Laser structured Cu foil for high-performance lithium-ion battery anodes
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To improve interface adhesion between anode film and Cu foil, ultrafast laser structuring was implemented to construct dot patterns with a variety of periodic spacing (25, 50, and 75 µm) on Cu foil. The microstructure and electrochemical performance of anode films coated on those structured Cu foils were characterized. It was shown that adhesive force of the electrodes increased as periodic distance between the dots on the Cu foil decreased. Comparison of XRD patterns of the wet slurries with the dried anode films showed that after drying in the case of 50 µm period dot structured Cu foils the most graphite particles were aligned with the c-axis, vertical to the Cu foil surface. EIS, CV, and rate capability measurements confirmed that the anode film on the 50 µm dot period Cu foil had the lowest impedance, strongest lithiation and de-lithiation peaks, and highest discharge capacity. The cycling test carried out under C/2 rate confirmed that the cells with the 50 µm dot interval Cu foil showed the highest capacity retention. We inferred that this was due to the relatively shorter diffusive path in the anode due to vertical orientation of more graphite particles against the laser structured Cu foil.
KeywordsUltrafast laser ablation Adhesive force Anode films Orientation of graphite particles Li+ diffusion
Due to anode swelling, especially during repeated cycling, detachment of anode films from current collectors has been a long-term issue in lithium-ion batteries [1, 2]. Silicon has been investigated as anode material for the next generation of lithium-ion batteries; however, its massive volumetric expansion hinders its commercialization in the near future. Stronger binders for improving the adhesive force of anode films have been investigated, and a series of new polymers such as PAA (polyacrylic acid) have been confirmed to enhance the adhesive force among silicon particles [3, 4, 5]. On the other hand, modification of current collectors (Cu foils) provides an alternative solution for improving the interfacial adhesion between the powders of the active materials and Cu foil.
Ultrafast laser is a powerful tool to realize surface structuring at micro- and nanometer scales of metallic, polymer, and ceramic substrates, and has been developed successfully and extensively on different substrates and for different applications . Our previous researches on the treatment of Al foils by ultrafast laser have confirmed the increase of adhesive force of cathode films [7, 8, 9, 10, 11, 12]. Another result showed that laser structuring of LiCoO2, LiNi x Co y Mn1–x −y O2, LiNi x Mn1−x O3, and LiFePO4 electrodes contributed to a substantial increase of their high-C rate performance and cycling behavior, which helps to develop them into three-dimensional electrodes for enhanced electrochemical kinetics.
Current research is focused on the quantitative evaluation of electrochemical performance to identify the most appropriate electrode structuring via ultrafast laser ablation for state-of-the-art and next generation electrode materials. This kind of approach will be up-scaled and transferred to lithium-ion battery production with large areal footprints and lines. However, ultrafast laser ablation might impose residual stress onto the surface or into the bulk of the materials ; the influence of these kinds of stress on the electrochemical properties of electrodes needs to be evaluated carefully.
In this work, we treated Cu foils with ultrashort laser pulses to form a series of dot structures and investigated the electrochemical performances of graphite anode films coated onto those structured Cu foils as a function of the period of the dot pattern.
2.1 Ultrafast laser processing of Cu foils
Laser structuring of Cu foils was performed at KIT, while the battery grade Cu foils with a thickness of 10 µm were purchased from Oak-Mitsui (Japan). An ultrafast fiber laser source (Tangerine, Amplitude Systèmes, France) with a maximum average laser power of 35 W operated at a laser wavelength of 1030 nm was used in the experiments. Various types of dot patterns with varying pitch periods were generated by single pulse ultrafast laser ablation at a wavelength of 515 nm, a laser fluence of 10.7 J cm−2, and a pulse width of 380 fs. After ablation by single pulse laser, nano-sized concavities and surface ripples were formed, and different pitch distances were realized as a function of laser scanning speed and pulse repetition rate.
2.2 Anode preparation
All anode components were commercially available. Artificial graphite was purchased from MTIXTL USA, with a specific capacity of 338.6 mAh g−1, and its D50 was about 20 µm, CMC (carboxymethyl cellulose) and SBR (styrene-butadiene rubber) also from MTIXTL (USA), and carbon black (C45) from IMERYS (Switzerland).
The dried anode films were calendared at 120 °C. The final film thickness was almost half of the thickness before calendaring. Circle electrodes with a diameter of 15 mm were punched and weighed, the density of the punched electrodes was around 6.0 mg cm−2. The punched electrodes were dried in vacuum at 120 °C for 12 h before being transferred to a glove box (MBraun, MB 200G, Germany) to be assembled into coin cells.
2.3 Coin cell assembly
The 2032 type coin cells were assembled in the glove box under a controlled atmosphere of O2 (<0.6 ppm) and H2O (<0.1 ppm). Lithium (Li) foils with a diameter of 16 mm and a thickness of 45 µm (MTI Inc.) were used as counter electrodes, and three layers of separator (20 mm in diameter, Freudenberg GmbH, Germany) were sandwiched between punched anode films and Li foils. 12 drops (120 µL each) of electrolyte (LP30, EC:DMC = 1:1, 1 M LiPF6, Sigma-Aldrich GmbH, Germany) were applied to wet the working electrode and separators, and was confirmed to be enough to keep close contact between working electrode and counter electrode in the housings of the coin cells.
2.4 Microstructure analysis and electrochemical characterization
The electrochemical impedance spectra (EIS) of the coin cells were measured with an impedance analyzer (Princeton Application Inc., USA) in a frequency range of 10−2–106 Hz with 10 mV as the oscillation voltage. Cyclic voltammetry (CV) was performed by using an electrochemical station (AMTEK Inc., USA), with a sweeping rate of 50 µV s−1 between 1.5 and 5 × 10−3 V for three cycles. C-rate and cycling tests were performed with a battery tester (Model: Series 4000, Maccor Inc., USA). A harsh C-rate test protocol was adopted: the coin cells were charged to 0.01 V, then discharged to 1.5 V both with the predefined currents, no constant voltage charging and no rest procedures were included. Cycling tests were performed between 1.5 and 0.01 V at C/2, where during charging a constant current procedure (CC) was adopted until the cell voltage reached 0.01 V; the cell was then fixed at this voltage till the charging current decreased to 0.01 C (CV). During discharging, a CC was adopted; a rest for 5 s was performed between charging and discharging procedures.
3 Results and discussions
In Fig. 2c a dot pattern with a period of 50 µm is shown and the actual distance between the edges of neighboring dots is about 30 µm. This value almost disappears in the case of the dot pattern with a period of 25 µm as shown in Fig. 2d. In Fig. 2f, the Cu foil is exposed after peeling-off of the anode film and two dots could be found, as labeled by the red dashed circles.
3.2 XRD patterns
Another phenomenon related to the evolution of the diffraction peaks of graphite (100) and (101) is observed in Fig. 3. After drying, the diffraction peaks in the 50 µm sample are the strongest, whereas in the wet films all those diffraction peaks almost have the same intensity, since the graphite particles are still floating freely in the suspension—all from the same slurry batch. Since the diffraction peaks of graphite (100) and (101) planes correspond to the layered structures in the graphite particles (or c axial) perpendicular to the surface of the diffraction plane (here the Cu foil), a higher diffraction intensity means a higher fraction of their c-axis oriented particles in the dried films . This orientation of graphite particles relative to Cu foil would help to reduce the tortuosity of the Li-ion diffusion path in the electrolyte, improving electrochemical performance . The capillary force generated during the drying process through the evaporation of solvent from the anode slurry might help to facilitate this re-orientation process.
3.3 Adhesion force of anode films
3.4 Electrochemical characterization
Based on the previous results of XRD, adhesive force, and electrochemical characterization, it is confirmed that the surface modification of Cu foil via ultrafast laser ablation benefits the adhesion between graphite particles and Cu foil, and reduces interfacial impedance. The best result is achieved with the 50 µm dot period. Since the particle size of our graphite powders is around 20 µm , which is similar to the space between dots in the 50 µm dot period Cu foil, this might indicate geometrical matching between the graphite particles and the patterning configuration of the Cu foil substrate. This kind of geometric consistency would ease re-orientation of graphite particles towards the direction perpendicular to the Cu foils, thus favoring the diffusion of Li+ in the anode. It is therefore suggested that for finding the optimal structuring parameters by ultrafast laser ablation of current collectors for further application in lithium-ion batteries, the characteristic parameters (here particle size) of the active materials should be considered and evaluated. A theoretical analysis and practical measurements of particle orientation and its evolution during coating, drying, calendaring, and the following electrochemical processes might help to build up knowledge in advanced electrode engineering.
On the other hand, the results of all electrochemical analysis are not fully consistent with each other, and also with the adhesive force. For electrochemical characterization in relatively steady processes (CV and EIS), the graphite electrode with a periodic spacing of 75 µm showed the reasonable performances; for relatively fast processes (C-rate and cycling test), it turned to be the worst.
We believed that these phenomena might be ascribed to the complicate physical pictures and electrochemical processes in the anode films on laser structured Cu foils based on a systematic view.
In Fig. 4, the adhesion force was improved after the formation of pitches due to laser ablation, which was ascribed to the high concentration of SBR in contact with the Cu foils with increased surface. Since the recipe was identical in all cases, therefore a higher concentration of SBR at the interface between graphite particles and Cu foil meant that a relatively low concentration of SBR in the rest of the anode bulk. The nano-sized carbon black particles are usually associated with SBR, the segregation of SBR along the cross-section of anode film would contribute to a corresponding distribution or segregation of CB. The re-distributions of those components in the anode bulk are not simply linear or monotonic, which results in some complicate changes to electrochemical performances of electrodes. To acquire the accurate distribution of those components cross the anode film further characterization such as element mapping through EDAX technique are needed, which was not included in this study. It is obvious that the introduction of laser structured Cu foils changed the distribution of some components especially in small amount in the anode films during drying process. A comprehensive evaluation on electrodes on structured metallic foils from interface to electrode bulk is necessary to find the optimized/compromised processing and configuration parameters. Such a situation is usually encountered in the mass production of lithium-ion batteries, when the good manufacturability which high concentration of binder favors and high energy density which high concentration of active materials favors are both exerted as two controversial constraints to one recipe. Besides this, the adsorption behavior of binder to the structured pitches formed through laser ablation needs to be investigated in depth to provide a theoretical basis for the application of laser ablation technology in the production of lithium-ion batteries.
In summary, ultrafast laser ablation has been successfully applied for the surface modification of Cu foil for preparation of anode films for lithium-ion batteries. Improved adhesive force is confirmed in anode films on laser structured Cu foils. The best electrochemical performance as well as the adhesive force between anode composites and Cu foil is found in the dot period of 50 µm. This is probably due to the improved Li-ion diffusion resulting from the preferred orientation of the graphite particles along the c-axis, i.e. perpendicular to the Cu foil in the dried anode film. The potential of this ultrafast laser ablation technique for lithium-ion battery improvement merits further investigation.
Ningxin Zhang and Atanaska Trifonova from AIT acknowledge the financial support from the Austrian Ministry of Transport, Innovation and Technology (BMVIT). KIT has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant agreement No. 644971. In addition, the support for laser materials processing by the Karlsruhe Nano Micro Facility (http://www.knmf.kit.edu), a Helmholtz research infrastructure at KIT, is gratefully acknowledged.
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