Current State of Understanding of the Solid-Electrolyte Interphase (SEI) in Lithium-Ion Cells and Its Relationship to Formation Cycling

Abstract

The process of formation cycling refers to the electrochemical side reactions involved with creating a “passive” film on the anode active material, for instance graphite, known as the solid electrolyte interface (SEI) layer. This interfacial layer is formed during the first several charge-discharge cycles primarily by the reaction of electrolyte components with graphite at reducing potentials near the equilibrium potential of lithium metal (-3.045 V vs. SHE), and it plays a protective role by preventing the graphite from undergoing subsequent reaction with the electrolyte solvent and salt. [1] An ideal SEI layer should be thin, electrochemically inert, electronically conductive, and conductive to lithium ions. The commonly accepted hypothesis of the structure and composition of the SEI layer is an inner (interface with the graphite surface) inorganic layer of compounds such as LiF, Li2CO3, LiOH, LiO2, etc. and an outer (interface with the electrolyte solvent) organic layer primarily consisting of alkyl carbonates such as lithium ethylene dicarbonate (LiEDC). However, obtaining direct experimental evidence of SEI-layer composition and structure is extremely challenging. Information about thicknesses, constituent mass fractions, crystallinity, reactivity, and how these characteristics change from early cell life through thousands of charge-discharge cycles has remained elusive. Formation cycling is performed immediately after a lithium ion cell has been constructed and filled with electrolyte, and it has a profound economic impact on lithium ion battery manufacturing. The formation process requires that battery producers install many cycling stations to complete the process, which results in a heavy capital equipment investment and a much larger plant size. The irreversible capacity loss associated with anode SEI formation involves the consumption of lithium from the fresh cathode, resulting in a diminished battery lifetime. The formation process occurs in two successive stages, with the first stage involving formation of a highly resistive SEI layer at higher anode potentials (>0.25 V vs. Li/Li+). The second stage involves simultaneous intercalation of lithium into the graphite at potentials <0.25 V vs. Li/Li+ where the SEI layer is converted to a highly conductive film. [2] Fong et al. showed that the second stage involving the initial intercalation of lithium into graphite does not occur without the proper passivating electrolyte solvent and a sufficient coverage of SEI film. [3] There are three primary methods of formation cycling, which consist of a two-step current-charge formation, [4] pulse formation, [5] or the ageing process at elevated temperature. [6] In industry, combinations of these three approaches are often used. Electrode wettability plays a critical role in SEI layer formation. [7] After electrodes are coated and dried, they are usually calendered at high pressure to compact the composite structure, thus improving the energy density of the electrode layer. However, the electrode porosity is correspondingly reduced to only 30-35%, which has a significant impact on the pore-size distribution and the related wetting of the electrolyte.8 Wettability of the electrolyte to the electrode pores can be enhanced in two ways: 1) by supplying an additive to the electrolyte to lower its composite surface tension; and 2) by increasing the composite surface energy of the electrode. There is evidence that both of these approaches are effective, but the latter will be the focus of this presentation, especially as it relates to the SEI layer formation chemistry, growth in early cell life, and degradation in aged cells. Acknowledgment This research at Oak Ridge National Laboratory (ORNL), managed by UT Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy (EERE) Vehicle Technologies Office (VTO) Applied Battery Research (ABR) subprogram (Program Managers: Peter Faguy and David Howell).