Measurement of 18650 Batteries and Conclusion

I have now received my “9900mAh, 3.7V” batteries, and their bundled “4.2V” charger, which I first wrote about in this earlier posting. After receiving a full charge, their measured voltage while still inserted was 4.215V , immediately after removed at no load 4.195V , and after standing for 30 minutes, at no load, 4.138V . When new they require approximately 4h + 5min to charge.

I have to conclude that these batteries do not contain any series-connected, internal, over-voltage-protection chip. They seem to be based on the Layered Lithium-Manganese-Oxide:┬áLi2MnO3 . They differ from the “3400mAh, 3.7V” variety, in that the other kind are based on the Spinel Lithium-Manganese-Oxide: LiMn2O4 .

I must only use this charger, with the batteries it shipped with.

Also, I am reading from other sources, that while the stated voltages of lithium-ion batteries usually assumes a more-sophisticated negative electrode consisting of graphite, a gain of 0.2V can be obtained, if cheaper metallic lithium is used instead.

The standard electrode potential of metallic lithium is -3.04V , while that of the lithium-bearing graphite is -2.84V .

This can make the difference between a 3.5V and a 3.7V battery…

Further, whether it is actually ‘cheaper’ to manufacture negative electrodes with metallic lithium, will depend on whether they are for primary batteries, or for rechargeable batteries:

  • In the former case, the metallic anode is already fully-functional when shipped, and the cathode hungry for ions, ‘with a transition metal in its higher oxidation state’ . Or then, the cathode could just consist of a convenient oxide, as discharging it need not be reversible.
  • In the latter case, most of the lithium is in the positive electrode as ions, with the transition metal in its lower oxidation state, and the first charge draws about half those across to the cathode, thus activating the negative electrode for the first time, and incrementing the oxidation state of the transition metal in the positive electrode.

It could sometimes be cheaper just to manufacture an empty negative electrode ‘sponge’ of pure graphite, alternatively to which we could manufacture one of inert metal, onto which metallic lithium becomes deposited during a first charge…

(December 22, 2016)



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