Abstract
Polyethylene oxide (PEO)-based polymers are commonly studied for use as a solid polymer electrolyte for rechargeable Li-ion batteries; however, simultaneously achieving sufficient mechanical integrity and ionic conductivity has been a challenge. To address this problem, a customized polymer architecture is demonstrated wherein PEO bottle-brush arms are hyperbranched into a star architecture and then functionalized with end-grafted, linear PEO chains. The hierarchical architecture is designed to minimize crystallinity and therefore enhance ion transport via hyperbranching, while simultaneously addressing the need for mechanical integrity via the grafting of long, PEO chains (M n = 10,000). The polymers are doped with lithium bis(trifluoromethane) sulfonimide (LiTFSI), creating hierarchically hyperbranched (HB) solid polymer electrolytes. Compared to electrolytes prepared with linear PEO of equivalent molecular weight, the HB PEO electrolytes increase the room temperature ionic conductivity from ∼2.5 × 10–6 to 2.5 × 10−5 S/cm. The conductivity increases by an additional 50% by increasing the block length of the linear PEO in the bottle brush arms from M n = 1,000 to 2,000. The mechanical properties are improved by end-grafting linear PEO (M n = 10,000) onto the terminal groups of the HB PEO bottle-brush. Specifically, the Young’s modulus increases by two orders of magnitude to a level comparable to commercial PEO films, while only reducing the conductivity by 50% below the HB electrolyte without grafted PEO. This study addresses the trade-off between ion conductivity and mechanical properties, and shows that while significant improvements can be made to the mechanical properties with hierarchical grafting of long, linear chains, only modest gains are made in the room temperature conductivity.
Highlights
Solid polymer electrolytes based on polyethylene oxide (PEO) have been widely explored to replace liquid or gel electrolytes in rechargeable Li-ion batteries
For the HB MI core, the apparent number-average molecular weight based on linear PMMA standards in size exclusion chromatography (SEC) with DMF as mobile phase is Mn 132,000, which is lower than its absolute value Mn 328,000, determined by multi-angle laser light scattering (MALLS) detector coupled in THF SEC system, suggesting a high degree of branching (DB) in the HB MI
The approach taken in this study is to tailor the architecture of HB Polyethylene oxide (PEO) electrolytes to create a hierarchical structure with endgrafted PEO attached to the bottle-brush core
Summary
Solid polymer electrolytes based on polyethylene oxide (PEO) have been widely explored to replace liquid or gel electrolytes in rechargeable Li-ion batteries. The ionic conductivity of (PEO)10:LiClO4 is on the order of 10–6 S/cm Croce et al (1998); Reddy et al (2006), which is three orders of magnitude lower than required (Shin et al, 2003). One problem with PEO-based electrolytes is their tendency to recrystallize, which inhibits segmental mobility and decreases ionic conductivity. It can take up to 3 days for the electrolyte (PEO)8:LiClO4, to recrystallize when the water concentration in the electrolyte is
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