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Cite this: Chem. Commun., 2015, 51, 5448
Li-ion battery shut-off at high temperature caused by polymer phase separation in responsive electrolytes†
Received 24th December 2014, Accepted 23rd February 2015
Jesse C. Kelly, Nicholas L. Degrood and Mark E. Roberts*
DOI: 10.1039/c4cc10282g www.rsc.org/chemcomm
For the purpose of realizing inherently safe high-power Li-ion batteries, a model Li4Ti5O12/LiFePO4 rechargeable battery is investigated using the thermally responsive polymer, poly(benzyl methacrylate), in an ionic liquid. At high temperature, battery operation is inhibited as a result of increased internal resistance caused by polymer and ionic liquid phase separation. Li-ion concentration is shown to affect the phase transition temperature and the extent to which batteries are deactivated.
The development of new materials and chemistries has accelerated improvements in batteries over the past few decades, providing exciting opportunities for high-power and high-energy density devices.1 Lithium-ion batteries (LIBs), in particular, exhibit many properties amenable to energy storage for transportation and stationary systems for intermittent, renewable energy generation.2 Existing safety hazards associated with LIBs, however, prevent their use in large-format systems, as the potential for thermal failure significantly increases with cell size.3 Stable LIB operation is often compromised through several scenarios as hazardous conditions reside outside a narrow range of temperatures and voltages.4 Whether from over-charging, high discharge current loads, or changes in temperature, abuse of LIBs can lead to thermal failure which is manifested as fires or explosions.5 While the stability of electrode materials continues to improve, a major concern is the reactivity and flammability of electrolytes (e.g. lithium salts in organic solvents).6 Room temperature ionic liquids (ILs) have recently received consideration as electrolyte media for batteries and supercapacitors.7 Due to their unique physiochemical properties (negligible volatility, thermal stability, and high ionic conductivity) and a drive for safer and higher power forms of energy storage, ILs have attracted interest as alternatives to conventional electrolytes. Recent advances in IL
Department of Chemical & Biomolecular Engineering, Clemson University, 204 Earle Hall, Clemson, SC 29634, USA. E-mail: [email protected]; Fax: +1 864 656 0784; Tel: +1 864 656 6307 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cc10282g
5448 | Chem. Commun., 2015, 51, 5448--5451
electrolytes,8 IL-doped polymer electrolytes9 and ion gels10 have led to systems that avoid leakage and flammability while maintaining high conductivities, thermal stability, and a degree of functionality. Lodge and co-workers recently studied the use of thermally responsive polymers in ILs, where they utilized the phase separation to process highly conductive ion gels with block copolymers.11 Of particular interest is the class of poly(aryl methacrylates), such as poly(benzyl methacrylate) (PBMA), that exhibit a lower-critical solution temperature (LCST) and phase separate in imidazolium based ionic liquids.12 This solid-liquid phase transition occurs in the temperature range where thermal failure is initiated and most safety mechanisms, such as trilayer separators, are triggered in LIBs.13 Previously, we demonstrated that the phase transition of a responsive polymer electrolyte comprising poly(ethylene oxide) (PEO) and an IL causes a reduction in conductivity and charge transfer at elevated temperatures.14 While state-of-the-art safety mechanisms render LIBs inoperable (melting separators), utilization of responsive electrolytes that reversibly increase internal resistances may provide an opportunity to mitigate thermal failure while extending LIB lifetime and operating conditions. Additionally, the use of responsive polymers may provide a mechanism whereby local microscopic hotspots can be mitigated by coating the electrode surface while allowing the device to continue operation. In this communication, we describe how a responsive electrolyte of PBMA in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [EMIM][TFSI] inhibits LIB operation at elevated temperatures. As opposed to the PEO–IL system, where the mechanism is based on a change in conductivity (the polymer is still capable of coordinating and transporting Li ions), PBMA acts as an electronic insulator with very low Li-ion conductivity, thereby causing an increase in internal resistances at the electrode/ electrolyte interface when the polymer phase separates above the phase transition temperature. Responsive electrolytes were evaluated independently and within model Li-ion batteries, as shown in Fig. 1. Test cells were comprised of a Li4Ti5O12 (LTO) anode, LiFePO4 (LFP) cathode, a high-temperature stable nonwoven separator and the responsive
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Fig. 1 Schematic of the LIB setup (including assembled coin cell), the thermal-response mechanism at high temperatures at the electrode surface, and the structure of the responsive polymer, PBMA, and ionic liquid, [EMIM][TFSI].
polymer to form a low voltage (B1.7 V), high power Li-ion battery (see ESI,† for Experimental details). With the responsive electrolyte, the cell can deliver power over the voltage range of 1.0–2.5 V when below the electrolyte LCST. As this cell is heated above B120–140 1C, the battery ceases to operate when the polymer phase separates from the electrolyte creating a barrier to charge transfer (lithium intercalation) at the electrode/electrolyte interface and to some extent, ion transport through the separator. The mechanism for inhibiting device operation is discussed below. At 60 1C, cells without the responsive polymer additive were evaluated using constant current charge–discharge testing with various C rates (1–4 C) over the voltage range of 1.0 to 2.5 V (Fig. 2a). Capacity values are reported based on the mass of LTO because LFP is used in excess. The discharge voltage of the LTO/LFP system (1.5–1.7 V) is lower than conventional LIBs; however, it is capable of delivering high power rates while maintaining high capacities.
Fig. 2 LFP and LTO in a 1 M LiTFSI in [EMIM][TFSI] (a) at 60 1C for various C-rates and (b) at various temperatures at a 3.6 C rate. Cell capacity is on a per mass LTO basis as LFP is used in excess. (c) Nyquist plot showing the imaginary part vs. real part of impedance over the temperature range of 60 1C to 160 1C.
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To determine the stability of the cell over the temperature range of interest, charge–discharge measurements were performed with increasing temperature up to 160 1C at a discharge rate of 3.6 C (B1000 s discharge). As expected, the cell capacity significantly increases with temperature up to 140 1C as the conductivity of the electrolyte increases due to the decreasing viscosity (Fig. 2b). At 160 1C, however, the discharge potential slightly decreases, resulting in a loss of capacity and decrease in power (P = IV). Resistance values within the battery were investigated using electrochemical impedance spectroscopy (EIS) measurements. The Nyquist plot shows that each resistive component of the cell decreases as the temperature is increased up to 140 1C (Fig. 2c). A slight increase in charge transfer resistance is observed at 160 1C presumably due to either impurities or electrode/electrolyte degradation, which results in a lower discharge voltage and slight decrease in capacity. For this reason, subsequent battery testing with the responsive PBMA–IL electrolytes was kept below this temperature. Previous work shows that the thermally activated phase transition of PBMA in [EMIM][TFSI] occurs around 105 1C.15 However, we found that Li-ion concentration has a significant influence on the phase transition temperature of PBMA–IL systems, ranging between 100–180 1C for Li-ion concentrations of 0 to 1 M. Here, optical transmission is used to identify the phase transition temperature of electrolytes, which were prepared by a co-solvent evaporation method whereby 5 wt% PBMA (100 000 Mw) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) were mixed with [EMIM][TFSI] and dried under vacuum (see ESI,† for details). Fig. 3 shows the optical transmittance of 5 wt% solutions of PBMA in [EMIM][TFSI] as the LiTFSI concentration was increased from 0 to 1.0 M. The abrupt decrease in the optical transmittance indicates the temperature at which polymer phase separation occurred. Without the lithium salt, the PBMA–IL mixture displayed an LCST of 97 1C, which is slightly lower than values reported by Watanabe et al. due to the difference in Mw (100k vs. 70k). Increasing LiTFSI concentration in the PBMA–IL led to an increase in LCST from 106 1C up to 135 1C for the 0.2 M to 1.0 M solutions. The so-called ‘‘salting in’’ effect inhibits the polymer from phase
Fig. 3 Cloud point measurements for 5 wt% PBzMA in [EMIM][TFSI] with increasing LiTFSI salt concentration.
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separating and increases its solubility limit, thus increasing the temperature at which the phase transition occurs. As a result, it is important to balance battery performance, which increases with salt concentration, and the electrolyte phase-transition temperature. In addition to the increase in the LCST, it was observed visually that polymer aggregation decreased as the concentration of LiTFSI increased. LTO/LFP batteries were fabricated using thermally responsive electrolytes with LiTFSI concentrations between 0.2 M and 1.0 M and tested over the temperature range of 60 1C to 150 1C. Battery testing involved EIS measurements at the open circuit voltage (OCV) and constant current charge–discharge measurements over the voltage range of 1.0 to 2.5 V at a C rate of 3.6, which was determined independently for each electrolyte at 60 1C. The OCV is shown for cells with various salt concentrations from 60 1C up to 140 1C (Fig. 4a). In general, the OCV decreases with increasing temperature, except in the cell with 1 M LiTFSI, which has an LCST of 135 1C and was visually confirmed to show minimal polymer aggregation, resulting in minimal charge resistance. The decrease in OCV observed in the cells with lower electrolyte concentration (0.8 M or less) is attributed to the increase in cell internal resistance due to polymer phase separation, which leads to a resistive coating on the electrode. The formation of the resistive polymer coating that impedes ion diffusion is verified using EIS measurements, as shown in the Nyquist plot in Fig. 4b for the 0.5 M electrolyte. The diffusional resistance (low frequencies) increases above 100 1C and a drastic increase in charge transfer resistance (semi-circle at mid- to high-frequencies) occurs when the solution is heated at 150 1C for 3 h. EIS revealed similar changes in cell resistances with increasing temperature for the 0.2 M and 0.8 M electrolytes, however, the 0.8 M solution showed only a minimal change in
Fig. 4 Effect of increasing temperature on the (a) OCV for the 0.2 M, 0.5 M, 0.8 M, and 1.0 M LiTFSI in 5 wt% PBMA–IL solutions, (b) Nyquist plot for 0.5 M LiTFSI in a 5 wt% PBMA–IL, and charge–discharge curves at 60 1C and 150 1C for the responsive electrolytes with LiTFSI concentrations of (c) 0.2 M, (d) 0.5 M and (e) 0.8 M. Discharge measurements were performed at a C-rate of B3.6 at 60 1C and the same currents were applied at 150 1C.
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charge transfer resistance, even after 6 hours at 150 1C. As shown above, the extent to which PBMA aggregates decreases with increasing LiTFSI concentration, which prevents the polymer from forming a resistive coating on the electrode at higher salt concentrations. Furthermore, even though PBMA–IL system exhibits reversible phase behavior in a beaker (polymer re-dissolves in solution upon cooling), we did not observe a decrease in resistance as the cell cooled below the LCST (ESI,† Fig. S2). The phase behavior of the mixture in the presence of the separator is the subject of ongoing investigation. The constant current charge–discharge measurements are shown in Fig. 4c–e for cells with varying salt concentration at the initial temperature of 60 1C and after heating to 150 1C for 3 h. Almost complete shut-off occurs in cells with 0.2 M LiTFSI as a result of an increase in internal resistance above the LCST (at 150 1C), which decreases the OCV and prevents the LTO/LFP battery from delivering the current set point between 1.0 and 2.5 V. When the LiTFSI concentration is increased to 0.5 M and 0.8 M, a significant potential drop is observed; however, complete battery shutoff is not achieved. While the performance of the cell with 0.5 M LiTFSI is reduced at 150 1C (Fig. 4d), devices with 0.8 M LiTFSI exhibits an increase in capacity, albeit with lower power output (Fig. 4e). The change in capacity observed in this system is less than what is achieved in the absence of the polymer additive (Fig. 2). In traditional IL electrolytes (Fig. 2), an increase in temperature will increase the capacity of the device due to the increase in Li-ion conductivity. Here, we show that the addition of a thermally responsive polymer to the electrolyte impedes this increase in performance and causes nearly complete shutoff in battery operation at high temperature. We concluded that the polymer aggregation and coating on the electrode was the primary mechanism for battery inhibition because of the following. (i) The charge transfer resistance was shown to increase above the LCST (EIS), which is one indication that the phase separation is causing something to occur on the surface. (ii) The conductivity of the 5 wt% PBMA electrolyte continues to increase with increasing temperature indicating that the shut-off mechanism is not occurring in the bulk (ESI,† Fig. S1). (iii) Without the polymer in the electrolyte, no shut-off occurs, so it cannot be due to the IL, the electrode interfaces with the current collector, or chemistry within the electrodes. (iv) PBMA does not thermally decompose over the tested temperature range in beaker experiments. It is possible that the polymer is absorbing on the separator; however, based on the pore size and porosity, it is unlikely that the polymer would completely fill the pores to decrease the ion-conductivity enough to inhibit the battery performance. In summary, we have demonstrated that a LTO/LFP battery can operate at high power rates in an [EMIM][TFSI] electrolyte with LiTFSI and that the addition of a thermally responsive polymer, PBMA, can be used to inhibit battery operation at elevated temperatures. While the addition of the polymer affects performance, high power rates at relatively high capacities can still be maintained. The LiTFSI concentration is shown to increase the polymer’s LCST in [EMIM][TFSI], which affects the
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device’s upper temperature limit and the temperature-dependent internal resistance of the battery. While the reversibility of the responsive behavior of LTO/LFP devices has yet to be observed upon cooling (see Fig. S2 in ESI†), this aspect is currently under further investigation as the polymer is known to redissolve in solution upon cooling. Nonetheless, the use of responsive polymers provides a new approach to preventing thermal runaway in Li-ion batteries because the polymer phase separation can impede further lithium reactions when local hotspots (thermal failures) form, which will likely prevent the entire cell from heating beyond an unsafe temperature. Funding for this work was provided by 3 M Non-Tenured Faculty Grant and NSF CMMI Scalable Nanomanufacturing Award 1246800. The authors are thankful to Prof. Stephen Creager for valuable discussions and Brian Morin at Dreamweaver International for providing high temperature, mechanically stable nonwoven separators for Li-ion batteries.
Notes and references 1 (a) H. Ibrahim, A. Ilinca and J. Perron, Renewable Sustainable Energy Rev., 2008, 12, 1221; (b) M. S. Whittingham, Proc. IEEE, 2012, 100, 1518; (c) Z. Yang, J. Zhang, M. C. Kintner-Meyer, X. Lu, D. Choi, J. P. Lemmon and J. Liu, Chem. Rev., 2011, 111, 3577; (d) C. Liu, F. Li, L. P. Ma and H. M. Cheng, Adv. Mater., 2010, 22, E28. 2 (a) B. Scrosati, J. Hassoun and Y.-K. Sun, Energy Environ. Sci., 2011, 4, 3287; (b) J. R. Croy, A. Abouimrane and Z. Zhang, MRS Bull., 2014,
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3 4 5 6 7
8 9 10
11
12 13 14 15
39, 407; (c) B. Scrosati and J. Garche, J. Power Sources, 2010, 195, 2419. X. Feng, M. Fang, X. He, M. Ouyang, L. Lu, H. Wang and M. Zhang, J. Power Sources, 2014, 255, 294. (a) J. Remmlinger, M. Buchholz, M. Meiler, P. Bernreuter and K. Dietmayer, J. Power Sources, 2011, 196, 5357; (b) L. Lu, X. Han, J. Li, J. Hua and M. Ouyang, J. Power Sources, 2013, 226, 272. D. Doughty and E. P. Roth, Electrochem. Soc. Interface, 2012, 21, 37. E. P. Roth and C. J. Orendorff, Electrochem. Soc. Interface, 2012, 21, 45. (a) D. R. MacFarlane, N. Tachikawa, M. Forsyth, J. M. Pringle, P. C. Howlett, G. D. Elliott, J. H. Davis, M. Watanabe, P. Simon and C. A. Angell, Energy Environ. Sci., 2014, 7, 232; (b) M. Armand, F. Endres, D. R. MacFarlane, H. Ohno and B. Scrosati, Nat. Mater., 2009, 8, 621. A. Lewandowski and A. ´ Swiderska-Mocek, J. Power Sources, 2009, 194, 601. Y.-S. Ye, J. Rick and B.-J. Hwang, J. Mater. Chem. A, 2013, 1, 2719. (a) S. Noor, P. Bayley, M. Forsyth and D. MacFarlane, Electrochim. Acta, 2013, 91, 219; (b) Y. Kitazawa, K. Iwata, S. Imaizumi, H. Ahn, S. Y. Kim, K. Ueno, M. J. Park and M. Watanabe, Macromolecules, 2014, 47, 6009; (c) M. W. Schulze, L. D. McIntosh, M. A. Hillmyer and T. P. Lodge, Nano Lett., 2013, 14, 122. (a) Y. He and T. P. Lodge, Chem. Commun., 2007, 2732; (b) Y. He and T. P. Lodge, Macromolecules, 2008, 41, 167; (c) Y. Kitazawa, T. Ueki, S. Imaizumi, T. P. Lodge and M. Watanabe, Chem. Lett., 2014, 43, 204. T. Ueki and M. Watanabe, Langmuir, 2007, 23, 988. C. J. Orendorff, Electrochem. Soc. Interface, 2012, 21, 61. J. C. Kelly, R. Gupta and M. E. Roberts, J. Mater. Chem. A, 2015, 3, 4026. T. Ueki, A. A. Arai, K. Kodama, S. Kaino, N. Takada, T. Morita, K. Nishikawa and M. Watanabe, Pure Appl. Chem., 2009, 81, 1829.
Chem. Commun., 2015, 51, 5448--5451 | 5451
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Cite this: Chem. Commun., 2015, 51, 5448
Li-ion battery shut-off at high temperature caused by polymer phase separation in responsive electrolytes†
Received 24th December 2014, Accepted 23rd February 2015
Jesse C. Kelly, Nicholas L. Degrood and Mark E. Roberts*
DOI: 10.1039/c4cc10282g www.rsc.org/chemcomm
For the purpose of realizing inherently safe high-power Li-ion batteries, a model Li4Ti5O12/LiFePO4 rechargeable battery is investigated using the thermally responsive polymer, poly(benzyl methacrylate), in an ionic liquid. At high temperature, battery operation is inhibited as a result of increased internal resistance caused by polymer and ionic liquid phase separation. Li-ion concentration is shown to affect the phase transition temperature and the extent to which batteries are deactivated.
The development of new materials and chemistries has accelerated improvements in batteries over the past few decades, providing exciting opportunities for high-power and high-energy density devices.1 Lithium-ion batteries (LIBs), in particular, exhibit many properties amenable to energy storage for transportation and stationary systems for intermittent, renewable energy generation.2 Existing safety hazards associated with LIBs, however, prevent their use in large-format systems, as the potential for thermal failure significantly increases with cell size.3 Stable LIB operation is often compromised through several scenarios as hazardous conditions reside outside a narrow range of temperatures and voltages.4 Whether from over-charging, high discharge current loads, or changes in temperature, abuse of LIBs can lead to thermal failure which is manifested as fires or explosions.5 While the stability of electrode materials continues to improve, a major concern is the reactivity and flammability of electrolytes (e.g. lithium salts in organic solvents).6 Room temperature ionic liquids (ILs) have recently received consideration as electrolyte media for batteries and supercapacitors.7 Due to their unique physiochemical properties (negligible volatility, thermal stability, and high ionic conductivity) and a drive for safer and higher power forms of energy storage, ILs have attracted interest as alternatives to conventional electrolytes. Recent advances in IL
Department of Chemical & Biomolecular Engineering, Clemson University, 204 Earle Hall, Clemson, SC 29634, USA. E-mail: [email protected]; Fax: +1 864 656 0784; Tel: +1 864 656 6307 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cc10282g
5448 | Chem. Commun., 2015, 51, 5448--5451
electrolytes,8 IL-doped polymer electrolytes9 and ion gels10 have led to systems that avoid leakage and flammability while maintaining high conductivities, thermal stability, and a degree of functionality. Lodge and co-workers recently studied the use of thermally responsive polymers in ILs, where they utilized the phase separation to process highly conductive ion gels with block copolymers.11 Of particular interest is the class of poly(aryl methacrylates), such as poly(benzyl methacrylate) (PBMA), that exhibit a lower-critical solution temperature (LCST) and phase separate in imidazolium based ionic liquids.12 This solid-liquid phase transition occurs in the temperature range where thermal failure is initiated and most safety mechanisms, such as trilayer separators, are triggered in LIBs.13 Previously, we demonstrated that the phase transition of a responsive polymer electrolyte comprising poly(ethylene oxide) (PEO) and an IL causes a reduction in conductivity and charge transfer at elevated temperatures.14 While state-of-the-art safety mechanisms render LIBs inoperable (melting separators), utilization of responsive electrolytes that reversibly increase internal resistances may provide an opportunity to mitigate thermal failure while extending LIB lifetime and operating conditions. Additionally, the use of responsive polymers may provide a mechanism whereby local microscopic hotspots can be mitigated by coating the electrode surface while allowing the device to continue operation. In this communication, we describe how a responsive electrolyte of PBMA in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [EMIM][TFSI] inhibits LIB operation at elevated temperatures. As opposed to the PEO–IL system, where the mechanism is based on a change in conductivity (the polymer is still capable of coordinating and transporting Li ions), PBMA acts as an electronic insulator with very low Li-ion conductivity, thereby causing an increase in internal resistances at the electrode/ electrolyte interface when the polymer phase separates above the phase transition temperature. Responsive electrolytes were evaluated independently and within model Li-ion batteries, as shown in Fig. 1. Test cells were comprised of a Li4Ti5O12 (LTO) anode, LiFePO4 (LFP) cathode, a high-temperature stable nonwoven separator and the responsive
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Fig. 1 Schematic of the LIB setup (including assembled coin cell), the thermal-response mechanism at high temperatures at the electrode surface, and the structure of the responsive polymer, PBMA, and ionic liquid, [EMIM][TFSI].
polymer to form a low voltage (B1.7 V), high power Li-ion battery (see ESI,† for Experimental details). With the responsive electrolyte, the cell can deliver power over the voltage range of 1.0–2.5 V when below the electrolyte LCST. As this cell is heated above B120–140 1C, the battery ceases to operate when the polymer phase separates from the electrolyte creating a barrier to charge transfer (lithium intercalation) at the electrode/electrolyte interface and to some extent, ion transport through the separator. The mechanism for inhibiting device operation is discussed below. At 60 1C, cells without the responsive polymer additive were evaluated using constant current charge–discharge testing with various C rates (1–4 C) over the voltage range of 1.0 to 2.5 V (Fig. 2a). Capacity values are reported based on the mass of LTO because LFP is used in excess. The discharge voltage of the LTO/LFP system (1.5–1.7 V) is lower than conventional LIBs; however, it is capable of delivering high power rates while maintaining high capacities.
Fig. 2 LFP and LTO in a 1 M LiTFSI in [EMIM][TFSI] (a) at 60 1C for various C-rates and (b) at various temperatures at a 3.6 C rate. Cell capacity is on a per mass LTO basis as LFP is used in excess. (c) Nyquist plot showing the imaginary part vs. real part of impedance over the temperature range of 60 1C to 160 1C.
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To determine the stability of the cell over the temperature range of interest, charge–discharge measurements were performed with increasing temperature up to 160 1C at a discharge rate of 3.6 C (B1000 s discharge). As expected, the cell capacity significantly increases with temperature up to 140 1C as the conductivity of the electrolyte increases due to the decreasing viscosity (Fig. 2b). At 160 1C, however, the discharge potential slightly decreases, resulting in a loss of capacity and decrease in power (P = IV). Resistance values within the battery were investigated using electrochemical impedance spectroscopy (EIS) measurements. The Nyquist plot shows that each resistive component of the cell decreases as the temperature is increased up to 140 1C (Fig. 2c). A slight increase in charge transfer resistance is observed at 160 1C presumably due to either impurities or electrode/electrolyte degradation, which results in a lower discharge voltage and slight decrease in capacity. For this reason, subsequent battery testing with the responsive PBMA–IL electrolytes was kept below this temperature. Previous work shows that the thermally activated phase transition of PBMA in [EMIM][TFSI] occurs around 105 1C.15 However, we found that Li-ion concentration has a significant influence on the phase transition temperature of PBMA–IL systems, ranging between 100–180 1C for Li-ion concentrations of 0 to 1 M. Here, optical transmission is used to identify the phase transition temperature of electrolytes, which were prepared by a co-solvent evaporation method whereby 5 wt% PBMA (100 000 Mw) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) were mixed with [EMIM][TFSI] and dried under vacuum (see ESI,† for details). Fig. 3 shows the optical transmittance of 5 wt% solutions of PBMA in [EMIM][TFSI] as the LiTFSI concentration was increased from 0 to 1.0 M. The abrupt decrease in the optical transmittance indicates the temperature at which polymer phase separation occurred. Without the lithium salt, the PBMA–IL mixture displayed an LCST of 97 1C, which is slightly lower than values reported by Watanabe et al. due to the difference in Mw (100k vs. 70k). Increasing LiTFSI concentration in the PBMA–IL led to an increase in LCST from 106 1C up to 135 1C for the 0.2 M to 1.0 M solutions. The so-called ‘‘salting in’’ effect inhibits the polymer from phase
Fig. 3 Cloud point measurements for 5 wt% PBzMA in [EMIM][TFSI] with increasing LiTFSI salt concentration.
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separating and increases its solubility limit, thus increasing the temperature at which the phase transition occurs. As a result, it is important to balance battery performance, which increases with salt concentration, and the electrolyte phase-transition temperature. In addition to the increase in the LCST, it was observed visually that polymer aggregation decreased as the concentration of LiTFSI increased. LTO/LFP batteries were fabricated using thermally responsive electrolytes with LiTFSI concentrations between 0.2 M and 1.0 M and tested over the temperature range of 60 1C to 150 1C. Battery testing involved EIS measurements at the open circuit voltage (OCV) and constant current charge–discharge measurements over the voltage range of 1.0 to 2.5 V at a C rate of 3.6, which was determined independently for each electrolyte at 60 1C. The OCV is shown for cells with various salt concentrations from 60 1C up to 140 1C (Fig. 4a). In general, the OCV decreases with increasing temperature, except in the cell with 1 M LiTFSI, which has an LCST of 135 1C and was visually confirmed to show minimal polymer aggregation, resulting in minimal charge resistance. The decrease in OCV observed in the cells with lower electrolyte concentration (0.8 M or less) is attributed to the increase in cell internal resistance due to polymer phase separation, which leads to a resistive coating on the electrode. The formation of the resistive polymer coating that impedes ion diffusion is verified using EIS measurements, as shown in the Nyquist plot in Fig. 4b for the 0.5 M electrolyte. The diffusional resistance (low frequencies) increases above 100 1C and a drastic increase in charge transfer resistance (semi-circle at mid- to high-frequencies) occurs when the solution is heated at 150 1C for 3 h. EIS revealed similar changes in cell resistances with increasing temperature for the 0.2 M and 0.8 M electrolytes, however, the 0.8 M solution showed only a minimal change in
Fig. 4 Effect of increasing temperature on the (a) OCV for the 0.2 M, 0.5 M, 0.8 M, and 1.0 M LiTFSI in 5 wt% PBMA–IL solutions, (b) Nyquist plot for 0.5 M LiTFSI in a 5 wt% PBMA–IL, and charge–discharge curves at 60 1C and 150 1C for the responsive electrolytes with LiTFSI concentrations of (c) 0.2 M, (d) 0.5 M and (e) 0.8 M. Discharge measurements were performed at a C-rate of B3.6 at 60 1C and the same currents were applied at 150 1C.
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charge transfer resistance, even after 6 hours at 150 1C. As shown above, the extent to which PBMA aggregates decreases with increasing LiTFSI concentration, which prevents the polymer from forming a resistive coating on the electrode at higher salt concentrations. Furthermore, even though PBMA–IL system exhibits reversible phase behavior in a beaker (polymer re-dissolves in solution upon cooling), we did not observe a decrease in resistance as the cell cooled below the LCST (ESI,† Fig. S2). The phase behavior of the mixture in the presence of the separator is the subject of ongoing investigation. The constant current charge–discharge measurements are shown in Fig. 4c–e for cells with varying salt concentration at the initial temperature of 60 1C and after heating to 150 1C for 3 h. Almost complete shut-off occurs in cells with 0.2 M LiTFSI as a result of an increase in internal resistance above the LCST (at 150 1C), which decreases the OCV and prevents the LTO/LFP battery from delivering the current set point between 1.0 and 2.5 V. When the LiTFSI concentration is increased to 0.5 M and 0.8 M, a significant potential drop is observed; however, complete battery shutoff is not achieved. While the performance of the cell with 0.5 M LiTFSI is reduced at 150 1C (Fig. 4d), devices with 0.8 M LiTFSI exhibits an increase in capacity, albeit with lower power output (Fig. 4e). The change in capacity observed in this system is less than what is achieved in the absence of the polymer additive (Fig. 2). In traditional IL electrolytes (Fig. 2), an increase in temperature will increase the capacity of the device due to the increase in Li-ion conductivity. Here, we show that the addition of a thermally responsive polymer to the electrolyte impedes this increase in performance and causes nearly complete shutoff in battery operation at high temperature. We concluded that the polymer aggregation and coating on the electrode was the primary mechanism for battery inhibition because of the following. (i) The charge transfer resistance was shown to increase above the LCST (EIS), which is one indication that the phase separation is causing something to occur on the surface. (ii) The conductivity of the 5 wt% PBMA electrolyte continues to increase with increasing temperature indicating that the shut-off mechanism is not occurring in the bulk (ESI,† Fig. S1). (iii) Without the polymer in the electrolyte, no shut-off occurs, so it cannot be due to the IL, the electrode interfaces with the current collector, or chemistry within the electrodes. (iv) PBMA does not thermally decompose over the tested temperature range in beaker experiments. It is possible that the polymer is absorbing on the separator; however, based on the pore size and porosity, it is unlikely that the polymer would completely fill the pores to decrease the ion-conductivity enough to inhibit the battery performance. In summary, we have demonstrated that a LTO/LFP battery can operate at high power rates in an [EMIM][TFSI] electrolyte with LiTFSI and that the addition of a thermally responsive polymer, PBMA, can be used to inhibit battery operation at elevated temperatures. While the addition of the polymer affects performance, high power rates at relatively high capacities can still be maintained. The LiTFSI concentration is shown to increase the polymer’s LCST in [EMIM][TFSI], which affects the
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device’s upper temperature limit and the temperature-dependent internal resistance of the battery. While the reversibility of the responsive behavior of LTO/LFP devices has yet to be observed upon cooling (see Fig. S2 in ESI†), this aspect is currently under further investigation as the polymer is known to redissolve in solution upon cooling. Nonetheless, the use of responsive polymers provides a new approach to preventing thermal runaway in Li-ion batteries because the polymer phase separation can impede further lithium reactions when local hotspots (thermal failures) form, which will likely prevent the entire cell from heating beyond an unsafe temperature. Funding for this work was provided by 3 M Non-Tenured Faculty Grant and NSF CMMI Scalable Nanomanufacturing Award 1246800. The authors are thankful to Prof. Stephen Creager for valuable discussions and Brian Morin at Dreamweaver International for providing high temperature, mechanically stable nonwoven separators for Li-ion batteries.
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