Molecular Level Differences in Ionic Solvation and Transport Behavior in Ethylene Oxide-Based Homopolymer and Block Copolymer Electrolytes

Abstract

Block copolymer electrolytes (BCE) such as polystyrene-block-poly(ethylene oxide) (SEO) blended with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and composed of mechanically robust insulating and rubbery conducting nanodomains are promising solid-state electrolytes for Li batteries. Here, we compare ionic solvation, association, distribution, and conductivity in SEO-LiTFSI BCEs and their homopolymer PEO-LiTFSI analogs toward a fundamental understanding of the maximum in conductivity and transport mechanisms as a function of salt concentration. Ionic conductivity measurements reveal that SEO-LiTFSI and PEO-LiTFSI exhibit similar behaviors up to a Li/ EO ratio of 1/12, where roughly half of the available solvation sites in the system are filled, and conductivity is maximized. As the Li/EO ratios increase to 1/5 the conductivity, of the PEO-LiTFSI drops nearly 3-fold, while the conductivity of SEO-LiTFSI remains constant. FTIR spectroscopy reveals that additional Li cations in the homopolymer electrolyte are complexed by additional EO units when the Li/EO ratio exceeds 1/12, while in the BCE, the proportion of complexed and uncomplexed EO units remains constant; Raman spectroscopy data at the same concentrations show that Li cations in the SEO-LiTFSI samples tend to coordinate more to their counteranions. Atomistic-scale molecular dynamics simulations corroborate these results and further show that associated ions tend to segregate to the SEO-LiTFSI domain interfaces. The opportunity for "excess" salt to be sequestered at BCE interfaces results in the retention of an optimum ratio of uncompleted and complexed PEO solvation sites in the middle of the conductive nanodomains of the BCE and maximized conductivity over a broad range of salt concentrations.

Polyply; a python suite for facilitating simulations of macromolecules and nanomaterials

“…Ion conducting polymers for the application in lithium ion batteries have been a very active field of research for many years. 11,35,48 Block-copolymer systems, consisting of one conducting polymer and one polymer improving the mechanical stability, are a very promising route to new and enhanced batteries. Simulation of ion conduction in these systems, however, are less common as creating the initial coordinates poses serval technical challenges: 1) The system has to be obtained in the phase separated state.…”

“…Website implementations 28,30 have the added problem that performance relies on server-load and it involves human-time having to interface with the website. In addition, coordinates for more complex systems such as micro phase separated polymers and hybrid nanoparticle blends are frequently generated by (multi-scale) self-assembly [31][32][33] or custom inhouse building scripts 34,35 . The general lack of programs supporting all-atom and CG polymer simulations limits the use of MD simulations for both large versatile systems and high throughput research of (bio-) macromolecular systems.…”

Orientation-Dependent Distortion of Lamellae in a Block Copolymer Electrolyte under DC Polarization

“…The foundation of our understanding of ion transport through nanostructured electrolytes is based on numerous studies which examine structure-property relationships. 15,[18][19][20][21][22][23][24][25][26] We extend this approach by considering the dynamic nature of these relationships when a dc current is applied.…”

“…The results are consistent with measurements on other block copolymer electrolytes which have been reported on in the literature, and we defer discussion of the relationship between κ and r to those references. 14,21,32,41,42 We fit the conductivity following the functional form presented in ref 43 and obtain:…”

Crosslinked Polymer‐Brush Electrolytes: An Approach to Safe All‐Solid‐State Lithium Metal Batteries at Room Temperature

“…[16] Thus, scalable polymer engineering strategies have developed to improve their ionic conductivity. One eminent strategy is based on block copolymer architectures, i. e., poly(ethylene oxide)-bpolystyrene (PEO-b-PS), [17][18][19] in which the ion-conductive PEO block melts at elevated temperature to fast transport ions and the stiff PS block provides requisite mechanical support. Unfortunately, this method mainly aims to overcome the tradeoff between ionic conductivity and mechanical properties at high temperatures, limiting their application in numerous stateof-the-art electrical devices that need to function at room temperature.…”