Silicon Nanowires: Pushing Energy Storage Capacity in Li Based Battery Systems
Authors: Grube Matthias , Krause Andreas , Weber Walter M. , Mikolajick Thomas , Dörfler Susanne , Piwko Markus , Jaumann Tony , Wisser Florian Michael , Langklotz Ulrike 
Published: February 2020

More than ever, there are increasing requirements for rechargeable energy storage in today’s mobile society, which are accompanied by a high demand in flexibility. This reflects in a large field of applications ranging from low power consumption devices like portable electronics up to high power draining systems like electrical powered vehicles. Li-Ion batteries are more and more dominating the market for these applications. Especially, the high power sector including electromobile applications shows a tremendous growth. In this sector, new materials are needed, which result in battery systems with high energy, high power densities and increased lifetime. One promising candidate is silicon because of its enormous theoretical capacity of nearly 4000 mAh/g­Si [1]. Unfortunately, a large volume expansion of 400% accompanies the charging of this anode material with Li ions [2], which leads to a disintegration of the electrode after cycling. Using nanostructures is one way of relaxing stress induced by the expansion. For example, amorphous Si films in the nanometer range show promising long term performance during slow cycling. Unfortunately, they are uninteresting for rapid charging/discharging in electrical vehicles [3]. Another disadvantage is the low capacity and low energy density due to the small amount of active material. Si Nanowires (SiNWs) are able to circumvent the drawbacks of thin films. Their inherent geometry possesses the ability to accommodate for the volume expansion stress [4, 5], (fig. 1, left) and deliver sufficient active material for a reasonable area capacity. A process was developed to integrate SiNWs and its advantages on various different current collectors [4]. The electrochemical investigation of the anode showed an increased lifetime at a high capacity approaching values with relevance for commercial application (fig. 1, right). The anode has a starting capacity of about 4000 mAh/g with respect to Si which is close to the theoretical value. The capacity still remains at more than 2000 mAh/g(Si) after more than 200 cycles. The change of morphology before and after cycling of these structures has been investigated with various methods. Those experiments point to a strong SEI formation, which is mainly responsible for the decrease of capacity. Furthermore, they show a remaining SiNW mesh after various cycles, no immanent degradation was seen. Additionally, the performance of the anode will be shown in a full cell setup, which unfolds insides related to real world battery operation.
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Fig. 1: Charging and discharging of Si-Anodes. Left: Sketch of the volume expansion and contraction during lithiation and delithiation of Si-Nanowires. Right [4]: The integration of Si-Nanowires as anode material leads to starting capacity of almost the theoretical value, after more than 200 cyles the capacity remains at 2000 mAh/g, which is still 5 times higher than the standard carbon electrode [6].

Figure 1