Single-Crystal Si Microwire Anodes for Li Ion Batteries and Their Necessary Conditioning during the First Charge/Discharge Cycles: A Voltammetry Study
Authors: Quiroga-González Enrique , Nöhren Sandra 
Published: February 2020

In the past years it has been proved that it s posible to make use of the high storage capacity of Si (4200 mAh/g) in real Li ion batteries. Micro- or nano-structuring the material has improved its mechanical properties, which a couple of decades ago were unthinkable for batteries (Si suffers of an expansion of up to 300 % when lithiated, causing its pulverization if it is in bulk). One of the most important concepts, due to its high areal capacity, is the one developed by us consisting of single-crystal Si microwire arrays with optimum spacing for allowing Si expansion. The fabrication method is unique and scalable, based on a series of electrochemical and chemical etching steps of Si wafers [1, 2].
It is known that Si could reach a very high capacity, but its performance is much dependent on the charging/discharging conditions. It has been seen that th first cycles must be done at slow rates of C/10 or slower, to assure the formation of a stable SEI and to minimize the mechanical stress in the wires [3]. Nevertheless, a systematic and fast analytic approach for confirming the descibed findings and particularizing to Si wire of specific dimensions.
Cyclic voltammetry is a technique commonly used for identifying redox pairs and the electric potentials at which electrochemical events occur. However, during this kind of test on Li ion batteries, the electrodes get lithiated/delithiated up to certain degree regulated by the mesuring time. In this way, this technique could be used for identifying different phenomena occurring during the lithiation/delithiation of Si.
Fort he tests of the present report, Si microwires of 1.8 µm in diameter and 60 µm in length have been prepared. 4.5 mg of the material have been mixed with an equal weight of Carbon black and 10 % of CMC, and have been casted on a Cu current collector to make anodes. The tests have been done in half cells, with Li metal as counter electrode.
Cyclic voltammograms of the anodes can be observe don the left figure. The sweeping rate for the measurement has been 0.1 mV/s in the range of 20 mV to 1 V. Fort the analysis, special attention has been paid to the delithiation process, considering that the SEI is not delithiated (delithiation just related with Si is observed). The right figure presents the calculated values of capacity and C rate from the data. The C rate has been calculated from the amplitude of the first delithiation peak as (height [mA]/weight [g])/4200 mAh/g. The total delithiation capacity has been obtained from the area under the whole positive current curve as (Area [mA*V]/(rate [V/h] * weight [g]). As can be observed the values tend to stabilize after 5 cycles, indicating that after cycle 5 the lithiated/delithiated portion (about 3.7 %) reaches a stable amorphous state. The final C rate is 0.0344 (about C/29), very slow, but necessary for self-organization at the given potential of the delithiation peak. After cycle 5 the capacity does not increase since the measurement time, given by the sweep rate, is fast (2.7 h) in comparison to the ideal C rates (29 h).
The implications of the right conditioning in the performance of the Si microwire anodes will be discussed in detail in the paper at the conference. Cyclic voltammetry is a simple and fast technique with high analytic potential for different unrevealed phenomena.
[1] E. Quiroga-González, E. Ossei-Wusu, J. Carstensen, H. Föll, “How to Make Optimized Arrays of Si Wires Suitable as Superior Anode for Li-ion Batteries,” J. Electrochem. Soc. 158(11) (2011) E119-E123.
[2] E. Quiroga-González, J. Carstensen, H. Föll, "Scalable processing and capacity of Si microwire array anodes for Li ion batteries", Nanoscale Res. Lett.9 (2014) 417.
[3] E. Quiroga-González, J. Carstensen, H. Föll, “Good cycling performance of high-density arrays of Si microwires as anodes for Li ion batteries,” Electrochim. Acta 101 (2013) 93-98.

Figure 1