So far, polymer or inorganic solid electrolytes that can meet the requirements of practical lithium-ion batteries are still very limited. Lithium-ion battery electrolyte is composed of organic solvents and inorganic salts, using LiPFs' ethylene carbonate (EC), propylene carbonate (PC) and low-viscosity diethyl carbonate (DEC) and other alkyl carbonates. Solvent system.
Lithium-ion battery electrolyte performance requirements
Ionic conductivity: good ionic conductivity, the conductivity should reach 10
The ion migration number is 2×103S/cm3 order of magnitude. The ideal lithium ion migration number should be close to 1 stability
Effectively block the battery at high temperatures
The room temperature conductivity is about -1×103s/cm on average, which is nearly two orders of magnitude lower than the aqueous electrolyte. Therefore, in order to enable commercial lithium-ion batteries to be charged and discharged at higher currents, the electrodes must be very thin to increase the total area of the electrodes and reduce the actual working current density of the electrodes.
The role of the electrolyte is to form a good ion conductive channel between the positive and negative electrodes inside the battery. Aqueous solutions, organic solutions, polymers, molten salts or solid substances can all be used as electrolytes.
Aqueous solution is currently the most widely used electrolyte.
The output power of organic electrolyte batteries is relatively low.
The use of molten inorganic salts as electrolytes is only used at high temperatures.
Conventional inorganic anion conductive salts mainly include: LiClO4, LiAsF6, LiBF4, LiPF4 these four types.
In the DMC electrolyte system, the order of the oxidation potential of several electrolyte lithium salts: LiPF6>LiBF4>LiAsF6>LiClO4
The change law of conductivity in EC/DMC electrolyte system: LiAsF6=LiPF6>LiClO4>LiBF4LiCIO4 is a strong oxidant due to the presence of high-valence chlorine, which can cause safety problems and cannot be commercialized.
LiBF4 has a relatively low electrolyte conductivity and can be used as an additive.
LiAsF6 electrolyte has high conductivity, but As is toxic and harmful to the environment, thus limiting its application.
Compared with other inorganic lithium salts, LiPFs has poor thermal stability and may decompose at room temperature 80°C. However, due to its high oxidation potential and conductivity, it is the main commercial lithium salt.
The properties of conventional solvents:
EC is an important component of the current electrolyte, with good film-forming properties, its dielectric constant is the highest, it can fully dissolve the lithium salt, and it is very beneficial to improve the conductivity of the electrolyte. However, the melting point of EC is 36.4°C, the boiling point is 238°C, and the viscosity is too high, which is not conducive to use under low temperature conditions.
EMC can improve the low temperature conductivity of the electrolyte and the low temperature performance of the battery.
The melting point of GBL is -43.5°C and the boiling point is 204°C. The reduction product is generally V-alkoxy-β-ketoester, which generates less gas and is beneficial to the safety of the battery.
EA has the lowest freezing point and low viscosity, so it can significantly improve the low temperature performance of the electrolyte.