Cationic Covalent Organic Framework with Ultralow HOMO Energy Used as Scaffolds for 5.2 V Solid Polycarbonate Electrolytes

Abstract Solid polymer electrolytes (SPEs) have become promising candidate to replace common liquid electrolyte due to highly improved security. However, the practical use of SPEs is still restricted by their decomposition and breakage at the electrode interfacial layer especially at high voltage. Herein, a new cationic covalent organic framework (COF) is designed and synthesized as a reinforced skeleton to resist the constant oxidative decomposition of solid polycarbonate electrolyte, which can stabilize cathode electrolyte interphase layer to develop long‐term cycle solid lithium metal battery. The ultralow HOMO energy (−12.55 eV according to density functional theory (DFT) calculations), reflecting its oxidation resistance at positive potential, would be responsible for the high decomposition voltage of 5.2 V versus Li+/Li of solid polycarbonate electrolyte. Furthermore, the smooth surface of interfacial layer and inhibited decomposition reaction at cathode side is confirmed in solid LiCoO2 cell, which realizes high initial capacity up to 160.3 mAh g−1 at 0.1 C and greatly improved stability in 4.5 V class solid polymer lithium metal battery with high capacity retention over 200 cycles. This new type of high‐voltage resistant solid polymer electrolyte promotes the realization of high‐voltage cathode materials and higher energy density lithium metal battery.


Characterizations
Fourier Transform infrared spectroscopy (FTIR) studies were carried out on the BRUKER spectrometer in the range 2300 -400 cm -1 . The FTIR spectra of COF is shown in Figure S1. Solid-State 13 C Nuclear Magnetic Resonance (NMR) tests were recorded on Bruker Avance spectrometer, with a working frequency of 400 MHz for 13 C nuclei. X-ray photoelectron spectroscopy (XPS) spectra were observed in an ultra-high vacuum ESCALAB 250 set-up equipped with a monochromatic Al Ka X-ray source (1486.6 eV; anode operating at 15 kV and 20 mA) and equipped with an Ar + sputtering gun (Thermo Fisher). Ar + etching was conducted at an argon partial pressure of 10 -8 Torr in the x-y scan 3 mode at ion acceleration of 2 kV and ion beam current density of 1 mA mm -2 .
The morphology and size of COF was determined with a TEM (FEI TECNAI G2 F20) and high resolution TEM (FEI TECNAI G20). All samples were scattered on a carbon-coated copper grid. Scanning electron microscopy (SEM) images were measured using HITACH SU8010. The Young's modulus of sample was analyzed by atomic Force Microscope (AFM) Dimension Icon.

Electrochemical Measurements
The EIS measurement was carried out on CHI660E with the frequency range 1000 kHz to 0.1 Hz. Electrochemical stability window was determined with linear sweep voltammetry (LSV, CHI660E) utilizing Li metal|SPE|stainless (SS) cells. The lithium ion conductivity (σ) was calculated by the following equation:

σ =
Where d represents the thickness of C-SPE, R is the resistance obtained from EIS measurements, and S represents the working area of electrode. The Li-ion transform number (t + ) was calculated by following equation: Where ΔV is the polarization voltage (10 mV), I s and R s represent the steady current and resistance value after polarization, and I 0 and R s are initial current and resistance value.
The polarization of SPE and the cycling performance of LiCoO 2 |SPE|Li cells were measured utilizing the CT2001A cell test instrument (Wuhan LAND Electronic Co. Ltd).

Computational method
Density functional theory (DFT) describes the electronic properties based on the electron charge density of the system, and has been used widely in material simulations 4 studying systems from crystal to organic molecules. However, due to their high order scaling, the DFT method can only be applied to relatively small systems. For the polymer molecules, the large size system beyond the capability of the straight forward DFT calculations. 1  Three-dimensional periodic boundary conditions were used in all simulations.