Currently, solid-state lithium batteries can be divided into two types: inorganic solid-state electrolyte batteries and polymer solid-state lithium batteries. The performance of the electrolyte is crucial to the overall performance of the battery. It has a significant impact on battery cycle performance, operating temperature range, and battery durability. For lithium-ion batteries, the composition of the electrolyte involves at least two aspects: solvent and lithium salt.
The role of electrolyte in lithium-ion batteries
Electrolytes are one of the core materials for the capacity of both lithium-ion secondary and primary batteries, acting as a medium to improve the fluidity between the moving anode and cathode. As an important component of lithium-ion batteries, electrolytes play a crucial role in transporting ions and conducting current between the positive and negative electrodes. Selecting a suitable electrolyte is key to obtaining lithium-ion secondary batteries with high energy density and power density, long cycle life, and good safety performance.
The electrolyte in a lithium battery is the interaction between the electrolyte and the electrode materials. It undergoes decomposition reactions and participates in almost all reaction processes occurring within the battery. Currently, the electrolytes in lithium-ion batteries are mostly organic systems. Under conditions of abuse such as overcharging, over-discharging, short circuits, and thermal shock, the battery temperature rises rapidly, and the electrolyte is generally flammable, often leading to battery fires or even explosions.
Electrolytes are a crucial component of lithium-ion batteries, playing a role in transporting ions and conducting current between the positive and negative electrodes. Based on phase state, lithium-ion battery electrolytes can be classified into three types: liquid, solid, and molten salt electrolytes.
Electrolyte requirements for lithium-ion batteries
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.
In summary, both liquid electrolytes and gel electrolytes, which are currently the mainstream technologies in the field of power lithium-ion batteries, pose certain safety risks. Therefore, developing electrolytes that are resistant to short circuits, overcharge, thermal runaway, combustion, and non-flammable is key to ensuring the safety of power batteries.
