Self‐Enhancing Gel Polymer Electrolyte by In Situ Construction for Enabling Safe Lithium Metal Battery

Abstract Lithium metal battery (LMB) possessing a high theoretical capacity is a promising candidate of advanced energy storage devices. However, its safety and stability are challenged by lithium dendrites and the leakage of liquid electrolyte. Here, a self‐enhancing gel polymer electrolyte (GPE) is created by in situ polymerizing 1,3‐dioxolane (DOL) in the nanofibrous skeleton for enabling safe LMB. The nanofiber membrane possesses a better affinity with poly‐DOL (PDOL) than commercial separator for constructing homogeneous GPE with enhanced ion conductivity. Furthermore, polydopamine is introduced on nanofiber membrane to form hydrogen bonding with PDOL and bis((trifluoromethyl)sulfonyl)imide anion, dramatically improving the mechanical strength, ionic conductivity, and transference number of GPE. Besides, molecular dynamic simulation is used to reveal the intrinsic factors of high ionic conductivity and reinforcing effect in the meantime. Consequently, the LiFePO4//Li batteries using self‐enhancing GPE show extraordinary cyclic stability over 800 cycles under high current density of 2 C, with a capacity decay of 0.021% per cycle, effectively suppressing the growth of lithium dendrites. This ingenious strategy is expected to manufacture advanced performance and high safety LMBs and compatible with the current battery production.


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(Ph=8.5, water: ethanol=1:1 volume ratio) to obtain a clear and transparent solution, and the PVDF-HFP membrane was impregnated into it at room temperature for two days. [2] The nanofiber porous membrane modified by PDA (PDA/PVDF-HFP) was successfully prepared after washed for several times with deionized water and transferred to a vacuum oven at 80 °C for 24 hours. The content of PDA in membranes was controlled about 0.17 mg cm -2 .

Preparation of the cathode
A certain amount of LiFePO 4 and Super p (the mass ratio was 8:1) were first ground for 30 minutes, then 3.5 wt% PVDF solution was added, continuing to grind for another 30 minutes to obtain a uniformly mixed slurry. The mass ratio of LiFePO 4 : Super p: PVDF was 8:1:1. [3] Final slurry was coated evenly on the aluminum foil with a scraper (thickness of 150 μm). The electrode sheet was transferred into a vacuum oven at 80 °C for 24 h, then cut into a disc (diameter of 12 mm), and the mass loading was about 2.2 mg cm -2 .

Structure characterization
The chemical structures of DOL monomer and PDOL were analyzed by Fourier Transform Infrared spectroscopy (FTIR, Nexus 670), while other composite electrolytes were tested with ATR-FTIR (attenuated total reflection-FTIR). The measurement spectra were recorded around from 500 to 4000 cm -1 , scanning 128 times.
Meanwhile, 1 H-NMR and 13 C-NMR spectra of PDOL were characterized by Nuclear Magnetic Resonance Spectroscopy (NMR, Bruker AVANCE III HD 400 MHz) to qualitatively analyze of the composition and structure of the sample, with tetramethylsilane (TMS) as an internal reference and DMSO-d 6 as the deuterated solvent. The PDOL was dissolved in tetrahydrofuran (THF) solvent for Gel Permeation Chromatography (Breeze2 GPC) measurement to analyze the relative molecular mass and relative molecular weight distribution (polydispersity index) of polymer samples.
Wetting performance and contact angle between the electrolyte precursor solution and framework membranes were conducted on the Contact Angle Meter (XG-CAMB).
The samples were immersed into n-butanol for 2 h to obtain the porosity (P), which was calculated based on equation 1 below: [2] % = where W 1 and W 2 are the membrane weights before and after soaked into n-butanol, V represents the volume of membranes, ρ n is the density of n-butanol (0.8098 g cm -3 ).
Microscopic morphologies and microstructures of as-prepared samples were where σ, L and S are the ionic conductivity, the thickness and the area of GPEs, respectively. R b is the bulk impedance.
The calculation of reaction activation energy (Ea) follows the formula 3: where the E a represents the activation energy that needed for Li + conduction. R is the molar gas constant and T is the measurement temperature. A is the pre-exponential factor and σ represents Li + conductivity [4a, 5] .
The Li//Li symmetric cells were measured to obtain the Li + transference number (t + ).
The AC impedance frequency was 200 kHz-0.1 Hz. Chronoamperometry method was used to determine the initial and steady-state current (I 0, I S ). The applied polarization voltage (△V) was 2 mV. t + is calculated according to the formula: [6] where 0 and are the resistance values before and after polarization, respectively.   for 0.5 h, and the results were shown in Figure S4. The PP membrane underwent serious shrinkage, and it was alleviated after being modified by PDA. The PVDF-HFP membrane was slightly deformed. Furthermore, the PDA@PVDF-HFP membrane showed almost no shrinkage, which featured the best thermal stability among those membranes. Figure S4. The thermal stability testing of different membranes at 155 ℃ for 0.5 h with PDA ( Table S1). The modulus of PDOL@PDA/PVDF-HFP was further increased after PDOL was integrated owing to the outstanding interaction between PDOL and PDA. This was beneficial to inhibit the growth of lithium dendrites during cycling. In Figure S5a  simulation equilibrated rapidly, and we can get some useful information in she short time. For PVDF-HFP, the typical random molecular chain was produced by Material studio's Polymer Builder. PDA's structure was not fully researched, so the most stable dimer structure suggested by Chun-Teh Chen was used in this paper. [8] The interaction in the system was described using the powerful forcefield COMPASS which was developed for the condensed-phase simulation. It had been widely used in various situation such as common organic molecules, macromolecules, and inorganic materials.
The simulations were prepared in the following way: (Ⅰ) The three-dimensional, low-density, cubic system was produced by Amorphous cell at 298 K and 1 atm. (Ⅱ) The system was equilibrated by a series of successive NVT/NPT processes under high temperature and pressure, followed by a long-time simulation at 363 K and 1 atm. (Ⅲ) The barostat of produce phase was changed to velocity rescale barostat which can produce the correct NPT statistical ensemble. The Ewald summation method was used for treatment of long-range electrostatic forces between partial charges. The cutoff for dispersion term was 12.5 Å, while a correction was made for the van der Waals term neglected beyond the cutoff. The r was distance between two atoms, and g(r) was the probability of the corresponding. [9]     indicating a faster diffusion of lithium ion. The first three CV curves overlapped well shown in Figure S10b, reflecting a well-reversible electrochemical redox process, [10] which was obviously beneficial to the cycle stability of batteries. [11] With the scan rate increasing, the redox peaks became wider, relating to the gradual increase in the polarization of LiFePO 4 but still in an obvious peak shape (Figure S10c, S10d).
Through the relationship between different scan rates and corresponding peak current (Ip) (Figure S10e, f), the Li + diffusion coefficients (D Li ) were figured out via equation higher than that of PP with commercial electrolyte (~10 -13 cm 2 s -1 ). [12] Figure S11   C1s XPS spectroscopy of the above GPEs.
The peak at 399.4 eV belonged to C-N stretching vibration.