Reports: ND751623-ND7: Phase Behavior and Structure in Polymer/Lithium Salt Mixtures

Paschalis Alexandridis, PhD, State University of New York at Buffalo

Rechargeable lithium batteries are attractive for energy storage because of their high specific energy and efficiency, but cannot meet scaled up demands.  In particular, the liquid electrolytes commonly used impose serious safety concerns. 

Solid electrolytes would improve safety, stability, and cost, but there are several issues to overcome.  Block copolymer-based electrolytes offer multi-scale interactions and structure that can benefit the design of batteries.  Block copolymers allow rational molecular design and processing to provide desirable ion transport and mechanical stability.  The structure of the nanoscale block copolymer domains where ions are located, and their mesoscale topology, are important to this end.

We research phase behavior and structure in multi-component systems containing block copolymers and lithium salts, with a focus on the interplay between block copolymer organization and Li+ location/mobility.  This work contributes fundamental information that could inform the design of new/improved electrolytes and batteries for energy applications.

We employ block copolymers consisting of poly(ethylene oxide) (PEO) that enable three levels of organization: crystalline PEO, liquid crystalline block copolymer assemblies, and long-range orientation of ordered domains.  We also investigate the effects of additives such as silica nanoparticles and polyhedral oligomeric silsesquioxanes (POSS) that are known to affect PEO chain conformation and ion mobility. 

A typical poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer (Pluronic P105: EO37PO56EO37) at room temperature exhibits crystallinity emanating from the PEO block. X-ray diffraction patterns of Pluronic P105 exhibit two strong peaks at 2θ = 19.5 (120) and 23.7 (032) and few weak peaks at 2θ = 15.07 (110), 26.56 (131), and 27.31 (200). These diffraction peaks suggest a helical crystalline structure of PEO (the numbers in parenthesis are Miller indices for a helical model that comprises seven chemical units (two turns in each helix period)). Homopolymer PEO exhibits higher degree of crystallinity than the PEO-PPO-PEO block copolymers. The PEO crystal structure is not affected much by the addition of LiCl, suggesting the localization of LiCl in the amorphous domains.

The degree of crystallinity initially increases upon addition of LiCl but then gradually decreases. Such decrease and elimination of crystallinity is also borne by FTIR spectra (upon addition of LiCl, peaks at 1240 and 1235 cm-1 become broad with lower intensity, very similar to the FTIR spectra of PEO melt) and Raman spectra (as the concentration of LiCl increases, peaks at 2840 and 2890 cm-1 become weak). Crystalline PEO become amorphous due to complex formation (coordination) between Li+ ions and multiple ether oxygen atoms of PEO chains. The influence of LiCl on the degree of crystallinity depends on Li/EO ratio.

In work that is considering the effects of nanoparticles on the PEO-PPO-PEO block copolymer organization, we examine the co-assembly of polyhedral oligosilsesquioxanes (POSS) of two different surface chemistries, one hydrophilic, poly(ethylene oxide) (PEO), and the other hydrophobic, isobutyl (iB), in cylindrical structures formed by hydrated Pluronic P105. The incorporation of PEO-POSS increases the block copolymer degree of segregation and PEO-POSS is located in the PEO-rich domains of the block copolymer. iB-POSS appears to decrease the degree of block segregation by locating at the PEO-PPO interfacial regions. The nanoparticle versus molecule nature of POSS is discussed by comparing POSS effects to those of 10.6 nm diameter deprotonated silica nanoparticles and of glycerol or glucose.

Ongoing work addresses practical issues pertaining to the preparation of the polymer-salt blend (solvent casting following by vacuum drying vs melt mixing), and fundamental issues such as effects of Li+ counterion, availability of PEO for coordination with Li+ (competitive interaction with protonated silica), and effect on PEO crystallinity of addition of PEO-modified polyhedral oligomeric silsesquioxanes.