Effect of LiClO4 on the Structure and Mobility of PEO-Based Solid Polymer Electrolytes

The relationship between structure, PEO mobility, and ionic conductivity is investigated for the solid polymer electrolyte, PED/LiClO4. Amorphous and semicrystalline samples with ether-oxygen-to-lithium ratios ranging from 4:1 to 100:1 are measured. Previous X-ray diffraction results show that three crystalline phases can form in this system depending on the LiClO4 concentration: (PEO)3:LiClO4. pure PEO, and (PEO)(6): LiClO4. We use SANS to determine that the (PEO)(3):LiClO4 phase forms cylinders with a radius of 125 angstrom and a length of 700 angstrom. We also measure the amount and size of pure PEO lamellae by exploiting the neutron scattering length density contrast that arises because of crystallization. The samples are thermally treated such that the (PEO)(6):LiClO4 phase does not form. QENS is used to measure PEO mobility directly in amorphous and semicrystalline samples, and it reveals two processes. The first process at short times is attributed to the segmental mobility of PEO, and the second process at longer times is attributed to the restricted rotation of protons around the Li+ ions. The type of motion and the radius of rotation are consistent with a cylindrical structure observed by diffraction: two PEO chains wrapping around Li+ ions in an ether-oxygen-to-lithium ratio of 6:1. By directly comparing structure, mobility, and conductivity of the same samples, we determine that at 50 degrees C, a semicrystalline sample (concentration of 14:1) has the highest conductivity despite being less mobile, partially crystalline, and having less charge carriers than amorphous samples at the same temperature. The results suggest a decoupling of ionic conductivity and polymer mobility.