Publications by authors named "Tobias Placke"

20 Publications

  • Page 1 of 1

Opportunities and Limitations of Ionic Liquid- and Organic Carbonate Solvent-Based Electrolytes for Mg-Ion-Based Dual-Ion Batteries.

ChemSusChem 2021 Aug 2. Epub 2021 Aug 2.

MEET Battery Research Center, Institute of Physical Chemistry, University of Münster, Corrensstraße 46, 48149, Münster, Germany.

Dual-ion batteries (DIBs) offer a great alternative to state-of-the-art lithium-ion batteries, based on their high promises due to the absence of transition metals and the use of low-cost materials, which could make them economically favorable targeting stationary energy storage applications. In addition, they are not limited by certain metal cations, and DIBs with a broad variety of utilized ions could be demonstrated over the last years. Herein, a systematic study of different electrolyte approaches for Mg-ion-based DIBs was conducted. A side-by-side comparison of Li- and Mg-ion-based electrolytes using activated carbon as negative electrode revealed the opportunities but also limitations of Mg-ion-based DIBs. Ethylene sulfite was successfully introduced as electrolyte additive and increased the specific discharge capacity significantly up to 93±2 mAh g with coulombic efficiencies over 99 % and an excellent capacity retention of 88 % after 400 cycles. In addition, and for the first time, highly concentrated carbonate-based electrolytes were employed for Mg-ion-based DIBs, showing adequate discharge capacities and high coulombic efficiencies.
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http://dx.doi.org/10.1002/cssc.202101227DOI Listing
August 2021

Scalable Synthesis of MAX Phase Precursors toward Titanium-Based MXenes for Lithium-Ion Batteries.

ACS Appl Mater Interfaces 2021 Jun 1;13(22):26074-26083. Epub 2021 Jun 1.

Institute of Energy and Climate Research: Materials Synthesis and Processing (IEK-1), Forschungszentrum Jülich GmbH, Wilhelm-Johnen-Straße, 52428 Jülich, Germany.

MXenes have emerged as one of the most interesting material classes, owing to their outstanding physical and chemical properties enabling the application in vastly different fields such as electrochemical energy storage (EES). MXenes are commonly synthesized by the use of their parent phase, , MAX phases, where "M" corresponds to a transition metal, "A" to a group IV element, and "X" to carbon and/or nitrogen. As MXenes display characteristic pseudocapacitive behaviors in EES technologies, their use as a high-power material can be useful for many battery-like applications. Here, a comprehensive study on the synthesis and characterization of morphologically different titanium-based MXenes, , TiC and TiC, and their use for lithium-ion batteries is presented. First, the successful synthesis of large batches (≈1 kg) of the MAX phases TiAlC and TiAlC is shown, and the underlying materials are characterized mainly by focusing on their structural properties and phase purity. Second, multi- and few-layered MXenes are successfully synthesized and characterized, especially toward their ever-present surface groups, influencing the electrochemical behavior to a large extent. Especially multi- and few-layered TiC are achieved, exhibiting almost no oxidation and similar content of surface groups. These attributes enable the precise comparison of the electrochemical behavior between morphologically different MXenes. Since the preparation method for few-layered MXenes is adapted to process both active materials in a "classical" electrode paste processing method, a better comparison between both materials is possible by avoiding macroscopic differences. Therefore, in a final step, the aforementioned electrochemical performance is evaluated to decipher the impact of the morphology difference of the titanium-based MXenes. Most importantly, the delamination leads to an increased non-diffusion-limited contribution to the overall pseudocapacity by enhancing the electrolyte access to the redox-active sites.
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http://dx.doi.org/10.1021/acsami.1c05889DOI Listing
June 2021

Solvent Co-intercalation into Few-layered TiCT MXenes in Lithium Ion Batteries Induced by Acidic or Basic Post-treatment.

ACS Nano 2021 Feb 1;15(2):3295-3308. Epub 2021 Feb 1.

MEET Battery Research Center, Institute of Physical Chemistry, University of Münster, Corrensstraße 46, 48149 Münster, Germany.

MXenes, as an emerging class of 2D materials, display distinctive physical and chemical properties, which are highly suitable for high-power battery applications, such as lithium ion batteries (LIBs). TiCT (T = O, OH, F, Cl) is one of the most investigated MXenes to this day; however, most scientific research studies only focus on the design of multilayered or monolayer MXenes. Here, we present a comprehensive study on the synthesis of few-layered TiCT materials and their use in LIB cells, in particular for high-rate applications. The synthesized TiCT MXenes are characterized complementary XRD, Raman spectroscopy, XPS, EDX, SEM, TGA, and nitrogen adsorption techniques to clarify the structural and chemical changes, especially regarding the surface groups and intercalated cations/water molecules. The structural changes are correlated with respect to the acidic and basic post-treatment of TiCT. Furthermore, the detected alterations are put into an electrochemical perspective galvanostatic and potentiostatic investigations to study the pseudocapacitive behavior of few-layered TiCT, exhibiting a stable capacity of 155 mAh g for 1000 cycles at 5 A g. The acidic treatment of TiCT synthesized the formation of HF through LiF/HCl is able to increase the initial capacity in comparison to the pristine or basic treatment. To gain further insights into the structural changes occurring during (de)lithiation, XRD is applied for LIB cells in a voltage range from 0.01 to 3 V to give fundamental mechanistic insights into the structural changes occurring during the first cycles. Thereby, the increased initial capacity observed for acidic-treated MXenes can be explained by the reduced co-intercalation of solvent molecules.
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http://dx.doi.org/10.1021/acsnano.0c10153DOI Listing
February 2021

Exploiting the Degradation Mechanism of NCM523 || Graphite Lithium-Ion Full Cells Operated at High Voltage.

ChemSusChem 2021 Jan 29;14(2):491. Epub 2020 Dec 29.

University of Münster, MEET Battery Research Center, Institute of Physical Chemistry, Corrensstr. 46, 48149, Münster, Germany.

Invited for this month's cover is the group of Tobias Placke and Martin Winter at the MEET Battery Research Center (University of Münster). The image shows the failure mechanism of high-voltage operated NCM523 || graphite lithium-ion cells, that is, the dissolution of transition metals (Mn, Co, Ni) from the NCM523 cathode and subsequent deposition at the graphite anode, resulting in formation of Li metal dendrites. The Full Paper itself is available at 10.1002/cssc.202002113.
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http://dx.doi.org/10.1002/cssc.202002870DOI Listing
January 2021

Impact of the Crystalline LiSi Phase on the Self-Discharge Mechanism of Silicon Negative Electrodes in Organic Electrolytes.

ACS Appl Mater Interfaces 2020 Dec 1;12(50):55903-55912. Epub 2020 Dec 1.

School of Mathematics and Sciences, Chemistry Department, Carl von Ossietzky University of Oldenburg, D-26111 Oldenburg, Germany.

Because of their high specific capacity and rather low operating potential, silicon-based negative electrode materials for lithium-ion batteries have been the subject of extensive research over the past 2 decades. Although the understanding of the (de)lithiation behavior of silicon has significantly increased, several major challenges have not been solved yet, hindering its broad commercial application. One major issue is the low initial Coulombic efficiency and the ever-present self-discharge of silicon electrodes. Self-discharge itself affects the long-term stability of electrochemical storage systems and, additionally, must be taken into consideration for inevitable prelithiation approaches. The impact of the crystalline LiSi phase is of great interest as the phase transformation between crystalline () and amorphous () phases not only increases the specific surface area but also causes huge polarization. Moreover, there is the possibility for electrochemical over-lithiation toward the LiSi phase because of the electron-deficient LiSi phase, which can be highly reactive toward the electrolyte. This poses the question about the impact of the -LiSi phase on the self-discharge behavior in comparison to its amorphous counterpart. Here, silicon thin films used as model electrodes are lithiated to cut-off potentials of 10 mV and 50 mV Li|Li ( and ) in order to systematically investigate their self-discharge mechanism open-circuit potential () measurements and to visualize the solid electrolyte interphase (SEI) growth by means of scanning electrochemical microscopy. We show that the -LiSi phase is formed for the electrode, while it is not found for the electrode. In turn, the electrode displays an almost linear self-discharge behavior, whereas the electrode reaches a plateau at 380 mV Li|Li, which is due to the phase transition from -LiSi to the -LiSi phase. At this plateau potential, the phase transformation at the Si|electrolyte interface results in an electronically more insulating and more uniform SEI ( electrode), while the electrode displays a less uniform SEI layer. In summary, the self-discharge mechanism of silicon electrodes and, hence, the irreversible decomposition of the electrolyte and the corresponding SEI formation process heavily depend on the structural nature of the underlying lithium-silicon phase.
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http://dx.doi.org/10.1021/acsami.0c16742DOI Listing
December 2020

Exploiting the Degradation Mechanism of NCM523 Graphite Lithium-Ion Full Cells Operated at High Voltage.

ChemSusChem 2021 Jan 10;14(2):595-613. Epub 2020 Nov 10.

University of Münster, MEET Battery Research Center, Institute of Physical Chemistry, Corrensstr. 46, 48149, Münster, Germany.

Layered oxides, particularly including Li[Ni Co Mn ]O (NCMxyz) materials, such as NCM523, are the most promising cathode materials for high-energy lithium-ion batteries (LIBs). One major strategy to increase the energy density of LIBs is to expand the cell voltage (>4.3 V). However, high-voltage NCM graphite full cells typically suffer from drastic capacity fading, often referred to as "rollover" failure. In this study, the underlying degradation mechanisms responsible for failure of NCM523 graphite full cells operated at 4.5 V are unraveled by a comprehensive study including the variation of different electrode and cell parameters. It is found that the "rollover" failure after around 50 cycles can be attributed to severe solid electrolyte interphase growth, owing to formation of thick deposits at the graphite anode surface through deposition of transition metals migrating from the cathode to the anode. These deposits induce the formation of Li metal dendrites, which, in the worst cases, result in a "rollover" failure owing to the generation of (micro-) short circuits. Finally, approaches to overcome this dramatic failure mechanism are presented, for example, by use of single-crystal NCM523 materials, showing no "rollover" failure even after 200 cycles. The suppression of cross-talk phenomena in high-voltage LIB cells is of utmost importance for achieving high cycling stability.
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http://dx.doi.org/10.1002/cssc.202002113DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7894331PMC
January 2021

Porous Graphene-like Carbon from Fast Catalytic Decomposition of Biomass for Energy Storage Applications.

ACS Omega 2019 Dec 5;4(25):21446-21458. Epub 2019 Dec 5.

Dpto. Física de la Materia Condensada, Universidad de Sevilla, Avda. Reina Mercedes SN, 41012 Sevilla, Spain.

A novel carbon material made of porous graphene-like nanosheets was synthesized from biomass resources by a simple catalytic graphitization process using nickel as a catalyst for applications in electrodes for energy storage devices. A recycled fiberboard precursor was impregnated with saturated nickel nitrate followed by high-temperature pyrolysis. The highly exothermic combustion of in situ formed nitrocellulose produces the expansion of the cellulose fibers and the reorganization of the carbon structure into a three-dimensional (3D) porous assembly of thin carbon nanosheets. After acid washing, nickel particles are fully removed, leaving nanosized holes in the wrinkled graphene-like sheets. These nanoholes confer the resulting carbon material with ≈75% capacitance retention, when applied as a supercapacitor electrode in aqueous media at a specific current of 100 A·g compared to the capacitance reached at 20 mA·g, and ≈35% capacity retention, when applied as a negative electrode for lithium-ion battery cells at a specific current of 3720 mA·g compared to the specific capacity at 37.2 mA·g. These findings suggest a novel way for synthesizing 3D nanocarbon networks from a cellulosic precursor requiring low temperatures and being amenable to large-scale production while using a sustainable starting precursor such as recycled fiberwood.
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http://dx.doi.org/10.1021/acsomega.9b03142DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6921631PMC
December 2019

Surface Modification of Ni-Rich LiNiCoMnO Cathode Material by Tungsten Oxide Coating for Improved Electrochemical Performance in Lithium-Ion Batteries.

ACS Appl Mater Interfaces 2019 May 13;11(20):18404-18414. Epub 2019 May 13.

MEET Battery Research Center, Institute of Physical Chemistry , University of Münster , Corrensstr. 46 , 48149 Münster , Germany.

Ni-rich NCM-based positive electrode materials exhibit appealing properties in terms of high energy density and low cost. However, these materials suffer from different degradation effects, especially at their particle surface. Therefore, in this work, tungsten oxide is evaluated as a protective inorganic coating layer on LiNiCoMnO (NCM-811) positive electrode materials for lithium-ion battery (LIB) cells and investigated regarding rate capability and cycling stability under different operation conditions. Using electrochemical impedance spectroscopy, the interfacial resistance of uncoated and coated NCM-811 electrodes is explored to study the impact of the coating on lithium-ion diffusion. All electrochemical investigations are carried out in LIB full cells with graphite as a negative electrode to ensure better comparability with commercial cells. The coated electrodes show an excellent capacity retention for the long-term charge/discharge cycling of NCM-811-based LIB full cells, i.e., 80% state-of-health after more than 800 cycles. Furthermore, the positive influence of the tungsten oxide coating on the thermal and structural stability is demonstrated using postmortem analysis of aged electrodes. Compared to the uncoated electrodes, the surface-modified electrodes show less degradation effects, such as particle cracking on the electrode surface and improvement of the thermal stability of NCM-811 in the presence of electrolyte.
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http://dx.doi.org/10.1021/acsami.9b02889DOI Listing
May 2019

Hydrothermal-derived carbon as a stabilizing matrix for improved cycling performance of silicon-based anodes for lithium-ion full cells.

Beilstein J Nanotechnol 2018 5;9:2381-2395. Epub 2018 Sep 5.

University of Münster, MEET Battery Research Center, Institute of Physical Chemistry, Corrensstraße 46, 48149 Münster, Germany.

In this work, silicon/carbon composites are synthesized by forming an amorphous carbon matrix around silicon nanoparticles (Si-NPs) in a hydrothermal process. The intention of this material design is to combine the beneficial properties of carbon and Si, i.e., an improved specific/volumetric capacity and capacity retention compared to the single materials when applied as a negative electrode in lithium-ion batteries (LIBs). This work focuses on the influence of the Si content (up to 20 wt %) on the electrochemical performance, on the morphology and structure of the composite materials, as well as the resilience of the hydrothermal carbon against the volumetric changes of Si, in order to examine the opportunities and limitations of the applied matrix approach. Compared to a physical mixture of Si-NPs and the pure carbon matrix, the synthesized composites show a strong improvement in long-term cycling performance (capacity retention after 103 cycles: ≈55% (20 wt % Si composite) and ≈75% (10 wt % Si composite)), indicating that a homogeneous embedding of Si into the amorphous carbon matrix has a highly beneficial effect. The most promising Si/C composite is also studied in a LIB full cell vs a NMC-111 cathode; such a configuration is very seldom reported in the literature. More specifically, the influence of electrochemical prelithiation on the cycling performance in this full cell set-up is studied and compared to non-prelithiated full cells. While prelithiation is able to remarkably enhance the initial capacity of the full cell by ≈18 mAh g, this effect diminishes with continued cycling and only a slightly enhanced capacity of ≈5 mAh g is maintained after 150 cycles.
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http://dx.doi.org/10.3762/bjnano.9.223DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6142743PMC
September 2018

Pentafluorophenyl Isocyanate as an Effective Electrolyte Additive for Improved Performance of Silicon-Based Lithium-Ion Full Cells.

ACS Appl Mater Interfaces 2018 Aug 8;10(33):28187-28198. Epub 2018 Aug 8.

MEET Battery Research Center, Institute of Physical Chemistry , University of Münster , Corrensstraße 46 , 48149 Münster , Germany.

Due to its high specific and volumetric capacity and relatively low operation potential, silicon (Si) has attracted much attention to be utilized as a high-capacity anode material for lithium-ion batteries (LIBs) with increased energy density. However, the application of Si within commercial LIBs is still hindered by its poor cycling stability related to the huge volume changes of Si upon lithiation/delithiation, followed by continuous electrolyte decomposition and active lithium loss at the anode side. In this work, we present the application of pentafluorophenyl isocyanate (PFPI) as an effective electrolyte additive for lithium-ion full cells, containing a pure, magnetron-sputtered Si anode and a LiNiMnCoO (NMC-111) cathode. The performance of the Si/NMC-111 full cells is significantly improved in terms of capacity retention and Coulombic efficiency by the addition of 2 wt % PFPI to the baseline electrolyte and is compared to the well-known additives vinylene carbonate and fluoroethylene carbonate. Furthermore, it is revealed that the additive is able to reduce the active lithium losses by forming an effective solid-electrolyte interphase (SEI) on the Si anode. X-ray photoelectron spectroscopy investigations unveil that PFPI is a main part of the SEI layer, leading to less active lithium immobilized within the interphase. Overall, our results pave the path for a broad range of different isocyanate compounds, which have not been studied for Si-based anodes in lithium-ion full cells so far. These compounds can be easily adjusted by modifying the chemical structure and/or functional groups incorporated within the molecule, to specifically tailor the SEI layer for Si-based anodes in LIBs.
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http://dx.doi.org/10.1021/acsami.8b07683DOI Listing
August 2018

Iron-Catalyzed Graphitic Carbon Materials from Biomass Resources as Anodes for Lithium-Ion Batteries.

ChemSusChem 2018 Aug 12;11(16):2776-2787. Epub 2018 Jul 12.

Dpto. Física de la Materia Condensada and, Instituto de Ciencia de Materiales de Sevilla, Universidad de Sevilla-CSIC, Avda. Reina Mercedes SN, 41012, Seville, Spain.

Graphitized carbon materials from biomass resources were successfully synthesized with an iron catalyst, and their electrochemical performance as anode materials for lithium-ion batteries (LIBs) was investigated. Peak pyrolysis temperatures between 850 and 2000 °C were covered to study the effect of crystallinity and microstructural parameters on the anodic behavior, with a focus on the first-cycle Coulombic efficiency, reversible specific capacity, and rate performance. In terms of capacity, results at the highest temperatures are comparable to those of commercially used synthetic graphite derived from a petroleum coke precursor at higher temperatures, and up to twice as much as that of uncatalyzed biomass-derived carbons. The opportunity to graphitize low-cost biomass resources at moderate temperatures through this one-step environmentally friendly process, and the positive effects on the specific capacity, make it interesting to develop more sustainable graphite-based anodes for LIBs.
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http://dx.doi.org/10.1002/cssc.201800831DOI Listing
August 2018

Running out of lithium? A route to differentiate between capacity losses and active lithium losses in lithium-ion batteries.

Phys Chem Chem Phys 2017 Oct;19(38):25905-25918

University of Münster, MEET Battery Research Center, Institute of Physical Chemistry, Corrensstraße 46, 48149 Münster, Germany.

Active lithium loss (ALL) resulting in a capacity loss (Q), which is caused by lithium consuming parasitic reactions like SEI formation, is a major reason for capacity fading and, thus, for a reduction of the usable energy density of lithium-ion batteries (LIBs). Q is often equated with the accumulated irreversible capacity (Q). However, Q is also influenced by non-lithium consuming parasitic reactions, which do not reduce the active lithium content of the cell, but induce a parasitic current. In this work, a novel approach is proposed in order to differentiate between Q and Q. The determination of Q is based on the remaining active lithium content of a given cell, which can be determined by de-lithiation of the cathode with the help of the reference electrode of a three-electrode set-up. Lithium non-consuming parasitic reactions, which do not influence the active lithium content have no influence on this determination. In order to evaluate this novel approach, three different anode materials (graphite, carbon spheres and a silicon/graphite composite) were investigated. It is shown that during the first charge/discharge cycles Q is described moderately well by Q. However, the difference between Q and Q rises with increasing cycle number. With this approach, a differentiation between "simple" irreversible capacities and truly detrimental "active Li losses" is possible and, thus, Coulombic efficiency can be directly related to the remaining useable cell capacity for the first time. Overall, the exact determination of the remaining active lithium content of the cell is of great importance, because it allows a statement on whether the reduction in lithium content is crucial for capacity fading or whether the fading is related to other degradation mechanisms such as material or electrode failure.
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http://dx.doi.org/10.1039/c7cp05405jDOI Listing
October 2017

A Step toward High-Energy Silicon-Based Thin Film Lithium Ion Batteries.

ACS Nano 2017 05 1;11(5):4731-4744. Epub 2017 May 1.

Institute of Physical Chemistry, MEET Battery Research Center, University of Münster , Corrensstr. 46, 48149 Münster, Germany.

The next generation of lithium ion batteries (LIBs) with increased energy density for large-scale applications, such as electric mobility, and also for small electronic devices, such as microbatteries and on-chip batteries, requires advanced electrode active materials with enhanced specific and volumetric capacities. In this regard, silicon as anode material has attracted much attention due to its high specific capacity. However, the enormous volume changes during lithiation/delithiation are still a main obstacle avoiding the broad commercial use of Si-based electrodes. In this work, Si-based thin film electrodes, prepared by magnetron sputtering, are studied. Herein, we present a sophisticated surface design and electrode structure modification by amorphous carbon layers to increase the mechanical integrity and, thus, the electrochemical performance. Therefore, the influence of amorphous C thin film layers, either deposited on top (C/Si) or incorporated between the amorphous Si thin film layers (Si/C/Si), was characterized according to their physical and electrochemical properties. The thin film electrodes were thoroughly studied by means of electrochemical impedance spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. We can show that the silicon thin film electrodes with an amorphous C layer showed a remarkably improved electrochemical performance in terms of capacity retention and Coulombic efficiency. The C layer is able to mitigate the mechanical stress during lithiation of the Si thin film by buffering the volume changes and to reduce the loss of active lithium during solid electrolyte interphase formation and cycling.
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http://dx.doi.org/10.1021/acsnano.7b00922DOI Listing
May 2017

Anodic Behavior of the Aluminum Current Collector in Imide-Based Electrolytes: Influence of Solvent, Operating Temperature, and Native Oxide-Layer Thickness.

ChemSusChem 2017 02 27;10(4):804-814. Epub 2017 Jan 27.

University of Münster, MEET Battery Research Center, Institute of Physical Chemistry, Corrensstraße 46, 48149, Münster, Germany.

The inability of imide salts to form a sufficiently effective passivation layer on aluminum current collectors is one of the main obstacles that limit their broad application in electrochemical energy-storage systems. However, under certain circumstances, the use of electrolytes with imide electrolyte salts in combination with the aluminum current collector is possible. In this contribution, the stability of the aluminum current collector in electrolytes containing either lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) or lithium fluorosulfonyl-(trifluoromethanesulfonyl) imide (LiFTFSI) as conductive salt was investigated by electrochemical techniques, that is, cyclic voltammetry (CV) and chronocoulometry (CC) in either room-temperature ionic liquids or in ethyl methyl sulfone. In particular, the influence of the solvent, operating temperature, and thickness of the native oxide layer of aluminum on the pit formation at the aluminum current collector surface was studied by means of scanning electron microscopy. In general, a more pronounced aluminum dissolution and pit formation was found at elevated temperatures as well as in solvents with a high dielectric constant. An enhanced thickness of the native aluminum oxide layer increases the oxidative stability versus dissolution. Furthermore, we found a different reaction rate depending on dwell time at the upper cut-off potential for aluminum dissolution in TFSI- and FTFSI-based electrolytes during the CC measurements; the use of LiFTFSI facilitated the dissolution of aluminum compared to LiTFSI. Overall, the mechanism of anodic aluminum dissolution is based on: i) the attack of the Al O surface by acidic species and ii) the dissolution of bare aluminum into the electrolyte, which, in turn, is influenced by the electrolyte's dielectric constant.
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http://dx.doi.org/10.1002/cssc.201601636DOI Listing
February 2017

Suppression of Aluminum Current Collector Dissolution by Protective Ceramic Coatings for Better High-Voltage Battery Performance.

Chemphyschem 2017 Jan 29;18(1):156-163. Epub 2016 Nov 29.

University of Münster, MEET Battery Research Center, Institute of Physical Chemistry, Corrensstrasse 46, 48149, Münster, Germany.

Batteries based on cathode materials that operate at high cathode potentials, such as LiNi Mn O (LNMO), in lithium-ion batteries or graphitic carbons in dual-ion batteries suffer from anodic dissolution of the aluminum (Al) current collector in organic solvent-based electrolytes based on imide salts, such as lithium bis(trifluoromethanesulfonyl) imide (LiTFSI). In this work, we developed a protective surface modification for the Al current collector by applying ceramic coatings of chromium nitride (Cr N) and studied the anodic Al dissolution behavior. By magnetron sputter deposition, two different coating types, which differ in their composition according to the CrN and Cr N phases, were prepared and characterized by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and their electronic conductivity. Furthermore, the anodic dissolution behavior was studied by cyclic voltammetry and chronocoulometry measurements in two different electrolyte mixtures, that is, LiTFSI in ethyl methyl sulfone and LiTFSI in ethylene carbonate/dimethyl carbonate 1:1 (by weight). These measurements showed a remarkably reduced current density or cumulative charge during the charge process, indicating an improved anodic stability of the protected Al current collector. The coating surfaces after electrochemical treatment were characterized by means of SEM and XPS, and the presence or lack of pit formation, as well as electrolyte degradation products could be well correlated to the electrochemical results.
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http://dx.doi.org/10.1002/cphc.201601095DOI Listing
January 2017

Nanostructured ZnFe2O4 as Anode Material for Lithium-Ion Batteries: Ionic Liquid-Assisted Synthesis and Performance Evaluation with Special Emphasis on Comparative Metal Dissolution.

Acta Chim Slov 2016 ;63(3):470-83

In this work, a ZnFe2O4 anode material was successfully synthesized by a novel ionic liquid-assisted synthesis method followed by a carbon coating procedure. The as-prepared ZnFe2O4 particles demonstrate a relatively homogeneous particle size distribution with particle diameters ranging from 40 to 80 nm. This material, which is well known to offer an interesting combination of an alloying and conversion mechanism, is capable of accommodating nine equivalents of lithium per unit formula, resulting in a high specific capacity (≥ 1,000 mAh g-1). The resulting composite anode material displayed a stable capacity of ca. 1,091 mAh g-1 for 190 cycles at a medium de-lithiation potential of 1.7 V and at a charge/discharge rate of 1C. Furthermore, the material displays an excellent high rate capability up to 20C, displaying a reversible capacity of still 216 mAh g-1. Studies on Fe and Zn losses of the ZnFe2O4 active material by dissolution in the electrolyte were performed and compared to those of silicon-, germanium- and tin-based high-capacity anode materials. In conclusion, ion dissolution from metal containing anode materials should not be underestimated in view of its impact on the overall cell performance and cycling stability.
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http://dx.doi.org/10.17344/acsi.2016.2243DOI Listing
July 2017

Investigating the Mg-Si Binary System via Combinatorial Sputter Deposition As High Energy Density Anodes for Lithium-Ion Batteries.

ACS Appl Mater Interfaces 2015 Sep 2;7(36):20124-33. Epub 2015 Sep 2.

University of Muenster , MEET Battery Research Center, Institute of Physical Chemistry, Corrensstrasse 46, 48149 Muenster, Germany.

Mg-Si thin films with various elemental compositions ranging from 0≤x≤1 in MgxSi(1-x) were obtained via combinatorial magnetron sputter deposition of Si and Mg in order to improve the electrochemical lithiation/delithiation process of pure Si by embedding Si in an active Mg-Si matrix. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and Raman spectroscopy methods were used to investigate the morphology, stoichiometry, and structure of the different thin film samples. Constant current charge/discharge cycling revealed significant electrochemical changes depending on the Mg content in comparison to the pure Si active material improving the capacity retention to 96% over 400 cycles.
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http://dx.doi.org/10.1021/acsami.5b05382DOI Listing
September 2015

Facile synthesis and lithium storage properties of a porous NiSi2/Si/carbon composite anode material for lithium-ion batteries.

ACS Appl Mater Interfaces 2015 Jan 17;7(3):1508-15. Epub 2015 Jan 17.

MEET Battery Research Center, Institute of Physical Chemistry, University of Münster , Corrensstrasse 46, 48149 Münster, Germany.

In this work, a novel, porous structured NiSi2/Si composite material with a core-shell morphology was successfully prepared using a facile ball-milling method. Furthermore, the chemical vapor deposition (CVD) method is deployed to coat the NiSi2/Si phase with a thin carbon layer to further enhance the surface electronic conductivity and to mechanically stabilize the whole composite structure. The morphology and porosity of the composite material was evaluated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and nitrogen adsorption measurements (BJH analysis). The as-prepared composite material consists of NiSi2, silicon, and carbon phases, in which the NiSi2 phase is embedded in a silicon matrix having homogeneously distributed pores, while the surface of this composite is coated with a carbon layer. The electrochemical characterization shows that the porous and core-shell structure of the composite anode material can effectively absorb and buffer the immense volume changes of silicon during the lithiation/delithiation process. The obtained NiSi2/Si/carbon composite anode material displays an outstanding electrochemical performance, which gives a stable capacity of 1272 mAh g(-1) for 200 cycles at a charge/discharge rate of 1C and a good rate capability with a reversible capacity of 740 mAh g(-1) at a rate of 5C.
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http://dx.doi.org/10.1021/am506486wDOI Listing
January 2015

Investigation of PF6(-) and TFSI(-) anion intercalation into graphitized carbon blacks and its influence on high voltage lithium ion batteries.

Phys Chem Chem Phys 2014 Dec 22;16(46):25306-13. Epub 2014 Oct 22.

MEET Battery Research Center, Institute of Physical Chemistry, University of Muenster, Corrensstraße 46, 48149 Muenster, Germany.

Graphitized carbon blacks have shown a more promising electrochemical performance than the non-treated ones when being applied in small amounts as conductive additives in composite cathode electrodes for lithium ion batteries, due to the absence of surface functional groups which contribute to detrimental side-reactions with the electrolyte. Here, we report that at high potentials of >4.5 V vs. Li/Li(+), graphitic structures in carbon black can provide host sites for the partially reversible intercalation of electrolyte salt anions. This process is in analogy to the charge reaction of graphite positive electrodes in dual-ion cells. A standard furnace carbon black with small graphitic structural units, as well as slightly and highly graphitized carbon blacks, were characterized and analyzed with regard to anion intercalation. A LiPF6 containing organic solvent based electrolyte as well as a state-of-the-art ionic liquid based electrolyte composed of LiTFSI in PYR14TFSI were applied. The intercalation of both PF6(-) and TFSI(-) could be confirmed by cyclic voltammetry in electrodes made of carbon blacks. When exposed to high potentials, carbon blacks experienced strong activation in the 1st cycle, which promotes the perception for anion intercalation, and thus increases the anion intercalation capacity in the following cycles. The specific capacity from anion intercalation was evaluated by constant current charge-discharge cycling. The obtained capacity was proportional to the graphitization degree. As anion intercalation might be accompanied by decomposition reactions of the electrolyte, e.g., by co-intercalation of solvent molecules, it could induce the decomposition of the electrolyte inside the carbon and thus degradation of the carbon black graphitic structure. In order to avoid side reactions from surface groups and from anion intercalation, the thermal treatment of carbon blacks must be optimized.
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http://dx.doi.org/10.1039/c4cp04113eDOI Listing
December 2014

Synthesis and electrochemical performance of surface-modified nano-sized core/shell tin particles for lithium ion batteries.

Nanotechnology 2014 Sep 12;25(35):355401. Epub 2014 Aug 12.

University of Muenster, MEET Battery Research Center, Institute of Physical Chemistry, Corrensstr. 46, D-48149 Muenster, Germany.

Tin is able to lithiate and delithiate reversibly with a high theoretical specific capacity, which makes it a promising candidate to supersede graphite as the state-of-the-art negative electrode material in lithium ion battery technology. Nevertheless, it still suffers from poor cycling stability and high irreversible capacities. In this contribution, we show the synthesis of three different nano-sized core/shell-type particles with crystalline tin cores and different amorphous surface shells consisting of SnOx and organic polymers. The spherical size and the surface shell can be tailored by adjusting the synthesis temperature and the polymer reagents in the synthesis, respectively. We determine the influence of the surface modifications with respect to the electrochemical performance and characterize the morphology, structure, and thermal properties of the nano-sized tin particles by means of high-resolution transmission electron microscopy, x-ray diffraction, and thermogravimetric analysis. The electrochemical performance is investigated by constant current charge/discharge cycling as well as cyclic voltammetry.
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http://dx.doi.org/10.1088/0957-4484/25/35/355401DOI Listing
September 2014
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