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Researchers Develop Soft-acidic/hard-basic Zwitterion for Low-temperature Anode-free Zn Batteries

Posted: 2024-03-06

Aqueous Zn batteries are considered as safe and low-cost alternatives to commercial Li-ion batteries.

However, water molecules in the aqueous electrolytes tend to undergo a phase transition from a disordered state to an ordered ice by forming extra hydrogen bonds. Furthermore, the electrostatic interaction between cations and anions in the electrolyte is strengthened at low temperatures, resulting in the undesired solidification of the aqueous electrolytes at low temperatures.

Consequently, the electrolyte freezing hampers the mobility of ions and impairs the low-temperature (particularly below -30 ℃) electrochemical activity of aqueous Zn batteries.

Recently, a research group led by Prof. WU Zhong-Shuai from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS), in collaboration with Prof. Sang-Young Lee from Yonsei University and Prof. Sang Kyu Kwak from Korea University, have developed a soft-acidic/hard-basic zwitterion for low-temperature anode-free Zn ion batteries. This study was published in Energy & Environmental Science.

Schematic depicting the roles of soft-acidic/hard-basic 2-(trimethylazaniumyl)acetate in enabling the low-temperature aqueous electrolyte and the cyclability of the anode-free Cu||Znxa-V2O5@Graphene pouch cell (115 × 95 mm2 in size) and its photograph (inset) (Image by WANG Xiao)

The researchers have proposed a new electrolyte design using zwitterions based on the hard and soft acids and based principle to restructure aqueous electrolytes.

They found that the incorporation of a soft-acidic/hard-basic zwitterion into an aqueous electrolyte could disrupt the hydrogen bonds of water molecules, weakening of zinc-trifluoromethanesulfonate (Zn2+ - OTf-) interactions, and resulting in the destabilization of the Zn2+ solvation sheath. Therefore, this electrolyte could enhance the anti-freezing phenomena with a solid–liquid transition temperature of -95 ℃ and Zn2+ desolvation kinetics.

By using this electrolyte, the researchers developed an anode-free full cell (Cu||Znxa-V2O5@graphene) which exhibited high energy and power densities (142 W h kg-1 at 50 mA g-1 and 230 W kg-1 at 2 A g-1) with stable cyclability at -40 ℃. It was comparable to low-temperature Li–metal batteries.

The above study was supported by the National Natural Sciences Foundation of China, and the DICP Innovation Foundation.