Competitive Li+ Coordination in Ionogel Electrolytes for Enhanced Li‐Ion Transport Kinetics

Abstract Developing ionogel electrolytes based on ionic liquid instead of volatile liquid in gel polymer electrolytes is regarded to be effective to diminish safety concerns in terms of overheating and fire. Herein, a zwitterion‐based copolymer matrix based on the copolymerization of trimethylolpropane ethoxylate triacrylate (ETPTA) and 2‐methacryloyloxyethylphosphorylcholine (MPC, one typical zwitterion) is developed. It is shown that introducing zwitterions into ionogel electrolytes can effectively optimize local lithium‐ion (Li+) coordination environment to improve Li+ transport kinetics. The interactions between Li+ and bis(trifluoromethanesulfonyl)imide (TFSI−)/MPC lead to the formation of Li+ coordination shell jointly occupied by MPC and TFSI−. Benefiting from the competitive Li+ attraction of TFSI− and MPC, the energy barrier of Li+ desolvation is sharply decreased and thus the room‐temperature ionic conductivity can reach a value of 4.4 × 10−4 S cm−1. Besides, the coulombic interaction between TFSI− and MPC can greatly decrease the reduction stability of TFSI−, boosting in situ derivation of LiF‐enriched solid electrolyte interface layer on lithium metal surface. As expected, the assembled Li||LiFePO4 cells deliver a high reversible discharge capacity of 139 mAh g−1 at 0.5 C and good cycling stability. Besides, the pouch cells exhibit a steady open‐circuit voltage and can operate normally under abuse testing (fold, cut), showing its outstanding safety performance.


Synthesis of GPE precursor
The thermal curing agent was prepared by mixing 200 μl ETPTA with 140 mg MPC powder (molar ratio of 1:1) uniformly. Then, 1.2 ml ionic liquid containing LiTFSI was added to the thermal curing agent. Firstly, LiTFSI concentration is changed from 1 to 4 M (group 1) to adjust the ion coordination architectures. For another group, the content of ionic liquid is decreased from 1.2 to 0.2 ml (1.2, 0.8, 0.4 and 0.2 ml) while keeping the LiTFSI concentration at 2 M (group 2). From the series of samples, ionogels featuring free TFSI, contact ion pairs and aggregated ion clusters are selected and named as GPE-1, 2 and 3 respectively.
The LiTFSI concentration is based on the volume of ionic liquid. By comparing the FTIR and Raman results, the ionogels featuring free TFSI, contact ion pair and aggregated ion clusters are selected and the composition is shown in Table S1.

Cell Preparation
For in-situ formed batteries, 0.01g AIBN initiator was added into 1 g gel precursor and mixed them uniformly.
Then, 100 μL gel electrolyte precursor was dropped on the cellulose film between the electrodes within a cell. The hydraulic battery packer is used to assemble coin cells (CR 2025), followed by thermostatically heating at 60 ℃ for more than 2h to obtain in-situ solid battery.
The ionogel electrolyte used in sandwiched batteries is disassembled from the in-situ formed SS||SS batteries to exclude the influence of electrolyte thickness.
As for the ex-formed independent membrane prepared for microstructure analysis, the gel precursor was dripped on release liner and heated at the same condition.
For the Li| |LFP cell, a slurry of LFP, super-P, and poly(vinylidene fluoride) (mass ratio of 8:1:1) dissolved in N-methly-2-pyrrolidone was cast onto an aluminum foil. The cast film was dried in a vacuum oven at 80 °C for 24 h. The active material weight of cathode applied in this experiment is ≈3.5 mg cm −2 .

Materials characterization
The crystal phases of the synthesized polymer were identified by X-ray diffraction method (XRD, Rigaku

Electrochemistry tests
The Lithium metal foils were used as both work electrode and counter electrode, with diameter of 10 mm. The batteries for electrochemistry tests were assembled by dripping liquid precursor onto a porous cellulose membrane between electrolytes in Ar-filled glovebox with O 2 and H 2 O content below 0.5 ppm, followed by heating at 80 °C for 2 h to obtain the solid cells. The ionic conductivity (σ) of the GPE was detected by EIS in a frequency range from 1 MHz to 0.1 Hz and a temperature range from 25 to 80 °C with stainless steel (SS) as symmetric electrodes.
And the σ was calculated depending on the Equation 1:

σ =
Where R is the resistant value of ss||ss cells (the intercept on the x-axis in EIS results), L and S are the thickness and area of GPEs. The corresponding was calculated based on EIS results obtained at different temperature and Equation 2 (Arrhenius formula): where A is the pre-exponential factor, E a is the activation energy of activated ion-hopping conduction process, and T is the absolute temperature.
The linear sweep voltammetry (LSV) was carried out using the cell configuration of SS||Li cells at the scanning rate of 1 mV s −1 and 25 °C. The + was conducted by direct-current (DC) polarization of the Li||Li symmetric cell with the DC voltage of 10 mV at 25 °C. And + was calculated by Equation 3: where ΔV is the applied voltage (10 mV), I 0 and I S are the initial and steady current through the cell, respectively, R 0 and R S are the initial and steady resistant value of the Li| |Li obtained by AC impendence, respectively.

DFT calculation
The first-principles calculations were conducted in Gaussian 09 (G09) program with Becke's three-parameter hybrid method using the Lee-Yang-Parr correlation functional (B3LYP) at 6-311G++G (d, p) level. Frequency analysis was performed to further confirm the ground state of cation-solvent complexes. The binding energy (E b ) between cations and molecule is defined as following: where E Complexis is the total energy of cation-molecule complex, E M is the total energy of cation, E molecule is the total energy of solvent, and the n represent the number of molecular in the complex. Figure S1. Fourier-transform infrared spectroscopy (FTIR) of liquid precursor and GPE-2 electrolyte.    Figure S4. Optimized geometric configurations of free TFSI -, contact ion pairs and aggregated ion clusters.  Table S1), in which the ionic liquid/ETPTA ratio was 6:1. The GPE-2 and GPE-3 were prepared with salt concentration of 2M and 4M respectively.  Table S2), in which the salt concentration was 2M. The GPE-1 was obtained when the volume ratio of ionic liquid and ETPTA was decreased to 1:1.          ignition test.mp4

Supporting figures
Video S1. Ignition test of the fabricated GPE-2 gel electrolyte, which exhibits desirable non-flammability.