Meaning
A volumetric expansion phenomenon occurs when the physical dimensions of a battery cell increase as it moves from a depleted state to a fully charged condition. Scientists attribute state of charge swelling to the intercalation of lithium ions into the crystal structure of the anode material, which causes the lattice to expand. The process is largely reversible during discharge, although a small amount of permanent growth often accumulates over many cycles.
In prismatic and pouch cells, this change is most noticeable in the thickness of the unit, while cylindrical cells constrain the movement through their rigid steel casings.
Material Behavior
Electrode materials like graphite typically expand by about ten percent when fully lithiated. Silicon-based materials exhibit much larger state of charge swelling which can reach several hundred percent if not properly managed. The magnitude of this effect is a primary consideration in the design of the cell housing and the battery pack enclosure.
Mechanical Pressure
Mechanical pressure from expansion exerts force against the neighboring cells and the pack frame. If the design does not account for state of charge swelling with compressible foam pads or sufficient clearance, the pressure can reach levels that damage the cooling plates or the electrical connections. Engineers use load cells to measure the pressure generated at various stages of the charge cycle.
Managing Expansion
Managing the physical movement of the cells is necessary to prevent premature mechanical fatigue of the internal components. Software algorithms sometimes limit the maximum charge level to reduce the amount of state of charge swelling and extend the life of the battery. The relationship between the electrical energy stored and the physical volume of the cell must be accurately mapped for every new chemistry.
