J. Pröll

A new approach for rapid electrolyte wetting in tape cast electrodes for lithium-ion batteries

An innovative concept for significantly decreasing the time-span for the electrochemical formation step after lithium-ion cell assembly is presented. Laser structuring has been developed for the formation of capillary microstructures in tape cast electrodes resulting in a tremendous acceleration of electrolyte wetting in comparison to unstructured electrodes. Pouch cells with laser-structured electrodes show an increase in capacity retention and an enhancement in cell cycle life-time at high charging and discharging rates. We assume that laser structuring of tape cast electrodes could be a further step towards economic and zero failure production of high-power lithium-ion batteries.

Surface micro-structuring of intercalation cathode materials for lithium-ion batteries: a study of laser-assisted cone formation

Strong efforts are currently undertaken in order to further improve the electrochemical performance of high energy lithium-ion batteries containing thick composite electrode materials. The properties of these electrode materials such as active surface area, film thickness, and film porosity strongly impact the cell life-time and cycling stability. A rather new approach is to generate hierarchical architectures into cathode materials by laser direct ablation as well as by laserassisted formation of self-organized structures. It could be shown that appropriate surface structures can lead to a significant improvement of lithium-ion diffusion kinetics leading to higher specific capacities at high charging and discharging currents. In this paper, the formation of self-organized conical structures in intercalation materials such as LiCoO2 and LiNi1/3Mn1/3Co1/3O2 is investigated in detail. For this purpose, the cathode materials are exposed to excimer laser radiation with wavelengths of 248 nm and 193 nm leading to cone structures with outer dimensions in the micrometer range. The process of cone formation is investigated using laser ablation inductively coupled plasma mass spectrometry and laser-induced breakdown spectroscopy (LIBS). Cone formation can be initiated for laser fluences up to 3 J/cm2 while selective removal of lithium was observed to be one of the key issues for starting the cone formation process. It could be shown that material re-deposition supports the cone-growth process leading to a low loss of active material. Besides the cone formation process, laser-induced chemical surface modification will be analysed by LIBS.

Fs-laser microstructuring of laser-printed LiMn2O4 electrodes for manufacturing of 3D microbatteries

Lithium manganese oxide composite cathodes are realized by laser-printing. The printed cathode is a composite and consists of active powder, binder and conductive agents. Laser-printed cathodes are first calendered and then laser structured using femtosecond-laser radiation in order to form three-dimensional (3D) micro-grids in the cathode material. Three-dimensional micro-grids in calendered/laser structured cathodes exhibit improved discharge capacity retention at a 1 C discharging rate. Calendered but unstructured cathodes indicate the poorest cycling behavior at 1 C discharge. The improved capacity retention and the reduced degradation of calendered/structured cathodes can be attributed to both the increased electrical contact through calendering as well as shortened Li-ion pathways due to laser-induced 3D microgrids.

Laser adjusted three-dimensional Li-Mn-O cathode architectures for secondary lithium-ion cells

Three-dimensional cathode architectures for rechargeable lithium-ion cells can provide better Li-ion diffusion due to larger electrochemical active surface area and therefore, may stabilize the cycling behaviour of an electrochemical cell. This features show great importance when aiming for long-life batteries, e.g. in stationary or portable power devices. In this study, lithium manganese oxide thin films were used as cathode material with the goal to stabilize their cycling behavior and to counter degradation effects which come up within the lithium manganese oxide system. Firstly, appropriate laser ablation parameters were selected in order to achieve defined three-dimensional structures with features sizes down to micro- and sub-micrometer scale by using mask imaging technique. Laser annealing was also applied onto the laser structured material in a second step in order to form an electrochemically active phase. Process development led to a laser annealing strategy for a flexible adjustment of crystallinity and grain size. Laser annealing was realized using a high power diode laser system operating at a wavelength of 940 nm. Information on the surface composition, chemistry and topography as well as studies on the crystalline phase of the material were obtained by using Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and X-ray diffraction analysis. The electrochemical activity of the laser modified lithium manganese oxide cathodes was explored by cyclic voltammetry measurements and galvanostatic testing by using a lithium anode and standard liquid electrolyte.

Laser modification and characterization of Li-Mn-O thin film cathodes for lithium-ion batteries

The development of future battery systems is mainly focused on powerful rechargeable lithium-ion batteries. To satisfy this demand, current studies are focused on cathodes based on nano-composite materials which lead to an increase in power density of the LIB primarily due to large electrochemically active surface areas. Electrode materials made of lithium manganese oxides (Li-Mn-O) are assumed to replace commonly used cathode materials like LiCoO2 due to less toxicity and lower costs. Thin films in the Li-Mn-O system were synthesized by non-reactive r.f. magnetron sputtering of a LiMn2O4 target on silicon and stainless steel substrates. In order to enhance power density and cycle stability of the cathode material, direct laser structuring methods were investigated using a laser system operating at a wavelength of 248 nm. Therefore, high aspect ratio micro-structures were formed on the thin films. Laser annealing processes were investigated in order to achieve an appropriate crystalline phase for unstructured and structured thin films as well as for an increase in energy density and control of grain size. Laser annealing was realized via a high power diode laser system. The effects of post-thermal treatment on the thin films were studied with Raman spectroscopy, X-ray diffraction and scanning electron microscopy. The formation of electrochemically active and inactive phases was discussed. Surface chemistry was investigated via X-ray photoelectron spectroscopy. Interaction between UV-laser radiation and the thin film material was analyzed through ablation experiments. Finally, to investigate the electrochemical properties, the manufactured thin film cathodes were cycled against a lithium anode. The formation of a solid electrolyte interphase on the cathode side was discussed.

Manufacturing of advanced Li(NiMnCo)O2 electrodes for lithium-ion batteries

Lithium-ion batteries require an increase in cell life-time as well as an improvement in cycle stability in order to be used as energy storage systems, e.g. for stationary devices or electric vehicles. Nowadays, several cathode materials such as Li(NiMnCo)O2 (NMC) are under intense investigation to enhanced cell cycling behavior by simultaneously providing reasonable costs. Previous studies have shown that processing of three-dimensional (3D) micro-features in electrodes using nanosecond laser radiation further increases the active surface area and therefore, the lithium-ion diffusion cell kinetics. Within this study, NMC cathodes were prepared by tape-casting and laser-structured using nanosecond laser radiation. Furthermore, laser-induced breakdown spectroscopy (LIBS) was used in a first experimental attempt to analyze the lithium distribution in unstructured NMC cathodes at different state-of-charges (SOC). LIBS will be applied to laser-structured cathodes in order to investigate the lithium distribution at different SOC. The results will be compared to those obtained for unstructured electrodes to examine advantages of 3D micro-structures with respect to lithium-ion diffusion kinetics.

Femtosecond laser patterning of lithium-ion battery separator materials: impact on liquid electrolyte wetting and cell performance

High capacity Li-ion batteries are composed of alternating stacked cathode and anode layers with thin separator membranes in between for preventing internal shorting. Such batteries can suffer from insufficient cell reliability, safety and electrochemical performance due to poor liquid electrolyte wetting properties. Within the electrolyte filling process, homogeneous wetting of cathode, separator and anode layers is strongly requested due to the fact that insufficient electrolyte wetting of battery components can cause limited capacity under challenging operation or even battery failure. The capacity of the battery is known to be limited by the quantity of wetting of the electrode and separator layers. Therefore, laser structuring processes have recently been developed for forming capillary micro-structures into cathode and anode layers leading to improved wetting properties. Additionally, many efforts have been undertaken to enhance the wettability and safety issues of separator layers, e.g. by applying thin coatings to polymeric base materials. In this paper, we present a rather new approach for ultrafast femtosecond laser patterning of surface coated separator layers. Laser patterning allows the formation of micro-vias and micro-channel structures into thin separator membranes. Liquid electrolyte wetting properties were investigated before and after laser treatment. The electrochemical cyclability of batteries with unstructured and laser-structured separators was tested in order to determine an optimal combination with respect to separator material, functional coating and laser-induced surface topography.

Laser generated microstructures in tape cast electrodes for rapid electrolyte wetting - new technical approach for cost efficient battery manufacturing

Three-dimensional (3D) battery architectures are under current scientific investigation since they can achieve large areal energy capacities while maintaining high power densities. A main objective of surface patterning is the enhancement of lithium-ion diffusion which is often a limiting factor in lithium-ion cells. By using a rather new approach, laser material processing of thick-film electrodes has been investigated for the precise adjustment of 3D surface topography. Besides lithium-ion diffusion in electrode materials as an electrochemically limited process, a critical step in lithium-ion pouch cell manufacturing is the homogeneous electrolyte wetting of stacked electrodes and separators. This process requires cost expensive and time-consuming vacuum and storage processes at elevated temperatures. A new and cost efficient laser process has been successfully applied in order to significantly improve the electrode wetting and the battery operation. Preliminary investigations for testing the process on pouch cell geometry revealed higher capacities and increased cell life-time compared to standard cells without storage processes at elevated temperatures. The laser structuring process can be applied to commercial electrode materials and integrated into existing production lines.

Ultrafast laser microstructuring of LiFePO4 cathode material

LiFePO4 is a very promising material to be used as positive electrode for future lithium-ion batteries. Nevertheless, a reduced rate capability at high discharging and charging currents is the main drawback. In this work, a 3D structure was made in LiFePO4 composite electrodes by applying ultrafast laser ablation. The change of the electrochemical properties in a lithium-ion half-cell due to laser structuring was studied in detail and will be discussed. The main challenging goal is to correlate cell properties such as capacity retention with laser parameters and laser generated microstructure. For microstructuring electrode materials an ultrafast as well as a ns fiber laser were used. The pulse duration was varied in the range from 350 fs to 200 ns. With ultrashort laser radiation, the ablation efficiency was increased. Electrochemical characterisations were performed. For this purpose, Swagelok® test cells with lithium metal as counter electrode were assembled. Main electrochemical parameters such as specific capacity and cycle stability were determined for the cells with structured and unstructured cathodes. It was shown that the rate capability for the cells with structured cathodes in comparison to cells with unstructured cathodes was significantly enhanced, especially for high charging and discharging rates.

Formation of nanostructures by femtosecond laser processing

The formation of laser-induced periodic surface structures (LIPSS) on metallic surfaces by femtosecond laser radiation was investigated. Various types of LIPSS could be formed on metallic surfaces such as copper and stainless steel. Nano-ripple formation was investigated as function of laser scanning speed, laser pulse number, and laser fluence. Hierarchical structures were generated on stainless steel surfaces and hydrophobic properties with water contact angles of about 145° could be achieved. It could be shown that direct femtosecond laser surface processing is a perfect tool for designing surface properties on micro- and nanometer scale which can be used for optical, biomedical, and tribological applications.