I am using LiFePO4 battery, I measured DCIR using the discharge pulse. I found that the measured value of DCIR changes with respect to both pulse current and SOC. I provided adequate rest period before the pulse and after.
I dont understand the following behaviour. Could anyone give some hints to understand?
1. Why DCIR values are high at both low SOC (90%)? I assume that the high resistance in low SOC is due to lower lithium concentration. What about high SOC?
2. Why DCIR value changes with pulse current? is it an error? Duration of the pulse, temperature remains the same.
Thanks in advance.
1. DCIR values (for LFP electrodes, LixFePO4), are high at both low SOC (~0%, Limx=1) and high SOC (~100%, Limx=0), because both (pure) cath. electrode materials "LiFePO4" and "FePO4" are common stoichiometric[1] insulators, for both the mixed carriers : Li+ and e- .
2. Yes, DCIR value changes (increases) with pulse current value; it is not an error; both the carriers : Li+ and e-, are correlated with the LFP-grain's mechanical stress; this electrochemically induced mechanical stress increases along with Li-gradient, due to high(er) current pulses. Very high charge (and current) pulses might form (inside a nano-grain) a textured locus of local stoichiometric insulator strings. Extremely high charge (and current) pulses might transform the open locus into closed loops (of the textured) locus e.g. internal islands of stress; this (island mechanism) results towards loosing, island by island, piece by piece, a single grain's activity. Upon cycling, and aging, we might loose, not only partially, but fully, "this" aged grain, resulting to an inactive "grain"; so, we have, finally, a grain (electrically isolated and) detached from the (macroscopic) LFP electrode; this a model illuminating how a (microscopic) "grain by grain" loss results into the (macroscopic active) material's progressive loss.
1. This is why we can form this stable (stoichiometric-LFP) electrode (not the LFP-battery) in a dry room.
Dear Dr. Samaras,
Thanks for such a detailed answer.
Regarding the second question, how can I decide which pulse is suitable for measuring the DCIR. Can I take average of different current pulse or 1C/2C/3C pulse with a shortest pulse possible? The lumped resistance which I measured for DCIR also differs with pulse duration due to fast/slow dynamicsi.e. Rct and Rd. Can I assume that DCIR should always be mentioned with the pulse current and duration for repeatabiliy?
Thank you once again.
Dear Dr. Raam Kasinathan,
as you, already, know the DCIR depends on SOC. A good tool for this (which pulse) decision is EIS, G-EIS is better.
No [1], not for LFP.
Yes[2].
1. Q1: " Can I take average of different current pulse or 1C/2C/3C pulse with a shortest pulse possible ? "
2. Q2: " The lumped resistance which I measured for DCIR also differs with pulse duration due to fast/slow dynamicsi.e. Rct and Rd. Can I assume that DCIR should always be mentioned with the pulse current and duration for repeatabiliy? "
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