Lithium-ion batteries have the advantages of long service life, large energy density and stable charge and discharge performance. They have been widely used in daily electronic products and are also one of the major candidate power supplies for many large-scale mobile devices. Increasing the working voltage of the battery is an effective way to obtain a high specific energy lithium battery, so a high voltage electrolyte system needs to be developed. This article will summarize several high-pressure electrolyte systems.

New carbonate high-voltage electrolyte

When the working voltage of the traditional carbonate electrolyte reaches 4.5V or above, a violent oxidation and decomposition reaction will occur, resulting in the process of taking off or embedding the lithium being unable to proceed normally. The new carbonate electrolyte can effectively improve the high pressure performance of carbonate electrolyte:

Improved generation

—Fluorocarbonates: Polyfluoroalkyl carbonates are chemically stable, hydrophobic and oleophobic. They produce a double passivation film on the electrode surface to reduce electrode surface degradation and electrolyte breakdown. Moreover, the longer the carbon chain of the perfluorocarbon substituent, the stronger the nucleophilic ability is. The more the passivation film is formed on the surface of the electrode, but the intermolecular force also increases accordingly, resulting in the increase of the viscosity and the decrease of the conductivity.

Compound generation

—Phosphorus-containing carbonate: adding appropriate amount of additives such as tris(2,2,2-trifluoroethyl)phosphite (TTFP) into carbonate: stable CEI passivation film can be formed on cathode surface; the phosphorus (III) atoms in TTFP center have a pair of lone pairs of electrons, can form a stable lithium salt complex in the electrolyte containing LiPF6; phosphorus (III) atoms are not in the highest valence state, easily oxidized to generate soluble phosphate compounds to effective inhibite the oxidative decomposition of carbonate, which further improve the battery cycle performance.

—Boron-containing carbonates: Boron-containing compounds can form stable CEI membranes on different cathode surfaces and improve the stability of other electrolytes on the electrode surface.

The new carbonate electrolyte can effectively improve the high pressure stability of the electrolyte, but how to improve the ignition point of the carbonate electrolyte, reduce the volatility, and further improve the battery safety performance, researchers still need to continue their efforts.

Nitrile high-voltage electrolyte

The non-protonic aliphatic dinitrile compounds NC-(CH)n-CN (n=3~8) have high voltage resistance and safety characteristics as electrolytes and exhibit good electrochemical stability at voltage of 7~8V, which also have high flash point and flash point. Addition of EC or DMC to nitrile solvents can significantly improve the compatibility of nitrile electrolyte with graphite electrodes and improve the solubility of lithium salts. In addition, 1,3,6-hexanetricanetricitrile has great advantages in high-voltage lithium battery electrolyte due to its high temperature storage and cycling performance. However, how to reduce the toxicity and the production cost of nitrile solvents is still an urgent problem to be solved in the application of such electrolytes.

Sulfone organic matter has dielectric constant of 40 or more, they are in a stable state at a voltage of 5.5V. For example, sulfolane (SL) is a common solvent with high dielectric constant, wide electrochemical window and strong polarity. However, sulfone organics have high viscosity, high melting point and poor compatibility with graphite anode materials. It often need to add additives to reduce viscosity and improve the conductivity of the electrolyte. Therefore, improving the safety performance of sulfone electrolyte and reducing the viscosity of sulfone are still the research directions.

Ionic Liquid High Pressure Electrolyte

Ionic liquids (ILs) are room-temperature molten salts composed of anion and cation. They have the characteristics of high flash point, high ignition point, low volatility, high dielectric constant and wide electrochemical stability window. In the present study, the anions are mostly defined as TFSI-, which is easily reduced to insoluble Li+ compounds at low potentials and forms a passive film on the surface of lithium and graphite anodes. Pyrrole and piperidine electrolytes have good cycle stability at 2.5~4.8V, but the conductivity is relatively low, the resistance is higher; imidazole electrolyte has high conductivity, but its cycle stability is poor, it needs further improvement.

Lithium-ion batteries are the most effective energy storage devices. The oxidation and decomposition of common Li-ion battery electrolytes at high voltage limits the development of high-voltage lithium-ion batteries. By developing a new type of high pressure resistant electrolyte, the electrolyte can show the advantages of high cycle stability, high conductivity and good compatibility. However, how to improve the safety of new organic solvents and optimize the reaction conditions still need further study.