In lithium-based batteries, the solid electrolyte interphase (SEI) is a layer of material that forms between the negative electrode and the liquid electrolyte . It is well known that this layer covers the anode surface and acts as a passivation layer between the electrode and the electrolyte . It contains lithium carbonate (Li2CO3), lithium methyl carbonate (LMC) and lithium ethylene dicarbonate (LEDC) and results from reactions between the electrolyte and the active material of the anode.The SEI is probably the most important factor controlling the efficiency, safety and lifespan of lithium batteries . Thus, the (SEI) has solid electrolyte properties, namely it allows ions to pass but prevents electron transport. Although it ensures the stability of the cell, its growth consumes active material and causes an increase in internal resistance. Therefore, the SEI growth leads to capacity and performance loss. It is also known to be the most regular and persistent form of anode degradation .
The formation of SEI
During the charging process of a lithium ion battery, lithium ions move to the anode and the battery voltage increases . Due to the electrochemical decomposition of the electrolyte on the anode, the (SEI) grows on the surface of the anode. This normally operates at voltages below the electrochemical stability window of the electrolyte, accelerating redox processes that irreversibly degrade the electrolyte and lead to electrolyte loss . The SEI layer initially forms in the first 4–6 cycle , resulting in a ca. 10% reduction in capacity, but then serves to stop further reaction of the electrolyte at the negative electrode . However, the thickness of the SEI layer increases as the cell ages. After the initial cycles, the SEI layer continues to thicken over the cycles, eventually blocking the interaction between graphite and electrolyte and leading to battery failure . SEI thickness has been reported to increase linearly with the square root of time .

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