A method for preparing an electrolyte for bio-based solid-state batteries

Abstract

This invention belongs to the field of lithium-ion battery electrolyte technology, specifically relating to a method for preparing an electrolyte for bio-based solid-state batteries. The invention uses microcrystalline cellulose as a raw material, which is modified by bromination followed by nucleophilic substitution to obtain trifluoromethanesulfonated cellulose, which is then used as a green, high-performance, and highly safe battery electrolyte. The bio-based solid-state electrolyte prepared using this method exhibits physical and electrochemical stability, good room-temperature ionic conductivity and lithium-ion transfer number, and the ion migration in the prepared Cell.-TFSI solid-state electrolyte is mainly characterized by ion hopping, with low contribution to relaxation conductivity. This invention demonstrates the rational design of modified cellulose as a solid-state electrolyte, explores the ion transport mechanism in cellulose-based solid-state electrolytes, and provides new insights into the understanding of cellulose ion transport mechanisms, showing great potential for application in high-performance batteries and other energy devices.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery electrolyte technology, specifically relating to a method for preparing an electrolyte for bio-based solid-state batteries. Background Technology

[0002] Energy is a key issue for sustainable development, making the development of environmentally friendly energy materials crucial. With the continued growth of portable electronic devices, electric vehicles, and grid-scale energy storage markets, the demand for energy storage batteries with high charge density, stable output, high charge-discharge cycle life, and robust safety features is constantly increasing. Among various batteries, alkaline ion batteries such as lithium-ion batteries (LIBs) have high energy density and long cycle life, and have been widely used in various industries. However, rocking chair lithium-ion batteries have an energy density of less than 300 Wh·kg⁻¹. -1 The limited energy density of lithium metal batteries restricts their use in high-energy-density industries. Therefore, attention has shifted to developing lithium metal batteries (LMBs) with high energy density and theoretical capacity. Their electrolytes are mostly mixtures of carbonates (EC, DMC, MC, etc.) and lithium salts (LiTFSI, LiPF6, LiBF4, LiFSI, LiClO4, etc.). Unfortunately, due to safety concerns such as the flammability of the electrolyte and dendrite growth, metal batteries are currently mostly in the laboratory research stage. Therefore, it is necessary to develop next-generation safe, reliable, and highly compatible high-energy-density batteries, especially solid-state polymer batteries and their key component, solid-state electrolytes. Solid-state electrolytes, due to their high stability, wide temperature range applicability, and strong designability, have become one of the most promising candidates for realizing next-generation high-energy-density battery electrolytes. In recent years, the performance of solid-state electrolyte materials has been continuously improving, but considering the non-renewable and non-degradable nature of polymer raw materials, it is necessary to use green and friendly natural polymers (such as cellulose) as electrolytes for solid-state batteries. For example, Tian Lei's team used cellulose acetate as a substrate to prepare a quasi-solid-state composite electrolyte with a room-temperature ionic conductivity of 6.17 × 10⁻⁶. -4 S·cm -1 In LFP / Li (lithium iron phosphate / lithium) full cells, the 1C rate (equivalent to a current density of 170 mA·g) -1 It achieved stable cycling for over 1200 cycles (D. Wang, H. Xie, Q. Liu, K. Mu, Z. Song, W. Xu, L. Tian, ​​C. Zhu, J. Xu, Angewandte Chemie International Edition 2023, DOI 10.1002 / anie.202302767.). Furthermore, Hu et al. prepared a solid-state electrolyte using cellulose-coordinated copper to expand ion transport channels, achieving ultrafast lithium-ion transport (1.5 × 10⁻⁶) along the cellulose chain direction at room temperature. -4 S·cm-1 (C. Yang, Q. Wu, W. Xie, X. Zhang, A. Brozena, J. Zheng, M. Ngaraga, B. HKo, Y. Mao, S. He, Y. Gao, P. Wang, M. Tyagi, F. Jiao, R. Briber, P. Albertus, C. Wang, S. Greenbaum, Y. Hu, A. Isogai, M. Winter, K. Xu, Y. Qi, L. Hu, “Copper-coordinated cellulose ion conductors for solid-state batteries,” 2021.). However, it requires mixing with oxide ceramic electrolytes or strictly controlling the orientation of CNFs, increasing production costs and process complexity. Therefore, it is necessary to design and develop novel bio-based solid-state battery electrolytes to promote the development of high-performance batteries and other energy devices. Summary of the Invention

[0003] To overcome the shortcomings of the prior art, this invention uses microcrystalline cellulose (MCC) as raw material, and designs and synthesizes Cell-TFSI solid electrolyte with characteristic functional groups through bromination modification and trifluoromethane sulfonation. This electrolyte has high stability, high ionic conductivity, and excellent ion transfer number, and has great potential for application in high-performance batteries and other energy devices (such as metal batteries).

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

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

[0006] S1. Dissolve microcrystalline cellulose in DMF to prepare a cellulose solution, then add an acid-binding agent, stir at room temperature, and then bathe in an ice water bath.

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

[0008] S3. The solution obtained from reaction S2 is introduced into water to precipitate the reactants. The solution is then washed with water, filtered, washed, and dried to constant weight to obtain the light yellow product Cell.-Br bromide.

[0009] S4. Dissolve Cell.-Br in DMF, add an acid-binding agent, stir, then add trifluoromethanesulfonamide, continue stirring, precipitate the reactants with water, wash with water, filter, wash, and dry to constant weight to obtain the white product trifluoromethanesulfonated cellulose Cell.-TFSI.

[0010] S5. Disperse Cell-TFSI in DMF, then 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 stirring speed at room temperature 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, each 2 mL of bromoacetyl bromide is diluted with 10-20 mL of LDMF, 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 an ice-water bath is 1-3 hours, and the reaction time at room temperature is 3-5 hours.

[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 molar equivalents 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 this invention provides the application of the bio-based solid electrolyte described in the second aspect in metal batteries.

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

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

[0022] Compared with the prior art, the beneficial effects of the present invention are:

[0023] This invention uses MCC as a raw material, which is modified by bromination and then nucleophilically substituted to obtain trifluoromethane sulfonated cellulose, which is then used as a green, high-performance, and highly safe battery electrolyte. The large-radius fluorinated groups grafted onto the cellulose chain can restrict the migration of anions, and simultaneously form a stable and dense SEI after reacting with the electrode, extending battery life. Furthermore, based on the contribution of ions to the electrolyte's conductivity through relaxation processes, dielectric spectroscopy and AC impedance spectroscopy were used to explore the influence of two transport mechanisms—ion hopping and polymer relaxation—on the electrolyte's ionic conductivity. The bio-based solid electrolyte prepared using this method exhibits physical stability (tensile strength: 21.65 MPa) and electrochemical stability (electrochemical stability window ESW: 5.62 V), as well as good room-temperature ionic conductivity (1.12 × 10⁻⁶). -4 S·cm -1 The ion transfer number (0.794) and the ion transfer rate in the prepared Cell.-TFSI solid electrolyte are both high, and the ion migration in the prepared Cell.-TFSI solid electrolyte are mainly ion hopping with low relaxation conductivity contribution. This invention demonstrates the rational design of modified cellulose as a solid electrolyte, explores the ion transport mechanism in cellulose-based solid electrolytes, and provides new insights into the ion transport mechanism of cellulose. It has great potential for application in high-performance batteries and other energy devices (such as metal batteries). Attached Figure Description

[0024] Figure 1 a: Preparation principle of Cell.-TFSI cellulose solid electrolyte; b: Optical photographs 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 at loading rates.

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

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

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

[0028] Figure 5 a: Nyquist spectra of commercial electrolyte membranes (PP) in solid and liquid states, Cellulose, and Cell.-TFSI; b: Current response of PP membranes, Cellulose, and Cell.-TFSI electrolytes to time under 1V voltage and changing current; c: EIS of Cell.-TFSI electrolyte as a function of assembly pressure (50–1000 kgf); d: Bulk resistance R of Cell.-TFSI electrolyte at different pressures. ohm and charge transfer resistance R ct change.

[0029] Figure 6 a: Nyqusitity of Cell-TFSI electrolyte as a function of temperature; b: Ionic conductivity of Cell-TFSI and Cellulose electrolytes as a function of temperature; c: LSV curves of Cell-TFSI, Cellulose, and PP solid electrolytes in Li / SPE / SS semi-blocking cells; d: Current-time curves of Cell-TFSI in Li / SPE / Li symmetric cells at 10 mV, with the inset showing the Nyqusit curves before and after polarization.

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

[0031] Figure 8 Solvation structures of cellulose-based solid electrolytes: a: Spatial model of Cell-TFSI / LiTFSI electrolyte; b: Li + With Cell.-TFSI, TFSI - Radial distribution function and coordination number of ether oxygen (-O-); c: Li+ The radial distribution function and coordination number of F atoms in the electrolyte vary with radius, with fluorine atoms in Cell.-TFSI electrolyte originating from modified groups and TFSI-, and fluorine atoms in Cellulose originating from TFSI-. - ;d:Li + The radial distribution function of N atoms in the electrolyte and the variation of coordination number with radius; e: Li + The radial distribution function of S atoms in the electrolyte and the variation of coordination number 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 oxygen atoms on the cellulose molecular backbone.

[0032] Figure 9 Typical structures extracted from molecular dynamics simulations: a: Li + The coordination structure with Cellulose is shown here, but hydrogen atoms are omitted from the model for a more intuitive observation of the solvated structure; b: Li + Coordination structure with Cell.-TFSI.

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

[0034] Figure 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 Implementation

[0035] The specific embodiments of the present invention will be further described below. It should be noted that these descriptions are for the purpose of aiding understanding the present invention, but do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0036] Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods, and the experimental materials used in the following embodiments are all available through conventional commercial channels.

[0037] Example: Design, preparation, and performance study of a novel electrolyte for bio-based solid-state batteries

[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(trifluoromethanesulfonylimide) (LITFSI), lithium metal discs (D: 16 mm, h: 0.8 mm), commercial electrolyte film (PP, purchased from Selge Technologies, USA, used as a control by immersing in the matching electrolyte during the experiment), dimethyl carbonate.

[0040] 1.2. Test Equipment:

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

[0042] 1.3. Preparation of electrolyte thin films:

[0043] (1) Dissolve 2g of microcrystalline cellulose in 100mL of DMF and stir at 300rpm at room temperature for 10h to obtain a 2wt.% cellulose solution. Add 340uL of dry pyridine to the cellulose solution as an acid-binding agent, stir at room temperature for 2h, and then in an ice-water bath for at least 15min. Dilute 2mL of bromoacetyl bromide to 10mL of pre-frozen (refrigerator temperature -4℃), and then slowly add the diluted bromoacetyl bromide solution to the cellulose solution. React in an ice-water bath for 1h, then at room temperature for 4h, and add an appropriate amount of glycerol to terminate the reaction. Then, introduce the obtained solution into 1L of deionized water to precipitate the reactants, then wash with deionized water, filter, wash 3 times, and dry in a 70℃ oven to constant weight to obtain the light yellow product cellulose bromide (Cell.-Br).

[0044] (2) Dissolve 1g of cellulose bromide in 50mL of DMF and stir at room temperature until completely dispersed. Then add 12 molar equivalents of diisopropylethylamine as an acid-binding agent to the solution. After stirring for 2 hours, add 12 molar equivalents of trifluoromethanesulfonamide. After stirring for 4 hours, slowly pour into 1L of deionized water. After stirring for 15 minutes, filter, rinse, and wash 3 times. Then place in a 70℃ oven to dry to constant weight to obtain the white product trifluoromethanesulfonated cellulose (Cell.-TFSI).

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

[0046] In addition, the entire process of preparing a 1M LiTFSI electrolyte using DMC for impregnating commercial PP diaphragms was carried out in a glove box.

[0047] 1.4. Battery Assembly: The solid electrolyte film prepared above is cut into circular pieces with a diameter of 19 mm. They are then assembled in an argon glove box into blocking batteries (stainless steel / electrolyte / stainless steel), semi-blocking batteries (lithium / electrolyte / stainless steel), and symmetric batteries (lithium / electrolyte / lithium) of the CR-2032 or CR-2016 type. The assembly pressure is not less than 5 MPa, and the holding time is 1 hour. The assembled button batteries are then placed in a 60°C oven for 24 hours to ensure that the electrolyte and electrode materials are in full contact and form a complete ion pathway.

[0048] 1.5. Test Method: Fourier Transform Infrared Spectroscopy (FTIR): The prepared solid electrolyte sample was dried to remove excess water or solvent, mixed and ground with KBr (mass ratio 0.5–1.0 wt.%), pressed into a film, and then placed in the instrument for testing. The wavelength range was 600–4000 cm⁻¹. -1 .

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

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

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

[0052] High-Temperature Dielectric Spectroscopy: Ion transport in polymer electrolytes includes ion hopping and polymer chain relaxation-driven ion transport. However, the dominant mechanism and its specific contribution rate require further investigation. Therefore, we investigated the ion transport mechanism in Cell-TFSI electrolytes based on dielectric spectroscopy and electrochemical impedance spectroscopy measurements. In the high-frequency range, ions move within space, and their power-law behavior represents AC conductivity. In the mid-frequency range, ions begin to drift away from the surrounding shielding groups (Coulomb cages), contributing to DC conductivity by overcoming the shielding effect. In the low-frequency range, corresponding to polarization effects, ions drifting to the electrode will not migrate further. According to the stochastic barrier model (RBM), ions randomly hop in the space market, significantly influencing AC conductivity. Once the maximum energy barrier is overcome, ions drift and directly contribute to DC conductivity. The ion relaxation time (τ) is... RBM The relationship between ) and conductivity satisfies the following (corresponding reference: JCDyre, PhysLett 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, Let σ0 be the imaginary part of the AC conductivity in the dielectric spectrum, σ0 be the critical DC conductivity in the dielectric spectrum, a constant independent of frequency, and v be the frequency, τ0 be the frequency. RBMLet π represent the relaxation time of ions in the stochastic barrier model, i be an imaginary number, and π be 3.142. Therefore, we use the above equation to describe the conductivity and polarization relaxation process of aggregates formed after the interaction of polar groups and ions in polymer electrolytes. This invention explores the electrolyte ion transport mechanism by using the dielectric properties and frequency spectra of the electrolyte 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 testing process and parameters are as follows: Before testing, the prepared solid electrolyte film sample is cut into a circular sheet with a diameter of 10 mm, and circular conductive adhesive (6 mm in diameter) is attached to both sides. The film thickness is measured using an electronic micrometer. The dielectric test system was used to perform tests within a frequency range of 20 Hz to 10 MHz and a bias voltage of 0.5 V. The test temperature range was 26℃, 30℃, 40℃, 50℃, 60℃, 70℃, 80℃, 90℃, and 100℃ (299K, 303K, 313K, 323K, 333K, 343K, 353K, 363K, and 373K), with a heating rate of 2℃·min. -1 Measurements were taken after each temperature point was kept at that temperature for 30 minutes.

[0055] Electrochemical performance characterization: Electrochemical resistance spectroscopy (EIS), linear voltammetry (LSV), cyclic voltammetry (CV), and constant voltage chronoamperometry (CA) were all performed on the Princeton Electrochemical Integrated Test System. Electrolyte temperature and pressure testing was performed using an electrolyte constant temperature and pressure testing device from Ionic New Energy Technology in conjunction with the electrochemical integrated test system.

[0056] Ionic conductivity: The ionic conductivity of polymer electrolytes was tested under the following conditions: the measurement frequency range was 1 MHz to 0.1 Hz, the amplitude was 10 mV, and the temperature range was 23℃ to 80℃. Each temperature point was maintained for 1 hour, and each pressure point was pressurized and held for 1 hour before measurement. Blocked batteries were used as the test subjects. Electrochemical impedance spectroscopy (EIS) was performed, and the ionic conductivity was calculated using the following formula (corresponding reference: S.-A. Safavi-Mirmahalleh, S.N. Eliseeva, ARMoghaddam, H. Roghani-Mamaqani, M. Salami-Kalajahi, ACS ApplEnergy Mater 2023, 6, 9624):

[0057]

[0058] Where R is the electrolyte resistance, fitted from EIS test results, in ohms (Ω); L is the electrolyte film thickness, in centimeters (cm); and S is the effective contact area of ​​the electrolyte, in square centimeters (cm²).2 ).

[0059] Electronic conductivity: Using the constant voltage chronoamperometry (CA) method, a control voltage of 1V was applied across the electrolyte blocking electrode, and the response current versus time curve was recorded. When the current reached a steady state, ion migration was restricted by the blocking electrode, and only electrons were available for circuit transport. Analyzing the time-current curve, the current at the last moment was selected as the steady-state current I, and its electronic conductivity was calculated using the following formula:

[0060]

[0061] Where U is the control potential in volts (V), I is the steady-state current in amperes (A), L is the electrolyte film thickness in centimeters (cm), R is the measured resistance in ohms (Ω), and S is the effective contact area of ​​the electrolyte in square centimeters (cm²). 2 ).

[0062] Electrochemical stability window: The stability of the electrolyte in a semi-blocked battery was evaluated using the LSV (Laser-Scanning-Volume) method. The scan rate was 1 mV·s. -1 The scanning range is: the battery open-circuit voltage is 6V.

[0063] Lithium-ion transfer number: The lithium-ion transfer number (t) of the electrolyte film was measured using a combination of chronoamperometry and AC impedance spectroscopy. + A symmetrical battery (lithium / electrolyte / lithium) is set with a certain bias potential ΔU (10mV), and the response current is recorded as a function of time. Once the current reaches a steady state, it is assumed that the current contribution is entirely from cations, and the contribution from anions in the circuit is zero. The steady-state current I is then recorded. s By analyzing the EIS spectra of the initial and steady states, the impedance of the passivation layer in the initial state was obtained by fitting the circuit. and the impedance of the steady-state passivation layer The current at the start 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 number of lithium ions transferred can then be calculated.

[0064]

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

[0066] 1.6. Calculation method:

[0067] In this work, the DFT calculations used employed the generalized gradient approximation (GGA) and the Perdew-Burke-Ernzerhof (PBE) functional to determine the exchange correlation energies of interacting electrons, and were implemented in the DMol3 module of Materials Studio.2020 software. After geometric optimization of the molecular model, point charge, charge density, and electrostatic potential distribution were simulated using a Cell.-TFSI substitution degree of 1 molecular model. The adsorption energies of lithium ions with specific groups were investigated using DFT calculations, and the following formulas were used for calculation (corresponding to the references):

[0068]

[0069] Where E Abs ., E Base , They are Li + Electrolytes in Cell.-TFSI or Cellulose and substrate materials Cell.-TFSI or Cellulose and Li + 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 functional groups. A simulation system was constructed using 5 LiTFSI, 35 Cellulose, or Cell.-TFSI disaccharide molecules (calculated dielectric properties of cellulose were closest to experimental results, with a difference of only 3.1% between the simulated and experimental dielectric properties). The molecules were initially randomly packaged in a cube with dimensions of length (x), width (y), and height (z). The dimensions of the simulation chamber after isothermal and isobaric pre-equilibration are: Cellulose electrolyte Cell.-TFSI electrolyte is Simultaneously, a condensed phase optimized molecular potential II (COMPASS II) force field was assigned to all atomic simulation studies. This force field can handle systems containing both organic and inorganic molecules, accurately simulating the structure and properties of condensed matter and meeting the requirements of the electrolyte system in the experiment. Both cellulose-based electrolyte systems were initially stabilized for 1000 steps using the algorithm. The models were first geometrically optimized, pre-equilibrated for 1 ns in an isothermal-isobaric ensemble (NPT, 333 K, 1 atm), and then equilibrated for 1 ns in a canonical ensemble (NVT). All simulation time steps (NPT or NVT) were 1 fs. Based on the motion trajectory file (in .xtd format) output after 1 ns NVT equilibration, the atomic (group) interactions were analyzed using the atomic radial distribution function (RDF) (all data analysis ranges were selected within the density and energy stability range: 101–1000 steps).

[0071] 2. Results and Discussion

[0072] 2.1.Material properties:

[0073] (1) Reaction mechanism and thin film properties

[0074] The substrate material for Cell.-TFSI electrolytes is obtained by uniformly dispersing microcrystalline cellulose in DMF solvent, brominating it, and then substituting bromine atoms with a nucleophilic reagent. The reaction mechanism is as follows: Figure 1 As shown in Figure a, the yield of this preparation process is relatively high, at 84.7%. The solid electrolyte film formed after adding LiTFSI is stable under normal conditions and is a semi-transparent film, exhibiting slight deformation after folding. Figure 1 b). Low-load tensile tests were performed on PP / LiTFSI and Cell.-TFSI / LiTFSI using a general-purpose testing machine. The tensile test results were obtained at 1 mm·min⁻¹. -1 Test results at the loading rate are as follows Figure 1 As shown in c, the tensile strength of the Cell.-TFSI electrolyte is 21.65 MPa, the elastic modulus is 461.76 MPa, and the elongation at break is 8.067%, which is 3.15 MPa higher than that of electrolytes composed of commercial PP separators. This indicates that the Cell.-TFSI solid electrolyte can withstand the large volume change effect of lithium metal and suppress dendrite growth.

[0075] (2) Phase and morphology

[0076] Before investigating the physical stability and microstructure of Cell-TFSI / LITFSI solid electrolytes, the dispersibility and dissociation ability of different lithium salt concentrations in the Cell-TFSI substrate were examined. The electrochemical impedance spectroscopy and room-temperature ionic conductivity of Cell-TFSI electrolytes with different lithium salt concentrations are shown below. Figure 2As shown, when the LiTFSI concentration is less than 25 wt.%, the ionic conductivity gradually increases with increasing lithium salt concentration, indicating that when the lithium salt concentration is less than 25 wt.%, the Cell-TFSI substrate has sufficient solvation units (polar groups) to promote lithium salt dissociation. When the lithium salt concentration is 25 wt.%, the room temperature ionic conductivity reaches its maximum value of 1.12 × 10⁻⁶. -4 S·cm -1 However, as the lithium salt concentration increases, the ionic conductivity gradually decreases. A possible reason is that once Cell-TFSI's ability to dissociate LiTFSI reaches a threshold, subsequently incorporated lithium salts cannot dissociate and instead aggregate into crystals, failing to participate in the Li-ionization process. + The precipitation of these undissociated molecules or crystals affects the originally formed solvated structure, hindering ion transport channels and reducing ionic conductivity. Therefore, in subsequent studies on the electrochemical performance of Cell-TFSI, the lithium salt concentration in the preparation of Cell-TFSI electrolytes was always 25 wt.%.

[0077] In this study, the structures of the synthesized target product Cell.-TFSI and intermediate product (Cell.-TFSI) were characterized. Figure 3 As shown in figure a, the structure of Cell.-TFSI was characterized by FTIR. Cell.-Br and Cell.-TFSI were etched at 1748 cm⁻¹. -1 The cellulose sample exhibits a distinct -C=O stretching vibration absorption peak, but after the reaction, the -OH absorption peak decreases significantly (to 2450 cm⁻¹). -1 Near the bromination site, only a few absorption peaks remain, due to the large-scale substitution of cellulose hydroxyl groups after bromination. Cell.-TFSI shows an absorption peak at 3466.8 cm⁻¹. -1 An -NH- absorption peak appeared at [location missing]. To further verify the structure of Cell.-TFSI, the sample was further characterized by XPS. The results are as follows... Figure 3 As shown in c, d, e, f, where Figure 3 c and d are the C1s and Br3d spectra of Cell.-Br. Figure 3e and f are the C1s and F1s spectra from Cell.-TFSI. All spectra were peak-calibrated based on the signal from the indeterminate carbon. The C1s in Cell.-Br was fitted and assigned to the indeterminate carbon (CC, reference value 284.8 eV), the carbon-oxygen bond (-CO-, reference value 287.5 eV), and the carbonyl group at the substitution site (-C=O, reference value 288.6 eV). The Br3d orbital has an overlapping split peak near 68.7 eV, ΔE = 1.03 eV, with an intensity ratio of 0.67. In Cell.-TFSI, the signal of the indeterminate carbon in C1s is 284.7 eV, which is assigned to the carbon-nitrogen bond at the trifluoromethane sulfonamide reaction site (286.2 eV) and the carbonyl group at the bromination reaction site (-C=O, 288.3 eV). The signal peak of F1s appears at 687.5 eV, which is 0.5 eV shifted from the organic fluorine reference value of 688-689 eV, which is within a reasonable range.

[0078] In addition, the XRD patterns of dried films (PP / LiTFSI), Cellulose, Cell.-TFSI, and Cell.-TFSI / LiTFSI solid electrolytes after solvent evaporation following impregnation of commercial electrolyte membranes (PP) were analyzed. Figure 3 (b) Due to the regeneration of cellulose after modification and the grafting of trifluoromethanesulfonamide groups onto its surface, the hydrogen bonds between Cell.-TFSI molecular chains are significantly reduced, making them more amorphous. This will facilitate the transport of lithium ions along the polymer chains. The modified polar groups promote the dissociation of lithium salts. No obvious LiTFSI crystallization peak is observed in Cell.-TFSI / TFSI, indicating that lithium salts in the Cell.-TFSI solid electrolyte exhibit good dispersion and dissociation.

[0079] (3) Adsorption energy and electronic structure calculation

[0080] This study, through density functional theory (DFT) calculations and frontier orbital theory analysis, revealed significant differences in lithium-ion adsorption properties and interfacial electrochemical behavior between cellulose backbone grafted with trifluoromethanesulfonylimide (-TFSI) and hydroxyl groups (-OH). The established cellulose molecular model simulates cellulosic disaccharides, and the model used in the calculations is as follows: Figure 4 As shown in figures a through d. DFT calculations show that the adsorption energy (ΔE) of -TFSI for lithium ions is... Abs. = -7.167 eV) is significantly lower than the adsorption energy of lithium ions by hydroxyl groups (ΔE). Abs. =-4.457eV), such as Figure 4 As shown in e. This phenomenon originates from the cooperative coordination effect of multiple active sites in the TFSI molecule, where the lone pair electrons of the oxygen atom of the sulfonyl group (-SO2-) interact with Li. +Strong coordination bonds are formed, and the strong electron-withdrawing effect of trifluoromethane (-CF3) enhances the electron cloud density of the sulfonyl group through inductive interaction, thus creating a dynamic interaction. Furthermore, the three-dimensional spatial configuration of the TFSI group provides multiple coordination sites (OSO triangular plane and F atom working synergistically), which is more conducive to the formation of a stable polydentate coordination structure compared to the single coordination mode of the hydroxyl group. This multiple coordination mechanism not only allows Cell.-TFSI to exhibit superior lithium-ion adsorption but may also lower the energy barrier for decoupling during lithium-ion migration through flexible conformational adjustments.

[0081] Frontier orbital theory calculations reveal a selective tendency in the interfacial reactions: Cell-TFSI has the lowest LUMO level (-1.673 eV), while LiTFSI has a LUMO level of -1.320 eV. Figure 4 f) The HOMO energy level of Cell.-TFSI (-6.156 eV) is higher than that of LiTFSI in the electrolyte (-7.125 eV), which will lead to preferential reactions at the anode interface. The lower ester bond (-COO-) cleavage energy (1.28 eV, main chain CO bond cleavage energy is 3.15 eV) in the modified group promotes the preferential exposure of the terminal -SO2CF3 group and its reaction with metallic lithium, which may generate inorganic phases containing LiF (XPS F 1s peak 683.3 eV), Li2S and organic phases containing fluorocarbons, forming a gradient SEI layer and extending battery life. The chemical modification of the cellulose molecular structure can effectively regulate the thermodynamic binding strength and transport kinetics of lithium ions, and at the same time optimize the interfacial reaction path through energy level matching, providing an important theoretical basis for the design of novel cellulose solid electrolytes.

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

[0083] To compare the electrochemical performance of different electrolytes and the optimal battery assembly pressure, the AC impedance spectra and current-time response changes of commercially available separators impregnated with liquid electrolyte (PP / LiTFSI, Liquid), commercially available electrolyte separators with solvent evaporation after impregnation with liquid electrolyte (PP / LiTFSI), cellulose electrolyte (Cellulose / LiTFSI), and trifluoromethane sulfonamide cellulose electrolyte (Cell.-TFSI) were investigated. The results are shown in Table 1. Figure 5 As shown in a and b, at room temperature (26°C), the modified groups in Cell.-TFSI can promote the dissociation of lithium salts and act as transport sites, with an ionic conductivity as high as 1.12 × 10⁻⁶. -4 S·cm -1The conductivity of PP electrolyte is approximately 1 / 25 that of liquid electrolyte. Since the PP membrane primarily functions to support and absorb the electrolyte, its contribution to ion conduction is very weak. Therefore, under the same conditions, the ionic conductivity of the liquid electrolyte (after solvent evaporation) in the PP membrane is much lower. Similarly, the ionic conductivity of cellulose electrolyte is about 5 times lower than that of Cell.-TFSI electrolyte, possibly due to the limited dissociation of lithium salts by hydroxyl groups, resulting in undissociated lithium salts in the cellulose electrolyte. Constant voltage current-time tests show that all three types of electrolytes have very low electronic conductivity, making them suitable as battery separators without short circuits. Furthermore, to explore the optimal assembly pressure of the electrolytes, different pressures (50–1000 kgf) were applied to the Cell.-TFSI electrolyte to simulate its assembly pressure as a battery separator. Its AC impedance was recorded, and its charge transfer resistance and membrane bulk resistance were analyzed. The test results are as follows: Figure 5 As shown in c and d, the volume resistance (R) of the Cell-TFSI electrolyte increases with increasing applied pressure. ohm The resistance initially decreases slightly, then remains stable. This is because when pressure is initially applied, the increased pressure promotes contact between the electrolyte and electrode interface, reducing the interfacial resistance. However, as the pressure continues to increase, the interfacial contact stabilizes, resulting in only a slight change in interfacial resistance. It is worth noting that as the applied pressure increases, the electrolyte charge transfer resistance (Rc) increases. ct The resistance is constantly increasing. Since charge transfer resistance is related to the heterogeneous electrochemical reactions of the system, applying pressure causes polymer chain sliding and deformation, increasing the proportion of heterogeneous components in the electrolyte system. Therefore, during battery assembly, higher pressure is not always better; rather, pressure should be minimized while ensuring good interfacial contact. Based on the changes in applied pressure and impedance, a reference range for Cell-TFSI electrolyte assembly pressure has been calculated: 25–31 MPa.

[0084] The AC impedance of the Cell.-TFSI solid electrolyte changes with temperature as follows: Figure 6 As shown in a and b, by analyzing and calculating the ionic conductivity and the change of ionic conductivity of Cellulose / LiTFSI electrolyte with temperature, based on the Arrhenius equation, the preliminary calculation shows that the ion transport activation energy of Cell-TFSI / LiTFS is 57.43 kJ·mol⁻¹. -1 The activation energy is 22.97 kJ·mol lower than that of Cellulose / LiTFSI. -1 The activation energy for ion transport in the Cellulose / LiTFSI electrolyte is 80.40 kJ·mol⁻¹. -1 On the one hand, the modified trifluoromethane group promotes the dissociation of lithium salt, providing more charge carriers for electrolyte ion migration. On the other hand, the trifluoromethane sulfonamide group interacts with L... i+It exhibits lower adsorption energy, enabling rapid ion transport. The electrochemical stability of the electrolyte was studied using linear voltammetry (LSV), and the results are as follows: Figure 6 As shown in Figure c, the Cell.-TFSI electrolyte boasts a high electrochemical stability window (ESW) of 5.62V, making it compatible with most electrode materials, allowing for higher operating voltages, increasing battery energy density, and enhancing cycle stability. In contrast, the electrochemical stability of PP electrolytes is limited by the oxidative stability of carbonates in the electrolyte, with an ESW of only 4.45V. Exceeding this voltage poses certain safety risks. Notably, the diameter of the trifluoromethanesulfonamide groups grafted onto cellulose differs from that of the TFSI groups dissociated from LiTFSI. - With diameters on the same order of magnitude, the migration of anions can be effectively restricted, while the transport of cations can be promoted, thus increasing the ion transfer number (t) of Cell-TFSI electrolytes. + Up to 0.794 ( Figure 6 d) This helps reduce concentration polarization at the interface and improve the power density and charge / discharge efficiency of the battery.

[0085] To investigate the contact performance of the prepared Cell.-TFSI / LiTFSI solid electrolyte with lithium metal electrodes and its adaptability to electrode volume changes, Li / Cellulose / Li and Li / Cell.-TFSI / Li symmetric cells were assembled for comparison, and long-term lithium plating / stripping cycle tests were conducted. The cycle results of the symmetric cell using Cellulose electrolyte are as follows: Figure 7 As shown in figures a through c, during the first 250 hours of cycling, with a constant current density, the voltage signal showed significant changes, indicating that during this cycling phase, the electrolyte and lithium metal had a hard contact conduction with limited contact, resulting in poor contact stability and voltage instability. The solid electrolyte interphase (SEI) had not yet fully formed. At 707 hours, the symmetrical cell with the Cellulose electrolyte experienced a short circuit, and the cell resistance decreased sharply, as shown in the figure. Figure 7 As shown in c. Uncontrolled dendrite growth leads to interface instability, causing electrolyte perforation and short circuits, resulting in battery failure. This may be due to insufficient F in the lithium salt to react and form a stable and dense SEI layer. In contrast, the Cell.-TFSI / LiTFSI solid electrolyte exhibits a lower polarization voltage, stabilizing around a voltage plateau of 0.09V, likely due to its lower electrolyte resistance. Fluctuations in the voltage plateau during the initial cycling period (within 40 hours) (7 days) indicate that the SEI layer was not fully formed during this stage. Subsequently, the Cell.-TFSI battery maintains a stable voltage plateau during cycling, even after 950 hours, and eventually cycles for over 1000 hours. Figure 7(d~e). The ability to achieve stable lithium plating / stripping cycles over a long period of time is due to the composition of the Cell.-TFSI electrolyte promoting the formation of a fluorine-rich SEI layer and the inherent strength of the electrolyte itself hindering further dendrite growth, indicating that Cell.-TFSI, as a solid electrolyte substrate material, can adapt to changes in electrode volume.

[0086] Table 1. Ionic conductivity parameters of different samples at room temperature (26℃)

[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 an atom to the average density around the central lithium ion at a distance r. Here, based on molecular dynamics simulations using the Forcite module in the MS software package, we analyzed the RDFs between lithium ions and various components in Cellulose / LiTFSI and Cell.-TFSI / LiTFSI electrolytes to elucidate the solvated sheath structure in cellulose-based electrolytes and its influence on ion migration. Figure 8 For simulation detail models and Li + For RDFs with other atoms, the Cellulose, Cell.-TFSI, and LiTFSI models used are the models established and geometrically optimized in section 1.6, where... Figure 8 a represents the constructed spatial model structure. From 8b, we can see that the Cell.-TFSI molecule and the TFSI- group... Nearby and Li + Coordination occurs, with the radial distribution function g(r) peak of TFSI- being higher than that of Cell.-TFSI, indicating a stronger interaction than that of Cell.-TFSI, classifying it as a strong coordination. However, it is worth noting that Li... + With Cell.-TFSI molecules and TFSI-coordination number in All values ​​are less than 1 within the specified range. Furthermore, in the Cell-TFSI electrolyte system, -O- exists only within the Cell-TFSI molecule, and its interaction with Li+ occurs in the middle layer of the solvation structure. The coordination number is similar to the variation in Cell.-TFSI.

[0090] In Cell-TFSI / LiTFSI electrolyte, Li + The innermost layer of the solvated sheath consists of non-ether oxygen atoms (such as oxygen atoms in hydroxyl, carbonyl, and sulfonyl groups), located at... The number of left and right coordination sites is 1, which is consistent with the oxygen atom positions reported in the literature [corresponding reference: Generation of a highly conductive and stable solid electrolyte interphase atlithium anode under additional electric filed Author links open overlay, Chemical Engineering Journal Volume 446, 137435]. In the vicinity, the coordination number of the oxygen atom rapidly increases to 3, and further increases with increasing distance. Coordination with F, N, and S atoms begins to appear. Nearby, in The coordination number in the range all reach 1, such as Figure 8 As shown in d and e. In contrast, Li in the Cellulose / LiTFSI electrolyte + The solvation structure formed with surrounding groups is relatively simple; the inner layer is the same as Cell-TFSI, consisting of O atoms. Within a range of 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-shell coordinated O atom in Cell.-TFSI is lower than that in Cellulose, indicating that the former and Li + The coordination between the two layers is weak, which is why Cell-TFSI electrolytes have high ionic conductivity. The weakly coordinated solvated inner layer structure results in a lower activation energy for ion hopping-coupling migration, leading to lower electrolyte resistance.

[0091] The lithium-ion solvation structure of the cellulose-based electrolyte was extracted from the trajectory file, such as... Figure 9 As shown, hydrogen atoms are omitted. Figure 9 The inner layer of the Li-Cellulsoe structure formed by a and b is a hydroxyl oxygen, while the inner layer of the Li-Cell.-TFSI structure contains carbonyl oxygen, oxygen from a sulfonyl group, and fluorine atoms. Both structures belong to solvation-separated ion pairs, which, compared to contact ion pairs, will improve ion migration efficiency. A small amount of contact ion pairs in the electrolyte can promote the contact electrolysis reaction of fluoride-containing anions, forming HF, which then reacts with carbonates in the SEI layer to produce LiF. However, the Li-Cell.-TFSI structure contains F atoms, which may also form a fluoride-containing SEI layer at the interface during ion migration. The brominated -O- in Cell.-TFSI contributes relatively little to the formation of the solvation structure. Figure 9b. The reason is that the steric hindrance is relatively large, and the carbonyl group and the trifluoromethane group at the end of the branch first react with Li. + Function. Before using BRM fitting to 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 constant analyzer. The dielectric constant-frequency spectrum changes with temperature as follows: Figure 10 As shown in Figures a, b, and 11a, b, in the low-frequency range, the dielectric constant of the Cellulose electrolyte increases rapidly with increasing temperature, reaching 10 at 363 K. 5 In contrast, the dielectric constant of Cell-TFSI solid electrolyte increases more slowly with increasing temperature, reaching 10⁻⁶ at 373 K. 3 The quantity may be related to the different solvation structures formed in the two electrolytes.

[0092] Further analysis and fitting of the relationship between molecular chain segment relaxation or solvation structure relaxation and conductivity of cellulose-based electrolytes requires calculating the electrolyte dielectric loss, real part AC conductivity, imaginary part AC conductivity, and the response behavior of the imaginary part dielectric modulus as a function of temperature. The calculation results for Cellulose electrolytes are as follows: Figure 10 As shown in c~f. In this electrolyte, with increasing , a typical high-temperature loss peak shifts ( ). Figure 10 c) This is because, with increasing temperature, the solvated structure is more prone to long-range migration, leading to losses, easier dipole orientation, and a higher dielectric constant. To reduce the influence of DC conductivity and electrode polarization on the material relaxation process in the low-to-mid-frequency range, introducing the imaginary part dielectric modulus-frequency spectrum to analyze the dielectric relaxation characteristics of the Cellulose electrolyte solvated structure is highly effective. The AC conductivity in the low-frequency region submerges the relaxation polarization process, and the relaxation change is not significant. If this influence were not present, the AC conductivity would exhibit significant relaxation. The change in dielectric modulus-frequency curve with temperature is as follows... Figure 10 As shown in f, obvious relaxation behavior occurs. At lower temperatures (T < 323 K), dipole orientation is not obvious, and distinct low-frequency and mid-to-high-frequency relaxation peaks appear. As the temperature increases, the two relaxation peaks gradually couple and shift towards the high-frequency region. Therefore, we believe that the relaxation of Cellulose electrolyte in the low-frequency, low-temperature range is caused by the interaction structure between Cellulose segments and undissociated LiTFSI- polar groups in the electrolyte, while in the mid-frequency, low-temperature range, it is mainly caused by the solvation structure formed by cellulose solvation units and ions. In the high-temperature region (T > 333 K), the dynamic behavior of various dipoles in Cellulose electrolyte is thermally activated and intensified, increasing the AC conductivity. Figure 10d,e), the mutual coupling effect of relaxation behavior is enhanced, gradually transforming into thermal motion, and the relaxation peak of the imaginary part of the dielectric modulus gradually transforms into a single peak, such as Figure 10 As shown in f, at 343 K, the Cellulose electrolyte exhibits strong coupling due to polarization relaxation around 100 kHz.

[0093] In the dielectric spectroscopy analysis of Cellulose / LiTFSI electrolyte, the relaxation behavior in the low-frequency region mainly stems from two interaction polarization mechanisms: (1) self-relaxation polarization of cellulose molecular chain segments, which is a dielectric response caused by the local movement of polymer chains; (2) relaxation polarization of the interaction structure formed between incompletely dissociated LiTFSI and cellulose polar groups (mainly hydroxyl groups). The relaxation phenomenon observed in the mid-to-low frequency region is closely related to the solvation structure of lithium ions, indicating that the hydroxyl groups on the cellulose molecular chain do indeed promote the dissociation of lithium salt to a certain extent and participate in the construction of the solvation sheath. However, further analysis reveals that this promoting effect has certain limitations. The coordination interaction between hydroxyl groups and lithium ions is limited, making it difficult to achieve complete dissociation of lithium salt. The dipole stability of the formed solvation structure 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 characteristics of Cell-TFSI solid electrolytes are as follows: Figure 11 As shown, a significant shift in the high-temperature loss peak also occurs. Figure 11 c) However, under the same temperature conditions, the frequency of the dielectric loss peak is lower than that of the Cellulose electrolyte, indicating that the dipole turning speed in this material is slower. This may be because the trifluoromethanesulfonylimide groups grafted onto the modified cellulose act as solvation units and react with Li. + The diameter of the formed solvated structure is larger than that of the hydroxyl group and Li. + The diameter of the formed solvated structure results in a larger dipole moment length in the former. In Cell-TFSI solid electrolytes, the process of AC conductivity being submerged by DC conductivity still exists, and the conductivity relaxation process remains insignificant in the low-frequency region. Figure 11 d,e). It is worth noting that the imaginary dielectric modulus of the Cell-TFSI electrolyte always exhibits only one polarization relaxation peak, such as Figure 11 f indicates that there is only one type of relaxation in this electrolyte within this frequency range, namely, only the solvation structure relaxation, and -TFSI and Li + The formed solvated structure is of a single and stable type. However, it is noteworthy that at lower temperatures (T≤313K), the dielectric modulus relaxation peak has a wider peak width than in the high-temperature range, indicating that -TFSI reacts with Li... +The diameter distribution of the formed solvated structures spans a wide range. As temperature increases, the peak width of the relaxation peak gradually decreases and migrates to higher frequencies, indicating that thermal motion can accelerate the uniformity of solvated structure size. This is one reason why polymer solid-state batteries require thermal activation after assembly. Therefore, a well-developed solvation unit (-TFSI) in the Cell.-TFSI solid electrolyte can effectively promote the complete dissociation of lithium salt and the formation of solvated structures, which is beneficial for ion transport. It can be concluded that in the Cell.-TFSI solid electrolyte, relaxation conductivity is extremely low and difficult to achieve, while ion hopping conductivity is dominant and easily achieved.

[0095] In summary, this invention has successfully designed and synthesized a Cell-TFSI solid electrolyte with specific functional groups, and found that this electrolyte exhibits high stability (physical stability tensile strength: 21.65 MPa, electrochemical stability ESW: 5.62 V) and high ionic conductivity (1.12 × 10⁻⁶ V). -4 S·cm -1 It exhibits an excellent ion transfer number (0.794). This is primarily due to the structural stability of the cellulose backbone itself, and the interaction between the trifluoromethanesulfonamide group and Li. + A highly consistent and stable solvation structure was formed. Furthermore, exploration of the ion transport mechanism in the Cell-TFSI electrolyte revealed that relaxivity is extremely low and difficult to achieve, while ion hopping conductivity is dominant and readily obtained. Therefore, this invention provides new insights into the development of green, high-performance solid-state electrolytes and the study of their ion transport mechanisms.

[0096] The embodiments of the present invention have been 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 can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. A method for preparing a bio-based solid electrolyte, characterized in that, Includes the following steps: S1. Dissolve microcrystalline cellulose in DMF to prepare a cellulose solution, then add an acid-binding agent, stir at room temperature, and then bathe in an ice water bath. S2. After diluting bromoacetyl bromide with DMF, add it to the cellulose solution of S1, react in an ice-water bath, and then react at room temperature. S3. The solution obtained from reaction S2 is introduced into water to precipitate the reactants. The solution is then washed with water, filtered, washed, and dried to constant weight to obtain the light yellow product Cell.-Br bromide. S4. Dissolve Cell.-Br in DMF, add an acid-binding agent, stir, then add trifluoromethanesulfonamide, continue stirring, precipitate the reactants with water, wash with water, filter, wash, and dry to constant weight to obtain the white product trifluoromethanesulfonated cellulose Cell.-TFSI. S5. Disperse Cell-TFSI in DMF, then 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, The concentration of the cellulose solution in S1 is 1-3 wt.%.

3. The method for preparing a bio-based solid electrolyte according to claim 1, characterized in that, The stirring speed at room temperature described 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 in S1 is pyridine, and the acid-binding agent 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 the diluted bromoacetyl bromide to the cellulose solution 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 hours, and the reaction time at room temperature is 3-5 hours.

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 molar equivalents 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 any one of claims 1-8.

10. The application of the bio-based solid electrolyte of claim 9 in a metal battery.

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