Publications Fraunhofer R&D Center for Electromobility Bavaria

Ultrasound spectroscopy (up to 6 MHz) on a 12 Ah Lithium-ion cell reveals absorption and resonance components of attenuation. Two resonances are identified. The attenuation of the first resonance linearly decreases with state of charge. Matching interrogation frequency to this resonance yields a 5x signal amplitude increase across 0-100 % charge, potentially enabling robust state of charge determination.
 

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The photostability of roll-to-roll-processed, sidechain-modified poly(3,4-ethylenedioxythiophene) (PEDOT-EthC6) on indium tin oxide coated polyethylene terephthalate (PET-ITO) is improved by excluding ambient atmosphere. In the colored and bleached states, degradation is prevented by 420 and 500 nm long pass filters, respectively. Nickel oxide, as a counter electrode and UV protection, results in ECDs that show no loss of EC performance after 350 h of light exposure.

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In this study, we present a 5,8-bis(3,4-ethylenedioxythiophene)quinoxaline monomer with two 4-(octyloxy)phenyl side chains (EDOTPQ) that can be electropolymerized on ITO glass in standard electrolytes containing lithium salts and propylene carbonate as solvent. The electrochemically deposited PEDOTPQ layers show very good adhesion and homogeneity on ITO. The green-colored polymer thin films exhibit promising electrochromic (EC) properties and are interesting for applications such as adaptive camouflage, as well as smart displays, labels, and sensors. Novel organic-inorganic (hybrid) EC cell configurations were realized with Prussian blue (PB) or titanium-vanadium oxide (TiVOx) as ion storage electrodes, showing a highly reversible and fast color change from green to light yellow.

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This study investigates the impact of water ratio on the direct aqueous recycling of NMC811. Three different ratios of NMC811 to water were examined. The results demonstrate that the water ratio significantly affects the electrochemical performance of NMC811. Capacity fading is observed in all water-exposed samples, with the sample having the lowest water ratio showing less fading compared to the samples processed with higher water ratios. Both samples with higher water ratios exhibit similar performance, suggesting an equilibrium at the NMC811-water interface is established. Characterization of the cathode materials reveals variations in the amount and type of surface species. The pristine sample, not exposed to water, only shows Li₂CO₃ and NiO as surface species, while the water-exposed NMC811 samples exhibit nickel carbonates and hydroxides along with associated water. The poorer performance of samples exposed to higher water ratios is likely due to higher amounts of these species forming on the particle surface. Additionally, lithium, cobalt, and manganese carbonates, as well as lithium hydroxide with associated water, are detected and could further contribute to the poorer performance.

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Metallopolymers (MEPEs) show a strong absorption band attributed to a metal-to-ligand charge transfer (MLCT) transition. Fe-MEPE and Ru-MEPE thin films switch reversibly from blue-purple (λ_max=584 nm, Fe²⁺) to colorless (Fe³⁺) or orange (λ_max=518 nm, Ru²⁺) to pale green (Ru³⁺). The addition of multi-walled carbon nanotubes (MWCNTs) enhances the electrochromic (EC) properties of Fe- and Ru-MEPE electrodes in several ways: (1) faster response for bleaching/coloring; (2) enhanced Coulombic efficiency; and (3) improved cycling stability, particularly for Ru-MEPE. Moreover, the charge density of the thin film electrodes can be increased upon the addition of the MWCNTs. This performance improvement is demonstrated in electrochromic devices (ECDs) with titanium-doped vanadium oxide (TiVO_x) as the optically-passive ion storage layer. The visible light transmittance τ_v values of the ECDs are improved by the addition of the MWCNTs from 20 %/56 % to 17 %/56 % and from 31 %/47 % and 34 %/55 % in the dark/bright state, for Fe-MEPE and Ru-MEPE, respectively. Thus, the addition of MWCNTs to Fe- or Ru-MEPE is a very promising route for future fast-switching EC applications, e. g., displays, sunroofs, and rear-view mirrors.

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This study compares two electrode architectures, one with a porous carbon current collector (PC) and the other with a state-of-the-art aluminum foil current collector (AF). The results show that the porous carbon outperforms the aluminum foil at high loadings (∼8 mAh/cm²) during cycling, likely due to better adhesion. The characterization methods used in the study include electrochemical cycling, electrochemical impedance spectroscopy (EIS), galvanostatic intermittent titration technique (GITT), and scanning electron microscopy (SEM). These methods provide insight into the performance of the two electrode architectures and allow for a comprehensive comparison between them. The findings of this study indicate that the use of porous carbon as a current collector can lead to improved performance in high-loading electrode applications, making it a promising alternative to traditional aluminum foil current collectors.

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Electrochromic (EC) windows on glass for thermal and glare protection in buildings, often referred to as smart (dimmable) windows, are commercially available, along with rearview mirrors or windows in aircraft cabins. Plastic-based applications, such as ski goggles, visors and car windows, that require lightweight, three-dimensional (3D) geometry and high-throughput manufacturing are still under development. To produce such EC devices (ECDs), a flexible EC film could be integrated into a back injection molding process, where the films are processed into compact 3D geometries in a single automized step at a low processing time. Polycarbonate (PC) as a substrate is a lightweight and robust alternative to glass due to its outstanding optical and mechanical properties. In this study, an EC film on a PC substrate was fabricated and characterized for the first time. To achieve a highly transmissive and colorless bright state, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) was used as the working electrode, while titanium dioxide (TiO₂) was used as the counter electrode material. They were deposited onto ITO-coated PC films using dip- and slot-die coating, respectively. The electrodes were optically and electrochemically characterized. An ECD with a polyurethane containing gel electrolyte was investigated with regard to optical properties, switching speed and cycling behavior. The ECD exhibits a color-neutral and highly transmissive bright state with a visible light transmittance of 74% and a bluish-colored state of 64%, a fast switching speed (7 s/4 s for bleaching/coloring) and a moderately stable cycling behavior over 500 cycles with a decrease in transmittance change from 10%to 7%.

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The development of solid-state lithium-ion batteries is a promising step to meet the increasing demand for safe batteries with a high energy density. In this work, silicon electrodes containing an organic/inorganic hybrid polymer electrolyte (HPE) are reported. Depending on the conducting salt and the inclusion of an ionic liquid (IL), the HPE exhibits an ionic conductivity between 0.24 ± 0.11 mS cm⁻¹ and 0.94 ± 0.07 mS cm⁻¹ at 60 °C. The achievable capacity in Si/HPE/Li cells depends strongly on the C-rate and the areal capacity of the electrodes, as well as on the electrolyte and electrode composition. Among the cells tested, those with an HPE containing lithium bis(fluorosulfonyl)imide (LiFSI) and no IL exhibit the highest capacity retention and average coulombic efficiency. The use of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and/or the addition of an IL enables higher electrode loadings, however at the expense of capacity retention. Si electrodes with a higher Si content show an improved cell performance compared to those with less Si. A combination of electrodes containing 75 wt% silicon with an HPE with LiFSI and IL reaches a high capacity of approx. 1500 mA h g_Si⁻¹ at 0.1 C with a capacity retention of 74% after 100 cycles.

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UV irradiation is used to precharge sputtered tungsten trioxide (WO₃) on polyethylene terephthalate enhancing the photochromic response with organic solvents. A comparison between the optical and electrochemical properties of photochemically and electrochemically charged WO₃ results in a correlation of the transmittance to the respective charge density. This allows for a precise control of the charge density in the precharging process monitored by UV–Vis spectroscopy. A proof-of-concept flexible electrochromic device combining precharged WO₃ (charge density: 20 mC cm⁻²) and Prussian blue (15 mC cm⁻²) exhibits a superior change in the visible light transmittance (τ_v) from 8% (dark) to 79% (bleached) and a coloration efficiency of 139 cm² C⁻¹ at 716 nm.

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The interplay between the internal mechanical properties and external mechanical conditions of a battery cell, e. g., Young's modulus and thickness change, has a crucial impact on the cell performance and lifetime, and thus, needs to be fully understood. In this work, 12 Ah lithium-ion battery pouch cells were studied during cycling and aging by non-invasive operando ultrasonic and dilation measurements. The effective Young's modulus increases and the thickness varies the most within a single cycle during the graphite transition from stage 1L to 4, at the beginning of the 2 to 1 stage transition and at the phase transition of the nickel-rich NCM from H2 to H3. After 1000 cycles of aging, the overall effective Young's modulus of the lithium-ion battery decreases by ∼11 %–12 % and the cell thickness increases irreversibly by ∼3 %–4 %, which is mostly related to a thicker and possibly softer, more porous solid electrolyte interphase layer.

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Lithium metal is considered as the ‘holy-grail’ among anode materials for lithium-ion batteries, but it also has some serious drawbacks such as the formation of dendritic and dead lithium. In this study, the interplay of external pressure and different carbonate- and ether-based electrolytes on the (ir)reversible expansion of lithium metal during cycling against lithium titanate and lithium iron phosphate is studied. In carbonate-based electrolytes without any additives, lithium metal shows tremendous irreversible expansion and significant capacity reduction at elevated current densities due to the formation of mossy and dead lithium. The addition of fluoroethylene carbonate can reduce irreversible expansion and capacity reduction, especially when a high external pressure is applied. When an ether-based electrolyte is used, the irreversible dilation of the lithium metal is suppressed when applying increased external pressures. Overall, increased external pressure appears to reduce the formation of mossy and dead lithium and improve the performance.

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State-of-the-art methods to monitor the degree of wetting of lithium-ion batteries during production, which are applicable at an industrial scale, are not capable of determining the spatial electrolyte distribution in the porous network of electrodes and separator. Ultrasound can bridge this gap as it has previously been proven to be a suitable method to monitor or evaluate the infiltration of porous structures, for example, epoxy resin in carbon fiber composite. Herein, the acoustic properties of different battery cell materials are evaluated over a wide frequency range in both the dry and wet states, for the first time. Furthermore, a collection of input parameters, from the experimental setup and the evaluation routine to the acoustic parameters of the battery materials themselves, are provided. These parameters are needed for the adaptation of ultrasonic nondestructive testing to the monitoring of the wetting process of a lithium-ion battery. Most importantly, this work demonstrates that a large change in the sound velocity (up to 100%) is observed when the porous structure of a lithium-ion battery is filled with electrolyte and thus can serve as an appropriate evaluation figure of merit.

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