Accelerated Selective Li+ Transports Assisted by Microcrack‐Free Anionic Network Polymer Membranes for Long Cyclable Lithium Metal Batteries

Abstract Rechargeable Li metal batteries have the potential to meet the demands of high‐energy density batteries for electric vehicles and grid‐energy storage system applications. Achieving this goal, however, requires resolving not only safety concerns and a shortened battery cycle life arising from a combination of undesirable lithium dendrite and solid‐electrolyte interphase formations. Here, a series of microcrack‐free anionic network polymer membranes formed by a facile one‐step click reaction are reported, displaying a high cation conductivity of 3.1 × 10−5 S cm−1 at high temperature, a wide electrochemical stability window up to 5 V, a remarkable resistance to dendrite growth, and outstanding non‐flammability. These enhanced properties are attributed to the presence of tethered borate anions in microcrack‐free membranes, which benefits the acceleration of selective Li+ cations transport as well as suppression of dendrite growth. Ultimately, the microcrack‐free anionic network polymer membranes render Li metal batteries a safe and long‐cyclable energy storage device at high temperatures with a capacity retention of 92.7% and an average coulombic efficiency of 99.867% at 450 cycles.


Material synthesis
General information Syntheses of borate monomer were conducted under a dry Ar atmosphere via standard Schlenk techniques.Anhydrous diethyl ether, tetrahydrofuran (THF), 1,4-dioxane and SH-PEG-SH (Mn=~1k) were purchased from Sigma-Aldrich.Organic solvents were dried with molecular sieves (4 Å) to further remove water trace.5-Hexen-1-ol was purchased from TCI and then store with 4 Å molecular sieves at least 48 h before use.SH-PEG-SH (Mn=~1.5k,2k, 3.4k, 5k) were bought from Aladdin.All other chemicals were purchased from commercial vendors and used as received without further purification.The 1 H, 13 C, and 19 F nuclear magnetic resonance (NMR) spectra were recorded by Bruker AV 400 and 600 MHz spectrometer at room temperature.Samples were dissolved in MeCN-d3, CDCl3   or DMSO-d6.FT-IR spectra were collected using PerkinElmer Spectrum two.
Thermogravimetric analysis (TGA) was carried out at a heating rate of 10 K min −1 in a nitrogen flow (20 mL min −1 ).Differential scanning calorimetry (DSC) was measured by from 4 to 250 °C at a heating rate of 5 °C min −1 under nitrogen atmosphere.X-ray diffraction (XRD) spectra was collected by Rigaku.Strain stress was conducted by Rheometer AERS G2 tensile tester.

Synthesis of 2,3,5,6-tetrafluorobenzyl chloride
In an oven-dried Schlenk flask, 2,3,5,6tetrafluorobenzyl alcohol (10.0 g), tetrabutylammonium chloride (7.8 g), and thionyl chloride (23.0 mL) were added separately.The solution was mixed at 85 °C for 2 hours.After cooling down to room temperature, the flask was placed in a 0 °C ice bath.40 mL of deionized water was added to the flask to quench the reaction.Then, concentrated aqueous Na2CO3 (40 mL) was slowly added to the solution, followed by solid Na2CO3 to adjust the pH to 6.The solution was extracted with 40 mL of diethyl ether four times.The organic layer was collected, washed with 40 mL of brine, and dried with MgSO4.After removing the diethyl ether using rotary evaporation under reduced pressure, a yellow oil was obtained.Finally, the oil was harvested in a flask placed in liquid N2 through vacuum distillation, resulting in a transparent oil.Yield: 8.9 g (86%).

Synthesis
of lithium tetrakis(4-(chloromethyl)-2,3,5,6-tetrafluorophenyl)borate (Monomer) In a dry Schlenk flask, tetrafluorobenzyl chloride (2.01 g, 10.1 mmol) was added using a syringe.The flask was then filled with 100 mL of anhydrous diethyl ether through a cannula.The flask was placed in a dry ice bath at -78 °C.Slowly, 4.93 mL (9.8 mmol) of 2 M n-butyllithium in hexanes was added to the solution using a syringe.After one hour, 1.0 M boron trichloride in heptanes (2.24 mL) was added dropwise using a syringe.The solution was stirred at -78 °C for 2 hours and then allowed to warm up to room temperature as the dry ice evaporated.After 18 hours, the reaction was quenched by adding 30 mL of a 0.1 M aqueous LiCl.The organic layer was collected, washed twice with 30 mL of 0.1 M LiCl aqueous solution, and then dried with MgSO4.The resulting pale yellow oil was concentrated using a rotary evaporator.The oil was transferred to a vial and dissolved in 1 mL of dichloromethane.To purify the solution, it was precipitated in hexane three times.Finally, the trace solvent was removed under vacuum, resulting in a white-yellow solid powder.Yield: 1.3 g (65 %).
And the vial was rinsed with additional anhydrous 1,4-dioxane (1 mL) and the solution was then added into the flask with stirring.The flask was placed into an oil bath at 80 °C while stirring under Ar.After 48 h, the solution was then filtered, and yellow solution was transferred into a vial.After evaporating solvent, the remained yellow product was dissolved in 1 mL of dichloromethane.The solution was purified by precipitation in hexane three times.
yellow solid product was obtained after the removal of trace solvent under vacuum.Finally, the product was dried at 60 °C under vacuum overnight.Yield: 42 mg (42 %).S1] Electrochemical Characterizations

ANP-
Sample preparations All electrolyte membrane were totally dried at 120 °C under vacuum at least 24 h before being transferred to the glove box filled with Ar.Before electrochemical or battery measurements, all membrane should be stored at least 24 h in the glove box.

Ionic Conductivity Measurement
Measuring the ionic conductivity of lithium-ion polymer electrolytes commonly involves using the AC impedance method.This method entails applying a small sine wave of specific amplitude to the system and obtaining the impedance spectrum by varying the frequency.Typically, the Amiral Squidstat Plus is used to investigate the ionic conductivity within an argon-filled glove box.The sample is enclosed within stainless-steel electrodes placed in a Swagelok cell.By applying a 100-mV ac to the Swagelok cell within a frequency range of 1 MHz to 1 Hz, the ionic conductivity of the sample can be calculated using the provided equation.

𝜎 = 𝑙 𝑅𝑆
where l is the sample thickness, S indicates the area of sample, and R refers to the bulk resistance.The Swagelok cell, containing the electrolyte, was assembled and then placed in the Belektronig BTC-LAB-A20 temperature controller for temperature-dependent measurements.Impedance spectra were collected at 28 °C, 38 °C, 48 °C, 58 °C, 68 °C, 78 °C, and 88 °C, with three measurements taken at each temperature.The variable-temperature ionic conductivities were determined using the Arrhenius and Nernst-Einstein equations.
where R is gas constant.Ea and T mean the activation energy and absolute temperature, respectively.

Lithium Transference Number (LTN)
The ability of immobilizing anions is shown by LTN, which is an important factor in single-ion polymer electrolytes.It is ideal for the lithium transference number to be close to one, and this can be achieved through structural and electrochemical methods.Determination of LTN in polymer electrolytes is done using the steady-state current method.Ar-filled glove box was used to build symmetric Li | electrolyte| Li Swagelok cells.After allowing the Swagelok cell to reach equilibrium overnight, the impedance spectrum was recorded at 100 mV ac.Subsequently, a dc voltage of 100 mV was applied and the current response was measured for 2 h.Following the DC polarization, the resistances of the electrolyte and the interface were measured using AC impedance.The LTN value was obtained using the given equation.
in which ∆V means the DC voltage;   0 and    are the bulk resistance before and after applying the voltage;  0 and   indicate the initial current and steady-state current;   0 is the charge transfer resistance before the voltage step while    is the resistance after polarization.

Cyclic voltammetry
To determine the electrochemical working window of polymer electrolytes, a three-electrode Swagelok cell was utilized.The stainless steel served as the working electrode, while the counter electrode and reference electrode consisted of lithium chips.The investigation of the electrochemical stability window involved performing voltage sweeps from -0.5 to 5.5 V at a rate of 0.3 mV s −1 .

Galvanostatic polarization
The interfacial stability of electrolytes was studied by using a Li | electrolyte membrane | Li Swagelok cell charged/discharged at selected current densities (0.1 and 0.2 mA cm −2 ) for 3 h per interval at 60 and 100 °C.

Molecular dynamics (MD) simulation
MD simulations were carried out using Materials Studio 2020, and the Universal force field was used to simulate interatomic interactions because of its flexibility to a broad spectrum of systems such as organic molecules and metal complexes [S2] .The whole borate group and Li + cation were set as one negative charge and one positive charge, respectively.The partial charges of the borate nodes and polyether linkers were calculated by RESP method [S3] implanted in Multiwfn software [S4] .The wave function and the optimized structure for the RESP calculation were generated in wB97M-V/def2-TVZP level by the ORCA package [S5] .
After setting the charges, the simulations were carried out in the isothermal-isobaric condition (with a constant numbers of atoms, constant pressure and constant temperature, NPT).Unless otherwise specified, a time step of 1fs was used for all simulations.The Andersen thermostat [S6] was utilized for temperature control, while the Berendsen barostat [S7] was used for pressure control.Newton's equation was integrated using the Verlet algorithm.Van der Waals interactions were computed using atom-based summation with a cut-off distance of 9.5 Å, and electrostatic interactions were computed using the Ewald summation.
The initial simulation structures were built by Materials visualizer which is embedded inside the software.Each simulation contained 8 Li + atoms and 8 anionic networks.The systems were heated to 343 K for at least 2 ns to simulate the experimental preparation temperature.After the structure shrinks to the experimental density, to achieve ion equilibrium, annealing procedures were employed.The systems were carried out from 298 to 600 K 5 times with each interval of 50 K lasting 50 ps.Simulations of Li transport under applied voltage used potentials of strength between 4 and 8 V nm −1 along the z axis.
Snapshots of the trajectory were recorded every 1 fs.
In order to study the effective of the Li + transportation, the mean square displacement (MSD) method was carried out to investigate the movements of atoms and molecular segments of the systems [S8] .The MSD can be obtained from the position change of particles where τ represents the total production time and r(t) is the position at time t.
The radial distribution function g(r) was calculated as: where nr is the atom number in the spherical shell, ρ is the number density of the whole system and r is the distance.
in unit time in a molecular dynamics (MD) simulation by following equation: