Electrolytes are compounds that can conduct electricity when dissolved in aqueous solutions or in a molten state. They can dissociate into freely moving ions when dissolved in water or heated. Electrolytes are an important component of lithium-ion batteries, playing a role in transporting ions and conducting current between the positive and negative electrodes.
Electrolytes, as a key material in lithium-ion batteries, influence and even determine the battery's specific energy, lifespan, safety performance, and charge/discharge rate performance. A practical electrolyte for lithium-ion batteries should meet the following conditions:
1. Lithium-ion conductivity: Electrolytes do not have electronic conductivity, but must have good ionic conductivity. Generally, within a certain temperature range, the conductivity of electrolytes is between 1×10⁻³ and 2×10⁻³ S/cm. As an electrolyte, it must possess excellent ionic conductivity and electronic insulation to function as an ion transport medium while reducing its own self-discharge.
2. Ion transference number: The transport of charge inside a lithium battery depends on the migration of ions. A high ion transference number can reduce concentration polarization during electrode reactions, enabling the battery to achieve high energy density and power density. Ideally, the lithium ion transference number should be as close to 1 as possible.
3. Stability: When the electrolyte is in direct contact with the electrode, side reactions should be avoided as much as possible. This requires the electrolyte to have certain chemical and thermal stability.
4. Mechanical Strength: Lithium-ion battery electrolytes need to have sufficiently high mechanical strength to meet the requirements of large-scale battery production and packaging processes. Li et al. used trimethyl phosphate (TMP) as an additive in high-voltage electrolytes, and tested the battery with Li1.2Mn0.54Ni0.13Co0.13O2 as the positive electrode. The results showed that adding 1% TMP to the electrolyte can significantly improve the rate performance and cycle performance of the battery.
5. Excellent mechanical properties: Since it is in direct contact with the positive and negative electrodes, the polymer lithium battery electrolyte should have strong toughness to withstand stress changes during battery assembly, storage, and use without becoming brittle. Simultaneously, as a separator, it must also possess sufficient mechanical strength to suppress the formation and penetration of lithium dendrites, preventing short circuits between the positive and negative electrodes.
