Nanosecond and picosecond laser structuring of electrode materials for lithium-ion batteries
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Nanosecond and picosecond laser structuring of electrode materials for lithium-ion batteries Robert Kohler1, Johannes Proell1, Heino Besser1, Maika Torge1, Steffen Scholz2, Todor Dobrev2, Sven Ulrich1, Wilhelm Pfleging1,3 1 Karlsruhe Institute of Technology, Institute for Applied Materials (IAM-AWP) P.O. Box 3640, 76021 Karlsruhe, Germany 2 Manufacturing Engineering Centre, Cardiff University, CF24 3AA, UK 3 Karlsruhe Nano Micro Facility, H.-von-Helmholtz-Platz 1, 76344 Egg.-Leopoldshafen, Germany ABSTRACT A comparative study for picosecond and nanosecond laser structuring was performed in order to identify structure geometries and dimensions that efficiently reduce the significant volume changes during electrochemical cycling of SnO2, a promising anode material. Line structures with widths of 20 µm could significantly improve cycling stability of 3 µm thick magnetron sputtered SnO2 thin films. A reduction of structure size led to further improvement of capacity retention. Free-standing conical micro-structures exhibited the best cycling behavior. INTRODUCTION The development of high performance micro-batteries is of major importance for the ongoing miniaturization of mobile electronic devices [1,2]. Lithium-ion batteries have shown great potential as light weight energy storage devices, due to their high energy density, power density and cycling stability [3]. The modification and improvement of electrode materials play a dominant roles in supplying decisive electrochemical properties [4]. SnO2 is one promising anode material for lithium-ion batteries [5] and is the focus of this work. The reaction mechanisms that occur during electrochemical cycling of SnO2 are well known from literature [6]. The two different reaction steps can be described by the following reaction equations [6]: SnO 2 + 4Li + 4e − ⎯ ⎯→ 2Li 2 O + Sn
(1)
⎯→ Li 4.4Sn Sn + 4.4Li + 4.4e − ←
(2)
In a first reaction step, metallic tin is formed within a Li2O matrix. This reaction occurs during the first charging cycle of the battery. It is assumed that this matrix partially compensates for the large volume expansion of up to 359 %, which results from the formation of the intermetallic phase Li4.4Sn in a second reaction step. The theoretical capacity resulting from both reactions therefore, reaches 1491 mAh/g. In this case, statistically an amount of 8.4 lithium atoms can be assigned to one tin atom. Yet, the formation of Li2O is irreversible. Thus, a theoretical capacity of 782 mAh/g is available for battery operation, which can be obtained for the alloying of Sn to Li4.4Sn. Nevertheless, there are still problems with irreversible capacity loss when using SnO2 as anode material. The huge change in volume leads to the formation of cracks
and to a pulverization of the electrode combined with a subsequent reduction of available active material due to loss of electrical contact [7]. The goal of this research was to investigate the influence of different structure designs on the performance of SnOx thin films anodes. By reducing the structure dimensi
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