Comprehensive optical spectroscopy of solid-state polymer electrolytes: Harmonic and anharmonic vibrational activity and charge dynamics from terahertz to ultraviolet

We investigate the vibrational and electronic properties of solid-state polymer electrolytes (SPEs) using optical spectroscopy spanning an ultrabroadband range of frequencies extending from terahertz (THz) to ultraviolet (UV). The employed techniques include THz time-domain spectroscopy (THz-TDS), Fourier-transform infrared spectroscopy, and photometric IR-UV spectroscopy. Our study demonstrates that incorporating salts into the host polymer matrix to form an electrolyte gives rise to characteristic optical properties of SPEs, depending on the type, content, and concentration of the added salt, which we describe as follows. In the THz range, distinctive absorption bands are observed, which we attribute to THz-resonant vibrations of polymer chains. In contrast, sharp resonant absorption bands and lines due to IR-active vibrations of chemical bonds (inter- and intra-molecular vibrations) within the polymer chains and lithium salts are observed in the IR range. Density functional theory calculations were performed to provide insight into these vibrational properties of SPEs and helped to assign the relevant vibrational modes. In the UV range, we focus on characterizing the energy gap associated with transitions from bonding to antibonding states. Optical spectroscopy, as a powerful noninvasive characterization tool, thus provides valuable insights into the charge and molecular dynamics in SPEs. These insights are crucial for their technological applications and can support chemistry-based efforts to improve the properties of SPEs.