a cross-linked soft matter polymer electrolyte for rechargeable lithium-ion batteries

DOI: 10.1002/cssc.201000249

A Cross-Linked Soft Matter Polymer Electrolyte for Rechargeable Lithium-Ion Batteries

Monalisa Patel, Manu U. M. Patel, and Aninda Jiban Bhattacharyya*[a]

Over the last few decades, there has been an upsurge in re-search activity to develop alternatives for conventional liquidelectrolytes (e.g. , lithium hexafluorophosphate (LiPF6) in 1:1volume ratio of ethylene carbonate (EC) and dimethyl carbon-ate (DMC)). One such direction has been to synthesize softmatter[1–16] electrolytes based on polymers,[1–2, 4] ionic liquids,[5, 6]

and plastic crystalline materials.[7, 8] This has facilitated the ideaof development of an all solid-state lithium ion battery.[3] Solidpolymer electrolytes (SPEs) are the most widely studied softmatter electrolyte comprising essentially of a salt (say lithium)solvated in a polyether matrix. The major drawback of SPE islow ambient temperature ionic conductivity(ca. 10�6 W�1 cm�1). Several attempts have been made to over-come this drawback. Notable among them are the gel electro-lytes[9–11] (non-aqueous liquid solvent + polymer electrolyte)and composite polymer electrolytes[13, 14] (heterogeneousdoping with oxide filler). However, both approaches havefailed to generate materials with optimized ionic conductivity,mechanical, and electrochemical properties simultaneously. Ithas been proposed that special polymer architecture, copoly-merization[15] may additionally aid in optimization of physicalproperties such as ionic conductivity and mechanical strengthof polymer and hence generate superior polymer electrolytes.We demonstrate here for the first time, a soft matter electro-lyte obtained from free-radical polymerization of vinyl mono-mers in a liquid solution matrix comprising of dinitrile[16] adipo-nitrile (N�C�(CH2�CH2)2�C�N, ADPN) and lithium salt [bis(tri-fluoromethanesulfonimide), LiTFSI] . ADPN was chosen as it is acolorless, moderately viscous (ca. 10�2 Pa) liquid with a dielec-tric constant (e) of 30. It readily dissolves a wide variety ofionic lithium salts and demonstrated to be non-corrosive to-wards metallic current collector in Ref. [16] We present herethe electrical, thermal, mechanical, and electrochemical proper-ties of the cross-linked polymer electrolytes obtained from thenew methodology.

For acrylonitrile (AN)/ADPN >0.20 (w/w), free radical poly-merization in LiTFSI-ADPN solution resulted in gel-like solidelectrolytes (Figure 1 D, E). For all concentration regimes, nophase separation was observed and electrolyte samples werehomogeneous and mechanically stable. Figure 2 shows thethermogravimetric analysis (TGA) for ADPN and AN/ADPN (0.25and 0.31). The TGA trace for ADPN showed a 100 % weightloss in a single step between 120 8C (onset) and 230 8C. Thegel electrolytes on the other hand showed a two step weightloss of total 83 % in the temperature range between 100 8C to

450 8C. The initial loss of approximately 65 % (2–4 % below100 8C and 61 % within 100-200 8C) corresponds to the decom-position of unpolymerized monomers and adiponitrile presentin the polymer-gel electrolyte and the final step of 18 % (200–450 8C) correspond to the cross-linked polymer network. Thus,formation of a polymer network in the LiTFSI–ADPN leads toan electrolyte with higher thermal stability. Mechanical proper-ties of the neat and polymer-gel electrolytes were studiedusing static and dynamic rheology (Figure 3 a, b). Figure 3 ashows the variation of viscosity (h) as a function of shear rate(g) for LiTFSI–ADPN and AN/ADPN >0.20. For the liquid elec-trolyte (i.e. , 0 wt % AN), the viscosity is nearly independent ofthe shear rate, indicating a Newtonian behavior. On the otherhand, the viscosity decreases with an increase in shear rate for

Figure 1. Photographs of the polymer electrolytes with various AN/ADPNratios: (A) 0, (B) 0.15, (C) 0.20, (D) 0.25, and (E) 0.31.

Figure 2. Thermal gravimetric analysis for ADPN and solid samples with AN/ADPN = 0.25 and 0.31 from 25 8C to 550 8C at a heating rate of 5 8C min�1.

[a] M. Patel, M. U. M. Patel, Dr. A. J. BhattacharyyaSolid State and Structural Chemistry Unit, Indian Institute of ScienceBangalore, Karnataka, 560012 (India)Fax: (+ 91 8023101360)E-mail : aninda_jb@sscu.iisc.ernet.in

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AN/ADPN >0.20. The gel electrolytes corresponding to AN/ADPN = 0.25 and 0.31 at lower shear rates show a power lawdecease with a slope of �1 implying the existence of a con-nected network structure. The viscoelastic behavior was addi-tionally studied using dynamic measurements (Figure 3 b)under constant strain (i.e. , the magnitude of which was withinthe linear viscoelastic regime). For polymer-gel electrolyteswith AN/ADPN >0.20, G’ (elastic modulus)>G’’ (loss modulus)and both modules were independent of frequency. The obser-vation G’>G’’ suggests that polymerization of AN in LiTFSI–ADPN liquid electrolyte resulted in gel electrolytes with intrin-sic elasticity. The elasticity of the gel electrolytes was observedto be a function of initial monomer concentration. At 1 rad s�1

(1 rad = 57.3 8), G’ was 355 kPa and 161 kPa for AN/ADPN = 0.31and AN/ADPN = 0.25, respectively, whereas for AN/ADPN<0.20, G’<G’’ was similar to that of sols. Thus, we proposedthat the cross-linked polymer-gel electrolytes with AN/ADPN>0.20 may be assembled in lithium batteries without any con-ventional separators.

Figure 4 shows the temperature variation of ionic conductiv-ity (s) for various electrolytes. The conductivity versus temper-ature traces showed a curvature in the Arrhenius plotting pro-cedure typically observed for an amorphous polymer or liquidsystem. In case of solid samples such as AN/ADPN >0.20, the

curvature in the s versus 1/T plot suggests existence of aliquid-like environment at smaller length scales. The ionic con-ductivity decreased with increase in polymer concentrationdue to increase in viscosity (Figure 3 b) and hence ionic mobili-ty. The highest conductivity was observed at room tempera-ture (25 8C) for electrolyte with AN/ADPN = 0.15 that is, 2.4 �10�3 W�1 cm�1 and with increasing polymer concentration itdecreased up to 1.2 � 10�3 W�1 cm�1(AN/ADPN=0.31) whichwas lower than neat LiTFSI-ADPN electrolyte (3 �10�3 W�1 cm�1). Thus, the generation of polymer network inthe dinitrile liquid matrix did not result in drastic decrease inionic conductivity for the gel electrolyte. The ionic conductivityof the cross-linked polymers was higher by several orders ofmagnitude compared to conventional polymers[4]

(ca. 10�6 W�1 cm�1) and also comparable to some of the roomtemperature ionic liquid systems[12] (10�2–10�3 W�1 cm�1). Inter-estingly, the polymer-gel electrolytes also showed significantlyhigh ionic conductivity at subambient temperature(ca. �20 8C). The conductivities were in the range of 1 � 10�4–2 � 10�4 W�1 cm�1, making it suitable for low temperature bat-tery applications.

The bulk (Rb) and interfacial (Ri) resistance of the pureLiTFSI–ADPN and AN/ADPN = 0.25 were measured using ac-im-pedance spectroscopy in a Li/electrolyte/Li symmetrical cellunder open circuit conditions at room temperature. Ac-impe-dance response is composed of a distorted semicircle (Fig-ure 5 a, b) and a spike at the low frequency region which ischaracteristic of a diffusion process. Figures 5 c and d showbulk and interfacial resistance versus time for both LiTFSI–ADPN and AN/ADPN = 0.25. It was found that the diameter ofthe semicircle increased drastically with time for pure LiTFSI–ADPN. This was attributed to the increase in interfacial resist-ance (Ri). However, polymer-gel electrolyte with AN/ADPN =

0.25 showed almost no increase in Ri in the same period of

Figure 3. a) Steady-shear viscosity versus shear rate for LiTFSI–ADPN andpolymer-gel electrolyte with AN/ADPN = 0.25 and 0.31 and b) variation ofelastic (G’) and viscous (G”) moduli as a function of frequency for LiTFSI–ADPN and AN/ADPN = 0.25 and 0.31 obtained from dynamic frequencysweep measurements at 25 8C.

Figure 4. Ionic conductivity for various electrolytes with AN/ADPN = 0–0.31versus temperature (range: �208–60 8C).

Figure 5. Ac-impedance spectra of a) LiTFSI-ADPN, b) AN/ADPN = 0.25 in asymmetrical cell (Li/Li cell) at room temperature under open circuit condi-tions. Time evolution spectra of c) bulk electrolyte resistance, and d) interfa-cial resistance of LiTFSI–ADPN and AN/ADPN = 0.25. Z’: real part of impe-dance; Z“: imaginary part of impedance.

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time. The difference in the interfacial stability among the twoelectrolytes was attributed to the difference in the growth be-havior of the passive layer formed at interface of Li electrodeand electrolyte. From the comparative interfacial resistanceplot, it can be concluded that polymer-gel electrolyte has sig-nificantly less detrimental effect towards lithium metal com-pared to the liquid electrolyte. As opposed to the generalbelief, we found here that polymerization of AN in LiTFSI–ADPN improves greatly the interface stability of the electrolytetowards lithium metal.

Figures 6 a and b show the cyclic voltammograms of neatLiTFSI–ADPN and gel electrolytes (AN/ADPN = 0.25) using stain-less steel (SS) as working and lithium as counter and reference

electrode. Among the whole series of polymer-gel electrolytes(AN/ADPN = 0.15–0.31), AN/ADPN = 0.25 showed the best elec-trochemical performance. The polymer-gel electrolyte as wellas the neat LiTFSI–ADPN showed a stable voltage window upto 4.5 V. However, in the case of neat LiTFSI–ADPN liquid elec-trolyte, the magnitude of current for lithium stripping(Li�e�!Li+) peak at approximately 0.3 V decreased for subse-quent cycles.[16] This suggests that the LiTFSI–ADPN/lithium in-terface was less stable compared to the polymer electrolyte/lithium interface. Preliminary corrosion studies showed poly-mer-gel electrolyte to be less corrosive towards aluminum cur-rent collector (working electrode) compared to the neat elec-trolyte.[16] Galvanostatic cycling was done in home-built Swage-lok cells with LiTFSI–ADPN and polymer-gel electrolyte (AN/ADPN = 0.25) assembled with lithium as anode and carbon-loaded porous LiFePO4 (LFP) as cathode.[17, 18] The beneficialsoft matter consistency of the polymer electrolyte allowed cellassembly without a conventional polymer separator. Voltageplateau corresponding to Fe3+/Fe2 + redox couple was ob-served at 3.5 V during oxidation and at 3.4 V during reduction(Figure 6 c) for the gel electrolyte. Figure 5 d shows the charge/

discharge cycling and columbic efficiency of LiTFSI–ADPN andAN/ADPN = 0.25 for few tens of cycles. The charging–discharg-ing capacity for the AN/ADPN = 0.25 almost coincided witheach other and stabilized at a capacity of 140 mAhg�1 after the15th cycle (Figure 6 d) at a current rate of 37 mAg�1 with a co-lumbic efficiency of nearly 100 % for both the charge and dis-charge cycles. The initial increase in capacity up to the 15th

cycle is attributed to the differential wetting of the pores ofLiFePO4 over a period of time. The LiTFSI–ADPN liquid electro-lyte showed very poor performance[16] with the same currentrate showing capacity of 40 mAhg�1 which further decreasedwith increasing cycle number. We believe that the synthesisconditions need to be further optimized for improvements inthe physicochemical properties, which will offer wider flexibili-ty for battery operation (such as wider compositional rangeand various current rate capabilities).

In summary, the generation of polymer network in ADPN byfree radical polymerization results in a soft matter electrolytewith superior electrochemical, thermal, and mechanical proper-ties. The electrolytes in addition to rechargeable lithium batter-ies may also have potential for other electrochemical devicessuch as solar cells.

Experimental Section

Acrylonitrile (CH2=CH�C�N) was used as the monomer. Radicalpolymerization was done at various ratios of AN to ADPN maintain-ing the salt concentration at a constant molar ratio with respect toADPN that is, ADPN/LiTFSI = 9:1. The mixture of AN, ADPN, andLiTFSI was heated under argon atmosphere (MBraun glove box,H2O <0.1 ppm) at 50–60 8C for 4 h in the presence of azobisisobu-trylonitrile (AIBN; 1 wt % of AN) as the initiator. In the regime ofAN/ADPN = 0–0.20 w/w, the macroscopic physical appearance ofthe electrolyte varied from a highly viscous liquid to a semisolid(Figure 1 A, B, C). For AN/ADPN >0.20 w/w, free radical polymeri-zation in 1 m LiTFSI-ADPN solution resulted in gel-like solid electro-lytes. Thermal properties of the pristine and gel electrolytes wereperformed using TGA. Temperature-dependent ionic conductivitywas obtained from ac-impedance spectroscopy (NovocontrolAlpha-A). Rheological measurements of gel and liquid samplesunder oscillatory shear (in the linear viscoelastic (LVE) region) wereperformed using cone and plate test geometries. Cyclic voltamme-try was carried out with stainless steel as working and lithiummetal as reference and counter electrode. Galvanostatic charge/dis-charge cycling (Arbin Instruments, S/N 164189-B) was done at dif-ferent current rates with pristine plastic crystalline and gel electro-lytes using lithium metal as counter and reference electrode andporous carbon coated LiFePO4 as working electrode.

Acknowledgements

We thank CSIR, India for SRF and DST (SR/S1/PC-07/2007), Indiafor research grant.

Keywords: conducting materials · mechanical properties ·radical reactions · polymerization · polymers

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Figure 6. Cyclic voltammograms of a) LiTFSI–ADPN, b) AN/ADPN = 0.25 at25 8C (scan rate 1 mV s)�1, c) galvanostatic charge/discharge of AN/ADPN = 0.25, d) capacity and columbic efficiency versus cycle number forAN/ADPN = 0.25 and LiTFSI–ADPN in Li jLiFePO4 cell (current rate = 37mAg�1).

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Received: August 8, 2010

Published online on October 28, 2010

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