Electrolyte Decomposition Reactions on Tin- and Graphite-Based Anodes are Different

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...

“…7,[17][18][19] The upper panel of Figure 2 shows the current-potential profile of the cyclic voltammetric (CV) formation of SLP30//NMC full-cells with 120 μl LP57 at 25…”

Section: Resultsmentioning

confidence: 99%

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

Metzger

Strehle

Solchenbach

et al. 2016

321

507

“…ethylene carbonate and propylene carbonate [15][16][17][18][19] ) and linear carbonates (e.g. dimethyl-, diethyl-and ethyl-methyl carbonate).…”

Section: -14mentioning

confidence: 99%

Electrochemical Performance and Thermal Stability Studies of Two Lithium Sulfonyl Methide Salts in Lithium-Ion Battery Electrolytes

Murmann

Schmitz

Nowak

et al. 2015

Electrolyte solutions, containing the lithium sulfonyl methide salts lithium-tris(trifluoromethanesulfonyl)methide (LiTFSM) and lithium-[bis(trifluoromethylsulfonyl)-pentafluoroethylsulfonyl]methide (LiPFSM) dissolved in organic carbonate solvents, were electrochemically investigated in Li/graphite, Li/LiNi 1/3 Co 1/3 Mn 1/3 O 2 (NCM) half-cells and compared to the LiPF 6 based electrolyte with regard to their ionic conductivity, electrochemical stability, thermal stability at 60 • C and the anodic dissolution behavior vs. Al. While the investigated salts show almost the same performance in Li/graphite half-cells compared to the LiPF 6 containing electrolyte, the methide salts show very promising results in Li/NCM half-cells, which depict superior capacity retention as well as higher Coulombic efficiencies compared to the LiPF 6 containing electrolyte. Taking the limited anodic stability as well as the occurring anodic dissolution into account, both salts but especially LiTFSM indicate the applicability in lithium ion battery electrolytes for cells with a cutoff potential up to 4. In the last 25 years the demand for lithium-ion batteries has grown tremendously. As a consequence, fundamental research has been carried out regarding the improvement and understanding of technological and safety related aspects. [1][2][3][4][5][6][7][8] Individual applications demand specifically tailored batteries in order to maximize the performance of the device which depicts a great challenge and opportunity for the scientific community to diversify the spectrum of compounds that can be utilized and to adapt the setup in a fast and efficient manner. 9,10 In this respect, the electrolyte, as a multifunctional battery component, can consist of a wide array of compounds such as different solvents, conductive salts and numerous additives. The combination of various electrolyte compounds opens up the possibility to tailor the electrolyte and thus, to a significant extent, the battery cell performance. 11-14The current state-of-the-art electrolyte composition is comprised of the conductive salt lithium hexafluorophosphate (LiPF 6 ) dissolved in a mixture of cyclic (e.g. ethylene carbonate and propylene carbonate [15][16][17][18][19] ) and linear carbonates (e.g. dimethyl-, diethyl-and ethyl-methyl carbonate). 11,[19][20][21][22][23][24][25] However, with increasing demands on the electrolyte regarding thermal and/or electrochemical stability, the limitations of such electrolyte mixtures are unraveled. 11,26,27 Alternative electrolyte compositions that depict certain improvements compared to the state-of-the-art electrolyte are still required to fulfil the following properties: a sufficient ionic conductivity to transport the lithium ions, the ability to form an effective solid electrolyte interphase (SEI) on the graphitic anode 28-37 which enables stable cycling in the low potential range as well as inertness toward aluminum to avoid anodic dissolution of the current collector on the cathode side. 38,39 In addition to performance, more...

“…Solvent decomposition reactions proceed differently on graphite and lithium storage alloys. 15 Electrode materials exhibiting large volume change, i.e., silicon, fail to form a stable SEI. SEI needs to be flexible to accommodate volume changes of the underlying substrate without damage by cracking or rupture.…”

mentioning

confidence: 99%

Revealing SEI Morphology: In-Depth Analysis of a Modeling Approach

Single

Horstmann

Latz

2017

109

134

In this article, we present a novel theory for the long term evolution of the solid electrolyte interphase (SEI) in lithium-ion batteries and propose novel validation measurements. Both SEI thickness and morphology are predicted by our model as we take into account two transport mechanisms, i.e., solvent diffusion in the SEI pores and charge transport in the solid SEI phase. We show that a porous SEI is created due to the interplay of these transport mechanisms. Different dual layer SEIs emerge from different electrolyte decomposition reactions. We reveal the behavior of such dual layer structures and discuss its dependence on system parameters. Model analysis enables us to interpret SEI thickness fluctuations and link them to the rate-limiting transport mechanism. Our results are general and independent of specific modeling choices, e.g., for charge transport and reduction reactions. In the near future, automotive and mobile applications demand power storage with large energy and power density. Currently, lithiumion batteries (LIBs) are the technology of choice for devices with these demands. They operate at high cell potentials and offer high specific capacities while providing long lifetimes. The latter is a consequence of the stable chemistry of modern LIB systems. A significant part of this stability can be attributed to the passivation ability of the solid electrolyte interphase (SEI). This thin layer forms between the negative electrode and the electrolyte. Hence contact between these phases is prevented and the continuous reduction of electrolyte molecules is suppressed. These reduction processes occur because the operating potential of the negative electrode lies well below the stability window of the electrolyte.1 They are suppressed because reduction products quickly form the SEI during the first charge of a pristine electrode. The self passivating ability is one of the most important distinctions between a well and a badly performing lithium-ion battery chemistry. It is of such importance because the reduction reactions consume lithium-ions, directly reducing battery capacity. However, a real SEI is not perfectly passivating and electrolyte reduction is never completely suppressed. Consequently, the lifetime of a battery is directly related to the long-term passivating ability of the SEI.Numerous studies on SEI have been conducted since Peled reported on this correlation in 1979.2 Most of these studies are experimental, investigating cycling stability as well as SEI impedance and composition. Theoretical studies are scarce in comparison, despite established methods such as DFT and DFT/MD derivatives. This can be partially explained with the chemical diversity of SEI, which has been investigated by Aurbach et al. for decades. Results are summarized in Refs. 3, 4 and include the study of SEI formation on graphite electrodes in organic solvent mixtures. The most significant finding of this time is that ethylene carbonate (EC) forms a stable SEI on graphite as opposed to propylene carbonate (PC). Another...