Lithium battery electrolyte formulation composition and performance
1. electrolyte formula is composed of solvent and solute.
Solvents commonly used in lithium battery pack electrolyte formulations include dimethylformamide, y-butyrolactone, ethylene glycol, glycerol, water, etc. Dimethylformamide is a solvent for military lithium battery pack capacitors. It has a wide operating temperature range. In Asia, the use of y-butyrolactone as a solvent for electrolytes is also very popular. It is mainly used in low-voltage small capacitors. Capacitors using this solvent as the main solvent have lower ESR. Ethylene glycol is the most widely used solvent, and low-voltage capacitors usually use a system of ethylene glycol plus some water as the solvent.
The solutes in the electrolyte are mainly boric acid, borate, carboxylate and a small amount of other inorganic salts, depolarizing hydrogen eliminators, corrosion inhibitors, hydrolysis inhibitors, etc.
For the low-pressure electrolyte, a part of water is added artificially to improve the conductivity of the electrolyte; for the medium and high-pressure electrolyte, anhydrous glycol system is usually used, even if it contains water, it is required to be below 3%.
Second, the type of electrolyte formulation
From the electrolyte water content, it can be divided into: water system and non-aqueous system, part of the non-aqueous system is g-butyrolactone (GBL) system. It can be divided into: low-voltage, medium-voltage, high-voltage electrolyte
3. Selection of lithium battery electrolyte formulation materials
Working electrolyte is one of the key processes of capacitors. As the actual cathode of the capacitor, it needs to have excellent electrochemical characteristics, continuously provide oxygen ions, and repair the damaged oxide film; it must also have excellent stability and not react or corrode with the constituent materials. The working electrolyte is composed of three parts: solvent, solute and additives.
1. The choice of solvent
The solvent determines the application range of the capacitor. The solvent is required to have a high boiling point, a low freezing point, a low vapor pressure and a low viscosity, and a good solubility for the solute. The electrical performance requirements of capacitors in the range of -40~+105℃, some of them are 125℃ or even above 150℃ (such as aluminum electrolytic capacitors for energy-saving lamps). At present, the commonly used main solvent is ethylene glycol with a boiling point of 196℃ and a freezing point. It is -12°C, and other auxiliary solvents include: DMF, y-butyrolactone, diethylene glycol, glycerol, etc. DMF and y-butyrolactone have good low temperature characteristics, and are often used in ultra-low temperature electrolytes such as -55°C and other products.
2. The choice of solute
The solute is the main component of the positive and negative ions or ionic groups that repair the oxide film and maintain the normal operation of the capacitor. Low pressure: ammonium formate, ammonium adipate, ammonium maleate, etc. as solutes; medium and high pressure: ammonium azelate, ammonium sebacate, ammonium adipate, ammonium dodecanoate, ammonium benzoate, and branched Ammonium salt is used as its main solute to ensure high and low temperature characteristics, provide oxidation efficiency and high temperature stability.
3. Additive selection
Additives are used to adjust electrolyte viscosity, conductivity, increase flashover voltage, passivate oxide film and inhibit hydrogen. Adding phosphoric acid and its salts, nitro compounds, hydrolysis inhibitors, etc. to the electrolyte can significantly improve the characteristics of the electrolyte.
Four. Detection of electrolyte formula:
The detection of electrolyte formula mainly has three parameters: conductivity, pH value, flash voltage, etc., if necessary Check the viscosity of C1-, SO42-, Fe3+, Cu2+, and electrolyte.
PH value: generally distributed in 5.5-9.0, mainly due to the characteristics of aluminum and alumina, can be measured with a pH meter.
Conductivity: generally distributed in 400uS/cm ~80000uS/cm. It can be measured by a conductivity meter. The conductivity has a great relationship with product loss and equivalent series resistance.
Flash voltage: generally distributed in 100-600V, oxidized with aluminum foil As measured by the membrane withstand voltage tester, the flash voltage of the electrolyte is a key parameter, especially for high-voltage electrolytes. The flash voltage is one of the breakthrough directions in manufacturing technology. The measurement method and material of the flash fire voltage are different for different companies: the external measurement current is mainly 10mA, 20mA, etc., while our company generally uses 1mA. The materials used for measurement include: high-voltage formed foil (voltage is generally above 590VF), Plain foil, anode foil strip, etc., the measurement area used is also different.
Measure temperature, generally normal temperature, 30℃ is relatively accurate, especially conductivity, which is greatly affected by temperature, so the loss of capacitance at different temperatures is also quite obvious, and the pH and flash voltage are also Will change slightly with temperature.
Appearance inspection: Whether there are impurities, dirt, crystals, turbidity, whether the color is uniform, transparent, and whether there is any peculiar smell.
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