Passivation is a chemical phenomenon affecting lithium battery performance. It is a film that forms on the negative electrode, serving to prevent discharge after removal of load. This is a positive arrangement within healthy limits, but can have negative consequences. We examine the chemistry behind passivation on negative battery electrodes.
How Does Passivation Apply to Negative Battery Electrodes?
Passivation is a chemical process that renders a material less likely to be affected / corroded by the environment. This may be achieved by applying a suitable coating, or through natural oxidation with the air.
Many metals passivate naturally to form a relatively inert surface layer. Out of interest, the dark tarnish (silver sulfide) that appears on silver is one example of this phenomenon. Iron, by comparison forms a rough coating of rust, which readily sloughs off exposing the metal to further oxidation.
A negative battery anode oxidizes naturally during the chemical reaction that releases electrons to the external circuit. This beneficially helps prevent the battery self-discharging after removal of load. But how does this affect lithium battery calendar life?
Lithium Chloride Battery Passivation
A passivation SEI layer of lithium chloride prevents the lithium from spontaneously discharging, after removal of load. However, this causes voltage delay upon reconnection.
Once past this point, the reaction gradually removes the layer, allowing voltage to optimally flow, according to Spectrum. The passivation layer then reinstates, once the load is again removed.
The thickness of the passivation on the negative battery electrode should reach a stable level after several iterations. However, elevated temperatures may cause it thicken further. Wikipedia explains this consumes lithium ions, and reduces overall charge and discharge efficiency.
Cycling lithium cells at high temperatures, or fast rates may damage the SEI layer, or lithium plating leading to battery degradation. Charging lithium batteries above 80% of their capacity may rapidly accelerate this process.
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