All-printed Substrate-versatile Microsupercapacitors with

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All-printed Substrate-versatile Microsupercapacitors with Thermoreversible Self-protection Behavior Based on Safe Sol-gel Transition Electrolytes Shaoshuai Ma, Yunhui Shi, Yan Zhang, Liting Zheng, Qian Zhang, and Xinhua Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09498 • Publication Date (Web): 19 Jul 2019 Downloaded from pubs.acs.org on July 20, 2019

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ACS Applied Materials & Interfaces

All-printed Substrate-versatile Microsupercapacitors with Thermoreversible Self-protection Behavior Based on Safe Sol-gel Transition Electrolytes Shaoshuai Maa, Yunhui Shia, Yan Zhanga, Liting Zhenga, Qian Zhanga, Xinhua Xua, b* a

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, P.

R. China b

Tianjin Key Laboratory of Composite and Functional Materials, Tianjin 300072, P.

R. China *E-mail: [email protected] KEYWORDS: Thermoreversible electronic devices; self-protection; sol-gel transition; Microsupercapacitors; substrate-versatile; 3D printing. ABSTRACT: Thermal runaway has always been a significant safety issue that high performance electronic devices urgently need to solve. These existing strategies are limited by lack of reversibility and low conductivity. Here we propose a novel thermoreversible

self-protection

microsupercapacitor

(TS-MSC)

based

on

thermoresponsive polymer electrolyte to prevent thermal runaway. When heating above the low critical solution temperature (LCST), a gelation process occurs in the smart electrolyte and effectively inhibits the migration of ions, leading to a decreased specific capacity and an increased internal resistance of the MSC. However, the electrolyte transform to solution state at room temperature in which ions can freely migrate. Benefiting by sol-gel transition of smart electrolyte, the TS-MSCs can exhibit different 1

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electrochemical performance at elevated temperature, demonstrating an active method of achieving thermoreversible and dynamic self-protection. In addition, 3D printing technology and substrate versatility provide an attractive method in the design of integrated micropower devices. Therefore, such functional TS-MSCs offer a promising strategy to solve the safety issues of the nowadays portable micro- electronic devices.

Introduction In recent years, electrochemical energy storage devices are rapidly developing in the direction of lightweight, miniaturization and high performance1-3. A wide variety of technologies and materials have accelerated the improvement in integrated high performance electrochemical energy storage devices, however, their safety still remain an important issue4-8. High-performance electrochemical energy storage devices generate a great deal of heat during rapid charge and discharge, and cause internal pressure change in electronic devices, even the risk of explosion or spontaneous combustion. Besides, working at high temperature for a long time will damage the electronics when the generated heat cannot be eliminated in time9-10. Therefore, solving the thermal runaway problem is of great significance for the study of safe electrochemical energy storage devices with high power and current density. In order to cope with the thermal runaway in the electronic devices, considerable countermeasures such as extinguishing agents, shutdown separators and safety vents have been proposed11-14. However, these strategies only provide a one-time protection and are difficult to be widely used owing to the low conductivity of the electronic devices with addition of a large number of additives. Moreover, the application of 2


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alternative polymer gel or solid-state electrolytes compared with the aforementioned measures, are considered to be more effective15-21. But it must be noted that these electrolytes lead to low charge and discharge rate over the entire temperature range, which accordingly reduces the overall electrochemical performance of electronics22-25. Clearly, electronic devices employing the above measures are short of a smart and reversible response to the rapid temperature change owing to the absence of selfadaptive. Recently, thermoresponsive hydrogel electrolyte with sol-gel transition upon heating/cooling cycle have received extensive attention to dissipating the heat accumulated in electronics26-32. The movement of conductive ions exhibits temperature dependence owing to the phase transition of thermoreversible electrolyte, thus the electronic equipment could experience different current density and charge-discharge rates at dynamic temperature and restore to original electrochemical state when temperature decrease to normal state33. Clearly, this strategy is a promising and active measure to enable electronics to achieve thermoresponsive self-protection at elevated temperature. Strategies such as poly (N-isopropylacrylamide) (PNIPAM) based copolymers were investigated for this goal. However, the low LCST at around 37 ℃ of PNIPAM limits its practical applications34-35. Although, the pluronic aqueous solution were proposed as a thermoresposive self-protection electrolyte with suitable LCST, the electrolyte cannot shutdown the electrochemical devices at elevated temperatures and decrease the electrochemical performance at room temperature due to the concentration up to 30 wt % 36. What is noteworthy is that the existing research is limited to large 3



scale energy supply equipment and button cell system, but thermal runaway of nowadays popular portable micro-powers have not attracted much attention. In addition, micro-integration is an important development direction of high energy density devices, but the micro-structure accelerates the ion transmission rate, which will lead to the thermal runaway problem more serious37-39. Therefore, such temperature-dependent electrolyte provide a promising solution to address the safety concerns of the portable electronic devices. Furthermore, if other components of an integrated circuit in the portable devices generate heat that elevates the temperature of environment around the thermoresponsive supercapacitors it would be the active option to shut down its operation. Microsupercapacitors (MSCs) are state-of-the-art micro-sized power sources with the superiority of ultrahigh performance, fast charge-discharge rate as well as facilitated ions transport38, 40-43. However, complicated fabrication process leads to high costs and low feasibility for scalability. In addition, existing fabrication strategies with issues of the substrate and mold needs to be designed in advance. In addition, the electrode and electrolyte cannot be integrated and prepared simultaneously, which can not compatible with nowadays electronic devices. Thus, a simple fabrication strategy for substrateversatile and durable MSCs is of great significance. Additive manufacturing, also called as 3D printing, is a promising strategy to prepare products bottom-up with complicated design. Direct ink writing (DIW) is one of 3D printing technologies that has received considerable attention owing to its low-cost fabrication processes, massive-scalable and substrate versatile44-45. Benefiting from the above advantages of DIW technology, the 4


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high energy density substrate-versatile MSCs were prepared, which may open the door for the development of next generation of portable micro-powers. Furthermore, considerable investigation has been conducted in MSCs with extraordinary power density and fast rate capability, however, attention has been rarely paid to the active and self-adaptive electrolytes. Notably, it would be fascinating to research the 3D printed high performance MSC with adjustable electrochemical performance at elevated temperature. Herein, we demonstrate a 3D printed thermoreversible self-protection MSC (TSMSC) based on reversible sol-gel transition thermosensitive electrolyte, a H2SO4dissolved polymer solution, poly (ethylene oxide)-g-methylcellulose (MC-g-PEO). Mechanistic research shows that MC-g-PEO based electrolyte is in the solution state at lower temperature, where conductive ion is able to migrate freely. However, once the electrolyte change to a gel state at elevated temperature that inhibits the migration of ions, which can achieve 100 % switch-off of the original capacitance. Interestingly, the TS-MSCs exhibit dynamic charge/discharge rate upon heating/cooling cycles. This smart thermosreversible self-protection electrolyte proposes an active method for switching the MSCs between on and off according to the temperature. Notably, the TSMSCs reflect several promising features: (1) This TS-MSCs are able to sensitively adjust electrochemical performance at elevated temperature and the self-protection ability is reversible and extraordinary effective even achieved 100% switch-off the capacitance at 80 ℃. (2) This TS-MSCs have excellent room temperature conductivity and can play a self-protection role at elevated temperature. (3) Notably, compared with 5



the high addition amount of some original thermosensitive polymers, the polymer only needs to add 2%, which reduces the cost. (4) The TS-MSCs prepared by DIW can replace various substrates such as paper, textile and plastic film, which provides direction for the development of safe flexible wearable micro-devices. To the best of our knowledge, this is the first report of all 3D printing MSCs with thermoreversible self-protection function. This results reveal that the TS-MSCs propose smart and active strategies to attain reversible and dynamic protection and provide great promise for novel safe portable wearable TS-MSCs.

EXPERIMENTAL SECTION Chemical. Poly (oxyethylene) capped with methyl group at one end (designated as MePEO, Mw=5000 g/mol) and methylcellulose (MC) were purchased from Aladdin. Ammonium persulfate (APS) from Sigma-Aldrich were recrystallized in water. N, N, N’, N’- tetramethylethylenediamine (TEMED) from Aladdin was used without further purification. Sulfuric acid (H2SO4) was purchased from Tianjin Yuanli Chemical Co., Tianjin, China. Carbon nanotube aqueous dispersion was purchased from Chinese Academy of Sciences Chengdu Organic Chemistry Co., Ltd., Chengdu, China. All reagents were analytical grade. Synthesis of MC-g-PEO based smart electrolyte The MC-g-PEO polymer was synthesized by free radical copolymerization.46-47 The PEO macromonomers end-capped with methacrylate (PEO-MA) were prepared according to the previous literature48. Before starting the reaction, MC was dissolved in distilled water (100 mL) at 5 ℃ overnight. First, PEO was grafted to MC by using 6


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APS as initiator and TEMED as accelerator. Then, PEO-MA (0.35 g), MC solution, 50 mL of distilled water and 1% of APS were added into a four-round-bottom flask. After magnetic stirring and degassing with argon for 30 min, 25 μL of TEMED was added. The reaction was carried out at room temperature for 8 h under argon. Finally, asproduced PEO/MC graft copolymer was dialyzed in deionized water for 5 days to remove possible unreacted monomers and impurities. Then a portion of dried copolymer was added into 0.5 M H2SO4 solution and the mixture was placed at 0 ℃ under vigorous stirring 2 hours to prepare the smart electrolyte. Fabrication of the substrate-versatile TS-MSCs via 3D printing A certain amount of aqueous dispersion of carbon nanotube was added to the mortar and ground vigorously for 4 hours to obtain a uniform printable electrode inks. As shown in Figure S4, the rheological properties of the carbon nanotube aqueous dispersion were investigated to demonstrate the printability of the electrode material. The carbon nanotube ink was put in a 1 mL syringe connected to a plastic head with inner diameter of 340 μm, while the inner diameter of the plastic dispensing head used to print the electrolyte was 840 μm. The ink was directly printed on various substrate to form electrode via the DIW 3D-printer, which the printing speed was 5 mm/s. Subsequently, the electrolyte was printed directly on the surface of the electrode when the electrode layer was completely dried. After a period of rest, a substrate-versatile TS-MSC was successfully prepared. Characterization The chemical structure of MC-g-PEO polymer was evaluated by Fourier transform 7



infrared spectroscopy (FTIR, Nicolet Magna 560) spectra with a spectral resolution of 2 cm−1. Variable temperature infrared spectroscopy was measured by Nicolet IS50 with a heating rate of 10 ℃/min. The temperature dependent rheological tests (TA Instruments DHR-2) were used to measure the thermoreversible behavior of asprepared solutions at 1 rad s-1 and 0.5% strain as well as 10 ℃/min heat rate. Gel permeation chromatography (GPC Instrucments Waters GPC 1515) data was measured to evaluate the weight-average molecular weight and the molecular weight distribution of the MC-g-PEO polymer. Electrochemical Characterization Cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) were conducted to assess the electrochemical performances of the TS-MSCs in a potential window from 0 to 0.8 V using electrochemical working station (CHI 660E). Electrochemical impedance spectra (EIS) were measured at CHI 660E with frequencies ranging from 0.1-1 Hz. However, the impedance frequency range when measuring ionic conductivity of thermosensitive electrolyte was 1 Hz to 106 Hz. The capacitances (CMSC) of the TS-MSCs were calculated from GCD data dependent on the following equations: CMSC, GCD = I ∆t/∆V where ∆V is the voltage drop during discharge, I (A) is the voltammetric current and ∆t (s) is the discharge time. Accordingly, areal capacitance (CMSC) of TS-MSC were calculated from the CV by the equation: CMSC, CV = ∫ 𝐼𝑑𝑡/(2𝑣∆𝑉) 8


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where 𝑣 is the scan rate. The specific capacitance of TS-MSCs were derived from the equations: Careal = CMSC, GCD/A Cmass = CMSC, GCD/m Cvolumetric = Careal/h where m is the total area of the electrodes, h reveal the thickness of the electrodes and A represents the area of electrodes.

RESULTS AND DISCUSSION The thermoresponsive copolymer MC-g-PEO was successfully synthesized through radical polymerization. Figure S5 a shows the infrared spectrum of mPEO and mPEO-MA. 3700-3300 cm-1 is the O-H stretching vibration peak of alcohols, 30002700 cm-1 is the C-H stretching vibration peak on the long chain of polyoxyalkyl alcohol, and 1000-800 cm-1 is the C-O-C stretching vibration peak of ether linkage. In addition, 1642 cm-1is a stretching vibration peak of C=C in methacrylic acid, and 17001750 cm-1 is a C=O stretching vibration characteristic peak47, 49. Both mPEO-MA and mPEO contain C=C and ester-based functional groups, however, the difference is that the ethoxylated product has an ether bond. The stretching vibration peaks at 1719 cm-1 (C=C) and 1644 cm-1 (C=O) are significantly enhanced, and a new peak at about 1065 cm−1 (C-O-C stretching vibration peak) is occurred. The results suggest that the mPEOMA was successfully synthesized. The infrared spectrum of MC and MC-g-PEO were shown in the Figure 1c. The intense peak at about 3475 cm-1 represents the O-H stretching vibration peak of MC. The absorption peak intensity of the graft copolymer 9



at 3463 cm-1 becomes higher compared with mPEO-MA. In addition, it can be observed that two bands situated at around 1117 cm-1 and 1060 cm-1 were the stretching of the ether bonds in MC. Contrasting the characteristic peaks of grafted copolymer with MC and mPEO-MA, it is obvious that there are both the peaks of MC and those of mPEOMA. Therefore, it is certain that PEO has been grafted to the chains of MC.

Figure 1. (a) Schematic illustration of reversible self-protection for thermoresponsive electrolyte in electrochemical storage devices. The migration of conductive ions were inhibited by hydrophobic crosslink effect due to the gelation of electrolyte at 10


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overheating state, thus achieving shut-off of the devices. After cooling down, the electrolyte reversibly transformed to solution state where ions could freely migrate. (b) Photograph of MC-g-PEO copolymer solution reversible sol-gel transition. (c) Infrared spectra of MC-g-PEO and MC. (d) IR differential spectrums of MC-g-PEO copolymer at 25 ℃ and 80 ℃. The prepared H2SO4 solution with the addition of 2 wt % MC-g-PEO can reversibly transform between sol and gel, which the LCST is around 80 ℃. The mechanism could be concluded from the hydrophilic and hydrophobic segments of the copolymer driving the microstructure of the polymer to transform between freely movable micelles and intertwined equilibrium clusters or even physical gels as temperature changes. The hydrophilic interaction force plays a dominant role in the low temperature state, and the copolymer generates hydrogen bond force with water molecules, propelling the copolymer to generate freely movable micelle, and the polymer solution can flow freely. The MC main chain collapses to mutually independent collapsed nuclei at the elevated temperature, however, the more hydrophilic PEO chain is never embedded and is located on the periphery of the collapsed core. The hydrophobic interaction of the collapsed nuclei causes the PEO side chains to entangle with each other to form micro-domains, while the peripheral hydrophilic PEO chains repel each other and the existence of physical crosslinks of the collapsed nuclei prevents them from further association, thereby generating a balanced cluster and further form the physical gel. After cooling down, the hydrophilic interaction in the system reoccupies the leading role, and the intertwining between the 11



molecular chains gradually disintegrates into micelles that can move freely, and the polymer reforms hydrogen bonding force with water molecules driving the gel to be freely movable solution state, therefore the sol-gel transition is reversed. In addition, variable temperature FTIR spectra method was used to research the effect of temperature changes on the molecular structure of the electrolyte based on 0.5 M H2SO4 solution with addition of 2 wt % MC-g-PEO copolymer. Since the characteristic peaks of water molecules in the electrolyte is too strong to clearly analyze the characteristic peak of the copolymer (as shown in Figure S5), FTIR differential spectrum technology is conducted to analyze the molecular structure of MC-g-PEO copolymer at 25 ℃ and 80 ℃. As depicted in Figure 1 d, 1050 cm-1 is the C-O-C stretching vibration peak of ether linkage, and 1638 cm-1 is a stretching vibration peak of C=C in methacrylic acid. 2932 cm-1 is the C-H stretching vibration peak on the long chain of polyoxyalkyl alcohol. The above analyses indicate that differential spectrum technology obtains successfully the infrared spectrum of the MC-g-PEO copolymer. More importantly, there is no significant change in the infrared absorption frequency and peak shape of the main functional groups of the copolymer at 25 ℃ and 80 ℃. These results indicate that no chemical reaction occurred during the sol-gel transition of the electrolyte, but the conformation of the polymer molecular chain changed. Interestingly, the sol-gel transition of MC-g-PEO H2SO4 solution is relevant to temperature, hence it is a fascinating approach to use it to prepare a smart thermoreponsive self-protection electrochemical storage device. The electrolyte is a flowable liquid at room temperature where conductive ions can migrate freely. However, 12


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when the temperature increase above the LCST, physical gel network formed by intertwined molecular chains can hinder the movement of ions, then shutting down the electronics. In addition, the sol-gel transition is sensitive and reversible. The mechanism diagram is shown as Figure 1a. To demonstrate the concept of smart self-protection of thermoreversible microsupercapacitors, a substrate-versatile TS-MSC based on the as-synthesized MCg-PEO copolymer electrolyte was constructed via 3D printing technology as depicted in Figure 2a. During the DIW printing process, the three-axis direction stage migrates with a steady speed according a programmed printing routine. The interdigitated microelectrodes of MSC were shown in Figure 2b. The width of interdigitated microelectrodes were 1 mm, while the gap between two electrodes was 650 μm. And the length of single microelectrode was 7 mm. The area of electrode layer was 0.3 cm2

. More information on the size of the interdigitated microelectrode was shown in

Figure S6. The magnified scanning electron microscopy (SEM) diagram (Figure 2c) exhibited the morphology of microelectrode printed on the slide, which revealed that the carbon nanotubes were uniformly distributed on the substrate. In addition, the profile SEM morphology (Figure 2d) of the carbon nanotube printed on the slide exhibited that the thickness of carbon nanotube layer was 13.5 ±0.2 μm. Notably, this work allow the TS-MSC to be printed onto various substrates (paper, plastic adhesive tape, glass sheet, textile and so on) as depicted in Figure 2 e-h. Moreover, the flexible TS-MSC with complex environmental adaptability can be directly printed on the plastic adhesive tape (Figure 2h). Then removing the printed tape-type device and pasting it to 13



the desired surface in different circumstances, which adequately increases the convenience of constructing a micro-power. More importantly, supported by the extensive applicability to different substrates, the plug and play capability was achieved by transferring or printing the TS-MSCs to all sorts of micro-powers. The electrochemical performance of TS-MSC using 0.5 M H2SO4 solution with the addition of 2 wt % MC-g-PEO as electrolyte at room temperature is shown in Figure 3. All the MSCs used in electrochemical testing in this work were printed on the substrate of glass plates. As depicted in the electrochemical data of the prepared TS-MSC, the CV curves of MSCs under different scan rates exhibit an excellent rectangular shape owing to the development of electrochemical double layers. The GCD measurements were further studied to evaluate the capacity performance of the TS-MSCs at different current densities from 0.5 A/g to 3 A/g, and it can be seen from the GCD data that MSC has excellent coulombic efficiency. Clearly, it can be concluded that the addition of the MCg-PEO polymer did not significantly alter the original electrochemical behavior of the MSC.

14


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Figure 2. (a) Schematic diagram of preparation process of TS-MSCs with different substrates. (b) The image of interdigitated microelectrodes of MSC printed on slide. Top-view (c) and profile (d) SEM morphologies of MSC printed on the slide. (e-h) Photographs of MSCs printed on slide (e), printed on paper (f) and being rolled up (g) and printed on plastic tape (h) and being pasted on the wall. 15



To evaluate the temperature-dependent of TS-MSCs, CV measurements were tested at different temperatures using 0.5M H2SO4 solution as electrolyte with addition of 2 wt % MC-g-PEO. Notably, the capacitances of the TS-MSCs gradually decay during the heating treatment at the same scan rate of 50 mV s-1 as shown in Figure 3c. Meanwhile, the specific capacitance of the TS-MSCs decreased from 59.5 F g-1 to 0 F g-1, even attain 100% switch-off of the capacitance when temperature increased to 80 ℃. When the temperature returns to 25 ℃, the capacitance of TS-MSCs is promptly restored to the original level. As depicted in Figure S7 b, the addition of thermosensitive polymer did not cause excessive loss of the original electrochemical performance of MSCs demonstrating that the smart electrolyte has excellent ionic conductivity at room temperature. Usually, the electrochemical performance of MSCs will increase to some extent at the elevated temperature owing to the accelerated ions movement in solution at high temperature. The TS-MSCs, however, can dynamically regulate their capacitance behavior with the increase of temperatures. These results indicate that the physical gel network formed by MC-g-PEO polymer at high temperatures effectively binds the movement of conductive ions, and even shut down the MSCs.

16


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Figure 3. (a) CV curves and GCD profiles (b) of TS-MSCs using electrolyte with addition of 2 wt % MC-g-PEO copolymer at 25 ℃. (c) Thermoresponsive CV curves at different temperatures from 25 ℃ to 80 ℃. (d) Temperature dependent rheological test of 2 wt % MC-g-PEO copolymer solution upon heating from 25 ℃ to 80 ℃. Interestingly, the extent of capacity suppression over LCST can be adjusted by using MC-g-PEO copolymer solution with different concentration. As depicted in Figure S7 a, the MSC using 1 wt % copolymer solution as electrolyte exhibits a different suppression effect towards capacitance compared with that of 2 wt % MC-g-PEO solution at elevated temperature. The MSC with 1 wt % MC-g-PEO copolymer still has a specific capacity of 19.25 F g-1 at 80 ℃, only suppressing 72 % of the original capacity. However, the system with 2 wt % copolymer even can attain 100 % shut-off the capacity at 80 ℃. Notably, these results reveal that the suppression effect on the capacitance is related to the concentration of the polymer solution and the strength of the physical gel 17



network formed after the sol-gel transition. The higher the concentration of the copolymer solution, the easier it is to form a perfect physical gel network at high temperature which can inhibit migration of conductive ions. Rheological measurement were studied upon heating MC-g-PEO copolymer solution to attain direct experimental evidence. Figure 3d shows the temperature-relevant storage modulus and loss modulus (G′-G″) of 2 wt % MC-g-PEO copolymer solution and G′ values at different concentration of copolymer. The G′-G″ curve demonstrated three different regions: G′ was lower than G″ before the intersection, G′ rises sharply at around 70 ℃, and reached a peak at about 80 ℃. The change was in accord with gelation process of threshold, molecular chains intertwined with each other and forming physical gel network. In addition, as shown in Figure S10, the temperature-relevant storage modulus of MC-gPEO copolymer solution with different concentrations, which the modulus significantly increased with the rise of concentrations. And the storage modulus of different concentrations of solution increased at the elevated temperature. These results indicated that electrolytes with higher concentration exhibit more solid-like in the gel state, exhibiting increased possibility of inhibiting the migration of ions, even achieving 100 % shut-off capacitance. Therefore, based on the above conclusions, it should be concluded that the TS-MSCs can dynamically adjust the migration of conductive ions according to changes in temperature by sol-gel transition and the suppression of capacity is related to the strength of gel network formed above LCST.

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Figure 4 Thermoresponsive GCD curves of TS-MSCs at the current density of 1A g-1. (a) Temperature-dependent GCD curves from 25 ℃ to 70 ℃. (b) The in situ thermal responsive GCD curves of TS-MSCs at elevated temperature. (c, d) The GCD curves at RT, 25 ℃ and HT, 70 ℃. The

temperature-relevant

charge-discharge

curves

of

TS-MSCs

using

thermosensitive electrolyte with addition of 2 wt % MC-g-PEO copolymer were depicted in Figure 4a. It is noteworthy that the thermoresponsive electrolyte enables the MSCs exhibit dynamic charge-discharge rate with the increase of temperature, demonstrating the feasibility of suppressing thermal runaway of MSCs. Clearly, the charge-discharge time of TS-MSCs demonstrates a gradually reduction in the temperature range from 25 ℃ to 70 ℃, the charge and discharge time declined from 38 s to 3.6 s as shown in Figure 4 c and d. Interestingly, the GCD curve of the TS-MSCs 19



can not be effectively measured at 80 ℃ since this TS-MSCs even attain 100% switchoff of the capacitance. Furthermore, the in situ thermoresponsive GCD measurements were studied to evaluate the thermal response of the TS-MSCs (Figure 4b). As depicted in this picture, the TS-MSCs can dynamically and quickly adjust its charge and discharge behavior when temperature increases from 25 ℃ to 70 ℃ exhibiting the prompt and efficient thermoresponsive self-protection, which is consistent with the capacitance behavior. Notably, the results mentioned above revealed that the TS-MSCs can attain 100 % switch-off of the electrochemical performance. Thus far, the majority of research concentrated on NIPAM based polymers, but their suppression of capacitance at elevated temperature was low. Only ≈ 85 % and ≈ 35 % loss of capacitance was attained in Roberts’ and Chen’s work, respectively. Although, the degree of capacity suppression achieve 90 % in Yu’s work, the electrolyte with addition of 30 wt % Pluronic significantly reduced it’s conductivity at low temperatures and increased the cost. Interestingly, the TS-MSCs using 2 wt % MC-g-PEO copolymer as electrolyte present a wide temperature range (25-80 ℃) and totally switch-off function at 80 ℃ as depicted in Table 1. More importantly, the TS-MSCs have effective thermoresponsive self-protection function in neutral, acidic and alkaline electrolytes as shown in Figure S12 and S13, which has never been mentioned in previous relevant publications. Therefore, the TS-MSCs provide an active and promising strategy to address thermal runaway of electronic devices.

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Table 1. Thermoresponsive self-protection performance of electronic devices Type

Addition T amount of /℃ polymer

Capaci -ty loss

Applicable solvent system

Threeelectrode supercapacitor Threeelectrode supercapacitor Coin-type supercapacitor Coin-type supercapacitor

20 wt %

50

85 %

Acid solution

40

93 %

20 wt %

70

65 %

30 wt %

60

Nearly 100%

Zinc battery

20 wt %

70

85 %

SodiumBromine Battery MSC

20 wt %

65

95 %

80

100 %

MSC

2 wt %

80

100 %

Specific capacity

Ref

35

32

Alkaline solution Acidic and neutral solutions Acid solution Neutral solution

52.5 F g-1 at 34 50 mv s-1 110 F g-1 at 36 -1 50 mv s 110 mAh g-1 33 at 0.2 A g-1 231 mAh g-1 29 at 20 mA g-1

Alkaline 2.4 mF cm-2 31 and neutral at 20 μA cm -2 solutions Acidic, This work This neutral and work alkaline solutions

According to the above discussions, in addition to the ability of adjusting electrochemical performance at elevated temperature, the most significant advantage was the outstanding reversibility of the thermal response compared with traditional safe strategies. To validate the reversibility of TS-MSCs, cyclic specific capacity measurements with alternately heating and cooling the TS-MSC for multiple times were studied. As shown in Figure 5a, the TS-MSC illustrated a stable and excellent capacitance of 59.5 F g-1 at room temperature and could actively shut-down the electronics once heated to a specific temperature then the specific capacitance 21



recovered during the cooling process due to the reversible sol-gel transition. There was no obvious vibration in the specific capacity of TS-MSC during repetitive circulations of sol-gel-sol transition. In contrast to original countermeasures with only one-time protection, the TS-MSCs with smart electrolyte can provide active reversible selfprotection with the change of temperature, illustrating the outstanding reversibility of the TS-MSCs.

Figure 5. The thermoresponsive electrochemical performance of TS-MSCs under 22


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various temperatures. (a) The thermoreversible specific capacitance behavior of the TSMSCs upon heating-cooling cycles. (b) The temperature-dependent impedance spectra of TS-MSCs upon hearing from 25 ℃ to 80 ℃. Comparison of Nyquist plots (c, d and e) at different temperatures. (f) Ionic conductivity of the electrolyte with addition of 2 wt % MC-g-PEO copolymer responding to temperature. Electrochemical impedance spectroscopy (EIS) was conducted to research the temperature-dependent self-protection of TS-MSCs. The temperature-relevant Nyquist impedance plots of TS-MSCs were shown in Figure 5. Normally, the radius of semicircle in high-frequency region represents the charge transfer resistance at the interface between the electrode and the electrolyte, while the linear slope of the low frequency region is relevant to the transfer resistance of ions in the electrolyte. As depicted in Figure 5, the impedance related to the migration of ions gradually increase at elevated temperature, exhibiting that the sol-gel transition of MC-g-PEO can dynamically modulate the diffusion of conductive ions to prevent thermal runaway. In addition, to further investigate the suppression of thermoresponsive electrolyte on conductive ions migration, the temperature-dependent ions conductivities were calculated from EIS spectrum based on 2 wt % MC-g-PEO electrolyte as shown in Figure S14. Upon heating, the ionic conductivities were decreased from 1.74 × 10-2 at 25 ℃ to 1.27 × 10-4 at 80 ℃ in conformity to the sol-gel transition demonstrating that the thermoresponsive electrolyte can restrain the movement of conductive ions and dynamically adjust its electrochemical behavior at different temperature, even attain 100 % shut-off its capacitance. The temperature-dependent ions conductivities were 23



consistent with the CV, GCD and EIS measurements mentioned above, thus verifying the feasibility of thermoreversible self-protection MSCs.

CONCLUSIONS In summary, we demonstrated a substrate-versatile thermoreversible selfprotection MSC with thermoresponsive sol-gel transition electrolyte, which provided a promising way to address thermal runaway of portable micro-powers. The heatingtriggered gelation of the smart electrolyte with MC-g-PEO copolymer suppressed the migration of conductive ions while conductive ions were again able to move freely at cooling state. Supported by temperature-dependent ionic conductivity, the TS-MSCs can dynamically adjust its electrochemical performance at elevated temperature and even attain 100 % shut-off the capacitance at 80 ℃. Interestingly, compared with previous strategies, the TS-MSCs exhibit high conductivity at room temperature and excellent self-adaptive stability at heating-cooling cycles, which can promptly and actively solve the thermal runaway of electronic devices. Furthermore, the suppression efficiency of capacitance can be readily adjusted by optimizing the concentration of MC-g-PEO based solution. We believe it would be a promising strategy to combine the thermosensitive electrolyte and substrate-versatile flexible MSCs. More importantly, benefiting from the extensive applicability to different substrates, the plug-and-play function and thermoswitchable behavior were achieved by attaching the TS-MSCs to various electronic devices. Interestingly, several polymers which exhibit relevant temperature sensitivity and sol-gel transition in different solvents or even ionic liquids provide the prospect of thermoresponsive electrolyte for non-aqueous electrochemical 24


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systems such as lithium-ion batteries. In conclusion, the concept of substrate-versatile TS-MSCs provide promising and active strategies for addressing thermal runaway of nowadays portable micro-power devices. ASSOCIATED CONTENT

Supporting Information Supporting Information is available from the ACS Publicarions website or from the author. Electrochemical performances of AC electrodes of supercapacitor (Figure S1); Temperature-dependent GCD curves of supercapacitors at the current density of 1 A g-1 (Figure S2); Illustration of thermal switching behavior of the coin cell supercapacitor with the MC-g-PEO copolymer sol-gel electrolyte (Figure S3); Rheological properties of the electrode ink (Figure S4); FTIR of smart electrolyte and polymers (Figure S5); The size diagram of integrated electrode all mentioned above (Figure S6); Electrochemical performances of MSCs under different circumstance (Figure S7); Comparison of CV curves of TS-MSCs at 25 ℃ and 80 ℃ (Figure S8); GPC curve of the MC-g-PEO graft copolymer (Figure S9); The temperature-dependent G′ value curves for MC-g-PEO copolymer solution with different

concentrations

(Figure

S10); Temperature-dependent

capacitive

performance of the MSCs (Figure S11); Temperature-dependent electrochemical performances of TS-MSCs based on 0.5 M LiNO3 electrolyte with addition of 2 wt % MC-g-PEO copolymer (Figure S12); Thermoresponsive electrochemical performances of TS-MSCs using smart electrolyte (Figure S13); The assembly 25



drawing of Swagelok cell (Figure S14); The comparison of electrochemical performance of MSCs after 30 heating-cooling cycles and original MSCs (Figure S15); and the impedance spectra of TS-MSCs at 25 ℃ (Figure S16). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The work was financially supported by the National Natural Science Foundation of China (Grant no. 51873147)

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