Significance of Additive Selection for Effective SEI during Prelithiation and Its Influence on Long-term Cyclability

Summary:

Pre-lithiation, a method of doping an active material with lithium prior to being assembled into a battery cell, has been regarded as one of the most prominent ways to compensate for the amount of active lithium loss during the initial few cycles. The precedent researches have successfully verified the effectiveness of prelithiation in diverse ways [1, 2]. Nevertheless, the majority of them focused more on increasing coulombic efficiency and specific capacity of initial cycles, meaning that the need for advanced cyclability still exists [3, 4]. Furthermore, even though the number of reversible lithium increases by prelithiation, an incomplete process or a mismatched cell system may deteriorate the performance. The robustness of solid electrolyte interphase (SEI) during prelithiation significantly affects the entire cycle life. This study demonstrates how the properly-selected additives refine the SEI and determine the electrochemical performance.

The prelithiated cell without additive had a lower coulombic efficiency (CE), reduced energy density, and less lithium inventory than the additive-mediated prelithiated cell. This lithium loss is attributed to non-stable SEI as less efficient passivation entails additional electrolyte decomposition and lithium consumption. Since the SEI built up during prelithiation is partially converted to another chemical species consuming additional lithium during formation, good passivation is critical to maintaining the number of reversible lithium inside the cell. By introducing a proper additive, SEI can become stable enough to prevent further reduction better.

Four different additives, such as carbon dioxide (CO2), fluoroethylene carbonate (FEC), vinylene carbonate (VC), and (2-cyanoethyl)triethoxysilane (TEOSCN) were examined in this study. From dQ/dV plots, it was shown that VC and FEC lead to less SEI formation but more lithium intercalation, resulting in higher retrievable capacities after prelithiation. On the other hand, TEOSCN has the smallest lithium reservoir and the highest Rct. VC and FEC have kept the small charge transfer resistance (Rct) since right after prelithiation. However, as the number of cycles increased, Rct of the VC-containing cell became slightly lower than that of the FEC-containing cell, which may end up being excellent long-term cyclability. In addition, the SEM images of the prelithiated anode with VC showed a thin layer encompassing the individual active material particles, while the prelithiated anode without additive seemed to have a thick and blunt layer. CO2 has a minor improvement in reversible lithium and Rct after prelithiation; however, CE and Rct after 1st cycle became worse than the prelithiated cell without additive. Therefore it gave rise to poor capacity retention. It is presumed that there was more decomposition during formation in the CO2- or TEOSCN-containing cells. Throughout all the results, it was proven that a small amount of but appropriately selected additives during prelithiation positively influenced the long-term cycling properties.

References
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[4] Shen, Y. et al. „Effective Chemical Prelithiation Strategy for Building a Silicon/Sulfur Li-Ion Battery,“ ACS Energy Lett. 4, 1717–1724 (2019).