The Operation Window of Lithium Iron Phosphate/Graphite Cells Affects their Lifetime

[upnorthandpersonal]

Came across this article:

Radware Bot Manager Captcha

(also attached to this post)

They cycle LFP cells in different SoC windows ((0%–25%, 0%–60%, 0%–80%, 0%–100%, and 75%–100%) and test degradation. The conclusion is that cycling at high state of charge windows leads to more degradation than cycling at low states of charge.

Full Abstact:

Lithium iron phosphate (LFP) battery cells are ubiquitous in electric vehicles and stationary energy storage because they are cheap and have a long lifetime. This work compares LFP/graphite pouch cells undergoing charge-discharge cycles over five state ofcharge (SOC) windows (0%–25%, 0%–60%, 0%–80%, 0%–100%, and 75%–100%). Cycling LFP cells across a lower average SOC results in less capacity fade than cycling across a higher average SOC, regardless of depth of discharge. The primary capacity fade mechanism is lithium inventory loss due to: lithiated graphite reactivity with electrolyte, which increases incrementally with SOC, and lithium alkoxide species causing iron dissolution and deposition on the negative electrode at high SOC which further accelerates lithium inventory loss. Our results show that even low voltage LFP systems (3.65 V) have a tradeoff between average SOC and lifetime. Operating LFP cells at lower average SOC can extend their lifetime substantially in both EV and grid storage applications.

 

[Solar Guppy]

Did a quick read:

First is these are not batteries we use in typical home storage, they are only 2.4 Ah batteries. I'm not a material scientist but do question a linear relationship what is measured with this cell would scale to the typical 100-320 Ah cells used in home storage. For example, larger cells can dissipate heat much better internally due to the increased mass of the cells as well as surface area. Additionally, these cells used in the research are pouch, not prismatic. The latter have been documented to have significantly better thermal properties.

Second, and I quote from the paper:

It is important to note that we cannot make conclusive lifetime extrapolation statements, because these cells were stopped for destructive analysis after only 2500 h of testing. The best performing cells retained 97% capacity at that time, while the worst performing cells retained 76% capacity. Some preliminary unpublished results on smaller sample sizes suggest that LFP cells cycled at high average SOC may not experience a continuously rapid capacity fade and could recover in later cycles. More work will be done to investigate this mechanism after longer cycling times.

Having my last career in higher education and first hand experience at "research" they do and being a doctoral student myself, the pressures to find a unique topic is priority 1, not the content of the research. I'm all for good data, but it would likely come from the cell manufactures, they are the experts in the material science, not higher education.

 

[upnorthandpersonal]

Yeah, I saw that quote. I don't want to give my opinion in this thread, but I thought it worthwhile to share.

 

[Solar Guppy]

Yeah, I saw that quote. I don't want to give my opinion in this thread, but I thought it worthwhile to share.

It's unfortunate that the companies making the cells don't publish, but for obvious reason we will never know. The money industry spends on technology and research is many, many orders of magnitude greater than higher education does ( always with government grants )

 

[OffGridInTheCity]

I have a large (121kwh) 18650 powerwall rather than LifePo4 and I'm in year 7 of operation. So this general topic is of great interest to me personally! I was originally (7 yrs ago) struck by Battery University's page "How to Prolong Lithium-based Batteries" - https://batteryuniversity.com/article/bu-808-how-to-prolong-lithium-based-batteries - and this chart has provided a key element of my powerwall goals....

This includes SoC that embodies DoD and voltage ranges. I've achieved 37% overall DoD between the 3.54v (22% SoC) and 4.0v (80% SoC) which is roughly between the purple line (75-45% SoC) and the blue-green line (75-25% SoC). Still actively working to achieve the 75-45% SoC purple line by adding more capacity to the powerwall. Notice the purple line goes out to nearly 7000 cycles.

To date, 40% of my powerall is >2,000cycles and no sign of issues. But of course, I'm wondering how many cycles thru low-stress (on cells) and what degradation/failure will look like. Will it be primarily a balance issue on individual packs (cells in LifePo4 terms) or just gradual degradation to say 60% of original capacity but still working or something catastrophic at the pack/cell level - or some combination.