Analysis of Vinylene Carbonate (VC) as Additive in Graphite/LiNi 0.5 Mn 1.5 O 4 Cells

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157

In this study, we will show how the oxygen release depends on the Li 2 MnO 3 content of the material and how it affects the actual voltage fading of the material. Thus, we compared overlithiated NCMs (x Li 2 MnO 3 • (1-x) LiMeO 2 ; Me = Ni, Co, Mn) with x = 0.33, 0.42 and 0.50, focusing on oxygen release and electrochemical performance. We could show that the oxygen release differs vastly for the materials, while voltage fading is similar, which leads to the conclusion that the oxygen release is a chemical material degradation, occurring at the surface, while voltage fading is a bulk issue of these materials. We could prove this hypothesis by HRTEM, showing a surface layer, which is dependent on the amount of oxygen released in the first cycles and leads to an increase of the charge-transfer resistance of these materials. Furthermore, we could quantitatively deconvolute capacity contributions from bulk and surface regions by dQ/dV analysis and correlate them to the oxygen loss. As a last step, we compared the gassing to the base NCM (LiMeO 2 , Me = Ni, Co, Mn), showing that surface degradation follows a similar reaction pathway and can be easily To face future issues, as global warming, air pollution, as well as the consumption of fossil fuels, an alternative is required to cover the future demand of energy and mobility in an environmentally friendly and sustainable way. In this context, lithium-ion batteries are viable options for large scale energy storage and for electric vehicles, as they have been used to power consumer electronics for many years. 1,2Since graphite is an excellent anode material at potentials of ≈0.1 V vs. Li + /Li with a roughly 2-fold higher specific capacity of about 360 mAh/g compared to currently used cathode active materials (CAMs), many efforts have been undertaken to increase the specific capacity and energy density of CAMs. As first practical cathode active material Lithium-Cobalt-Oxide (LCO) was investigated by Goodenough et al. in the 1980s, exhibiting a specific capacity of about 140 mAh/g and having a layered structure composed of lithium and transition metal layers.3 As these layered structures showed good structural stability during lithium extraction and insertion, and therefore good capacity retention, many attempts have been undertaken to further develop alternative layered structures which would offer higher capacity. One promising attempt that led to the currently used Lithium-NickelCobalt-Manganese-Oxides (NCMs) is to change the occupancy of the transition metal layer by not using exclusively cobalt, but also introducing nickel and manganese into the transition metal layer; hereby it was found that nickel shows a high redox activity, while manganese helps to stabilize the structure during lithium extraction.4-6 By using different transition metals and metal compositions, a playground has been created that allows to tune the properties of the material: while initially a Ni:Co:Mn ratio of 1:1:1 was used (also referred to as NCM-111), trends nowadays favor the so-called Ni...

Section: Resultsmentioning

confidence: 97%

Section: Resultsmentioning

confidence: 99%

Oxygen Release and Surface Degradation of Li- and Mn-Rich Layered Oxides in Variation of the Li₂MnO₃Content

Teufl

Strehle

Müller³

et al. 2018

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157

“…This is largely related to the fact that in commercialscale cells, the ratio of active materials to electrolyte solution and void volume is typically ∼10 times higher compared to the lab-scale cells used here. 37,56 Strehle et al 16 recently showed that under these conditions, the majority of CO 2 released from VC reduction would remain dissolved in the electrolyte solution instead of being released into the gas headspace of the cell. This is illustrated by first estimating the amount of dissolved CO 2 by Henry's law 2: where n CO2(el) is the amount of CO 2 dissolved in the liquid electrolyte solution, V el is the volume of the electrolyte solution, c el is the total molar concentration of the electrolyte solution (i.e., solvent and salt) and K H is the Henry constant of CO 2 in the electrolyte solution in units of pressure.…”

Section: Discussionmentioning

confidence: 99%

Lithium Oxalate as Capacity and Cycle-Life Enhancer in LNMO/Graphite and LNMO/SiG Full Cells

Solchenbach

Wetjen

Pritzl

et al. 2018

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In the present study, we explore the use of lithium oxalate as a "sacrificial salt" in combination with lithium nickel manganese spinel (LNMO) cathodes. By online electrochemical mass spectrometry (OEMS), we demonstrate that the oxidation of lithium oxalate to CO 2 (corresponding to 525 mAh/g) occurs quantitatively around 4.7 V vs. Li/Li + . LNMO/graphite cells containing 2.5 or 5 wt% lithium oxalate show an up to ∼11% higher initial discharge capacity and less capacity fade over 300 cycles (12% and 8% vs. 19%) compared to cells without lithium oxalate. In LNMO/SiG full-cells with an FEC-containing electrolyte solution, lithium oxalate leads to a better capacity retention (45% vs 20% after 250 cycles) and a higher coulombic efficiency throughout cycling (∼1%) compared to cells without lithium oxalate. When CO 2 from lithium oxalate oxidation is removed after formation, a similar capacity fading as in LNMO/SiG cells without lithium oxalate is observed. Hence, we attribute the improved cycling performance to the presence of CO 2 in the cells. Further analysis (e.g., FEC consumption by 19 F-NMR) indicate that CO 2 is an effective SEI-forming additive for SiG anodes, and that a combination of FEC and CO 2 has a synergistic effect on the lifetime of full-cells with SiG anodes. Lithium nickel manganese spinel (LiNi 0.5 Mn 1.5 O 4 , LNMO) is a promising cathode material for high energy lithium ion batteries due to its high operating potential around 4.7 V vs. Li/Li + , its high rate capability, structural stability and the absence of cobalt. However, its lower specific capacity (146 mAh/g LNMO ) compared to layered oxide materials (e.g. lithium nickel manganese cobalt oxide (NMC), specific capacity 150-250 mAh/g NMC ) 1 is regarded as a major drawback. In full-cells, the practically achievable capacity of LNMO is even lower, as the formation of the solid-electrolyte interphase (SEI) on the graphite anode consumes active lithium. For many layered oxide cathodes, however, the first cycle irreversible capacity of the cathode is similar to the capacity needed for SEI formation (∼20 mAh/g NMC ), and hence the practical discharge capacity of the cathode and the remaining active lithium are more or less balanced again. 2,3 In contrast, the first cycle irreversible capacity of LNMO (∼6 mAh/g LNMO ) is much lower than the capacity needed for SEI formation. This leads to a mismatch between active lithium and practical cathode capacity, i.e., there is not enough active lithium available to fully discharge the LNMO cathode during subsequent cycling.In cells with silicon-based anodes, active lithium losses on the anode are even higher compared to graphite, as the expansion of the silicon particles during the first lithiation leads to a continuous exposure of fresh, unpassivated silicon surface. 4 On this new surface, electrolyte reduction occurs instantaneously, which reduces the total lithium reservoir in the cell. Therefore, different ideas to increase the amount of active lithium in lithium ion full-cells have been suggested, ...

“…This increase of the LNMO cathode's contact resistance fits very well to the observation that a delamination of the LNMO electrode from the current collector occurs during cycling at elevated temperatures (60 • C), 31 the underlying mechanism of which will be a subject of a future work. 32 Compared to the contact resistance, the pore resistance (R Pore , green symbols) increases from an initial value of ≈3.5 to ≈5 after 85 cycles, while the charge transfer resistance at 4.4 V FC (R CT-non-blocking , purple symbols) starts at ≈0.5 and increases to only ≈0.7 after 85 cycles. Thus, the contact resistance shows the strongest increase over the 85 charge/discharge cycles of +300%, while the pore and charge transfer resistance at 4.4 V only increase by ≈45% and ≈30%, respectively (see Figure 9b).…”

Section: Lnmo Cathode Impedance Evolution In Lnmo/graphite Cells-mentioning

confidence: 99%

An Analysis Protocol for Three-Electrode Li-Ion Battery Impedance Spectra: Part I. Analysis of a High-Voltage Positive Electrode

Landesfeind

Pritzl

Gasteiger

2017

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141

199

A key for the interpretation of porous lithium ion battery electrode impedance spectra is a meaningful and physically motivated equivalent-circuit model. In this work we present a novel approach, utilizing a general transmission line equivalent-circuit model to exemplarily analyze the impedance of a porous high-voltage LiNi 0.5 Mn 1.5 O 4 (LNMO) cathode. It is based on a LNMO/graphite full-cell setup equipped with a gold wire micro-reference electrode (GWRE) to obtain impedance spectra in both, non-blocking conditions at a potential of 4.4 V cell voltage and in blocking configuration achieved at 4.9 V cell voltage. A simultaneous fitting of both spectra enables the deconvolution of physical effects to quantify over the course of 85 cycles at 40 • C: a) the true charge transfer resistance (R CT ), b) the pore resistance (R Pore ), and c) the contact resistance (R Cont. ). We demonstrate that the charge transfer resistance would be overestimated significantly, if the spectra are fitted with a conventionally used simplified R/Q equivalent-circuit compared to our full transmission line analysis. Advanced analysis techniques for lithium ion batteries are a key requirement to deconvolute the complex interplay between the aging mechanisms occurring at the anode and the cathode. In principle, this can be accomplished by electrochemical impedance spectroscopy (EIS), if the individual contributions of anode and cathode to the overall cell impedance can be determined, and if this EIS response can be fitted unambiguously to physically motivated equivalent-circuit models. In general, the measured cell and/or electrode impedances are usually fitted with a serial connection of an ohmic resistor (R), with a parallel circuit of a resistor and a capacitor (C), commonly referred to as R/C element and often also modified to an R/Q element (Q representing a constant-phase element), as well as with a Warburg element (W). [1][2][3][4][5] Recently, more elaborate equivalentcircuits using a transmission line model are getting more and more attention. [6][7][8] In order to independently obtain the impedance of anode and cathode, there are two possible options: i) the assembly of symmetric cells as shown by Chen et al. 9 or Petibon et al., 10 where coin cells out of two anodes (impedance of the negative electrode) or two cathodes (impedance of the positive electrode) are assembled in a glove box or dry-room from two (aged) full-cells at a specified state-of-charge (SOC); ii) the use of three-electrode setups consisting of a working electrode (WE), a counter electrode (CE) and a reference electrode (RE), which allows to individually determine the impedance of the anode and the cathode of a lithium ion battery full-cell. The latter is a more convenient approach, as individual impedance spectra can be recorded continuously during battery cycling, so that anode and cathode impedance can be monitored during cycle-life studies on a fullcell instead of obtaining only one set of anode and cathode impedance spectra after disassembly of a full-cell...