Theoretical Physics Advances in Research on Solid-Electrolyte Interface Layers for Li-Ion Batteries

[ Instrument Network Instrument R & D ] Lithium-ion batteries have already entered every aspect of people's lives. How to design more safe and high-performance lithium batteries is a question of great concern to everyone. During the charge and discharge of lithium-ion batteries, the electrode material and the electrolyte solution react on the solid-liquid interface to form a passivation layer covering the surface of the electrode material. Floor. Researchers have found that the SEI layer determines the performance of most batteries, but people's understanding of the structure and properties of the SEI layer is still very limited.

Researcher Wang Yankun and Ph.D. student Su Mao from the Institute of Theoretical Physics of the Chinese Academy of Sciences collaborated with Zhu Zihua's group from the National Laboratory of the Pacific Northwest (PNNL) to use the secondary ion mass spectrometry (SIMS) experimental method combined with molecular dynamics simulation methods to form and Physical and chemical properties were studied. The results were published in the journal Nature Nanotechnology.

Electrolyte is a term with a wide meaning, which is widely used in different industries. There are electrolytes in the body (also called electrolytes), as well as electrolytes used in the battery industry, and electrolytes in electrolytic capacitors and supercapacitors.

The composition of electrolytes used in different industries varies greatly, or even completely different.

For example, the electrolyte of the human body is mainly composed of moisture and PH buffer substances. The electrolytic solution of aluminum electrolytic capacitors contains GBL and other major solvents. The super capacitor electrolytic solution contains propylene carbonate or acetonitrile as the main solvent. The lithium manganese primary battery electrolyte contains propylene carbonate. Major solvents such as esters, ethylene glycol dimethyl ether, and lithium ion battery electrolytes contain major solvents such as ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. Their respective conductive salts are also completely different, For example, tetraethylammonium tetrafluoroborate in supercapacitor electrolyte, lithium perchlorate or lithium triflate is commonly used in lithium manganese primary batteries, and lithium hexafluorophosphate is used in lithium ion batteries.

When a lithium-ion battery is first charged, before any phase interface chemistry occurs, solvent molecules will rapidly self-assemble at the electrode-electrolyte interface to form an electrical double layer due to the effects of lithium ions and the surface potential of the electrode. The structure of this electrical double layer determines the phase interface chemistry of the battery. In particular, when the electrode is negatively charged, the electric double-layer structure on the surface will empty out anions (such as fluoride ions), thereby forming a thin, dense, inorganic SEI inner layer. The main function of this dense layer is to conduct lithium ions and insulate electrons. After the SEI inner layer is formed, it will further form an outer layer that is rich in organic molecules and can penetrate into the electrolyte. The study found that the main component of the SEI inner layer was lithium oxide, which denied the traditional view that lithium fluoride was contained at the phase interface. For some types of batteries, since fluorine ions play a very important role at the phase interface, fluorine-containing solvents or additives must be introduced.

This research solves the problem of the characteristics of the SEI layer that has long plagued people.

Source: Encyclopedia, Institute of Theoretical Physics