High-Precision Coulometry Studies of the Impact of Temperature and Time on SEI Formation in Li-Ion Cells

Abstract

Electrolyte reacts at the surfaces of charging electrodes during the first cycle of a Li-ion cell (formation). This creates the initial solid electrolyte interphases (SEIs), which passivate the electrodes against further reactions with electrolyte. The quality of the SEIs improve over time as a cell matures. The coulombic efficiencies (CEs) of fresh cells (measured by high-precision coulometry, (HPC)) stabilize after a certain period of time (up to 600 hours), after which different cell chemistries can then be ranked for their quality. The purpose of this work is to accelerate the maturation of SEIs during the first cycle, allowing for stabilized CE measurements to be taken after less time. The effects of formation cycle temperature, voltage and duration on subsequent CE are explored. Undergoing formation at 60 • C accelerates the maturity of cells. XPS analysis suggests that organic SEI species are replaced by inorganic SEI species as a cell matures. These results give insight into the mechanisms of cell maturity. Application of the formation protocol described in this work may decrease the time needed for HPC experiments. The first cycle of a lithium-ion cell is when the initial solid electrolyte interphases (SEIs) form from the decomposition of electrolyte at the surfaces of the charging electrodes. If an SEI is ionically conductive and electronically insulating, it passivates the electrode against further reactions with the electrolyte. SEI quality is related to both the SEI thickness 1-4 and the SEI chemistry. [5][6][7] The SEI layers formed in the first cycle are not completely passivating but become more and more passivating over time. Over the past years, the Dahn lab has cycled thousands of new Li-ion cells on its ultra high precision chargers (UHPCs), used to evaluate cell chemistries and rank them in order of coulombic efficiency. [8][9][10][11][12][13] In every case, the CE of a fresh cell increases toward the ideal value of 1.0000. . . during testing but never actually reaches 1.0000. . . After about 15-20 cycles at C/20 (600-800 hours) and 40• C the coulombic efficiencies reach relatively stable values where reliable comparisons between cells can be made.12,14-18 One must wait a sufficient amount of time in an UHPC experiment to rank cells in order of their CEs. Storing cells for extended periods of time before UHPC measurements can also impact the CEs measured in early charge-discharge cycles as the SEI layers can mature to varying degrees and lithium can be incorporated within the anode overhang region, which impacts early CE measurements. 19Ideally in a search for the optimum electrolyte chemistry for a particular cell type, a number of cells differing only in electrolyte composition should undergo formation under the same conditions and be mounted for UHPC testing after the same time interval after formation. This is the way experiments are carried out in our laboratory where we are fortunate to be able to fill pouch cells with electrolyte and perform the formation steps ourselves. Even with th...

Degradation Mechanisms and Mitigation Strategies of Nickel-Rich NMC-Based Lithium-Ion Batteries

“…As aging continues during cycling, researchers believe that the inorganic portions of SEIs also increase as LiF (can be used as an indicator of SEI maturation) is produced through LiPF 6 reacting with lithium carbonates (Eqs. 7, 8) [37].…”

“…Based on this, this review will also provide a complete picture of the aging of graphite anodes that can affect both electrodes in battery cells based on degradation mechanisms and effective mitigation strategies. Furthermore, the solid electrolyte interface (SEI) is a film that forms on graphite anode surfaces during initial charging [36][37][38][39] and thickens in subsequent cycles, which can protect and impede anodes and is vital in the discussion of the aging behaviors of graphite anodes. Therefore, detailed mechanisms of anode SEIs are also reviewed to supplement previously published reviews.…”

Towards a Mechanistic Model of Solid-Electrolyte Interphase Formation and Evolution in Lithium-ion Batteries

“…In fact, previously developed SEI formation protocols involve holding cells at elevated temperatures for hours to optimize this evolution for improved battery cell performance. 60 In order to probe SEI evolution, we performed simulations beginning with equal amounts of Li + EC, LEDC, LEMC, Li 2 CO 3 , and Li 2 C 2 O 4 at an elevated temperature of 423.15K (150 °C) to accelerate decomposition reactions with a tunneling barrier of D = 10 Å, approximating an already-formed and partially electronically insulating SEI. The rapid decoordination/re-coordination reactions with Li + were removed in order to allow us to access longer time scales of ≈ 1s; because all initial species are fully lithiated, this should not adversely affect the availability of Li + in the simulation.…”

The role of metal substitutions in the development of Li batteries, part I: cathodes

“…The CE typically begins smaller and increases approaching 1, as the initial formation of the SEI causes a large discrepancy, which decreases as the SEI matures. 34 Typical values of the CE are 499% after initial SEI formation and state-of-the-art Li-ion batteries show CEs 499.9% and these are the batteries that can last up to 10 years in commercial settings. Thus, it is completely uninformative to present the CE on a scale of 0-100%, but nonetheless this practice is common and leaves the reader unable to evaluate the CE properly.…”