H.J. Seifert

Direct laser interference patterning and ultrafast laser-induced micro/nano structuring of current collectors for lithium-ion batteries

Laser-assisted modification of metals, polymers or ceramics yields a precise adjustment of wettability, biocompatibility or tribological properties for a broad range of applications. Due to a specific change of surface topography on micro- and nanometer scale, new functional properties can be achieved. A rather new scientific and technical approach is the laserassisted surface modification and structuring of metallic current collector foils for lithium-ion batteries. Prior to the thick film electrode coating processes, the formation of micro/nano-scaled surface topographies on current collectors can offer better interface adhesion, mechanical anchoring, electrical contact and reduced mechanical stress during cycling. These features in turn impact on the battery performance and the battery life-time. In order to generate the 3D surface architectures on metallic current collectors, two advanced laser processing structuring technologies: direct laser interference patterning (DLIP) and ultrafast laser-induced periodic surface structuring (LIPSS) were applied in this study. After laser structuring via DLIP and LIPSS, composite electrode materials were deposited by tape-casting on the modified current collectors. The electrode film adhesion was characterized by tensile strength measurements. The impact of various surface structures on the improvement of adhesive strength was discussed.

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.

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.

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.

Fabrication and characterization of silicon-based 3D electrodes for high-energy lithium-ion batteries

For next generation of high energy lithium-ion batteries, silicon as anode material is of great interest due to its higher specific capacity (3579 mAh/g). However, the volume change during de-/intercalation of lithium-ions can reach values up to 300 % causing particle pulverization, loss of electrical contact and even delimitation of the composite electrode from the current collector. In order to overcome these drawbacks for silicon anodes we are developing new 3D electrode architectures. Laser nano-structuring of the current collectors is developed for improving the electrode adhesion and laser micro-structuring of thick film composite electrodes is applied for generating of freestanding structures. Freestanding structures could be attributed to sustain high volume changes during electrochemical cycling and to improve the capacity retention at high C-rates (> 0.5 C). Thick film composite Si and Si/graphite anode materials with different silicon content were deposited on current collectors by tape-casting. Film adhesion on structured current collectors was investigated by applying the 90° peel-off test. Electrochemical properties of cells with structured and unstructured electrodes were characterized. The impact of 3D electrode architectures regarding cycle stability, capacity retention and cell life-time will be discussed in detail.

Femtosecond laser modification of Li(NiCoMn)O 2 electrodes for lithium-ion batteries

Ultrafast laser micromachining processes for modification and formation of three-dimensional architectures in cathode materials were developed. The electrochemical properties were investigated applying cell test with current densities in the range of (0.05-28.79) mA/cm2.

Investigation of micro-structured Li(Ni1/3Mn1/3Co1/3)O2 cathodes by laser-induced breakdown spectroscopy

Lithium nickel manganese cobalt oxide (Li(Ni1/3Mn1/3Co1/3)O2, NMC) thick film electrodes were manufactured by using the doctor-blade technique (tape-casting). Ultrafast laser-structuring was performed in order to improve the electrochemical performance. For this purpose, three-dimensional (3D) micro-structures such as free standing micropillars were generated in NMC cathodes by using femtosecond laser ablation. Laser-induced breakdown spectroscopy (LIBS) was used for post-mortem investigation of the lithium distribution of unstructured and femtosecond laser-structured NMC electrodes. For achieving a variable State-of-Health (SoH), both types of electrodes were electrochemically cycled. LIBS calibration was performed based on NMC electrodes with defined lithium amount. Those samples were produced by titration technique in a voltage window of 3.0 V - 5.0 V. Elemental mapping and elemental depth-profiling of lithium with a lateral resolution of 100 μm were applied in order to characterize the whole electrode surface. The main goal is to develop an optimized 3D cell design with improved electrochemical properties which can be correlated to a characteristic lithium distribution along 3D micro-structures at different SoH.

Laser interference patterning and laser-induced periodic surface structure formation on metallic substrates

Laser-assisted modification of metals, polymers or ceramics yields a precise adjustment of wettability, bio-compatibility or tribological properties for a broad range of applications. Two types of advanced laser processing technologies — direct laser interference patterning and ultrafast laser-induced periodic surface structuring — were applied in this study. Formation of laser-induced periodic surface structures on metallic substrate was investigated systematically as function of wavelength, pulse duration, laser fluence and scanning speed. Line-like periodic patterns with adjustable periodicity were successfully formed on metallic substrates. For lithium-ion batteries, composite electrode materials were deposited by tape-casting on laser micro/nano-structured metallic current collectors. Tensile strength measurements revealed a tremendous improvement of film adhesion.

Laser-induced breakdown spectroscopy as a powerful tool for characterization of laser modified composite materials

Laser-Induced Breakdown Spectroscopy (LIBS) was applied in order to investigate the elemental composition in laser modified battery materials. In a first approach, thick film electrodes — all incorporating the same active material (lithium nickel manganese cobalt oxide (Li(Ni1/3Mn1/3Co1/3)O2, NMC)) — were manufactured using the tape-casting technique. In a further approach, femtosecond (fs) laser radiation was used for the generation of three dimensional (3D) micro-structures. 3D micro-grids were successfully implemented into thick film electrodes applying a fs-laser wavelength of 515 nm. The electrochemical performance in lithium-ion cells with untreated and laser-modified NMC electrodes was determined by galvanostatic cycling at high charging- and discharging currents. Finally, LIBS was used as a powerful characterization tool in order to investigate the change in lithium distribution after electrochemical cycling at different State-of-Health. Evaluation of lithium distribution will be used to analyze degradation mechanisms and to find the most appropriate 3D electrode architecture. For this purpose, results of electrochemical cycling and LIBS measurements of untreated and laser-structured NMC thick films have to be correlated.