In this study, I systematically investigated the swelling force behavior of large-format lithium iron phosphate (LFP) energy storage battery modules during cyclic aging. The research focused on a 280 Ah prismatic LFP cell, assembled into modules with different series configurations (1P8S and 1P12S), and tested under constrained fixture conditions that simulate real-world energy storage battery module applications. By conducting long-term cycle durability tests, I analyzed the evolution of swelling forces across the full state of charge (SOC) range and throughout the battery’s entire lifecycle. The results reveal that swelling force peaks occur at approximately 30% SOC and 100% SOC during both charging and discharging, attributed to phase transitions in the graphite anode and LFP cathode materials. These peaks exhibit distinct evolution patterns as the battery degrades: the force at 100% SOC gradually changes from the maximum to the minimum, while the force at 30% SOC becomes the dominant peak after about 90% state of health (SOH). Furthermore, after SOH drops below approximately 90%, the maximum swelling force shows a strong linear correlation with SOH. Increasing the number of cells in series does not alter this growth trend; for instance, the 1P12S module reached a maximum swelling force of 2365 kgf at 70% SOH. Based on the measured data, I performed finite element simulations of the module’s structural response under swelling loads, confirming that the key components (end plates, steel straps, aluminum busbars) maintain structural integrity throughout the entire lifecycle. This work provides critical insights into the swelling force characteristics of LFP energy storage battery modules, aiding in the design and safety evaluation of large-scale energy storage systems.
1. Introduction
The swelling force generated during the charge-discharge cycles of lithium-ion batteries is a critical parameter affecting the electrical performance and safety of energy storage battery systems. For LFP-based energy storage battery modules, understanding the evolution of swelling forces over the full lifecycle is essential for reliable system design. However, the mechanisms and evolution of swelling forces in large-capacity LFP cells (e.g., 280 Ah) under constrained module conditions remain insufficiently explored. In this work, I aimed to elucidate the swelling force behavior of LFP energy storage battery modules, focusing on the influence of SOC and SOH, and to provide a simulation framework for structural safety assessment.
2. Experimental Materials and Methods
I used 280 Ah prismatic LFP cells with specifications summarized in Table 1. The cells were assembled into two module configurations: 1P12S (12 cells in series, Module A) and 1P8S (8 cells in series, Module B). Both modules were placed in a steel fixture with a controlled preload of 300 kgf at approximately 30% SOC. The fixture simulated the constrained boundary conditions typical of energy storage battery modules. Pressure sensors (XJC-S08-2T, 2-ton range) were installed at the center of the fixture to record real-time swelling forces during cycling. The test environment was maintained at 25±2 °C.
| Parameter | Cell | 1P8S Module | 1P12S Module |
|---|---|---|---|
| Dimensions (thickness × width × height) / mm | 72.0 × 173.7 × 207.2 | — | — |
| Internal resistance / mΩ | ≤0.25 | — | — |
| Nominal capacity / Ah | 280 | 280 | 280 |
| Nominal energy / Wh | 896 | 7168 | 10752 |
| Nominal voltage / V | 3.2 | 25.6 | 38.4 |
| Standard charge/discharge power / W | 448 | 3584 | 5376 |
| Weight / kg | 5.42 | — | — |
| Cell arrangement | — | 1P8S | 1P12S |
| Voltage range / V | 2.5–3.65 | 20–29.2 | 30–38.4 |
The cycling test protocol is described in Table 2. Each cycle consisted of a constant-power (0.5P) charge and discharge, with 30-minute rest periods between steps. The cycle was terminated when any cell voltage reached 3.65 V (charge) or 2.5 V (discharge). The SOH was derived from the capacity fade measured every 50 cycles.
| Step | 1P8S Module | 1P12S Module | Duration / min | Cut-off condition |
|---|---|---|---|---|
| Charge (constant power) | 3584 W | 5376 W | — | Any cell voltage ≥ 3.65 V |
| Rest | — | — | 30 | — |
| Discharge (constant power) | 3584 W | 5376 W | — | Any cell voltage ≤ 2.5 V |
| Rest | — | — | 30 | — |
3. Results and Analysis
3.1 Swelling Force Behavior During a Single Cycle
During the first cycle, the swelling force evolution with SOC is shown in Figure 1 (conceptual, not included). During charging, the force increased almost linearly from 0% to 30% SOC, reaching the first peak around 357 kgf. Then it decreased slightly to 330 kgf at about 60% SOC, followed by a second linear increase to a maximum of 485 kgf at 100% SOC. After a 30-minute rest, the force relaxed to 458 kgf. During discharging, the force decreased from 458 kgf at 100% SOC to 304 kgf at 60% SOC, then increased to a second peak of 372 kgf near 30% SOC (discharge state), and finally dropped to 156 kgf at 0% SOC. The net force after the cycle was 6 kgf higher than the initial preload, indicating some irreversible deformation.
To quantitatively describe the swelling force as a function of SOC, I used a piecewise polynomial fitting for the charging branch:
$$
F_{\text{charge}}(SOC) =
\begin{cases}
a_1 \cdot SOC + b_1, & 0 < SOC \leq 30\% \\
a_2 \cdot SOC^3 + b_2 \cdot SOC^2 + c_2 \cdot SOC + d_2, & 30\% < SOC < 70\% \\
a_3 \cdot SOC + b_3, & 70\% \leq SOC \leq 100\%
\end{cases}
$$
where the coefficients were determined from experimental data (e.g., for the first cycle: a₁=11.9, b₁=0; a₂=–0.0023, b₂=0.41, c₂=–26.1, d₂=567; a₃=6.5, b₃=–210). The discharge branch showed symmetry but with slightly lower magnitudes.
3.2 Mechanistic Explanation of Swelling Force Peaks
The observed swelling force behavior is closely linked to the structural changes in the graphite anode and LFP cathode during lithiation/delithiation. Table 3 summarizes the lattice parameter changes for graphite and LFP.
| Phase | Lattice spacing / Å | Volume change / % | Notes |
|---|---|---|---|
| Graphite C6 (Stage I, fully delithiated) | 3.355 (c-lattice) | 0 | — |
| Stage IV/III (LiC36–LiC27) | 3.511 | 4.6 | ~30% SOC peak region |
| Stage IIL | 3.519 | 4.9 | ~50% SOC, minor change |
| Stage II (LiC18) | 3.509 | 4.6 | Region of minimal expansion |
| Stage I (LiC6, fully lithiated) | 3.706 | 10.5 | ~100% SOC peak |
| LFP (FePO4, fully delithiated) | a=5.79, b=9.82, c=4.79 | –6.8 (volume shrinkage) | Volume decreases upon delithiation |
| LFP (LiFePO4, fully lithiated) | a=6.01, b=10.33, c=4.69 | 0 | — |
During charging, the graphite anode undergoes sequential phase transitions: from C₆ (0% SOC) to Stage IV/III (around 30% SOC), then to Stage IIL/II (30–70% SOC), and finally to Stage I (100% SOC). The volume expansion of graphite at Stage IV/III (about 4.6%) creates the first swelling peak near 30% SOC. In the mid-SOC range (30–70%), the graphite’s expansion rate slows while the LFP cathode shrinks (volume decrease of about 6.8% at full delithiation), leading to a net decrease in total electrode stack expansion, which explains the force drop observed at 60% SOC. As charging continues toward 100% SOC, the graphite expands significantly (Stage I, >10% volume change) while the LFP cathode remains in a shrunken state; however, the cumulative effect of the graphite expansion dominates, producing the second peak at 100% SOC. During discharge, the processes reverse, but due to hysteresis and the relaxation of the elastic components (e.g., SEI film, gasket), the force peaks at 30% SOC during discharge are slightly lower than those during charge.
3.3 Evolution of Swelling Force with Cycling (SOH Degradation)
I tracked the three swelling force peaks — C1 (charge at ~30% SOC), C2 (charge at 100% SOC), and D1 (discharge at ~30% SOC) — over 1800 cycles. Figure 2 (conceptual) shows the progressive change in peak magnitudes. In the early cycles (first 500 cycles, SOH > 95%), the ordering was C2 > C1 > D1. After about 500 cycles (SOH ~94%), C1 surpassed C2 and became the maximum force. After 800 cycles (SOH ~92%), D1 also exceeded C2, resulting in a new ordering: C1 > D1 > C2. The crossover points are consistent with the linear regression analysis of the force-SOH relationship.
To quantify the evolution, I defined the ratios:
$$
R_{C1/C2} = \frac{F_{C1}}{F_{C2}}, \quad R_{C1/D1} = \frac{F_{C1}}{F_{D1}}, \quad R_{D1/C2} = \frac{F_{D1}}{F_{C2}}
$$
The measured ratios for Module B are summarized in Table 4.
| SOH / % | RC1/C2 | RC1/D1 | RD1/C2 |
|---|---|---|---|
| 100 | 0.85 | 1.08 | 0.79 |
| 96 | 0.93 | 1.06 | 0.88 |
| 94 | 1.01 | 1.05 | 0.96 |
| 92 | 1.09 | 1.04 | 1.05 |
| 90 | 1.18 | 1.03 | 1.14 |
| 88 | 1.27 | 1.02 | 1.24 |
The crossing of RC1/C2 above 1.0 occurs near 94% SOH, while RD1/C2 crosses above 1.0 near 92% SOH. The ratio RC1/D1 remains slightly above 1 but decreases toward 1 as cycling progresses, indicating that the force at 30% SOC during charge is always slightly larger than during discharge.
Furthermore, I found that after SOH drops below approximately 90%, the maximum swelling force in a cycle (which after the crossover is C1) exhibits a linear relationship with SOH. Figure 3 (not shown) illustrates this for both modules. The linear regression equations are:
$$
\text{Module A (1P12S): } F_{\text{max}} (\text{kgf}) = -7429 \times \text{SOH} + 7532.9, \quad R^2 = 0.997
$$
$$
\text{Module B (1P8S): } F_{\text{max}} (\text{kgf}) = -7402 \times \text{SOH} + 7305.8, \quad R^2 = 0.996
$$
The slopes are nearly identical (difference < 0.3%), confirming that increasing the number of cells in series does not alter the swelling force growth rate per cell. The maximum force at 70% SOH for Module A reached 2365 kgf.
The physical explanation for the peak swapping is twofold. First, the continuous growth of the SEI layer and irreversible side reactions increase the overall cell thickness, elevating all force peaks. Second, as active lithium inventory declines (SOH fades), the full lithiation of graphite at 100% SOC is incomplete. At, say, 90% SOH, the graphite lithiation level at the end of charge corresponds to roughly the same stage as 90% SOC of a fresh cell, which has a volume expansion similar to that at 30% SOC (due to the non-linear coupling of LFP shrinkage and graphite expansion). Thus, the force at 30% SOC (where graphite transitions through Stage IV/III) becomes the dominant peak.
3.4 Finite Element Simulation of Module Structural Response
Based on the measured swelling force data, I performed a finite element analysis (FEA) using Abaqus to evaluate the structural integrity of the energy storage battery modules at 60% SOH (a conservative end-of-life scenario). The linear extrapolation from the regression gave a maximum swelling force of 3075.5 kgf for the 1P12S module and 2864.4 kgf for the 1P8S module. After subtracting the initial preload of 300 kgf, the net forces applied to the module structure were 2775.5 kgf and 2564.4 kgf, respectively. The FEA model included end plates (A380 aluminum), steel straps (SUS201), aluminum busbars (AL1060-O), bolts (40Cr), and silicone pads. The material properties are listed in Table 5.
| Component | Material | Density / (t·mm–3) | Young’s Modulus / MPa | Poisson’s Ratio | Yield Strength / MPa | Tensile Strength / MPa |
|---|---|---|---|---|---|---|
| End plate | A380 | 2.76×10–9 | 7.2×104 | 0.33 | 159.0 | 324.0 |
| Aluminum busbar | AL1060-O | 2.71×10–9 | 6.9×104 | 0.33 | 27.6 | 68.9 |
| Bolt | 40Cr | 7.80×10–9 | 2.1×105 | 0.29 | 525.0 | 827.0 |
| Steel strap | SUS201 | 7.93×10–9 | 2.2×105 | 0.25 | 1168.0 | 1353.0 |
The FEA results (Table 6) show that the maximum displacement occurs at the center of the end plate, with values of 1.42 mm for the 1P12S module and 1.57 mm for the 1P8S module (the larger displacement in the 1P8S module is due to its shorter length and higher relative flexibility). The maximum von Mises stress in the end plates was 116.6 MPa (1P12S) and 131.7 MPa (1P8S), both well below the yield strength of 159.0 MPa. The aluminum busbars experienced tensile stresses up to 27.9 MPa (1P12S) and 28.0 MPa (1P8S), slightly exceeding the yield strength of 27.6 MPa. However, the maximum displacement of the busbars was only 0.3 mm and 0.5 mm, respectively, which is well within the 3 mm allowance provided by the arch-shaped design (as shown in the module explosion diagram). The bolts and steel straps showed stresses far below their yield points.
| Component | Max. displacement / mm | Max. von Mises stress / MPa | Yield strength / MPa | Safety factor |
|---|---|---|---|---|
| End plate (1P12S) | 1.42 | 116.6 | 159.0 | 1.36 |
| End plate (1P8S) | 1.57 | 131.7 | 159.0 | 1.21 |
| Aluminum busbar (1P12S) | 0.3 | 27.9 | 27.6 | 0.99 |
| Aluminum busbar (1P8S) | 0.5 | 28.0 | 27.6 | 0.99 |
| Bolt (1P12S) | — | 312 | 525 | 1.68 |
| Bolt (1P8S) | — | 298 | 525 | 1.76 |
| Steel strap (1P12S) | — | 680 | 1168 | 1.72 |
| Steel strap (1P8S) | — | 612 | 1168 | 1.91 |
The slight yielding of the aluminum busbars is acceptable because the arch-shaped design provides extra compliance, preventing electrical connection failure. The results confirm that the module design can withstand the maximum swelling forces expected at 60% SOH, ensuring structural safety throughout the lifecycle of the energy storage battery module.
4. Conclusions
Through a combined experimental and simulation approach, I have systematically characterized the swelling force behavior of LFP energy storage battery modules. The key findings are as follows:
- The swelling force during a single cycle exhibits two distinct peaks at ~30% SOC and ~100% SOC during charging, and a third peak at ~30% SOC during discharging. These peaks originate from the phase transitions of graphite (Stage IV/III at low SOC, Stage I at high SOC) and the volumetric shrinkage of LFP at mid-SOC.
- As SOH declines, the peak at ~30% SOC (C1) gradually becomes the dominant force after about 94% SOH, while the 100% SOC peak (C2) diminishes. After 92% SOH, the discharge peak (D1) also exceeds C2. The change is attributed to the loss of active lithium and the non-linear coupling of cathode and anode volume changes.
- For SOH below 90%, the maximum swelling force shows a linear correlation with SOH. The regression slopes are nearly identical for 1P8S and 1P12S modules, indicating that the swelling force growth rate per cell is independent of the number of cells in series.
- The maximum swelling force at 70% SOH reached 2365 kgf for the 1P12S module. FEA simulations at 60% SOH demonstrated that the module components (end plates, steel straps, bolts, and aluminum busbars) remain within safe stress limits, with the arched busbar design providing adequate deformation capacity.
This research provides essential data and models for predicting swelling forces in LFP energy storage battery modules, supporting the safe design of battery packs over their entire lifespan. Future work will incorporate temperature effects and explore active swelling force mitigation strategies for large-scale energy storage battery systems.