In Situ Polymerization in COF Boosts Li-Ion Conduction in Solid Polymer Electrolytes for Li Metal Batteries

Abstract

Solid polymer electrolytes (SPEs) have garnered considerable interest in the field of lithium metal batteries (LMBs) owing to their exceptional mechanical strength, excellent designability, and heightened safety characteristics. However, their inherently low ion transport efficiency poses a major challenge for their application in LMBs. To address this issue, covalent organic framework (COF) with their ordered ion transport channels, chemical stability, large specific surface area, and designable multifunctional sites has shown promising potential to enhance lithium-ion conduction. Here, we prepared an anionic COF, TpPa-COOLi, which can catalyze the ring-opening copolymerization of cyclic lactone monomers for the in situ fabrication of SPEs. The design leverages the high specific surface area of COF to facilitate the absorption of polymerization precursor and catalyze the polymerization within the pores, forming additional COF-polymer junctions that enhance ion transport pathways. The partial exfoliation of COF achieved through these junctions improved its dispersion within the polymer matrix, preserving ion transport channels and facilitating ion transport across COF grain boundaries. By controlling variables to alter the crystallinity of TpPa-COOLi and the presence of -COOLi substituents, TpPa-COOLi with partial long-range order and -COOLi substituents exhibited superior electrochemical performance. This research demonstrates the potential in constructing high-performance SPEs for LMBs.

Keywords: Covalent organic framework; In situ polymerization; Lithium metal batteries; Ring-opening polymerization; Solid polymer electrolyte.

Fig. 1

a Chemical structure and SEM image of TpPa-COOLi. b PXRD patterns and c FTIR spectra of TpPa-COOH and TpPa-COOLi. d Solid-State ¹³C NMR spectrum of TpPa-COOLi. e XPS (Li 1s) spectrum of TpPa-COOLi

Fig. 2

a Process of loading COF onto expanded pore cellulose separators through freeze-drying. b SEM section view image of cellulose separator after freeze-drying treatment. c, d SEM section view image of cellulose separator after freeze-drying treatment. e EDS characterization of the section view image of cellulose separator loaded COF. f, g Local magnification SEM section view image of cellulose separator loaded COF and polymer

Fig. 3

a Digital images of polymer@TpPa-COOLi before polymerization and after polymerization. ¹H NMR spectra (CDCl₃) of P(4CL-TMC) catalyzed by TpPa-COOLi at b 80 °C and c 100 °C. d ¹H NMR spectra (CDCl₃) of 4CL-TMC monomer precursor solution with 30 wt% LiTFSI. e FTIR spectra of carbonyl groups at 1800–1650 cm⁻¹ in 4CL-TMC monomer precursor solution with 30 wt% LiTFSI

Fig. 4

SEM images of surface morphologies of a ex situ b and in situ P(4CL-TMC)@COF films. AFM bitmap diagram of c ex situ and d in situ P(4CL-TMC)@COF films. AFM phase diagram of e ex situ and f in situ P(4CL-TMC)@COF films. AFM image with roughness of g ex situ and h in situ P(4CL-TMC)@COF films

Fig. 5

a ⁷Li solid-state magic-angle-spinning (MAS) NMR spectra comparison of TpPa-COOLi, in situ P(4CL-TMC)@COF, and ex situ P(4CL-TMC)@COF. b The plot of chemical shift versus ⁷Li signal peak half-width curves of TpPa-COOLi, in situ P(4CL-TMC)@COF, and ex situ P(4CL-TMC)@COF. c DFT calculations of electrostatic potentials, lithium desolvation energies of COF-polymer composite and energy level of PTMC, PCL, P(CL-TMC)

Fig. 6

a Molecular dynamics simulations for in situ P(4CL-TMC)@COF. Radial distribution function in b Li–O (Total COF and Polymer), c Li–O (–COOLi in COF) and Li–O (C=O–Li in polymer), and d Li–N (Li–N in LiTFSI)

Fig. 7

a Temperature dependence of ionic conductivity for polymer, polymer@TpPa-COOLi, polymer@NCTpPa-COOLi, and polymer@TpPa. b Lithium-ion transference number bar chart for polymer, polymer@TpPa-COOLi, polymer@NCTpPa-COOLi, and polymer@TpPa. c Tafel curves of polymer, polymer@TpPa-COOLi, polymer@NCTpPa-COOLi, and polymer@TpPa. d Voltage profiles of Li/Li symmetric cell of polymer, polymer@TpPa-COOLi, polymer@NCTpPa-COOLi, and polymer@TpPa with the current density of 0.1 mA cm⁻² at 60 °C

Fig. 8

a SEM image and XPS spectra of F 1s, C 1s, O 1s, and N 1s of Li surface after cycling 1500 h from Li/Polymer/Li. b SEM image and XPS spectra of F 1s, C 1s, O 1s, and N 1s of Li surface after cycling 4200 h from Li/Polymer@TpPa-COOLi/Li. Finite element analysis to simulate the lithium-ion concentration, electric field distribution, and lithium dendrite growth of c ex situ P(4CL-TMC)@COF and d in situ P(4CL-TMC)@COF

Fig. 9

a Cycle performance of Li//LFP of polymer, polymer@TpPa-COOLi, polymer@NCTpPa-COOLi, and polymer@TpPa electrolyte at 1C under 60 °C. b Specific capacities of polymer@TpPa-COOLi at 0.1C, 0.3C, 0.5C, 1C, 2C, and 3C under 60 °C. c Cycle performance and d charge–discharge plot of Li/polymer@TpPa-COOLi/LFP at 0.5C under 60 °C. e Cycle performance of Li/polymer@TpPa-COOLi/NCM622 at 0.5C under 60 °C. f Specific capacities verse voltage curves of Li//NCM622 half-cells of polymer@TpPa-COOLi at 0.5C

Fig. 10

a Structure diagrams of COF, in situ formed polymer@COF, and ex situ formed polymer@COF. Radar chart comparing the performance of this work with that of some previously reported works about b COF electrolytes and c polymer@COF electrolytes