Poly(ethylene oxide)-ethylene carbonate solid binary electrolyte with higher conductivity, lower operating temperature and fully impregnated separator for all solid-state lithium ion batteries

Abstract

Poly(ethylene oxide) (PEO) solid electrolyte suffers from the low conductivity and the high operating temperature owing to the cation is trapped in the highly crystalline domains and enmeshed in amorphous domains. Therefore, a tactic was attempted by forming the solid binary electrolyte with the solid cosolvent, ethylene carbonate (EC). X-ray diffraction revealed the crystallinity of the electrolyte progressively degraded at room temperature as EC was added, eventually disappeared, accompanying the endothermic peak continuously shifted to the lower temperature and then vanished in differential scanning calorimetry. Fourier transform infrared spectroscopy also indicated the surroundings around PEO chains mutated after addition of 50% EC (in EO ratio). More importantly, the conductivity was significantly improved ten times after 60% EC was added, still three times at 25 °C than the pristine electrolyte at 60 °C, which was further corroborated with cyclic voltammogram of LiFePO4 micro electrode. Based on the rate discharge, no less than 60% EC content was recommended for the application in the medium rate (<2C) at 25 °C. Additionally, another barrier, the poor wettability of polyolefin separator with the solid state electrolyte was also overcome completely by the facile surface modification.