Upgrading Ion Migration and Interface Chemistry via a Cyano-Containing COF in a Single-Ion Conductive Polymer toward High-Voltage Lithium-Metal Batteries

Abstract

Concentration polarization-triggered dendrite growth hinders the practical application of solid-state polymer lithium batteries, which is caused by the uncontrolled anion migration in conventional dual-ion electrolytes. Single-ion conductive polymer electrolytes (SICPEs) offer a promise to mitigate dendrite growth via reducing concentration polarization and prohibiting salt depletion, yet they are highly challenging for successful implementation due to their narrow electrochemical window and poor ionic conductivity, which result from the deficient dissociation of Li⁺ polyanions and sluggish chain relaxation. Here, a cyano-containing covalent organic framework (COF) is designed to fuse with SICPEs, promising fast Li⁺ transport and remarkable interfacial stability toward high-voltage lithium-metal batteries. The electron-withdrawing cyano group on the COF facilitates the dissociation of the polyanions via ion-dipole interactions, resulting in more free-moving Li⁺. Rapid ion migration then occurs along the long-range cooperative ion transport pathways between the COF and SICPE. Additionally, the cyano group robustly bonds with transition metal ions of NCM cathodes to inhibit the catalytic decomposition of SICPE and guarantee the structural integrity of NCM. Hence, the as-prepared SICPE exhibits a significantly enhanced ionic conductivity of 9.2 × 10^-4 S cm^-1 and an improved Li⁺ transference number of 0.94 at room temperature. Accordingly, the NCM622||Li quasi-solid-state cell achieves an exceptional capacity retention of 92.0% over 1000 cycles at 0.5 C, while the cell pairing with the 4.8 V NCM622 cathode delivers a remarkable capacity of 149.5 mAh g^-1 after 200 cycles at 0.5 C. This study provides a new perspective for facilitating ionic conductivity and interface chemistry toward the practical feasibility of single-ion conductive polymer electrolytes.

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(a) Regulation mechanisms of COF316 on the ionic migration and interfacial chemistry of SICPEs. (b) Top view of the space-filling model, (c) the electrostatic potential (ESP), and (d) high-resolution transmission electron microscopy (TEM) images and corresponding selective area electron diffraction (SAED) pattern of COF316.

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Synthesis of the PLF@COF316 electrolyte. (a) The preparation of the PLF@COF316 electrolyte. (b) XRD spectrum and (c) nitrogen adsorption–desorption isotherms of COF316. (d) Digital photographs for illustrating the flexibility of PLF@COF316. (e) Cross-sectional SEM image and corresponding EDS element mappings of PLF@COF316. (f) FT-IR spectra of COF316, PLF, and PLF@COF316. (g) X-ray photoelectron spectroscopy (XPS) Li 1s spectra, (h) glass transition temperatures (T _g), and (i) stress–strain curves of PLF and PLF@COF316.

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Simulation of the Li⁺ migration in PLF@COF316. (a) Electrostatic potential calculations and (b) deformation charge density of the SSLi-COF316 structure. (The red and blue clouds represent the electron concentration and dissipation area, respectively.) (c) Calculated Li⁺ dissociation energy barriers of SSLi and SSLi-COF316 configurations. The top view of the conformation evolution of the PLF@COF316 system at (d) 0 ns and (e) 20 ns based on molecular dynamics (MD) simulations. 2D number density distribution of (f) Li⁺ and (g) PLF near COF316. (h) Simulation snapshots of the Li⁺ migration in the PLF@COF316 system at room temperature. (i) The mean-squared displacement (MSD) of Li⁺ in the PLF@COF316 and PLF systems calculated from MD simulations.

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Li⁺ transport in PLF@COF316. (a) XPS N 1s spectra of PLF@COF316 and COF316. (b) ⁷Li solid NMR, (c) linear sweep voltammetry (LSV) curves, and (d) ionic conductivity of PLF@COF316 with different COF316 contents at 30 °C. (e) Arrhenius plots of PLF@COF316 and PLF. (f) Chronoamperometry polarization curve and the impedance spectra before and after polarization of the Li|PLF@COF316|Li symmetric cell. (g) Summarized σ_Li⁺ and t _Li⁺ of PLF@COF316 and PLF electrolytes. (h) Tafel plots, (i) critical current density (CCD) tests, and (j) voltage–time plots at 0.2 mA cm^–2 and 0.2 mAh cm^–2 of Li||Li symmetric cells with PLF@COF316 and PLF electrolytes.

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Li-storage performance of PLF@COF316. (a) Cycling performance of NCM622||Li cells with PLF@COF316 and PLF electrolytes at 0.5 C with a cutoff voltage of 4.3 V. (b) Rate performance of NCM811||Li cells with PLF@COF316 and PLF electrolytes at current densities from 0.1 to 2 C. (c) Cycling performance of the NCM622|PLF@COF316|Li cell at 0.5 C with a cutoff voltage of 4.8 V. (d) Cycling performance of the NCM811|PLF@COF316|Li cell at −20 °C and 0.1 C. (e) Cycling performance of the NCM811|PLF@COF316|Li pouch cell at 0.2 C. (f) Comparison of σ_Li⁺ , t _Li⁺ , cutoff voltage, and cycling performance with reported SICPEs. (The capacity retentions only select from the cycling performance over 200 loops.) (g) Molecular dynamics simulation snapshots of the Li⁺ migration in the PLF@COF316 system at 0 °C. (h) The MSD of Li⁺ for PLF@COF316 at 0 and −20 °C.

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Characterization of the CEI. STEM images and corresponding FFT patterns for the NCM811 particles cycled in the (a) NCM811|PLF|Li and (b) NCM811|PLF@COF316|Li cells. XPS depth profiles of (c) F 1s and (d) Ni 2p of cycled NCM811 cathodes in the NCM811|PLF@COF316|Li and NCM811|PLF|Li cells. (e) Comparison of LiF, organic C, and reduced Ni content distribution in the cycled NCM811 cathodes. (The reduced Ni content is magnified 10 times for clear display in the figure.) (f) Time-of-flight secondary-ion mass spectrometry (TOF-SIMS) three-dimensional (3D) mappings in the formed CEI by PLF@COF316 (up) and PLF (bottom) electrolytes. (g) The corresponding TOF-SIMS depth profiles of various elemental segments. (h) TM dissolution measured by inductively coupled plasma mass spectrometry (ICP-MS) after 100 cycles.

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Characterization of the SEI. Cryo-TEM images of SEI on the Li anode surface formed by the PLF@COF316 electrolyte at (a) low magnification and (b) high resolution. (c) EDS mappings of the surface of the deposited Li. The enlarged high-resolution TEM images and corresponding FFT patterns of (d) LiF, (e) Li₃N, and (f) Li₂O. XPS depth profiles of (g) C 1s, (h) F 1s, and (i) N 1s of Li anodes cycled in NCM811|PLF@COF316|Li and NCM811|PLF|Li cells.