Scalable processing and capacity of Si microwire array anodes for Li ion batteries
© Quiroga-González et al.; licensee Springer. 2014
Received: 18 May 2014
Accepted: 16 July 2014
Published: 21 August 2014
Si microwire array anodes have been prepared by an economical, microelectronics compatible method based on macropore etching. In the present report, evidence of the scalability of the process and the areal capacity of the anodes is presented. The anodes exhibit record areal capacities for Si-based anodes. The gravimetric capacity of longer anodes is comparable to the one of shorter anodes at moderate lithiation/delithiation rates. The diffusion limitation of the lithium ions through the electrolyte in depth among the wires is the limiting factor for cycling longer wires at high rates.
82.47.Aa; 82.45.Vp; 81.16.-c
KeywordsSilicon microwire Battery anode High storage capacity Scalability Lithium ion battery Micromachining
With the first technique, nanowires usually in a random arrangement are obtained. This production process is limited with respect to the wire density, diameter control, wire length, and array stability. Moreover, an efficient low-resistivity connection to a current collector is not easy with this technique. Method 2 overcomes some problems of technique 1, and may be easier than method 3 from a process point of view, but has a number of limits with respect to optimizing the array geometry and attaching to a current collector. For the moment, there are no reports of pores or wires with modulated diameter by method 2, and thus, for the moment, it is not possible to fabricate interconnected wires forming a free-standing array of long wires. Having a free-standing array is important for the deposition of a mechanically stable metal contact at one side.
A new concept of Si anodes has been developed by technique 3, which consists of an array of Si microwires embedded at one end in a Cu current collector. The capacity of the anodes is very stable over 100 cycles and breaks all the records when considering the capacity per area (areal capacity). In the present work, the scalability of the production process will be discussed. As will become clear in the following lines, the capacity of the anodes is also scalable, with certain limits in the cycling rate.
Battery cycling tests were performed using half-cells, with Li metal as counting and reference electrode. The separator was a glass fiber filter from Whatman (Piscataway, NJ, USA), with pores of 1 μm. The electrolyte was LP-30, consisting of dimethyl carbonate and ethylene carbonate (1:1) plus 1 mol/L of LiPF6. The tests were done with a BatSMALL battery charging system from Astrol Electronic AG (Othmarsingen, Switzerland). The anodes were cycled in a galvanostatic/potentiostatic mode, for which the voltage limits 0.11 V for lithiation and 0.7 V for delithiation were set. By this mode, when the voltage limit is reached, the cycling is switched to potentiostatic mode, and this mode finishes when the current has decreased to 10% of its initial value or when the capacity limit is reached.
SEM observations were performed with an Ultra Plus SEM from Zeiss (Oberkochen, Germany).
Results and discussion
Performance limitations after scaling
The amount of Li used for the formation of the solid electrolyte interface (SEI), normalized to the weight of Si, also scales when scaling the size of the wires. The sum of the irreversible Li losses (difference between the lithiation and delithiation capacities) during the first four cycles amounts to 1,606 mAh/g for the short wires and 3,087 mAh/g for the long wires (1.92 times the value for short wires). The SEI forms mainly during these first cycles, being the losses minimal afterwards. Considering the active portion of the wires with lengths 70 and 130 μm, the scaling factor is 2, value very close to the value 1.92 of the proportion of SEI. Thus, one may say that the SEI scales with the length, but tests with other wire lengths are necessary to confirm the theory. For the moment, the reason of this scaling is not clear. The SEI is an important structural component of the anode, which may be a decisive factor for the mechanical stability of the anode. Nevertheless, the amount of Li necessary to form it has to be carefully considered when scaling up the length of the wires; one needs an excess of Li of the order as the scaling factor.
Producing Si microwire anodes out of macroporous Si is a fully scalable process. Mainly, the current for the electrochemical processes has to be scaled according to the desired area of the anodes. Having longer wires enables the storage of larger amount of charge per area (areal capacity), while larger anode areas represent larger amounts of active material and thus higher total capacities.
Scaling up the capacity pays, however, with a demerit in the performance of the anodes. Due to diffusion limitation of Li when scaling up the length of the wires, the capacity fades monotonically when cycling at high rates. On the other hand, the amount of Li necessary for the formation of the solid electrolyte interface scales up with the scaling factor.
EQG is a professor for materials science at the University of Puebla. He led the project for the development of high capacity Si wire anodes for Li ion batteries at the University of Kiel (‘general materials science’ group) until 2013. He is also a specialist in the synthesis and characterization of photoactive materials and microstructured electrodes for Li ion batteries. JC is a senior scientist in materials science. Since 1993, he coordinates the academic and scientific activities of the ‘general materials science’ group of the Institute for Materials Science of the University of Kiel. He is an expert in electrochemical pore etching in semiconductors, FFT impedance spectroscopy, and general characterization of solar cells. HF is a professor for materials science at the University of Kiel. He is the leader of the ‘general materials science’ group of the Institute for Materials Science. He is one of the co-finders of the electrochemical etching process of pores in n-type Si in 1990. His expertise includes silicides, electrochemical processes with semiconductors, and solar cells.
The authors acknowledge the German Federal Ministry of Education and Research (BMBF) for the economical support provided through the ‘AlkaSuSi’ project. The company Siltronic AG is also gratefully acknowledged for providing us Si wafers for the experiments.
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