A New Strategy for Construction of Flexible Solid Electrolytes Based on Single Lithium-Ion Conductive Polymers
Authors: Lian Fang , Li Hao , Zhang Hongnan , Meng Nan 
Published: February 2020

The flexible solid electrolytes attract more attention because (1) their light weight is necessary to further improve the gravimetric energy density (Wh/ kg); (2) good flexibility to develop roll to roll processing, and (3) easy to form films and good adhesion to allow a low-impedance contact with the solid electrodes [1]. Generally lithium salt is dissolved in the polymer matrix to achieve solid polymer electrolytes (SPEs). The involved lithium salt should have high solubility and good dissociation in the polymers, which are desired to possess low glass transition temperatures Tg and structural compliance for decoupled cation motion. Such ‘salt in polymer’ electrolyte have dual ion conductivity of the anion X and the lithium ions, in which Li+ diffusion induced by segmental motion is much more difficult than the counter anions. About 80% of the ionic current is carried by anions, leading to the high concentration polarization and low Li+ ion transference number [2].

In recent years, novel single-ion conductors has been prepared by fixing the anions such as alkyl carboxylate (RCO2
−) or sulfonate (RSO3
−) to the polymeric backbones, in which the transference number (tLi+) of Li+ cations is close to one. However, these single-ion conductors still suffer from a low ionic conductivity due to their low lithium-ion concentration and low degrees of negative charge distribution in these anions. Inspired by the design strategies for the flexible solid electrolyte, we try to exploring SPEs with high ambient ionic conductivity and simultaneously high lithium ion transference number (tLi
+, higher than 0.5) from single lithium-ion conductive polymers [3]. In our study, a polyvinyl formal (PVFM) based single-ion SPEs (shorted for LiPVFM) has been proposed with wide electrochemical stability window and high lithium ion transference number of 0.87, in which the Li+ coordinated PVFM via coupling suitable amount of oxalate-chelated borate units to vinyl hydroxyl in the host polymer backbone [4]. Combined by constructing the tight and loose Li+ coupling on the polymer framework, the polymer membrane possesses a high concentration of highly mobile Li+, resulting in a significantly improved ambient ionic conductivity in two orders of magnitude to 5.7 × 10−4 S cm−1. Moreover, LiPVFM membranes exhibit an ultimate tensile strength 25 MPa and a high elongation-at-break value 266.7%. The promotion of LiPVFM with excellent mechanical and electrochemical properties opens up the rational design of advanced polymer electrolytes for room temperature operational solid-state batteries.

Acknowledgements

This work was financially supported by the National Key Research and Development Program of China (Grant No. 2018YFB0104302) and the National Natural Science Foundation of China (Grant No. 51872026).

Reference

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[2] L. Z. Long, S. J. Wang, M. Xiao, Y. Z. Meng, J. Mater. Chem. A. 2016, 4, 10038.

[3]Fang Lian, Hong-yan Guan, Yan Wen, Xiao-rong Pan, Journal of membrane science, 2014, 469:67-72.

[4] Hongnan Zhang, Fang Lian, Lijuan Bai, Nan Meng, Chunzhong Xu, Journal of Membrane Science, 2018, 552: 349–356.