Electrochemical Storage Degradation Mechanisms in Prismatic Lithium Iron Phosphate Cells
Prismatic LFP degradation stems primarily from loss of active lithium to anode SEI growth, accelerated by high state-of-charge storage and stack pressure.
Jellyroll Breathing
Meaning
Periodic expansion and contraction of the wound electrode assembly inside a battery occurs as ions move in and out of the host materials during normal charge and discharge cycles. A rhythmic change in the volume of the internal components happens every time the battery is used. This jellyroll breathing must be accommodated by the cell design to prevent mechanical fatigue and the failure of internal connections.
It governs the choice of materials for the separator and the thickness of the electrode coatings. The term applies specifically to cells where the electrodes are rolled into a cylindrical or flattened coil.
Cyclic Motion
Physical movement of the layers is a direct result of the atomic scale changes in the electrode structure. During jellyroll breathing, the diameter or thickness of the coil increases during charging and decreases during discharging. This movement creates friction between the layers and can lead to the wear of the separator.
If the layers are wound too tightly, the expansion can cause the metal current collectors to wrinkle or tear. If they are too loose, the contact between the active material and the collector may be lost. Manufacturers use specialized winding machines to ensure that the tension is consistent and that there is enough room for this growth.
Internal Stress
Mechanical strain within the coil can lead to the formation of gaps or the misalignment of the electrodes. As jellyroll breathing proceeds over hundreds of cycles, the cumulative stress can cause the ends of the roll to deform. This deformation might put pressure on the tabs that connect the electrodes to the external terminals.
A broken tab will disconnect a portion of the cell, leading to an immediate loss of capacity and an increase in resistance. The design must also account for the electrolyte, which acts as a lubricant but can also be squeezed out of the active areas. Managing these internal forces is a core challenge for battery engineers.
Failure Mode
Long term fatigue from repeated volumetric changes can eventually lead to the breach of the cell containment or an internal short circuit. Jellyroll breathing contributes to the degradation of the solid electrolyte interphase, as the expanding surface of the graphite can crack the protective layer. This exposes fresh lithium to the electrolyte and consumes the battery’s energy reserves.
In extreme cases, the mechanical stress can cause the separator to fail at the edges of the roll. Once the separator is compromised, the cell can enter thermal runaway due to the high current flow at the shorted point. Understanding the limits of this expansion is necessary for predicting the total number of cycles a battery can survive.