Cell electrode pressure and expansion are elements that we need to consider when designing any battery pack. This is perhaps even more important when looking at large cell to pack designs.
During the charge and discharge processes, Lithium Iron Phosphate (LFP) cells experience highly restricted cell volume variation. This structural stability is fundamentally attributed to the robust PO4 tetrahedral structure and strong covalent bonds within the crystal lattice, which prevent significant physical expansion but also contribute to the material’s slow ion diffusivity.
Lithium iron phosphate crystal structure in view of Li+ one-dimensional diffusion channels [1]
During charging and discharging, the LFP cathode undergoes a first-order phase transition between LiFePO4 and FePO4. Due to the stable olivine crystal structure and strong covalent bonds between the octahedra, the LFP cathode experiences minimal structural changes and limited cell volume variation (less than 7%) during these cycles.
While intrinsic volume changes during a single charge-discharge cycle are minimal, repeated cycling over time leads to irreversible expansion and stress accumulation, which are influenced by several operational factors:
- SEI Formation: As the cells age through continuous cycling, the ongoing formation and growth of the solid electrolyte interphase (SEI) cause an accumulation of irreversible physical expansion within the cell.
- State of Charge (SOC) Windows: The extent of this expansion is highly dependent on the depth of the cycling window. Wider cycling ranges—particularly those extending all the way to 100% SOC—induce significantly greater irreversible expansion and result in larger increases in internal force. Conversely, using narrower SOC windows that avoid deep charging reduces this stress accumulation.
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This robust structural integrity is the primary reason LFP batteries exhibit an exceptionally long cycle life.
Comparing LFP to NMC we see that NMC cells expand 2 to 5x more than LFP cells over their lifetime.
Single Cycle Expansion (Reversible Breathing) During a single charge-discharge cycle, the intrinsic volume change at the material level for LFP is often less than 2%, whereas NMC materials can experience volume changes between 6% and 10%.
- LFP: The minimal cell-level expansion in LFP is due to a unique balancing effect. During charging, the graphite anode expands by 10% to 12%. However, the LFP cathode actually contracts by approximately 6.7% during this same phase. This offsetting effect results in a net cell thickness increase (reversible breathing) of only about 0.5% to 3.1% per cycle.
- NMC: NMC lacks the rigid 3D framework of LFP and undergoes significant anisotropic strain (uneven expansion and contraction). For example, the NMC lattice exhibits roughly 3.5% expansion just during the initial charge to 4.2 V. Because NMC’s structural changes do not neatly balance out like LFP’s, the cells undergo much more mechanical “working” and strain during each equivalent cycle.
Hence, the structural design of modules and packs containing NMC pouch and prismatic format cells need to more carefully take this expansion into account. Cell to Pack designs with LFP are easier to design and engineer robustly. Perhaps NMC cell to pack designs are best approached with cylindrical cells (Tesla and BMW).
References
- Boyan Shang, Advances in Lithium Iron Phosphate Cathode Materials for Lithium-Ion Batteries, Vol. 116 (2024): 7th International Conference on Chemical Engineering and Advanced Materials (CEAM 2024)
- Yang, Z.; Li, X.; Li, J.; Li, H.; Shi, J.; Fan, X.; Cong, Z.; Feng, X.; Yang, X.-G. Study on Influencing Factors of Calendar Aging and Cycle Aging of LFP Batteries. Appl. Sci. 2025, 15, 12749
- Maddipatla, S.; Rauf, H.; Osterman, M.; Arshad, N.; Pecht, M. Swelling Mechanisms, Diagnostic Applications, and Mitigation Strategies in Lithium-Ion Batteries. Batteries 2025, 11, 356.