A Survey of In Situ Gas Evolution during High Voltage Formation in Li-Ion Pouch Cells

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Different concentrations of LiPF 6 (0.3 M-2 M) in ethyl methyl carbonate (EMC) electrolyte and ethylene carbonate (EC)-based electrolyte were studied in LiNi 0.4 Mn 0.4 Co 0.2 O 2 (NMC442)/graphite pouch cells. Fresh cells containing 0.3 M LiPF 6 in EMC electrolyte showed extremely large charge transfer resistance while those with 0.3 M LiPF 6 in EC/EMC electrolyte did not. Impedance spectra taken on symmetric cells and ionic conductivity measurements suggest this difference is due to difficulty in dissociating and desolvating Li + ions from the EMC-based electrolyte to intercalate into both the electrodes. After elevated temperature storage experiments at 4.5 V, cells with 0.3 M LiPF 6 in EC/EMC showed a large increase in positive electrode charge transfer impedance, presumably caused by electrolyte oxidation. With salt concentrations greater than 1 M, charge transfer resistance was much smaller in EMC-based electrolytes and was stable during storage for both electrolyte types. Conductivity and cycle testing measurements suggest that 1.5 M LiPF 6 should be used in EC-free EMC-based electrolytes to optimize cell performance. Using high potential positive electrodes can increase the energy density of Li-ion cells. [1][2][3][4] Layered LiNi x Mn y Co z O 2 (x + y + z = 1) (NMC) electrodes have been extensively studied because they are cheaper materials than LiCoO 2 (LCO) and can operate at high working potentials.5-15 LiNi 0.4 Mn 0.4 Co 0.2 O 2 (NMC442) is a possible choice for high voltage application due to its high working potential, high specific capacity, moderate cost and excellent safety. [16][17][18][19] NMC442 has been shown to be structurally stable up to 4.7 V 16,17 and its specific capacity increases almost linearly as the upper cutoff voltage increases from 4.1 V to 4.7 V. At 4.7 V, NMC442 can deliver a reversible specific capacity of ∼207 mAh/g. NMC442 shows attractive safety features compared to other NMC grade materials as well. 19It is difficult to create NMC442/graphite cells that show long lifetime when operated continually to an upper cutoff potential above 4.4 V. Electrolyte additives such as pyridine boron fluoride (PBF), 20,21 triallyl phosphate (TAP) 22 and additive blends such as prop-1-ene-1,3-sultone (PES) + 1,3,2-dioxathiolane-2,2-dioxide (DTD) + tris-(trimethyl-silyl) phosphite (TTSPi) [23][24][25][26][27] in EC-based electrolyte have been used to improve charge-discharge cycle life and calendar life of cells operated to 4.4 V. Sulfone-based and fluorinated electrolyte systems have also shown to be of value in getting NMC442/graphite cells to operate beyond 4.4 V. 28,29 In spite of these improvements, further improvement is required. Recently, it was found that EC-free electrolyte using only ethyl methyl carbonate as the solvent can enhance high voltage operation and lifetime of NMC442/graphite pouch cells compared to EC-based electrolyte.30-32 However, only a salt concentration of 1 M LiPF 6 in this novel electrolyte was investigated.The concentration of LiPF 6 in EC-based electrolyte...

Section: Methodsmentioning

confidence: 99%

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confidence: 99%

Dramatic Effects of Low Salt Concentrations on Li-Ion Cells Containing EC-Free Electrolytes

Xiong¹,

Hynes²,

Dahn³

2017

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“…Details of the Archimedes volume measurement instruments at Dalhousie University can be found in several publications. 1,9,10 Quantification and identification of gases using GC-TCD.-Quantification and identification of the gases inside lithium-ion cells were done using a gas chromatograph (GC, Bruker 436-GC) and a thermal conductivity detector (TCD, Bruker). The instrumentation, detector calibration, sample preparation and analysis methods were the same used by Petibon et al 11 Lithium-ion cells were placed in an air-tight gas extraction device (GED).…”

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confidence: 99%

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

Ellis

Allen

Thompson

et al. 2017

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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...

“…Some of this gas is then consumed by reduction on the negative electrode (e.g., reduction of CO 2 to oxalate). 15,17 It has been shown that for given electrolyte, additives and electrode pairs, an increase in temperature causes gas evolution to occur at lower voltages and the total amount of gas generated to increase significantly.Xia et al 16 investigated formation, cycling and storage of graphite/NMC pouch cells (cells balanced for a 2.8-4.2 V voltage window) with different electrolyte additives and observed that the volume of gas generated during the formation cycles (up to ∼13 mL Ah −1 ) was roughly correlated to the amount of irreversible capacity loss. This implied that the additives affected the amount of irreversible capacity loss and gas formation similarly.…”

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confidence: 99%

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

Mao

Farkhondeh

Pritzker

et al. 2017

The calendar life of a commercial graphite/NMC-LMO lithium-ion battery is assessed under various storage temperature (35 • C and 60 • C) and state-of-charge (0% and 100%) conditions. Virtually no degradation is observed for cells stored at 0% SOC as expected, whereas those aged for 9 months in a fully charged state at 35 • C and 60 • C lose ∼10% and ∼43% of their capacity, respectively. Differential-voltage analysis of periodic cell cycling data and post-mortem examination of the aged electrodes are used to identify degradation modes. Three main sources of high-temperature capacity loss are identified: i) electrode dry-out due to gas formation (∼30%), ii) loss of cyclable lithium (∼10%) and iii) cathode active material loss (∼3%). Cells stored at a lower temperature of 35 • C experience little gas generation and degrade primarily through the loss of Li inventory. The analysis shows that similar amounts of cyclable Li are consumed by side reactions during calendar aging at both storage temperatures of 35 • C and 60 • C. Non-destructive compression (1.0-5.0 psi) of the aged pouch cells during discharge is shown to improve their capacity by ∼15%. This effect is attributed to the redistribution of gas bubbles inside the pouch cell by the applied pressure. After undergoing several generations of development as energy storage devices, lithium-ion batteries (LIBs) are now widely used in consumer electronic products and their designs are being broadened for use in the automotive industry. The introduction of commercial LIBs consisting of LiNi x Mn y Co 1-x-y O 2 -LiMn 2 O 4 (NMC-LMO) blended cathodes and graphite anodes has been so successful that they now dominate the burgeoning market of automotive LIBs due to their superior performance and lower price compared to conventional LiCoO 2 cathodes.1 In order to continue optimizing the design of these batteries, a number of experimental and modeling studies on the operating behavior, life cycle, degradation mechanism, etc., of blended cathodes have been conducted. Jung 2 developed a mathematical model for a graphite-soft carbon/NMC-LMO cell for the purpose of designing new electrodes and predicting the performance of the cell with different cathode compositions. In our previous studies, 3,4 we developed a model-based approach to identify the composition of an unknown blended cathode and presented a multi-particle model to describe the electrochemical performance at different charge/discharge rates that accounts for the effects of particle size distribution and solid-state diffusivities in a commercial NMC-LMO (70:30 wt%) cathode. The model also was used to explain the asymmetry between charge and discharge behavior of the blended electrode 5 and describe its internal dynamics during intermittent operation. 6 Several research groups have studied the cycling 7-10 and calendar aging 11,12 of commercial cylindrical graphite/NMC-LMO batteries and identified the loss of capacity due primarily to SEI growth and secondarily to the loss of active materials (e.g., active particle br...