Mechanical Degradation

How particle expansion, cracking, and silicon swelling cause mechanical degradation and accelerate capacity fade.

Mechanical degradation refers to the structural damage that occurs in batteries due to repeated expansion and contraction during cycling. While SEI growth and lithium plating are electrochemical processes, mechanical degradation causes physical damage—cracking particles, breaking electrical connections, and deforming cell structures—that leads to capacity loss and resistance increase.

Volume Changes During Cycling

During each charge and discharge cycle, lithium ions move in and out of the active material particles in each electrode. This insertion and extraction cause the particles to expand and contract, introducing mechanical stresses and deformations.

Material-Dependent Expansion

The extent of this expansion varies depending on the material:

Material	Volume Change	Notes
Graphite	Moderate (~10%)	Standard anode material
Silicon	Extreme (~400%)	High capacity but significant challenges

Silicon is well known for its extreme expansion—swelling up to four times its original volume during lithiation. While silicon offers much higher capacity than conventional graphite anodes, this significant volume change presents a major challenge, leading to mechanical instability and faster degradation.

Understanding the mechanics of batteries is not only important to extend battery life but also to enable new chemistries.

Types of Mechanical Degradation

The following are some specific examples of how mechanical effects degrade batteries:

1. Particle Cracking

   Repeated expansion and contraction creates stress within active material particles, leading to surface cracks that expose fresh material to the electrolyte, promoting additional SEI formation and increasing resistance.

2. Binder Cracking

   Stress can also crack the binder that holds particles together, reducing electrical conductivity and causing loss of active material as sections of the electrode become electrically isolated.

3. Jellyroll Collapse

   In cylindrical cells, the wound electrode assembly (jellyroll) can deform or collapse due to internal pressure changes and electrode swelling, leading to uneven current distribution and localized degradation.

4. Electrolyte Pumping

   Repeated volume changes during cycling can pump electrolyte in and out of electrode pores, leading to electrolyte redistribution, dry spots, and accelerated local degradation.

Related Topics

• Degradation Overview—how mechanical effects contribute to LAM
• SEI Growth—accelerated by particle cracking exposing fresh surfaces
• Lithium Plating—another major degradation mechanism
• Electrode Essentials—electrode materials and their properties
• State of Health—tracking capacity fade from active material loss