Abstract:
Lithium-ion batteries (LIBs) have been extensively utilized in energy storage systems due to their high energy density, long lifespan, and relatively low cost. This paper focuses on the strain evolution and state of charge (SOC) estimation of Graphite-Lithium Iron Phosphate (Graphite-LFP) pouch cells during cycling. By employing Fiber Bragg Grating (FBG) sensors, we monitored the strain changes in situ and analyzed the electrochemical-mechanical behavior of the cells. Furthermore, based on the strain measurements, we estimated the SOC of the cells, demonstrating the potential of FBG sensors in battery management systems.
1. Introduction
Lithium-ion batteries (LIBs) have revolutionized the energy storage landscape, particularly in consumer electronics, electric vehicles, and grid energy storage systems [1-3]. However, the thermal and mechanical stability of LIBs is crucial for their safety. During cycling, the anodes and cathodes undergo periodic volume changes, leading to stress and strain accumulation. Monitoring these strain changes in real-time is essential for ensuring the long-term safe and stable operation of batteries.
2. Literature Review
Traditional methods for battery strain monitoring include strain gauges and pressure sensors. However, these sensors are mostly electrically connected, susceptible to electromagnetic interference, and difficult to integrate into battery modules. Advanced techniques such as high-energy X-ray diffraction, computed tomography [20], and digital image correlation [21] provide precise in-situ strain monitoring but require expensive equipment and complex laboratory environments [22]. Recently, Fiber Bragg Grating (FBG) sensors have emerged as a promising alternative due to their small size, electromagnetic interference resistance, chemical corrosion resistance, and multiplexing capabilities.
3. Materials and Methods
3.1 Battery Assembly
The Graphite-LFP pouch cells used in this study were purchased from Hunan Lifang New Energy Company with a capacity of 2 Ah. The cells were assembled using a winding process . The lithium iron phosphate (LFP) electrode was prepared by coating a slurry of LFP, conductive carbon, and binder onto a substrate and drying it . Similarly, the graphite (Graphite) electrode was prepared. The dried electrodes were assembled into pouch cells in an argon-filled glove box after being baked in a vacuum oven.
3.2 FBG Sensor Integration
To monitor the strain evolution of the pouch cells, FBG sensors were bonded to the cells using UV glue. Two fibers were used: one fully encapsulated with the cell for strain measurement and the other loosely placed for temperature compensation. The strain and temperature changes were decoupled using the following equation:
ΔλB = K_TΔT + K_εΔε (1)
where ΔλB is the wavelength shift of the FBG, K_T and K_ε are the temperature and strain sensitivity coefficients, respectively, ΔT is the temperature change, and Δε is the strain change.
4. Results and Discussion
4.1 Strain Evolution During Cycling
The strain evolution of the Graphite-LFP pouch cell. The strain changes exhibit a periodic pattern correlated with the battery voltage. However, the direct bonding method led to fiber breakage due to stress concentration.
To address this issue, a brass strain gauge with an I-beam design was developed. This design protected the fiber from mechanical damage and enhanced strain sensitivity [37]. The strain evolution measured using this method. The strain changes correspond well with the battery voltage, demonstrating reversible electrochemical and mechanical processes.
4.2 Electrochemical-Mechanical Behavior Analysis
Further analysis of the voltage and strain data revealed that the strain platforms correspond to phase transitions in the electrodes. The dV/dt and d(strain)/dt plots highlight the regions of phase transitions.
To investigate the electrochemical-mechanical behavior of each electrode, FBG sensors were implanted into single-layer pouch cells. The strain evolution of the LFP and Graphite electrodes.
The strain of the LFP electrode exhibits a linear relationship with time during charging and discharging, without a strain platform. In contrast, the Graphite electrode shows a complex strain profile related to phase transitions. These results suggest that the strain platforms observed in the full cell are primarily caused by phase transitions in the Graphite electrode.
4.3 SOC Estimation Based on Strain
The strain magnitude is correlated with the amount of lithium ions intercalated into the electrodes, which directly relates to the battery capacity and state of charge (SOC). The strain signal provides a more accurate estimation of SOC compared to the voltage signal, especially during the flat voltage plateau of the LFP electrode.
A piecewise fitting model was developed to estimate the SOC based on the strain signal. The model showed good agreement with the actual SOC, with an estimation error of less than 10%.
5. Conclusion
This study presents a comprehensive analysis of strain evolution and SOC estimation in Graphite-LFP pouch cells during cycling. The main findings are summarized in Table 1.
| Main Findings | Details |
|---|---|
| FBG Sensor Integration | I-beam design for mechanical protection and strain enhancement |
| Strain Evolution | Periodic strain changes corresponding to battery voltage |
| Electrochemical-Mechanical Behavior | Strain platforms related to phase transitions in Graphite |
| SOC Estimation | Strain-based SOC estimation with an error of <10% |
The optical sensing technique employed in this study provides a powerful tool for in-situ monitoring of battery stress evolution, offering insights into battery expansion, internal mechanical changes, and SOC estimation. This technology has the potential to be applied in smart lithium-ion battery energy storage stations, ensuring their safe and stable operation.