A Novel On-Line Mass Spectrometer Design for the Study of Multiple Charging Cycles of a Li-O 2 Battery

153

179

The gas evolution during the formation of graphite electrodes is quantified by On-line Electrochemical Mass Spectrometry (OEMS) for dry electrolyte (< 20 ppm H 2 O) and 4000 ppm H 2 O containing electrolyte to mimic the effect of trace water during the formation process. While the formation in dry electrolyte mainly shows ethylene (C 2 H 4 ) from the reduction of ethylene carbonate (EC) and small amounts of hydrogen (H 2 ), the formation in water-containing electrolyte yields large amounts of H 2 and considerable amounts of CO 2 in addition to the expected C 2 H 4 evolution. We could show that a protective solid-electrolyte interphase (SEI) layer formed by pre-cycling the graphite electrode in 2% vinylene carbonate (VC) containing electrolyte can reduce the H 2 evolution in watercontaining electrolyte by a factor of 7.5 compared to a pristine graphite electrode. Consequently, the ability of graphite electrodes to form an SEI prevents excessive gassing from trace water, which, e.g., is observed for non-SEI forming lithium titanate ( Today's Li-ion batteries usually employ graphite or lithium titanate Li 4 Ti 5 O 12 (LTO) as anode material. Graphite electrodes are known to have an irreversible capacity loss upon the first charge, i.e., during the first Li + -ion intercalation into the layered graphite structure. 1 This effect is related to the formation of the so called solid-electrolyte interphase (SEI) and depends strongly on the electrolyte composition and the electrode potential.2 The capacity loss is explained by the reduction of electrolyte components on the graphite surface, which irreversibly consumes Li + -ions and forms the protective and passivating SEI layer on the negative electrode. On the one hand, the SEI inhibits further reduction reactions on the anode surface due to its electronically insulating nature, on the other hand it is still permeable for Li + -ions, allowing for Li + intercalation. [3][4][5][6][7][8][9][10][11] Recent work by the group of Brett Lucht showed that lithium ethylene dicarbonate (LEDC) and LiF are the main constituents of the SEI layer on graphite, when ethylene carbonate-based electrolytes are used, independent of the type of lithium salt. 12 The main gaseous components formed during electrolyte reduction on graphite anodes are ethylene (C 2 H 4 ), hydrogen (H 2 ) and other hydrocarbons like propylene (depending on the cyclic carbonate, which is used in the electrolyte mixture).13-15 When employing the common alkyl carbonate electrolytes containing ethylene carbonate (EC), dimethylcarbonate (DMC), diethylcarbonate (DEC) and/or ethyl methyl carbonate (EMC) with LiPF 6 , no carbon dioxide (CO 2 ) formation is observed during the SEI formation. 10In contrast to graphite, LTO is generally believed to possess no SEI protection, since its lithiation potential (1.55 V Li ) is too high to allow reduction of electrolyte components. Thus, LTO is more prone to gassing from trace water and other impurities in the alkyl carbonate electrolyte.16 Gassing studies with LTO as anode material...

Section: Methodsmentioning

confidence: 99%

“…23,29 Figure 1 represents the schematic arrangement of cell components in this setup. For both, dry (< 20 ppm H 2 O) and wet electrolyte (4000 ppm H 2 O), the same volume of 120 μl was pipetted into the cell.…”

Section: On-line Electrochemical Mass Spectrometry (Oems)-mentioning

confidence: 99%

Gas Evolution at Graphite Anodes Depending on Electrolyte Water Content and SEI Quality Studied by On-Line Electrochemical Mass Spectrometry

Bernhard

Metzger

Gasteiger

2015

153

179

“…25 For OEMS measurements a custom-made cell is used; the cell design as well as the OEMS setup were reported previously. 39 OEMS cells were assembled with Li metal counter electrode, two porous polyolefin separators (2500, Celgard, USA), a HE-NCM or NCM working electrode and 120 μl of electrolyte composed of FEC:DEC (2:8 g:g) and 1 M LiPF 6 (BASF SE, Germany). The cells were connected to the mass spectrometer, held for 4 h at OCV (open circuit voltage), and then charged to 4.8 V vs. Li + /Li at a C/10 rate, followed by a 1 h CV step at 4.8 V vs. Li + /Li; the discharge to 2.0 V vs. Li + /Li and the second charge/discharge cycle were conducted at C/5 rate between 4.8 V vs Li + /Li (+1 h CV) and 2.0 V vs Li + /Li (C-rates here are calculated based on a nominal capacity of 250 mAh/g).…”

Section: On-line Electrochemical Mass Spectrometry (Oems)-formentioning

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

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...

“…19 In this study, we use self-standing LFP_ 13 C workingelectrodes for the following reasons: (i) a high compression is needed to optimize the electrochemical performance of LFP, especially at high C-rates, which would destroy the porosity of the afore-mentioned porous electrode supports; and, (ii) high working-electrode loadings are needed in order to obtain sufficient signal intensity from the anodic oxidation of the isotopically labeled carbon coating which accounts for only 1 wt% of the electrode mass.…”

Section: On-line Electrochemical Mass Spectrometry (Oems)mentioning

confidence: 99%

“…19 To avoid signal fluctuations due to minor pressure/temperature changes, all mass signals were normalized to the ion current of the 36 Ar isotope. The cell is first held at OCV for 2 h, followed by a linear sweep voltammetry (LSV) procedure at a scan rate of 0.05 mV/s (Series G300 potentiostat, Gamry, USA) from 3.3 to 5.3 V vs. Li/Li + .…”

Section: On-line Electrochemical Mass Spectrometry (Oems)mentioning

confidence: 99%

Carbon Coating Stability on High-Voltage Cathode Materials in H₂O-Free and H₂O-Containing Electrolyte

Metzger

Sicklinger

Haering

et al. 2015

Carbon coatings on cathode materials with low electrical conductivity like phospho-olivines LiMPO 4 (M = 3d-transition metal) are known to improve their performance in Li-ion batteries. However, at high potentials and in the presence of water, the stability of carbon coatings on high-voltage materials (e.g., LiCoPO 4 ) may be limited due to the anodic oxidation of carbon. In this work, we describe the synthesis of LiFePO 4 (LFP) with an isotopically labeled 13 C carbon coating (characterized by Raman spectroscopy, electrical conductivity, and charge/discharge rate capability tests) as a model compound to study the anodic stability of carbon coated cathode materials in ethylene carbonate-based electrolytes. We characterize the degradation of the 13 C carbon coating by On-line Electrochemical Mass Spectrometry (OEMS) through the 13 CO 2 and 13 CO signals in order to differentiate the anodic oxidation of the coating ( 13 C) from the oxidation of electrolyte, conductive carbon, and binder (all 12 C) in the electrode. The oxidation of the carbon coating takes place at potentials ≥ 4.75 V for electrolyte without H 2 O (< 20 ppm) and ≥ 4.5 V for electrolyte with 4000 ppm H 2 O, and it is strongly enhanced for H 2 O-containing electrolyte. The extent of carbon coating oxidation over 100 h at 4.8 and 5.0 V vs. Li/Li + (25 • C) is projected on the basis of our OEMS data, suggesting that carbon coatings have insufficient stability at such high cathodic potentials. Furthermore, our results prove the in situ formation of H 2 O during the anodic decomposition of ethylene carbonate-containing electrolyte. The H 2 O formation is monitored via the detection of gaseous POF 3 , which is formed from the reaction of Li-ion batteries are extensively investigated as energy storage devices for electric vehicles (EVs) due to their high energy density and reasonable life time.1 In order to make EVs competitive with gasoline or diesel cars, and to eventually reduce CO 2 emissions by the electrification of personal mobility, many fundamental challenges still need to be overcome. In contrast to NMC and other layered oxides, phospho-olivines like LFP and LCP suffer from very poor electrical conductivity which limits their rate capability, i.e., their performance at high charge/discharge rates. 7 In the case of LFP, its poor electrical conductivity can be overcome by using small primary particles (0.1-0.5 μm) in combination with an electrically conductive very thin carbon coating (thickness 1-2 nm) 8 applied to the primary particles using different kinds of precursors.7,9 While uncoated LFP samples either show very poor rate capability or low capacity at rates as low as 0.1 C, 10-12 Lou et al. 13 showed that carbon coated LFP can reach 116 mAh/g LFP at high rates of 10 C (theoretical capacity: 170 mAh/g LFP ).3 Even when the current is increased to 30 C, the discharge capacity can still reach 75 mAh/g LFP , and in the 100 th cycle the capacity loss is only 2.3%. 14 presented a carbon coated LFP which is able to reach discharge capacities of 1...