MobilNachhaltigTC Plattling MoMoZeitschriftenartikel
E. dos Santos Sardinha, Michael Sternad, H. Wilkening, H. Martin, G. Wittstock
Nascent SEI-Surface Films on Single Crystalline Silicon Investigated by Scanning Electrochemical Microscopy
ACS Applied Energy Materials, vol. 2, no. 2, pp. 1388-1392
2019
DOI: 10.1021/acsaem.8b01967
Abstract anzeigen
Silicon is a promising high-capacity host material for negative electrodes in lithium-ion batteries with low potential for the lithiation/delithiation reaction that is outside the stability window of organic carbonate electrolytes. Thus, the use of such electrodes critically depends on the formation of a protective solid electrolyte interphase (SEI) from the decomposition products of electrolyte components. Due to the large volume change upon charging, exposure of the electrode material to the electrolyte must be expected, and facile reformation of SEI is a scope for improving the stabilities of such electrodes. Here, we report the formation of incipient SEI layers on monocrystalline silicon by in situ imaging of their passivating properties using scanning electrochemical microscopy after potentiodynamic charging to different final potentials. The images show a local initiation of the SEI growth at potentials of around 1.0 V vs Li/Li+ in 1 M LiClO4 in propylene carbonate.
MobilNachhaltigTC Plattling MoMoZeitschriftenartikel
M. Uitz, Michael Sternad, S. Breuer, C. Täubert, T. Traußnig, V. Hennige, I. Hanzu, M. Wilkening
Aging of Tesla's 18650 Lithium-Ion Cells: Correlating Solid-Electrolyte-Interphase Evolution with Fading in Capacity and Power
Journal of The Electrochemical Society, vol. 164, no. 14
2017
DOI: 10.1149/2.0171714jes
Abstract anzeigen
The long-term performance of commercial lithium-ion batteries used in today's electric vehicles is of utmost importance for automotive requirements. Here, we use Tesla's 18650 cells manufactured by Panasonic to elucidate the origins of capacity fading and impedance increase during both calendar and cycle aging. Full cell testing is systematically carried out at three different temperatures (25°C, 40°C, 60°C). The cells are galvanostatically cycled at different C-rates (0.33 C – 1 C) and calendar aging is monitored at 4 different state-of-charges (SOC). Operation at high temperatures turns out to have the largest effect on both the capacity and direct current (DC) impedance. As an example, after 500 cycles at 25°C and 40°C capacity fading is approximately 12%, while at 60°C the fading reaches 22%. Our DC impedance measurements reveal the same trend. Post mortem analysis indicate that aging is strongly related to changes of the solid electrolyte interphase (SEI). Hence, the changes in performance are correlated with the change in composition (and thickness) of the SEI formed. In particular, we quantitatively measure the formation of electrically insulating LiF and find a correlation between overall DC impedance of the cells and lithium fluoride of the SEI.
MobilNachhaltigTC Plattling MoMoZeitschriftenartikel
R. Janski, M. Fugger, M. Forster, M. Sorger, A. Dunst, I. Hanzu, Michael Sternad, M. Wilkening
Lithium barrier materials for on-chip Si-based microbatteries
Journal of Materials Science: Materials in Electronics, vol. 28, no. 19, pp. 14605-14614
2017
DOI: 10.1007/s10854-017-7325-4
Abstract anzeigen
The integration of lithium-ion batteries, featuring ultra-high discharge rates, directly into silicon-based semiconductor devices opens unique paths towards the development of new mobile micro-electronics applications. Nevertheless, the small and mobile lithium ions have to be confined within the battery area of the silicon chip, otherwise the nearby fine microelectronics devices will be irreversibly damaged. Hence, a barrier material that blocks Li+ transport from the active components of the battery into the surrounding crystalline Si is needed. Here we evaluated the capability of magnetron sputtered barrier films of nitrides and alloys of refractory metals to prevent lithium ion diffusion and, thus, the formation of Li–Si phases outside the battery area. In order to determine the Li profiles in the barrier layer and in the silicon substrate, time-of-flight secondary ion mass spectroscopy was applied for profiling the first microns. In combination with electrochemical testing it turned out that titanium nitride as well as tantalum nitride barriers are able to significantly block Li ion migration.
MobilNachhaltigTC Plattling MoMoZeitschriftenartikel
Michael Sternad, M. Forster, M. Wilkening
The microstructure matters: breaking down the barriers with single crystalline silicon as negative electrode in Li-ion batteries
Scientific Reports (Nature Publishing Group), vol. 6, no. Article number: 31712 (2016)
2016
DOI: 10.1038/srep31712
Abstract anzeigen
Silicon-based microelectronics forms a major foundation of our modern society. Small lithium-ion batteries act as the key enablers of its success and have revolutionised portable electronics used in our all everyday's life. While large-scale LIBs are expected to help establish electric vehicles, on the other end of device size chip-integrated Si-based μ-batteries may revolutionise microelectronics once more. In general, Si is regarded as one of the white hopes since it offers energy densities being ten times higher than conventional anode materials. The use of monocrystalline, wafer-grade Si, however, requires several hurdles to be overcome since it its volume largely expands during lithiation. Here, we will show how 3D patterned Si wafers, prepared by the sophisticated techniques from semiconductor industry, are to be electrochemically activated to overcome these limitations and to leverage their full potential being reflected in stable charge capacities (>1000 mAhg(-1)) and high Coulomb efficiencies (98.8%).
MobilNachhaltigTC Plattling MoMoZeitschriftenartikel
A. Dunst, Michael Sternad, M. Wilkening
Overall conductivity and NCL-type relaxation behavior in nanocrystalline sodium peroxide Na 2 O 2 —Consequences for Na-oxygen batteries
Materials Science and Engineering: B, vol. 211, no. September, pp. 85-93
2016
DOI: 10.1016/j.mseb.2016.06.002
Abstract anzeigen
Metal air batteries are considered as promising candidates for room-temperature batteries with high-energy densities. On discharge, atmospheric oxygen is reduced at the positive electrode which, in the ideal case, forms the discharge products in a reversible cell reaction. In Na-O2 batteries upon discharge either sodium peroxide (Na2O2) or sodium superoxide (NaO2) is reported to be formed. So far, the charge carrier transport remains relatively unexplored but is expected to crucially determine the efficiency of such energy storage systems. Na2O2 is predicted to be an electrical insulator wherein the transport presumably is determined by very slow hopping processes. Understanding the basic fundamental properties of the overall charge carrier transport, including also nanostructured forms of Na2O2, is key to developing high-energy metal oxygen batteries. The present study answers the question how overall, i.e., total, conductivity changes when going from microcrystalline to nanocrystalline, defect-rich Na2O2. Nanocrystalline Na2O2 was prepared via a top-down approach, viz by high-energy ball milling. Milling does not only shrink the average crystallite diameter but also introduces a large amount of defects which are anticipated to influence total conductivity. It turned out that even after vigorous mechanical treatment the conductivity of the sample is only increased by ca. one order of magnitude. The activation energy remains almost untouched. Thus, the increase seen might be attributed to an enhanced number of charge carriers. Low-temperature data reveals nearly constant loss relaxation behavior which has frequently explained in terms of strictly localized electrical relaxation processes.
MobilNachhaltigTC Plattling MoMoZeitschriftenartikel
H. Bülter, Michael Sternad, Santos Sardinha, Eduardo dos, J. Witt, C. Dosche, M. Wilkening, G. Wittstock
Investigation of the Electron Transfer at Si Electrodes: Impact and Removal of the Native SiO 2 Layer
Journal of The Electrochemical Society, vol. 163, no. 3
2016
DOI: 10.1149/2.0731603jes
Abstract anzeigen
Silicon is considered as one of the promising alternatives to graphite as negative electrode material in lithium-ion batteries. The electron transfer at uncharged microstructured and planar Si was characterized using the feedback mode of scanning electrochemical microscopy (SECM) and 2,5-di-tert-butyl-1,4-dimethoxybenzene as redox mediator. Approach curves and images demonstrate that the electron transfer rate constants at pristine Si are relatively small due to the native SiO2 surface layer. In addition, the electron transfer rate constants show local variations because of the heterogeneous coverage of SiO2. The SiO2 layer is at least partially removed by mechanical contact and abrasion with the microelectrode probe. After SiO2 removal by the microelectrode or by a hydrofluoric acid dip, the electron transfer rate constants increase strongly and remain heterogeneous. Moreover, the surface of the Si electrodes is at least stable over hours after SiO2 removal. The consequences for investigating the formation of the solid electrolyte interphase (SEI) on Si are discussed.
MobilNachhaltigTC Plattling MoMoZeitschriftenartikel
W. Schmidt, P. Bottke, Michael Sternad, P. Gollob, V. Hennige, M. Wilkening
Small Change—Great Effect: Steep Increase of Li Ion Dynamics in Li 4 Ti 5 O 12 at the Early Stages of Chemical Li Insertion
Chemistry of Materials, vol. 27, no. 5, pp. 1740-1750
2015
DOI: 10.1021/cm504564k
Abstract anzeigen
Lithium titanate (LTO) is one of the most promising anode materials for large-scale stationary electrochemical storage of energy produced from renewable sources. Besides many other aspects, such as negligible formation of passivation layers and no volume expansion during lithiation, the success of LTO is mainly based on its ability to easily accommodate and release Li ions in a fully reversible way. This feature is tightly connected with Li self-diffusion. As yet, little information is available about microscopic Li diffusion properties and elementary steps of Li hopping at low intercalation levels, i.e., at values of x being significantly smaller than 1. Here, we used 7Li spin-locking NMR relaxometry to probe absolute hopping rates of LTO (homogeneous) solid solutions in quasi-thermodynamic equilibrium. As a result, the largest increase of Li diffusivity is observed when small amounts of Li are inserted. Strong Coulomb repulsions caused by the simultaneous occupation of neighboring 8a and 16c sites serve as an explanation for the enhanced Li diffusivity found. At even larger values of x, Li mobility slows down but is still much faster than in the host material with x = 0. Our results experimentally corroborate the outcome of recently published calculations on the DFT level focusing on both dynamic and structural aspects. The findings favor the formation of LTO solid solutions upon chemical lithiation; the steep increase in Li diffusivity found might also help with understanding the flat insertion potential observed.
MobilNachhaltigTC Plattling MoMoZeitschriftenartikel
J. Langer, V. Epp, Michael Sternad, M. Wilkening
Diffusion-induced 7Li NMR relaxation of layer-structured tin disulphide — Li diffusion along the buried interfaces in Li0.17SnS2
Solid State Ionics, vol. 276, no. August, pp. 56-61
2015
DOI: 10.1016/j.ssi.2015.03.039
Abstract anzeigen
7Li NMR relaxation has been used to study lithium-ion diffusion in layer-structured SnS2. Keeping the Li intercalation degree in LixSnS2 below x = 0.49, the Li ions preferentially occupy sites in the van der Waals gap between the SnS2 sheets. In contrast to conventional NMR spin-lattice relaxation (SLR) rate measurements in the laboratory frame of reference, which are sensitive to rather fast Li exchange processes, with the help of spin-locking SLR NMR slower Li motions were extracted from characteristic diffusion-induced rate peaks. The latter contain information on both Li+ activation energies Ea and Li ion jump rates τ− 1 characterizing the elementary steps of Li+ hopping. Our results point to two different diffusion processes (Ea(I) = 0.38 eV; Ea(II) = 0.28 eV), a slower and a faster one, observable directly after chemical Li insertion. Interestingly, the diffusion behaviour irreversibly changes when the sample has been exposed to temperatures as high as 573 K. Diffusion-induced NMR rates and corresponding line shapes are discussed with respect to an inhomogenous distribution of Li ions in SnS2, which seems to be present directly after Li intercalation.
MobilNachhaltigTC Plattling MoMoZeitschriftenartikel
A. Dunst, Michael Sternad, V. Epp, M. Wilkening
Fast Li+ Self-Diffusion in Amorphous Li–Si Electrochemically Prepared from Semiconductor Grade, Monocrystalline Silicon: Insights from Spin-Locking Nuclear Magnetic Relaxometry
The Journal of Physical Chemistry C, vol. 119, no. 22, pp. 12183-12192
2015
DOI: 10.1021/acs.jpcc.5b02490
Abstract anzeigen
Silicon is one of the most promising anode materials for lithium-based rechargeable batteries. Provided the volume changes during Li uptake can be brought under control, Li ion diffusivity is expected to crucially determine the performance of such next-generation energy storage systems. Therefore, studying diffusion properties in e.g. amorphous Li–Si underpins applied research that is being directed toward the development of powerful storage devices. So far, only little information is available on Li+ self-diffusion in amorphous Si. Here, we used 7Li NMR spectroscopy to precisely quantify microscopic activation energies and Li jump rates in amorphous Li–Si which is primarily formed if monocrystalline Si is lithiated electrochemically. Surprisingly, our results reveal relatively fast Li ion diffusivity with low activation energies for localized Li+ motions being in agreement with results from theory. The average activation energy for long-range ion transport is as high as ca. 0.65 eV; jump rates turn out to be in the order of 2.5 × 105 s–1 at 246 K. Our results point to complex dynamics that is most likely governed by nonexponential motional correlation functions originating from a distribution of activation energies. The data obtained might help optimizing Li-based silicon batteries whose performance critically depend on fast Li ion transport.
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