Battery Basics IV: Electrolytes

This is the first post in our new Electrolyte Essentials series. We start with an overview of battery electrolytes and formation of the solid electrolyte interphase (SEI) in li-ion battery applications. 

What is an electrolyte?

An electrolyte is a solution composed of solvents and salt, similar to Gatorade except you wouldn’t want to drink battery electrolyte. 

The solvent mixture has the role of balancing stability with conductivity and influences properties such as viscosity and solubility, while the salt aids in ion conduction. 

The electrolyte itself is the medium through which ions can travel from the cathode to the anode in a lithium-ion battery, and vice versa. This movement is what enables charging and discharging in a lithium-ion battery.

Are we talking lithium metal or lithium-ion?

Lithium - ion

Lithium-ion electrolytes conventionally consist of a solvent mixture of organic carbonates with lithium hexafluorophosphate as the salt. Lithium salt contributes to the ionic conductivity of the electrolyte and boosts li-ion transport from the cathode. It will dissociate into lithium cations and anions (Li+ and PF6-). The typical solvents include dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, propylene carbonate. Vinylene carbonate is used as an additive in a formulation to assist with stable SEI formation.

Lithium Metal

Lithium metal electrolytes can also consist of lithium salts and organic carbonates like ethylene carbonate, dimethyl carbonate, and fluoroethylene carbonate. Additionally, ethers, such as 1,2-dimethoxyethane, are a popular choice due to performance optimization. Ether and carbonate-based formulations can lead to improved li-ion solvation and SEI formation. Ethers can suppress dendritic growth of lithium and enable inorganic SEI components to form. The polar carbonates have a high dielectric constant, allowing them to solvate lithium ions. Ultimately, a robust SEI aids in lithium deposition and stripping during charge/discharge cycles, preventing dendrite formation.

Read more on the SEI here. 

Some Resources We Love:

• This paper on electrolytes is an oldie but a goodie: Kang Xu’s review