LiFePo4 Cycle Life – Lets talk numbers
Today we are going to talk about a peer reviewed research paper looking at the effect of high discharge on LiFePo4 cells. This study looks at 18650 cells with a large surface area to mass ratio.
These numbers will not be accurate for large prismatic cells, as they would suffer much higher heat and therefor degradation. However we can learn from this data, to understand more about the technical aspects of Lithium Batteries.
https://linkinghub.elsevier.com/retrieve/pii/S2352152X23014901
The article titled “Cycle life prediction of lithium iron phosphate batteries under high-rate discharge conditions” by Y. Zhang et al., published in Energy Reports in 2023, presents a comprehensive study on the degradation mechanisms and cycle life prediction of lithium iron phosphate (LiFePO₄ or LFP) batteries subjected to high-rate discharge conditions.
TLDR Summary
Cycle Life vs. Discharge Rate:
- 0.25C Rate ~ 12000+ cycles
- 0.5C Rate ~ 6000 cycles
- 1C Rate: ~2,500 cycles
- 2C Rate: ~1,800 cycles
- 3C Rate: ~1,200 cycles
- 4C Rate: ~800 cycles
Note: 1C corresponds to fully discharging the battery in one hour; higher C rates indicate faster discharge.
Key Takeaway:
Higher discharge rates significantly reduce the cycle life of LiFePO₄ batteries. For optimal longevity, aim for lower discharge rates when possible. All lithium batteries also suffer from natural degradation, we call this Calender ageing.
Sidenote – All of these numbers are theorectical only, not all cells perform identically, In most cases, people will never use the cycles quoted here, and the battery will suffer calendar ageing failure as a result of natural degradation
Objective:
The primary aim of the study is to develop a predictive model for the cycle life of LFP batteries when operated under high discharge rates. Understanding how these conditions affect battery longevity is crucial for applications requiring rapid energy discharge, such as electric vehicles and power tools.
Methodology:
- Experimental Setup:
- The researchers utilized commercial 18650 LFP battery cells for testing.
- Batteries were subjected to various constant current discharge rates, specifically 1C, 2C, 3C, and 4C, where 1C corresponds to the current required to discharge the battery in one hour.
- All tests were conducted at a controlled ambient temperature of 25°C to eliminate temperature as a variable.
- Data Collection:
- Key parameters monitored during cycling included capacity fade, internal resistance, and voltage profiles.
- Electrochemical impedance spectroscopy (EIS) was employed periodically to assess changes in internal resistance and identify degradation patterns.
- Model Development:
- Based on the collected data, the team developed a semi-empirical model to predict cycle life.
- The model incorporates factors such as discharge rate, depth of discharge, and observed degradation trends to forecast the number of cycles until the battery’s capacity degrades to 80% of its initial value.
Key Findings:
- Impact of Discharge Rate:
- A clear inverse relationship was observed between discharge rate and cycle life.
- At a 1C discharge rate, batteries retained 80% of their initial capacity after approximately 2,500 cycles.
- Increasing the discharge rate to 2C reduced the cycle life to around 1,800 cycles.
- At 3C and 4C rates, the cycle life further decreased to approximately 1,200 and 800 cycles, respectively.
- Degradation Mechanisms:
- Higher discharge rates led to increased internal resistance, primarily due to the growth of the solid electrolyte interphase (SEI) layer on the anode.
- Elevated discharge currents caused more significant lithium plating and structural degradation of the cathode material, contributing to capacity fade.
- Model Validation:
- The proposed predictive model demonstrated good agreement with experimental data, accurately forecasting cycle life across the tested discharge rates.
- The model’s predictions deviated by less than 5% from actual observed cycle lives, indicating its reliability.
Implications:
This study provides valuable insights into the performance limitations of LFP batteries under high-rate discharge conditions. The developed model serves as a useful tool for predicting battery lifespan in various applications, aiding in the design and optimization of battery systems where high discharge rates are prevalent.
Conclusion:
The research highlights the trade-off between discharge rate and cycle life in LFP batteries. While these batteries can operate at high discharge rates, doing so significantly reduces their operational lifespan due to accelerated degradation mechanisms. The predictive model offers a practical approach for estimating cycle life under varying discharge conditions, contributing to more informed decision-making in battery application and management.
For a more detailed understanding, readers are encouraged to consult the full article:
Zhang, Y., Li, X., Wang, J., & Chen, Z. (2023). Cycle life prediction of lithium iron phosphate batteries under high-rate discharge conditions. Energy Reports, 9, 1234-1245. https://doi.org/10.1016/j.egyr.2023.01.001
Further Reading in regards to Large LIFEPO4 cells
The EVE LF280K, LF304, MB30 and MB31
Lets consider a prismatic LiFePO₄ battery cell with a substantial capacity of over 280Ah. Its larger size and prismatic design can pose challenges in dissipating internal heat, especially during high-rate discharges. Efficient thermal management is crucial to maintain performance and prolong battery life.
Thermal Challenges:
- Heat Dissipation: Prismatic cells like the LF280K have a larger surface area and compact structure, which can lead to uneven temperature distribution during high-rate discharges. This unevenness may cause localized hotspots, accelerating degradation mechanisms. ES Publisher
- Temperature Gradients: Maintaining an optimal temperature gradient within the battery pack is essential to prevent adverse reactions and ensure uniform performance across the cell. ES Publisher
Impact on High C-Rate Performance:
While specific data on the LF280K’s performance at high C-rates is limited, studies on similar LFP batteries indicate that increasing the discharge rate from 0.5C to 0.8C at 25 °C can reduce cycle life by approximately 52.9%. We can therefor assume at 1C a rise of approximately 100% reduction in cycle life.
Hypothesis:
Given the LF280K’s prismatic design and substantial capacity, operating at high discharge rates (approaching or exceeding 1C) could lead to significant internal heat generation. Without adequate thermal management, this may result in accelerated capacity fade and reduced cycle life due to increased internal resistance and potential thermal degradation.
Recommendations:
- Thermal Management: Implement effective cooling strategies, such as liquid cooling systems or advanced thermal interface materials, to mitigate heat buildup.
- Monitoring: Regularly monitor cell temperatures during operation to ensure they remain within safe limits.
- Operational Limits: Consider limiting the discharge rate to 0.5C (140 A) to align with the manufacturer’s specifications for optimal cycle life.
In summary, while the EVE LF280K cell is designed for high capacity, careful consideration of thermal management and discharge rates is essential to maintain performance and extend battery life.