The very first charge is crucial for a battery. At the moment the battery is first charged, a very thin layer which protects the electrolyte and the anode is formed on the surface of the anode. This layer is the “SEI (Solid Electrolyte Interphase) layer”, which affects the battery’s lifespan and performance.

How is the SEI layer formed?

The final stage in the battery manufacturing is the formation process, often described as the step that brings the battery to life. This is because it imparts electrical properties to the battery cell, checks for defects, and ensures the battery functions as intended.

When charging begins during the formation process, lithium ions leave the cathode and move toward the anode. At this point, the electrolyte acts as a pathway for the lithium ions to travel. However, some of the electrolyte that reaches the anode surface decomposes instead of remaining intact. The substances generated by this decomposition gradually accumulate on the anode surface, eventually forming a very thin layer. This solid layer is the SEI layer that separates the anode from the electrolyte.

[The SEI layer is formed on the surface of the anode]

Why does SEI layer determine battery life and performance?

The SEI layer protects both the electrolyte and the anode. If the SEI layer does not form properly, the electrolyte continues to decompose every time the battery is charged, and the anode surface is quickly damaged. The SEI layer acts as a protective barrier that prevents these problems, helping the battery last longer and function more reliably.

Another important role of the SEI layer is blocking electron movement while allowing only lithium ions to pass through. As a result, the battery remains stable even when it undergoes repeated charging and discharging cycles. 

[The role of the SEI layer]

Just because there’s an SEI layer doesn’t mean it’s perfect

Not all batteries perform the same just because they have an SEI layer. A thin, even SEI layer helps the battery maintain its performance over a long period. In contrast, an SEI layer that is too thick or unevenly formed can disrupt the internal flow in the battery and lead to earlier degradation in lifespan or performance. In the end, what matters most is how well the SEI layer is formed.

SAMSUNG SDI strives to improve the quality of the SEI layer through technological innovation in formation process and the development of electrolyte materials.

Such fine invisible details in battery technology can make all the difference. [Battery Talk Talk] will be back with another insightful topic!