An Introduction to Battery Binders

Battery binders are typically polymeric materials that are used like glue to hold together electrode components. Crucial for providing stability, they ensure even coatings and robust adhesion. Carefully selected electrochemical properties prevent unwanted chemical reactions with the active electrode materials. Importantly, they allow efficient electron and ion movement within the battery system.

Whilst they are considered inactive components, they play a pivotal role in ensuring battery lifespan and quality. When used in electrode fabrication these materials must have sufficient strength, elasticity, flexibility, and hardness to maintain structural integrity.

The roles of a battery binder are:

• Facilitate the formation of uniform electrode slurries
• Aid in binding and adhering of the slurry to the surface of the current collectors
• Prevent the active materials from falling off during the charge/discharge processes of intercalation/de-intercalation.

Cathode Requirements

• Binder HOMO should be lower than the chemical potential of the cathode.
• The binder should help prevent volume expansion and corrosion of the cathode active material.
• Ideally the binder should help improve the relative low conductivity of cathode active materials.

Anode Requirements

• Binder LUMO should be higher than the chemical potential of the anode.
• Must be compatible with solid electrolyte interphase (SEI) and passivation layer formation which dominates some anode surface chemistry (particularly graphite based anodes).
• Silicon powder based anodes require extra support due to the large volume change during charging and discharging cycles.

Mechanical Properties

The mechanical properties of battery binders vary depending on the polymer type, molecular structure, and crystallinity. Mechanical characteristics determine how well the electrode and resulting battery can withstand the intercalation and de-intercalation mechanisms. The expansion and contraction of the electrode active materials are reliant on the tensile strength, flexibility, elasticity, and adhesive strength of the polymer binder.

Stability

Binders play a large role, especially in intercalation type batteries, in maintaining chemical and thermal stability. Binders should have reversible redox activity or be redox-inactive, meaning that they should not oxidize or be reduced at highly positive or low potentials. The binder should be chemically stable in order to prevent the build up of unwanted by-products which lead to at best poorer efficiency and at worst increased risk of battery damage and fire-risk.

Electrochemical Properties

Electrochemical stability is crucial to endure electrochemical reactions and prevent the corrosion of electrolytes during battery operation. Binders should have reversible redox activity or be redox-inactive. The electrochemical activity of binders can be beneficial for increasing the specific capacity of the electrodes. The lowest unoccupied molecular orbital (LUMO) of the binder used in the anode should be positioned higher than the chemical potential of the active material. Alternatively, the highest occupied molecular orbital (HOMO) of the binder should be lower than the chemical potential of the cathode.

Conductivity

The electrical and ionic conductivity of binders is essential for high performance batteries. Some binders are not inherently electronically conductive so must be used alongside conductive additives.

Ionic conductivity controls the movement of solvated ions through the binder. This is dependent on porosity, viscosity and crystallinity. Highly crystalline binders tend to have lower conductivities due to the lower free volume for ion transport. The contact and wettability relationship to the electrolyte component is also important in determining ionic conductivity.