Researchers from the Institute of Metal Research (IMR) of the Chinese Academy of Sciences have developed a versatile compatibilizing-solvent plasticization strategy that resolves the longstanding incompatibility between electrochemically stable plasticizers and polymer matrices, paving the way for practical high-energy, high-safety solid-state lithium metal batteries.

The study was published in the Journal of the American Chemical Society on May 21.

Polymer electrolytes, particularly poly(vinylidene fluoride) (PVDF)-based systems, are promising for solid-state lithium metal batteries due to their high oxidative stability and ionic conductivity. Conventional Plasticizers, such as dimethylformamide, are crucial for facilitating ion transport. Yet, their inherent electrochemical instability causes them to continuously decompose at the electrode interfaces. Conversely, electrochemically stable plasticizers such as sulfolane are thermodynamically incompatible with PVDF, preventing the formation of homogeneous electrolyte films.

The researchers, led by Profs. LI Feng, SUN Zhenhua and CHENG Huiming, have now broken this barrier by proposing a compatibilizing-solvent-enabled plasticization strategy. By introducing a volatile compatibilizing solvent, they effectively lowered the Flory–Huggins interaction parameter of the mixed system, overcoming thermodynamic repulsion and yielding a homogeneous precursor solution.

As the membrane forms, the rapid evaporation of the compatibilizing solvent sharply increases the system viscosity, locking the plasticizer (sulfolane) within the three-dimensional polymer network and achieving uniform plasticization of otherwise immiscible components.

Molecular dynamics simulations and spectroscopic analyses further revealed that the PVDF-HFP copolymer interacts with sulfolane through atypical hydrogen bonding. This interaction not only restricts the free migration of the plasticizer, suppressing interfacial side reactions, but also reconstructs the solvation structure into an anion-aggregate-dominated configuration. Consequently, a stable LiF-rich interphase layer forms at both the anode and cathode interfaces, significantly enhancing electrode compatibility.

The resulting solid-state lithium metal batteries demonstrated exceptional performance. When paired with a 4.7 V high-nickel cathode, the battery cycled stably for 700 cycles at an ultrahigh rate of 20 C, equivalent to a full charge-discharge cycle in about three minutes, while retaining 81.9% of their capacity.

The researchers also fabricated an ampere-hour-scale pouch cell using a thin lithium anode with an N/P ratio of 1.1. The pouch cell operated stably for over 100 cycles and achieved an energy density of 451.5 Wh kg^-1, far exceeding commercial lithium iron phosphate cells (about 200 Wh kg^-1). In addition, the pouch cell passed nail penetration testing, demonstrating inherent safety.

This work overcomes the limitations of polymer-plasticizer combinations, expanding the design possibilities for polymer electrolytes and providing essential technological support for the practical implementation of high-safety, high-energy solid-state lithium batteries.