Sodium-metal batteries (SMB) are considered a promising replacement for lithium-ion batteries: sodium is abundant and cheap. But their widespread use is limited by safety and stability problems. Sodium is chemically active, and dendrites - thread-like formations capable of short-circuiting the battery - can grow on the electrodes. Conventional liquid electrolytes are flammable and toxic, and at high temperatures catastrophic failure is possible.
A group led by Professor Chang-Jin Park of Chonnam University (South Korea) has developed a polymerizable deep eutectic solvent (PDES) as an electrolyte. A deep eutectic solvent is a mixture of substances that melts at a lower temperature than each component separately. The results were published in the journal Energy Storage Materials.
The electrolyte consists of sodium perchlorate (NaClO4), N-isopropylacrylamide (NIPAM) and succinonitrile (SN). In liquid form the mixture does not burn and is chemically stable, but it is prone to leakage. To eliminate this drawback, the scientists polymerized the mixture directly inside the battery and obtained a solid polymer matrix resistant to leakage. The components were selected to improve sodium ion transport: sodium perchlorate serves as the ion source, NIPAM promotes their separation and forms a strong polymer network, and succinonitrile provides plasticity.
The optimized electrolyte showed an ionic conductivity of 1.39 mS/cm at 30 degrees Celsius, a sodium ion transference number of 0.58 and a wide electrochemical stability window. It also demonstrated high flame resistance under direct flame exposure and resistance to leakage. In full cells the electrolyte formed a stable interface with the cathode, suppressing side reactions and irregular sodium growth.
In tests, a full cell with the optimized PDES showed an initial discharge capacity of 110.2 mAh/g at a rate of 0.1 C and almost 99% Coulombic efficiency - the share of charge actually used in a cycle. After 1000 cycles at a rate of 2 C, 90.4% of capacity was retained. In addition, the scientists made a flexible cell that continued to power an LED even after being folded and cut, which confirms its safety and mechanical strength.
"Our electrolyte design for sodium-metal batteries could lead to safer, cheaper and more sustainable batteries," Park noted. "In the future such batteries could support renewable energy storage at the power grid level, backup power and low-cost rechargeable systems."
The limitation of the work is that it is a laboratory prototype, not a finished product: commercial production is still far off. Nevertheless, the proposed approach combines efficient sodium ion transport with increased safety, resistance to leakage and interface stability, which makes it promising for practical sodium-metal batteries.