M. Wohlfahrt-Mehrens's research while affiliated with Zentrum für Sonnenenergie und Wasserstoff-Forschung Baden-Württemberg and other places
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In Lithium-ion batteries, the graphite anode is known to undergo a noticeable chromatic change during lithiation and de-lithiation by forming graphite intercalation compounds. Additionally, the graphite anode primarily contributes to the volume change of the battery. Using a novel in-situ optical microscopy setup for imaging cross-sections of Li-ion full cells, both effects can be studied simultaneously during charging and discharging. In this work, we describe feature extraction methods to quantify these effects in the image data (3730 images in total) captured during the lithiation and de-lithiation process. Automated and manual evaluations are compared. The images show graphite anodes and NMC 622 cathodes. For colorfulness, we evaluate different methods based on classical image processing. The metrics calculated with these approaches are compared to the results of ColorNet, which is a trainable colorfulness estimator based on deep convolutional neural networks. We propose a supervised semantic segmentation approach using U-Net for the layer thickness measurement and the anode dilation derived from it.
Water-based processing of positive Li-ion battery electrodes is becoming increasingly important to enable green and sustainable electrode production. Although already widely established for carbon-based anodes, the water-based coating process still poses challenges if applied to cathode materials containing high contents of nickel. Here, positive electrodes using Ni-rich cathode materials with areal capacities of 2.6 mAh/cm ² were prepared either with epoxy, a polyisocyanate-based (ICN) binder, or polyacrylic acid (PAA). All three binders can cross-link with Na-carboxymethyl cellulose used in the formulation. In bi-layer pouch-cells, such cathodes based on epoxy or ICN binders reach an excellent long-term 1 C charge/discharge capacity retention of 85% and 88% after 1000 cycles, whereas electrodes with PAA only reach 65%. Post-mortem analysis of cells after cycling suggests aging of the cathode electrode as the main source of deactivation. Scanning electron microscopy data shows that aqueous processing does not lead to a stronger cracking of the secondary CAM particles and no enhanced dissolution of transition metals was found on the anode side. However, a stronger increase in charge-transfer impedance is observed for the aged water-based cathodes. Thus, the formation of a blocking surface layer appears to be the major reason for performance deterioration with increasing cycle number.
In this work, a method to characterize reliably the porosity and pore size distribution of ultra-thick LiNi0.6Co0.2Mn0.2O2 (NCM622) electrodes with mercury intrusion porosimetry is presented. In terms of sample preparation, a large sample area and low sample mass are essential to prevent inter-sample voids during analysis. The sample mass must be high enough to achieve a low relative error in terms of intrusion volume. Moreover, it was demonstrated that during analysis the protective oxide layer of the aluminum foil appears to be breached due to the high applied pressure and Al⋅Hg alloy is formed. The influence of this effect on the mercury intrusion porosimetry results was analyzed by investigations with blank aluminum foil. Thick electrodes with pore size distributions varied by the overall density, mechanical perforation and preparation with a pore forming agent were analyzed with the developed method and the results obtained for the very different structures are discussed.
The use of concentrated aprotic electrolytes in lithium batteries provides numerous potential applications, including the use of high-voltage cathodes and Li-metal anodes. In this paper, we aim at understanding the effect of salt concentration on the variation of the Li/Li+ Quasi-Reference Electrode (QRE) potential in Tetraglyme (TG)-based electrolytes. Comparing the obtained results to those achieved using Dimethyl sulfoxide DMSO-based electrolytes, we are now able to take a step forward and understand how the effect of solvent coordination and its donor number (DN) is attributed to the Li-QRE potential shift. Using a revised Nernst equation, the alteration of the Li redox potential with salt concentration was determined accurately. It is found that, in TG, the Li-QRE shift follows a different trend than in DMSO owing to the lower DN and expected shorter lifespan of the solvated cation complex.
Gas evolution caused by undesirable side reactions in liquid electrolyte-based Li-ion batteries are one of the essential interest because gas formation might cause internal pressure build-up, cell bulging, de-lamination of the electrode and de-contacting the active materials. There are two sources of gas evolution in LIBs. The first one is due to the reductive decomposition of the electrolyte during the solid electrolyte interphase (SEI) films formation and it depends on the electrolyte and type of the conductive carbon. The second source is the oxidative decomposition at the positive electrode especially when fully charged. Using the high voltage spinel (LiNi 0.5 Mn 1.5 O 4 , LMNS) 1,2 and layered oxide LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC 811) as cathode active materials in half-cell lithium-ion batteries, their effect on the chemical and electrochemical stability of the carbonate electrolyte containing LiPF 6 was studied with an online electrochemical mass spectrometer (OEMS) 3-6. Porous electrodes are manufactured as freestanding films or slurry coated separator. After they are validated in coin cell setup the gas evolution detected in home-built cell setup for OEMS. H 2 , C 2 H 4 , and CO 2 are the main reaction products. The rate of CO 2 formation in the initial three cycles at 0.1C is higher than its formation rate during the soaking of the cell. The reported data shows that the POF 3 gas evolution depends on the electrode potential and on the cell composition. Finally, the mechanism of gases evolution will be suggested. Acknowledgment: The research leading to these results has been performed within the Li-EcoSafe project and received funding from the German Federal Ministry of Education and Research (BMBF) under contract number 03X4636A. The authors would like to thank the Project Management Agency Forschungszentrum Jülich (PTJ). References:
High concentrated liquid aprotic electrolytes have been proposed as a new class of electrolytes and received intense attention for use in Li-ion and Li-metal batteries. We investigate the effect of the Li-salt concentration (up to 3M) in the supporting electrolyte on the redox potential of lithium metal in Dimethyl Sulfoxide (DMSO)-based electrolytes. Measuring the electromotive force (emf) of concentration cells allowed to experimentally assess the change of the Li/Li + potential as function of the salt concentration in the electrolyte. In the low salt concentration region, a linear change of the Li/Li + potential is observed whereas at higher salt concentrations it rises exponentially. Experimental findings are elucidated using two forms of the Nernst equation and remarkable agreement is obtained, thus allowing for theoretical prediction of the change in the alkali metal redox potential. The shift of the Li potential is found to be greatly influenced by the concentration and availability of uncoordinated solvent, which can be estimated from Raman analysis. Finally, the effect of the Li-salt concentration on the diffusion coefficient (D) and heterogeneous rate transfer kinetics (k 0) of Ferrocene (Fc) is investigated by rotating disk electrode (RDE) and cyclic voltammetry in the same electrolytes. Li-ion batteries (LIBs) are undoubtedly the leading technology easing the world energy revolution. Next to LIBs, the so called "beyond" Li-ion technologies (e.g. Li-metal, Li-S and Li-air) have garnered much attention as next generation energy storage devices because of their potential to offer higher energies. 1-3 Most of these battery technologies use liquid electrolytes based on organic aprotic solvents. 4-10 Therefore, contrary to aqueous electrochemistry where the electrode potential is usually measured and reported against the normal hydrogen electrode (NHE) potential, in Li-batteries the electrode potential is usually referred against the Li/Li + couple. To note that the Li electrode is a quasireference electrode (QRE). Its potential is supposedly stable during a series of measurements in a well-defined system. Nonetheless, the actual potential of the QRE should be calibrated against a true reference electrode. Such calibration can be performed in nonaqueous electrolytes against the Fc/Fc + couple, which shows a Nernstian behavior and is suggested by IUPAC as among the best example of solvent independent redox system. In this context, Calvo et al. recently published a perspective paper in which they addressed the issue of having a clear potential scale for the nonaqueous solvents pointing out that the Li/Li + potential strongly depends on the solvent used. 11 For instance, using solvents with different Donor Number (DN) can drastically change the Li +-solvent interaction, which in turn defines the Li/Li + equilibrium potential occurring at the QRE. Indeed, a shift of about 0.5 V was observed when moving from a high DN solvent, such as DMSO (29.8 kcal/mol), to a low DN solvent , such as acetonitrile (14.1 kcal/mol). 12-13 Obviously the reference electrode potential is also affected by the salt concentration in the supporting electrolyte, as predicted by the Nernst equation. To be noted that the Nernst equation applies for low concentrated electrolytes. In highly concentrated electrolytes, not only the salt concentration but also the activity of free solvent plays a key role. Moon et al. 14 found that in highly concentrated glyme-based electrolytes, the shift of the Li potential is greatly influenced by the concentration and availability of uncoordinated solvent. Thus, the Li/Li + equilibrium potential not only depends on the activities of the solvated Li(solvent) n + cation (or Li + activity), but also on that of the free solvent. 15 In highly concentrated electrolytes, activity of the free solvent becomes very low 14-16 thus becoming a dominant factor in defining the electrode potential. High concentrated electrolytes have been proposed as a new class of liquid electrolytes and received intense attention for use in Li-ion and Li-metal batteries. 16-19 The advantages include extended anodic and * Electrochemical Society Member. z E-mail: mario.marinaro@zsw-bw.de cathodic stability, high thermal stability, low volatility etc. 16,17,20-22 These advantages make them as a possible candidate for use in next generation Li-ion and beyond Li-ion batteries. Despite their proven performance in Li-ion batteries, electrolytes based on carbonate solvents are not a suitable choice for post Li-ion technologies, such as Li-air, where the intermediates/products formed during the oxygen reduction/evolution reactions promote fast decomposition of these solvents. 23,24 In this context, the high DN of DMSO, together with its chemical/electrochemical stability, serves advantageous for use in Li-air cells by stabilizing the reaction intermediate. 25-29 In this manuscript, we report a study on the influence of the Li-salt concentration (up to 3M) on the potential of the Li/Li + QRE in DMSO-based electrolytes and, by revising the work of Moon et al., 14 provide an effective and accurate way to theoretically predict such dependence. Moreover, we show that the potential of the Fc/Fc + couple is unaffected by the salt concentration in the supporting electrolyte and that therefore it can well serve as calibrating redox couple. The introduction of Fc and its derivates as redox shuttles/redox mediator (RM) for overcharge protection 30,31 in Li-ion batteries extended its applications much farther than just a calibrating redox couple. By tuning the substituents in the cyclopentadienyl rings, upper potential limits in the range of 3.0-3.5 V vs. Li/Li + can be achieved. 32 Although Fc cannot work as RM for current high voltage cathodes, it can still be used as a model molecule for analyzing the behavior of other RMs 33-37 under varying Li-salt concentration. Moreover, RMs are becoming more and more popular and find applications in next generation elec-trochemical cells (i.e. Li-air). Many studies over last decades have reported on the electrochemical behavior of Fc in nonaqueous elec-trolyte with salt concentration in the range of 0.1-1 M. 38-40 However, reports on more fundamental investigations such as the heterogeneous electron transfer rate and diffusivity properties have been far less common , and that is particularly true when super concentrated electrolytes are considered. In view of the advantages of concentrated solutions, we aim at shedding light on the effect of the Li-salt concentration on diffusion coefficient and heterogeneous rate transfer kinetics of Fc. Materials and Methods Electrolyte preparation.-Bis(trifluoromethylsulfonyl)amine (LiTFSI; Solvionic, 99.9% pure) was dried in a Büchi oven at 140°C for 72h under vacuum. Molecular sieves (4Å) that were used for drying DMSO (Sigma-Aldrich, >99.9% pure) were washed with distilled water, ultra-sonicated to remove dust, and dried in a vacuum oven at 300°C for 48 hours and later transferred to a Büchi oven and) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 193.197.66.72 Downloaded on 2019-05-08 to IP
High concentrated liquid aprotic electrolytes have been proposed as a new class of electrolytes and received intense attention for use in Li-ion and Li-metal batteries. We investigate the effect of the Li-salt concentration (up to 3M) in the supporting electrolyte on the redox potential of lithium metal in Dimethyl Sulfoxide (DMSO)-based electrolytes. Measuring the electromotive force (emf) of concentration cells allowed to experimentally assess the change of the Li/Li⁺ potential as function of the salt concentration in the electrolyte. In the low salt concentration region, a linear change of the Li/Li⁺ potential is observed whereas at higher salt concentrations it rises exponentially. Experimental findings are elucidated using two forms of the Nernst equation and remarkable agreement is obtained, thus allowing for theoretical prediction of the change in the alkali metal redox potential. The shift of the Li potential is found to be greatly influenced by the concentration and availability of uncoordinated solvent, which can be estimated from Raman analysis. Finally, the effect of the Li-salt concentration on the diffusion coefficient (D) and heterogeneous rate transfer kinetics (k⁰) of Ferrocene (Fc) is investigated by rotating disk electrode (RDE) and cyclic voltammetry in the same electrolytes.
Lithium deposition on graphite anodes is an unwanted side reaction in lithium ion batteries, which significantly contributes to accelerated ageing of the cells. Lithium deposition is connected not only to a drastic decrease of lifetime , but also limits fast-charging capability and can cause severe safety issues due to increased exothermic reactions [1]. The reason for lithium deposition are polarization effects, which lead to negative anode potentials vs. Li/Li + , which can be determined by reference electrode measurements [2]. Lithium plating is promoted by charging at high rates, high SOCs and low temperature. Local variations of the anode potential can for example be caused by temperature gradients, differences in current density or by local inhomogeneity in lithium ion cells. These local differences of the anode potential can lead to non uniform lithium deposition. The presentation will summarize results from different types of lithium ion cells, which have been cycled using various operation conditions and have been analysed by post mortem analysis using complimentary analytical methods. The presentation will discuss causes, hints and proofs for lithium deposition, the morphology of lithium deposition/plating, the impact of lithium plating on ageing mechanisms and shapes of capacity fade curves and the influence of lithium plating on safety. Although often discussed, safety issues regarding Li deposition are not only limited to dendrite growth and internal short circuits, but also to exothermic reactions and other properties of Li metal. Furthermore, routes to predict and reduce lithium plating in cells will be discussed including strategies for optimized electrode microstructure, cell design and optimized charging profiles.
The valence energy loss spectrum which is characterized by valence-to-conduction band and plasmon excitations is rarely used in spectroscopy of Li-ion battery materials. One reason being the large number of different excitations observed in this region as well as the difficulty in interpreting their nature and origin. We have determined the nature and origin of spectral features observed in Li-Mn-Ni-O spinel oxides with respect to Mn valency changes during the insertion of lithium ion. The lithiation process is accompanied by a Mn valency change from Mn 4+ in LiNi0.5Mn1.5O4 to Mn 3+ in lithium rich Li2Ni0.5Mn1.5O4. The valence energy loss spectrum of LiNi0.5Mn1.5O4 is characterized by sharp peaks in the 7–10 eV energy loss range whose intensity decrease with lithiation to Li2Ni0.5Mn1.5O4. Using electronic structure calculations and molecular orbital considerations we show that the intense peaks in the valence loss spectra of LiNi0.5Mn1.5O4 have a large contribution from ligand-metal charge transfer transitions. These transitions arise from the mainly O 2p nonbonding t2u and bonding t1u orbitals to the mainly Mn 3d antibonding t2g* and eg* orbitals. We discuss the origins of the observed valence spectra differences between the two phases in relation to peaks shift, variations in occupancy, and variations in covalency as a result of Mn valency changes occurring during lithiation.
... 15 The main issue in the case of high-energy NMC cathode materials is the sensitivity of the active material to water. 72,73 As schematized in Fig. 3, protons may exchange with the lithium ions on the particle surface. As a consequence of this process, known as lithium leaching, lithium hydroxide (LiOH) and lithium carbonate (Li 2 CO 3 ) are released as a sub product. ...
... Consequently, it appears that minimising water content and reducing mixing time contribute to enhanced electrochemical performance. 161,175 Moreover, the formulation employing Na-CMC and SBR as binders in a 1 : 1 ratio, demonstrated successful upscaling from 50 g to 5 kg slurries. This upscaling process facilitated the production of 140 m double-sided cathodes, utilising carbon-coated aluminium as the current collector and phosphoric acid as a pH regulator. ...
... Radloff et al. examined the effects of the sample's geometry and mass on the MIP result. [86] As the calendering process ultimately determines porosity, this measuring system is conventionally applied to the calendered electrode. However, it can also be adopted to characterize uncalendered electrodes to thoroughly investigate the effects of drying conditions or slurry formulation, which have been shown to impact porosity. ...
... Designing new electrolyte compositions could be a valid approach to overcome the aforementioned issues [17] as solvents, salts, and additives have together strong effects on the electrochemical performance of lithium-sulfur cells [18,19]. In this regard, the tuning of the lithium-salt anion donicity could positively affect the Li 2 S passivation film morphology, preventing the insulation of the electrode surface and, thus, enhancing the sulfur utilization within the active material [20][21][22][23]. Additionally, more electro-donating anions have been shown to yield the production of a stable electrolyte interphase layer on both anode and cathode surfaces, which could reduce the shuttle effect phenomenon [24,25]. ...
... For establishing the design strategy, the thermodynamic lithiation/delithiation potential of SiOx, which is dominated by the chemical potential of Li + in the electrolyte, should be highlighted as a critical factor in the reduction stability of the electrolyte. [15][16][17][18] In particular, the reductive decomposition of the electrolyte at the SiOx anode can be significantly suppressed by upshifting its inherent lithiation/delithiation potential close to the potential window (reduction limit) of the electrolyte, thereby disburdening the kinetic support of SEI. However, this simple strategy is yet to be applied for high-voltage batteries because the mechanisms behind the potential shift are still unclear, although it is well known that the redox potential of an electrode depends on the electrolyte. ...
... The rate capability (RC) of the electrodes was evaluated by charging/discharging the cells at different rates (C/10 − 5 C) 5 times followed by 50 cycles at C/2. Finally, given the sensitivity of quasi-reference electrodes to any change of electrolyte and salt concentration, 48 we would like to specify that all potential values in this paper are reported versus the Na/Na + redox potential of the Na-metal quasi-reference electrode which was immersed in the exact same electrolyte solution and cell compartment used to evaluate the electrochemical performance of the WE. ...
... The former is one of the most studied materials and is close to being industrially used as cathode material [5]. LiNi 0.5 Mn 1.5 O 4 is characterized by a theoretical capacity of 147 mAh g −1 due to reversible Ni 2+ /Ni 4+ redox, a two-stage voltage-composition plateau, indicating the occurrence of two two-phase transitions at an average potential of 4.75 V, and high rates of 3D Li + diffusion within the structure [6][7][8][9][10][11]. However, the use of a standard Li-ion electrolyte inevitably leads to the rapid degradation of LiNi 0.5 Mn 1.5 O 4 -based half-and full-cells. ...
... Therefore, lithium can be used either in powder form or as a lithium foil. A commonly used method is the application of so-called stabilized lithium metal powders (SLMPs) in a mixture with anode materials [33][34][35][36][37][38][39][40][41][42][43]. Through the application of pressure and electrolytes, the lithium intercalates into the anode material. ...
... Besides information regarding the accuracy of edge contours, the recovery time, or in general the viscosity values at low shear rates, allows for estimations regarding the dewetting behavior on current collector foil defects. [15,20,91,92] Figure 6 shows the 3ITT for a pilot-scale NMC622 slurry (%65 kg batch size) after a dwell time of 15 h, right before coating. The viscosity recovery in the third interval is in the range of 10 s after applying a sharp shear rate, which indicates an excellent processability For pilot-scale NMC811 slurries (%65 kg batch size), viscosity recovery times of only 5 s were reported. ...
... Taking into account the suppression of degradation that we observed when the lower potential limit was increased from 1.9 to 2.6 V, we can conclude that these structural changes can appear due to the Mn 3+ /Mn 4+ redox associated with significant volume change. Previously, similar behavior was observed for lithium-containing spinels during lithiation of LiNi 0.5 Mn 1.5 O 4 [34][35][36]. ...
