An electrolyte is a chemical compound that facilitates the movement of lithium ions between the positive and negative electrodes and resist electrons during the battery's charge and discharge. The electrolyte can be liquid, solid or a gel-like substance that contains lithium ions, which can move under a potential. The electrolyte plays a critical role in the performance of a lithium-ion battery, as it affects the battery's capacity, voltage, and safety.

The composition of the electrolyte in a lithium-ion battery can vary depending on the specific type of battery and its application, it is carefully designed and optimized to balance the requirements for high energy and power density, good stability, low toxicity, and safety. In general, the electrolyte is composed of a solvent, a lithium salt, and sometimes additives.
  • The solvent is usually an organic solvent which can be either one or the combination of many, such as ethylene carbonate (EC), polyethylene carbonate (PC), dimethyl carbonate (DMC), or diethyl carbonate (DEC). These solvents are chosen because they have a high dielectric constant, which allows them to dissolve the lithium salt and facilitate the movement of lithium ions.
  • The lithium salt is typically a lithium-ion conductor, such as lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), or lithium tetrafluoroborate (LiBF4). The lithium salt is dissolved in the solvent to form the electrolyte solution.
  • Additives may also be added to the electrolyte to improve its performance. For example, some additives can improve the stability of the electrolyte, prevent unwanted side reactions, or enhance the battery's safety.

SEI Layer & Electrolyte
The electrolyte plays an important role in the formation and stability of the SEI (solid electrolyte interface) layer in a lithium-ion battery. The SEI layer is formed on the surface of the negative electrode as a result of the reaction between the electrolyte and the electrode material during the first few cycles of charging and discharging. The SEI layer is a critical component of the battery, as it stabilizes the electrode-electrolyte interface and prevents further reaction between the electrolyte and the electrode.

The formation and properties of the SEI layer depend on the nature and composition of the electrolyte. The electrolyte should have good compatibility with the electrode material to promote the formation of a stable and uniform SEI layer. The solvent in the electrolyte should be able to dissolve the lithium salt and facilitate the movement of lithium ions, while minimizing unwanted side reactions that can lead to the degradation of the SEI layer.

In addition, the properties of the electrolyte can affect the stability of the SEI layer over time. For example, a highly stable electrolyte can prevent the degradation of the SEI layer and improve the long-term performance of the battery.

 Optimization of electrolyte
 A good electrolyte for a lithium-ion battery depends on the specific application and performance requirements of the lithium-ion battery. In general they can be as follows
  • High ionic conductivity: The electrolyte should have a high ionic conductivity to facilitate the movement of lithium ions between the positive and negative electrodes.
  • High electrochemical stability: The electrolyte should be stable under the operating voltage range of the battery to avoid decomposition, which can lead to the formation of unwanted byproducts that can degrade the performance of the battery.
  • Low viscosity: The electrolyte should have low viscosity to reduce the resistance to ion flow and improve the battery's power density.
  • Low volatility: The electrolyte should have low volatility to minimize the risk of the electrolyte evaporating, which can cause a loss of capacity or even a safety hazard.
  • Wide electrochemical window: The electrolyte should have a wide electrochemical window to allow for a high operating voltage range, which can increase the energy density of the battery.
  • Good compatibility with electrode materials: The electrolyte should be compatible with the electrode materials to prevent undesirable side reactions or degradation of the electrodes.
  • Good thermal stability: The electrolyte should be stable at high temperatures to avoid thermal degradation, which can lead to the formation of unwanted byproducts or even a safety hazard.


Other Electrolytes
Conventional liquid electrolyte is used with the general separator made of polypropylene or polyethylene. These polymers doesn't soak any electrolyte and remain distinctive physically. Few polymers like PEO and PANI soak and form a gel and thus the combination named as gel polymer electrolyte. Dry polymer like PEO has low ionic conductivity and high internal resistance. These properties are enhanced at relatively high temperatures of above 30C which is not practicable to use everyday.

Even though liquid electrolytes are quite popular in the present industry, they have severe limitations like (i) Safety (ii) Voltage window, research is moving towards solid electrolytes but the ionic conductivity is too low at room temperature for replacing liquid electrolyte. Different varieties of solid oxide and polymer electrolytes are been researched vigorously to replace them. Even organic polymers also have similar conductivity but huge amount of research is going on to make them successful.