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

Graphical Take cover : The shielding effects of ether oxygen atoms in Poly(ethylene glycol) (PEG) molecular chains on the B atoms can be alleviated by introducing siloxane oxygens in between them (B-Si x -PEGs). The deshielding effects of siloxane spacing groups on B atoms increase along with the increase of x in the B-Si x -PEGs polymer electrolytes. 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 ( ) 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 -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-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 and values, both the ionic conductivities and values of B-Si x -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 7 Li 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-Si x -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-Si x -PEGs polymer electrolytes, while it levels off when x exceeds 3.