The Role of Salt Concentration on the Ordering and Ionic Conduction of Zwitterionic Organic Solid Electrolytes

Progress toward durable and energy-dense lithium-ion batteries has been hindered by instabilities at electrolyte–electrode interfaces, leading to poor cycling stability associated with energy-dense lithium metal anodes. Although crystalline domains are conventionally detrimental to ion conduction, previous work suggests that properly designed semicrystalline organic solid electrolytes (OSEs) can exhibit excellent lithium conductivity and selectivity. Herein, we present the characterization of zwitterion-lithium salt blends as OSEs. At low salt concentrations, crystallinity of the zwitterion can be maintained up to 10wt% salt, above which the blend became amorphous, as determined by XRD. At salt concentrations less than 10 wt%, a strong relationship between the crystalline structure of the blend and ionic conductivity was found, namely an ionic conductivity decrease when approaching melting temperature of the crsytaline phase. Conductivities of crystalline samples at 100ºC exceeded 1.0 mS/cm, suggesting future promise as solid state electrolytes. Amorphous blends were found to have maximum conductivities exceeding 10 mS/cm, while remaining entirely nonvolatile. These results suggest that crystalline and amorphous forms of zwitterionic OSEs may have merit as electrolyte materials and further study of their conductivity and selectivity properties are merited.