Paper review

1. Lithium Distribution in Structured Graphite Anodes Investigated by Laser-Induced    Breakdown Spectroscopy

Yijing Zheng 1,*,Lisa Pfäffl 1,Hans Jürgen Seifert 1 and Wilhelm Pfleging 1,2
1

Institute for Applied Materials—Applied Materials Physics (IAM—AWP), Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany

2

Karlsruhe Nano Micro Facility (KNMF), H.-von-Helmholtz-Pl. 1, 76344 Eggenstein-Leopoldshafen, Germany

  For the development of thick film graphite electrodes, a 3D battery concept is applied, which significantly improves lithium-ion diffusion kinetics, high-rate capability, and cell lifetime and reduces mechanical tensions. Our current research indicates that 3D architectures of anode materials can prevent cells from capacity fading at high C-rates and improve cell lifespan. For the further research and development of 3D battery concepts, it is important to scientifically understand the influence of laser-generated 3D anode architectures on lithium distribution during charging and discharging at elevated C-rates. Laser-induced breakdown spectroscopy (LIBS) is applied post-mortem for quantitatively studying the lithium concentration profiles within the entire structured and unstructured graphite electrodes. Space-resolved LIBS measurements revealed that less lithium-ion content could be detected in structured electrodes at delithiated state in comparison to unstructured electrodes. This result indicates that 3D architectures established on anode electrodes can accelerate the lithium-ion extraction process and reduce the formation of inactive materials during electrochemical cycling. Furthermore, LIBS measurements showed that at high C-rates, lithium-ion concentration is increased along the contour of laser-generated structures indicating enhanced lithium-ion diffusion kinetics for 3D anode materials. This result is correlated with significantly increased capacity retention. Moreover, the lithium-ion distribution profiles provide meaningful information about optimizing the electrode architecture with respect to film thickness, pitch distance, and battery usage scenario. 

2. Dissimilar laser welding of a CoCrFeMnNi high entropy alloy to 316 stainless steel

J.P. Oliveira a, Jiajia Shen a, Z. Zeng b, Jeong Min Park d, Yeon Taek Choi d, N. Schell c, E. Maawad c, N. Zhou e, Hyoung Seop Kim d 

  a

UNIDEMI, Department of Mechanical and Industrial Engineering, NOVA School of Science and Technology, Universidade NOVA de Lisboa, Caparica 2829-516, Portugal

b

School of Mechanical and Electrical Engineering, University of Electronic Science and Technology of China, Sichuan 611731, China

c

Institute of Materials Physics, Helmholtz-Zentrum Hereon, Max-Planck-Str. 1, Geesthacht D-21502, Germany

d

Graduate Institute of Ferrous Technology, POSTECH (Pohang University of Science and Technology), Pohang 790-794, South Korea

e

Centre of Advanced Materials Joining, Department of Mechanical & Mechatronics Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada

In this work, laser welding of a rolled CoCrFeMnNi high entropy alloy to 316 stainless steel was performed. Defect-free joints were obtained. The microstructure evolution across the welded joints was assessed and rationalized by coupling electron microscopy, high energy synchrotron X-ray diffraction, mechanical property evaluation, and thermodynamic calculations. The fusion zone microstructure was composed of a single FCC phase, and a hardness increase at this location was observed. Such results can be attributed to the formation of a new solid solution (arising from the mixing of the two base materials). Moreover, the incorporation of carbon in the fusion zone upon melting of the stainless steel also aids in the strengthening effect observed. The welded joints presented good mechanical properties, with fracture occurring at the fusion zone. This can be ascribed to the non-favourable, i.e., large grain size, microstructure that developed at this location. 

3. Effect of μ-precipitates on the microstructure and mechanical properties of non-equiatomic CoCrFeNiMo medium-entropy alloys

 Jae Wung Bae, Jeong Min Park, Jongun Moon, Won Mi Choi, Byeong-Joo Lee, Hyoung Seop Kim 

 Department of Materials Science and Engineering, Center for High Entropy Alloys, POSTECH (Pohang University of Science and Technology), Pohang 37673, South Korea 

Non-equiatomic Co17.5Cr12.5Fe55Ni10Mo5 (Mo5) and Co18Cr12.5Fe55Ni7Mo7.5 (Mo7.5) medium-entropy alloys were synthesized by vacuum induction melting, cold rolling, and subsequent annealing treatment at various temperatures (900–1200 °C) and they were investigated to exploit the precipitation strengthening in addition to solid solution strengthening of alloys. The effect of annealing temperature and Mo content on the microstructure and mechanical properties are systematically analyzed. From the microstructural analysis, a Mo-rich μ phase is observed in the face-centered cubic (fcc) matrix. Increasing the Mo content and low annealing temperature enhance the formation of μ phase, which is consistent with the thermodynamic calculation results. The formation of μ phase effectively enhances the strength of the Mo7.5 alloy by precipitation strengthening, and suppression of grain growth and recrystallization by Zener pinning effect. These lead to superior combination of tensile strength as high as 1100 MPa and large ductility. Our results provide insights not only into μ-phase strengthening of fcc-structured alloys, but also into the future development of high-performance MEAs