Design of Solid Electrolyte Interphase to Improve Li-Ion Battery Cycle Efficiency

Among the many challenges to enable large scale commercialization of Li-ion batteries in plug-in hybrid electric vehicles (PHEVs) and full electric vehicles (EVs), formation of high quality solid electrolyte interphase (SEI) is one critical issue to achieve long term battery durability and safety. As a nano-meter scale thin film which natively forms on the negative electrode due to electrolyte reduction, SEI is a complex system composed of various lithium salts and organic reduction products. Most published works have focused on identifying the SEI composition, but the understanding of correlation between SEI properties and battery performance is still lacking.

Previously, we have identified that SEI impedance is closely related with its composition. Briefly, the inorganic components (e.g. Li₂O and Li₂CO₃, etc) constitute a low impedance and less porous part of SEI, while the organic reduction products tend to be more porous and resistive.¹ In this talk, we will advance the findings and evaluate characteristic Li salts using isotope exchange method. Typical SEI components, e.g. Li₂CO₃, LiF, etc. are found to behave differently in passing Li. With these understandings, we will also demonstrate that the SEI composition can be modified by surface functionalization and coating on graphite electrode. By designing the SEI chemistry, the electrode/electrolyte interface is more stable and the cycling efficiency can be improved.

Figure. Isotope profiles of three Li salt thin films after soaked in 1M ⁶LiClO4 (DMC) solution for 30s. The natural abundance of ⁶Li:⁷Li is ~0.08. The different isotope ratios in the films indicate that SEI components may behave differently in passing Li.

Reference 1. Lu, P., Li, C., Schneider, E. W., Harris, S. J., J. Phys. Chem. C, 2014, 118, 896-903