The rapid development of renewable energy systems has elevated the importance of electrochemical energy storage technologies. Among these, lithium-ion batteries, particularly lithium iron phosphate (LFP) batteries, dominate due to their high safety, long cycle life, and cost-effectiveness. A critical challenge in energy storage battery systems is managing the mechanical stresses induced by swelling forces during charge-discharge cycles. These forces arise from volume changes in electrode materials and significantly impact module structural integrity and safety. This study investigates the swelling force evolution in LFP battery modules under varying states of charge (SOC), health (SOH), and module configurations, supported by experimental data and finite element simulations.
1. Introduction
Energy storage batteries are pivotal for stabilizing power grids and integrating renewable energy sources. However, repeated lithium-ion intercalation/deintercalation in electrodes generates internal stresses, leading to cell swelling. Over time, accumulated mechanical strain degrades module components, threatening safety and longevity. While prior studies have explored swelling forces in small-format cells, large-capacity energy storage battery modules (e.g., 280 Ah LFP cells) remain underexamined. This work addresses this gap by analyzing swelling force dynamics across SOC ranges, SOH degradation, and module configurations.
2. Experimental Methodology
2.1 Battery and Module Specifications
The study employed 280 Ah prismatic LFP cells with the following parameters:
| Parameter | Value | Unit |
|---|---|---|
| Nominal Capacity | 280 | Ah |
| Nominal Voltage | 3.2 | V |
| Thickness | 72.0 | mm |
| Weight | 5.42 | kg |
| Internal Resistance | ≤0.25 | mΩ |
Modules were assembled in 1P12S (12 cells in series) and 1P8S (8 cells in series) configurations.
2.2 Testing Conditions
Cycling tests were conducted at 25±2°C using 0.5P constant-power charge/discharge protocols:
| Module Type | Charge/Discharge Power (W) | Cutoff Voltage (V) | Rest Time (min) |
|---|---|---|---|
| 1P8S | 3584 | 2.5–3.65 | 30 |
| 1P12S | 5376 | 2.5–3.65 | 30 |
Swelling forces were measured using XJC-S08-27 pressure sensors under a 300 kgf preload.
3. Swelling Force Dynamics
3.1 Single-Cycle Swelling Force Variation
Swelling forces exhibited nonlinear dependence on SOC (Fig. 1):
- Charging: Two peaks at ~30% SOC (357 kgf) and 100% SOC (485 kgf).
- Discharging: Peaks at 100% SOC (450 kgf) and ~30% SOC (372 kgf).
The force evolution aligns with graphite staging transitions during lithiation:
- Stage IV→III→II: Volume expansion (~5.33%) at low SOC.
- Stage II→I: Maximal expansion (~12.4%) at full charge.
The coupling of LFP cathode contraction and graphite anode expansion explains intermediate force reduction (30–70% SOC):ΔVtotal=ΔVgraphite−ΔVLFPΔVtotal=ΔVgraphite−ΔVLFP
3.2 Long-Term Swelling Force Evolution
Swelling forces increased with cycle count and SOH degradation (Fig. 2):
| Cycle Range | Dominant Peak Transition | Key Observation |
|---|---|---|
| 0–500 cycles | C2 (100% SOC) > C1 (~30% SOC) | SEI growth and irreversible Li loss. |
| 500–800 cycles | C1 > D1 (~30% SOC) > C2 | SOH <90%: Linear correlation established. |
| >800 cycles | C1 ≈ D1 > C2 | Peak hierarchy stabilizes. |
For 1P12S modules, the maximum swelling force reached 2365 kgf at 70% SOH. Post-90% SOH, swelling force (FswellFswell) correlated linearly with SOH (xx):Fswell=−7429x+7532.9(R2=0.997)Fswell=−7429x+7532.9(R2=0.997)
4. Mechanistic Insights
4.1 Graphite Staging and Volume Changes
Graphite undergoes four distinct lithiation stages (Table 1):
| Stage | LiC66 Phase | Interlayer Spacing (Å) | Volume Change |
|---|---|---|---|
| IV | LiC2424 | 3.511 | +5.33% |
| III | LiC1818 | 3.519 | +5.53% |
| II | LiC1212 | 3.509 | +5.57% |
| I | LiC66 | 3.706 | +12.4% |
Stage transitions govern force peaks:
- 30% SOC: IV→III→II transitions dominate.
- 100% SOC: II→I transition maximizes expansion.
4.2 LFP Cathode Contribution
LFP contracts during delithiation (charging):LiFePO4→Li1−xFePO4+xLi++xe−LiFePO4→Li1−xFePO4+xLi++xe−
Lattice parameters shrink from a=6.01 A˚a=6.01A˚, b=10.33 A˚b=10.33A˚ (LiFePO44) to a=5.79 A˚a=5.79A˚, b=9.82 A˚b=9.82A˚ (FePO44). This contraction counteracts anode expansion at mid-SOC ranges.
5. Simulation and Structural Safety
Finite element analysis (Abaqus) evaluated module components under 60% SOH swelling forces:
| Component | Max Stress (MPa) | Yield Strength (MPa) | Safety Margin |
|---|---|---|---|
| End Plate (A380) | 131.7 | 159.0 | 17% |
| Aluminum Busbar | 28.0 | 27.6 | 1.5%* |
| Steel Strap | 1168 | 1353 | 14% |
*Busbar stress exceeded yield strength but remained within allowable deformation limits (<3 mm).
6. Implications for Energy Storage Battery Design
- Module Configuration: Higher cell counts (1P12S vs. 1P8S) amplify swelling forces but retain linear SOH dependence.
- Material Selection: Steel straps and end plates (A380) withstand cyclic loads, while arched busbars accommodate displacements.
- Safety Protocols: Swelling force thresholds can serve as early warnings for SOH degradation or thermal risks.
7. Conclusion
This study elucidates the intricate relationship between SOC, SOH, and swelling forces in energy storage battery modules. Key findings include:
- Swelling forces peak at ~30% and 100% SOC due to graphite staging transitions.
- Post-90% SOH, forces linearly correlate with health degradation.
- Simulation validates structural robustness of module designs up to 60% SOH.
These insights enhance predictive models for swelling forces, guiding safer and more durable energy storage battery systems.