Yijing Zheng

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.

Laser processes and analytics for high power 3D battery materials

Laser processes for cutting, modification and structuring of energy storage materials such as electrodes, separator materials and current collectors have a great potential in order to minimize the fabrication costs and to increase the performance and operational lifetime of high power lithium-ion-batteries applicable for stand-alone electric energy storage devices and electric vehicles. Laser direct patterning of battery materials enable a rather new technical approach in order to adjust 3D surface architectures and porosity of composite electrode materials such as LiCoO2, LiMn2O4, LiFePO4, Li(NiMnCo)O2, and Silicon. The architecture design, the increase of active surface area, and the porosity of electrodes or separator layers can be controlled by laser processes and it was shown that a huge impact on electrolyte wetting, lithium-ion diffusion kinetics, cell life-time and cycling stability can be achieved. In general, the ultrafast laser processing can be used for precise surface texturing of battery materials. Nevertheless, regarding cost-efficient production also nanosecond laser material processing can be successfully applied for selected types of energy storage materials. A new concept for an advanced battery manufacturing including laser materials processing is presented. For developing an optimized 3D architecture for high power composite thick film electrodes electrochemical analytics and post mortem analytics using laser-induced breakdown spectroscopy were performed. Based on mapping of lithium in composite electrodes, an analytical approach for studying chemical degradation in structured and unstructured lithium-ion batteries will be presented.

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.

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.

Laser in battery manufacturing: impact of intrinsic and artificial electrode porosity on chemical degradation and battery lifetime

The main goal is to develop an optimized three-dimensional (3D) cell design with improved electrochemical properties, which can be correlated to a characteristic lithium distribution along 3D micro-structures at different State-of-Health (SoH). 3D elemental mapping was applied for characterizing the whole electrode as function of SoH. It was demonstrated that fs-laser generated 3D architectures improves the battery performance regarding battery power and lifetime. It was quantitatively shown by laser-induced breakdown spectroscopy that 3D architectures act as attractor for lithium-ions. Furthermore, lateral intrinsic porosity variations were identified to be possible starting points for lithium plating and subsequent cell degradation. Results achieved from post-mortem studies of cells with laser structured electrodes (intrinsic and artificial porosity variation), and unstructured lithium-nickel-manganese-cobalt-oxide electrodes will be presented.

Laser-Materials Processing for Energy Storage Applications

This chapter will review the use of laser-based material processing techniques, such as pulsed laser deposition (PLD), laser-induced forward transfer (LIFT), and material processing via 3D laser structuring (LS) and laser annealing (LA) techniques for energy storage applications. PLD is a powerful tool for fabricating highquality layers of materials for cathodes, anodes, and solid electrolytes for thin film microbatteries. LIFT is a versatile technique for printing complex materials with highly porous structures for the fabrication of micropower sources, such as ultracapacitors and thick-film batteries. LS is a recently developed technique for modifying the active material by forming advanced 3D electrode architectures and increasing the overall active surface area. LA is a rapid technology for adjusting the crystalline battery phase and for controlling the grain size on the micro- and nanoscale. This chapter will review recent work using these laser processing techniques for the fabrication of micropower sources and lessons learned from the characterization of their electrochemical properties.

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.

Laser structuring for improved battery performance

Within the last two decades, lithium-ion batteries (LIBs) have emerged as the power source of choice in the high-performance rechargeable battery market.1, 2 LIBs are high-capacity batteries that are able to store electricity converted from green energy sources (e.g., solar and wind power), and they can act as energy sources in pollution-free electric vehicles. Nevertheless, there are several drawbacks to state-of-the-art LIBs, including high production costs, short battery lifetimes, safety issues, and time-consuming charging periods. The main issue in cell production is the electrolyte wetting of LIBs, which is realized through timeand cost-consuming vacuum and storage processes at elevated temperatures. Insufficient wetting of electrodes results in a production failure rate, accompanied by a reduced cell capacity and cell lifetime. The development of 3D electrode architectures in LIBs is a relatively new approach for overcoming the problems related to battery performance (e.g., power losses or high interelectrode ohmic resistances3) and mechanical degradation4 during battery operation. 3D batteries can be used to achieve large areal energy capacities, while simultaneously maintaining high power densities. A common approach is 3D structuring of the electrode substrate (the ‘current collector’) before deposition of the thin-film electrode. Unfortunately, this method is in a very early stage of development for model electrodes in thin-film microbatteries. Furthermore, it is generally not feasible for up-scaling to thick-film composite electrodes or for large electrode footprint areas. At the Karlsruhe Institute of Technology, we have thus developed a new technical approach for the generation of 3D electrode designs that can be applied to all types of LIBs (i.e., thin-film batteries, as well as high-energy and high-power LIBs).5 In this approach, for the first time, we have applied laser-assisted processing to the active electrode material itself. For this Figure 1. Scanning electron microscope images of laser-generated microstructures in composite electrode materials. (a) Self-organized microstructures (produced with the use of an excimer laser) and (b) micro-pillars obtained from direct laser structuring with an ultrafast (femtosecond) laser.