Key factors for the cycling stability of graphite intercalation electrodes for lithium-ion batteries

Strehle

Solchenbach

et al. 2016

321

507

Gassing in lithium-ion batteries (LIBs) is a serious challenge, especially at high voltage and elevated temperature. In this study, we use On-line Electrochemical Mass Spectrometry (OEMS) and a two-compartment cell with a newly developed aluminum edge-seal to elucidate the origin of H 2 evolution in LIBs. We demonstrate that the new sealing is entirely impermeable for gaseous and liquid species, thus allowing us to measure the true H 2 evolution from H 2 O reduction at a graphite electrode, without interference from the lithium counter-electrode. We further report that graphite//NMC full-cells without any diffusion barrier between anode and cathode show enhanced H 2 generation, especially for high charging potentials and at elevated temperature. We propose that the diffusion of protic electrolyte oxidation species (R-H + ) from the cathode to the anode and their subsequent reduction is the origin of enhanced H 2 gassing. To prove this hypothesis, methanesulfonic acid is added to the electrolyte as a chemical source of protons. At the negative graphite electrode, all H + can be quantitatively reduced to H 2 . By the use of the electrolyte additives vinylene carbonate (VC) and lithium bis(oxalato) borate (LiBOB), less H 2 evolution is observed, since the reduction of both H 2 O and R-H + is hindered by a more effective SEI on graphite. Finally, we demonstrate that the Al-sealed diffusion barrier between anode and cathode can stop the diffusion of oxidation products to the anode and therefore essentially eliminates the generation of H 2 caused by high cathode potentials. years after the commercial introduction of lithium-ion batteries (LIBs), a technological turning point is marked by the use of LIBs in battery electric vehicles (BEVs).1 In order to meet the high energy density demands for BEVs, high-voltage cathode materials are required, 2-4 so that it is essential to understand the degradation phenomena at high voltage, e.g., electrolyte oxidation and gas generation. Hydrogen (H 2 ) is one of the main gases found in lithium-ion cells after long term cycling or storage at elevated temperature. 5,6 H 2 gassing in LIBs is generally attributed to residual moisture, coming from improper drying of electrodes, separators and other cell components, as well as H 2 O contamination in the electrolyte. At the negative electrode, H 2 O can be reduced to form hydroxide and H 2 according to H 2 O + e − → OH − + 1 2 H 2 . Bernhard et al. 7 showed recently that an intact solid-electrolyte interphase (SEI) layer derived from vinylene carbonate (VC) in the electrolyte is able to inhibit H 2 O reduction on the graphite surface. In contrast to graphite, lithium titanate (LTO) is generally believed to possess no SEI protection, since its lithiation potential (1.55 V vs. Li/Li + ) is too high to establish an effective SEI by reduction of electrolyte components. Thus, LTO is more prone to gassing from trace water. 8 In fact, pouch bag cells with LTO anodes frequently tend to swell during long term cycling or storage in the charged sta...

Section: Gas Evolution At a Graphite Electrode At 25contrasting

confidence: 79%

Origin of H₂Evolution in LIBs: H₂O Reduction vs. Electrolyte Oxidation

Strehle

Solchenbach

et al. 2016

321

507

“…In Li-ion batteries, the effect of water has mostly been evaluated with regards to battery formation, i.e., with regards to its effect on the solid electrolyte interphase (SEI) formation on graphite anodes. [24][25][26][27] For example, Joho et al performed DEMS measurements in half-cells with different water concentrations, and demonstrated that the evolution of ethylene during the SEI formation is inversely proportional to the water concentration in the electrolyte. 26 Recently, Bernhard et al examined the frequently observed gassing of Li-ion batteries based on lithium titanate Li 4 Ti 5 O 12 (LTO) anodes, showing a direct correlation between the amount of evolved hydrogen during charge/discharge and the water content of the electrolyte.…”

mentioning

confidence: 99%

“…[24][25][26][27] For example, Joho et al performed DEMS measurements in half-cells with different water concentrations, and demonstrated that the evolution of ethylene during the SEI formation is inversely proportional to the water concentration in the electrolyte. 26 Recently, Bernhard et al examined the frequently observed gassing of Li-ion batteries based on lithium titanate Li 4 Ti 5 O 12 (LTO) anodes, showing a direct correlation between the amount of evolved hydrogen during charge/discharge and the water content of the electrolyte. 28 Two recent articles by the group of Jeff Dahn looked also into the effect of intentionally added water to the electrolyte and compared the implications for the performance and cycle-life of LiCoO 2 /Li 4 Ti 5 O 12 (LCO/LTO) cells and LiCoO 2 /graphite cells.…”

mentioning

confidence: 99%

Anodic Oxidation of Conductive Carbon and Ethylene Carbonate in High-Voltage Li-Ion Batteries Quantified by On-Line Electrochemical Mass Spectrometry

Marino

Sicklinger

et al. 2015

175

246

The anodic oxidation stability of battery components like the conductive carbon black (Super C65) and the co-solvent ethylene carbonate (EC) is of great relevance, especially with regards to high-voltage cathode materials. In this study, we use On-line Electrochemical Mass Spectrometry (OEMS) to deconvolute the CO and CO 2 evolution from the anodic oxidation of carbon and electrolyte by using a fully 13 C-isotope labeled electrolyte based on ethylene carbonate with 2 M LiClO 4 . We present a newly developed two-compartment cell, which provides a tight seal between anode and cathode compartment via a solid Li + -ion conducting separator, and which thus allows us to examine the effect of trace amounts of water on the anodic oxidation of carbon ( 12 C) and ethylene carbonate ( 13 C) at high potentials (> 4.5 V) and 10 to 60 • C. Moreover, we report on the temperature dependence of the water-driven hydrolysis of ethylene carbonate accompanied by CO 2 evolution. Finally, by quantifying the evolution rates of 12 CO/ 12 CO 2 and 13 CO/ 13 CO 2 at 5.0 V, we demonstrate that the anodic oxidation of carbon and electrolyte can be substantial, especially at high temperature and in the presence of trace water, posing significant challenges for the implementation of 5 V cathode materials. The requirements placed on Li-ion battery technology have changed from powering small portable electronics to applications demanding high energy and high power density, such as hybrid and plug-in electric vehicles.1,2 As a result, high-voltage cathode materials have been developed that raise the cell voltage from 3.7 V in the case of a traditional LiMO 2 (M = Co, Ni, Mn) cathode to 4.8 V in new cathodes such as the high-voltage spinel LiNi 0.5 Mn 1.5 O 4 (LNMO) or LiCoPO 4 (LCP).3-6 The state-of-the-art electrolyte, a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC), dimethyl carbonate (DMC), and/or ethyl methyl carbonate (EMC) with dissolved LiPF 6 salt, tends to decompose on the surface of the delithiated cathode at potentials higher than 4.5 V vs. Li/Li + , especially at high temperature.7-12 Thus, the practical application of these high-voltage materials remains hindered by several obstacles: [13][14][15][16][17][18] (i) the limited anodic stability of electrolyte solvents and salts as well as of binders, (ii) the loss of active Li + -ions, (iii) the corrosion of the aluminum current collector, and (iv) the instability of conductive carbon additives due to anion intercalation and/or carbon oxidation.While the corrosion of conductive carbons is suggested by the frequently observed increase of the electrical resistivity of long-term cycled high-voltage cathodes, 15 quantitative measurement on the anodic decomposition of carbons have only been made in the context of Li-air battery research, making use of isotopically labeled battery components to decouple electrode and electrolyte related CO 2 evolution. For example, Thotiyl et al. used a 13 C carbon cathode in DMSO and tetraglyme-based electrolyte to study CO 2 evolution from...

“…3,[16][17][18][19][20][21] In contrast to that, pouch cells containing graphite electrodes do not show such strong gassing upon cycling or storage. 13,22 Joho et al 8 performed DEMS measurements on graphite half-cells with different concentrations of water (250 ppm, 1000 ppm and 4000 ppm H 2 O), demonstrating that the amount of C 2 H 4 decreases with increasing H 2 O concentration and that the formation of H 2 increases with the water content.…”

mentioning

confidence: 99%

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

Bernhard

Gasteiger

2015

155

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...