Study of Electrolyte Additives Using Electrochemical Impedance Spectroscopy on Symmetric Cells

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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...

mentioning

confidence: 99%

“…R ct was taken to be the diameter of the mid-frequency semicircle in the Nyquist plot. 13,15,16 High voltage storage.-The effect of gas consumption on selfdischarge was tested in a high voltage storage experiment. In this experiment, only the voltage of the cells was monitored as a function of time.…”

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

“…12,26 Symmetric cells were constructed from electrodes after dissembling some of these pouch cells. The detailed procedures for symmetric cell construction have been described by Petibon et al 26 The pouch cells were charged or discharged to 3.8 V and held at 3.8 V for one hour (approx. 50% state of charge) before they were cut open in an argon-filled glove box.…”

mentioning

confidence: 99%

Effects of Electrolyte Additives and Solvents on Unwanted Lithium Plating in Lithium-Ion Cells

Liu

Petibon

et al. 2017

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Unwanted lithium plating on the graphite anode of lithium ion batteries can reduce the cycle life and safety of lithium ion batteries. Increased charging rates, lower temperatures, thicker electrodes, lower Li-ion diffusion constant and larger graphite particles all increase the propensity for unwanted lithium plating. In this work, a variety of electrolyte additives and electrolytes, which extend lifetime during low rate cycling, were used in Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 /graphite (NMC111/graphite) pouch cells subjected to high rate charging at 20 • C. It was found that additives and electrolytes which increased the negative electrode area-specific resistance, R negative , decreased the onset current, I u , for unwanted lithium plating. Here, the processes of ion desolvation, electron and ion transport through the solid electrolyte interphase and contact resistance are lumped into the R negative . Under conditions where R negative is the dominant factor determining when unwanted Li plating occurs, the onset current for lithium plating could be well predicted by the expression: I u = 0.080 V x S/R negative , where S is the geometric electrode surface area. R negative is easily determined using negative electrode coin-type symmetric cells. This simple rule-of-thumb relation will help guide researchers seeking to select electrolyte additives that simultaneously increase lifetime and also allow fast charging. Unwanted lithium plating can occur in Li-ion cells at high charge rates due to the close working potential of the graphite anode to metallic lithium. If the lithium ions and corresponding electrons cannot intercalate within the graphite particles fast enough, then metallic lithium can deposit on the surface of the graphite anode. Unwanted lithium plating is one mechanism that can limit the lifetime of lithiumion cells. Even worse, plated Li can form dendrites which may pierce the separator and affect the safety of lithium-ion cells. 1,2The propensity for unwanted lithium plating depends on many factors such as electrolyte components, 3 cell design (e.g., anode/cathode ratios, thickness and porosity of the graphite anode), 4 cell history and age (e.g., thickening of the negative electrode solid electrolyte interphase (SEI)) 5 and harsh charging conditions (e.g., high charge rates, low temperature and overcharge).6-8 Among these factors, the electrolyte used in lithium-ion cells not only determines the ionic conductivity but also plays a significant role in determining the properties of the SEI films.9,10 Therefore, the electrolyte components (i.e., Li salt, solvents and additives) can strongly affect unwanted lithium plating.The choice of appropriate electrolyte additives is an efficient and simple way to improve the lifetime of Li ion batteries by modifying the SEI properties.11 Vinylene carbonate (VC) has been commonly used as an additive for forming a robust negative electrode SEI and has been shown to improve the cycle life and calendar life of Li-ion cells.12-14 Ethylene sulfite (ES) was reported to form a...

“…Symmetric cells were made by the procedures described by Petibon et al and Burns et al 9,16 The pouch cells were charged or discharged to 3.80 V (approx. 50% state of charge) before they were opened in an argonfilled glove box.…”

Section: Methodsmentioning

confidence: 99%

Succinic Anhydride as an Enabler in Ethylene Carbonate-Free Linear Alkyl Carbonate Electrolytes for High Voltage Li-Ion Cells

Xia

Liu

Hebert

et al. 2017

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Ethylene carbonate-free electrolytes containing 1 M LiPF 6 in ethyl methyl carbonate with succinic anhydride as an enabler exhibited promising cycling and storage performance in Li(Ni 0.4 Mn 0.4 Co 0.2 )O 2 /graphite pouch type Li-ion cells tested to 4.5 V. Although cells using 1 M LiPF 6 in EMC can barely function due to the poor passivation of the graphite electrode, the addition of 1% succinic anhydride allows cells to operate well. Compared to other enablers such as vinylene carbonate, succinic anhydride provides similar cycling and storage performance at 40 • C and improved storage or cycling performance at 60 • C. Succinic anhydride also limits gas evolution, especially at high temperatures where cells with vinylene carbonate normally produces large amounts of gas. Symmetric cell studies showed that adding succinic anhydride to cells greatly increased the impedance of the graphite electrode after formation. However, the impedance of the graphite electrode decreased substantially during cycling, leading to a significant decrease of impedance in the full pouch cells. With succinic anhydride as an enabler, ethylene carbonate-free linear alkyl carbonate electrolytes may be suitable for high temperature applications in some Li-ion cells. Long life and energy dense Li-ion cells are attractive to battery manufacturers and electric vehicle (EV) makers. Cycling a Li-ion cell to higher voltage increases its energy density but normally decreases cell lifetime. This is because electrolyte solvents are unstable at high voltages and the positive electrode/electrolyte interface becomes more reactive as the potential increases.1,2 Recent results have shown that ethylene carbonate (EC) itself is responsible for many issues associated with the operation of NMC/graphite cells to high potentials. 3,4 Reactions involving EC at the positive electrode cause gas evolution and impedance growth, leading to cell failure. Such reactions become very problematic above 4.5 V vs. Li/Li + regardless of the electrolyte additives used. 5The removal of EC has been shown to enhance high voltage performance of NMC(442)/graphite cells at both room temperature and high temperature.4 When used at small loadings, several cyclic carbonates including vinylene carbonate (VC), fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DiFEC), can enable the use of linear alkyl carbonate-based electrolytes without ethylene carbonate.3 The cycling performance of these EC-free electrolytes was determined to be dependent upon the initial loading of passivating agent added, which must be high enough to passivate the graphite electrode, yet low enough to ensure low impedance (for VC and DiFEC) and low gassing during cell use. The addition of co-additives helps lower polarization growth during high voltage cycling. 4 Problems with EC-free ethyl methyl carbonate (EMC)-based electrolytes include gas evolution during both cycling and storage at high temperatures. Figure S1 (supporting information) shows open circuit voltage (OCV) versus time as well as gas ...