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drees
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Why do lithium batteries die and how to improve them?

Wed Feb 05, 2014 11:46 pm

Ran across this great presentation from Professor Jeff Dahn (Dalhousie University) talking about various issues in engineering batteries for EVs (and other applications) that last a long time. Professor Dahn has been working with GM for a year or two now helping them in their battery testing lab.



Notes:
  • Coulombic efficiency is an easy way to compare cycle/calendar life of a lithium batteries without having to run very prolonged cycling tests - the better the efficiency, the more durable the battery.
  • Chemistry (and additives) are vital in determining cycle/calendar life of a lithium battery.
  • The colder the battery, the longer it will last (Arrhenius' equation).
  • The lower the SOC the battery is charged, the longer it will last (example was storing battery at ~20% or 3.5V resting voltage).
  • LiMn (as used in the LEAF/Volt) has horrendous cycle/calendar life at temperatures of 30C+ compared to say LiCo (as used by Tesla). LiCo is even better than LiFe (A123).
  • Additives can have a very strong effect on coulombic efficiency (and thus durability)
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Re: Why do lithium batteries die and how to improve them?

Thu Feb 06, 2014 12:15 am

Amazing...I just got done watching and came here to post a link and there it was already posted. Fantastic presentation.

Really shows why the guys in Arizona who charged more than once a day were having so much degradation. Also interesting his explanation of how Nissan missed it by doing the accelerated testing. Problems occur with the length of time you spend charging or discharging at high temperature. With the accelerated testing the cells did not spend much time at a high SOC. Actual use with a 3kW charger they spend a lot of time (relatively) charging. Discharge (driving) is probably similar to the accelerated testing rates.
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Re: Why do lithium batteries die and how to improve them?

Thu Feb 06, 2014 8:09 am

Very impressive. 3 weeks to get predictive results instead of 8 years. The rate of improvement should.... improve.
I noticed you're still working with polymers.

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Re: Why do lithium batteries die and how to improve them?

Thu Feb 06, 2014 8:23 am

All this begs the question, what did Nissan change in the chemistry of the "hot battery" to allegedly reduce degradation? And how successful will it really be in the real world?
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Now driving a 2016 Volt Premier.

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Re: Why do lithium batteries die and how to improve them?

Thu Feb 06, 2014 9:46 am

drees wrote:[*]The lower the SOC the battery is charged, the longer it will last (example was storing battery at ~20% or 3.5V resting voltage).


3.5V is pretty darn close to turtle. Is the implication that it's better to leave a Leaf sitting with 8 Gids vs 80 or 180?
We all know higher voltages are bad (although not nearly as bad as higher temperatures), but is there any accelerated degradation mechanism at 3.0-3.5V (turtle)?
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Re: Why do lithium batteries die and how to improve them?

Thu Feb 06, 2014 9:59 am

GregH wrote:
drees wrote:[*]The lower the SOC the battery is charged, the longer it will last (example was storing battery at ~20% or 3.5V resting voltage).


3.5V is pretty darn close to turtle. Is the implication that it's better to leave a Leaf sitting with 8 Gids vs 80 or 180?
We all know higher voltages are bad (although not nearly as bad as higher temperatures), but is there any accelerated degradation mechanism at 3.0-3.5V (turtle)?


Iirc, at very low SOC, the Copper anode starts to be attacked, enters the electrolyte and Copper plating starts ruining the cell. I don't know what the critical voltage that represents with the LEAF chemistry.
I noticed you're still working with polymers.

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Re: Why do lithium batteries die and how to improve them?

Thu Feb 06, 2014 10:33 am

Nubo wrote:Very impressive. 3 weeks to get predictive results instead of 8 years. The rate of improvement should.... improve.

Hopefully AESC is using this technique. At least we know that GM is.

TomT wrote:All this begs the question, what did Nissan change in the chemistry of the "hot battery" to allegedly reduce degradation? And how successful will it really be in the real world?

So far all we know is that they are very likely going to a ceramic coated separator likely from Polypore. They did make slight chemistry tweaks for 2013, but who knows what other tweaks they have in store for the "hot battery". We need more data from our real world accelerated testing lab (also known as Phoenix).

GregH wrote:3.5V is pretty darn close to turtle. Is the implication that it's better to leave a Leaf sitting with 8 Gids vs 80 or 180?
We all know higher voltages are bad (although not nearly as bad as higher temperatures), but is there any accelerated degradation mechanism at 3.0-3.5V (turtle)?

Yeah, the 3.5V probably shouldn't apply directly to the LEAF's batteries, as at a 3.5V resting voltage it's well below 20% SOC.

Somewhere around 3.75V resting voltage (~360V pack voltage) or around LBW would be a good suggestion for the LEAF, but realistically, anything between VLBW and 80% is good enough for short term storage. For longer term ideally you'd want somewhere between LBW and 40% and to keep it as cold as possible.
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Re: Why do lithium batteries die and how to improve them?

Thu Feb 06, 2014 11:10 am

Nubo wrote:
GregH wrote:
drees wrote:[*]The lower the SOC the battery is charged, the longer it will last (example was storing battery at ~20% or 3.5V resting voltage).


3.5V is pretty darn close to turtle. Is the implication that it's better to leave a Leaf sitting with 8 Gids vs 80 or 180?
We all know higher voltages are bad (although not nearly as bad as higher temperatures), but is there any accelerated degradation mechanism at 3.0-3.5V (turtle)?


Iirc, at very low SOC, the Copper anode starts to be attacked, enters the electrolyte and Copper plating starts ruining the cell. I don't know what the critical voltage that represents with the LEAF chemistry.

IIRC (and my recollection is foggy at best).. low voltage degradation is at extremely low voltages (0-2V) and not any voltage a Leaf battery would ever be allowed to see (3.0-3.5V.. although I have seen screenshots showing 2.7xV)
So I guess my question would be: Is it ANY worse (for battery longevity) to leave a Leaf sitting at 3.5V (say, 10-12 Gids or so) than at 50% SOC?
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Re: Why do lithium batteries die and how to improve them?

Thu Feb 06, 2014 11:12 am

drees wrote:Somewhere around 3.75V resting voltage (~360V pack voltage) or around LBW would be a good suggestion for the LEAF, but realistically, anything between VLBW and 80% is good enough for short term storage. For longer term ideally you'd want somewhere between LBW and 40% and to keep it as cold as possible.

But not VLBW (24 Gids)? Not 10-15 Gids?
If not, why not? (with respect to battery longevity)
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Re: Why do lithium batteries die and how to improve them?

Thu Feb 06, 2014 11:17 am

drees wrote:[*]LiMn (as used in the LEAF/Volt) has horrendous cycle/calendar life at temperatures of 30C+ compared to say LiCo (as used by Tesla). LiCo is even better than LiFe (A123).
[*]Additives can have a very strong effect on coulombic efficiency (and thus durability)[/list]

Thanks drees. Marvelous informative video. Everyone that wants to understand lithium ion batteries should watch this :!: :D

The truly horrendous coulombic efficiency of the LiMn battery as used by the LEAF and Volt does raise interesting questions about why Nissan and GM went that way.
LiMn does have advantages, but it is primarily in safety, markedly lower run away potential.
There haven't been any LEAF HV pack related fires, and only the one post accident Volt fire, where no one did the right thing and failed to remove the fire potential post accident.
Who would leave gasoline in a damaged vehicle :?: :?: :?:

While Tesla with 18650 commodity cells, which they took extensive efforts to protect and eliminate fire hazard, have still encountered a couple of unfortunate post road hazard / post accident fires.
And use of LiCo on an airplane (which in my own personal opinion was ill advised and which the FAA should still take a much harder look at) had serious problems.
Time will tell if Boeing has fixed the problem and adequately minimized the hazard.

GM did take the approach of providing temperature management for their LiMn version, which as the professor acknowledges in the video has resulted in much better capacity retention than the LEAF.
The real question is whether you can tweak the electrolyte chemistry with the right combination of trace additives to get a reasonable capacity life in a LiMn battery without a temperature management system :?:

Nissan may be correct that not having a temperature management system is the right design approach.
As Dr. Edward Buiel of Coulometrics pointed out in his presentation to the Chattanooga Engineers Club recently, a battery temperature management system can be a huge energy drain. Research on an electric bus a few years back showed the battery temperature management system taking as much energy as what the bus was using for propulsion :!:

But not having a temperature management system is only the right cost effective design approach if you have a battery that can maintain capacity for 100,000+ miles (and while being sufficiently safe). Nissan did NOT get the capacity side of the question correct for the product they have been selling in 2011 / 2012 / 2013 / early 2014. We can only hope they have come up with the magic five or more part additive for the electrolyte to get there with the HOT battery. And that they will offer it to buyers of the early defective HV pack LEAFs at a reasonable pro-rated / variable cost only price, and to the people they are now attempting to sell used LEAFs to that are coming off of lease. If they don't, they may have destroyed the Nissan EV brand :shock: :( :shock: :(

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