A typical liquid electrolyte for lithium secondary batteries dissolves lithium salt in an organic solvent. Although there are many types of organic solvents and lithium salts, not all solvents and lithium salts are suitable for use in lithium secondary batteries. The liquid electrolyte used in lithium secondary batteries needs to have the following characteristics:
1) The electrolyte should have high ionic conductivity.
The battery with high ionic conductivity electrolyte has excellent electrochemical performance. The migration of lithium ions in the electrode and the diffusion in the electrolyte are very important for the rapid charge and discharge of lithium secondary batteries. At room temperature, the ionic conductivity of the liquid electrolyte of the lithium secondary battery should be higher than 10-3S/cm.
2) There must be good chemical and electrochemical stability between the electrolyte and the electrode.
Since the electrochemical reaction occurs at the negative electrode and the positive electrode of the lithium secondary battery, the electrolyte should have electrochemical stability within the voltage range of the redox reaction of the two electrodes. In addition, the electrolyte should also be chemically stable to different metals and polymers, which are an important part of the positive electrode, negative electrode and battery.
3) The electrolyte should be used in a wide temperature range.
Lithium-ion batteries using liquid electrolyte are generally widely used in mobile devices, so they need to meet the above requirements in the range of -20-60°C. At higher temperatures, the electrochemical stability of the liquid electrolyte will decrease and the ionic conductivity will increase.
4) The electrolyte must have high safety performance.
The organic solvent in the electrolyte is easy to burn, and when a short circuit occurs, it will be heated to a higher temperature, causing combustion or explosion. A high ignition point or high flash point is advantageous, and non-flammable materials can be used if possible. The electrolyte should also have low toxicity to prevent leakage and waste.
5) The electrolyte should be low cost.
If the cost of high-performance electrolytes is too high, they may be difficult to commercialize. Taking into account the fierce market competition of lithium-ion batteries, high-cost materials are unlikely to be adopted.
As described above, the electrolyte of a lithium secondary battery needs to have high ionic conductivity in a wide temperature range and maintain electrochemical stability in a voltage range wider than the operating voltage of a lithium battery. The properties of the electrolyte are determined by the solvent and the lithium salt, and vary with the combination.