Asymmetrical Sp
3 Boron Atoms with Poly(Ethylene Glycol) Bridges As Single Lithium Ion Conducting Polymer Electrolytes
Authors: Guzmán Gregorio , Ávila-Paredes Hugo Joaquin , Ramos-Sánchez Guadalupe , Rivera Ernesto , González I. 
Published: February 2020

Single lithium ion conducting polymers electrolytes (SLICPE) are characterized by a high lithium ion transference number and low concentration polarization [1]. However, the ionic conductivity values need to be increased in order to obtain better performance in batteries. In general, two different approaches are used to increase the ionic conductivity in SLICPE, the first consists of a simple process of mixing lithium polymer salts with flexible polymers such as Poly(ethylene oxide) (PEO) . However, in this type of SLICPE phase separation between the polymer matrix and the lithium polymer salt can be generated, causing loss of contact between the electrode and the electrolyte, as well as conductivity decrease. The second form of elaboration of SLICPE consists of the synthesis of copolymers containing units with a relatively large segmental movement, which have anions covalently bonded to the polymer. However, the synthesis of this type of SLICPE is generally complicated and expensive [2].

In this work, we report the electrochemical characterization of a series of SLICPE based on sp3 boron atoms, homogeneously distributed in a way by means of poly(ethylene glycol) PEG bridges of different lengths that allow easily modulating the physical, electrochemical and ionic conduction properties. SLICPE were synthesized by a conventional fusion method, and samples were characterized by NMR, and electrochemical impedance spectroscopy (EIS) in symmetrical cells in the range between 20 and 90°C. In addition, the effect of charge delocalization present in BO4
- groups, associated to strong electron-withdrawing substituents and their effect on the ionic conductivity in SLICPE, was studied.

Results indicate a decrease on the activation energy for Li+ transport as function of the ethoxy chains length from 0.053 to 0.023 eV for the PEG chain of one and eight ethoxy groups, respectively. These results suggest that in short ethoxy chains the ion conduction mechanism is predominantly owed to hopping between boron centers (B-EO-B) while in larger chains it evolves to a hopping mechanism between boron atoms and ethoxy groups (B-(EO)n-B). In general, the analysis of the results indicates that the ionic conductivity (10-4 Scm-1 @ 60°C). However, these values can increase in a function of the degree of delocalization of negative charge on the receptor anion and make this class of polymer electrolytes promising candidates for LIB specially at elevated temperatures [3].

Acknowledgements This work was supported by Fondo de Sustentabilidad project 245754 “Predicción, síntesis, elaboración y calibración de celdas fotovoltaicas y baterías de flujo. ER thanks CONACYT (project 253155), GR thanks (project seciti/080/2017) IG thanks CONACYT (proyect 237343) for financial support. G. Guzman is grateful to CONACYT for the scholarship granted to pursue his doctoral studies.

Reference
s
:

[1] V. Di Noto, S. Lavina, G.A. Giffin, E. Negro, B. Scrosati, Electrochimica Acta, 57 (2011) 4-13.

[2] H. Zhang, C. Li, M. Piszcz, E. Coya, T. Rojo, L.M. Rodriguez-Martinez, M. Armand, Z. Zhou, Chemical Society Reviews, 46 (2017) 797-815.

[3] G. Guzmán-González, H. J. Ávila-Paredes1, E. Rivera, I González. ACS Applied Materials & Interfaces
- under reviews.