Studies of Gas Generation, Gas Consumption and Impedance Growth in Li-Ion Cells with Carbonate or Fluorinated Electrolytes Using the Pouch Bag Method

Abstract

Li [Ni 0.42 Mn 0.42 Co 0.16 ]O 2 (NMC442)/graphite pouch cells with an ethylene carbonate-containing or a fluorinated electrolyte were used to prepare charged electrodes for studies using "pouch bags". Sealed pouch bags containing either lithiated graphite or delithiated NMC442 electrodes taken from pouch cells, and also "sister" pouch cells, were subjected to 500 h storage at elevated temperature. The electrodes recovered from the pouch bags and pouch cells after storage were studied using electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy while the gases generated were quantified using gas chromatography. The fluorinated electrolyte suppressed impedance growth of the positive electrode during storage but caused a large initial negative electrode impedance compared to the carbonate electrolyte. The solid electrolyte interface (SEI) formed by the fluorinated electrolyte at the graphite electrode hinders the consumption of CO 2 generated at the delithiated NMC442 electrode, leading to more CO 2 in pouch cells with fluorinated electrolyte than in cells with carbonate electrolyte. Hydrogen gas was only observed in pouch cells after storage and not in pouch bags which contained either a single negative electrode plus electrolyte or a single positive electrode plus electrolyte, suggesting the H 2 results from a species created at one electrode which reacts at the other in a pouch cell. , one of the layered NMC series, has attracted attention due to its high working potential, high specific capacity and excellent safety. 11-14This material can be operated to up to 4.7 V without any substantial structural change.11,12 Its discharge specific capacity increases almost linearly with the upper cutoff potential from 4.1 V to 4.7 V. At 4.7 V, a reversible specific capacity of ∼207 mAh/g can be obtained from this material. In addition. NMC442 shows higher onset temperatures for exothermic reactions with electrolyte compared to other NMC materials such as LiNi 0. 16-27 A similar approach was employed to improve NMC442/graphite cell performance at high voltage as well.28-39 For example, pyridine boron trifluoride (PBF), [28][29][30] triallyl phosphate (TAP), 31 and a blend of additives containing prop-1-ene-1,3-sultone (PES), 1,3,2-dioxathiolane-2,2-dioxide (DTD) + tris-(trimethyl-silyl) phosphite (TTSPi) [32][33][34][35][36] have been used to improve high voltage performance of NMC442/graphite cells to some extent. Sulfone-based electrolytes can improve high voltage performance of NMC442/graphite cells as well. 38 Compared to the options of additives in conventional EC-based electrolyte and sulfone-based electrolyte, fluorinated electrolytes may be the best option for improving NMC442/graphite cell cycle life when cells are operated to 4.5 V and above.39 This is because the cell impedance can be controlled when fluorinated electrolytes are used while it increases dramatically with carbonate solvents operated above 4.5 V.Recently, a pouch cell and pouch bag method has been put forward to study the inte...

Lithium Oxalate as Capacity and Cycle-Life Enhancer in LNMO/Graphite and LNMO/SiG Full Cells

“…Hence, under the present conditions, the CO 2 /oxalate shuttle effect is either negligible or nonexistent and does not contribute to the side reactions in the cell. This is in agreement with Xiong et al, 40,43 who showed that there is no re-generation of CO 2 from a lithium oxalate/CO 2 shuttle detectable in NMC422/graphite cells.…”

“…As many commercial-scale cells are vented during or after formation, the high pressure/volume increase is only a matter of the very first cycles. We have further not considered the consumption of CO 2 on the graphite or silicon/graphite anode: Strehle et al 16 showed that up to 40 μmol (≈1 mL) CO 2 can be consumed per square meter graphite surface area during the first formation cycle, which agrees well with previous reports by Xiong et al 40 The graphite electrodes used in the present study have a specific surface area of 0.034 m BET 2 /cm geom 2 , hence 40 μmol/m BET 2 would correspond to a CO 2 consumption of 1.4 μmol/cm geom 2 , which is about ∼20% of the evolved CO 2 (6 μmol/cm geom 2 ). As long as the ratio of lithium oxalate to graphite or the specific surface area of the graphite do not change drastically, the same fraction of CO 2 would also be consumed during formation in other cell formats.…”

“…12,13,39 Xiong et al found that almost all CO 2 generated on the cathode can be consumed on the graphite anode in commercial-scale NMC422/graphite fullcells, given that no other strong SEI-forming additives are present. 40 Krause et al showed that CO 2 can have a similar effect on the cycling stability of graphite electrodes in EC-free electrolyte solutions as VC. 18 Recently, our group demonstrated that CO 2 can stop the transesterification reactions occurring from alkoxides generated through the reduction of linear carbonates.…”

Calendar Aging and Gas Generation in Commercial Graphite/NMC-LMO Lithium-Ion Pouch Cell

“…Also, hydrogen was found in pouch cells, but not pouch bags, suggesting that both electrodes in the cell are required for its appearance. 20 Considering the detrimental effect of gas formation on battery life, Michalak et al 25 suggests that proper cell formation procedure is necessary to improve battery durability.…”

“…In a series of publications, Dahn and co-workers used gas chromatography (GC-MS and GC-TCD) and a lab-made apparatus based on Archimedes' principle to investigate gas generation during the formation cycles of batteries filled with various electrolyte formulations. [14][15][16][17][18][19][20] The gaseous species were shown to be mostly generated at the onset of the first charging step (i.e., low voltages) and then almost entirely consumed throughout the remainder of the formation period.…”