What happens when you abuse an 18650 rechargeable LiMn2O4 battery (refer to next photo)? We address this question by explaining why the electronic IR (internal resistance) of a lithium battery becomes much higher if abused. Additional topic about the increasing ionic IR in a Li-ion battery is beyond the scope of this blog post. We may bring that back later on. But for now, let us dissect an abused 18650 rechargeable lithium ion battery.
The reality about most high drain cells sold in the market today
The picture above shows a high drain LiMn2O4 battery that’s designed to drive a heavy electrical load up to its advertised 35 A. They are popular in the Vaping & MODS industry. However, the cycle count only lasts approximately 150 ~ 200 cycles. Their lifetime capacity may accumulate no more than 300 Ah, which is only about 12% lifetime capacity of a well-designed 18650 such as the AmeriBatt Heavy Duty NMC cell. There are several factors contributing to the short cycle life of your typical high drain cell. Unfortunately, most consumers don’t understand this fact and continue to pay high prices for it.
Let’s look at the following two photos as we dissect an abused 18650. Note that this dissection is from an abused IMR18650 35A high drain LiMn2O4 battery.
Structure of negative electrode of an abused 18650
The above photo shows the abused negative electrode. The black layer consists a mixture of conductive powder, additive, binder and anode active (graphite) material, which should adhere to the RA (rolled copper alloy) copper foil seamlessly. Unfortunately, many large areas of the mixed binder material just simply peel off. The effective areas of the electron collector at negative electrode is substantially lost, which results in higher resistance in the electronic path.
Structure of positive electrode of an abused 18650
The above photo shows the abused positive electrode. The black layer consists a mixture of lithium carbonate, additives, binder and cathode active material, which should adhere to the aluminum foil seamlessly. Unfortunately, many large areas of the mixed binder material just simply peel off. The effective areas of electron collector at the positive electrode is substantially lost, which results in higher resistance in the electronic path.
Binder composition used in 18650 lithium ion cells
The binder material is made of PVDF (polyvinylidene difluoride) and SBR (styrene-butadiene rubber). They are highly abrasion resistant and have good aging stability. When mixed into slurry in a vacuum, they exhibit very strong adhesive property and good tensile strength. But what makes their adhesion so flimsy? The answer is in continuous cycling of the battery at very high discharge currents such as 20 ~ 30 A. When discharged continuously at these high currents, the cell gets hot within a few minutes. A hot cell experiences immense strain between binder and metal foil due to their different temperature coefficients. The effect of this strain is more pronounced as the battery body temperature goes above 75 °C.
How to prolong lifespan of your 18650 high drain cell?
To extend the life of your battery, you need to reduce the electronic IR increase by preventing the battery body temperature from going above 60 °C when you discharge it. Do not take the max continuous discharge current of the advertised 35 A at face value! Always measure the battery body temperature to determine the proper discharge current so that it does not exceed 60 °C. Improper discharge practices may instantly increase the electronic IR by 5~10 mOhm, which has a similar effect as a cell that’s undergoing ionic IR aging by 6~12 months! Both the electronic IR aging and the ionic IR aging are irreversible!