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

Doping 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 (tLi+) 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 (Si-x) are introduced in between the B atom and ether oxygen in the B-PEG macromolecules (B-Si-x-PEGsx=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-Si-x-PEGs polymer electrolytes will also reduce the neat contents of both B and PEG, which has the opposite effect on the whole ionic conductivities andtLi+values, both the ionic conductivities andtLi+values of B-Si-x-PEGs electrolytes first increase then decrease along withxincreases from 0 to 5 when the [Li+]:[EO] mole ratio is fixed at 1 : 10. The peak value exists atx=1. Meanwhile, both the mobilities of Li(+)ions (confirmed via(7)Li nuclear magnetic resonance (NMR) results) and bis(trifluoromethane) sulfonimide (TFSI-) anions (confirmed from Raman spectra analysis) get their maximum values whenx=1. However, if both the concentrations of TFSI(-)and B atoms in B-Si-x-PEGs electrolytes are taken into consideration, the normalized diffusion coefficient of TFSI(-)anions obtained by molecular dynamic (MD) simulation decrease monotonically whenxincreases 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 withx. Namely, the deshielding effects of siloxane spacing groups on B atoms increase along with the increase ofxin the B-Si-x-PEGs polymer electrolytes, while it levels off whenxexceeds 3.