Abstract
This study investigates the influence of formation charge states (62%, 72%, and 82% SOC) on the performance of LiFePO₄ batteries. By analyzing electrode morphology, first-cycle efficiency, impedance, high-temperature storage, and cycling stability, we demonstrate that a 62% SOC formation process optimizes the uniformity and compactness of the solid electrolyte interphase (SEI) film on the graphite anode. This results in the highest first-cycle Coulombic efficiency (90.26%), lowest DC internal resistance (DCR), minimal SEI impedance, superior high-temperature capacity retention (96.41%), and extended cycle life (81.52% capacity retention after 1,731 cycles at 45°C). These findings highlight the critical role of formation charge state in enhancing the electrochemical performance and longevity of LiFePO₄ battery systems.
Introduction
LiFePO₄ batteries have gained prominence in electric vehicles (EVs) and energy storage systems due to their high energy density, long cycle life, and thermal stability. A pivotal step in LiFePO₄ battery manufacturing is the formation process, where a controlled initial charge activates electrochemical materials and forms the SEI film on the anode. The SEI film governs lithium-ion transport, mitigates electrolyte decomposition, and prevents lithium dendrite growth. However, suboptimal formation protocols can lead to non-uniform SEI layers, increased impedance, and accelerated capacity fade.
Key parameters such as charge state (SOC), current density, temperature, and pressure influence SEI quality. Among these, SOC during formation directly impacts lithium-ion intercalation kinetics and SEI morphology. Excessive SOC may induce lithium plating, while insufficient SOC results in incomplete SEI formation. This study systematically evaluates three formation charge states (62%, 72%, and 82% SOC) to identify the optimal protocol for LiFePO₄ battery performance.
Experimental Methodology
1. Battery Fabrication
LiFePO₄ batteries (26 mm × 148 mm × 91 mm, prismatic design) were assembled using:
- Cathode: LiFePO₄ (92.5 wt%), SuperCarbon conductive agent (4 wt%), and PVDF binder (3.5 wt%).
- Anode: Graphite (94.5 wt%), SuperCarbon (2 wt%), CMC thickener (1 wt%), and SBR binder (2.5 wt%).
- Electrolyte: 1M LiPF₆ in EC:DMC:DEC (1:1:1 vol%).
The formation process involved three protocols (Table 1), differentiated by charge duration at 2,500 mA to achieve 62%, 72%, and 82% SOC.
Table 1: Formation protocols for LiFePO₄ batteries
| Step | Protocol 1 (62% SOC) | Protocol 2 (72% SOC) | Protocol 3 (82% SOC) |
|---|---|---|---|
| 1 | 5 min rest | 5 min rest | 5 min rest |
| 2 | 500 mA CC, 60 min | 500 mA CC, 60 min | 500 mA CC, 60 min |
| 3 | 1,250 mA CC, 240 min | 1,250 mA CC, 240 min | 1,250 mA CC, 240 min |
| 4 | 2,500 mA CC, 240 min | 2,500 mA CC, 300 min | 2,500 mA CC, 360 min |
2. Characterization Techniques
- SEM Imaging: Graphite anode surfaces were analyzed using a JEOL JSM-6510 microscope (5,000× magnification).
- Electrochemical Testing:
- DC Internal Resistance (DCR): Measured at 50 Hz using a HIOKI BT3562-01.
- EIS: Conducted with a Zennium workstation (10 μHz–8 MHz frequency range).
- High-Temperature Storage: Batteries stored at 55°C (100% SOC) for 7 days; capacity retention calculated as:Retention (%)=Post-Storage CapacityInitial Capacity×100Retention (%)=Initial CapacityPost-Storage Capacity×100
- Cycle Life Testing: 1C charge/discharge (2.0–3.65 V) at 45°C until 80% capacity retention.
Results and Discussion
1. SEI Morphology and Anode Surface Analysis
SEM images revealed distinct SEI structures across protocols:
- 62% SOC: Uniform, thin, and compact SEI layer (Figure 2a).
- 72% SOC: Discontinuous SEI with partial coverage (Figure 2b).
- 82% SOC: Thick SEI with excessive lithium deposition (“purple spots”) due to localized current density variations (Figure 2c).
The formation of “purple spots” at higher SOC (82%) correlates with uneven lithium-ion intercalation, leading to SEI heterogeneity and increased impedance.
2. First-Cycle Coulombic Efficiency
First-cycle efficiency decreased with higher SOC (Table 2), attributed to irreversible lithium loss from SEI formation and lithium plating.
Table 2: First-cycle efficiency and capacity loss
| Protocol | Formation Capacity (mAh) | Discharge Capacity (mAh) | Efficiency (%) |
|---|---|---|---|
| 62% SOC | 15,491.4 | 13,522.6 | 90.26 |
| 72% SOC | 17,996.2 | 15,554.5 | 89.69 |
| 82% SOC | 20,495.1 | 18,086.9 | 87.73 |
3. Impedance Analysis
DC Internal Resistance (DCR):
- 62% SOC: 1.78 mΩ (mean)
- 72% SOC: 1.86 mΩ (7.2% higher)
- 82% SOC: 2.14 mΩ (17.2% higher)
EIS Parameters:
- SEI resistance (RSEIRSEI): 62% SOC < 72% SOC < 82% SOC
- Charge transfer resistance (RctRct): Followed the same trend.
The lower impedance at 62% SOC aligns with the compact SEI structure, facilitating faster lithium-ion diffusion.
4. High-Temperature Storage Performance
After 7 days at 55°C:
- 62% SOC: 96.41% capacity retention
- 72% SOC: 94.12% capacity retention
- 82% SOC: 91.05% capacity retention
The superior retention at 62% SOC stems from stable SEI suppressing electrolyte decomposition.
5. Cycle Life at 45°C
- 62% SOC: 1,731 cycles (81.52% retention)
- 72% SOC: 1,379 cycles (80.04% retention)
- 82% SOC: 711 cycles (81.84% retention, rapid “capacity跳水”)
Higher SOC protocols exhibit accelerated degradation due to SEI instability and lithium plating.
Theoretical Model of SEI Formation
The relationship between SOC, SEI quality, and impedance can be modeled as:Rtotal=RSEI+Rct+RdiffusionRtotal=RSEI+Rct+Rdiffusion
where RSEIRSEI is inversely proportional to SEI compactness. At 62% SOC, minimal RSEIRSEI and RctRct enhance overall kinetics.
Conclusion
Optimizing the formation charge state is critical for LiFePO₄ battery performance. A 62% SOC protocol yields a uniform SEI film, minimal impedance, and exceptional cycling stability. These insights provide actionable guidelines for improving LiFePO₄ battery manufacturing, emphasizing the need to balance lithium intercalation kinetics with SEI quality. Future work will explore dynamic formation protocols to further enhance energy density and lifespan.