WO2022116497A1 - 一种含防冻两性离子水凝胶电解质的超级电容器及其制备方法 - Google Patents

一种含防冻两性离子水凝胶电解质的超级电容器及其制备方法 Download PDF

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WO2022116497A1
WO2022116497A1 PCT/CN2021/097104 CN2021097104W WO2022116497A1 WO 2022116497 A1 WO2022116497 A1 WO 2022116497A1 CN 2021097104 W CN2021097104 W CN 2021097104W WO 2022116497 A1 WO2022116497 A1 WO 2022116497A1
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electrolyte
polysh
supercapacitor
licl
solution
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French (fr)
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刘利彬
杨健波
班青
盖利刚
李学林
李梅
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齐鲁工业大学
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
    • H01G11/54—Electrolytes
    • H01G11/56—Solid electrolytes, e.g. gels; Additives therein
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
    • H01G11/22—Electrodes
    • H01G11/30—Electrodes characterised by their material
    • H01G11/32—Carbon-based
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
    • H01G11/84—Processes for the manufacture of hybrid or EDL capacitors, or components thereof
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/13—Energy storage using capacitors

Definitions

  • the invention belongs to the field of supercapacitors, and relates to a supercapacitor containing a zwitterionic polymer hydrogel electrolyte and a preparation method thereof.
  • Supercapacitors also known as electrochemical capacitors, in the energy density-power density correlation diagram, are a class between batteries (high energy density and low power density) and traditional dielectric capacitors (high power density and low energy density). electrochemical energy storage devices.
  • the working principle of supercapacitors is that under the action of an external electric field, the electrolyte ions are electrostatically separated at the electrode/electrolyte interface, and electrical energy is stored in the form of electrochemical double-layer capacitors.
  • supercapacitors have high power density and long cycle life, their low energy density compared to batteries limits their application range. Therefore, improving the energy density of supercapacitors without compromising their power density and cycle life is a key challenge in the field of electrochemical energy storage.
  • Electrolytes including liquid electrolytes and solid electrolytes, are indispensable components of supercapacitors. Compared with liquid electrolytes, the low electrical conductivity of solid electrolytes is a major disadvantage that affects the high performance of energy storage devices. Due to the high conductivity of polymer hydrogel electrolytes at room temperature, it has gradually become a hot spot in the research of high-performance solid-state supercapacitors. However, the conductivity of polymer hydrogel electrolytes is greatly reduced at 0 °C because the large amount of water in the hydrogel network is bound to freeze at subzero temperatures.
  • Incorporating organic liquids into hydrogels is one way to obtain antifreeze hydrogels.
  • Commonly used organic liquids include ethylene glycol, glycerol, dimethyl sulfoxide, and the like.
  • these binary/ternary systems the interaction of organic liquids with water molecules is considered to be the main reason for the inhibition of ice lattice formation.
  • these hydrogels are either non-conductive or have low conductivity due to the presence of organic liquids.
  • the volatility and high spontaneous combustion of organic liquids pose serious safety hazards to organohydrogel electrolytes. How to realize the low temperature antifreeze of hydrogel electrolyte supercapacitor is a big problem.
  • the purpose of the present invention is to provide a supercapacitor containing an antifreeze zwitterionic polymer hydrogel electrolyte and a preparation method thereof in view of the deficiencies of the prior art.
  • the zwitterionic polymer hydrogel electrolyte supercapacitor assembled using activated carbon as the electrode not only has good antifreeze performance, but also has a wide operating temperature range and the ability to work at low temperature for a long time.
  • a supercapacitor characterized by comprising a zwitterionic polymer hydrogel electrolyte.
  • the capacity retention rate at extreme temperatures is less than 25% relative to the area specific capacitance at 25°C.
  • the area-specific capacitances changed to 178 mF cm -2 and 134 mF cm -2 , which were approximately 104% and 78% of that, indicating good electrochemical performance at extreme temperatures.
  • the extreme temperature refers to high temperature (eg higher than 50°C) or low temperature (eg lower than 0°C).
  • the hydrogel will freeze due to the freezing of water at low temperature; the hydrogel will lose water at high temperature, which will make the supercapacitor unable to work.
  • the extreme temperature is a broad definition.
  • the low temperature is generally 0 °C, and the high temperature has no specific temperature, but the electrochemical performance is generally greatly affected at 50 °C. In this application, unless otherwise specified, extreme temperatures refer to higher than 50°C or lower than 0°C.
  • the zwitterionic antifreeze hydrogel electrolyte is a poly(SBMA-HEA) electrolyte in the presence of LiCl, and Li + undergoes hopping migration through zwitterionic groups on the polymer chain, thereby improving ionic conductivity ;
  • the stretching vibration of CH3 in -N + (CH 3 ) 2 is 2953cm-1 ⁇ 2957cm-1, indicating that the amphiphilic
  • DSC demonstrated the formation of a Li+(H2O)n solvated structure with H2O, which improves the antifreeze performance of the hydrogel electrolyte, which still exhibits 325% stretch at -40°C It has excellent ionic conductivity at low temperature and can be used in ionic conductors and energy storage devices.
  • the conductivity of the polySH electrolyte is 114-147 mS cm -1 .
  • the storage modulus G' of the polySH electrolyte is greater than the loss modulus G".
  • the low-temperature conductivity of the electrolyte is 11-13 mS cm -1 .
  • the electrolyte can be stretched to 300 ⁇ 13mS cm -1 at -40°C 350% strain. When compressed to 75% strain, the compressibility curve of the electrolyte is similar to that at room temperature.
  • the freezable water content of the electrolyte is less than 10% (polySH-5, polySH-7); preferably, the freezable water content of the electrolyte is zero (polySH-7).
  • the water molecule diffusion coefficient of the electrolyte is less than or equal to 11.21*10 -6 cm 2 s -1 .
  • the water molecule diffusion coefficient of the electrolyte is less than or equal to 1.02*10 -6 cm 2 s -1 .
  • the preparation method of the hydrogel electrolyte is, in the presence of LiCl salt, using SBMA and HEA to prepare a zwitterionic polySH electrolyte by a one-pot random polymerization method; including the following steps:
  • the concentration of the LiCl deionized aqueous solution in step 1) is 3-7 mol L -1 ; more preferably, the concentration of the LiCl deionized aqueous solution in step 1) is 5-7 mol L -1 .
  • the amount of the initiator added is equivalent to 1 wt% of the total mass of the monomers.
  • the ultrasonic in step 3 is ultrasonic for 10 min to remove air bubbles.
  • the polymerization in the sealed environment described in step 3) is to seal and place in a 38° C. environment for polymerization for 12 hours.
  • the obtained hydrogel is abbreviated as polySH-x, where x is the molar concentration of LiCl.
  • the preparation method of the supercapacitor comprises the following steps:
  • activated carbon electrode Preparation of activated carbon electrode: activated carbon AC, conductive carbon black and PVDF (mass ratio 8:1:1) were dispersed in NMP to prepare a uniform dispersion slurry. The slurry was coated on the carbon cloth and placed in a vacuum oven at 80 °C for 24 h, and dried to obtain an AC electrode. The loading of active material on each electrode is about 2.5 mg cm -2 ;
  • the prepared capacitors were sealed with tape to prevent moisture evaporation before electrochemical testing.
  • the polySH electrolyte precursor solution refers to the monomer mixture before polymerization, and its preparation method is as follows: 1) Dissolving LiCl in a first solvent to prepare a 1-7 mol L -1 LiCl deionized aqueous solution.
  • the supercapacitor assembled by using activated carbon as an electrode in the present invention has a specific capacitance of 178 mF cm -2 at 60° C. and a specific capacitance of 134 mF cm -2 at -30° C., showing a wide operating temperature range. Its good antifreeze property enables the capacitor to keep 95.5% of the initial value even if the capacitor is placed at -30°C for 30 days.
  • the retention rates of the specific capacitance after 10,000 cycles were 81% and 71% of the initial capacitance, respectively, giving the capacitor the ability to work at low temperatures for a long time.
  • the flexibility and adhesion of the hydrogel electrolyte enable the supercapacitor to not suffer structural damage in practical applications, and the specific capacitance of the capacitor can still maintain 78.5% even after 500 twists.
  • electrical conductivity and frost resistance are a pair of contradictions, and the method of improving frost resistance often reduces electrical conductivity; the supercapacitor of the present invention can simultaneously improve the frost resistance and electrical conductivity of products, and at low temperatures Ability to work long hours. Break through the limitations of existing technology and the prejudice of existing technology.
  • the present invention constructs an antifreeze hydrogel electrolyte (polySH) by simulating biological macromolecules and introducing zwitterions into the polymer network.
  • polySH antifreeze hydrogel electrolyte
  • the content of freezable water in polySH is less than 10%, and it has good antifreeze and electrical conductivity.
  • the addition of LiCl destroys the freezing process of water, especially the high concentration of LiCl has a more obvious effect on lowering the freezing point of the hydrogel electrolyte. This allows capacitors to operate at low temperatures for long periods of time.
  • the electrostatic interaction between zwitterions and salt ions makes the salts in the system very easy to dissociate, thereby increasing the number of free ions in the system.
  • the zwitterionic group can provide a migration channel for the separated positive and negative ions under the action of an applied electric field, thereby enhancing the conductivity, with a high ionic conductivity of 12.6 mS cm -1 at -40 °C.
  • the polySH electrolyte can even stretch to 325% strain and compress to 75% strain at ⁇ 40 °C, which makes polySH electrolytes promising in the fields of ionic conductors and responsive materials.
  • Figure 1 (a) Schematic diagram of polySH hydrogel electrolyte and its network structure.
  • Figure 2 (a) DFT calculations of the interaction of different components in the polySH electrolyte. (b) MSD of salt ions in polySH electrolyte.
  • Figure 3 (a) Ionic conductivity of polySH, polyHEA and PVA electrolytes. (b) Viscoelastic properties of polySH electrolytes with different LiCl contents. Raman spectra of -SO 3 - (c) and -N + (CH 3 ) 2 (d) in polySH electrolytes with different salt concentrations.
  • Figure 4 (a) Raman spectra of water in different polySH electrolytes. (b) DSC results of polySH electrolytes with different LiCl concentrations. (c) MSD of water in different polySH electrolytes. (d) Determination of electrochemical stability windows of various electrolytes by LSV method.
  • Figure 5 (a) Ionic conductivity of polySH electrolytes at different temperatures.
  • Figure 6 Tensile (a) and compressible (b) stress-strain curves of polySH-7 electrolyte at different temperatures.
  • Figure 7 (a) Photo of polySH-7 electrolyte circuit connected to an LED lamp. (b) The resistive response of polySH-7 electrolyte at different temperatures. (c) Resistive response of polySH-7 electrolyte under continuous stretch-release cycles at ⁇ 40 °C.
  • Figure 8 Photographs of polySH-7 electrolytes in network circuits at room temperature and low temperature (-20°C).
  • Figure 9 (a) EIS spectrum and (b) adhesion test of polySH-7 based capacitors and PVA based capacitors.
  • Figure 10 CV curves of polySH-based supercapacitors at different temperatures.
  • Figure 11 GCD curves of polySH-based supercapacitors at different temperatures.
  • Figure 12 Electrochemical performance of polySH supercapacitors at different temperatures: (a) CV curves at a scan rate of 100 mV s -1 . (b) GCD curve at a current density of 1 mA cm -2 . (c) EIS spectra of polySH-based supercapacitors at different temperatures. (d) Area-specific capacitance of polySH-based supercapacitors calculated from GCD curves.
  • Figure 13 Capacitance retention of poySH-based capacitors at different temperatures.
  • Figure 14 Photo of a polySH-based supercapacitor lighting up a small lamp in a freeze-thaw state.
  • Figure 15 (a) Capacitance retention of polySH-based supercapacitors after 10 freeze-thaw cycles. (b) Capacitance retention of supercapacitors after freezing at -30 °C for 30 days. (c) Cycling stability of polySH-based supercapacitors after 10,000 cycles at different temperatures.
  • Figure 16 (a) CV curves of polySH-based supercapacitors under 360° twist. (b) Capacitance retention of the supercapacitor after 500 twist cycles. (c) Photograph of the supercapacitor lighting up the LED lamp in the twisted state.
  • Figure 17 CV curves of polySH-based supercapacitors under heavy load state (a) and perforated state (b). (c) GCD curves of two supercapacitors in series and in parallel.
  • Methacryloylethylsulfobetaine SBMA
  • HAA hydroxyethyl acrylate
  • AIBA azobisisobutyramidine hydrochloride
  • LiCl lithium chloride
  • Polyvinyl alcohol 1799 PVA
  • PVDF polyvinylidene fluoride
  • NMP methylpyrrolidone
  • Carbon cloth was purchased from Taiwan Carbon Energy Company.
  • Activated carbon (AC) was purchased from Kuraray Corporation of Japan. Carbon black was purchased from Alfa Aesar.
  • SBMA methacryloyl ethyl sulfobetaine
  • HEA hydroxyethyl acrylate
  • polySH poly(SBMA-HEA) electrolyte; ie the zwitterionic polymer hydrogel electrolyte of the present invention.
  • polyHEA electrolyte Poly(HEA) electrolyte.
  • PVDF polyvinylidene fluoride
  • AIBA azobisisobutyramidine hydrochloride
  • PVA electrolyte polyvinyl alcohol 1799 electrolyte.
  • PVA-1799 represents polyvinyl alcohol with a degree of polymerization of 1700 and a degree of alcoholysis of 99%.
  • Ionic conductivity was measured by electrochemical impedance spectroscopy (EIS) using an electrochemical workstation (CHI660E). First, the polySH electrolyte was placed between two steel sheets, and then the electrolyte was stabilized at different temperatures for 5 h, and then the EIS test was performed. Each sample was measured three times to reduce errors.
  • the ionic conductivity ( ⁇ , mS cm -1 ) is obtained by the following formula:
  • R is the resistance ( ⁇ )
  • S is the contact area of the electrolyte (cm 2 )
  • L is the thickness (cm) of the test electrolyte.
  • Electrolyte linear voltammetry was measured in the range of -1.2to 1.3V with Ag/AgCl as the reference electrode.
  • the electrochemical properties of the capacitors were measured with a two-electrode system on a CHI 660E electrochemical workstation. Cyclic voltammetry (CV) was obtained by different scan ranges in the 0-1V voltage range.
  • Electrochemical Impedance Spectroscopy (EIS) was measured at 10 mV in the range of 0.01 Hz to 100 kHz.
  • ECD Charge-discharge cycling
  • Cycling Stability Cycling 10000 times in GCD. Before the test, the supercapacitor devices were placed at different temperatures to stabilize for 5h.
  • the area specific capacitance C sp (mF cm -2 ) of a single electrode was calculated by GCD, and the calculation formula was:
  • I is the applied current (mA)
  • ⁇ t is the discharge time (s)
  • S device is the total area of the capacitor electrodes (cm 2 )
  • ⁇ V represents the discharge voltage (V).
  • the electrolyte sample is a cylinder with a diameter of 5 mm and a length of 40 mm with a strain rate of 100 mm min ⁇ 1 .
  • the compression test adopts a cylindrical specimen with a diameter of 10 mm and a height of 15 mm, the compression strain is 75%, and the compression speed is 10 mm min -1 . All samples were stabilized at low temperature for 24 hours prior to low temperature testing.
  • the T-peel experiment was measured with a universal testing machine at room temperature with a tensile speed of 100 mm min-1. One electrode of the supercapacitor was fixed, and the other electrode was peeled off, covering an area of 4mm ⁇ 65mm.
  • Raman spectra were recorded using a LabRAM tHR800 Raman spectrometer (HORIBA JY, France) with a laser excitation wavelength of 532 nm.
  • Differential scanning calorimetry uses a TAQ-10D supercapacitor instrument with a temperature range of -80°C to 50°C, a heating rate of 10°C min -1 , and a sample mass of 5-10 mg each time.
  • Zwitterionic polySH electrolytes were prepared by a one-pot random polymerization method using SBMA and HEA in the presence of LiCl salts.
  • LiCl was dissolved in deionized water (8 ml) to prepare a 3 mol L -1 LiCl solution.
  • 0.75g SBMA and 1.25g HEA were dissolved in 8ml LiCl solution, the solution was placed in an ice bath and stirred for 1h, and then 0.02g of initiator AIBA (equivalent to the total amount of monomers) was added. 1 wt% of the mass), then placed in an ice bath and stirred for 1.5 h.
  • the hydrogel obtained by polymerization is abbreviated as SH-3, where 3 is the molar concentration of LiCl.
  • activated carbon electrode Preparation of activated carbon electrode: activated carbon AC, conductive carbon black and PVDF (mass ratio 8:1:1) were dispersed in NMP to prepare a uniform dispersion slurry. The slurry was coated on the carbon cloth and then placed in a vacuum oven at 180 °C for 24 h, and dried to obtain an AC electrode. The loading of active material on each electrode is about 2.5 mg cm -2 .
  • Assembling the supercapacitor Take two AC electrodes with the same load area (0.5cm ⁇ 1.2cm) and cover them on both sides of the electrolyte to form a sandwich structure to prepare a supercapacitor. Then, a drop of polySH electrolyte precursor solution was added dropwise to the electrodes on both sides of the supercapacitor to wet the electrodes, thereby making the electrode and electrolyte bond more firmly.
  • the total thickness of the fabricated supercapacitor is about 1 mm, in which the thickness of the polySH electrolyte is 0.4 mm.
  • the prepared capacitors were sealed with tape to prevent moisture evaporation before electrochemical testing.
  • LiCl concentration in the hydrogel changes.
  • Zwitterionic polySH electrolytes were prepared by a one-pot random polymerization method using SBMA and HEA in the presence of LiCl salts. First, LiCl was dissolved in deionized water (8 ml) to prepare a 7 mol L -1 LiCl solution. Then 0.75g SBMA and 1.25g HEA (total mass of 2g, molar ratio of 1:4) were dissolved in 8ml LiCl solution, the solution was placed in an ice bath and stirred for 1h, and then 0.02g of initiator AIBA (equivalent to the total amount of monomers) was added.
  • AIBA initiator AIBA
  • the hydrogel obtained by polymerization is abbreviated as SH-7, where 7 is the molar concentration of LiCl.
  • polyHEA electrolyte and PVA electrolyte As a comparison, we also prepared polyHEA electrolyte and PVA electrolyte, and the preparation method was the same as that of Example 1.
  • the polyHEA electrolyte is obtained only by polymerizing HEA monomer, and the preparation method is the same as that of the above-mentioned polySH electrolyte.
  • PVA-based supercapacitors use PVA as the electrolyte and are fabricated in the same way as polySH electrolyte supercapacitors.
  • PVA electrolyte is obtained by dissolving in high temperature water bath: 2g PVA is added to 8ml LiCl solution (1-7mol L -1 ), then placed in 85°C water bath and stirred, and PVA electrolyte is obtained when PVA is completely dissolved.
  • the co-existing anionic and cationic groups on SBMA are the binding sites for Li + and Cl- , which are favorable for the dissociation of lithium salts.
  • DFT density functional theory
  • the mean square displacement (MSD) of Li + and Cl- is linear with the time interval.
  • the slopes of Li + and Cl- are larger than those in the polyHEA electrolyte, indicating a faster diffusion rate in the polyelectrolyte, which is benefited from the presence of zwitterionic groups in SBMA, which provide a channel for ion migration.
  • SBMA zwitterionic groups
  • Li ions can combine with H 2 O molecules to form Li + (H 2 O) n structures. Therefore, the migration of Li ions should also be related to the Li + (H 2 O) n solvation structure, which also contributes to the antifreeze properties of polySH, which have been verified by Raman, differential scanning calorimetry (DSC) and molecular simulations .
  • the ionic conductivity of polySH electrolytes at different salt concentrations was determined at room temperature.
  • the conductivity of polySH electrolyte is higher than that of polyHEA at a certain salt concentration.
  • the conductivity of the polySH electrolyte also increases.
  • the LiCl concentration reaches about 5 mol L -1 , a threshold is observed, indicating that the binding of anions and cations to Li + and Cl- is saturated.
  • the conductivity of the polySH electrolyte is 146 mS cm -1 , indicating that the introduction of amphoteric groups indeed provides a migration channel for Li + , which greatly improves the migration rate of Li + .
  • the polySH electrolyte has higher electrical conductivity compared with the conventional PVA electrolyte, indicating that the polySH electrolyte has the potential to be applied in energy storage devices.
  • the addition of salt also changed the rheological properties of the polySH electrolyte.
  • the linear viscoelastic region of polySH electrolytes was first measured in the range of 0.1-100% under a strain of 10 rad s -1 . All subsequent viscoelastic tests were performed at 1% strain to ensure the validity and sufficient sensitivity of the linear viscoelasticity.
  • the storage modulus G' of each polySH electrolyte is larger than the loss modulus G", indicating that the electrolyte is mainly elastically deformed and exhibits solid-state behavior.
  • the electrostatic interaction between anions and cations can increase the degree of crosslinking of the network to a certain extent, so that the polySH-0 hydrogel has the highest modulus.
  • the modulus of the polySH electrolyte decreases gradually with the increase of salt concentration.
  • the change in the mechanical modulus of the electrolyte indicates that LiCl destroys the interaction between the cationic and anionic groups and reduces the crosslink density of the polymer network.
  • we Raman spectroscopy studies were carried out on polySH electrolyte.
  • the CH 3 stretching vibrations in (CH 3 ) 2 gradually changed from 2953 cm -1 to 2957 cm -1 , respectively (Fig. 3c, 3d).
  • the changes of these peaks indicate that the original electrostatic balance between anions and cations in zwitterions is broken, A new electrostatic equilibrium appears between anions and cations.
  • the added LiCl not only interacts with the polySH polymer chains, but also affects the structure of the solvent water in the system.
  • the most intuitive manifestation of this effect is the reduction of the freezing point of the electrolyte.
  • the Raman spectra show the stretching vibrational changes of OH in H2O at different LiCl concentrations.
  • the peak at 3230cm -1 belonging to the hydrogen bond between water molecules gradually weakened.
  • the asymmetric stretch band at 3420cm -1 became sharper with the increase of LiCl concentration, and the corresponding peak also shifted to a certain extent.
  • non-freezable bound water and freezeable water.
  • non-chilled water which is produced due to the interaction of water with other components in the system, does not exhibit a phase transition in calorimetric analysis.
  • the DSC method can determine the relative content of water in different states, which provides a tool for quantitative analysis of the frozen state of polySH electrolytes. From Figure 4b, it can be seen that in the absence of LiCl, polySH-0 has similar melting properties to pure water, and a melting peak appears near 0 °C, which means that polySH-0 hydrogel contains a large amount of frozen water. After the introduction of LiCl, the melting peak of water in polySH electrolyte starts to shift to subzero temperature with the increase of LiCl concentration.
  • the formula for calculating the content of freezable water in the polySH electrolyte is as follows:
  • W f is the freezable water content in the electrolyte
  • ⁇ H m is the melting enthalpy of the freezing water in the electrolyte, obtained by integrating the DSC melting peak
  • ⁇ H m 0 is the melting enthalpy of pure water
  • ⁇ H m 0 333.5J g -1
  • W H2O is the relative content of water in the electrolyte
  • W H2O m H2O /m total .
  • the freezable water content of the polySH-0 hydrogel was calculated to be 80.5%.
  • the freezable water content of the polySH-5 electrolyte gradually decreased to 8.2%.
  • the freezable water content of the electrolyte with high salt concentration is less than 10% (polySH-5, polySH-7); preferably, the freezable water content of the electrolyte is zero (polySH-7).
  • the water molecule diffusion coefficient of the electrolyte is shown in Table 2.
  • Table 2 The water molecule diffusion coefficient of the electrolyte is shown in Table 2.
  • MSD MSD simulations on several electrolytes at room temperature to understand the diffusion properties of water molecules. As shown in Fig. 4c, the MSD is linear with the time interval. As the LiCl concentration increases, the slope of the curve decreases, indicating that the diffusion of water molecules is more restricted at high LiCl concentrations.
  • the diffusion coefficient of water molecules was calculated from MSD, and due to the absence of LiCl in polySH-0, a high diffusion coefficient of water molecules of 19.08 ⁇ 10 -5 cm 2 s -1 could be observed.
  • the water diffusion coefficient of polySH-7 gradually decreased to 0.4 ⁇ 10 -5 cm 2 s -1 , which was only 2.1% of that of polySH-0.
  • the formation of the Li + (H 2 O) n solvation structure and the reduction of the freezable water content in the electrolyte are demonstrated, which endow the polySH electrolyte with good antifreeze properties.
  • the state of water in the polymer hydrogel is also reflected in the electrochemical stability window of the electrolyte.
  • the electrochemical window of the polySH electrolyte as measured by linear sweep voltammetry (LSV) broadened from 2.0 V for polySH-0 to 2.2 V for polySH-7 with increasing LiCl concentration.
  • LSV linear sweep voltammetry
  • FIG. 5 shows the variation trend of the conductivity of the polySH electrolyte at different temperatures. It can be seen that the curve is divided into two ranges: above zero temperature and below zero temperature. At above-zero temperature, the conductivity of all electrolytes decreases slowly with decreasing temperature because all electrolytes are in a non-frozen state at this time. In the subzero temperature range, the conductivity of polySH-1 and polySH-3 electrolytes decreased rapidly, while the conductivity of polySH-5 and polySH-7 electrolytes decreased less.
  • the ionic conductivity is linearly related to the reciprocal absolute temperature, indicating that the conductivity of the electrolyte obeys Arrhenius' law.
  • the activation energy of each electrolyte in the subzero temperature range was calculated, and the activation energy of the electrolyte decreased with the increase of salt concentration.
  • the activation energy of polySH-1 decreased from 33.5kJ mol-1 to 9.5kJ mol- 1 of polySH-7.
  • the activation energy is the energy barrier that ion migration must overcome, and the smaller the activation energy, the easier the ion migration. Therefore, the conductivity of polySH-1 increases from 0.11 mS cm -1 to 12.6 mS cm -1 .
  • the high concentration of LiCl improves the antifreeze performance of the electrolyte, so that its low temperature conductivity can reach 12.6 mS cm -1 at -40 °C. More importantly, the polySH-7 electrolyte still maintains good flexibility at -40 °C.
  • the polySH-7 electrolyte can be stretched to a strain of 325%, which is slightly higher than that at room temperature (Fig. 6a). Meanwhile, when compressed to 75% strain, the compressibility curve of the electrolyte is similar to that at room temperature (Fig. 6b), indicating that the polySH-7 electrolyte can still maintain good mechanical properties and stability at low temperature. At -40°C, the low temperature conductivity of the electrolyte is 11-13 mS cm -1 .
  • the electrolyte can be stretched to a strain of 300-350% at -40°C. When compressed to 75% strain, the compressibility curve of the electrolyte is similar to that at room temperature.
  • polySH electrolytes Since polySH electrolytes have good electrical conductivity and flexibility at low temperatures, they can be used as low-temperature conductors. When polySH-7 is connected to the LED lamp circuit, the LED lamp can be lit at low temperature with a brightness similar to that at room temperature (Fig. 7a). Due to the change in conductivity at different temperatures, polySH electrolytes can also be used as temperature-responsive materials. As shown in Figure 7b, the polySH-7 electrolyte was sealed and placed in the temperature range of -40°C to 40°C, and its resistance change was recorded. We found that the resistance of the polySH-7 electrolyte changed significantly when the temperature was changed and remained stable when the temperature was kept constant.
  • the resistance of the polySH-7 electrolyte can still recover to its original state, showing excellent resistance reversibility.
  • the polySH-7 electrolyte can be stretch-released at a constant rate at ⁇ 40°C, with a corresponding resistance change uniform and a good symmetrical shape (Fig. 7c).
  • the resistance change remains stable even after several consecutive stretch-release cycles, which is of great significance for low-temperature strain sensor applications.
  • polySH-7 electrolytes into metal mesh wires for network signal transmission. As shown in Figure 8a, after connecting to the network circuit, the computer can successfully identify the network signal and work stably.
  • the GCD curves at different temperatures also show a regular inverted triangle with smaller voltage drop.
  • the CV curve of 100mV s -1 in the range of -40°C ⁇ 40°C is shown in Fig. 12a.
  • the CV curve maintains a rectangular shape similar to room temperature. Due to the unique water retention of zwitterionic hydrogels, supercapacitors also have good performance at high temperatures. As the temperature decreases, the CV curve shows a trend of gradual deformation.
  • the GCD curve is a regular inverted triangle, and there is only a small voltage drop (0.1V) at -30°C (Fig. 12b).
  • the EIS curves at different temperatures reflect the mobility of ions.
  • the EIS curves in the low frequency region are almost parallel to the imaginary axis, indicating that the polySH electrolyte has good ion diffusion behavior (Fig. 12c).
  • the supercapacitor has a series resistance of 6.3 ⁇ , while this value increases to 12.2 ⁇ at -30°C and decreases to 5.9 ⁇ at 60°C.
  • the change of temperature affects the diffusion rate of electrolyte ions, thus affecting the electrochemical performance of supercapacitors.
  • the supercapacitor also exhibited smaller interfacial resistance, only 1.8 ⁇ at 25 °C, which indicated adequate contact between electrode and electrolyte and faster charge transfer rate.
  • the area specific capacitance values at different current densities were calculated according to the GCD curve (Fig. 12d). (See Table 4 for the specific capacitance data) At 25°C, the specific capacitance of the supercapacitor is 171mF cm-2 at 1mA cm -2 and 152mF cm -2 at 5mA cm - 2 , which is only 11% weaker, indicating that Supercapacitors have good rate capability. As shown in Figure 13 for the capacity retention at different temperatures, when the temperature is increased to 60°C or decreased to -30°C, the area specific capacitance becomes 178mF cm -2 and 134mF cm -2 , which is about 104% of that at 25°C and 78%, indicating good electrochemical performance at extreme temperatures.
  • a "QLU" type LED bulb (operating voltage 2.5V) can be illuminated by three supercapacitors in series. After 24 hours of freezing at low temperatures, the supercapacitors still functioned normally. After the supercapacitor was thawed, the brightness of the LED bulb did not change significantly compared with the initial state. This suggests that low temperatures will not cause irreversible damage to supercapacitors.
  • the electrochemical performance of the supercapacitor was measured several times at 25°C and -30°C. As shown in Fig. 15a, the specific capacitance can still recover to the initial level after thawing at room temperature.
  • solid-state supercapacitors In addition to antifreeze performance, solid-state supercapacitors also have good flexibility, which can avoid structural damage and extend service life in practical applications.
  • Fig. 16a after twisting the polySH-based supercapacitor by 360°, the CV curve shows almost the same capacitive behavior as the initial state. Even after 500 twisting cycles, the capacitance retention of the supercapacitor was still 78.5% (Fig. 16b). After connecting the supercapacitor into the circuit, the brightness of the LED light did not change significantly before and after twisting the supercapacitor (Fig. 16c), indicating that the large deformation does not affect the electrochemical performance.

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Abstract

一种含两性离子聚合物水凝胶电解质的超级电容器及其制备方法。超级电容器，包含两性离子聚合物水凝胶电解质。该超级电容器，在极端温度下的容量保持率，相对于25℃时的面积比电容，变化率小于25％。例如当温度增加到60℃或减少到-30℃时，面积比电容变到178mF cm -2和134mF cm -2 ­，大约为25℃时的104％和78％，表明在极端温度下良好的电化学性能。

Description

一种含防冻两性离子水凝胶电解质的超级电容器及其制备方法 技术领域
本发明属于超级电容器领域，涉及一种含两性离子聚合物水凝胶电解质的超级电容器及其制备方法。
背景技术
随着全球能源需求的不断增长，对开发高效储能装置提出了更高的要求。超级电容器作为一种新型的储能器件，因其充放电速度快、工作寿命长而受到人们的广泛关注。在2012～2019年期间，中国的超级电容器市场规模年均复合增长率大约在35％左右，去年中国超级电容器市场规模更是高达120亿元左右，其中，交通运输用超级电容器是支撑整个行业发展的最主要动力。
超级电容器，又称为电化学电容器，在能量密度-功率密度关联图中，是一类介于电池(能量密度高而功率密度低)与传统介电电容器(功率密度高而能量密度低)之间的电化学储能器件。超级电容器的工作原理为：在外加电场作用下，电解质离子在电极/电解质界面处静电分离、以电化学双层电容器的形式储存电能。虽然超级电容器其功率密度高、循环寿命长，但相比于电池，其能量密度低，这限制了其应用范围。因此，在不损害超级电容器功率密度和循环寿命的前提下提高其能量密度，是电化学储能领域的关键难题。
电解质包括液体电解质和固体电解质，是超级电容器不可缺少的组成部分。与液体电解质相比，固体电解质电导率低是影响储能器件高性能的主要缺点。而由于聚合物水凝胶电解质在室温下较高的导电性，逐渐成为高性能固态超级电容器研究中的热点。但是，由于水凝胶网络中大量的水在零下必然会结冰，所以在0℃下聚合物水凝胶电解质的电导率会大大降低。
在水凝胶中加入有机液体是获得防冻水凝胶的一种方法。常用的有机液体包括乙二醇、甘油、二甲基亚砜等。在这些二元/三元体系中，有机液体与水分子的相互作用被认为是抑制冰晶晶格形成的主要原因。然而，由于有机液体的存在，这些水凝胶要么不导电，要么导电率低。此外，有机液体的挥发性和高自燃性对有机水凝胶电解质造成了严重的安全危害。如何实现水凝胶电解质超级电容器的低温防冻，是一大难题。
发明内容
本发明的目的，是针对现有技术的不足，提供一种含防冻两性离子聚合物水凝胶电解质的超级电容器及其制备方法。使用活性碳为电极组装的、两性离子聚合物水凝胶电解质的超级电容器不仅具有良好的防冻性能，而且具有较宽的工作温度范围和在低温下长时间工作的能力。
一种超级电容器，其特征在于，包含两性离子聚合物水凝胶电解质。
超级电容器，在极端温度下的容量保持率，相对于25℃时的面积比电容，变化率小于25％。例如当温度增加到60℃或减少到-30℃时,面积比电容变到178mF cm -2和134mF cm -2，大约为的104％和78％,表明在极端温度下良好的电化学性能。
所述极端温度，指高温(比如高于50℃)或低温(比如低于0℃)。一般而言，水凝胶低温下会由于水结冰而冻住；高温下水凝胶失水；都会使得超级电容器无法工作。极端温度是个宽泛定义，低温一般0℃下，高温并无详细温度，但是一般50℃电化学性能就受较大影响。本申请中，如无特殊说明，极端温度是指高于50℃或低于0℃。
当超级电容器在-30℃的低温下放置30天以上时，其电容仍保留原来的95％以上(见图15b)。对不同温度下的循环稳定性进行了测试。如图15c所示，经过10000次循环后，超级电容器在-30℃、25℃、60℃下的电容保持率分别为71％、81％、76.4％。这种优良的循环性能表明，polySH基超级电容器可以在低温和高温下长期工作。
所述两性离子防冻水凝胶电解质(polySH)，为在LiCl存在下的聚(SBMA-HEA)电解质，Li +通过聚合物链上的两性离子基团进行跳跃式迁移，从而提高了离子导电性；拉曼光谱中，-SO 3 -中的S＝O伸缩振动为1044cm -1～1054cm -1，-N +(CH 3) 2中的CH3伸缩振动为2953cm-1～2957cm-1，说明两性离子集团的存在并为Li+迁移提供位点；DSC证明与H2O形成一种Li+(H2O)n溶剂化结构，提高了水凝胶电解质的防冻性能，在-40℃下仍然具有325％的拉伸量；在低温下具有优异的离子导电率，可以在离子导电体和储能设备中应用。
优选的，在室温条件下，polySH电解质的电导率为114～147mS cm -1。polySH电解质的储能模量G’大于损失模量G”。在-40℃时，所述电解质的低温电导率为11～13mS cm -1。所述电解质在-40℃可以拉伸到300～350％的应变。当被压缩到75％应变时，电解质的可压缩曲线与室温下相似。
优选的，所述电解质的可冷冻水含量低于10％(polySH-5，polySH-7)；优选的，所述电解质的可冷冻水含量为零(polySH-7)。
优选的，所述电解质的水分子扩散系数小于等于11.21*10 -6cm 2s -1。优选的，所述电解质的水分子扩散系数小于等于1.02*10 -6cm 2s -1。
所述水凝胶电解质的制备方法为，在LiCl盐的存在下，使用SBMA和HEA采用一锅法无规聚合法制备了两性离子型polySH电解质；包括下列步骤：
1)将LiCl溶解在第一溶剂中，制备1-7mol L -1的LiCl去离子水溶液。
2)将SBMA和HEA溶解于LiCl溶液中，SBMA和HEA的摩尔比为1:(3-5)。将溶液置于冰浴中搅拌1h后加入引发剂AIBA，引发剂的加入量相当于单体总质量的0.5～2wt％。随后置于冰浴中搅拌1.5h。
3)待溶液完全溶解后超声8～15min以移除气泡，随后将溶液注入模具中，密封置于30-50℃环境中聚合8-16h。
优选的，步骤1)中LiCl去离子水溶液的浓度为3-7mol L -1；更优选的，步骤1)中LiCl去离子水溶液的浓度为5-7mol L -1。
优选的，所述步骤2)中，引发剂的加入量相当于单体总质量的1wt％。
优选的，步骤3)所述超声为，超声10min以移除气泡。
优选的，步骤3)中所述的密封环境中聚合为，密封置于38℃环境中聚合12h。
聚合得到的水凝胶简记为polySH-x，其中x为LiCl的摩尔浓度。
所述超级电容器的制备方法，包括下列步骤：
1)制备活性碳电极：活性碳AC，导电碳黑和PVDF(质量比8:1:1)分散于NMP以制备成均匀的分散物料浆。将料浆涂覆于碳布上后置于80℃真空烘箱中24h，干燥后得到AC电极。活性材料在每一片电极上的负载量约为2.5mg cm -2；
2)组装超级电容器：取负载面积相同(0.5cm×1.2cm)的两片AC电极，将其分别覆盖于电解质两侧组成三明治结构来制备超级电容器。随后在超级电容器两侧电极上分别滴加一滴polySH电解质前体溶液以润湿电极，从而使电极和电解质结合更加牢固。制备的超级电容器总厚度约为1mm，其中polySH电解质的厚度为0.4mm。
电化学测试前将制备的电容器用胶带密封以防止水分蒸发。
所述polySH电解质前体溶液，指进行聚合前的单体混合物，其制备方法为：1)将LiCl溶解在第一溶剂中，制备1-7mol L -1的LiCl去离子水溶液。
2)将SBMA和HEA溶解于LiCl溶液中，SBMA和HEA的摩尔比为1:(3-5)。将溶液置于冰浴中搅拌1h后加入引发剂AIBA，引发剂的加入量相当于单体总质量的0.5～2wt％。随后置于冰浴中搅拌1.5h。
3)待溶液完全溶解后超声8～15min以移除气泡，得到polySH电解质前体溶液。
有益效果
本发明使用活性碳为电极组装的超级电容器在60℃时比电容为178mF cm -2，在-30℃时比电容为134mF cm -2，表现了较宽的工作温度范围。其良好的防冻性使电容器即使在-30℃下放置30天，电容也能保持初值的95.5％。在循环10000次后比电容的保留率分别为初始电容的81％和71％，使得电容器具有在低温下长时间工作的能力。此外，水凝胶电解质的柔韧性和附着力使超级电容器在实际应用中不会受到结构损伤，即使在经过500次扭曲后电容器比电容仍然能保持78.5％。而现有技术中，导电性和抗冻性是一对矛盾，提高抗冻性的方法，往往使得导电性降低；本发明的超级电容器可以同时提高产品的抗冻性和导电性，且在低温下有长时间工作能力。突破了现有技术的局限和现有技术的偏见。
本发明通过模拟生物大分子，将两性离子引入聚合物网络中来构建防冻水凝胶电解质(polySH)。polySH中可冻水含量小于10％，具有好的防冻性和导电率。LiCl的加入破坏了水的冻结过程，尤其是高浓度的LiCl，对降低水凝胶电解质的凝固点的作用更为明显。这使得电容器可以在低温下长时间工作。
事实上，除两性离子所具有的防冻性能外，两性离子与的盐离子之间的静电相互作用，使得体系中的盐变得十分容易解离，从而增加了体系中自由离子的数量。此外，两性离子基团在外加电场作用下可以为分离的正离子和负离子提供迁移通道，从而提高导电率，在-40℃下具有和12.6mS cm -1的高离子导电率。polySH电解质甚至可以在-40℃下可以拉伸至325％的应变和压缩至75％的应变，这使得polySH电解质在离子导体和响应材料领域中具有一定的应用。这些研究结果表明，使用两性离子可能是制备高导电性防冻固体电解质的一种可行策略。
附图说明
图1：(a)polySH水凝胶电解质及其网络结构示意图。
图2：(a)polySH电解质中不同组分相互作用的DFT计算。(b)polySH电解质中盐离子的MSD。
图3：(a)polySH、polyHEA和PVA电解质的离子电导率。(b)不同LiCl含量的polySH电解质的粘弹性特性。不同盐浓度polySH电解质中-SO 3 -(c)和-N +(CH 3) 2(d)的拉曼光谱。
图4：(a)不同polySH电解质中的水的拉曼光谱。(b)不同LiCl浓度的polySH电解质的DSC结果。(c)水在不同polySH电解质中的MSD。(d)用LSV法测定各种电解质的电化学稳定窗口。
图5：(a)polySH电解质在不同温度下的离子电导率。
图6：polySH-7电解质在不同温度下的拉伸(a)和可压缩(b)应力-应变曲线。
图7：(a)连接到LED灯的polySH-7电解质电路照片。(b)polySH-7电解质在不同温度下的电 阻响应。(c)polySH-7电解质在-40℃连续拉伸释放周期下的电阻响应。
图8：网络电路中polySH-7电解质在常温和低温(-20℃)下的照片。
图9：(a)EIS谱图和(b)polySH-7基电容器和PVA基电容器的粘附试验。
图10：polySH基超级电容器在不同温度下的CV曲线。
图11：polySH基超级电容器在不同温度下的GCD曲线。
图12：polySH超级电容器在不同温度下的电化学性能：(a)扫描速率为100mV s -1时的CV曲线。(b)电流密度为1mA cm -2时的GCD曲线。(c)基于polySH的超级电容器在不同温度下的EIS谱图。(d)由GCD曲线计算的polySH基超级电容器的面积比电容。
图13：poySH基电容器在不同温度下的电容保持率。
图14：polySH基超级电容器在冻融状态下点亮小灯照片。
图15：(a)polySH基超级电容器10次冻融循环后的电容保持情况。(b)超级电容器在-30℃下冷冻30天后的电容保持。(c)polySH基超级电容器在不同温度下10,000个循环后的循环稳定性。
图16：(a)polySH基超级电容器在360°扭曲下的CV曲线。(b)500个扭曲周期后超级电容器的电容保持情况。(c)超级电容器在扭曲状态下点亮LED灯的照片。
图17：polySH基超级电容器在重载状态(a)和穿孔状态(b)下的CV曲线。(c)两个超级电容器串联和并联的GCD曲线。
具体实施方式
甲基丙烯酰乙基磺基甜菜碱(SBMA)，丙烯酸羟乙酯(HEA)，偶氮二异丁脒盐酸盐(AIBA)，氯化锂(LiCl)从阿拉丁购得。聚乙烯醇1799(PVA)，聚偏氟乙烯(PVDF)，甲基吡咯烷酮(NMP)从麦克林购得。碳布从台湾碳能公司购得。活性碳(AC)从日本可乐丽公司购得。碳黑从阿法埃莎公司购得。
名称解释：
SBMA：甲基丙烯酰乙基磺基甜菜碱；
HEA：丙烯酸羟乙酯；
polySH：聚(SBMA-HEA)电解质；即本发明的两性离子聚合物水凝胶电解质。
polyHEA电解质：聚(HEA)电解质。
AC：活性碳；
PVDF：聚偏氟乙烯；
AIBA：偶氮二异丁脒盐酸盐；
PVA电解质：聚乙烯醇1799电解质。PVA-1799表示聚合度为1700、醇解度为99％的聚乙烯醇。
电化学测试
离子导电性使用电化学工作站(CHI660E)通过电化学阻抗谱(EIS)测得。首先将polySH电解质置于两片钢铁片之间，然后将电解质置于不同温度下稳定5h，然后进行EIS测试。为减小误差对每种样品都进行了三次测量。离子导电率(σ,mS cm -1)通过以下公式求得：
Figure PCTCN2021097104-appb-000001
在这里，R是电阻(Ω),S是电解质的接触面积(cm 2)，L是测试电解质的厚度(cm)。
电解质线性伏安曲线(LSV)以Ag/AgCl为参比电极在-1.2to 1.3V范围内测试得到。电容器的电化学性能在CHI 660E电化学工作站上用两电极系统测得。循环伏安(CV)在0-1V电压范围内通过不同扫描范围得到。电化学阻抗谱(EIS)在0.01Hz到100kHz范围内通过10mV测得。充放电循环(GCD)在0-1V范围内使用不同电流密度测得。循环稳定性在GCD中循环10000次。测试前将超级电容器device分别放置在不同温度下稳定5h.单电极的面积比电容C sp(mF cm -2)通过GCD计算得到，计算公式为：
Figure PCTCN2021097104-appb-000002
其中I为施加电流(mA)，Δt为放电时间(s)，S device为电容器电极的总面积(cm 2)，ΔV代表放电电压(V)。
机械性能测试
拉伸试验是用万能测试仪器(Hensgrand,WDW-02,中国)进行的。电解质样品直径为5mm，长度为40mm的圆柱体，应变速度为100mm min -1。压缩试验采用直径为10mm，高度为15mm的圆柱形试样，压缩应变为75％，压缩速度为10mm min -1。在低温测试之前，所有样品在低温下稳定24小时。
T-peel实验是用万能试验机在室温下以100mm min-1的拉伸速度进行测量的。将超级电容器的一个电极固定，另一个电极进行剥离，覆盖面积为4mm×65mm。
流变学测量采用ARES-G2流变仪，使用直径为25mm的平行板。首先，利用角频率为10rad s -1,在0.1-100％范围内的动态应变扫描确定线性粘弹性区域。频率扫描在0.1-100rad s -1的频率范围内进行，固定应变为1％。
其他表征
拉曼光谱使用LabRAM tHR800拉曼光谱仪(HORIBA JY，法国)记录，激光激发波长为532nm。差示扫描量热法(D超级电容器)使用TAQ-10D超级电容器仪器，温度范围从-80℃到50℃，加热速率为10℃min -1，每一次样品质量为5-10mg。
实施例1
制备polySH水凝胶电解质
在LiCl盐的存在下，使用SBMA和HEA采用一锅法无规聚合法制备了两性离子型polySH电解质。首先，将LiCl溶解在去离子水(8ml)中，制备出3mol L -1的LiCl溶液。然后将0.75g SBMA和1.25gHEA(总质量为2g，摩尔比为1:4)溶解于8ml LiCl溶液中，将溶液置于冰浴中搅拌1h后加入0.02g引发剂AIBA(相当于单体总质量的1wt％)，随后置于冰浴中搅拌1.5h。待溶液完全溶解后超声10min以移除气泡，随后将前体溶液注入模具中，密封置于38℃环境中聚合12h。聚合得到的水凝胶简记为SH-3，其中3为LiCl的摩尔浓度。
组装超级电容器
制备活性碳电极：活性碳AC，导电碳黑和PVDF(质量比8:1:1)分散于NMP以制备成均匀的分散物料浆。将料浆涂覆于碳布上后置于180℃真空烘箱中24h，干燥后得到AC电极。活性材料在每一片电极上的负载量约为2.5mg cm -2.
组装超级电容器：取负载面积相同(0.5cm×1.2cm)的两片AC电极，将其分别覆盖于电解质两侧组成三明治结构来制备超级电容器。随后在超级电容器两侧电极上分别滴加一滴polySH电解质前体溶液以润湿电极，从而使电极和电解质结合更加牢固。制备的超级电容器总厚度约为1mm，其中polySH电解质的厚度为0.4mm。电化学测试前将制备的电容器用胶带密封以防止水分蒸发。
实施例2
其他同实施例1，不同之处在于，水凝胶中LiCl浓度有所变化。在LiCl盐的存在下，使用SBMA和HEA采用一锅法无规聚合法制备了两性离子型polySH电解质。首先，将LiCl溶解在去离子水(8ml)中，制备出7mol L -1的LiCl溶液。然后将0.75g SBMA和1.25gHEA(总质量为2g，摩尔比为1:4)溶解于8ml LiCl溶液中，将溶液置于冰浴中搅拌1h后加入0.02g引发剂AIBA(相当于单体总质量的1wt％)，随后置于冰浴中搅拌1.5h。待溶液完全溶解后超声10min以移除气泡，随后将前体溶液注入模具中，密封置于38℃环境中聚合12h。聚合得到的水凝胶简记为SH-7，其中7为LiCl的摩尔浓度。
将不同量的LiCl溶解在去离子水(8ml)中，制备出不同浓度的1-7mol L -1的LiCl溶液，聚合得到的水凝胶简记为polySH-x，其中x为LiCl的摩尔浓度。
对比例1-2
作为对比，我们还制备了polyHEA电解质和PVA电解质，制备方法与实施例1相同。polyHEA电解质仅使用HEA单体聚合得到，其制备方法与上述polySH电解质制备方法相同。PVA基超级电容器使用PVA作为电解质，制备方法与polySH电解质超级电容器相同。
PVA电解质通过在高温水浴中溶解得到:2g PVA加入8ml LiCl溶液(1-7mol L -1)中，随后置于85℃水浴中搅拌，待PVA完全溶解便得到PVA电解质。
结果分析
如图1所示，SBMA上同时存在的阴离子和阳离子基团是与Li +和Cl -结合的位点，有利于锂盐的解离。为了验证这一假设，对体系进行了密度泛函理论(DFT)计算。图2a显示了使用LiCl盐的SBMA片段的最佳配置。在加入锂盐之前，由于静电作用，SBMA上的阴离子和阳离子将与其他SBMA上的阳离子和阴离子相互作用(ESBMA-SBMA:-4.025Kcal mol -1)形成内盐，以保持体系的中性。加入LiCl后，与-N +(CH 3) 2SO 3 -相比，-SO 3 -Li +的结合能更低(-18.15Kcal mol-1)，说明-SO 3 -更倾向于与Li +结合(图2a)。因此，两性基团的引入确实促进了LiCl的解离，并为Li +的迁移提供了位点支持。用分子动力学方法研究了polySH电解质中的离子迁移机制。一般认为，对于聚合物电解质，Li +可以通过与聚合物链上的O、S、N等极性基团连续络合和分解而跳跃迁移。如图2b所示，在polySH和polyHEA电解质中，Li +和Cl -的均方位移(MSD)与时间间隔呈线性关系。在polySH电解质中，Li +和Cl -的斜率大于polyHEA电解质，表明在多聚电解质中扩散速率更快，这得益于SBMA中两性离子基团的存在，为离子迁移提供了通道。考虑到SO 3 -和Li +的较低结合能以及Li +较快的扩散速率，Li +的输运应该在SO 3 -基团的位置进行连续的络合和分解(图1e)。更重要的是，在高浓度的LiCl中，Li离子可以与H 2O分子结合形成Li +(H 2O) n结构。因此，Li离子的迁移也应该与Li +(H 2O) n溶剂化结构有关，这也有助于polySH的防冻性能，这已被拉曼、差示扫描量热法(DSC)和分子模拟验证。
在室温下测定了不同盐浓度下polySH电解质的离子电导率。为了比较，我们还测量了polyHEA和传统PVA电解质的电导率。如图3a(具体数据见表1)所示，在一定盐浓度下，polySH电解质的电导率高于polyHEA。随着盐浓度的增加，polySH电解质的电导率也随之增加。当LiCl浓度达到约5mol L -1时，观察到一个阈值，表明阴离子和阳离子与Li +和Cl -的结合达到饱和。在室温条件下，polySH电解质的电导率为146mS cm -1，说明两性基团的引入确 实为Li +提供了迁移通道，大大提高了Li +的迁移速率。此外，与传统PVA电解质相比，polySH电解质具有较高的电导率，表明polySH电解质具有应用于储能装置的潜力。
表1 polySH、polyHEA和PVA电解质的离子电导率
Figure PCTCN2021097104-appb-000003
盐的加入也改变了polySH电解质的流变学性质。首先在10rad s -1的应变下在0.1-100％范围内测polySH电解质的线性粘弹性区域。之后所有的粘弹性试验都在1％的应变下进行，以保证线性粘弹性的有效性和足够的灵敏度。如图3b所示，各polySH电解质的储能模量G’均大于损失模量G”，说明电解质主要呈弹性变形，呈现固态行为。如前文所述，在没有LiCl的情况下，SBMA链上阴离子和阳离子之间的静电相互作用可以在一定程度上增加网络的交联度，从而使polySH-0水凝胶具有最高的模量。随着盐浓度的增加，polySH电解质的模量逐渐降低。电解质力学模量的变化说明LiCl破坏了阳离子和阴离子基团之间的相互作用，降低了聚合物网络的交联密度。为了进一步了解聚合物链上的离子基团与盐离子的相互作用，我们在polySH电解质上进行了拉曼光谱研究。随着盐浓度从polySH-0增加到polySH-7，-SO3-中的S＝O伸缩振动分别从1044cm -1逐渐变化到1054cm -1，-N +(CH 3) 2中的CH 3伸缩振动分别从2953cm -1逐渐变化到2957cm -1(图3c,3d)。这些峰的变化表明，两性离子中原有的阴离子和阳离子间的静电平衡被破坏，阴离子和阳离子间出现了新的静电平衡。
添加的LiCl不仅与polySH聚合物链相互作用，还会影响体系中溶剂水的结构。这种效应最直观的表现是电解质冰点的降低。如图4a所示，拉曼光谱显示了在不同LiCl浓度下H 2O中O-H的伸缩振动变化。3230cm -1处属于水分子间氢键的峰逐渐减弱，相应地，3420cm -1处的不对称伸缩带随着LiCl浓度的增加而逐渐尖锐，而相应的峰值也发生了一定的位移。这些变化表明，添加的LiCl破坏了水分子之间的氢键，并与H 2O形成一种Li +(H 2O) n溶剂化结构。一般认为，亲水聚合物水凝胶中的水状态至少可以分为两大类:不可冻结的结合水和可冻结的水。而由于水和系统中其他组分的相互作用而产生的非冷冻水，在量热分析中不表现出相变。DSC法可以测定不同状态下水的相对含量，为polySH电解质的冻结状态的定量分析提供了工具。 从图4b可以看出，在没有LiCl的情况下，polySH-0与纯水具有相似的熔融性质，在接近0℃时出现熔融峰，这意味着在polySH-0水凝胶中含有大量的可冻水。引入LiCl后，随着LiCl浓度的增加，polySH电解质中水的熔融峰开始向零下温度转移。polySH电解质中可冷冻水的含量计算公式如下：
Figure PCTCN2021097104-appb-000004
式中，W f为电解质中的可冷冻水含量，ΔH m为电解质中可冻水的熔化焓，通过积分DSC熔融峰得到，ΔH m 0为纯水的熔化焓,ΔH m 0＝333.5J g -1。W H2O是电解质中水的相对含量，W H2O＝m H2O/m total。经计算，polySH-0水凝胶的可冷冻水含量为80.5％。polySH-5电解质的可冷冻水含量逐渐下降到8.2％。有趣的是，polySH-7的量热图中没有发现熔融峰，说明在-80～50℃的温度范围内，polySH-7电解质中几乎没有可冷冻水。这些数据表明，LiCl浓度越高，电解质中可冷冻水的数量越少。综上分析，LiCl的加入确实破坏了水的冻结过程，尤其是高浓度的LiCl，对降低水凝胶电解质的凝固点的作用更为明显。所述高盐浓度电解质的可冷冻水含量低于10％(polySH-5，polySH-7)；优选的，所述电解质的可冷冻水含量为零(polySH-7)。
所述电解质的水分子扩散系数见表2。为了进一步研究Li +-H 2O的相互作用，我们在室温下对几种电解质进行了MSD模拟，以了解水分子的扩散特性。如图4c所示，MSD与时间间隔呈线性关系。随着LiCl浓度的增加，曲线的斜率减小，说明在LiCl浓度高的情况下，水分子的扩散受到更大的限制。根据MSD计算水分子扩散系数，由于polySH-0中不存在LiCl，可以观察到水分子的高扩散系数19.08×10 -5cm 2s -1。随着LiCl浓度的增加，polySH-7的水分子扩散系数逐渐降低到0.4×10 -5cm 2s -1，仅为polySH-0的2.1％。证明了电解质中Li +(H 2O) n溶剂化结构的形成和可冷冻水含量的降低，使polySH电解质具有良好的防冻性能。聚合物水凝胶中水的状态也反映在电解质的电化学稳定性窗口中。如图4d所示，随着LiCl浓度的增加，线性扫描伏安法(LSV)测得的polySH电解质的电化学窗口从polySH-0的2.0V拓宽到polySH-7的2.2V。Li +与H 2O分子的结合降低了水分子的活性，从而抑制了高压下水分子的分解。
表2不同盐浓度polySH电解质中水分子的扩散系数
Figure PCTCN2021097104-appb-000005
LiCl的加入极大地影响了电解质体系中水分子和聚合物结构，使polySH电解质在低温下具有良好的抗冻性和高的离子电导率。图5显示了polySH电解质在不同温度下电导率的变化趋势。可以看出，曲线分为两个范围：零上温度和零下温度。在零上温度下，所有电解质的电导率随着温度的降低而缓慢降低，因为此时所有的电解质都处于非冻结状态。在零下温度范围内，polySH-1和polySH-3电解质的电导率下降较快，而polySH-5和polySH-7电解质的电导率下降幅度较小。此外，在零度以下的温度范围内，离子电导率与绝对温度倒数呈线性关系，说明电解质的电导率服从阿伦尼乌斯定律。计算了各电解质在零下温度范围内的活化能，随着盐浓度的增加，电解质的活化能降低。例如，polySH-1的活化能从33.5kJ mol-1下降到polySH-7的9.5kJ mol -1。活化能是离子迁移必须克服的能垒，活化能越小，离子迁移越容易。因此，polySH-1的导电性从0.11mS cm -1增加到12.6mS cm -1。高浓度的LiCl提高了电解质的防冻性能，使得在-40℃时，其低温电导率可达12.6mS cm -1。更重要的是，polySH-7电解质在-40℃仍能保持良好的柔韧性。polySH-7电解质可以拉伸到325％的应变，略高于室温下的应变(图6a)。同时，当被压缩到75％应变时，电解质的可压缩曲线与室温下相似(图6b)，说明polySH-7电解质在低温下仍能保持良好的力学性能和稳定性。在-40℃时，所述电解质的低温电导率为11～13mS cm -1。所述电解质在-40℃可以拉伸到300～350％的应变。当被压缩到75％应变时，电解质的可压缩曲线与室温下相似。
由于polySH电解质在低温下具有良好的导电性和柔韧性，可作为低温导体使用。polySH-7连接到LED灯电路后，可以在低温下点亮LED灯，亮度与室温下相差无几(图7a)。由于在不同温度下导电率的变化，polySH电解质也可以作为温度响应材料使用。如图7b所示，将polySH-7电解质密封，放置在-40℃～40℃的温度范围内，记录其电阻变化。我们发现，当温度变化时，polySH-7电解质的电阻变化明显，当温度保持不变时，电阻则保持稳定。在完成一个高低温循环后，polySH-7电解质的电阻仍能恢复到原来的状态，表现出优异的电阻可逆性。与大多数报道的工作温度在0℃以上的应变传感器不同，polySH-7电解质可以在-40℃以恒定速度拉伸释放，相应的电阻变化均匀且具有良好的对称形状(图7c)。即使经过几个连续的拉伸-释放周期，电阻变化仍然保持稳定，这对于低温应变传感器的应用具有重要意义。为了进一步验证polySH电解质在导体应用中的稳定性，我们将polySH-7电解质连接到金属网线中进行网络信号传输。如图8a所示，连接到网络电路后，计算机可以成功识别网络信号并稳定工作。在网络延时测试中，发现网络时延维持在24ms的低水平，且波动较小。更重要的是， 将测得的polySH-7电解质置于低温环境(-20℃)(图8b)时，计算机仍能正常工作，网络时延仅比室温下(25ms)降低1ms。说明polySH-7电解质具有与金属网线几乎相同的传输效果，是一种良好的信号传输材料。
表3不同温度下polySH-7电解质的电阻变化率
温度(℃) 40 20 0 -20 -40
电阻变化率 0 0.7 2.0 5.6 19.5
我们的polySH电解质也被组装成一个固体超级电容器，使用活性碳电极来评估电化学性能。两性离子中的带电基团和极性基团使电解质与电极紧密结合，减少了对超级电容器结构的破坏，有效提高了超级电容器在低温下的电化学性能。为了比较，我们首先进行了室温下polySH电解质和PVA电解质组装超级电容器的EIS图(图9a)。可以看出polySH基电容器具有较小的界面电阻比PVA基电容器(1.8vs 6.8Ω)。对两种电容器的黏附试验表明，多糖电解质的黏附力高于PVA电解质(85N m -1vs 42N m -1)(图9b)。这说明两性离子确实有助于改善电解质与电极之间的粘附，这对于降低界面电阻，提高超级电容器的电化学性能具有重要意义。在0-1V电压范围内，进一步研究了不同温度下polySH基电容器的CV曲线。如图10所示，对固体超级电容器的扫描率为1000mV s -1，远高于其他AC电极固体超级电容器的扫描率。如图11所示，不同温度下的GCD曲线也呈规则的倒三角形，具有较小的电压降。-40℃～40℃范围内100mV s -1的CV曲线如图12a所示。在60℃时，CV曲线保持与室温相似的矩形形状。由于两性离子水凝胶独特的保水性，使得超级电容器在高温下也具有良好的性能。随着温度的降低，CV曲线呈现逐渐变形的趋势。而且GCD曲线呈规则的倒三角形，只有在-30℃时才有一个小的电压降(0.1V)(图12b)。不同温度下的EIS曲线反映了离子的迁移能力。可以看出，在低频区域EIS曲线几乎都平行于虚轴，表明polySH电解质具有良好的离子扩散行为(图12c)。25℃时,超级电容器具有6.3Ω的串联电阻,而这一数值在-30℃时增加到12.2Ω，在60℃时减小到5.9Ω。温度的变化影响电解质离子的扩散速率,从而影响超级电容器的电化学性能。值得注意的是,超级电容器也表现了较小的界面阻力，在25℃时只有1.8Ω,这表明了电极和电解质间充分的接触和更快的电荷转移率。
表4不同电流密度下超级电容器的比电容
Figure PCTCN2021097104-appb-000006
根据GCD曲线计算不同电流密度下的面积比电容值(图12d)。(比电容数据见表4)25℃时，超级电容器的比电容在1mA cm -2时为171mF cm -2，在5mA cm -2时为152mF cm -2，只变弱了11％，说明了超级电容器良好的倍率性能。如图13不同温度下的容量保持率所示，当温度增加到60℃或减少到-30℃时,面积比电容变到178mF cm -2和134mF cm -2，大约为25℃时的104％和78％,，表明在极端温度下良好的电化学性能。如图14所示，“QLU”型LED灯泡(工作电压2.5V)可以由三个超级电容器串联照明。在低温下冻结24小时后，超级电容器仍能正常工作。超级电容器解冻后,LED灯泡的亮度相比初始状态没有明显变化。这表明低温不会对超级电容器造成不可逆的损害。为进一步证实超级电容器的防冻性能,在25℃和-30℃下多次测量了超级电容器的电化学性能。如图15a所示，比电容在室温下解冻后仍然可以恢复到初始水平。即使经过10次冻融循环，超级电容器的电容值仍与室温下相同(图10a)。更重要的是，当超级电容器在-30℃的低温下放置30天以上时，其电容仅下降到原来的95.5％(图15b)。对不同温度下的循环稳定性进行了测试。如图15c所示，经过10000次循环后，超级电容器在-30℃、25℃、60℃下的电容保持率分别为71％、81％、76.4％。这种优良的循环性能表明，polySH基超级电容器可以在低温和高温下长期工作。
固态超级电容器除具有防冻性能外，还具有良好的灵活性，在实际应用中可避免结构损坏，延长使用寿命。如图16a所示，将polySH基超级电容器扭转360°后，CV曲线显示出与初始状态几乎相同的电容行为。即使在500个扭曲周期后，超级电容器的电容保持率仍为78.5％(图16b)。将超级电容器连接到电路中后，扭曲超级电容器前后LED灯的亮度没有明显变化(图16c)，说明大变形并不会影响电化学性能。这不仅是由于整个超级电容器的灵活性，而且由于电极与电解质之间的良好的粘附力，使得超级电容器在面临变形时仍能保持结构的完整性。当负载4000倍于其自身重量时，polySH基超级电容器的电容略有增加，这可能是由于高负载下电解质中离子输运路径缩短所致(图17a)。polySH电解质良好的力学性能使超级电容器在重载时避免了短路。另外，当我们的超级电容器被利器刺穿时，超级电容器仍能正常工作，并保持电容稳定(图17b)。在实际应用中，通过串联和并联多个超级电容器可以获得更高的电压和更高的能量密度。在我们的工作中，与单个超级电容器相比，两个超级电容器串联展示了2.0V的电压窗口，并且在并联连接时，放电时间几乎增加了2倍(图17c)。因此，我们的polySH电解质的防冻性能和良好的机械强度拓展了储能装置的应用领域。

Claims (10)

  1. 一种超级电容器，其特征在于，包含两性离子聚合物水凝胶电解质。
  2. 如权利要求1所述的超级电容器，其特征在于，超级电容器，在极端温度下的容量保持率，相对于25℃时的面积比电容，变化率小于25％。
  3. 如权利要求1所述的超级电容器，其特征在于，
    当超级电容器在-30℃的低温下放置30天以上时，其电容仍保留原来的95％以上；经过10000次循环后，超级电容器的电容保持率大于70％。
  4. 如权利要求1-3任一项所述的超级电容器，其特征在于，所述两性离子防冻水凝胶电解质(polySH)，为在LiCl存在下的聚(SBMA-HEA)电解质，Li +通过聚合物链上的两性离子基团进行跳跃式迁移，从而提高了离子导电性；拉曼光谱中，-SO 3 -中的S＝O伸缩振动为1044cm -1～1054cm -1，-N +(CH 3) 2中的CH3伸缩振动为2953cm-1～2957cm-1，与H 2O形成一种Li+(H 2O)n溶剂化结构，在-40℃下仍然具有325％的拉伸量。
  5. 如权利要求4所述的超级电容器，其特征在于，在室温条件下，polySH电解质的电导率为114～147mS cm -1；在-40℃时，所述电解质的低温电导率为11～13mS cm -1。所述电解质在-40℃可以拉伸到300～350％的应变。
  6. 如权利要求4所述的超级电容器，其特征在于，所述电解质的可冷冻水含量低于10％(polySH-5，polySH-7)；
    所述电解质的水分子扩散系数小于等于11.21*10 -6cm 2s -1。优选的，所述电解质的水分子扩散系数小于等于1.02*10 -6cm 2s -1。
  7. 如权利要求4所述的超级电容器，其特征在于，所述水凝胶电解质的制备方法为，在LiCl盐的存在下，使用SBMA和HEA采用一锅法无规聚合法制备了两性离子型polySH电解质；包括下列步骤：
    1)将LiCl溶解在第一溶剂中，制备1-7mol L -1的LiCl去离子水溶液。
    2)将SBMA和HEA溶解于LiCl溶液中，SBMA和HEA的摩尔比为1:(3-5)。将溶液置于冰浴中搅拌1h后加入引发剂AIBA，引发剂的加入量相当于单体总质量的0.5～2wt％。随后置于冰浴中搅拌1.5h。
    3)待溶液完全溶解后超声8～15min以移除气泡，随后将溶液注入模具中，密封置于30-50℃环境中聚合8-16h。
  8. 如权利要求7所述的超级电容器，其特征在于，，步骤1)中LiCl去离子水溶液的浓 度为3-7mol L -1；更优选的，步骤1)中LiCl去离子水溶液的浓度为5-7mol L -1。
  9. 如权利要求1-8任一项所述超级电容器的制备方法，包括下列步骤：
    1)制备活性碳电极：活性碳AC，导电碳黑和PVDF(质量比8:1:1)分散于NMP以制备成均匀的分散物料浆。将料浆涂覆于碳布上后置于80℃真空烘箱中24h，干燥后得到AC电极。活性材料在每一片电极上的负载量约为2.5mg cm -2；
    2)组装超级电容器：取负载面积相同(0.5cm×1.2cm)的两片AC电极，将其分别覆盖于电解质两侧组成三明治结构来制备超级电容器。随后在超级电容器两侧电极上分别滴加一滴polySH电解质前体溶液以润湿电极。
  10. 如权利要求9所述的制备方法，其特征在于：
    所述polySH电解质前体溶液，指进行聚合前的单体混合物，其制备方法为：1)将LiCl溶解在第一溶剂中，制备1-7mol L -1的LiCl去离子水溶液。
    2)将SBMA和HEA溶解于LiCl溶液中，SBMA和HEA的摩尔比为1:(3-5)。将溶液置于冰浴中搅拌1h后加入引发剂AIBA，引发剂的加入量相当于单体总质量的0.5～2wt％。随后置于冰浴中搅拌1.5h。
    3)待溶液完全溶解后超声8～15min以移除气泡，得到polySH电解质前体溶液。
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