Reducing crystallinity in solid polymer electrolytes for lithium-metal batteries via statistical copolymerization

Vincent St-Onge,Mengyang Cui,Sylviane Rochon,Jerome P Claverie,Jean-Christophe Daigle

doi:10.1038/s43246-021-00187-2

Abstract

The discovery that polyethylene oxide promotes ionic conductivity led to the development of solid polymer electrolytes. However, their conductivity is severely reduced by crystallinity. Here, statistical copolymerization is used to design macromolecular architectures where crystallinity is disrupted by a minimal amount of non-ethylene oxide comonomer units. Using the Flory exclusion model, we demonstrate that polymers containing 18 mol% comonomer and 18 wt% LiTFSI are devoid of crystallinity. A 10 mol% comonomer content is sufficient to reach a conductivity of 0.3 × 10−4 S cm−1 at 25 °C. The Li+ transference number is 0.6, indicating that the comonomer units not only limit the crystallinity but also weaken the strength of the Li+ coordination to the polymer. The resulting solid polymer electrolyte is effective in an all-solid LFP|Li-metal battery operating at 25 °C, demonstrating that statistical copolymerization is an efficient tool for polymer electrolyte design.

Highlights

The discovery that polyethylene oxide promotes ionic conductivity led to the development of solid polymer electrolytes
Analysis of the copolymer composition by 1H nuclear magnetic resonance (NMR) demonstrates that each copolymer contains around 300 EO units and a few comonomer units which increases with comonomer feed
Two or more PO comonomers in a row would reduce the ionic conductivity because a comonomer unit is not able to solvate the Li+ cation as well as EO units, as suggested by the much lower ionic conductivity of poly(propylene oxide) (PPO) under similar conditions as opposed to that of polyethylene oxide (PEO) with lithium bis-(trifluoromethane sulfonyl)imide (LiTFSI)[33,34]

Summary

Introduction

The discovery that polyethylene oxide promotes ionic conductivity led to the development of solid polymer electrolytes. The use of additives such as organic solvents (carbonates) to form gel polymer electrolytes (GPEs)[14,15,16,17] or longer organic molecules (small PEG, DOP) acting as plasticizers have been demonstrated to successfully disrupt the crystallinity Such materials, which are no more solvent-free, no longer provide the desired mechanical properties offered by all-solid-state PEO SPEs. A few all-solid-state polymer electrolytes (ASSPEs) can be found in the literature, being the results of composite polymer electrolytes (CPEs)[18], polymer blends (like PEO with PVDF)[19] or (semi-) interpenetrating networks[20]. A fundamental question that is addressed here lies in the determination of the smallest amount of comonomer needed to break crystallinity From this knowledge, a PEO rich material, which favors ionic conductivity, can be prepared with a minimal amount of comonomer units (defects) to prevent crystallization[28]

Methods

Results

Conclusion