Interaction Mechanism Between SEI Lithium Salts and Glymes or Surfactant as Additive in Li-Ion Batteries

Lithium-ion batteries (LIB) are undoubtedly the most popular and mature electrochemical storage devices and, for future use, a higher degree of safety, a longer cycle life, and a higher specific energy are desirable. In order to achieve this goal, numerous publications have been devoted to new active materials and new electrolytes [1] but far less to interfaces. As a matter of fact the solid-electrolyte interphase (SEI) formed through the reductive decomposition of solvent molecules plays a crucial role in the performances of LIB. The robustness and efficiency of the SEI [2] therefore significantly affect the power capability, stability, safety and cycle life of a LIB.

In this study, we investigate the impact of several lithium salts, already present in the SEI like LiF, Li₂O, LiOH, Li₂CO₃, LiOCH₃ and LiOC₂H₅, on the cycling ability of a graphite or LiNiMnCoO₂ electrodes [3] in Li half cells containing the standard (EC/PC/3DMC + 1 M LiPF₆) electrolyte. These lithium salts are soluble (LiF), slightly soluble (LiOCH₃ and LiOC₂H₅) or almost insoluble (Li₂O, LiOH, Li₂CO₃) but present in solution as a colloidal suspension at saturation. Results show that these lithium salts, dissolved or in suspension in the electrolyte have a non-negligible impact on the electrode capacity during cycling, depending on the type of electrode and nature of the salt. Mineral salts like LiOH, Li₂O and LiF (at high concentration) degrade battery performances by forming a resistive film at the electrode surface. On the contrary, LiOCH₃ and Li₂CO₃ are beneficial to cycling performances owing to the formation of a Li conductive coating. In order to avoid the negative impact of dissolved LiF, glymes like diglyme (G2), triglyme (G3) and tetraglyme (G4) have been added as these compounds are known to complex strongly Li⁺ ions. A complementary approach has been the use of fluorinated surfactant additives able to modify the interfacial material/electrolyte free energy. Electrochemical characterization by galvanostatic cycling and cyclic voltammetry, combined with XPS and SEM surface characterization have been performed to evaluate the impact of each additive and to clarify its mechanism of action. Results show that the addition of glymes limits the negative effect of SEI lithium salts, while the surfactant improves the wettability of the electrodes leading to better cycling performances.

[1] J.B. Goodenough, Y. Kim, Chemistry of Materials, 22 (2010) 587-603.

[2] P. Verma, P. Maire, P. Novák, Electrochimica Acta, 55 (2010) 6332-6341.

[3] Fabien Chrétien, Jennifer Jones, Christine Damas, Daniel Lemordant, Patrick Willmann, M. Anouti, Journal of Power Sources, 248 (2014) 969-977.