Structural factors to improve physico-chemical properties of zwitterions as ion conductive matrices

Thermal properties and ion conductive characteristics of equimolar mixtures of zwitterion and lithium salts (Z/L mixtures) have been investigated. Through a systematic study of their structure–property relationship, we describe a zwitterion structure that produces high ionic conductivity when the zwitterion is mixed with lithium salt. We found that a pure zwitterion containing imide anion melted at 87 °C and the melting point of pure zwitterions was lowered in proportion to the alkyl spacer chain length between the anion and cation sites. In Z/L mixtures, ionic liquid-like domain formed between the cation sites of zwitterions and the anions of lithium salts regardless of the anion structure of the zwitterions. However, the anion structure affected the transference number of the lithium cation in the Z/L mixtures. To improve the thermal stability and the ionic conductivity of Z/L mixtures, suitable alkyl chain length between the cation and anion sites was determined to be 5 to 7 CH2 units.