During the cycling life of lithium ion battery, the decline in the early stage is usually linear, but in the later stage, we often find that the decline exhibits an accelerating trend (as shown in the following picture). Many reasons lead to irreversible capacity loss of lithium ion batteries, such as loss of active material, increase of internal resistance and Li loss.
The analysis of the battery after severe nonlinear degradation shows that the boundary layer between the cathode and the separator is very thick. The composition analysis also shows that the film contains much lithium. All the above results show that the capacity decline acceleration of lithium ion battery in the late cycle has a great relationship with the deposition of lithium on the anode surface.
After cycles, lithium will precipitate on the anode surface. The precipitated Lithium metal may lead to electrolyte decomposition, resulting in SEI membrane thickening, the anode porosity decreasing and performance deterioration of lithium ion battery. Parts of lithium metal will lose connection with the conductive network, thus forming “dead” lithium. in severe cases, the metal lithium even formed lithium dendrites, threading the battery safety.
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