Recently, a research team led by Shen Yanbin, a researcher at the Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, proposed a design concept of anion-modulated polymer electrolyte (hereinafter referred to as AMPE), and developed a polymer electrolyte that takes into account the interface stability of high-voltage cathode and lithium anode and has a high room-temperature electrical conductivity. It has achieved good cycling stability in high-voltage solid-state lithium metal batteries. The relevant achievements were published in Angewandte Chemie International Edition.

Polymer electrolytes have good flexibility and can form low-impedance interfaces with electrode materials, thus having a promising application prospect in solid-state batteries. However, polymer electrolytes usually have a relatively low room-temperature conductivity and a narrow electrochemical window, making them unsuitable for high specific energy solid-state lithium metal batteries. Therefore, the development of polymer electrolytes with high ionic conductivity and good interfacial compatibility is one of the important research directions in the field of solid-state batteries. Based on previous research such as the development of single-ion polymer electrolytes, the study of interface transport mechanisms, and the construction of solid-state electrode transport networks, the team proposed a design method for polymer electrolytes to match high-voltage solid-state lithium batteries.

Specifically, this work employs ionic liquid monomers that can withstand high voltages and have high charge density as the polymer framework to ensure the high voltage tolerance of the polymer chain on the positive electrode side and sufficient carriers. To address the issues of concentrated charge density and weak lithium salt dissociation ability in polyionic liquid electrolytes, the team introduced anion receptors, which interact with the anions on the monomers of the ionic liquid by their electron-deficient groups, promoting the uniform distribution of anions in the electrolyte and increasing the lithium ion migration number of the electrolyte. At the same time, it helps to dissociate lithium salt anion and cation pairs and promotes the generation of free lithium ions.

It is worth mentioning that theoretical calculations and experimental studies have found that the interaction between the electron-deficient groups in the anion accepter and the lithium salt anion TFSI- reduces the LUMO (minimum unoccubed molecular orbital) of the lithium salt anion, making it easier to be reduced on the lithium metal anode, thereby decomposing to form a stable electrolyte interface layer and improving the cycling stability of the lithium metal anode. Through the above design, the obtained AMPE has a high ionic conductivity, a high lithium ion migration number, a wide electrochemical stability window, and can also effectively inhibit the growth of lithium dendrites.

In this study, anionic receptors were utilized to modulate the charge density of high-voltage-resistant polyionic liquids, and a high-performance polymer electrolyte was designed. Anion receptors assist in the dissociation of anion and cation pairs to promote the generation of free lithium ions, and the anion anchoring effect increases the lithium ion migration number from 0.13 to 0.41. Meanwhile, the strong interaction between electron-deficient groups and TFSI- promotes the reduction and decomposition of TFSI-, forming an electrostally stable SEI (Solid Electrolyte interface) on the surface of the lithium anode, which improves the cycling stability of high-voltage lithium metal batteries. This design method provides a new idea for the development of polymer electrolytes for high specific energy solid-state lithium batteries.