer electrolyte [22].

4 434 Ionics (2010) 16: Fig. 4 FTIR spectr of pure PAN PAN+10 wt.%licf 3 SO 3 c PAN+26 wt.%licf 3 SO 3 d PAN+30 wt.%licf 3 SO 3 e PAN+10 wt.%ncf 3 SO 3 f PAN+24 wt.%ncf 3 SO 3 g PAN+30 wt.% NCF 3 SO 3 films g f % Trnsmittnce e d c Wvenumer (cm -1 ) conductivity with temperture cn e linked to the decrese in viscosity nd hence incresed chin flexiility [21]. The ctivtion energy, E, cn e evluted from the slope of the plots [21]. The E for the PAN + 24 wt.% NCF 3 SO 3 film nd PAN + 26 wt.% LiCF 3 SO 3 film hve een clculted to e 0.23 ev nd 0.28 ev, respectively. It cn e oserved tht PAN + 24 wt.% NCF 3 SO 3 film hs higher ionic conductivity nd lower ctivtion energy compred to PAN + 26 wt.%licf 3 SO 3 film. This result cn e explined sed on the Lewis cidity of the lkli ions, i.e., the strength of the interction of ctions with the Lewis se of the polymer electrolyte [22]. The interction etween Li + ion nd the nitrogen tom of PAN is stronger thn tht of N + ion. Thus, Li + ion trnsfer requires higher ctivtion energy thn N + ion in polymer electrolytes. These results gree well with the works reported y Sgne et l. nd Ae et l. [22 24]. Figure 3 represents the XRD ptterns of pure PAN film, LiCF 3 SO 3, NCF 3 SO 3 nd the films with the highest ionic conductivity from PAN + LiCF 3 SO 3 nd PAN + NCF 3 SO 3 systems. XRD ptterns of LiCF 3 SO 3 exhiited the diffrction peks t 2θ=16.7, 19.8, 20.3, 22.5, 24.6, 25.5, 33.0, 33.6, nd 41.7 s shown in Fig. 3 while the diffrction pek of NCF 3 SO 3 cn e found t 2θ=23.7, 32.9, 34.8, 42.7, 48.7, 53.5 nd s shown in Fig. 3c. The XRD ptterns of pure PAN film nd films contining slts were found to e morphous showing no diffrction peks over the diffrction ngle, 2θ of 10 to 80 s oserved in Fig. 3, d, nd e. It cn e oserved tht most of the peks pertining to pure LiCF 3 SO 3 nd NCF 3 SO 3 re sent in the polymer electrolyte films nd indictes the complete dissolution of the slt in the polymer mtrix PAN. Thus, the XRD studies confirm tht complextion hs occurred in the polymer mtrices nd the complex formed re morphous. Berthier et l. [25] estlished tht ionic conductivity in polymer electrolytes is ssocited with the morphous phse of the studied smples. c Mgnifiction = 3000X Mgnifiction = 3000X Mgnifiction = 3000X Fig. 5 SEM imges of pure PAN film, PAN+26 wt.%licf 3 SO 3 film nd c PAN+24 wt.% NCF 3 SO 3 film

5 Ionics (2010) 16: In order to investigte the complex formtion in the polymer mtrices, FTIR studies hve een crried out. The FTIR spectr in the wvenumer rnge from 2,000 cm 1 to 4,000 cm 1 of pure PAN, PAN + LiCF 3 SO 3 system, nd PAN + NCF 3 SO 3 system re shown in Fig. 4. The nitrile nd, C N, ssigned to stretching nd in the FTIR spectrum ppered t 2,247 cm 1 for pure PAN. The nitrile nd is displced towrds the lower frequency round 2,244 cm 1 due to inductive effect creted y the interction etween the nitrogen tom in C N with Li + nd N + ions from the slts. It lso cn e oserved tht the intensity of this nd is reduced with increses of slts concentrtion. This shows tht the complextion hs occurred etween the polymer, PAN nd the slts. Similr FTIR spectr for pure PAN nd PAN contining EC, PC, nd LiClO 4 hve een oserved y Rjendrn nd his co-workers [26]. Figure 5 shows the SEM microgrphs of pure PAN PAN + 26 wt.% LiCF 3 SO 3, nd PAN + 24 wt.% NCF 3 SO 3. It cn e seen tht the surfce morphology of the pure PAN film is smooth nd homogenous. However, when LiCF 3 SO 3 nd NCF 3 SO 3 slts were dded the surfce ecomes rough nd uneven. It lso cn e oserved tht the pores pper in the slted films s shown in Fig. 3 nd c. This revels tht LiCF 3 SO 3 nd NCF 3 SO 3 slts strongly interct with polymer host s oserved in the FTIR spectr. Conclusions The ion-conducting PAN polymer electrolyte films contining LiCF 3 SO 3 nd NCF 3 SO 3 slts hve een prepred nd studied. The PAN film contining 24 wt.% NCF 3 SO 3 hs higher ionic conductivity nd lower ctivtion energy compred to the PAN film contining 26 wt.% LiCF 3 SO 3 film. The results indicte tht the interction etween Li + ion nd the nitrogen tom of PAN is stronger thn tht of N + ion. The plots of conductivity temperture dependence follow Arrhenius eqution in the temperture rnge of 303 to 353 K. XRD studies show tht the complextion hs occurred nd complexes formed re morphous. These results correspond with FTIR spectr nd SEM nlysis. Acknowledgements The uthors would like to thnk the University of Mly nd Acdemy of Sciences Mlysi for the grnts nd scholrship wrded. References 1. Le Nest JP, Gndini A (1990) In: Scrosti B (ed) Proc. 2nd Interntionl Symposium on Polymer Electrolytes. Elsevier, Amsterdm, p Scrosti B (1993) In: Scrosti B (ed) Applictions of electroctive polymers. Chpmn nd Hll, London, p Hooper A, Guthier M, Belnger A (1990) In: Linford RG (ed) Electrochemicl science nd technology of polymers, vol 2. Elsevier, London, p Murt K, Izuchi S, Yoshihis Y (2000) Electrochim Act 45: Wng C, Xi Y, Koumoto K, Ski T (2002) J Electrochem Soc 149:A967 A Chen T, Chen-Yng YW (2004) Recent development of ppliction of polymer electrolytes on polymer lithium secondry tteries. Huxue 62(4): Hymizu K, Aihr Y (2004) Electrochimic Act 49: Fenton BE, Prker JM, Wright PW (1973) Polymer 14: Armnd B (1979) In: Vshistht P et l (eds) Fst ion trnsport in solids. Elsevier, North-Hollnd 10. Bndr LRAK, Dissynke MAKL, Mellnder BE (1998) Electrochimic Act 43: Kumr B, Rodrigues SJ, Kok S (2002) Electrochimic Act 47: Wng Y-J, Pn Y, Wng L, Png M-J, Chen L (2005) Mter Lett 59: Vondrák J, Reiter J, Velick J, Sedlříkov M (2004) Solid Stte Ion 170: Osmn Z, Md Is KB, Ahmd A (2009) In: Arof AK, Mjid SR, Teo LP, Kufin MZ (eds) Proc. of Ntionl Workshop on Functionl Mterils (NWFM). (ISBN ), pp Arhm KM, Choe HS, Psquriello DM (1998) Electrochim Act 43: Chen-Yng YW, Chen HC, Lin FJ, Chen CC (2002) Solid Stte Ion 150: Yoon HK, Chung WS, Jo NJ (2004) Electrochim Act 50: Hung B, Wng Z, Li G, Hung H, Xue R, Chen L, Wng F (1996) Solid Stte Ion 85: Rmy CS, Selvsekrpndin S, Svith T, Hirnkumr G (2007) Physic B 393: Souquet JL, Levy M, Duclot M (1994) Solid Stte Ion 70 71: Rjendrn S, Sivkumr M, Sudevi R (2004) Mter Lett 58: Sgne F, Ae T, Iriym Y, Ogumi Z (2005) J Power Sources 146: Ae T, Fukud H, Iriym Y, Ogumi Z (2004) J Electrochem Soc 151(8):A1120 A Ae T, Ohtsuk M, Iriym Y, Ogumi Z (2004) J Electrochem Soc 151(11):A1153 A Berthier C, Gorecki W, Minier M, Armnd MB, Chgno JM, Rigud P (1983) Solid Stte Ion 11: Rjendrn S, Knnn R, Mhendrn O (2001) Mter Lett 48:

Electronic Supplementary Information

Electronic Supplementary Information Electronic Supplementry Informtion Assemling cron-coted hollow α-fe 2 nnohorns on the CNT ckone for superior lithium storge cpility Corresponding uthor: Xiong Wen (Dvid) Lou* School of Chemicl nd Biomedicl