The Use of Elevated Temperature Storage Experiments to Learn about Parasitic Reactions in Wound LiCoO2∕Graphite Cells

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The effect of mixtures of lithium hexafluorophosphate (LiPF 6 ) and lithium bis(fluorosulfonyl)imide (LiFSI) with different molar ratios in the electrolyte of Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 /graphite pouch cells was studied using the ultra-high precision charger (UHPC) at Dalhousie University, an automated storage system, electrochemical impedance spectroscopy (EIS) and gas evolution measurements. Ethylene carbonate: ethyl methyl carbonate (EC:EMC, 3:7 wt.% ratio) solvent was used as the base solvent in these studies. Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 /graphite pouch cells containing both LiPF 6 and LiFSI (with a total salt content of 1 M) with or without 2% VC showed smaller or similar self-discharge, lower charge transfer resistance (R ct ), and smaller amounts of gas evolution during formation and during storage at high temperature, compared to cells containing only 1 M LiPF 6 with and without 2% VC, respectively. For cells without 2% VC, cells with 0.3 M LiPF 6 + 0.7 M LiFSI showed the smallest self-discharge and the lowest R ct after 40 • C storage. For cells with 2% VC, cells with 0.5 M LiPF 6 + 0.5 M LiFSI + 2% VC showed the lowest voltage drop and the lowest R ct after 40 • C storage. The UPHC cycling data showed that cells containing LiPF 6 :LiFSI mixtures with 2% VC showed similar coulombic efficiency (CE), and similar charge end-point capacity slippage compared to cells with 1 M LiPF 6 + 2% VC. The combination of LiFSI and LiPF 6 in electrolytes that contain 2% VC can bring benefits of improved storage properties and reduced gas evolution at high temperature while maintaining all other properties of the cells in experiments limited to 4.2 V. However, preliminary experiments at voltages up to 4.45 V suggest that LiFSI may lead to increased transition metal dissolution (Ni, Mn and Co) compared to lithium bistrifluoromethane sulfonyl imide (LiTSFI), another salt additive used to improve storage properties and reduce gassing. The use of electrolyte additives is one of the most economical and effective ways to improve the performance of Li-ion cells.1,2 Vinylene carbonate (VC) has been shown to be an effective electrolyte additive for the improvement of cell performance.3,4 Burns et al. 5 showed that LiCoO 2 /graphite cells with 2% VC demonstrated improved life-time with a higher coulombic efficiency (CE) and lower charge and discharge end-point capacity slippage rates, compared to cells without VC. Lithium hexafluorophosphate (LiPF 6 ) is the most commonly used salt in Li-ion cells, due to balance of properties, such as high dissociation constant, high conductivity and good electrochemical stability against Al corrosion.1,2,6,7Recent studies [8][9][10] have shown that lithium bis(fluorosulfonyl)imide (LiFSI) can be thermally stable up to 200• C. However, Al current collector corrosion was observed in Li-ion cells with LiFSI-based electrolytes.10-14 The Al corrosion problem for LiFSI-based electrolytes can be readily solved by the addition of LiPF 6 , which can be decomposed to form a protective film of AlF 3 . ...

Section: Resultsmentioning

confidence: 99%

Effect of Mixtures of Lithium Hexafluorophosphate (LiPF₆) and Lithium Bis(fluorosulfonyl)imide (LiFSI) as Salts in Li[Ni_1/3Mn_1/3Co_1/3]O₂/Graphite Pouch Cells

Wang

Xiao²,

Wells³

et al. 2014

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“…Charge-end point capacity slippage is caused by electrolyte oxidation and correlates with self-discharge at the positive electrode. 19,41 Negative charge-end point capacity slippage is observed when the rate of capacity loss is greater than the rate of electrolyte oxidation. Negative charge-end point capacity slippage was observed for cells without electrolyte additives and without positive electrode coatings.…”

Section: Reactions Of Co-mentioning

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...

“…Smaller voltage drop during storage means cells have less self-discharge due to electrolyte oxidation at the positive electrode. 36 Figure 5a shows cells containing 2% VC, 2% PES or PES211 in EC:EMC all had smaller V drop than cells containing control electrolyte at 40…”

Section: Resultsmentioning

confidence: 99%

Improving Linear Alkyl Carbonate Electrolytes with Electrolyte Additives

Xia

Glazier

Petibon

et al. 2017

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The effectiveness of some selected electrolyte additives were studied in ethylene carbonate-free linear alkyl carbonate electrolyte mixtures containing 1 M LiPF 6 /ethylmethyl carbonate: vinylene carbonate (98:2 w:w). The electrolytes were used in Li(Ni 0.4 Mn 0.4 Co 0.2 )O 2 /graphite pouch type Li-ion cells tested to 4.5 V at elevated temperature. The results showed that cells with linear alkyl carbonate electrolytes outperformed cells with EC-based electrolytes having state-of-the-art additive blends and cells with fluorinated electrolytes during short-term and long-term cycling at elevated temperatures. The addition of triallyl phosphate or pyridine phosphorus pentafluoride as electrolyte additives to the EC-free electrolyte provided cells with better capacity retention during long-term cycling than cells containing vinylene carbonate alone. These results suggest that small quantities of electrolyte additives can be used to create EC-free, linear alkyl carbonate electrolytes that perform well at high voltages. Energy-dense, long-lasting lithium-ion cells are in high demand for electric vehicles, portable electronics and many other important technologies. The easiest way to increase the energy density of a conventional lithium-ion cell is to increase its upper charge cutoff potential when certain layered oxide positive electrode materials are used. However, increasing the upper cutoff potential normally decreases cell lifetime because the rate of unwanted parasitic reactions like electrolyte oxidation increase with potential. Parasitic reactions involving ethylene carbonate (EC)-a ubiquitous co-solvent in lithiumion electrolytes-at high potential has proven to be one of the most important factors to overcome. 1Until recently, EC has been regarded as an essential component of lithium-ion electrolytes and it has been used in nearly all commercial Li-ion cells produced today. EC was first considered as an electrolyte co-solvent by Elliot in 1964 2 and was used as an electrolyte component in cells with lithiated graphite electrodes by Fong et al. in 1990. 3 EC was found to preferentially react with Li + and electrons at the surface of the graphite electrode prior to lithium intercalation, forming a solid electrolyte interface (SEI) on the surface of the graphite electrode that then allowed the reversible intercalation and de-intercalation of lithium ions into the graphite electrode at very low potentials (<0.20 V).3-5 It was generally believed that the graphite electrode could not be electrochemically lithiated in most common solvents, including ethers, esters, butyrolactones and propylene carbonate, without the sacrificial reaction of EC to form the SEI. 6While the passivation of graphite is virtually complete during the first few cycles, the remaining EC in the electrolyte can be oxidized at high voltages, causing the performance of cells containing EC to diminish as the upper charge voltage increases above 4.4 V.7,8 Electrolyte oxidation at the positive electrode at high potentials can lead to salt consum...