Promising and Reversible Electrolyte with Thermal Switching Behavior

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 7171−7179

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Promising and Reversible Electrolyte with Thermal Switching Behavior for Safer Electrochemical Storage Devices Yunhui Shi,† Qian Zhang,† Yan Zhang,† Limin Jia,† and Xinhua Xu*,†,‡ †

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, P. R. China Tianjin Key Laboratory of Composite and Functional Materials, Tianjin 300072, P. R. China

‡

S Supporting Information *

ABSTRACT: A major stumbling block in large-scale adoption of high-energy-density electrochemical devices has been safety issues. Methods to control thermal runaway are limited by providing a one-time thermal protection. Herein, we developed a simple and reversible thermoresponsive electrolyte system that is efficient to shutdown the current flow according to temperature changes. The thermal management is ascribed to the thermally activated sol−gel transition of methyl cellulose solution, associated with the concentration of ions that can move between isolated chains freely or be restricted by entangled molecular chains. We studied the effect of cellulose concentration, substituent types, and operating temperature on the electrochemical performance, demonstrating an obvious capacity loss up to 90% approximately of its initial value. Moreover, this is a cost-effective approach that has the potential for use in practical electrochemical storage devices. KEYWORDS: thermoresponsive electrolyte, reversible self-protection, cellulose ester, sol−gel transition, electrochemical storage devices

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INTRODUCTION Advanced electrochemical devices, such as supercapacitors, lithium-ion batteries, and nickel−metal hydride batteries, have been large-scale applied in energy storage because of their properties such as high power and energy densities,1,2 stable cycling performance, and long cycle life.3−8 Delivering energy at high rates, however, could cause security issues with respect to considerable amounts of gases and heat generated by ultrafast charging and discharging processes, especially in some extreme conditions such as short circuiting and overcharging, thus resulting in catastrophic burning or explosion.9,10 With the increasing oil crisis and continuous development of new energy vehicles, safety problems of large battery packs with high specific energy density are in urgent need of technical breakthroughs. A series of exothermic reactions leading to a rapid rise in temperature and to thermal runaway can be initiated in addition to the charge/discharge cycle, including the thermal pyrolysis of electrodes and evolution of oxygen and hydrogen between the electrode/electrolyte interface, which in turn increases the internal cell temperature and pressure.11 Accordingly, effective suppression of thermal runaway is the premise of achieving safety application and plays a very important role in the research studies of high-energy storage devices.12 Considerable countermeasures are currently being © 2018 American Chemical Society

proposed to solve this problem, such as by using external/ internal thermal protection or solid-state electrolytes with high thermal stability, to prevent or absorb heat before thermal runaway. It must be noted, however, that external devices may not support instant response to rapid heat releases and pressure increases at high speeds.13 In addition, the use of alternative solid-state or polymer gel electrolytes is usually detrimental to ionic conductivity, which results in low charge/discharge rates and decreased energy density.14−17 Internal designs and solutions including incorporation of flame-retardant and overcharge additives,18,19 and shut-down separators20 compared with the aforementioned strategies, are considered to be more effective and prompt but have been limited in application in terms of low ionic conductivity and poor electrochemical performance. Clearly, the provisional strategies have been unable to meet the demand of efficient and rapid response to thermal runaway, thus calling for new ideas and approaches. Stimuli-responsive materials attract great attention as their properties may exhibit reversible change by external stimuli. Particular thermosensitive gels with sol−gel transition upon heating are finding increasing applications in biomaterial areas Received: December 28, 2017 Accepted: February 5, 2018 Published: February 5, 2018 7171

DOI: 10.1021/acsami.7b19726 ACS Appl. Mater. Interfaces 2018, 10, 7171−7179

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic illustration of the smart electrolyte with reversible thermoresponsive gelation properties for electrochemical energy storage devices. The system works normally at room temperature because of the free migration of ions in the interspaces. On heating, the hydrophobic crosslinked network leads to the sol−gel transition and absence of moving ions from solution, thus effectively shutting down the device above the LCST. Upon cooling, the electrolyte reversibly transforms to solution and recovers its ion motion. (b) Structures of MC, showing hydrophilic and hydrophobic groups. (c) Digital photograph of a MC solution below and above the LCST. (d) FTIR of pure MC and the mixture solution of MC and 1 M H2SO4-based electrolyte before and after 10 CV cycles, respectively.

such as tissue engineering and drug delivery.21,22 Recently, researchers proposed to develop smart reversible electrolytes based on thermoresponsive polymers, which have unique lower-critical solution temperature (LCST), to cope with the issue of thermal runaway. The migrations of conductive ions in the reversible electrolytes exhibit temperature dependence due to the phase separation of LCST polymers, thus providing or inhibiting conductive paths to achieve the thermoreversible protection. To date most of the research studies have focused on poly(N-isopropylacrylamide)-based copolymers because of its extensive mature application in biomedicine.23,24 However, their actual applications in electrochemical energy storage are restricted by the low LCST at around 32−34 °C and complicated synthesis with low productivity. Although commercial thermoplastic elastomer Pluronic [poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide)] have been dissolved in the electrolyte to shutdown the electrochemical devices upon heating, the high concentration up to 30 wt % influences the electrochemical properties at room temperature and increases the cost.25 Here, methyl cellulose (MC) is used as a stimuli-responsive material in the smart electrolyte because of its thermoreversible gelation property in aqueous solution at elevated temperatures. MC, a type of modified cellulose with a portion of hydroxyl groups substituted by hydrophobic groups, has been increasingly investigated as separators or binder materials instead of synthetic polymers for electrochemical devices owing to its characteristics such as abundance, regeneration, and low cost.26−31 In this study, MC molecules exist as randomly isolated chains in the electrolyte below the LCST when ions can move in the interspaces freely, enabling a high ionic conductivity, which is not possible with conventional flameretardant additives. When temperature increases, the electrolyte undergoes a thermally activated sol−gel transition, which results in a decreased ion concentration and broken circuit because the hydrophobic segment in MC molecular chains is tangled to inhibit the motion of ions. This process is reversible

and returns to high conductivity after cooling, as shown in Figure 1. We chose MC as the stimuli-responsive material in reversible electrolytes based on the following considerations: (1) MC solution exhibits relatively proper transition temperatures, so that system can be shutdown before or at the early stage of thermal runaway; (2) it is sensitive to temperature change in abnormal conditions and low concentration (∼2 wt %), making it a cost-effective system; (3) it is inert and stable within the electrochemical environment, having little impact on the electrochemical performance. Influences of concentration and substituent groups of cellulose ester upon the sol−gel transition temperature and self-protection behavior were researched. The results showed that the thermoresponsive electrolyte is a promising approach for electrochemical storage devices owing to its simplicity, reliability, and reversible operation.

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EXPERIMENTAL SECTION

Chemicals. Cellulose with different hydrophobic substituents, including methylcellulose (27.5−31.5 wt % methoxy) and hydroxypropyl methylcellulose (HPMC, methoxyl content of 19−24% and hydroxypropoxyl content of 7−12%) was supplied by Sigma-Aldrich and used after heating to 80 °C in a vacuum oven overnight. Sulfuric acid (H2SO4) and activated carbon (AC) were purchased from Tianjin Jiangtian Chemical Co., Tianjin, China. All of the above solvents are of analytical purity. Sample Preparation. To make the working electrodes, 80 wt % of AC, 10 wt % of Super P, and 10 wt % of polyvinylidene fluoride were mixed and coated on a copper foil or carbon paper. Then, the electrodes were placed in a vacuum oven and dried overnight at 90 °C. For the preparation of electrolytes, 50 mL of deionized water was heated to 70 °C and a chosen amount of MC or HPMC was dissolved into it under vigorous stirring for 8 h and cooled to room temperature overnight. After the cellulose solution was formed, 2.72 mL of H2SO4 was added into the solution under stirring at 0 °C for 30 min to obtain thermoresponsive electrolytes with desired ion concentrations. For the fabrication of symmetric coin cell supercapacitors (CR 2032), a cellulose spacer (NKK TF45) as a separator, AC-based electrodes coated on a copper foil, and a cellulose solution mixed with 7172


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Figure 2. Electrochemical properties of AC electrodes in 2 wt % MC-based electrolyte (1 M H2SO4). The CV curves were performed at scan rates of 10, 50, and 100 mV/s on AC electrodes at (a) 25 and (b) 70 °C. The charge/discharge characteristics in reversible electrolyte using a current density of 3 A g−1 at (c) RT, 25 °C and (d) HT, 70 °C. H2SO4 as a thermoresponsive electrolyte were assembled. The optimum content of electrolytes was around 300 μL. The particular process of making cell-structure supercapacitors was illustrated in the Supporting Information (Figure S5). Characterization. The electrolyte structure and chemical stability was measured by Fourier transform infrared spectroscopy (FTIR, Nicolet Magna 560) spectra with a spectral resolution of 2 cm−1 using the KBr pellets method at room temperature. The reversible transitions of the as-prepared solutions were evaluated by temperature-dependent rheological tests (TA Instruments DHR-2) at 1 rad s−1 and 0.5% strain, with a 5 °C/min heat rate. The differential scanning calorimetry (DSC, TA Instruments Hi-Res DSC 2920) was also used to investigate the sol−gel transition temperature at 5 °C/min under N2, and the samples were scanned from 25 to 80 °C. Electrochemical Characterization. Cyclic voltammetry (CV) and galvanostatic charge/discharge (3 A g−1) were carried out in a potential window of −0.2 to 0.8 V using a three-electrode electrochemical system with 1 M H2SO4 electrolyte, a platinum foil as the counter electrode, and Ag/AgCl electrode as the reference electrode. The temperature-dependent electrochemical analyses were performed by placing the system in a water bath at elevated temperatures using CHI 660. The frequency range of 0.1 Hz to 1 MHz was set for electrochemical impedance spectroscopy (EIS) measurements. Furthermore, the practicability of the thermal switching electrolyte in electrochemical storage devices was demonstrated by fabricating symmetric coin-type supercapacitors using AC as working electrodes. Ionic conductivities, σ (mS/cm), were determined by the corresponding impedance spectroscopy, which performed in a coin cell with stainless steel electrodes and a cellulose spacer (NKK TF45).31

protection. Our proposed smart electrolyte is based on the MC solution with a reversible sol−gel phase transition. Below the transition temperature, molecules are hydrated as a hydrophobic segment, packing of hydrophobic clustering of methyl groups in regions, surrounded by water to form an enclosed cagelike structure,32,33 causing the chains to be randomly coiled isolated in aqueous solution where ions could move freely with little impact. Upon increasing the temperature, the structures are gradually distorted by dehydration, resulting in the formation of hydrophobic conjunction and aggregate, eventually forming an infinite hydrophobically crosslinked gel network structure.34 The gelation process of MC solution decreases the amount of free ions along with the increased electronic interaction between chains and ions, thus shutting down the device above its switching temperature (LCST) as a result of phase transition and absence of moving ions that break the conductive paths.23−25 Figure 1c shows the reversible properties of a corresponding mixture solution of MC and 1 M H2SO4 in the sol and gel states. The electrolyte structure and chemical stability before and after electrochemical measurement was evaluated by FTIR, as shown in Figure 1d. All three samples exhibit a characteristic O−H stretching peak at 3200−3500 cm−1 and stretching vibration of C−O bond at 1050 cm−1. However, the spectrum band of pure MC solution is broader and shifts to a lower wavenumber, indicating the existence of strong hydrogen bonds between MC molecules and water, which could be further verified by the bending vibration of O−H at 1640 cm−1. For electrolytes, the characteristic peak at 1150 cm−1 is attributed to the stretching vibrations of SO. Compared to pure MC solution, the addition of mineral acid would lead to weaker hydrogen bonds and thus lower the sol−gel transition temperature, as proved by the weakened O−H bending vibration of the mixture solution of MC and 1 M H2SO4. Overall, all characteristic peaks of MC could be observed in the spectrum of mixture solution, indicating the stable chemical structure of the electrolyte. In addition, FTIR before and after

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RESULTS AND DISCUSSION The MC solution transforms into gel states at elevated temperatures because of hydrophobic association, and the transition is reversible-dependent on temperature changes as shown in Figure 1a. The mechanism relies on the switching between the dissolved hydrophilic section and conjuncted hydrophobic segment of cellulose ester (Figure 1b), demonstrating a promising approach to realize active thermal 7173


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Figure 3. Electrochemical tests of AC electrodes and 1 wt % MC (1 M H2SO4) as electrolyte: (a,b) CV curves at different scanning rates at 25 and 70 °C. (c,d) Charge/discharge curves at 25 and 70 °C. (e) Impedance tests at RT and HT (RT stands for room temperature, 25 °C and HT stands for high temperature, 70 °C).

to meet the demand of desirable devices by tuning the concentration of MC in solutions. Clearly, on the basis of the charge/discharge tests (Figure 3c,d), upon heating, the estimated specific capacitances decrease from 90 to 9 F g−1 with the addition of 1 wt % MC, displaying effective operation of approximately 90% capacity loss of its initial value at 25 °C. Besides, no clear differences can be observed between using thermal-reversible and conventional electrolytes in the charge/ discharge cycling, indicating good electrochemical stability of the smart electrolyte with 1 wt % MC (see Figure S1c in the Supporting Information). CV tests below and above the LCST (Figure 3a,b) have further confirmed the results aforementioned, revealing efficient suppression of thermal runaway in comparison with existing literature of only ≈35 and ≈85% capacity loss.23,24 More importantly, the initial specific capacitance is reasonably high compared to that with 2 wt % MC and the traditional liquid electrolyte system, suggesting that the system with 1 wt % MC can be operated as normal electrolytes without considerable sacrifice in electrochemical performance. This is owing to the fact that the average interspace between MC molecular chains increases as the concentration of MC aqueous solution decreases, thus leading to the reduced impact on the movement of ions. The EIS in Figure 3e was employed to further study the migration of conductive ions in the smart thermoresponsive electrolyte. Theoretically, the semicircle radius in high frequency of the Nyquist plot is associated with the interfacial charge transfer between the electrode/electrolyte, whereas the line at the low frequency region corresponds to the diffusion of conductive ions in the electrolyte. In this section, contrary to the conventional electrolyte system, the impedance controlled by the diffusion of conductive ions show a significant increase upon elevated temperature, suggesting that the thermoreversible gelation of smart electrolyte at high temperature can

10 CV cycles of the mixture solution of MC and 1 M H2SO4based electrolyte shows nearly the same infrared absorption peak and no significant difference in the fingerprint, demonstrating the chemical structural stability after the sol− gel transition and excellent reversibility. To evaluate the overtemperature protection of electrochemical storage devices, a three-electrode system was employed first. A chosen amount of MC dissolved with 1 mol/L H2SO4 aqueous solution served as the smart electrolyte and an AC coated on a carbon paper was used as the working electrode. We carried out CV (Figure 2a,b) and charge/ discharge tests (Figure 2c,d) initially in the presence of 2 wt % MC at room temperature 25 and 70 °C, respectively, which is much lower in additive concentration as compared to previous literature.23,25 According to the results of Figure 2, the specific capacitance decreases around 97% upon increasing the temperature to 70 °C, and the charge and discharge time decreases from 22 to 1 s when the voltage reached from −0.2 to 0.8 V, revealing the inhibited migration of ions and reduced current. This decrease measured above supports our hypothesis that the MC solution-based electrolyte do in fact exhibit distinctly different characteristics from the traditional electrolyte. Besides, a good thermoreversible transition can be obtained, and the system can operate again as normal while the temperature falls below the LCST. However, compared with the traditional liquid electrolyte system (see Figure S1a in the Supporting Information), a certain loss of specific capacitance about 30% at 25 °C was observed using the electrolyte with 2 wt % MC while excellent thermoresponsive switching behavior was obtained. Generally, to ensure that electrochemical storage devices can work effectively and be widely used, the capacitance should remain at a high level under ambient temperature. Thus, in this section, the initial and final specific capacitance was rationally designed 7174


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capacity loss can be achieved at an elevated temperature, so that we do not have to make a trade-off between the efficient control of thermal runaway and the electrochemical performance of desirable devices. To better demonstrate the thermoresponse influenced by temperature and concentration, temperature-dependent CV was conducted. The specific capacitances corresponding to the CV curves of the system with 1 wt % MC show an obvious loss (Figure 4a) upon heating, exhibiting distinctly different characteristics from the conventional electrolyte. System based on electrolyte containing 0.1 wt % MC as displayed in Supporting Information Figure S3a, however, shows a trend quite different from that of 1 wt % MC at low temperature from 25 to 40 °C. After that the specific capacitance begins to decrease as the temperature increases and a relatively high final specific capacitance is received. These observations are postulated to be closely related to the competition between the diffusion rate of ions in the electrolyte and electronic interaction of MC chains and ions, which is primarily affected by the temperature involving the sol−gel transition. Rheological tests and DSC were selected on heating MC solutions to obtain direct experimental evidence. The rheological data of 1 wt % MC as shown in Figure 4b, carried out at low shear strains and 1 rad s−1, illustrate at least three distinct regions: storage modulus G′ less than loss modulus G″ before the intersection, significant increase in storage modulus (G′) around 65 °C, and a plateau eventually at about 70 °C. The results are well consistent with the gelation process of threshold, forming hydrophobic conjunction and aggregate to the weak structure, and the construction of infinite hydrophobically cross-linked gel network structure. Accordingly, a strong endothermic peak located at 66 °C is observed in DSC traces (Figure 4c), and the values can roughly correspond to the sol−gel transition point of solution with 1 wt % MC. By contrast, Figure 4d shows the temperature-dependent G′ values for MC solutions at different concentrations, at which sudden increases in the G′ values are

increase the electronic interaction between MC molecules and ions, thereby resulting in the increased impedance and active thermal protection. As a control experiment, the electrochemical performance of the system with 0.5 wt % MC is also shown in Figure S2 (Supporting Information), showing a high initial specific capacitance whereas weakened overheating protection of only 80% decrease in final capacity at 70 °C. This further proves that the decrease of thermoresponsive materials can weaken the electronic interaction between MC chains and ions, thus enhancing the initial capacitance at room temperature. Meanwhile, it would decrease the conjunction interaction between hydrophobic segments, which is necessary to inhibit migration of ions, and finally result in an inefficient control of thermal runaway. For comparative analysis, Table 1 summarizes the electrochemical properties and thermal switching behavior of systems Table 1. Electrochemical Properties and Thermal Switching Behavior of Systems with Different Concentration of MC concentration of MC

initial specific capacitance at 25 °C (F/g)

final specific capacitance at 70 °C (F/g)

capacity loss

0 2 wt % 1 wt % 0.5 wt %

99 67 90 89

150 2 9 18

97% 90% 80%

with different concentration of MC systematically. The results indicate that although efficient suppression of thermal runaway can be achieved along with relatively high concentration of MC, a considerable sacrifice in electrochemical performance at room temperature should also be taken into account. However, by adjusting a suitable concentration of MC (e.g., 1 wt %), such a system can operate stable under ambient temperature with acceptable sacrifice in electrochemical performance while great

Figure 4. (a)Temperature-dependent CV tests of the system based on electrolyte with 1 wt % MC solution. (b) Storage modulus G′ (black) and loss modulus G″ (red) as a function of temperature in heating processes for a 1 wt % MC solution. (c) Illustrative DSC trace for 1 wt % MC solution on heating. (d) The temperature-dependent G′ value curves for MC with different concentrations. 7175


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Figure 5. (a) Storage modulus G′ (black) and loss modulus G″ (red) as a function of temperature in heating processes for a 1 wt % HPMC solution. (b) Illustrative DSC trace for 1 wt % HPMC solution on heating. (c,d) Charge/discharge curves at RT, 25 °C and HT, 70 °C.

Figure 6. (a) Illustration of thermal switching behavior of the coin cell supercapacitor using 1 wt % MC solution-based electrolyte: digital photographs show the decreased light intensity of the LED while heating to 70 °C. (b) The reversible specific capacitance summary of thermalresponsive supercapacitor cycling between 25 °C and shutdown. (c) Ionic conductivity response to temperature of 1 wt % MC solution-based electrolyte.

almost independent of the MC concentration. The gel modulus, however, appears to change with the increase of concentration. In addition, a rather weak endothermic peak of 0.1 wt % MC solution is illustrated by the DSC trace as displayed in the Supporting Information Figure S3b. It can be assumed that hydrophobic interactions to association occur at essentially the same temperature under different contents of

homogeneous cellulose ether, and more entanglements tend to form based on polymer chains at high concentration, thus leading to the stronger gel network and more efficient thermal switching behavior. Therefore, according to the above results, there are two factors influencing the suppression of thermal runaway for the thermal-reversible electrolyte with an increasing temperature: 7176


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In addition, to further understand how the sol−gel phase transition and gel strength affect the inhibition of ion migration, temperature-dependent conductivities were determined by the corresponding EIS using a electrochemical test cell with stainless steel electrodes. The impedance spectrum for electrolyte of 1 wt % MC solution upon heating was conducted (see in Supporting Information Figure S7), and corresponding ionic conductivities were calculated as shown in Figure 6c. With increasing temperature, the ionic conductivities are reduced by orders of magnitude from 1.65 × 10−2 to 1.60 × 10−4 S cm−1 in accordance with the phase transition of sol to gel process, indicating that the increase of temperature can restrict the motion of ions and thus leading to the dramatic decrease of electrolyte conductivity. In addition, the ionic conductivities of electrolytes with different concentration of MC are further summarized in Supporting Information Table S1 and Figure S8a,b. The dependences of ionic conductivities on temperature are consistent with the CV and charging/discharging measurements described above, thereby providing efficient thermoresponsive protection behavior by the inhibition effect of ion transport.

the diffusion rate of ions and the electronic interaction between entangled MC chains and ions. Generally for low concentration of MC (e.g., 0.1 wt %) below the LCST, the increase in temperature would mainly improve the diffusion rate of ions which move in the interspaces between unconjugated molecules freely; besides, there is no sufficient tangle and aggregation of MC to restrict the transport of ions, thus leading to the poor performance in thermal protection, as shown in Supporting Information Figure S3. In contrast, gel modulus increases as the concentration of MC increases, owing to the increased possibility to form entanglements; hence, the sol−gel transition and resultant stronger gel network counteracts the migration of ions at lower temperature and eventually leads to the efficient shutting down of the system upon heating. To systematically study the influence of substituent groups on thermoreversible gelation besides the adjusting of concentration, HPMC was also incorporated to the thermoresponsive electrolyte. Compared to the system comprising MC in electrolytes, solution containing 1 wt % HPMC exhibits an increase in temperature at which thermogelation occurred and thus decreased the gel modulus (Figure 5a,b). Also, an obvious decrease to nearly 88% of its initial value at 25 °C in discharge capacity, represented by the discharge time, was observed as shown in Figure 5c,d. From the results of rheological test and electrochemical performance aforementioned (see CV curves in Supporting Information Figure S4), we can deduce that the sol−gel transition of HPMC-based solution does not occur until higher temperature, and the control of thermal runaway for the resulting thermoresponsive electrolyte is substantially weaker in comparison with that of the methyl subtituents.35 This phenomenon can be explained by the fact that the incorporation of hydroxypropyl groups in HPMC is responsible for altering the sol−gel transition, for the more hydrophilic hydroxypropyl groups would facilitate the hydrogen-bonding interaction between molecular chains and water, and therefore restrict the intermolecular hydrophobic association, leading to an increase in the transition temperature and weakened overheating protection. The practicability of the thermal switching behavior in electrochemical storage devices was demonstrated by fabricating symmetric coin-type supercapacitors using AC as electrodes because of its wide application in commercial supercapacitors and by obeying electrical double layer capacitance where ions physically adsorbed on the interface of electrode/electrolyte without charge transfer (Figure 6a and Supporting Information Figure S5).36 The electrochemical storage performance was investigated at elevated temperatures based on the electrolyte with solution of 1 wt % MC, for its appropriate LCST and gel strength, excellent electrochemical performance, and active thermal self-protection as mentioned above. Cyclic experiments of the supercapacitors were conducted to measure the specific capacitance under multiple heating and cooling processes (see in Supporting Information Figure S6). It is shown that during each cycle, the electrochemical activity of the coin cells maintains high and stable initial capacitance, while a drastic decrease in capacity of approximately 85% loss is observed due to the absence of ions upon heating, as shown in Figure 6b. This implies that the thermoresponsive electrolyte system can guarantee the effective overheating self-protection of electrochemical storage devices with acceptable sacrifice in energy density and exhibit excellent reversibility that are promising for practical thermally safe devices compared to traditional disposable strategies.

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CONCLUSIONS In summary, the simplicity and reliability of the thermoreversible electrolyte, at low incorporation of MC, can be achieved and attributed to the sol−gel transition at elevated temperatures, resulting in efficient shutting down of electrical energy storage devices above the LCST. The use of the thermoresponsive electrolyte will inhibit the migration of ions upon heating because the increased electronic interaction of ions and MC chains involves the sol−gel transition, thus largely decreasing the conductivity at the electrolyte and electrode/ electrolyte interface on heating. Adopting AC electrodes, the gelation of 1 wt % MC solution-based electrolyte displays effective operation of approximately 90% capacity loss of its initial value, demonstrating a significant inhibition effect of ion transport and thermal switching. Furthermore, we demonstrate how the type of substitution, in addition to the concentration of MC, affects the thermoreversible transition temperature and the suppressing efficiency of thermal runaway. It is anticipated that this type of smart thermoreversible electrolytes can be used in electrochemical storage devices that may encounter safety issues. Specific materials which exhibit similar LCST behavior and ability to gelation in corresponding solvents or ionic liquids have the potential to serve as smart reversible electrolytes for nonaqueous electrochemical devices such lithium-ion batteries. This reliable and reversible strategy will facilitate easy protection strategy fabrication into electrochemical storage devices, and further development of similar responsive electrolytes will be needed for practical applications.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b19726. Electrochemical performances of AC electrodes in 1 M H2SO4 electrolyte; electrochemical performances of AC electrodes and 0.5 wt % MC (1 M H2SO4) as electrolytes; temperature-dependent CV tests of the system in electrolyte with 0.1 wt % MC solution; DSC trace for 0.1 wt % MC solution on heating; CV curves of AC electrodes and 1 wt % HPMC (1 M H2SO4) as 7177


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electrolytes; fabrication and photo image of a coin cell supercapacitor; selected two cycles CV curves in cyclic test of supercapacitor devices; temperature-dependent impedance tests of 1 wt % MC solution-based electrolyte; ionic conductivities of electrolytes with different concentration of MC; and temperature-dependent impedance tests of electrolytes with 2 wt % MC and 0.5 wt % MC (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xinhua Xu: 0000-0002-7864-3044 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the National Natural Science Foundation of China (grant nos. 51143009 and 51273145).

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REFERENCES

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Promising and Reversible Electrolyte with Thermal Switching Behavior

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