Solid Polymer Electrolytes with Increased Polymer Polarizability for Solid-State Lithium Batteries

Abstract

Solid polymer electrolytes (SPE) are light-weight, flexible, and non-flammable, thus providing a promising technical route to practical solid-state lithium batteries. Although a broad variety of molecular structures of the solid polymer matrices are described in the literature, commercially available materials are scarce. The mostly widely studied example of SPE are the poly(ethylene oxide) (PEO)-based electrolytes that contain a small molecule Li-salt (e.g., LiPF6, LiTFSI, etc). However, solid-state batteries made with a PEO-based SPE are only operational above 60 °C, corresponding to the melting of PEO, where their mechanical strength is poor. The room-temperature performance of PEO-SPEs are too low (< 10-5 S/cm) to be practical [1]. Thus, new polymer electrolytes that have high Li+ conductivity and high mechanical strength are needed.The migration of Li+ in PEO-SPE depends on the coordination of Li+ by the ethylene oxide (EO) segment, which normally involves 6-8 EO moieties. The interaction of Li+ with an EO segment is strongly influenced by the polarizability of the EO groups. The relative permittivity (ε) of a single ethylene oxide (CH2CH2O) is only about 13 – 14 [2]. This implies that Li ions experience much weaker interactions with EO groups than other polar molecules, like water (ε = 80) or cyclic carbonates (e.g., ethylene carbonate, ε = 90). Thus, one approach to increase the interaction of Li+ with the polymer chain could be to increase the polarizability of the polymers themselves. Many functional groups are highly polarizable. But only those groups that can be linked to the PEO main chain are worthy of consideration here. An example is vinylene carbonate (VC, ε = 126), which contains double bonds and can be added to the EO chains to form new polymer electrolytes.In order to link VC groups to EO chains, a PEO-like SPE may be formed using in-situ radical polymerization of the EO precursors. Examples of such precursors include poly(ethylene glycol) dimethacrylate, poly(ethylene glycol) diacrylate or their derivatives. Small molecule Li-salts may be incorporated in the precursor solution. The polymerization can be initiated by UV light irradiation or by thermal heating, depending on the type of radical initiator. In this method, the intimate interactions between Li-ions and EO groups are ensured and the crystallization of the EO segments is largely prevented.In this work, we combine these approaches to form a new solid polymer electrolyte that has good conductivity and high mechanical strength. The membrane is formed in-situ using UV irradiation of a blend of precursors and salts and chain additive functional molecules. This resulting SPE has high conductivity (0.3 mS/cm at room temperature and 1.1 mS/cm at 70 °C), a wide electrochemical stability window up to 4.8 V (vs. Li+/Li) and is thermally stable to ~200 °C, while resisting penetration of Li metal dendrites. The application of the polymer electrolyte in a SiO composite electrode is also demonstrated to achieve high capacity.