Influences on Cycle Life for Lithium-Ion Batteries and their Implications in Second Life Applications

Introduction:

Lithium-ion batteries have become ubiquitous in our modern world, powering everything from smartphones to electric vehicles. One critical aspect that shapes their performance and longevity is the cycle life. In this blog post, we'll delve into the key influences on the cycle life of lithium-ion batteries and explore their relevance in the realm of second life applications.

1. Temperature's Crucial Role

The impact of temperature on lithium-ion batteries is undeniable. High temperatures accelerate chemical reactions within the battery, leading to increased degradation. Research suggests that cycle life may be halved for every 8°C (15°F) rise in temperature. Managing temperature becomes especially critical in second life scenarios where batteries might be subjected to diverse environmental conditions.

2. Depth of Discharge

How deeply a battery is discharged in each cycle, known as Depth of Discharge (DoD), significantly affects its lifespan. Limiting discharge to approximately 10-20% DoD results in fewer stress cycles, preserving the battery's health and significantly prolonging its lifespan. It's crucial to strike a balance between maximizing capacity use and minimizing stress for optimal cycle life, a key consideration in second life applications. 

3. Charging Methods Matter

The charging approach plays a pivotal role in lithium-ion battery longevity. Avoiding high-voltage charging and overcharging is essential. Research indicates that charging to only 80% of the full capacity can significantly extend the cycle life. Implementing smart charging strategies becomes imperative when repurposing batteries in second life applications. This is why we set our systems up with tighter operating voltage ranges than new, and recommend that our customers do the same when installing our batteries. Slightly over-designing on capacity further helps to limit extremes on either end of the voltage range.

4. Chemical Composition and Design

The specific chemistry and design of lithium-ion batteries vary, influencing their cycle life. Cobalt-based cathodes, for instance, offer high energy density but may compromise lifespan. Lithium iron phosphate (LiFePO4) has much longer cycle life at the cost of lower energy density. Advancements in battery technologies, such as nickel-rich cathodes, aim to enhance both energy density and cycle life, making them promising for second life applications. We are constantly monitoring the state of lithium-ion energy storage systems to keep up with the latest developments in chemistries, as well as their potential impact on sustainability and recyclability.

5. Cycling Frequency

The frequency of charge and discharge cycles impacts the overall longevity of lithium-ion batteries. Batteries used infrequently might degrade due to self-discharge, while those cycling frequently may experience a more rapid decline. We are rarely given information on the first life, but we’ve developed models to estimate cycling based on the original application, which helps us estimate remaining life. We are almost always stepping down the load profile when going to second life, which can greatly increase life.

Implications for Second Life Applications

Repurposing lithium-ion batteries in second life applications requires a nuanced understanding of many factors. Implementing intelligent battery management systems, temperature control, and optimized charging/discharging strategies is critical to extending the useful life of these batteries. 

In Conclusion

The cycle life of lithium-ion batteries is a multifaceted interplay of factors. Navigating these influences becomes especially crucial when considering second life applications. By acknowledging and mitigating these aspects, we can unlock the full potential of lithium-ion batteries and contribute to a sustainable and efficient energy ecosystem.

We’ve touched on a few factors that we take into account when renewing batteries. We will dive a bit deeper into how we implement these strategies in a future blog post.

Sources

- Battery University. (batteryuniversity.com)

- Armand, M., & Tarascon, J. M. (2008). Building better batteries. Nature, 451(7179), 652-657.

- Goodenough, J. B., & Park, K. S. (2013). The Li-ion rechargeable battery: a perspective. Journal of the American Chemical Society, 135(4), 1167-1176.