Organic Modification of Eutectogels Enhances Electrolyte/Electrode Contact in Sodium-Ion Batteries

Na⁺ ion conducting deep eutectic solvents (DESs) hold promise as alternative electrolytes for future sodium-ion batteries (SIBs) because of their higher thermal stability compared to conventional liquid electrolytes, drastically improving safety characteristics. However, their liquid nature remains to pose a risk of potential leakage. In this study, the latter is resolved by the encapsulation of DESs in a solid host matrix, creating so-called eutectogels, which are promising alternatives to ionogels because of their cost-effectiveness. The nature of the host matrix heavily influences the mechanical properties of the gels, where completely inorganic host materials readily experience mechanical deterioration when stress is applied. In this work, organic modification of the inorganic host matrix enhances the pliability of eutectogels, decreasing their Young's modulus from 4.8 to 2.1 MPa. This results in an improved electrolyte/electrode contact (reduced charge-transfer resistance) without compromising ionic conductivity (up to 0.17 mS cm^-1) or electrochemical stability window (≈0.9 V vs. Na⁺/Na to ≈4.5 vs. Na⁺/Na). As such, the eutectogels outperformed conventional liquid SIB electrolytes in full cells.