In-Situ TEM Observation of Solid Electrolyte Interface Evolution during Li-Ion Battery Opeeration

Solid electrolyte interface (SEI) plays a critical role in the Li-ion battery. The formation of the SEI passivates the electrode surface and prevents excessive decomposition of the electrolyte. On the other hand, it can degrade the electrolyte and cause a battery to fail if the electrolyte continues to decompose due to insufficient passivation of the electrode. This can be severe for the high capacity anode with large volume changes during charge/discharge cycles. With the expansion and the contraction of the electrode, SEI formed on the surface can crack or delaminate exposing a fresh surface to the electrolyte and cause poor cyclability of the battery. Understanding how does the SEI response to the volume change of the electrode is important to design the electrode structure to improve the cycle performance. The thickness of the SEI is in the nano-meter scale. Therefore, high-resolution imaging using electron microscopy is essential for observing the reaction process.

In this work we present the in-situ transmission electro microscopy observation of the structural evolution of the SEI formed on a Si anode during lithium ion battery operations. Here, we developed a liquid confining cell to prevent electrolyte from evaporating into the high vacuum inside the TEM. We observed the formation of a SEI layer on the Si film during charge along with the volume expansion of the film. Upon discharge, the contraction of the film was observed. The shrinking of the film was not uniform across the film and the strain gradient caused a tension on the SEI layer resulting in the crack formation. New SEI will form on the freshly exposed Si surface on the following charge. This can cause continuous decomposition of the electrolyte and eventual failure due to the electrolyte consumption. The in-situ TEM experiment in this work shows the breaking of the SEI can take place during the discharge after it was formed on charging. Various combinations of the electrolyte, electrode, and the applied potential or current density are necessary to obtain the optimum battery design for producing adequate SEI layer or prevent them from forming. However, out observation provides important information toward designing a battery structure with improved performance.

Figure caption: Sequential TEM image of the SEI cracking on Si anode discharged at 0.0 V vs. LiCoO₂.