Synthesis and characterization of thin film polyelectrolytes for solid-state lithium microbatteries

Abstract

A critical challenge associated with the efficient fabrication of solid-state, three-dimensional microbatteries is the synthesis of conformal, solid-state thin film electrolytes. This study presents the synthesis of lithium ion conducting thin film polyelectrolytes based on poly(methacrylic acid-co-ethylene glycol diacrylate) using initiated chemical vapor deposition. These films are transformed to lithium-bearing polyelectrolytes through an H+/Li+ ion exchange reaction in a 1M LiOCH3 solution in methanol. Infrared spectroscopy of the vibrational stretching modes associated with the carbonyl bond in methacrylic acid confirmed that the ion exchange was successful, and quantification by x-ray photoelectron spectroscopy showed that ion exchange was nearly 100% complete. Incorporation of the crosslinking comonomer ethylene glycol diacrylate (EGDA) was necessary for physical stability of the film during the ion exchange process, and four polyelectrolyte compositions with varying crosslinking densities (ethylene glycol diacrylate content) were prepared to understand its role on ionic conductivity. The highest ionic conductivity was achieved in the polymer film with the lowest crosslinking density (9 mol. % EGDA); its conductivity was determined to be 6 × 10−9 S cm−1 at 20 °C and 1 × 10−5 S cm−1 at 100 °C. The activation energy for ionic conductivity was 0.88 eV. Using atomic force microscopy, the surfaces of thin film polyelectrolytes were shown to be continuous, smooth, and pinhole-free, suggesting that these films are suitable for integration into 3D microbatteries.