(Battery Division Student Research Award) Production and Characterization of Si Based Thin Films as Negative Electrodes For Secondary Lithium Ion Batteries
Authors: Polat Karahan Billur Deniz 
Published: February 2020

Big hurdle on portable equipment and electrical vehicles is storage capacity. Use of higher energy density LIBs would provide longer driving range with low cost and promise a green world for future. Thus, researchers are working to find new higher energy density materials. Recently, silicon (Si) based electrodes have been considered promising because Si offers high capacity (3579mAh/g). However, adaptation of Si into battery applications has been difficult due to high volumetric change that causes electrode to fracture and degrade after few cycles. Use of nanostructured composite Si electrodes is a solution as they can expand and contract without breaking. Moreover, their high surface area reduces polarization, hence improving rate performance. But, finding a cost-effective and environmentally friendly way to create these nanostructured composite electrodes is a challenge.
Thin film technology uses vacuum assistance, its thermodynamics gives an opportunity to invent new active phases. Moreover, different than lamination process; in thin film technology electrodes are deposited directly on a current collector in one step without a need of binders or any conductive additives. Finally, being environmentally benign and suitable for mass production this technology would be adapted for having electrodes, electrolytes and even complete solid state designs.
Our motivation is to use these advantages of thin film technology to design new anodes. So, we have used glancing angle electron beam (e-beam) evaporation to form different shaped nanostructured Si films (nanowires, chevrons, C-shaped structures and helices). The adhesion is one of the main concerns for thin film electrode; therefore, in our study innovatively ion assisted deposition has been adapted into e-beam to promote the adherence (Figure 1). In addition, to compare the results magnetron sputtering is used for producing Si based films. To improve both the cycle and the rate performance of the electrodes, compositionally graded nanostructured Si based films (rich in Cu at the bottom and rich in Si on top) are fabricated by controlling the deposition rates of Cu to Si atoms during the deposition.
In our study once we have achieved to deposit thin films, we have characterized their morphologic and structural properties by scanning electron microscope (SEM) and x-ray diffractometer. Then, we have applied extensive electrochemical analyses (electrochemical impedance spectroscopy, cyclic voltammetry, galvanostatic tests) to understand their performances. Plus, volumetric changes occurred in SiCu nanowires and solid electrolyte interface formation (SEI) in the 1stdischarged and 1stcharged samples have been analyzed by electrodilatometer and X-ray photoelectron spectroscopy (Figures2,3). Furthermore, an in-depth ex-situ morphological investigation via SEM after several cycles has been done to justify the lithiation mechanism.
Detailed electrochemical test results reveal that the existence of Cu is important since it creates electron conductive pathway along the film due to its high electron conductivity, it increases mechanical tolerance of the film with its ductile properties, and it changes morphology as well as structure since it affects the nucleation mechanism of the coating when Cu is co-deposited with Si.
Indeed, this project creates a new application area for thin film technology in LIB world. The strategy of using thin film technology in electrode design opens a new gateway for the development of LIB technology.

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Figure 1