Studies of the Effect of Varying Vinylene Carbonate (VC) Content in Lithium Ion Cells on Cycling Performance and Cell Impedance

Li-ion cells are used in a variety of mobile and stationary applications. Their use must be safe under all conditions, even for aged cells in second-life applications. In the present study, different aging mechanisms are taken into account for accelerating rate calorimetry (ARC) tests. 18650-type cells are cycled at 0 • C (Li plating expected) and at 45 • C (SEI growth expected). After extensive evaluation of the electrochemical results (voltage curve analysis, capacity fade, energy fade, Coulombic efficiency), the cells are tested by PostMortem analysis (CT, GD-OES, SEM) to reveal the main aging mechanisms and by ARC to test the safety behavior. Besides typical ARC results such as onset-of-self-heating, onset-of-thermal runaway and maximum temperatures, as well as acoustic responses of thermal runaway are evaluated and a method is developed to compare fresh cells and cells aged until different SOHs. It turns out that the safety of aged cells is not simply a function of the SOH. However, safety is strongly affected by the main aging mechanism and to the history of operating parameters during the life-time of the cell. Driven by the demand for higher capacity, lower volume, and lower weight, Li-ion technology was developed in the 1980s.1,2 Nowadays, Li-ion technology is used for energy storage in a large variety of applications, e.g. smartphones, tablets, or drones. In particular, electric vehicles in combination with renewable energy sources are promising for reducing climate change and the corresponding glacier melting and sea level rise which is influenced by burning fossil fuels.3-6 For all applications, the safety behavior under all specified operating conditions is most important.7-9 The same is true for aged Li-ion cells used in second-life applications, for example cells which are utilized in stationary energy storage applications after their usage in electric cars.In the past, failure of Li-ion cells led to product recalls which are very expensive for the manufacturers and unsettle customers, even if such incidents happen only in very few cases (ppm range).10 At the moment, safety tests have to be performed with Li-ion prototypes cells before market release. In these tests, fresh cells are always used, however, aged cells are usually neglected. Even if fresh cells show an acceptable safety behavior, this can change for aged cells. 7,[11][12][13][14][15] While some authors observed decreases for certain safety properties, 7,11,12 others found improvements. 12,13,15 It is well known that aging mechanisms change the properties of the materials inside Li-ion cells. [16][17][18][19] Operating conditions and materials have a strong influence on these aging mechanisms. 12,[20][21][22][23] We recently reported on a change of the aging mechanism with temperature for cycling aging of commercial 18650-type cells with graphite anodes and NMC/LMO cathodes. 20 This mechanism change is expressed by a V-shape in Arrhenius plots of the capacity fade rates obtained from cycling at different ambient temperatures. 20 This c...

Section: Resultsmentioning

confidence: 99%

Electrochemical, Post-Mortem, and ARC Analysis of Li-Ion Cell Safety in Second-Life Applications

Waldmann

Quinn

Richter

et al. 2017

“…However, since it is known that LCO cathodes can also form a resistive surface film in the presence of VC additive (presumably consisting of poly(VC) 8,9 ), an assignment of the overall cell impedance growth to the individual contributions from anode and cathode requires more advanced techniques, like the symmetric cell approach. 10 Thus, later on, Burns et al 10 investigated the effect of VC (0 -6 wt%) over extended charge/discharge cycling of graphite/NMC 18650 cells on anode and cathode impedance growth via symmetric cell measurements. They showed that indeed the impedance of the negative electrode increases nearly linearly with VC concentration, whereas the impedance of the positive electrode first decreases as the VC concentration is increased to 2 wt% and then only increases gradually at higher VC concentrations.…”

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confidence: 99%

Analysis of Vinylene Carbonate (VC) as Additive in Graphite/LiNi_0.5Mn_1.5O₄Cells

Pritzl

Solchenbach

Wetjen

et al. 2017

124

Vinylene Carbonate (VC) is an effective electrolyte additive to produce a stable solid electrolyte interphase (SEI) on graphite anodes, increasing the capacity retention of lithium-ion cells. However, in combination with LiNi 0.5 Mn 1.5 O 4 (LNMO) cathodes, VC drastically decreases cell performance. In this study we use on-line electrochemical mass spectrometry (OEMS) and electrochemical impedance spectroscopy (EIS) with a micro-reference electrode to understand the oxidative (in-)stability of VC and its effect on the interfacial resistances of both anode and cathode. We herein compare different VC concentrations corresponding to VC to graphite surface area ratios typically used in commercial-scale cells. At low VC concentrations (0.09 wt%, corresponding to 1 wt% in commercial-scale cells), an impedance increase exclusively on the anode and an improved capacity retention is observed, whereas higher VC concentrations (0.17 wt -2 wt%, corresponding to 2 wt -23 wt% in commercial-scale cells) show an increase in both cathode and anode impedance as well as worse cycling performance and overcharge capacity during the first cycle. By considering the onset potentials for VC reduction and oxidation in graphite/LNMO cells, we demonstrate that low amounts of VC can be reduced before VC oxidation occurs, which is sufficient to effectively passivate the graphite anode. During the first charge of a lithium ion battery (LiB), the so called solid electrolyte interphase (SEI) 1 is formed on the surface of the negative electrode. The standard electrolyte for LiBs consists of a mixture of cyclic and linear carbonates, e.g., ethylene carbonate (EC) and ethyl methyl carbonate (EMC), typically with lithium hexafluorophosphate (LiPF 6 ) as salt. Starting from a potential of ∼0.8 V vs. Li/Li + , EC is reduced electrochemically into ethylene gas and lithium ethylene dicarbonate (LEDC), which is a key component of the SEI.2,3 Vinylene carbonate (VC) is one of the most effective additives to modify the SEI on graphite anodes, as it is reduced at potentials more positive than 1.0 V vs. Li/Li + and hence suppresses the reduction of EC. 4,5Aurbach et al. have used VC as electrolyte additive in an EC/DMC (dimethyl carbonate) based electrolyte and that time reported a reduction of the irreversible capacity in the first cycles and an improved cycling stability at elevated temperatures for graphite anodes. The SEI resulting from the reduction of VC consists mainly of poly (vinylene carbonate) (poly(VC)). 4,6Important studies on the impact of different VC concentrations in graphite/NMC pouch cells have been carried out by the Dahn group. For example, Burns et al. 7 investigated the effect of different concentrations of VC (0, 1 and 2 wt%) on cycle life and impedance growth of full-cells with graphite anodes and either LCO (LiCoO 2 ) or NMC (Li(Ni 0.42 Mn 0.42 Co 0.16 )O 2 ) cathodes, employing galvanostatic cycling experiments coupled with high precision coulombic efficiency and electrochemical impedance spectroscopy (EIS) measurements. For cel...

“…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][13][14] Ethylene sulfite (ES) was reported to form a protective SEI film on the negative electrode. [15][16][17] Ternary additives of 2% propene sultone (PES), 1% ethylene sulfate (DTD) and 1% tris(-trimethylsilyl)-phosphite (TTSPi) (called PES211) improved the cycle and calendar life of NMC/graphite Li-ion cells under normal (i.e., 4.2 V) and high voltage (i.e., 4.4 V) operation.…”

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confidence: 99%

“…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.…”

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confidence: 99%

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

Liu

Petibon

et al. 2017

Self Cite

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...