Optimization Design of Fluoro‐Cyanogen Copolymer Electrolyte to Achieve 4.7 V High‐Voltage Solid Lithium Metal Battery

Abstract Raising the charging voltage and employing high‐capacity cathodes like lithium cobalt oxide (LCO) are efficient strategies to expand battery capacity. High voltage, however, will reveal major issues such as the electrolyte's low interface stability and weak electrochemical stability. Designing high‐performance solid electrolytes from the standpoint of substance genetic engineering design is consequently vital. In this instance, stable SEI and CEI interface layers are constructed, and a 4.7 V high‐voltage solid copolymer electrolyte (PAFP) with a fluoro‐cyanogen group is generated by polymer molecular engineering. As a result, PAFP has an exceptionally broad electrochemical window (5.5 V), a high Li+ transference number (0.71), and an ultrahigh ionic conductivity (1.2 mS cm−2) at 25 °C. Furthermore, the Li||Li symmetric cell possesses excellent interface stability and 2000 stable cycles at 1 mA cm−2. The LCO|PAFP|Li batteries have a 73.7% retention capacity after 1200 cycles. Moreover, it still has excellent cycling stability at a high charging voltage of 4.7 V. These characteristics above also allow PAFP to run stably at high loading, showing excellent electrochemical stability. Furthermore, the proposed PAFP provides new insights into high‐voltage resistant solid polymer electrolytes.

foil.The cathode was dried in a vacuum oven at 70°C for 12 h to remove the NMP solvent.The load of the prepared LiCoO2 cathode is about 1.0 mg cm −2 -1.5 mg cm −2 .
All coin cells used were CR2032 type, and their assemblies were conducted in an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm).The charge/discharge tests of coin-type cells (CR2032) were conducted on LAND testing system (Wuhan LAND electronics Co., Ltd.).

Materials characterization
The morphologies of the sample were examined by a scanning electron microscope (S-3400N, HITACHI).FTIR spectra using Bruker Germany to investigate the structure of the electrolytes.The thermal stability of solid electrolyte was monitored by differential scanning calorimetry (METTLER, Switzerland) and thermogravimetric analysis (METTLER, Switzerland) under a nitrogen flow with a heating rate of 10 °C/min.XPS was tested by the Thermo Scientific K-Alpha+ (Thermo Fisher Scientific).The TEM was tested by the Japanese electronics company JEM-2100 & X-Max80.Hydroxyl radicals were tested by an electron spin resonance spectrometer ESR, model Bruker A300.

Electrochemical characterization
Electrochemical impedance spectroscopy (EIS) tests were conducted at the Shanghai Chenhua Instrument Co., LTD in a frequency range from 0.1 Hz to 100 kHz.The testing temperatures were ranged from 25 to 100℃ and the ionic conductivity σ is calculated by the following equation: (1) where L represents the thickness (cm) of the electrolyte membrane, R represents the bulk resistance (Ω), and S corresponds to the contact area (cm 2 ) between SS and electrolyte.The impedance spectra were measured by scanning in a frequency range from 0.1 Hz to 100 kHz in the symmetrical Li|Li cells.
LSV measurements were conducted at a sweep rate of 1 mV s −1 from 2 to 6 V.
The Li-ion transfer number (tLi + ) was calculated by the following equation: where I0 and Iss are initial and steady-state current, which were recorded by chronoamperometry for 4000 s.Rbss and Rb0 are the initial and steady-state values of the bulk resistance.Ri0 and Riss are interfacial resistance between the electrode and electrolyte before and after the test.The Li||Li symmetric cells and LFP||Li full cells were charged and discharged on a LAND-CT3001A battery tester (Wuhan LAND Electronic Co. Ltd.).All electrochemical cell performances were tested under room temperature

Theoretical calculation
The geometric structures of all molecules are fully optimized by using the mixture B3LYP of 6-31G+ (d, p) basis set in Gauss software.The Density functional theory (DFT) was used to calculate the visualize the electrostatic potential maps, LUMO-HUMO energy level, and binding energy.In order to simplify the computational model, one repeating unit of one polymer chain was selected as the computational model for the polymer matrix.The effect of lithium anion on the system was ignored in the calculation of the interaction between lithium ion and polymer.Very tight convergence criteria were adopted for all computational optimizations.In the binding energy calculation, the exchange correlation interaction is described by the generalized gradient approximation (GGA) and the Perdew-Burke-Ernzerhof (PBE) functional.For the construction of the surface model, a 15 Å vacuum was used to eliminate interactions between periodic structures.The adsorption energies (Eabs) of PAFP or PEO on the surface of LCO (001) were calculated as follows.Eabs = Etotal-EPAFP/PEO-ELCO, where Etotal, EPAFP/PEO and ELCO are the energy of the whole system, the energy of PAFP or PEO molecules and the energy of the surface of LCO (001), respectively.1μm

Supplement Figures
Figure S1.The HOMO and LUMO energy levels of different polymers.

FigureFigure S4 .
Figure S2.a) SEM plot of PAFP after in situ polymerization in the separator.b)

Figure S8 .
Figure S8.Polarization curves, initial and steady-state impedance maps of TFOB and

Figure S9 .
Figure S9.Snapshot of a Molecular dynamics (MD) simulation of PAFP.

Figure S11 .
Figure S11.Constant current cycle curves of Li|PAFP|Li symmetric cells at different

Figure S12 .
Figure S12.Constant current cycle curves of Li|LE|Li symmetric cells at different

Figure S13 .
Figure S13.Constant current cycle curves of Li|TFOB|Li symmetric cells at different

Figure S17 .Figure S18 .
Figure S17.The EIS curve of Li|LCO cells with PAFP at before and after cycles.

Figure S19 .
Figure S19.SEM images of LCO particles before cycling.

Table S1 .
Comparison of battery cycle performance between this paper and other papers.