This is the first of 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. Detailed information on the SEI is provided below.

Solid Electrolyte Interphase (SEI)

SEI = A thin, passivating layer formed at the anode/electrolyte interface due to the electrochemical instability of the electrolyte. Ideally stable during cycling, electronically insulating, and permitting fast Li transport.

Additive packages within the electrolyte formulation are integral to the formation of a stable SEI. This film forms on the anode upon first charge of the battery – when lithium-ions shuttle from the cathode material to the anode. It is composed of degradation products from the electrolyte formulation.

An additive package highlights the importance of strategy and sacrifice in the sense that the chosen components will be subtracted from the overall electrolyte formulation in formation of the SEI – an unsung hero if you will. A battery requires an SEI to function, but a battery intended for high cycle life, power performance, and energy efficiency will have a carefully constructed SEI. To reduce ion transport distances, it is ideal for an SEI to be as thin as possible. And to ensure additional electrolyte degradation does not occur over the course of cycling, it is optimal for an SEI to be as stable as possible. This aids in preventing unnecessary electrolyte consumption and SEI formation as the battery cycles. Electrode materials are strong oxidizing and reducing agents, so the chemical passivation of the anode via SEI formation maintains the bulk electrolyte components over cell life.

Bulk electrolyte = the electrolyte solution unimpacted by electrodes or interfaces. So, by chemically passivating the anode, the electrolyte components will not be continuously consumed/degraded over cell life as lithium ions are allowed to pass through the film, but not electrons.

The formation of the SEI also accounts for the capacity lost during the first charge-discharge cycle. ICL stands for irreversible capacity lost and refers to the difference in the first charge capacity and first discharge capacity of a battery. The SEI formation consumes li-ions, which means they cannot participate in the discharge after the first charge, resulting in a loss of capacity or energy that is “irreversible.”

Formulation History Through Graphite Exfoliation

Historically, with lithium graphite intercalation compounds - a predecessor to graphite anodes that was studied as an alternative to lithium metal anodes - a major issue was the exfoliation of graphite by propylene carbonate – a component of li-ion electrolytes. PC reductively decomposes on the graphite surface at 0.8V, which leads to “physical disintegration of graphite”. This exfoliation would prevent any lithium ions from intercalation into the graphite as the intercalation occurs at a lower potential. Apparently, PC molecules co-intercalate with the li-ions into the graphite structure and their decomposition (gaseous byproducts like propylene) causes the weakly held together multi-layered graphite structure to fall apart. You can thank van de Waals forces for that weakness. Alternate organic molecules and solvents were utilized to make this chemistry work – Jeff Dahn and his team contributed significantly to understanding how changing the electrolyte formulation could address graphite exfoliation. Ethylene carbonate was utilized. EC and PC are very similar – PC just contains an additional methyl group. With EC added into the formulation, graphite exfoliation is prevented at 0.8V, while also encouraging the reversible intercalation and deintercalation of li-ions at a low potential. With PC still in the formulation, there is a small amount of graphite exfoliation that occurs, but much of it is suppressed thanks to our friend EC – what a solid rock you are (if you know, you know).

Source: Kang Xu’s Review

Electrolyte Essentials