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Cycling characteristics of lithium powder polymer cells assembled with cross-linked gel polymer electrolyte
Electrochimica Acta 132 (2014) 1–6
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Cycling characteristics of lithium powder polymer cells assembled with cross-linked gel polymer electrolyte Ji-Ae Choi a , Ji-Hyun Yoo a , Woo Young Yoon b , Dong-Won Kim a,∗ a b
Department of Chemical Engineering, Hanyang University, Seungdong-Gu, Seoul 133-791, South Korea Department of Materials Science and Engineering, Korea University, Seongbuk-Gu, Seoul 136-701, South Korea
a r t i c l e
i n f o
Article history: Received 13 January 2014 Received in revised form 8 March 2014 Accepted 17 March 2014 Available online 31 March 2014 Keywords: Gel polymer electrolyte In-situ cross-linking Lithium powder Lithium polymer cell Lithium dendrite
a b s t r a c t Lithium polymer cells composed of lithium powder anode and LiV3 O8 cathode were assembled with an insitu cross-linked gel polymer electrolyte, and their cycling performance was evaluated. The Li/LiV3 O8 cells exhibited better capacity retention and greater rate performance than the liquid electrolyte-based cell. The stable cycling characteristics of the lithium powder polymer cells resulted from the strong interfacial adhesion between the electrodes and the electrolyte as well as the suppression of the dendritic growth of lithium powder electrode during repeated cycling. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, the demand for rechargeable lithium batteries with high energy density and enhanced safety has increased to meet growing needs for smaller, lighter portable electronic devices and to accommodate growing interest in electric vehicles and energy storage systems [1–4]. As a result, lithium batteries using lithium metal as a negative electrode are the focus of substantial research interest because the lithium electrode offers a very high specific capacity (3,860 mAh g−1 ) [5], which is more than ten times that of the currently used carbon electrode. However, the use of lithium metal electrodes has been limited by the occurrence of dendrite growth during repeated charge and discharge cycles, as it gives rise to safety problems and gradual degradation of the cycling efficiency [6,7]. Therefore, the control of dendrite growth is very important for developing lithium metal batteries with enhanced safety and good capacity retention. In our previous studies, lithium powder instead of lithium foil was suggested as a new anode material to suppress dendritic growth, and distinct improvement in the electrochemical properties and safety of lithium powder electrodes was demonstrated [8,9]. Lithium vanadate (LiV3 O8 ) is a promising cathode active material for use in lithium metal batteries [10–15]. Based on theoretical calculations, lithium vanadate can deliver a
∗ Corresponding author. E-mail address: [email protected] (D.-W. Kim). http://dx.doi.org/10.1016/j.electacta.2014.03.119 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
high specific capacity (280 mAh g−1 for 3Li+ insertion/deinsertion) that is nearly double that of LiCoO2 . Additionally, lithium vanadate works in a potential range in which no side reactions due to electrolyte oxidation are expected. For the successful development of lithium metal batteries, there is also a pressing need for safer, more reliable electrolyte systems. Currently, gel polymer electrolytes are considered a promising alternative to the liquid electrolytes used in lithium batteries [16–20]. As a type of gel polymer electrolyte, chemically cross-linked gel polymer electrolytes can be synthesized by an in-situ cross-linking reaction of a liquid electrolyte with cross-linking agents [20–25], a technique which has been applied to the manufacture of commercialized lithium-ion polymer batteries. In this process, an electrolyte solution containing cross-linking agents is injected into the cell and gelation is performed by heating the cell, which resolves the leakage problem while maintaining good thermal and dimensional stability as well as high ionic conductivity. With the goal of developing high energy density lithium metal polymer cells with good capacity retention and enhanced safety, the lithium powder polymer cells composed of a lithium powder anode, a cross-linked gel polymer electrolyte and a LiV3 O8 cathode were assembled and their cycling performance was evaluated. The cross-linked gel polymer electrolytes were synthesized by in-situ chemical cross-linking in the cells, and the amount of the cross-linking agent necessary to achieve good cycling performance was suggested. The morphological analysis of the lithium powder electrode after repeated cycling demonstrated that the
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Fig. 1. Schematic presentation of lithium powder polymer cell assembled by in-situ cross-linking of a precursor electrolyte solution.
dendritic growth of lithium metal could be effectively suppressed in cross-linked gel polymer electrolytes, resulting in stable cycling characteristics.
2. Experimental 2.1. Synthesis of gel polymer electrolytes Poly(ethylene glycol) dimethacrylate (PEGDMA, Mn = 550) was purchased from Aldrich and used as a cross-linking agent after drying under a vacuum at 60 ◦ C for 24 hr. The water content in PEGDMA after vacuum drying was determined by Karl Fisher titration to be 18 ppm. PEGDMA and t-amyl peroxypivalate (Seki Arkema) as a thermal radical initiator were added to a liquid electrolyte to prepare the precursor electrolyte solution. The liquid electrolyte, which consisted of 1.0 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 by volume, battery grade) containing 1 wt.% vinylene carbonate (VC), was kindly supplied by Soulbrain Co. Ltd., and used without further treatment. VC was added as a solid electrolyte interphase (SEI) forming agent. In order to optimize the content of the cross-linking agent, PEGDMA was dissolved with different concentrations (0, 2.0, 4.0, 6.0 and 8.0 wt.%) in the liquid electrolyte. The cross-linked gel polymer electrolyte was then prepared by a radical-initiated reaction of the precursor electrolyte solution at 90 ◦ C for 20 min.
2.2. Electrode preparation and cell assembly Lithium powders were prepared by the droplet emulsion technique [8,9,26]. A mixture of molten lithium and silicone oil was sheared at approximately 25,000 rpm to produce an emulsion. As the emulsion was cooled to room temperature, the liquid lithium droplets solidified to form solid powders. The lithium powders were compacted by pressing to form an electrode. The LiV3 O8 electrode was prepared by coating a water-based slurry containing 80 wt.% lithium vanadate (GfE, Germany), 15 wt.% Ketchen black and 5 wt.% carboxymethyl cellulose (CMC) on Al foil. The electrode was roll pressed to enhance particulate contact and adhesion to the current collector. The lithium powder polymer cell was assembled by sandwiching the polypropylene (PP) separator (Celgard 2400) between the lithium powder anode and the LiV3 O8 cathode. The cell was enclosed in a pouch bag, injected with the gel electrolyte precursor and then vacuum-sealed. The cell assembly was performed in a dry box filled with argon gas. After cell assembly, the cells were maintained at 90 ◦ C for 20 min to induce in-situ thermal curing of the gel electrolyte precursor within the cell. The in-situ cross-linking enabled bonding of the separator firmly to the lithium powder anode and LiV3 O8 cathode together in the cell, as illustrated in Fig. 1.
2.3. Measurements The morphologies of the electrodes were examined using a scanning electron microscope (SEM, JEOL JSM-6300). Fourier transform infrared (FT-IR) spectra were recorded on JASCO 460 IR spectrometer in the range of 400-4000 cm−1 . The ionic conductivity of the liquid electrolyte was measured by a Cond 3210 conductivity meter (WTW GmbH, Germany), and the ionic conductivity of the crosslinked gel polymer electrolyte after thermal curing was determined from AC impedance measurements. AC impedance measurements were performed using a Zahner Electrik IM6 impedance analyzer over a frequency range of 100 Hz to 100 kHz with an amplitude of 10 mV. Charge and discharge cycling tests of the lithium powder polymer cells were conducted at a constant current over a voltage range of 2.0–3.6 V with battery test equipment (WBCS 3000, Wonatech) at room temperature. To observe the morphological changes of the lithium powder electrodes, the cells were disassembled after 100 cycles in a glove box and the electrodes were washed with dimethyl carbonate to remove the residual electrolyte. After drying in an argon-filled glove box, the morphology of the lithium powder electrodes was characterized using a field emission scanning electron microscope. 3. Results and discussion The cross-linked gel polymer electrolytes were synthesized by thermal curing of the liquid electrolyte with different crosslinking agent contents at 90 ◦ C for 20 min. Fig. 2-(a) shows the photo images of the cross-linked gel polymer electrolytes cured with different amounts of PEGDMA. As the content of PEGDMA increased at intervals of 2.0 wt.%, the electrolyte solution became highly viscous and finally non-fluidic, indicating that PEGDMA with multiple oligo(ethylene oxide) acrylate functional groups effectively induced the thermal cross-linking reaction. Gel polymer electrolytes without liquid flow were obtained at PEGDMA contents greater than 6 wt.%, as shown in Fig. 2-(a). Ionic conductivities of the gel polymer electrolytes after thermal curing were measured as a function of the PEGDMA content, and the results are shown in Fig. 2-(b). The ionic conductivity of the base liquid electrolyte was 7.0×10−3 S cm−1 . The ionic conductivities of the gel polymer electrolytes decreased with increasing PEGDMA content, since the thermal curing with cross-linking agent increased the viscosity of the resulting electrolytes and produced the three-dimensional electrolyte polymer networks. The large decrease in the ionic conductivity with increasing PEGDMA content from 4.0 to 6.0 wt.% can be ascribed to the abrupt reduction of ionic mobility due to the formation of cross-linked polymer electrolytes with high cross-linking density. Because the complete gelation of the liquid electrolyte failed at PEGDMA contents less than 6 wt.%, a gel electrolyte precursor containing 6.0 or 8.0 wt.% PEGDMA was applied to the lithium powder polymer cells. In order to confirm the chemical cross-linking reaction of PEGDMA, FT-IR analysis was carried out
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Fig. 2. (a) Photographs of gel polymer electrolytes cured with different amounts of PEGDMA, and (b) ionic conductivities of cross-linked gel polymer electrolytes as a function of the PEGDMA content.
before and after thermal curing, and the resulting FT-IR spectra are shown in Fig. 3. The characteristic peak of C = C for the methacrylate group observed at 1637 cm−1 [27,28] was found to disappear after thermal curing at 90 ◦ C for 20 min, indicating full cross-linking of PEGDMA. This result reveals that the curing condition is sufficient to complete the thermal cross-linking of PEGDMA in the precursor electrolyte solution.
Transmittance
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Wavenumber (cm ) Fig. 3. FT-IR spectra of (a) PEGDMA and (b) cross-linked gel polymer electrolyte cured by 6 wt.% PEGDMA.
Fig. 4. SEM images of (a) the compacted lithium powder electrode and (b) the positive electrode prepared with LiV3 O8 powders.
The morphology of a compacted lithium powder electrode is shown in Fig. 4-(a). The lithium powders were spherical in shape, and porous characteristics of the lithium electrode could be observed. The porous structure of the lithium powder electrode with a large surface area is expected to enhance the charge transfer reaction at the electrode surface. The larger reactive surface area can also reduce the apparent current density on the lithium electrode surface, which may suppress dendrite formation during charge and discharge cycling of the cells. Fig. 4-(b) shows a SEM image of the positive electrode prepared with LiV3 O8 powder, binder and a conducting agent. Ketchen blacks as the conductive material were homogeneously dispersed with active LiV3 O8 powders with lath-like structures in the electrode. The Li/LiV3 O8 cells were cycled in the voltage range of 2.0–3.6 V at a constant current rate of 0.2 C and room temperature. Fig. 5 (a) and (b) show the voltage profiles of the cell assembled with liquid electrolyte and cross-linked gel polymer electrolyte, respectively. The discharge plateau around 2.8 V corresponds to the single-phase insertion process and the 2.6 V plateau is ascribed to the two-phase transformation between Li1+x V3 O8 (1≤x≤2) and Li4 V3 O8 , as previously reported [29,30]. The Li/LiV3 O8 cell with liquid electrolyte delivered an initial discharge capacity of 240.5 mAh g−1 based on the LiV3 O8 active material in the positive electrode, and its discharge capacity declined to 150.3 mAh g−1 at the 100th cycle. For
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Fig. 5. Charge and discharge curves of the lithium powder polymer cell (Li/LiV3 O8 ) assembled with (a) liquid electrolyte and (b) cross-linked gel polymer electrolyte cured by 6.0 wt.% PEGDMA (0.2 C rate, cut-off: 2.0–3.6 V, 25 ◦ C).
the Li/LiV3 O8 cell assembled with cross-linked gel polymer electrolyte cured by 6.0 wt.% PEGDMA, an initial discharge capacity was reduced to 220.3 mAh g−1 , however, it delivered higher discharge capacity than liquid electrolyte-based cell at the 100th cycle. Fig. 6-(a) presents the discharge capacities as a function of the cycle number in the Li/LiV3 O8 cells assembled with the liquid electrolyte or the cross-linked gel polymer electrolytes. The initial discharge capacity of the cell decreased with increasing PEGDMA content, due to the increased resistance for ion migration in both electrolyte and electrodes. Note that the capacity retention was improved using the cross-linked gel polymer electrolyte, irrespective of the PEGDMA content. By thermal curing with the cross-linking agent, the liquid electrolyte becomes a chemically cross-linked gel polymer electrolyte within the cell. Additionally, curing allows effective encapsulation of the liquid electrolyte in the cell and promotes good interfacial adhesion between the separator and electrodes, which results in good capacity retention. Coulombic efficiencies of the cells with cross-linked gel polymer electrolyte initially increased and stabilized with cycle number, as shown in Fig. 6-(b). On the other hand, in case of the cell with liquid electrolyte, the coulombic efficiency initially increased but decreased again with cycling. High and stable coulombic efficiencies in the cells with cross-linked gel polymer electrolytes can be ascribed to the presence of ionic conductive cross-linked polymer layer covering the lithium powder electrode, which reduces the reductive decomposition of electrolyte on lithium electrode and suppress growth of lithium dendrite during cycling. Based on these results,
Fig. 6. (a) Discharge capacities and (b) coulombic efficiencies of the Li/LiV3 O8 cells assembled with liquid electrolyte or cross-linked gel polymer electrolytes (0.2 C rate, cut-off: 2.0–3.6 V, 25 ◦ C).
we believe that an in-situ cross-linking is very effective for enhancing cycling stability of the cell with lithium powder electrode as a negative electrode. To understand the interfacial behavior of Li/LiV3 O8 cells with different electrolytes, we obtained ac impedance spectra of the cells after 100 cycles. Fig. 7 shows the ac impedance spectra of
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ZRe(Ω) Fig. 7. AC impedance spectra of the Li/LiV3 O8 cells assembled with liquid electrolyte and cross-linked gel polymer electrolytes, which are measured after the repeated 100 cycles.
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Fig. 8. SEM images of the surface of the lithium powder electrodes after 100 cycles in (a) the liquid electrolyte, (b) gel polymer electrolyte cured by 6 wt.% PEGDMA, and (c) gel polymer electrolyte cured by 8 wt.% PEGDMA.
the Li/LiV3 O8 cells with liquid electrolyte or cross-linked gel polymer electrolytes. After charge and discharge cycles, ac impedance spectra showed a poorly separated semicircle. By previous works [31,32], the overlapped semicircle observed from high to low frequency regions corresponds to the resistance due to Li+ ion migration through the surface film on the electrode (Rf ) and charge
transfer resistance between the electrode and electrolyte (Rct ). Of particular our interest in the depressed semicircles is the total interfacial resistance, which is sum of Rf and Rct . When comparing the interfacial resistances, the cell cured by 6 wt.% PEGDMA exhibited the lowest value. It is plausible that the interfacial contact between electrodes and electrolyte is improved by in-situ chemical cross-linking, which is essential for efficient charge transport during charge-discharge cycling. However, an increase the PEGDMA content to 8 wt.% may suppress the ionic migration and the charge transfer reaction, though it can promote strong interfacial adhesion. These results imply that the proper control of cross-linking density in the cell is important to achieve good cycling performances. SEM analysis of the lithium powder electrodes was performed after 100 cycles of cells. Fig. 8 compares the SEM images of the surface of lithium powder electrodes after cycling in different electrolytes. For the lithium powder electrode cycled in the liquid electrolyte, the individual lithium powders did not retain their original spherical shapes, likely due to the repeated deposition and stripping of lithium on the powder surface during cycling. Some scattered and unevenly deposited lithium was observed on the powder surface, which may grow to dendrites with further cycling. Alternatively, the lithium powder electrode in the cell assembled by in-situ chemical cross-linking was covered by the gel polymer electrolyte layer. In these electrodes, dendritic morphology was hardly observed in the lithium powders, likely due to the formation of the protective cross-linked gel polymer electrolyte layer on
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the electrode surface. Thus, stable cycling characteristics of the cells with cross-linked gel polymer electrolytes shown in Fig. 6 can be ascribed to the presence of the ionic conductive polymer layer covering the lithium powders, which suppresses growth of lithium dendrites during cycling, as explained earlier. Choi et al. also reported that the cell assembled with a lithium electrode protected by a gel polymer electrolyte based on a semi-interpenetrating polymer network exhibited better cycling characteristics compared to the cell with an unprotected lithium electrode [33,34]. The rate capability of the Li/LiV3 O8 cells assembled with different electrolytes was evaluated. The cells were charged to 3.6 V at a constant current of 0.2 C and discharged at different current rates, from 0.2 to 2.0 C. Voltage profiles of the lithium powder polymer cell prepared with the gel polymer electrolyte cured by 6.0 wt.% PEGDMA are presented in Fig. 9-(a). Both the discharge voltage and discharge capacity were nearly the same up to a 1.0 C rate. At a 2.0 C rate, the discharge capacity decreased to 188.4 mAh g−1 , which corresponds to 85% of the capacity as delivered at the 0.2 C rate. Fig. 9-(b) compares the relative capacities of the Li/LiV3 O8 cells prepared with liquid electrolyte or cross-linked gel polymer electrolytes as a function of the current rate. The relative capacity is defined as the ratio of the discharge capacity at a specific C rate to the discharge capacity delivered at a 0.2 C rate. Notably, the relative capacity of the cell cured with 6 wt.% PEGDMA was greater than that of the cell assembled with a liquid electrolyte. This result demonstrates that the ionic conductivity of the electrolyte system is not the only factor in determining the high rate performance. Good interfacial contact between electrodes (the Li powder anode and LiV3 O8 cathode) and electrolyte is also important for improving the rate capability at high current rates. However, increasing the PEGDMA content to 8 wt.% decreased the relative capacity at a high current rate, which occurred from a reduction in the ionic mobility in both the electrolyte and electrodes as a result of excessive cross-linking, as explained earlier. 4. Conclusions Lithium polymer cells assembled with lithium powder anode, cross-linked gel polymer electrolyte and LiV3 O8 cathode delivered a relatively high discharge capacity and exhibited stable cycling characteristics. The morphological analysis of the lithium powder electrode demonstrated that the dendritic growth of lithium could be effectively suppressed by the protective cross-linked gel polymer electrolyte layer on the lithium powder surface. Our results revealed that the lithium powder polymer cell with high energy density and good capacity retention can be produced by insitu chemical cross-linking of the electrolyte with an appropriate amount of cross-linking agent. Acknowledgements This work was supported by Pohang Steel Corporation (POSCO) and the Research Institute of Industrial Science & Technology (RIST, No. 2011K128) and a grant from the Fundamental R&D Program for Core Technology of Materials, funded by the Ministry of Knowledge Economy, Korea. References [1] A.S. Arico, P. Bruce, B. Scrosati, J.-M. Tarascon, W.V. Schalkwijk, Nanostructured materials for advanced energy conversion and storage devices, Nature Materials 4 (2005) 366. [2] B. Dunn, H. Kamath, J.-M. Tarascon, Electrical energy storage for the grid: A battery of choices, Science 928 (2011) 334. [3] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Challenges in the development of advanced Li-ion batteries: a review, Energy and Environmental Science 4 (2011) 3243.
[4] N.S. Choi, Z. Chen, S.A. Freunberger, X. Ji, Y.K. Sun, K. Amine, G. Yushin, L.F. Nazar, J. Cho, P.G. Bruce, Challenges facing lithium batteries and electrical double-layer capacitors, Angewandte Chemie International Edition 51 (2012) 9994. [5] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359. [6] D. Aurbach, E. Zinigard, Y. Cohen, H. Teller, A short review of failure mechanisms of lithium metal and lithiated graphite anodes in liquid electrolyte solutions, Solid State Ionics 148 (2002) 405. [7] M. Rosso, C. Brissot, A. Teyssot, M. Dolle, L. Sannier, J.-M. Tarascon, R. Bouchet, S. Lascaud, Dendrite short-circuit and fuse effect on Li/polymer/Li cells, Electrochimica Acta 51 (2006) 5334. [8] W.S. Kim, W.Y. Yoon, Observation of dendritic growth on Li powder anode using optical cell, Electrochimica Acta 50 (2004) 541. [9] Y.S. Lee, J.H. Lee, J.A. Choi, W.Y. Yoon, D.W. Kim, Cycling characteristics of lithium powder polymer batteries assembled with composite gel polymer electrolytes and lithium powder anode, Advanced Functional Materials 23 (2013) 1019. [10] K. West, Z. Bachau-C, S. Skaarup, Y. Saidi, J. Barker, I.I. Olsen, R. Pynenburg, R. Koksbang, Comparison of LiV3 O8 cathode materials prepared by different methods, Journal of the Electrochemical Society 143 (1996) 820. [11] G.Q. Liu, C.L. Zeng, K. Yang, Study on the synthesis and properties of LiV3 O8 rechargeable lithium batteries cathode, Electrochimica Acta 47 (2002) 3239. [12] S. Jouanneau, A.L. La Salle, A. Verbaere, D. Guyomard, The origin of capacity fading upon lithium cycling in Li1.1 V3 O8 , Journal of the Electrochemical Society A1660 (2005) 152. [13] N. Tran, K.G. Bramnik, H. Hibst, J. Prolb, N. Mronga, M. Holzapfel, W. Scheifele, P. Novak, Spray-drying synthesis and electrochemical performance of lithium vanadates as positive electrode materials for lithium batteries, Journal of the Electrochemical Society 155 (2008) A384. [14] S.H. Ju, Y.C. Kang, Morphological and electrochemical properties of LiV3 O8 cathode powders prepared by spray pyrolysis, Electrochimica Acta 55 (2010) 6088. [15] Y.Q. Qiao, X.L. Wang, J.P. Zhou, J. Zhang, C.D. Gu, J.P. Tu, Synthesis and electrochemical performance of rod-like LiV3 O8 cathode materials for rechargeable lithium batteries, Journal of Power Sources 198 (2012) 287. [16] K.M. Abraham, H.S. Choe, D.M. Pasquariello, Polyacrylonitrile electrolyte-based Li ion batteries, Electrochimica Acta 43 (1998) 2399. [17] J.Y. Song, Y.Y. Wang, C.C. Wan, Review of gel-type polymer electrolytes for lithium-ion Batteries, Journal of Power Sources 77 (1999) 183. [18] K. Zaghib, P. Charest, A. Guerfi, J. Shim, M. Perrier, K. Striebel, Safe Li-ion polymer batteries for HEV applications, Journal of Power Sources 134 (2004) 124. [19] V.D. Noto, S. Lavina, G.A. Giffin, E. Negro, B. Scrosati, Polymer electrolytes: Present, past and future, Electrochimica Acta (2011) 4. [20] J.A. Choi, Y. Kang, D.W. Kim, Lithium polymer cell assembled by in-situ chemical cross-linking of ionic liquid electrolyte with phosphazene-based cross-linking agent, Electrochimica Acta 89 (2013) 359. [21] W. Kim, J.J. Cho, Y. Kang, D.W. Kim, Study on cycling performances of lithiumion polymer cells assembled by in situ chemical cross-linking with star-shaped siloxane acrylate, Journal of Power Sources 178 (2008) 837. [22] J.R. Nair, C. Gerbaldi, G. Meligrana, R. Bongiovanni, S. Bodoardo, N. Penazzi, P. Reale, V. Gentili, UV-cured methacrylic membranes as novel gel–polymer electrolyte for Li-ion batteries, Journal of Power Sources 178 (2008) 751. [23] J. Reiter, R. Dominko, M. Nadherna, I. Jakubec, Ion-conducting lithium bis (oxalato) boratebased polymer electrolytes, Journal of Power Sources 189 (2009) 133. [24] J.A. Choi, Y.K. Kang, H.J. Shim, D.W. Kim, E.H. Cha, D.W. Kim, Cycling performance of a lithium-ion polymer cell assembled by in-situ chemical crosslinking with fluorinated phosphorous-based cross-linking, Journal of Power Sources 195 (2010) 6177. [25] Y.W. Lee, W.K. Shin, D.W. Kim, Cycling performance of lithium-ion polymer batteries assembled using in-situ chemical cross-linking without a free radical initiator, Solid State Ionics 255 (2014) 6. [26] H.E. Park, I.W. Seong, W.Y. Yoon, Electrochemical behaviors of wax-coated Li powder/Li4 Ti5 O12 cells, Journal of Power Sources 189 (2009) 499. [27] A.C. Karmaker, A.T. Dibenedetto, A.J. Goldberg, Extent of conversion and its effect on the mechanical performance of Bis-GMA/PEGDMA-based resins and their composites with continuous glass fibres, Journal of Materials Science: Materials in Medicine 8 (1997) 369. [28] Z. Tang, L. Qi, G. Gao, Polymer electrolytes based on copolymer of poly(ethylene glycol) dimethacrylate and imidazolium ionic liquid, Solid State Ionics 180 (2009) 226. [29] H. Liu, Y. Wang, W. Yang, H. Zhou, Large capacity of LiV3 O8 cathode material for rechargeable lithium-based batteries, Electrochimica Acta 56 (2011) 1392. [30] J. Kawakita, T. Miura, T. Kishi, Lithium insertion and extraction kinetics of Li1+x V3 O8 , Journal of Power Sources 83 (1999) 79. [31] Y.C. Chang, J.H. Jong, T.G. Fey, Kinetic characterization of the electrochemical intercalation of lithium ions into graphite electrodes, Journal of the Electrochemical Society 147 (2000) 2033. [32] S.S. Zhang, K. Xu, J.L. Allen, T.R. Jow, Effect of propylene carbonate on the low temperature performance of Li-ion cells, Journal of Power Sources 110 (2002) 216. [33] N.S. Choi, Y.M. Lee, J.H. Park, J.K. Park, Interfacial enhancement between lithium electrode and polymer electrolytes, Journal of Power Sources 119–121 (2003) 610. [34] N.S. Choi, Y.M. Lee, W. Seol, J.A. Lee, J.K. Park, Protective coating of lithium metal electrode for interfacial enhancement with gel polymer electrolyte, Solid State Ionics 172 (2004) 19.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Cycling characteristics of lithium powder polymer cells assembled with cross-linked gel polymer electrolyte Ji-Ae Choi a , Ji-Hyun Yoo a , Woo Young Yoon b , Dong-Won Kim a,∗ a b
Department of Chemical Engineering, Hanyang University, Seungdong-Gu, Seoul 133-791, South Korea Department of Materials Science and Engineering, Korea University, Seongbuk-Gu, Seoul 136-701, South Korea
a r t i c l e
i n f o
Article history: Received 13 January 2014 Received in revised form 8 March 2014 Accepted 17 March 2014 Available online 31 March 2014 Keywords: Gel polymer electrolyte In-situ cross-linking Lithium powder Lithium polymer cell Lithium dendrite
a b s t r a c t Lithium polymer cells composed of lithium powder anode and LiV3 O8 cathode were assembled with an insitu cross-linked gel polymer electrolyte, and their cycling performance was evaluated. The Li/LiV3 O8 cells exhibited better capacity retention and greater rate performance than the liquid electrolyte-based cell. The stable cycling characteristics of the lithium powder polymer cells resulted from the strong interfacial adhesion between the electrodes and the electrolyte as well as the suppression of the dendritic growth of lithium powder electrode during repeated cycling. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, the demand for rechargeable lithium batteries with high energy density and enhanced safety has increased to meet growing needs for smaller, lighter portable electronic devices and to accommodate growing interest in electric vehicles and energy storage systems [1–4]. As a result, lithium batteries using lithium metal as a negative electrode are the focus of substantial research interest because the lithium electrode offers a very high specific capacity (3,860 mAh g−1 ) [5], which is more than ten times that of the currently used carbon electrode. However, the use of lithium metal electrodes has been limited by the occurrence of dendrite growth during repeated charge and discharge cycles, as it gives rise to safety problems and gradual degradation of the cycling efficiency [6,7]. Therefore, the control of dendrite growth is very important for developing lithium metal batteries with enhanced safety and good capacity retention. In our previous studies, lithium powder instead of lithium foil was suggested as a new anode material to suppress dendritic growth, and distinct improvement in the electrochemical properties and safety of lithium powder electrodes was demonstrated [8,9]. Lithium vanadate (LiV3 O8 ) is a promising cathode active material for use in lithium metal batteries [10–15]. Based on theoretical calculations, lithium vanadate can deliver a
∗ Corresponding author. E-mail address: [email protected] (D.-W. Kim). http://dx.doi.org/10.1016/j.electacta.2014.03.119 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
high specific capacity (280 mAh g−1 for 3Li+ insertion/deinsertion) that is nearly double that of LiCoO2 . Additionally, lithium vanadate works in a potential range in which no side reactions due to electrolyte oxidation are expected. For the successful development of lithium metal batteries, there is also a pressing need for safer, more reliable electrolyte systems. Currently, gel polymer electrolytes are considered a promising alternative to the liquid electrolytes used in lithium batteries [16–20]. As a type of gel polymer electrolyte, chemically cross-linked gel polymer electrolytes can be synthesized by an in-situ cross-linking reaction of a liquid electrolyte with cross-linking agents [20–25], a technique which has been applied to the manufacture of commercialized lithium-ion polymer batteries. In this process, an electrolyte solution containing cross-linking agents is injected into the cell and gelation is performed by heating the cell, which resolves the leakage problem while maintaining good thermal and dimensional stability as well as high ionic conductivity. With the goal of developing high energy density lithium metal polymer cells with good capacity retention and enhanced safety, the lithium powder polymer cells composed of a lithium powder anode, a cross-linked gel polymer electrolyte and a LiV3 O8 cathode were assembled and their cycling performance was evaluated. The cross-linked gel polymer electrolytes were synthesized by in-situ chemical cross-linking in the cells, and the amount of the cross-linking agent necessary to achieve good cycling performance was suggested. The morphological analysis of the lithium powder electrode after repeated cycling demonstrated that the
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Fig. 1. Schematic presentation of lithium powder polymer cell assembled by in-situ cross-linking of a precursor electrolyte solution.
dendritic growth of lithium metal could be effectively suppressed in cross-linked gel polymer electrolytes, resulting in stable cycling characteristics.
2. Experimental 2.1. Synthesis of gel polymer electrolytes Poly(ethylene glycol) dimethacrylate (PEGDMA, Mn = 550) was purchased from Aldrich and used as a cross-linking agent after drying under a vacuum at 60 ◦ C for 24 hr. The water content in PEGDMA after vacuum drying was determined by Karl Fisher titration to be 18 ppm. PEGDMA and t-amyl peroxypivalate (Seki Arkema) as a thermal radical initiator were added to a liquid electrolyte to prepare the precursor electrolyte solution. The liquid electrolyte, which consisted of 1.0 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 by volume, battery grade) containing 1 wt.% vinylene carbonate (VC), was kindly supplied by Soulbrain Co. Ltd., and used without further treatment. VC was added as a solid electrolyte interphase (SEI) forming agent. In order to optimize the content of the cross-linking agent, PEGDMA was dissolved with different concentrations (0, 2.0, 4.0, 6.0 and 8.0 wt.%) in the liquid electrolyte. The cross-linked gel polymer electrolyte was then prepared by a radical-initiated reaction of the precursor electrolyte solution at 90 ◦ C for 20 min.
2.2. Electrode preparation and cell assembly Lithium powders were prepared by the droplet emulsion technique [8,9,26]. A mixture of molten lithium and silicone oil was sheared at approximately 25,000 rpm to produce an emulsion. As the emulsion was cooled to room temperature, the liquid lithium droplets solidified to form solid powders. The lithium powders were compacted by pressing to form an electrode. The LiV3 O8 electrode was prepared by coating a water-based slurry containing 80 wt.% lithium vanadate (GfE, Germany), 15 wt.% Ketchen black and 5 wt.% carboxymethyl cellulose (CMC) on Al foil. The electrode was roll pressed to enhance particulate contact and adhesion to the current collector. The lithium powder polymer cell was assembled by sandwiching the polypropylene (PP) separator (Celgard 2400) between the lithium powder anode and the LiV3 O8 cathode. The cell was enclosed in a pouch bag, injected with the gel electrolyte precursor and then vacuum-sealed. The cell assembly was performed in a dry box filled with argon gas. After cell assembly, the cells were maintained at 90 ◦ C for 20 min to induce in-situ thermal curing of the gel electrolyte precursor within the cell. The in-situ cross-linking enabled bonding of the separator firmly to the lithium powder anode and LiV3 O8 cathode together in the cell, as illustrated in Fig. 1.
2.3. Measurements The morphologies of the electrodes were examined using a scanning electron microscope (SEM, JEOL JSM-6300). Fourier transform infrared (FT-IR) spectra were recorded on JASCO 460 IR spectrometer in the range of 400-4000 cm−1 . The ionic conductivity of the liquid electrolyte was measured by a Cond 3210 conductivity meter (WTW GmbH, Germany), and the ionic conductivity of the crosslinked gel polymer electrolyte after thermal curing was determined from AC impedance measurements. AC impedance measurements were performed using a Zahner Electrik IM6 impedance analyzer over a frequency range of 100 Hz to 100 kHz with an amplitude of 10 mV. Charge and discharge cycling tests of the lithium powder polymer cells were conducted at a constant current over a voltage range of 2.0–3.6 V with battery test equipment (WBCS 3000, Wonatech) at room temperature. To observe the morphological changes of the lithium powder electrodes, the cells were disassembled after 100 cycles in a glove box and the electrodes were washed with dimethyl carbonate to remove the residual electrolyte. After drying in an argon-filled glove box, the morphology of the lithium powder electrodes was characterized using a field emission scanning electron microscope. 3. Results and discussion The cross-linked gel polymer electrolytes were synthesized by thermal curing of the liquid electrolyte with different crosslinking agent contents at 90 ◦ C for 20 min. Fig. 2-(a) shows the photo images of the cross-linked gel polymer electrolytes cured with different amounts of PEGDMA. As the content of PEGDMA increased at intervals of 2.0 wt.%, the electrolyte solution became highly viscous and finally non-fluidic, indicating that PEGDMA with multiple oligo(ethylene oxide) acrylate functional groups effectively induced the thermal cross-linking reaction. Gel polymer electrolytes without liquid flow were obtained at PEGDMA contents greater than 6 wt.%, as shown in Fig. 2-(a). Ionic conductivities of the gel polymer electrolytes after thermal curing were measured as a function of the PEGDMA content, and the results are shown in Fig. 2-(b). The ionic conductivity of the base liquid electrolyte was 7.0×10−3 S cm−1 . The ionic conductivities of the gel polymer electrolytes decreased with increasing PEGDMA content, since the thermal curing with cross-linking agent increased the viscosity of the resulting electrolytes and produced the three-dimensional electrolyte polymer networks. The large decrease in the ionic conductivity with increasing PEGDMA content from 4.0 to 6.0 wt.% can be ascribed to the abrupt reduction of ionic mobility due to the formation of cross-linked polymer electrolytes with high cross-linking density. Because the complete gelation of the liquid electrolyte failed at PEGDMA contents less than 6 wt.%, a gel electrolyte precursor containing 6.0 or 8.0 wt.% PEGDMA was applied to the lithium powder polymer cells. In order to confirm the chemical cross-linking reaction of PEGDMA, FT-IR analysis was carried out
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Fig. 2. (a) Photographs of gel polymer electrolytes cured with different amounts of PEGDMA, and (b) ionic conductivities of cross-linked gel polymer electrolytes as a function of the PEGDMA content.
before and after thermal curing, and the resulting FT-IR spectra are shown in Fig. 3. The characteristic peak of C = C for the methacrylate group observed at 1637 cm−1 [27,28] was found to disappear after thermal curing at 90 ◦ C for 20 min, indicating full cross-linking of PEGDMA. This result reveals that the curing condition is sufficient to complete the thermal cross-linking of PEGDMA in the precursor electrolyte solution.
Transmittance
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Wavenumber (cm ) Fig. 3. FT-IR spectra of (a) PEGDMA and (b) cross-linked gel polymer electrolyte cured by 6 wt.% PEGDMA.
Fig. 4. SEM images of (a) the compacted lithium powder electrode and (b) the positive electrode prepared with LiV3 O8 powders.
The morphology of a compacted lithium powder electrode is shown in Fig. 4-(a). The lithium powders were spherical in shape, and porous characteristics of the lithium electrode could be observed. The porous structure of the lithium powder electrode with a large surface area is expected to enhance the charge transfer reaction at the electrode surface. The larger reactive surface area can also reduce the apparent current density on the lithium electrode surface, which may suppress dendrite formation during charge and discharge cycling of the cells. Fig. 4-(b) shows a SEM image of the positive electrode prepared with LiV3 O8 powder, binder and a conducting agent. Ketchen blacks as the conductive material were homogeneously dispersed with active LiV3 O8 powders with lath-like structures in the electrode. The Li/LiV3 O8 cells were cycled in the voltage range of 2.0–3.6 V at a constant current rate of 0.2 C and room temperature. Fig. 5 (a) and (b) show the voltage profiles of the cell assembled with liquid electrolyte and cross-linked gel polymer electrolyte, respectively. The discharge plateau around 2.8 V corresponds to the single-phase insertion process and the 2.6 V plateau is ascribed to the two-phase transformation between Li1+x V3 O8 (1≤x≤2) and Li4 V3 O8 , as previously reported [29,30]. The Li/LiV3 O8 cell with liquid electrolyte delivered an initial discharge capacity of 240.5 mAh g−1 based on the LiV3 O8 active material in the positive electrode, and its discharge capacity declined to 150.3 mAh g−1 at the 100th cycle. For
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Fig. 5. Charge and discharge curves of the lithium powder polymer cell (Li/LiV3 O8 ) assembled with (a) liquid electrolyte and (b) cross-linked gel polymer electrolyte cured by 6.0 wt.% PEGDMA (0.2 C rate, cut-off: 2.0–3.6 V, 25 ◦ C).
the Li/LiV3 O8 cell assembled with cross-linked gel polymer electrolyte cured by 6.0 wt.% PEGDMA, an initial discharge capacity was reduced to 220.3 mAh g−1 , however, it delivered higher discharge capacity than liquid electrolyte-based cell at the 100th cycle. Fig. 6-(a) presents the discharge capacities as a function of the cycle number in the Li/LiV3 O8 cells assembled with the liquid electrolyte or the cross-linked gel polymer electrolytes. The initial discharge capacity of the cell decreased with increasing PEGDMA content, due to the increased resistance for ion migration in both electrolyte and electrodes. Note that the capacity retention was improved using the cross-linked gel polymer electrolyte, irrespective of the PEGDMA content. By thermal curing with the cross-linking agent, the liquid electrolyte becomes a chemically cross-linked gel polymer electrolyte within the cell. Additionally, curing allows effective encapsulation of the liquid electrolyte in the cell and promotes good interfacial adhesion between the separator and electrodes, which results in good capacity retention. Coulombic efficiencies of the cells with cross-linked gel polymer electrolyte initially increased and stabilized with cycle number, as shown in Fig. 6-(b). On the other hand, in case of the cell with liquid electrolyte, the coulombic efficiency initially increased but decreased again with cycling. High and stable coulombic efficiencies in the cells with cross-linked gel polymer electrolytes can be ascribed to the presence of ionic conductive cross-linked polymer layer covering the lithium powder electrode, which reduces the reductive decomposition of electrolyte on lithium electrode and suppress growth of lithium dendrite during cycling. Based on these results,
Fig. 6. (a) Discharge capacities and (b) coulombic efficiencies of the Li/LiV3 O8 cells assembled with liquid electrolyte or cross-linked gel polymer electrolytes (0.2 C rate, cut-off: 2.0–3.6 V, 25 ◦ C).
we believe that an in-situ cross-linking is very effective for enhancing cycling stability of the cell with lithium powder electrode as a negative electrode. To understand the interfacial behavior of Li/LiV3 O8 cells with different electrolytes, we obtained ac impedance spectra of the cells after 100 cycles. Fig. 7 shows the ac impedance spectra of
60 liquid electrolyte GPE with 6 wt.% PEGDMA GPE with 8 wt.% PEGDMA
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ZRe(Ω) Fig. 7. AC impedance spectra of the Li/LiV3 O8 cells assembled with liquid electrolyte and cross-linked gel polymer electrolytes, which are measured after the repeated 100 cycles.
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C rate Fig. 9. (a) Discharge profiles of the lithium polymer cell cured by 6 wt.% PEGDMA at different current rates, and (b) relative capacities of the Li/LiV3 O8 cells assembled with liquid electrolyte or cross-linked gel polymer electrolytes, as a function of the current rate. The cells were charged to 3.6 V at a constant current of 0.2 C and discharged at different current rates, from 0.2 to 2.0 C.
Fig. 8. SEM images of the surface of the lithium powder electrodes after 100 cycles in (a) the liquid electrolyte, (b) gel polymer electrolyte cured by 6 wt.% PEGDMA, and (c) gel polymer electrolyte cured by 8 wt.% PEGDMA.
the Li/LiV3 O8 cells with liquid electrolyte or cross-linked gel polymer electrolytes. After charge and discharge cycles, ac impedance spectra showed a poorly separated semicircle. By previous works [31,32], the overlapped semicircle observed from high to low frequency regions corresponds to the resistance due to Li+ ion migration through the surface film on the electrode (Rf ) and charge
transfer resistance between the electrode and electrolyte (Rct ). Of particular our interest in the depressed semicircles is the total interfacial resistance, which is sum of Rf and Rct . When comparing the interfacial resistances, the cell cured by 6 wt.% PEGDMA exhibited the lowest value. It is plausible that the interfacial contact between electrodes and electrolyte is improved by in-situ chemical cross-linking, which is essential for efficient charge transport during charge-discharge cycling. However, an increase the PEGDMA content to 8 wt.% may suppress the ionic migration and the charge transfer reaction, though it can promote strong interfacial adhesion. These results imply that the proper control of cross-linking density in the cell is important to achieve good cycling performances. SEM analysis of the lithium powder electrodes was performed after 100 cycles of cells. Fig. 8 compares the SEM images of the surface of lithium powder electrodes after cycling in different electrolytes. For the lithium powder electrode cycled in the liquid electrolyte, the individual lithium powders did not retain their original spherical shapes, likely due to the repeated deposition and stripping of lithium on the powder surface during cycling. Some scattered and unevenly deposited lithium was observed on the powder surface, which may grow to dendrites with further cycling. Alternatively, the lithium powder electrode in the cell assembled by in-situ chemical cross-linking was covered by the gel polymer electrolyte layer. In these electrodes, dendritic morphology was hardly observed in the lithium powders, likely due to the formation of the protective cross-linked gel polymer electrolyte layer on
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the electrode surface. Thus, stable cycling characteristics of the cells with cross-linked gel polymer electrolytes shown in Fig. 6 can be ascribed to the presence of the ionic conductive polymer layer covering the lithium powders, which suppresses growth of lithium dendrites during cycling, as explained earlier. Choi et al. also reported that the cell assembled with a lithium electrode protected by a gel polymer electrolyte based on a semi-interpenetrating polymer network exhibited better cycling characteristics compared to the cell with an unprotected lithium electrode [33,34]. The rate capability of the Li/LiV3 O8 cells assembled with different electrolytes was evaluated. The cells were charged to 3.6 V at a constant current of 0.2 C and discharged at different current rates, from 0.2 to 2.0 C. Voltage profiles of the lithium powder polymer cell prepared with the gel polymer electrolyte cured by 6.0 wt.% PEGDMA are presented in Fig. 9-(a). Both the discharge voltage and discharge capacity were nearly the same up to a 1.0 C rate. At a 2.0 C rate, the discharge capacity decreased to 188.4 mAh g−1 , which corresponds to 85% of the capacity as delivered at the 0.2 C rate. Fig. 9-(b) compares the relative capacities of the Li/LiV3 O8 cells prepared with liquid electrolyte or cross-linked gel polymer electrolytes as a function of the current rate. The relative capacity is defined as the ratio of the discharge capacity at a specific C rate to the discharge capacity delivered at a 0.2 C rate. Notably, the relative capacity of the cell cured with 6 wt.% PEGDMA was greater than that of the cell assembled with a liquid electrolyte. This result demonstrates that the ionic conductivity of the electrolyte system is not the only factor in determining the high rate performance. Good interfacial contact between electrodes (the Li powder anode and LiV3 O8 cathode) and electrolyte is also important for improving the rate capability at high current rates. However, increasing the PEGDMA content to 8 wt.% decreased the relative capacity at a high current rate, which occurred from a reduction in the ionic mobility in both the electrolyte and electrodes as a result of excessive cross-linking, as explained earlier. 4. Conclusions Lithium polymer cells assembled with lithium powder anode, cross-linked gel polymer electrolyte and LiV3 O8 cathode delivered a relatively high discharge capacity and exhibited stable cycling characteristics. The morphological analysis of the lithium powder electrode demonstrated that the dendritic growth of lithium could be effectively suppressed by the protective cross-linked gel polymer electrolyte layer on the lithium powder surface. Our results revealed that the lithium powder polymer cell with high energy density and good capacity retention can be produced by insitu chemical cross-linking of the electrolyte with an appropriate amount of cross-linking agent. Acknowledgements This work was supported by Pohang Steel Corporation (POSCO) and the Research Institute of Industrial Science & Technology (RIST, No. 2011K128) and a grant from the Fundamental R&D Program for Core Technology of Materials, funded by the Ministry of Knowledge Economy, Korea. References [1] A.S. Arico, P. Bruce, B. Scrosati, J.-M. Tarascon, W.V. Schalkwijk, Nanostructured materials for advanced energy conversion and storage devices, Nature Materials 4 (2005) 366. [2] B. Dunn, H. Kamath, J.-M. Tarascon, Electrical energy storage for the grid: A battery of choices, Science 928 (2011) 334. [3] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Challenges in the development of advanced Li-ion batteries: a review, Energy and Environmental Science 4 (2011) 3243.
[4] N.S. Choi, Z. Chen, S.A. Freunberger, X. Ji, Y.K. Sun, K. Amine, G. Yushin, L.F. Nazar, J. Cho, P.G. Bruce, Challenges facing lithium batteries and electrical double-layer capacitors, Angewandte Chemie International Edition 51 (2012) 9994. [5] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359. [6] D. Aurbach, E. Zinigard, Y. Cohen, H. Teller, A short review of failure mechanisms of lithium metal and lithiated graphite anodes in liquid electrolyte solutions, Solid State Ionics 148 (2002) 405. [7] M. Rosso, C. Brissot, A. Teyssot, M. Dolle, L. Sannier, J.-M. Tarascon, R. Bouchet, S. Lascaud, Dendrite short-circuit and fuse effect on Li/polymer/Li cells, Electrochimica Acta 51 (2006) 5334. [8] W.S. Kim, W.Y. Yoon, Observation of dendritic growth on Li powder anode using optical cell, Electrochimica Acta 50 (2004) 541. [9] Y.S. Lee, J.H. Lee, J.A. Choi, W.Y. Yoon, D.W. Kim, Cycling characteristics of lithium powder polymer batteries assembled with composite gel polymer electrolytes and lithium powder anode, Advanced Functional Materials 23 (2013) 1019. [10] K. West, Z. Bachau-C, S. Skaarup, Y. Saidi, J. Barker, I.I. Olsen, R. Pynenburg, R. Koksbang, Comparison of LiV3 O8 cathode materials prepared by different methods, Journal of the Electrochemical Society 143 (1996) 820. [11] G.Q. Liu, C.L. Zeng, K. Yang, Study on the synthesis and properties of LiV3 O8 rechargeable lithium batteries cathode, Electrochimica Acta 47 (2002) 3239. [12] S. Jouanneau, A.L. La Salle, A. Verbaere, D. Guyomard, The origin of capacity fading upon lithium cycling in Li1.1 V3 O8 , Journal of the Electrochemical Society A1660 (2005) 152. [13] N. Tran, K.G. Bramnik, H. Hibst, J. Prolb, N. Mronga, M. Holzapfel, W. Scheifele, P. Novak, Spray-drying synthesis and electrochemical performance of lithium vanadates as positive electrode materials for lithium batteries, Journal of the Electrochemical Society 155 (2008) A384. [14] S.H. Ju, Y.C. Kang, Morphological and electrochemical properties of LiV3 O8 cathode powders prepared by spray pyrolysis, Electrochimica Acta 55 (2010) 6088. [15] Y.Q. Qiao, X.L. Wang, J.P. Zhou, J. Zhang, C.D. Gu, J.P. Tu, Synthesis and electrochemical performance of rod-like LiV3 O8 cathode materials for rechargeable lithium batteries, Journal of Power Sources 198 (2012) 287. [16] K.M. Abraham, H.S. Choe, D.M. Pasquariello, Polyacrylonitrile electrolyte-based Li ion batteries, Electrochimica Acta 43 (1998) 2399. [17] J.Y. Song, Y.Y. Wang, C.C. Wan, Review of gel-type polymer electrolytes for lithium-ion Batteries, Journal of Power Sources 77 (1999) 183. [18] K. Zaghib, P. Charest, A. Guerfi, J. Shim, M. Perrier, K. Striebel, Safe Li-ion polymer batteries for HEV applications, Journal of Power Sources 134 (2004) 124. [19] V.D. Noto, S. Lavina, G.A. Giffin, E. Negro, B. Scrosati, Polymer electrolytes: Present, past and future, Electrochimica Acta (2011) 4. [20] J.A. Choi, Y. Kang, D.W. Kim, Lithium polymer cell assembled by in-situ chemical cross-linking of ionic liquid electrolyte with phosphazene-based cross-linking agent, Electrochimica Acta 89 (2013) 359. [21] W. Kim, J.J. Cho, Y. Kang, D.W. Kim, Study on cycling performances of lithiumion polymer cells assembled by in situ chemical cross-linking with star-shaped siloxane acrylate, Journal of Power Sources 178 (2008) 837. [22] J.R. Nair, C. Gerbaldi, G. Meligrana, R. Bongiovanni, S. Bodoardo, N. Penazzi, P. Reale, V. Gentili, UV-cured methacrylic membranes as novel gel–polymer electrolyte for Li-ion batteries, Journal of Power Sources 178 (2008) 751. [23] J. Reiter, R. Dominko, M. Nadherna, I. Jakubec, Ion-conducting lithium bis (oxalato) boratebased polymer electrolytes, Journal of Power Sources 189 (2009) 133. [24] J.A. Choi, Y.K. Kang, H.J. Shim, D.W. Kim, E.H. Cha, D.W. Kim, Cycling performance of a lithium-ion polymer cell assembled by in-situ chemical crosslinking with fluorinated phosphorous-based cross-linking, Journal of Power Sources 195 (2010) 6177. [25] Y.W. Lee, W.K. Shin, D.W. Kim, Cycling performance of lithium-ion polymer batteries assembled using in-situ chemical cross-linking without a free radical initiator, Solid State Ionics 255 (2014) 6. [26] H.E. Park, I.W. Seong, W.Y. Yoon, Electrochemical behaviors of wax-coated Li powder/Li4 Ti5 O12 cells, Journal of Power Sources 189 (2009) 499. [27] A.C. Karmaker, A.T. Dibenedetto, A.J. Goldberg, Extent of conversion and its effect on the mechanical performance of Bis-GMA/PEGDMA-based resins and their composites with continuous glass fibres, Journal of Materials Science: Materials in Medicine 8 (1997) 369. [28] Z. Tang, L. Qi, G. Gao, Polymer electrolytes based on copolymer of poly(ethylene glycol) dimethacrylate and imidazolium ionic liquid, Solid State Ionics 180 (2009) 226. [29] H. Liu, Y. Wang, W. Yang, H. Zhou, Large capacity of LiV3 O8 cathode material for rechargeable lithium-based batteries, Electrochimica Acta 56 (2011) 1392. [30] J. Kawakita, T. Miura, T. Kishi, Lithium insertion and extraction kinetics of Li1+x V3 O8 , Journal of Power Sources 83 (1999) 79. [31] Y.C. Chang, J.H. Jong, T.G. Fey, Kinetic characterization of the electrochemical intercalation of lithium ions into graphite electrodes, Journal of the Electrochemical Society 147 (2000) 2033. [32] S.S. Zhang, K. Xu, J.L. Allen, T.R. Jow, Effect of propylene carbonate on the low temperature performance of Li-ion cells, Journal of Power Sources 110 (2002) 216. [33] N.S. Choi, Y.M. Lee, J.H. Park, J.K. Park, Interfacial enhancement between lithium electrode and polymer electrolytes, Journal of Power Sources 119–121 (2003) 610. [34] N.S. Choi, Y.M. Lee, W. Seol, J.A. Lee, J.K. Park, Protective coating of lithium metal electrode for interfacial enhancement with gel polymer electrolyte, Solid State Ionics 172 (2004) 19.