Preparation method of electrolyte for bio-based solid-state battery

Abstract

The invention belongs to the technical field of lithium ion battery electrolytes, and particularly relates to a preparation method of an electrolyte for a bio-based solid-state battery. According to the invention, microcrystalline cellulose is used as a raw material, and is subjected to bromination modification and nucleophilic substitution to obtain trifluoromethane sulfonamide cellulose, and then the trifluoromethane sulfonamide cellulose is used as an electrolyte for a green, high-performance and high-safety battery. The bio-based solid electrolyte prepared by the method disclosed by the invention has physical stability and electrochemical stability and good room-temperature ionic conductivity and lithium ion transfer number, and ion migration in the prepared Cell.-TFSI solid electrolyte is mainly ion jump, so that relaxation conductivity contribution is low. The reasonable design of the modified cellulose as the solid electrolyte is shown, the ion transmission mechanism in the cellulose-based solid electrolyte is explored, a new insight is provided for deep understanding of the cellulose ion transmission mechanism, and the cellulose-based solid electrolyte has huge potential in application of energy devices such as high-performance batteries and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery electrolytes, and in particular relates to a method for preparing a bio-based solid-state battery electrolyte. Background Art

[0002] Energy issues are a key concern for sustainable development, and the development of environmentally friendly energy materials is essential. As the market for portable electronic devices, electric vehicles, and grid-scale storage continues to grow, there is an increasing demand for energy storage batteries with high charge density, stable output, high number of charge and discharge cycles, and strong safety characteristics. Among many batteries, alkaline ion batteries such as lithium-ion batteries (LIBs) have been widely used in different industries due to their high energy density and long cycle life. However, the energy density of rocking-chair lithium-ion batteries is less than 300Wh·kg -1 , which limits its use in high energy density industries. To this end, people have turned their attention to the development of lithium metal batteries (LMBs) with high energy density and theoretical capacity. Its electrolytes are mostly mixtures of carbonates (EC, DMC, MC, etc.) and lithium salts (LiTFSI, LiPF6, LiBF4, LiFSI, LiClO4, etc.). Unfortunately, metal batteries are currently in the laboratory research stage due to safety issues such as the flammability of the electrolyte and the shuttling of dendrite growth. Therefore, it is necessary to develop the next generation of safe, reliable, and compatible high-energy density batteries, especially solid-state polymer batteries and their key components, solid electrolytes. Because of its high stability, wide temperature range applicability, and strong designability, it has become one of the most promising candidates for the realization of the next generation of high-energy battery electrolytes. In recent years, the performance of solid electrolyte materials has been increasing, but considering the non-renewable and non-degradable nature of polymer raw materials, green and friendly natural polymers (such as cellulose) need to be used as electrolytes for solid-state batteries. For example, Tian Lei's team used cellulose acetate as a substrate to prepare a quasi-solid composite electrolyte with a room temperature ionic conductivity of 6.17×10 -4 S cm -1 , in LFP / Li (lithium iron phosphate / lithium) full battery, 1C rate (equivalent to a current density of 170mA·g -1 ) for more than 1200 stable cycles under high temperature (D.Wang, H.Xie, Q.Liu, K.Mu, Z.Song, W.Xu, L.Tian, ​​C.Zhu, J.Xu, Angewandte ChemieInternational Edition 2023, DOI 10.1002 / anie.202302767.). In addition, Hu et al. prepared a solid electrolyte by using cellulose to coordinate copper to expand the ion transport channel, achieving ultrafast lithium ion transport along the cellulose chain at room temperature (1.5×10 -4 S cm-1 )(C.Yang, Q.Wu, W.Xie, X.Zhang, A.Brozena, J.Zheng, MN Garaga, BH Ko, Y.Mao, S.He, Y.Gao, P.Wang, M.Tyagi, F.Jiao, R.Briber, P.Albertus, C.Wang, S.Greenbaum, YYHu, A.Isogai, M.Winter, K.Xu, Y.Qi, L.Hu,“Copper-coordinatedcellulose ion conductors for solid-state batteries,” 2021.). However, it needs to be mixed with oxide ceramic electrolytes or the orientation of CNFs needs to be strictly controlled, which increases the production cost and process complexity. Therefore, it is necessary to design and develop new bio-based solid-state battery electrolytes to promote the development of energy devices such as high-performance batteries. Summary of the invention

[0003] In order to overcome the shortcomings of the above-mentioned prior art, the present invention uses microcrystalline cellulose (MCC) as a raw material, and designs and synthesizes a Cell.-TFSI solid electrolyte with characteristic functional groups through bromination modification and trifluoromethanesulfonamidation. The electrolyte has high stability and high ionic conductivity, excellent ion transfer number, and has great potential for application in energy devices such as high-performance batteries (such as metal batteries).

[0004] In order to achieve the above object, the technical solution adopted by the present invention is:

[0005] A first aspect of the present invention provides a method for preparing a bio-based solid electrolyte, the method comprising the following steps:

[0006] S1, dissolving microcrystalline cellulose in DMF to prepare a cellulose solution, then adding an acid binding agent, stirring at room temperature and then bathing in ice water;

[0007] S2, dilute bromoacetyl bromide with DMF, add it to the cellulose solution in S1, react in an ice-water bath and then react at room temperature;

[0008] S3, introducing the solution obtained from the reaction in S2 into water to precipitate the reactant, then washing with water, filtering, washing, and drying to constant weight to obtain a light yellow product, cellulose bromide Cell.-Br;

[0009] S4, dissolving Cell.-Br in DMF, adding an acid binding agent, stirring, and then adding trifluoromethanesulfonamide, stirring continuously, and then precipitating the reactant with water, and then washing with water, and then filtering, washing, and drying to constant weight to obtain a white product trifluoromethanesulfonyl cellulose Cell.-TFSI;

[0010] S5. Disperse Cell.-TFSI in DMF, add LiTFSI, and form a solid electrolyte film after molding.

[0011] Preferably, the concentration of the cellulose solution in S1 is 1-3 wt.%.

[0012] Preferably, the rotation speed of the room temperature stirring in S1 is 200-1000 rpm, and the stirring time is 8-15 h.

[0013] Preferably, the acid binding agent in S1 is pyridine, and the acid binding agent in S4 is diisopropylethylamine.

[0014] Preferably, in S2, every 2 mL of bromoacetyl bromide is diluted with 10-20 mL of DMF, and the volume ratio of the diluted bromoacetyl bromide to the cellulose solution is 7-15:90-120.

[0015] Preferably, in S2, the reaction time in the ice-water bath is 1-3 h, and the reaction time at room temperature is 3-5 h.

[0016] Preferably, in S4, the concentration of Cell.-Br in DMF is 1 g / 40-60 mL, and the amount of trifluoromethanesulfonamide used is 11-13 times the molar equivalent based on Cell.-Br.

[0017] Preferably, in S5, the concentration of LiTFSI is 20-30 wt.%.

[0018] The second aspect of the present invention provides a bio-based solid electrolyte prepared by the preparation method described in the first aspect.

[0019] The third aspect of the present invention provides the use of the bio-based solid electrolyte described in the second aspect in a metal battery.

[0020] Preferably, the metal battery includes a blocked battery (stainless steel / electrolyte / stainless steel), a semi-blocked battery (lithium / electrolyte / stainless steel), a symmetric battery (lithium / electrolyte / lithium), and a full battery (lithium / electrolyte / lithium iron phosphate).

[0021] Preferably, the reference value range of the assembly pressure of the bio-based solid electrolyte is: 25 to 31 MPa.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The present invention uses MCC as a raw material, and obtains trifluoromethanesulfonamidated cellulose after bromination modification and nucleophilic substitution, and then uses it as a green, high-performance, and high-safety battery electrolyte. The large-radius fluorine-containing group grafted on the cellulose chain can limit the migration of anions, and at the same time, after reacting with the electrode, a stable and dense SEI is formed to extend the battery life. In addition, based on the contribution of the relaxation process to the conductivity of ions in the electrolyte, the two types of transport mechanisms, ion hopping and polymer relaxation, are jointly analyzed using dielectric spectroscopy and AC impedance spectroscopy to explore the effect of the two types of transport mechanisms on the conductivity of electrolyte ions. The bio-based solid electrolyte prepared by the method of the present invention has physical stability (tensile strength: 21.65MPa) and electrochemical stability (electrochemical stability window ESW: 5.62V), good room temperature ionic conductivity (1.12×10 -4 S cm -1 ) and lithium ion transfer number (0.794), and the migration of ions in the prepared Cell.-TFSI solid electrolyte is mainly ion hopping, and the relaxation conductivity contribution is low. The present invention demonstrates the rational design of modified cellulose as a solid electrolyte, explores the ion transport mechanism in cellulose-based solid electrolytes, provides new insights for a deeper understanding of the cellulose ion transport mechanism, and has great potential for application in energy devices such as high-performance batteries (such as metal batteries). BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 a: Preparation principle of Cell.-TFSI cellulose solid electrolyte; b: Optical photos of Cell.-TFSI electrolyte (before folding, during folding and after folding); c: PP / LiTFSI and Cell.-TFSI / LiTFSI at 1 mm·min -1 Stress-strain curves under loading rate.

[0025] Figure 2 a: EIS curves of Cell.-TFS / LiTFSI solid electrolyte with lithium salt concentrations of 15wt.%, 20wt.%, 25wt.%, 30wt.% and 35wt.%; b: Room temperature ionic conductivity of Cell.-TFSI solid electrolyte with different lithium salt concentrations.

[0026] Figure 3 a: Infrared absorption spectra of Cellulose, Cell.-Br, and Cell.-TFSI; b: XRD of different samples; c, d: C1s and Br3d spectra of Cell.-Br; e, f: C1 and F1s spectra of Cell.-TFSI.

[0027] Figure 4The molecular structures and adsorption energy calculation results of cellulose, lithium salts, etc.: a: cellulose (Cellulose) structure; b: cellulose bromide (Cellulose-Br) structure; c: cellulose trifluoromethanesulfonyl imide (Cell.-TFSI) structure; d: lithium bis(trifluoromethanesulfonyl imide) (LiTFSI) structure; e: adsorption energy of Li+ with -OH and -TFSI groups, the inset is the electrostatic potential distribution of Cell.-TFSI molecule; f: MS theoretical calculation of HOMO and LUMO energy levels of LiTFSI, Cellulose, Cell.-Br and Cell.-TFSI.

[0028] Figure 5 a: Nyquist plots of commercial electrolyte membrane (PP) in solid and liquid state, Celluose, Cell.-TFSI; b: Current response of PP membrane, Cellulose, Cell.-TFSI electrolyte to time under the condition of 1V voltage, the response current changes with time; c: EIS of Cell.-TFSI electrolyte changes with assembly pressure (50-1000kgf); d: Bulk resistance R of Cell.-TFSI electrolyte under different pressures ohm and the charge transfer resistance R ct change.

[0029] Figure 6 a: Change of Nyqusity of Cell.-TFSI electrolyte with temperature; b: Relationship between ionic conductivity and temperature of Cell.-TFSI and Cellulose electrolytes; c: LSV curves of Cell.-TFSI, Cellulose, and PP solid electrolytes in Li / SPE / SS semi-blocked cells; d: Current-time curve of Cell.-TFSI in Li / SPE / Li symmetric cells at a voltage of 10 mV, the inset is the Nyqusit curve before and after polarization.

[0030] Figure 7 For lithium / solid electrolyte / lithium symmetric cells at a current density of 0.5 mA cm -2 , 1mAh cm -2 Cycling performance under different conditions: a~c: Cellulose solid electrolyte; d~f: Cell.-TFSI solid electrolyte.

[0031] Figure 8 Solvation structure of cellulose-based solid electrolyte: a: Spatial model of Cell.-TFSI / LiTFSI electrolyte; b: Li + With Cell.-TFSI, TFSI - , radial distribution function of ether oxygen (-O-) and coordination number (dashed line); c: Li+ The radial distribution function and coordination number of F atoms in the electrolyte vary with radius. The fluorine atoms in Cell.-TFSI electrolyte come from the modified groups and TFSI-, and the fluorine atoms in Cellulose come from TFSI - ;d:Li + The radial distribution function and coordination number of N atoms in the electrolyte vary with radius; e: Li + Radial distribution function of S atoms in electrolyte and coordination number change with radius; f: Li + The radial distribution function and coordination number of oxygen atoms in the electrolyte vary with radius, where Li + The RDF selection with oxygen atoms can participate in the coordination part, excluding the oxygen atoms on the cellulose molecular backbone.

[0032] Fig. 9 A typical structure extracted from molecular dynamics simulation: a: Li + The coordination structure with Cellulose. In order to observe the solvation structure more intuitively, the hydrogen atoms in the model are omitted here; b: Li + Coordination structure with Cell.-TFSI.

[0033] Fig.10 The dielectric relaxation characteristics of unmodified cellulose solid electrolyte vary with temperature: a: real dielectric constant, b: imaginary dielectric constant, c: dielectric loss, d: real AC conductivity, e: imaginary AC conductivity, f: imaginary dielectric modulus.

[0034] Fig.11 The dielectric relaxation characteristics of Cell.-TFSI solid electrolyte vary with temperature: a: real dielectric constant, b: imaginary dielectric constant, c: dielectric loss, d: real AC conductivity, e: imaginary AC conductivity, f: imaginary dielectric modulus. DETAILED DESCRIPTION

[0035] The specific embodiments of the present invention are further described below. It should be noted that the description of these embodiments is used to help understand the present invention, but does not constitute a limitation of the present invention. In addition, the technical features involved in each embodiment of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0036] The experimental methods in the following examples are conventional methods unless otherwise specified, and the experimental materials used in the following examples are commercially available unless otherwise specified.

[0037] Example: Design, preparation, and performance study of new bio-based solid-state battery electrolytes

[0038] 1. Experimental Materials and Methods

[0039] 1.1. Raw materials: microcrystalline cellulose (MCC), N,N-dimethylformamide (DMF), pyridine, bromoacetyl bromide, glycerol, N,N-diisopropylethylamine (DIPEA), trifluoromethanesulfonamide, lithium bis(trifluoromethanesulfonyl imide) (LITFSI), lithium metal disc (D: 16 mm, h: 0.8 mm), commercial electrolyte film (PP, purchased from Searle Technology, USA, immersed in the matching electrolyte as a control during the experiment), dimethyl carbonate.

[0040] 1.2.Test equipment:

[0041] Fourier transform infrared spectrometer (Nicolet 6700, Thermoelectric Nicolet, USA), X-ray diffractometer (Bruker, Germany), high temperature dielectric test system (Tonghui Electronic Technology, TONGHUI 4294-50), micrometer (Kaimei Chemical Engineering Equipment, 211-101K), X-ray photoelectron spectrometer (Axis SUPRA, Shimadzu, Japan), universal testing machine (UTM-4103, Sansi Zongheng Technology), solid electrolyte constant temperature and constant pressure test device (HWY-10H, Ionic New Energy Technology), electrochemical comprehensive tester (PARSTAT3000aA-DX, Princeton, USA), etc.

[0042] 1.3. Preparation of electrolyte film:

[0043] (1) Take 2g of microcrystalline cellulose and dissolve it in 100mL of DMF, stir at room temperature at 300rpm for 10h to obtain a 2wt.% cellulose solution. Then take 340uL of dry pyridine and add it dropwise to the cellulose solution as an acid-binding agent. Stir at room temperature for 2h and then place in an ice-water bath for more than 15min. Take another 2mL of bromoacetyl bromide and add it dropwise to 10mL of pre-frozen DMF (the refrigerator temperature is -4°C) to dilute it. Then slowly add the diluted bromoacetyl bromide solution to the cellulose solution. After reacting in an ice-water bath for 1h, react at room temperature for 4h, and add an appropriate amount of glycerol to terminate the reaction. Then, introduce the resulting solution into 1L of deionized water to precipitate the reactant, then rinse with deionized water, filter, wash 3 times, and place in a 70°C oven to dry to constant weight to obtain a light yellow product, cellulose bromide (Cell.-Br).

[0044] (2) Dissolve 1 g of cellulose bromide in 50 mL of DMF and stir at room temperature until it is completely dispersed. Then, add 12 molar equivalents of diisopropylethylamine as an acid binding agent to the solution. After stirring for 2 h, add 12 times the molar equivalents of trifluoromethanesulfonamide. After stirring for 4 h, slowly pour into 1 L of deionized water. After stirring for 15 min, filter, rinse, and wash three times, and then place in a 70 ° C oven to dry to constant weight to obtain a white product, cellulose trifluoromethanesulfonyl (Cell.-TFSI).

[0045] (3) Take 0.5g of Cell.-TFSI and disperse it in DMF, and add LiTFSI with concentrations of 5wt.%, 10wt.%, 15wt.%, 20wt.%, 25wt.%, 30wt.%, 35wt.%, and 40wt.% to the mixture. After sufficient stirring, cast it into a PVDF mold, dry it at 50℃ for 12h and then at 105℃ for 5h to obtain a solid electrolyte film, which is stored in an argon glove box for later use. Similarly, an unmodified cellulose electrolyte is prepared.

[0046] In addition, 1M LiTFSI electrolyte was prepared with DMC and used to impregnate the commercial PP separator. The whole process was carried out in a glove box.

[0047] 1.4. Battery assembly: The solid electrolyte film prepared above was cut into discs with a diameter of 19 mm and assembled into blocked cells (stainless steel / electrolyte / stainless steel), semi-blocked cells (lithium / electrolyte / stainless steel), and symmetrical cells (lithium / electrolyte / lithium) of CR-2032 or CR-2016 in an argon glove box. The assembly pressure was not less than 5 MPa and the holding time was 1 h. The assembled button cells were placed in a 60°C oven for 24 h to ensure full contact between the electrolyte and the electrode material to form a complete ion path.

[0048] 1.5. Test method: Fourier transform infrared spectroscopy (FTIR): The solid electrolyte sample prepared above was dried to remove excess water or solvent, mixed and ground with KBr (mass ratio of 0.5-1.0wt.%), and then pressed into a film and placed in the equipment for testing at a wavelength range of 600 to 4000cm -1 .

[0049] X-ray photoelectron spectroscopy (XPS): The solid electrolyte prepared above is placed in the equipment, the radiation source is single-frequency AL-Kα, the power is 450W mode, and the chemical composition, element content and bonding state of the material are analyzed.

[0050] X-ray diffraction analysis (XRD): The prepared solid electrolyte was placed on the XRD sample stage, and the 2θ angle range of X-ray (Cu, wavelength 0.1542nm, 50kV, 40mA) scanning was 5-60°, and the scanning speed was 1°·min -1 , the scanning mode is continuous scanning.

[0051] Tensile test: The solid electrolyte prepared above was cut into a 30 mm × 10 mm rectangle, and the film thickness was measured three times by a micrometer and the average value was obtained. A low-load tensile test was performed at room temperature with a clamping distance of 2 mm and a loading rate of 1 mm min. -1 .

[0052] High-temperature dielectric spectroscopy: The ion transport modes in polymer electrolytes include ion hopping and polymer chain relaxation driving ion transport. However, which mode is the main one and what is the specific contribution rate need further study. To this end, we studied the ion transport mechanism in Cell.-TFSI electrolyte based on the measurement information of dielectric spectroscopy and electrochemical impedance. In the high-frequency range, ions move in space, and their power-law behavior represents AC conductivity; in the medium-frequency range, ions begin to break away from the surrounding shielding groups (Coulomb cages) and drift, that is, they overcome the shielding effect and do work, contributing to DC conductivity; in the low-frequency range, corresponding to the polarization effect, ions drift to the electrode and will not migrate. According to the random barrier model (RBM), ions randomly jump in the space market, which highly affects the AC conductivity. Once the maximum energy barrier is overcome, the ions will drift and directly contribute to the DC conductivity. The ion relaxation time (τ RBM ) and conductivity satisfy the following relationship (corresponding references: JC Dyre, Phys Lett A 1985, 108, 457.; TB JCDyre,Phys Rev Lett 2008,101,025901.;EWStacy,CPGainaru,M.Gobet,Z.Wojnarowska,V.Bocharova,SGGreenbaum,APSokolov,Macromolecules 2018,51,8637.):

[0053]

[0054] in, is the imaginary AC conductivity in the dielectric spectrum, σ0 is the critical DC conductivity in the dielectric spectrum, a constant that is independent of frequency, v is the frequency, τ RBMis the relaxation time of ions in the random barrier model, i is the imaginary symbol, π is the circumference of the circle, and its value is 3.142. For this reason, we use the above formula to describe the conductivity and polarization relaxation process of the aggregates formed after the interaction between polar groups and ions in polymer electrolytes. The present invention explores the mechanism of electrolyte ion transport by using the dielectric properties and frequency spectra of electrolytes at different temperatures to study the polarization relaxation characteristics of solvated groups, and introduces the dielectric modulus to analyze its conductivity contribution and relaxation time. The test process and parameters are as follows: before the test, the prepared solid electrolyte film sample is cut into a circular sheet with a diameter of 10 mm, and circular conductive glue (with a diameter of 6 mm) is pasted on both sides, and the film thickness is measured by an electronic spiral micrometer. The dielectric test system was used to test in the frequency range of 20Hz to 10MHz and at a bias voltage of 0.5V. The test temperature range was 26℃, 30℃, 40℃, 50℃, 60℃, 70℃, 80℃, 90℃, 100℃ (299K, 303K, 313K, 323K, 333K, 343K, 353K, 363K, 373K), and the heating rate was 2℃·min -1 , and then start measuring after keeping each temperature point for 30 minutes.

[0055] Characterization of electrochemical performance: Electrochemical tests such as electrochemical impedance spectroscopy (EIS), linear voltammetry (LSV), cyclic voltammetry (CV), constant voltage chronoamperometry (CA) were all carried out on the Princeton electrochemical comprehensive tester. The electrolyte temperature and pressure test was carried out using the electrolyte constant temperature and constant pressure test device of Ionic New Energy Technology in combination with the electrochemical comprehensive tester.

[0056] Ionic conductivity: The test of ionic conductivity of polymer electrolyte is carried out under the conditions of application temperature, with a measurement frequency range of 1MHz to 0.1Hz, an amplitude of 10mV, a temperature test range of 23℃ to 80℃, and each temperature point is kept warm for 1h, and each pressure point is pressurized for 1h before starting the measurement. The test battery uses the blocked battery as the test object, tests the electrochemical impedance spectroscopy (EIS), and calculates its ionic conductivity according to the following formula (corresponding reference: S.-A.Safavi-Mirmahalleh, SN Eliseeva, ARM Moghaddam, H.Roghani-Mamaqani, M.Salami-Kalajahi, ACS Appl Energy Mater 2023, 6, 9624.):

[0057]

[0058] Where R is the electrolyte body resistance, which is read after fitting based on the EIS test results, in ohms (Ω), L is the electrolyte film thickness, in centimeters (cm), and S is the electrolyte effective contact area, in square centimeters (cm2 ).

[0059] Electronic conductivity: Use the constant voltage chronoamperometry (CA) to apply a 1V control voltage on both sides of the electrolyte blocking electrode and record the response current curve over time. When the current reaches stability, the ion migration is restricted by the blocking electrode, and only electrons exist in the circuit for circuit transmission. Analyze the time-current curve, select the current at the last moment as the steady-state current I, and calculate its electronic conductivity according to the following formula:

[0060]

[0061] Where U is the controlled potential in volts (V), I is the steady-state current in amperes (A), L is the thickness of the electrolyte film in centimeters (cm), R is the resistance measurement in ohms (Ω), and S is the effective contact area of ​​the electrolyte in square centimeters (cm 2 ).

[0062] Electrochemical stability window: The semi-blocked battery was tested using the LSV test method to evaluate the stability of its electrolyte. The scan rate was 1mV·s -1 , the scanning range is: battery open circuit voltage is 6V.

[0063] Lithium ion transfer number: The lithium ion transfer number (t + ). Set a certain bias potential ΔU (10mV) for the symmetrical battery (lithium / electrolyte / lithium), record the response current curve over time, and when the current reaches stability, it is considered that the current contribution at this time is all contributed by cations, and the anion contribution in the circuit is 0. Record the steady-state current I s , analyze the EIS spectra of the initial state and steady state, fit the circuit and read the impedance of the passivation layer in the initial state and the steady-state passivation layer impedance The current at the beginning of the CA test is selected as the initial current, and the current at the last moment is selected as the steady-state current I s , the lithium ion transfer number can be further calculated.

[0064]

[0065] In the above formula, ΔU is the bias voltage in volts (V), I 0 is the initial current in milliamperes (mA), I s is the steady-state current in milliamperes (mA), is the resistance of the passivation layer in the initial state, in ohms (Ω), is the resistance of the steady-state passivation layer in ohms (Ω).

[0066] 1.6. Calculation method:

[0067] In this work, the DFT calculations used were all based on the exchange correlation energy of interacting electrons determined by the generalized gradient approximation (GGA) and Perdew-Burke-Ernzerhof (PBE) functionals, and were implemented in the DMol3 module in the Materials Studio.2020 software. After the geometric optimization of the molecular model, the point charge, charge density, and electrostatic potential distribution were simulated and calculated using a molecular model with a degree of substitution of Cell.-TFSI of 1. The adsorption energy of lithium ions and specific groups was studied using DFT calculations, and the following formula was used for calculation (corresponding to reference:):

[0068]

[0069] Where E Abs ., E. Base , They are Li + Electrolyte and base material in Cell.-TFSI or Cellulose Cell.-TFSI or Cellulose and Li + The total energy.

[0070] Molecular dynamics simulations of cellulose electrolytes were performed using the Forcite module in the Material Studio software package to evaluate the coordination of lithium ions with different groups. The simulation system was constructed using 5 LiTFSI and 35 Cellulose or Cell.-TFSI disaccharide molecules (calculated that the dielectric properties of cellulose disaccharide are closest to the actual experimental test results, with only a 3.1% difference between the simulated calculation and the experimental test dielectric properties). The molecules were initially randomly packed in a cube with the dimensions of length (x), width (y), and height (z) all being The dimensions of the simulation box after isothermal and isobaric comprehensive pre-equilibrium are: Cellulose electrolyte Cell.-TFSI electrolyte is At the same time, a condensed phase optimized molecular potential II (COMPASS II) force field was assigned to all atomistic simulation studies. This force field can handle systems containing organic and inorganic molecules, accurately simulate the structure and properties of condensed matter, and meet the requirements of electrolyte systems in experiments. Both cellulose-based electrolyte systems were initially stabilized for 1000 steps using the algorithm. After geometric optimization, the model was pre-equilibrated in an isothermal-isobaric ensemble (NPT, 333K, 1atm) for 1ns, and then balanced in a canonical ensemble (NVT) for 1ns. The time step of all simulations (NPT or NVT) was 1fs. Based on the motion trajectory file (.xtd format) output after 1ns NVT equilibrium, the atomic (group) interactions were analyzed using the atomic radial distribution function (RDF) (all data analysis ranges selected the density and energy stability range: 101 to 1000 steps).

[0071] 2. Results and Discussion

[0072] 2.1.Material properties:

[0073] (1) Reaction mechanism and film properties

[0074] The base material of Cell.-TFSI electrolyte is obtained by uniformly dispersing microcrystalline cellulose in DMF solvent, brominating it and then replacing the bromine atom with a nucleophilic reagent. The reaction mechanism is as follows: Figure 1 As shown in a, the yield of the preparation process is relatively high, which is 84.7%. The solid electrolyte film formed after adding LiTFSI is normally stable and is a translucent film. After folding, there is a small amount of deformation ( Figure 1 b) Use a universal testing machine to perform low-load tensile tests on PP / LiTFSI and Cell.-TFSI / LiTFSI. -1 The test results at the loading rate are as follows Figure 1 c. The tensile strength of Cell.-TFSI electrolyte is 21.65MPa, the elastic modulus is 461.76MPa, and the elongation at break is 8.067%, which is 3.15MPa higher than the tensile strength of the electrolyte composed of commercial diaphragm PP, indicating that Cell.-TFSI solid electrolyte can withstand the large volume change effect of lithium metal and inhibit dendrite growth.

[0075] (2) Phase and morphology

[0076] Before studying the physical stability and micromorphology of Cell.-TFSI / LITFSI solid electrolytes, the dispersibility and dissociation ability of different lithium salt concentrations in the Cell.-TFSI matrix were investigated. The electrochemical impedance spectroscopy and room temperature ionic conductivity of Cell.-TFSI electrolytes with different lithium salt concentrations are shown in Figure 2. Figure 2When the concentration of LiTFSI is less than 25wt.%, the ionic conductivity increases with the increase of lithium salt concentration, indicating that when the lithium salt concentration is less than 25wt.%, there are enough solvation units (polar groups) in the Cell.-TFSI matrix to promote the dissociation of lithium salt. When the lithium salt concentration is 25wt.%, the room temperature ionic conductivity reaches a maximum value of 1.12×10 -4 S cm -1 As the lithium salt concentration increases, the ionic conductivity gradually decreases. The possible reason is that after the ability of Cell.-TFSI to dissociate LiTFSI reaches the threshold, the lithium salt added later cannot be dissociated but forms crystals and aggregates, which cannot participate in Li + The undissociated molecules or crystals precipitate and affect the originally formed solvation structure, resulting in the obstruction of ion transport channels and the reduction of ion conductivity. Therefore, in the subsequent study of the electrochemical performance of Cell.-TFSI, the lithium salt concentration of the Cell.-TFSI electrolyte was 25wt.%.

[0077] In this study, the target product Cell.-TFSI and the intermediate product (Cell.-TFSI) were structurally characterized. Figure 3 As shown in a, the structure of Cell.-TFSI was characterized by FTIR. Cell.-Br and Cell.-TFSI were detected at 1748 cm -1 There is an obvious -C=O stretching vibration absorption peak. After the reaction, the -OH absorption peak of the cellulose sample decreases significantly (2450cm -1 There are only a few absorption peaks, which is due to the fact that the cellulose hydroxyl groups are largely replaced after the bromination reaction. Cell.-TFSI is at 3466.8cm -1 In order to further verify the structure of Cell.-TFSI, the sample was further characterized by XPS. The results are shown in Figure 3 c, d, e, and f are shown, among which Figure 3 c, d are the C1s and Br3d spectra of Cell.-Br, Figure 3e, f are the C1s and F1s spectra of Cell.-TFSI. All spectra were peak calibrated according to the signal of adventitious carbon. C1s in Cell.-Br was fitted and assigned to adventitious carbon (CC, reference value 284.8eV), carbon oxygen bond (-CO-, reference value 287.5eV), and carbonyl group (-C=O, reference value 288.6eV) at the substitution site. Br3d orbital has an overlapping split peak near 68.7eV, ΔE=1.03eV, and the intensity ratio is 0.67. In Cell.-TFSI, the adventitious carbon signal in C1s is 284.7eV, which is assigned to the carbon-nitrogen bond of the trifluoromethanesulfonamide reaction site (286.2eV) and the carbonyl group of the bromination reaction site (-C=O, 288.3eV). The signal peak of F1s appears at 687.5eV, which is 0.5eV offset compared to the organic fluorine reference value of 688-689eV, which is within a reasonable range.

[0078] In addition, the XRD patterns of the commercial electrolyte separator (PP) impregnated with electrolyte and then the solvent evaporated (PP / LiTFSI), Cellulose, Cell.-TFSI, and Cell.-TFSI / LiTFSI solid electrolyte were analyzed. Figure 3 b) Due to the regeneration of cellulose after modification and the grafting of trifluoromethanesulfonamide groups on the surface, the hydrogen bonds between the Cell.-TFSI molecular chains are greatly reduced, tending to be more amorphous, which will be beneficial to the transmission of lithium ions on the polymer chain. The modified polar groups promote the dissociation of lithium salts, and there is no obvious crystallization peak of LiTFSI in Cell.-TFSI / TFSI, indicating that the lithium salts in the Cell.-TFSI solid electrolyte have good dispersion and dissociation.

[0079] (3) Calculation of adsorption energy and electronic structure

[0080] This study uses density functional theory (DFT) calculations and frontier orbital theory analysis to reveal the significant differences in lithium ion adsorption characteristics and interfacial electrochemical behavior between cellulose backbone grafted trifluoromethanesulfonyl imide (-TFSI) and hydroxyl (-OH). The established cellulose molecular model is simulated by cellobiose, and the model used in the calculation is as follows Figure 4 a~d are shown. The DFT calculation results show that the adsorption energy (ΔE Abs. =-7.167eV) is significantly lower than the adsorption energy of hydroxyl groups to lithium ions (ΔE Abs. =-4.457eV), such as Figure 4 This phenomenon is due to the synergistic coordination effect of multiple active sites in the TFSI molecule. The lone pair electrons of the oxygen atom of the sulfonyl group (-SO2-) and the Li +A strong coordination bond is formed, and the strong electron-withdrawing effect of the trifluoromethane group (-CF3) enhances the electron cloud density of the sulfonyl group through induction, thereby forming a dynamic effect. In addition, the three-dimensional spatial configuration of the TFSI group can provide multiple coordination sites (OSO triangular plane and F atom synergistic effect), which is more conducive to the formation of a stable multi-dentate coordination structure compared to the single coordination mode of the hydroxyl group. This multiple coordination mechanism not only allows Cell.-TFSI to have better lithium ion adsorption, but also may reduce the energy barrier of decoupling of the lithium ion migration process through flexible conformational adjustment.

[0081] Frontier orbital theory calculations reveal the selective tendency of the interfacial reaction: Cell.-TFSI has the lowest LUMO energy level (-1.673 eV), and LiTFSI has a LUMO energy level of -1.320 eV ( Figure 4 f), while the HOMO energy level of Cell.-TFSI (-6.156eV) is higher than the HOMO energy level of LiTFSI in the electrolyte (-7.125eV), which will cause the electrolyte to react preferentially at the anode interface. The low fracture energy of the ester bond (-COO-) in the modified group (1.28eV, the fracture energy of the main chain CO bond is 3.15eV) is even lower, which makes the terminal -SO2CF3 group preferentially exposed and react with metallic lithium, and may generate an inorganic phase containing LiF (XPS F 1s peak 683.3eV) and Li2S and an organic phase of fluorocarbon compounds, forming a gradient SEI layer and extending the battery life. Chemical modification of the cellulose molecular structure can effectively regulate the thermodynamic binding strength and transmission kinetics of lithium ions, and optimize the interface reaction path through energy level matching, which provides an important theoretical basis for the design of new cellulose solid electrolytes.

[0082] (4) Electrochemical performance of Cell.-TFSI / LiTFSI in metal battery applications

[0083] In order to compare the electrochemical performance of different electrolytes and the optimal pressure for battery assembly, the AC impedance spectra and current-time response changes of commercial separators impregnated with liquid electrolytes (PP / LiTFSI, Liquid), commercial electrolyte separators impregnated with liquid electrolytes and then evaporated (PP / LiTFSI), cellulose electrolytes (Cellulose / LiTFSI), and trifluoromethanesulfonamidated cellulose electrolytes (Cell.-TFSI) were studied. The results are shown in Table 1. Figure 5 a, b. At room temperature (26°C), the modified groups in Cell.-TFSI can promote the dissociation of lithium salts and serve as transport sites, with an ionic conductivity as high as 1.12×10 -4 S cm -1, which is about 1 / 25 of that of liquid electrolyte. Since the PP diaphragm mainly plays the role of supporting and absorbing the electrolyte, its own contribution to the conduction of ions is very weak. Therefore, under the same conditions, the ionic conductivity of the liquid electrolyte with volatilized solvent in the PP diaphragm is much lower. Similarly, the ionic conductivity of the cellulose electrolyte is about 5 times lower than that of the Cell.-TFSI electrolyte. It may be that the dissociation of the hydroxyl group to the lithium salt is limited, and there are undissociated lithium salts in the cellulose electrolyte. The constant voltage current-time test results show that the electronic conductivity of the three types of electrolytes is very low, and they can be used as battery diaphragms without short circuiting. In addition, in order to explore the optimal assembly pressure of the electrolyte, different pressures (50-1000kgf) are applied to the Cell.-TFSI electrolyte to simulate the pressure when it is assembled as a battery diaphragm, and its AC impedance is recorded, and its charge transfer resistance and diaphragm body resistance are analyzed. The test results are as follows Figure 5 c, d. As the applied pressure increases, the volume resistance (R ohm ) first decreases slightly and then remains stable. The reason is that when the pressure is applied, the pressure promotes the contact between the electrolyte and the electrode interface and reduces the interface resistance. However, as the pressure continues to increase, the interface contact tends to be stable, making the change in interface resistance weak. It is worth noting that as the applied pressure increases, the electrolyte charge transfer resistance (R ct ) is always increasing. Since the charge transfer resistance is related to the heterogeneous electrochemical reaction of the system, the applied pressure causes the polymer chain to slide and deform, expanding the proportion of heterogeneous phase in the electrolyte system. For this reason, when assembling batteries, the higher the applied pressure, the better. Instead, the pressure should be reduced as much as possible while ensuring good contact at the interface. Based on the change in applied pressure and impedance, a reference value range for the assembly pressure of the Cell.-TFSI electrolyte is calculated here: 25 to 31 MPa.

[0084] Cell.-TFSI solid electrolyte AC impedance changes with temperature Figure 6 As shown in a and b, by analyzing and calculating the change of ionic conductivity and ionic conductivity of Cellulose / LiTFSI electrolyte with temperature, based on the Arrhenius equation, it is preliminarily calculated that the ion transport activation energy of Cellulose-TFSI / LiTFS is 57.43 kJ·mol -1 , which is 22.97 kJ·mol lower than that of Cellulose / LiTFSI -1 (The ion transport activation energy of Cellulose / LiTFSI electrolyte is 80.40 kJ·mol -1 On the one hand, the modified trifluoromethane group promotes the dissociation of lithium salts and provides more carriers for electrolyte ion migration. On the other hand, the trifluoromethanesulfonamide group and L i+With lower adsorption energy, ions can be transported quickly. The electrochemical stability of the electrolyte was studied by linear voltammetry (LSV). Figure 6 As shown in Figure c, the electrochemical stability window (ESW) of Cell.-TFSI electrolyte is as high as 5.62V, which can adapt to most electrode materials, allow higher operating voltage, improve battery energy density, and enhance cycle stability. In contrast, the electrochemical stability of PP electrolyte is limited by the oxidative stability of carbonate in the electrolyte, and its ESW is only 4.45V. When the battery operating voltage exceeds this, there will be certain safety risks. It is noteworthy that due to the diameter of the trifluoromethanesulfonamide group grafted on cellulose and the TFSI dissociated from LiTFSI, - The order of magnitude of the diameter is comparable, which can effectively limit the migration of anions and promote the transport of cations, making the ion transfer number (t + ) up to 0.794( Figure 6 d), which is beneficial to reduce the concentration polarization at the interface and improve the power density and charge and discharge efficiency of the battery.

[0085] In order to study the contact effect of the prepared Cellulose-TFSI / LiTFSI solid electrolyte on the lithium metal electrode and the adaptability of the electrode volume change, Li / Cellulose / Li and Li / Cellulose-TFSI / Li symmetric cells were assembled for comparison and long-term lithium plating / stripping cycle tests were performed. The symmetric battery cycle results using Cellulose electrolyte are shown in Figure 2. Figure 7 As shown in a to c, in the first 250h cycle, the voltage signal changes significantly when the current density is constant, indicating that in this cycle stage, the electrolyte and lithium metal are in hard contact conduction and the contact is limited. The poor contact stability leads to unstable voltage, and the solid electrolyte interphase (SEI) is not fully formed. At 707h, the symmetrical battery of Cellulose electrolyte short-circuited, and the battery resistance decreased sharply, as shown in Figure 2. Figure 7 As shown in c. Due to the unstable interface caused by uncontrolled dendrite growth, the dendrites perforated the electrolyte and short-circuited the battery, which may be because the F element alone in the lithium salt was insufficient to react to form a stable and dense SEI layer. In contrast, the polarization voltage of the Cell.-TFSI / LiTFSI solid electrolyte is smaller and is stable near the 0.09V voltage platform, which is due to the smaller body resistance of the electrolyte. In the short period of time at the beginning of the cycle (within 40h), the voltage platform fluctuated (7d), indicating that the SEI layer was not fully formed at this stage. After that, the Cell.-TFSI battery maintained a stable voltage platform cycle, and the voltage platform remained stable even after 950h, and finally cycled for more than 1000h ( Figure 7d~e). The long-term stable lithium plating / stripping cycle is because the components in the Cell.-TFSI electrolyte promote the formation of the fluorine-rich SEI layer and the strength of the electrolyte itself hinders the further growth of dendrites, indicating that Cell.-TFSI as a solid electrolyte substrate can adapt to the volume change of the electrode.

[0086] Table 1 Room temperature (26°C) ionic conductivity parameters of different samples

[0087]

[0088] (5) Exploration of ion transport mechanism in Cell.-TFSI / LiTFSI

[0089] Radial distribution functions (RDFs) quantitatively describe the ratio of the local density of atoms to the average density around the central lithium ion at a distance r. Here, based on the molecular dynamics results simulated by the Forcite module in the MS software package, the RDFs between lithium ions and various components in Cellulose / LiTFSI and Cellulose-TFSI / LiTFSI electrolytes were analyzed to illustrate the solvation sheath structure in cellulose-based electrolytes and its effect on ion migration. Figure 8 For simulation details model and Li + For the RDF of other atoms, the Cellulose, Cell.-TFSI and LiTFSI models used are the models established and geometrically optimized in 1.6. Figure 8 a is the constructed spatial model structure. From 8b, we can see that Cell.-TFSI molecules and TFSI- groups are Nearby Li + The radial distribution function g(r) peak of TFSI- is higher than that of Cell.-TFSI, indicating that its interaction is stronger than that of Cell.-TFSI molecule, which belongs to strong interaction coordination. However, it is worth noting that Li + The coordination number of the Cell.-TFSI molecule and TFSI- In addition, in the Cell.-TFSI electrolyte system, -O- only exists in the Cell.-TFSI molecule, and the position of interaction with Li+ also belongs to the middle layer of the solvation structure. The coordination number changes are close to that of Cell.-TFSI.

[0090] In Cell.-TFSI / LiTFSI electrolyte, Li + The innermost solvation sheath of is non-ether oxygen atom (such as hydroxyl oxygen, carbonyl oxygen, oxygen atom in sulfonyl group), and the position is The number of left and right coordination is 1, which is consistent with the position of oxygen atoms reported in the literature [corresponding reference: Generation of a highly conductive and stable solid electrolyte interphase at lithium anode under additional electric filed Author links open overlay, Chemical Engineering Journal Volume 446, 137435]. Nearby, the coordination number of oxygen atoms rapidly increases to 3, and as the distance increases, the coordination number further increases. The coordination with F, N, and S atoms begins to appear Nearby, in The coordination number in the range reaches 1, such as Figure 8 d, e. In contrast, Li + The solvation structure formed with the surrounding groups is relatively simple, and the inner layer is the same as Cell.-TFSI, which is O atoms. The range is 1 N atom, a certain number of F atoms, and no S atoms coordinated, such as Figure 8 f. It is worth noting that in Figure 8 In c, the DRF peak of the inner-layer coordinated O atoms of Cellulose is lower than that of Cellulose, indicating that the former and Li + The coordination between them is weak strength coordination, which is also the reason why Cell.-TFSI electrolyte has high ionic conductivity. The weak coordination solvation inner layer structure makes the activation energy of ion hopping-coupling migration lower, and the electrolyte body resistance will be lower.

[0091] The lithium ion solvation structures of cellulose-based electrolytes were extracted from trajectory files, e.g. Fig. 9 As shown, hydrogen atoms are omitted. Fig. 9 a, b, the inner layer of the formed Li-Cellulsoe structure is hydroxyl oxygen, while the inner layer of the Li-Cell.-TFSI structure contains carbonyl oxygen, oxygen in the sulfonyl group and fluorine atoms. Both structures belong to solvated separated ion pairs, which will be beneficial to the improvement of ion migration efficiency compared with contact ion pairs. The presence of a small amount of contact ion pairs in the electrolyte can promote the contact electrolysis reaction of fluoride-containing anions to form HF and then react with SEI layer carbonate to produce LiF. However, the Li-Cell.-TFSI structure contains F atoms, and a fluorinated SEI layer may also be formed at the interface during the ion migration process. The bromide-grafted -O- in Cell.-TFSI accounts for a small proportion of the formation of the solvated structure, such as Fig. 9b. The reason is that the steric hindrance is large, and the carbonyl group and the trifluoromethane group at the end of the branch chain first react with Li + Before using BRM to fit and calculate the characteristic parameters of ion migration in Cell.-TFSI solid electrolyte, the dielectric properties of Cell.-TFSI and Cellulose solid electrolyte films were tested using a dielectric tester. The dielectric constant-frequency spectrum changes with temperature as shown in Fig.10 a, b, 11a, b. In the low frequency band, the dielectric constant of Cellulose electrolyte increases rapidly with the increase of temperature. At 363K, its dielectric constant reaches 10 5 In contrast, the dielectric constant of Cell.-TFSI solid electrolyte increases slowly with increasing temperature. At 373K, the dielectric constant is 10 3 This may be related to the different solvation structures formed in the two electrolytes.

[0092] To further analyze and fit the relationship between molecular segment relaxation or solvation structure relaxation and conductivity of cellulose-based electrolytes, it is necessary to calculate the response behavior of electrolyte dielectric loss, real AC conductivity, imaginary AC conductivity and imaginary dielectric modulus with temperature changes. The calculation results of Cellulose electrolyte are shown in the figure below. Fig.10 c~f. In this electrolyte, as the increase of, a typical high temperature loss peak migration appears ( Fig.10 c) The reason is that after the temperature rises, the solvated structure is more likely to migrate over a long distance, resulting in loss, dipole orientation is more likely to occur, and the dielectric constant is also larger. In order to reduce the influence of DC conductivity and electrode polarization in the medium and low frequency range on the relaxation process of the material, the imaginary dielectric modulus-frequency spectrum is introduced to analyze the dielectric relaxation characteristics of the solvated structure of Cellulose electrolyte. The effect is significant. The AC conductivity in the low frequency region submerges the relaxation polarization process, and the relaxation change is not obvious. Assuming that there is no such influence, the AC conductivity will show obvious relaxation. The dielectric modulus-frequency curve changes with temperature as shown in Fig.10 f, there is obvious relaxation behavior). When the temperature is low (T<323K), the dipole orientation is not obvious, and there are obvious low-frequency relaxation peaks and medium- and high-frequency relaxation peaks. As the temperature increases, the two relaxation peaks gradually couple and move to the high-frequency region. Therefore, we believe that the relaxation of Cellulose electrolyte in the low-frequency and low-temperature section is caused by the interaction structure between the Cellulose chain segments and the undissociated LiTFSI-polar groups in the electrolyte. In the medium-frequency and low-temperature section, it is mainly caused by the solvation structure formed by the cellulose solvation unit and the ions. In the high-temperature region (T>333K), the dynamic behaviors of various dipoles in the Cellulose electrolyte are intensified by thermal activation, and the AC conductivity increases ( Fig.10d,e), the mutual coupling of relaxation behavior is enhanced and gradually turns to thermal motion, and the relaxation peak of the imaginary part of the dielectric modulus gradually turns into a single peak, such as Fig.10 As shown in (f), at 343K, the polarization relaxation of Cellulose electrolyte near 100KHz is already strongly coupled.

[0093] In the dielectric spectrum analysis of Cellulose / LiTFSI electrolyte, the relaxation behavior presented in the low-frequency region is mainly derived from two interacting polarization mechanisms: (1) the self-relaxation polarization of the cellulose molecular chain segment, which is a dielectric response caused by the local movement of the polymer chain; (2) the relaxation polarization of the interaction structure formed between the incompletely dissociated LiTFSI and the cellulose polar groups (mainly hydroxyl groups). The relaxation phenomenon observed in the medium and low frequency regions is closely related to the solvation structure of lithium ions, which indicates that the hydroxyl groups on the cellulose molecular chain do promote the dissociation of lithium salts to a certain extent and participate in the construction of the solvation sheath. However, further analysis shows that this promotion effect has certain limitations. The coordination strength between hydroxyl groups and lithium ions is limited, making it difficult to achieve complete dissociation of lithium salts. The dipole stability of the solvation structure formed is relatively weak. This may be because the rigid structure of the cellulose molecular chain restricts the spatial orientation and freedom of movement of hydroxyl groups, thereby affecting their coordination ability with lithium ions.

[0094] Similarly, the dielectric relaxation properties of Cell.-TFSI solid electrolytes are shown in Fig.11 As shown, there is also an obvious high temperature loss peak migration ( Fig.11 c), but under the same temperature conditions, the frequency of the dielectric loss peak is lower than that of Cellulose electrolyte, indicating that the dipole turning speed in this material is slower. This may be because the trifluoromethanesulfonyl imide group grafted on the modified cellulose acts as a solvation unit and Li + The diameter of the solvated structure is larger than that of the hydroxyl group and Li + The diameter of the solvated structure formed leads to a larger dipole moment length in the former. In the Cell.-TFSI solid electrolyte, the process of AC conductivity being submerged by DC conductivity still exists, and the conductivity relaxation process is still not obvious in the low-frequency region ( Fig.11 d,e). It is worth noting that the imaginary dielectric modulus of Cell.-TFSI electrolyte always has only one polarization relaxation peak, e.g. Fig.11 f, indicating that there is only one type of relaxation in this electrolyte within this frequency range, that is, only the solvation structure is relaxed and -TFSI and Li + The solvation structure type formed is single and stable. However, it is worth noting that at low temperatures (T≤313K), the dielectric modulus relaxation peak has a peak width greater than that of the high temperature section, indicating that -TFSI and Li +The diameter size distribution of the formed solvated structure is a large range. As the temperature rises, the relaxation peak width gradually decreases and migrates to the high-frequency region, indicating that thermal motion can accelerate the unification of the solvated structure size. This is also one of the reasons why polymer solid-state batteries need to be kept warm and activated after assembly. Therefore, a good solvation unit (-TFSI) in the Cell.-TFSI solid electrolyte can effectively promote the complete dissociation of lithium salts and form a solvated structure, which is beneficial to the transport of ions. It can be concluded that in the Cell.-TFSI solid electrolyte, the proportion of relaxation conductivity is extremely low and difficult, while ion hopping conductivity is dominant and easy.

[0095] In summary, the present invention rationally designed and synthesized Cell.-TFSI solid electrolyte with characteristic functional groups, and found that the electrolyte has high stability (physical stability tensile strength: 21.65MPa, electrochemical stability ESW: 5.62V) and high ionic conductivity (1.12×10 -4 S cm -1 ), excellent ion transfer number (0.794). This is mainly due to the structural stability of the cellulose skeleton itself, and the interaction between the trifluoromethanesulfonamide group and Li + A highly consistent and stable solvation structure is formed. In addition, by exploring the ion transport mechanism in Cell.-TFSI electrolyte, it is known that the relaxation conductivity accounts for a very low and difficult proportion, while the ion hopping conductivity is dominant and easy. Therefore, the present invention provides new insights for developing green high-performance solid electrolytes and studying their ion transport mechanisms.

[0096] The embodiments of the present invention are described in detail above, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions and variations of these embodiments are made without departing from the principles and spirit of the present invention, and still fall within the protection scope of the present invention.

Claims

1. A method for preparing a bio-based solid electrolyte, characterized in that: The following steps are involved: S1, dissolving microcrystalline cellulose in DMF to prepare a cellulose solution, then adding an acid binding agent, stirring at room temperature and then bathing in ice water; S2, dilute bromoacetyl bromide with DMF, add it to the cellulose solution in S1, react in an ice-water bath and then react at room temperature; S3, introducing the solution obtained from the reaction in S2 into water to precipitate the reactant, then washing with water, filtering, washing, and drying to constant weight to obtain a light yellow product, cellulose bromide Cell.-Br; S4, dissolving Cell.-Br in DMF, adding an acid binding agent, stirring, and then adding trifluoromethanesulfonamide, stirring continuously, and then precipitating the reactant with water, and then washing with water, and then filtering, washing, and drying to constant weight to obtain a white product trifluoromethanesulfonyl cellulose Cell.-TFSI; S5. Disperse Cell.-TFSI in DMF, add LiTFSI, and form a solid electrolyte film after molding.

2. The method for preparing a bio-based solid electrolyte according to claim 1, characterized in that: S1: The concentration of the cellulose solution is 1-3 wt.%.

3. The method for preparing a bio-based solid electrolyte according to claim 1, characterized in that: The rotation speed of the stirring at room temperature in S1 is 200-1000 rpm, and the stirring time is 8-15 h.

4. The method for preparing a bio-based solid electrolyte according to claim 1, characterized in that: The acid binding agent described in S1 is pyridine, and the acid binding agent described in S4 is diisopropylethylamine.

5. The method for preparing a bio-based solid electrolyte according to claim 1, characterized in that: In S2, every 2 mL of bromoacetyl bromide is diluted with 10-20 mL of DMF, and the volume ratio of bromoacetyl bromide to cellulose solution after dilution is 7-15:90-120.

6. The method for preparing a bio-based solid electrolyte according to claim 1, characterized in that: In S2, the reaction time in an ice-water bath is 1-3 h, and the reaction time at room temperature is 3-5 h.

7. The method for preparing a bio-based solid electrolyte according to claim 1, characterized in that: In S4, the concentration of Cell.-Br in DMF is 1 g / 40-60 mL, and the amount of trifluoromethanesulfonamide used is 11-13 times the molar equivalent based on Cell.-Br.

8. The method for preparing a bio-based solid electrolyte according to claim 1, characterized in that: In S5, the concentration of LiTFSI is 20-30 wt.%.

9. A bio-based solid electrolyte prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the bio-based solid electrolyte according to claim 9 in metal batteries.

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