Mitigating the Shielding Effect of Ether Oxygen in Poly(ethylene glycol) on Boron Atoms in Boron‐Doped Poly(ethylene glycol) Hybrid Polymer Electrolyte by Introducing Siloxane Spacers

Abstract

AbstractDoping boron (B) atoms into poly(ethylene glycol) (PEG)‐based solid polymer electrolytes (B‐PEG) is believed to be an attractive method to increase the lithium‐ion transference number ( ) because of the interaction between the electropositive B atoms and the anions of lithium salts. However, this effect can be largely impeded by the interaction between B and the electronegative ether oxygen atoms in the attached PEG molecular chains, which is referred to as a shielding effect. Enlarging the distance between the center B atoms and the electronegative ether oxygens is believed to be an effective way to reduce such shielding effect. In this work, different number of siloxane spacing groups (Six) are introduced in between the B atom and ether oxygen in the B‐PEG macromolecules (B‐Six‐PEGs x=0,1,3,5). As the siloxane oxygen has less electronegativity to ether oxygen, the shielding effects should be weakened and the ability of B atoms to trap the anions of lithium salt will increase. As the introduction of siloxane groups into the B‐Six‐PEGs polymer electrolytes will also reduce the neat contents of both B and PEG, which has the opposite effect on the whole ionic conductivities and values, both the ionic conductivities and values of B‐Six‐PEGs electrolytes first increase then decrease along with x increases from 0 to 5 when the [Li+]:[EO] mole ratio is fixed at 1 : 10. The peak value exists at x=1. Meanwhile, both the mobilities of Li+ ions (confirmed via 7Li nuclear magnetic resonance (NMR) results) and bis(trifluoromethane) sulfonimide (TFSI−) anions (confirmed from Raman spectra analysis) get their maximum values when x=1. However, if both the concentrations of TFSI− and B atoms in B‐Six‐PEGs electrolytes are taken into consideration, the normalized diffusion coefficient of TFSI− anions obtained by molecular dynamic (MD) simulation decrease monotonically when x increases from 0 to 5. As the diffusion coefficient of TFSI− anions is determined by the interaction probability and strength of TFSI− and B atoms; these results confirm that the interaction between the B atoms and TFSI− anions increase with x. Namely, the deshielding effects of siloxane spacing groups on B atoms increase along with the increase of x in the B‐Six‐PEGs polymer electrolytes, while it levels off when x exceeds 3.