Interactions between Positive and Negative Electrodes in Li-Ion Cells Operated at High Temperature and High Voltage

Petibon

et al. 2016

Self Cite

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...

“…40 The delithiated NMC442 electrode placed in a sealed pouch bag with conventional carbonate electrolyte, * Electrochemical Society Member.…”

Section: -14mentioning

confidence: 99%

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confidence: 99%

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

Xiong

Petibon

et al. 2016

Self Cite

“…This was done because the reaction of electrolyte with charged positive electrodes in pouch bags (i.e. without the presence of the negative electrode) is known to cause significant gas evolution and impedance growth, 8 which would have interfered with the observation of gas consumption. Charged electrodes were then placed inside "pouch bags".…”

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confidence: 99%

“…Rinsed and dried positive electrodes were placed in pouch bags alone, without electrolyte (which would have oxidized to cause gas evolution). 8 Negative electrodes were placed in pouch bags with the addition of 0.4 mL electrolyte (to more accurately simulate the environment of a pouch cell). Rubber septa (Thermo Scientific, C4000-30) were adhered firmly to the outside of the pouch bags, using a generous amount of silicone sealant ('Silicone 1', General Electric).…”

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confidence: 99%

Quantifying, Understanding and Evaluating the Effects of Gas Consumption in Lithium-Ion Cells

Allen

Thompson

et al. 2017

Self Cite

114

Lithium-ion cells produce a considerable amount of gas in their first cycle. If the gases are not removed in a degassing step, most are consumed by the cell over time. This phenomenon has never been investigated explicitly in the literature. In this paper, the evolution and subsequent consumption of gas in typical lithium-ion cells are measured by Archimedes' principle and gas chromatography. It is found that all evolved gases are subsequently consumed to some degree, except for saturated hydrocarbons. The consumption of gas occurs predominantly at the negative electrode, where the gases are reduced to form part of the solid-electrolyte interphase (SEI). Changes to the negative electrode SEI upon gas consumption are investigated using X-ray photoelectron spectroscopy. The effect of gas consumption on cell performance is studied with ultra-high precision charging and high voltage storage experiments. It is found that gas consumption does not result in measurable adverse effects to cell performance. Lithium-ion cells can produce a significant amount of gas during the first charge (in the formation cycle), as electrolyte and additives react at the surfaces of the charging electrodes to form passivating films. If lithium-ion cells are packaged in a flexible casing, these gases are normally removed by the manufacturer in a degassing step, to prevent deformation of the cell and to ensure uniform stack pressure on the electrodes. If the degassing step is omitted, a large portion of the gas evolved is consumed over time.1 The reactions that consume gas are presumably prevalent in hard-cased cylindrical cells, such as 18650 s, which are often hermetically sealed before the first charge, and therefore cannot be easily degassed. The reactions that consume gas are presumably less prevalent in pouch-type cells, which are degassed.Several authors have speculated about the fates of gases in lithiumion cells.2-5 There has been no work explicitly dedicated to understanding the phenomenon of gas consumption. There is no consensus as to whether the effects of gas consumption are beneficial or harmful to cell performance. For example, it has been argued by some that the consumption of CO 2 is beneficial to cells, as it reacts to form a passivating film on the negative electrode. 3,4,6 However it has also been argued that the consumption of CO 2 is detrimental to cells, as it may reduce at the negative electrode to form Li 2 C 2 O 4 , which causes continual self-discharge at high voltage. 2It is important for both scientists and manufacturers of lithium-ion cells to understand the causes and the effects of gas consumption. If gas consumption is quick, benign, or even beneficial to cell performance, then the time-consuming degassing step for lithium-ion pouch cells might be skipped. 7 The gases evolved in lithium-ion pouch cells could be left for consumption within the cell, perhaps leaving the pouch cell flat and rigid after several hours if all the gases were consumed. If gas consumption in a cell produces undesirable effects, such...

“…A pouch bag method where the graphite electrode is not present has also been used to investigate the reactivity between delithiated NMC materials and electrolyte. [51][52][53] These various studies suggest that the oxidation of electrolyte is a major issue that increases significantly with cell voltage. Self et al recently noted that decomposition products from a sulfur-containing additive (prop-1-ene-1,3-sultone) and the organic carbonate solvent oxidation have been observed at lower electrode potentials (i.e., ∼4.2 -4.7 V vs. Li/Li + ) 42 than expected from voltammetric studies (>5.3 V vs. Li/Li + ) [54][55][56][57][58] and density functional theory (DFT) calculations (> 6.0 V vs. Li/Li + ).…”

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confidence: 99%

Measuring Oxygen Release from Delithiated LiNi_xMn_yCo_1-x-yO₂and Its Effects on the Performance of High Voltage Li-Ion Cells

Xiong

et al. 2017

Self Cite

There can be a trade-off between the lifetime and energy density of LiNi x Mn y Co 1-x-y O 2 (NMC)-containing cells that depends on their upper cutoff voltage. This work applies thermogravimetric analysis coupled with mass spectrometry (TGA-MS) to measure the release of oxygen from delithiated NMC electrode materials at high electrode potentials, i.e., low lithium content. This release is observed at relatively mild temperatures as low as 40 • C. The amount of oxygen released is greatly limited using single crystal NMC particles. Electrochemical measurements demonstrate a correlation between TGA-MS results and the cycling performance of NMC/graphite cells. X-ray photoelectron spectroscopy complements the TGA-MS results and provides evidence of the solid electrolyte interphase decomposition. The results in this work offer strong support that the release of oxygen from NMC can cause oxidative decomposition of the electrolyte and is a major reason why high voltage cells can generate gas and can have poor capacity retention. There is a need for longer-lasting and higher energy density lithium-ion cells for electrified vehicles and grid energy storage applications.1-3 Layered LiNi x Mn y Co 1-x-y O 2 (NMC) materials make good positive electrodes on account of their well-balanced properties of high specific capacity, moderate cost, and good safety.4-9 As a result, NMC/graphite cells have become a ubiquitous part of our society. However, to avoid electrolyte and material degradation, commercially available NMC-containing cells normally operate at a much lower voltage (typically ≤ 4.2 V) than their theoretical limit. This is because trade-offs exist between the lifetime and the energy density of a NMC cell that depends on its upper cutoff voltage. 10,11 One method to improve existing NMC technology is the development of new electrolyte solution chemistries. The use of new solvent blends and the introduction of electrolyte additives are two practical ways to improve the lifetime of high voltage NMC Li-ion cells. 12-22Ma et al. and Nelson et al. found that a ternary additive combination can greatly improve NMC442/graphite cell performance at high voltage.11-13 Xia et al. reported that fluorinated electrolyte and ECfree electrolyte also enhance NMC/graphite cell performance. [16][17][18][19] Another approach to improve cell stability is the application of an inorganic coating onto the NMC surface. [23][24][25][26][27][28][29] For example, Arumugam et al. reported that a surface coating of Al 2 O 3 greatly improves NMC622/graphite cell performance. 24 Whereas both of these approaches have proven to be quite valuable, further improvements to the lifetime of NMC-containing cells at high voltages are still desired.Great effort has been put forward to understand the chemical mechanisms behind the degradation of high voltage NMC cells. Analytical approaches have included monitoring impedance growth, 12,13,30-38 the volume and composition of gases produced, 39-43 the heat flow [44][45][46][47][48] and the leakage current 49,50 du...