Ammonium-Based Plastic Crystals as Solid-State Electrolytes for Lithium and Sodium Batteries

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• Organic ionic plastic crystals (OIPCs) are advanced solid materials comprising organic cations and inorganic anions.
• OIPCs are investigated as safer alternatives to liquid electrolytes in lithium-ion and sodium-ion batteries.
• Current research lacks significant demonstrations of true solid-state behavior in OIPCs with decoupled ion transport.

Purpose of the Study:

• To synthesize and characterize novel solid-state electrolytes based on OIPCs for enhanced electrochemical energy storage.
• To investigate the ionic conductivity, diffusion properties, and transference numbers of OIPCs doped with lithium and sodium salts.
• To evaluate the performance of these OIPCs as solid-state electrolytes in lithium and sodium symmetric cells.

Main Methods:

• Incorporation of lithium and sodium salts into tetramethylammonium bis(fluorosulfonyl)imide ([N1111][FSI]).
• Measurement of ionic conductivity, diffusion coefficients, and transference numbers at elevated temperatures.
• Testing of symmetric Li|Li and Na|Na cells to assess current densities and cycling stability.

Main Results:

• Achieved high ionic conductivities: 1.79 mS·cm⁻¹ for LiFSI-doped and 3.2 mS·cm⁻¹ for NaFSI-doped OIPCs at 80 °C.
• Observed elevated diffusion coefficients up to 3.83 × 10⁻¹¹ m²·s⁻¹ for Li⁺ at 80 °C.
• Demonstrated high transference numbers (0.8 for Li⁺, 0.4 for Na⁺) and impressive current densities (up to 3.5 mA·cm⁻² for Li|Li, 2.9 mA·cm⁻² for Na|Na) at room temperature.

Conclusions:

• The developed OIPCs exhibit promising properties for solid-state electrolytes, including high ionic conductivity and diffusion.
• These materials show potential for enabling true solid-state ion transport decoupled from host structure motion.
• The OIPCs offer a viable pathway for improving the efficiency and safety of both lithium and sodium electrochemical energy storage technologies.