Deciphering the role of tetrahydrofuran residue in the poly(ethylene oxide)/LiTFSI hybrid used for secondary battery electrolyte

Abstract

Residual solvent in polymer electrolytes recently has attracted great attention since it influences the performance and safety of the polymer-electrolyte based batteries and electric vehicles. In this work, we did a systematic study of exploring the role of the residual solvent in lithium-ion secondary battery electrolyte beyond the traditional understanding of plasticizing effect, based on the typical polymer electrolyte system of polyethylene oxide/tetrahydrofuran/lithium bis(trifluoromethanesulfonyl)imide (PEO/THF/LiTFSI). A series of complementary characterization techniques, i.e., small-angle neutron scattering (SANS), wide-angle X-ray scattering (WAXS), Raman, infrared, nuclear magnetic resonance, thermogravimetric analysis and electrochemical impedance spectroscopy, were applied to decipher the structure-function relationship. We found that the PEO/LiTFSI hybrid formed a complex with the THF residue, which was further confirmed by the Gaussian simulation. By using the oxygen-free solvent, acetonitrile, as a reference, we demonstrated that THF provided ambient oxygen bonding sites among PEO and LiTFSI, which leaded to PEO/LiTFSI/THF clusters and endowed the hybrid a ionic conductivity value of 1.1 × 10−4 S cm−1 at 30 °C. The battery showed a discharge capacitance of 905 mAh g−1 at 0.1C in a Li-S cell. Moreover, both ionic conductivity and cell performance deteriorate significantly with vigorous drying of the polymer electrolyte membrane in vacuum oven at 120 °C and argon-purged glove box at room temperature, although there are still around 10 wt% THF residues found in the dried membranes. It seems to be that the vigorous drying caused damage to the PEO/LiTFSI/THF clusters and the ion conductive network, especially around the electrode/electrolyte interfaces. Our findings shed light on improving the ionic conductivity of polymer electrolyte for lithium-ion secondary battery by carefully tailoring the solvent residue.