A unified approach to understanding conductivity enhancement in nanoparticle-filled solid polymer electrolytes for lithium-ion batteries

The relationship between structure, polyethylene oxide [PEO] mobility, and ionic conductivity is investigated for the solid polymer electrolyte, PEO/LiClO4, filled and unfilled with Al2O3 nanoparticles. Oxide nanoparticles are known to improve conductivity in solid polymer electrolytes; however, the mechanism is not well understood. We measure semi-crystalline and amorphous samples over a range of lithium and nanoparticle concentrations.;We use small-angle neutron scattering [SANS] to determine that the (PEO) 3:LiClO4 phase forms cylinders with radius 125°A and length 700°A. We measure the amount and size of pure PEO lamellae by exploiting the neutron scattering contrast that arises because of crystallization, and we learn that nanoparticles do not affect the extent or crystal structure of pure PEO. We also learn that crystalline (PEO)6:LiClO4 does not form immediately, but requires several days if the sample is dry, or weeks if the sample is exposed to moisture.;It is generally accepted that ion mobility is maximized in amorphous polymer electrolytes, because polymer mobility (and therefore ion mobility) is faster in amorphous regions. We measure structure and mobility in dry samples at high molecular weight where the (PEO)6:LiClO4 crystal phase has not formed; however, remnants of this structure are known to persist in the liquid phase.3 In fact, our SANS results yield scattering consistent with concentration fluctuations that may represent the remnants of (PEO)6:LiClO4 in the liquid phase.;By comparing structure, mobility and conductivity results on all the unfilled samples, we determine that a semi-crystalline sample (concentration of 14:1) has the highest conductivity at 50°C, despite being less mobile, partially crystalline, and having less charge carriers than amorphous samples at the same temperature. This result suggests a decoupling of ionic conductivity and polymer mobility. It is possible that the pure crystalline PEO in the 14:1 sample stabilizes the conductive (PEO)6:LiClO4 remnants, allowing them to persist long enough for conduction to occur.;We suggest that pure PEO and (PEO)6:LiClO4 form alternating layers extending away from the nanoparticle surface - consistent with the structure expected at a eutectic. This could provide a conductive pathway for lithium ions, accounting for the improved conductivity at this concentration. Above the eutectic temperature, the layers can fluctuate and rearrange easily, and are likely stabilized by the nanoparticle surface. These results suggests a new mechanism for increased lithium-ion transport in nanoparticle-filled solid polymer electrolytes.;Water boosts the conductivity in both filled and unfilled samples. This is attributed to the fact that water increases the segmental motion of the polymer, and therefore the ion mobility. When nanoparticles are added, the conductivity boost is unaffected at the 8:1 concentration, whereas nanoparticles decrease the conductivity boost at a concentration of 10:1. While we do not know for certain, it is possible that the 8:1 sample undergoes phase-separation into regions rich and poor in Li+/H2O, with nanoparticles located in the Li+/H2O-poor regions. Conduction will occur in the Li+/H2O-rich regions, meaning that nanoparticles will have no influence on the conductivity-boost with water. In contrast, it is likely that the 10:1 sample will not phase separate, owing to that fact that it is at the eutectic concentration where the energies of all phases are equivalent.;The results of this study suggest that structure could play an important role for improving ionic conductivity in solid polymer electrolytes, despite the fact that ion transport through structure is often dismissed in favor of transport through purely amorphous regions. We suggest that nanoparticles improve conductivity by stabilizing and aligning the conductive (PEO) 6:LiClO4 remnants. Understanding transport through the (PEO) 6:LiX structure is important for designing a solid polymer electrolyte with adequate conductivity to operate a portable device at room temperature. (Abstract shortened by UMI.)...