Electrolyte

The electrolyte is the medium that allows ionic transport between the electrodes during charging and discharging of a cell.

Electrolytes in lithium ion batteries may either be a liquid, gel or a solid. Lithium batteries use non-aqueous electrolytes because of reactivity of lithium with aqueous electrolytes and the inherent stability of non-aqueous electrolytes at higher voltages.

Liquid electrolytes are a combination of a solution of solvents, salts and additives. The liquid electrolyte in Li-ion cells is typically lithium hexafluorophosphate (LiPF₆) dissolved in a mixture of organic solvents.

Lithium Ion Cell

When discharge begins the lithiated carbon releases a Li⁺ ion and a free electron.

Electrolyte, that can readily transports ions, contains a lithium salt that is dissolved in an organic solvent. The Li⁺ ion, which moves towards the electrolyte, replaces another Li + ion from the electrolyte, which moves towards the cathode. At the cathode/electrolyte interface, Li⁺ ions then become intercalated into the cathode and the associated electron is used by the external device.

The role of the electrolyte [2] is to act as a medium for ionic conduction and a barrier for electronic conduction to avoid self-discharge of the cell. Other essential requirements are:

• viscosity of electrolytes, which influences ionic mobility, should be low (<2 cP);(2)
• Chemical and thermal stability in a wide range of voltage and temperatures
  • A wide electrochemical stability window (0.01–5 V vs. Li/Li⁺) of operation
  • stable against oxidation and reduction reactions
• able to withstand the potential window of the electrochemical reaction
• good Li+ conductivity
  • Ionic conductivity should be greater than 1 mS cm⁻¹
  • adequate diffusion of Li-ions
• does not dissolve the SEI
• Environment friendliness, cost-effectiveness, and safety in operating conditions.
  • low toxicity
• low cost
• high dielectric constant (>20) of solvents helps to dissociate salt
• Inert behavior towards other battery components such as separator, current collector, and packaging materials

The interplay between Electrolyte Mass, Capacity and C-Rate: as the C-rate increases (indicating faster charge/discharge), the electrochemical environment becomes more demanding.

Electrochemical Potential Window of Battery Electrolytes

The energy diagram [1] illustrates the electrochemical stability window of an electrolyte in a battery cell, relative to the energy levels of the anode and cathode electrodes. 

The overall cell voltage (𝐄_{𝐜𝐞𝐥𝐥}) is related to the difference in the electrochemical potentials of the cathode and anode. 

The diagram shows the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the electrolyte. 

For an electrolyte to be stable, the anode and cathode electrochemical potentials must fall within this stability window. 

Acetonitrile Electrolyte

An acetonitrile (CH₃CN) electrolyte with a salt and other additives has high ionic conductivity. This high ionic conductivity allows ease of movement of ions between the anode and cathode.

Additives

Anti-Perovskite Solid Electrolyte

A solid electrolyte such as Li3OCl with an inverted perovskite structure that is intrinsically stable against lithium metal.

Argyrodite

A family of Li₆PS₅X-type (X = Cl, Br, I) sulfide solid electrolytes prized for very high room-temperature ionic conductivity, though sensitive to moisture.

Cell Manufacturing

In the cell manufacturing process the filling of the cell with electrolyte and ensuring it fully encompasses all areas of the active materials is critical for the cell performance and lifetime.

What are the Electrolyte Fill Requirements for a cell versus chemistry, capacity, format, lifetime and other parameters?

The calculation is based on the porosity of the cathode, anode and separator. Added to this is the free volume and then a multiplier to account for losses in the filling process.

Image: Thomas Knoche, Florian Surek, Gunter Reinhart, A process model for the electrolyte filling of lithium-ion batteries, 48th CIRP Conference on MANUFACTURING SYSTEMS – CIRP CMS 2015, Procedia CIRP 41 ( 2016 ) 405 – 410

Cold Temperature Performance

The electrolyte plays a key part in the Cold Temperature Charge / Discharge performance of the Lithium-Ion cell.

Below 0 °C, the viscosity of the electrolyte increases while the Li⁺ conductivity decreases, limiting the process of Li⁺ diffusion.

The low-temperature restriction of Li⁺ transport in the electrolyte is much larger than that in the electrode.

At the electrode–electrolyte interface the wettability and compatibility of the viscous electrolyte with respect to the electrode and separator become worse, increasing the resistance of LIBs, while a thicker SEI on the anode side hinders transport on the interface, as well as the desolvation process.

Evan M. Erickson et al [4] show how the conductivity of the electrolyte increases with temperature.

The difference in conductivity of the electrolyte between -20°C and 20°C can be 4x higher.

Composite Solid Electrolyte

A solid electrolyte that blends a polymer matrix with ceramic filler particles to combine mechanical flexibility with higher ionic conductivity.

Decomposition

When thinking about electrolyte decomposition we should consider this in stages:

• decomposition of electrolyte in bulk storage
• decomposition of electrolyte prior to cell formation
• in use decomposition / aging
• decomposition of the electrolyte during thermal runaway

Deep Eutectic Solvent Electrolyte

An electrolyte based on a low-melting-point mixture of two or more components, explored as a lower-cost, less flammable alternative to conventional carbonate solvents.

Electrolyte Motion Induced Salt Inhomogeneity (EMSI)

Prof. Dahn emphasized that the quantity of liquid electrolyte has to be very carefully controlled in such high-energy cells to avoid ‘Electrolyte Motion Induced Salt Inhomogeneity’ (EMSI), which can lead to temporary capacity loss (reversible after letting cells rest for weeks) and lithium plating (risk of irreversible damage). Prof. Dahn referred to work by Dr. Sophie Solchenbach et al [1]. from BMW, who illustrate that an ‘electrolyte pore filling ratio’ in Si active material-containing cells of slightly below 1 should be targeted (‘sweet spot’, as the presence of too little electrolyte also can cause problems).

Fluorinated Electrolyte (Fluoroether)

An electrolyte solvent containing fluorine substitutions that improve oxidative and thermal stability compared with standard carbonate solvents.

Fluoroethylene Carbonate (FEC)

The most widely used electrolyte additive for forming a stable solid electrolyte interphase on silicon and lithium metal anodes.

Garnet Solid Electrolyte (LLZO)

A cubic, garnet-structured oxide solid electrolyte with high ionic conductivity and good stability against lithium metal, one of the most widely studied solid-state battery materials.

Gel Polymer Electrolyte

Liquid electrolyte immobilized within a polymer matrix, offering some of the safety benefits of a solid electrolyte while retaining liquid-like ionic conductivity.

High-Concentration Electrolytes

Engineered by combining a high concentration of lithium salt with weakly coordinating diluents. This unique chemical composition drives the creation of an anion-rich solvation structure. This tailored environment is highly advantageous because it enables the formation of dense, inorganic Solid Electrolyte Interphase (SEI) and Cathode Electrolyte Interphase (CEI) protective layers

Ionic Liquid Electrolytes

Ionic liquids offer a different starting point. They are salts that are molten at, or close to, room temperature, built from bulky, asymmetric organic cations (imidazolium, pyrrolidinium, piperidinium) paired with fluorinated anions such as TFSI or FSI. Because the entire electrolyte is composed of ions rather than a molecular solvent carrying dissolved ions, it has negligible vapour pressure and does not sustain a flame. 

LISICON

An early family of lithium-super-ionic-conductor solid electrolytes that helped establish the design principles later used in sulfide and oxide solid electrolytes.

Localised High Concentration Electrolytes

Characterised by a high concentration of lithium salt combined with weakly coordinating diluents. The key advantages and applications of LHCEs include:

• High Anodic Stability and Efficiency
• Enhanced Safety and Flame Retardancy
• Improved Operational Range

NASICON Solid Electrolyte

A sodium-super-ionic-conductor framework structure used both as a solid electrolyte and, in modified form, as the ceramic membrane in seawater battery designs.

Non-Aqueous Electrolyte

Any electrolyte based on organic solvents rather than water, the standard category for conventional lithium-ion cells.

Oxide Solid Electrolyte

The general family of solid electrolytes, including garnet, NASICON and perovskite types, that are more air-stable than sulfide electrolytes but harder to densify into thin layers.

Perovskite-Type Solid Electrolyte (LLTO)

A solid electrolyte with high bulk ionic conductivity but poor grain-boundary transport, and a tendency to reduce on contact with lithium metal.

Polymer-in-Salt Electrolyte

An electrolyte formulation where the salt concentration is high enough that the polymer becomes the minority component, boosting ionic conductivity.

Quasi-Solid-State Electrolyte

An electrolyte intermediate between liquid and fully solid, retaining a small amount of liquid for interfacial contact while improving on liquid-electrolyte safety.

Safety

Cell Temperature

The cell has to be operated within safe temperature.

If the temperature is too high the electrolyte will decompose, the result vapour and gases will result in a soft venting of the cell.

Hence one option for detecting a cell on the edge of thermal runaway is to have some form of Electrolyte Detection in the Battery Management System.

Do Non-Flammable Electrolytes make Batteries Safer? Work led by Siddhartha Nanda and Hadi Khani [2] published in Royal Society of Chemistry journal of Energy and Environmental Science, dives into the sources of heat generation for Triethyl Phosphate (TEP) with KFSI electrolyte and a potassiated graphite anode: Our findings demonstrate that low flammability alone does not ensure thermal safety; rather, interfacial reactivity governs risk.

Single-Ion Conducting Polymer Electrolyte

A polymer electrolyte with the anion immobilized on the polymer backbone, pushing the lithium-ion transference number toward 1.

Solid Polymer Electrolyte

Solid polymer electrolytes (SPEs), such as polyethylene oxide (PEO)–LiTFSI, polycarbonates, and block-copolymer systems, trade some conductivity (10⁻⁵–10⁻³ S cm⁻¹ at 60–90 °C) for exceptional processability. Moreover, SPEs can be cast as free-standing films, laminated at low pressure to electrodes, and formulated with ceramic fillers or ionic plasticizers to tune modulus and transport. 

Sulfide Solid Electrolyte

A solid electrolyte family, including argyrodites, offering the highest room-temperature ionic conductivity of any solid electrolyte class but sensitive to moisture.

Transference Number

The fraction of total ionic current carried by the lithium cation (Li⁺) in the electrolyte. A transference number close to 1.0 means Li⁺ carries nearly all the current, minimising concentration gradients across the cell. Typical liquid electrolytes have Li⁺ transference numbers of 0.3–0.4; single-ion conducting solid or polymer electrolytes aim for values approaching 1.0.

Water-in-Salt Electrolyte

An aqueous electrolyte in which the salt-to-water ratio is inverted from normal, widening the electrochemical stability window enough to support higher-voltage aqueous battery chemistries.

Wettability

The ability of the electrolyte to spread and penetrate the porous electrode materials, crucial for battery performance and safety. Poor wettability can lead to issues like reduced capacity, increased resistance and safety problems.

References

1. Christina Sauter, Raphael Zahn and Vanessa Wood, Understanding Electrolyte Infilling of Lithium Ion Batteries, Journal of The Electrochemical Society, 2020 167 100546
2. Yuliya Preger, Loraine Torres-Castro, Jim McDowall, Chapter 3 Lithium-ion Batteries, Sandia National Laboratories and Saft America Inc.
3. Das, Dhrubajyoti, Sanchita Manna, and Sreeraj Puravankara. 2023. Electrolytes, Additives and Binders for NMC Cathodes in Li-Ion Batteries—A Review Batteries 9, no. 4: 193
4. Evan M. Erickson, Elena Markevich, Gregory Salitra, Daniel Sharon, Daniel Hirshberg1, Ezequiel de la Llave, Ivgeni Shterenberg, Ariel Rosenman, Aryeh Frimer and Doron Aurbach, Review—Development of Advanced Rechargeable Batteries: A Continuous Challenge in the Choice of Suitable Electrolyte Solutions, Journal of The Electrochemical Society, Volume 162, Number 14

See also: Separator, Electrolyte Solvent Ratios, Solid Electrolyte Comparative Matrix, Designing Batteries for Winter: Why the Electrolyte is Everything