materials Article Fabrication of Si 3 N 4 @Si@Cu Thin Films by RF Sputtering as High Energy Anode Material for Li-Ion Batteries Hocine Merabet 1, * , Yannis De Luna 2 , Khadiga Mohamed 1 and Nasr Bensalah 2, *   Citation: Merabet, H.; De Luna, Y.; Mohamed, K.; Bensalah, N. Fabrication of Si 3 N 4 @Si@Cu Thin Films by RF Sputtering as High Energy Anode Material for Li-Ion Batteries. Materials 2021, 14, 2824. https://doi.org/10.3390/ma14112824 Academic Editor: Rosalinda Inguanta Received: 3 May 2021 Accepted: 20 May 2021 Published: 25 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Mathematics, Statistics and Physics, College of Arts and Sciences, Qatar University, Doha P.O. Box 2713, Qatar; km1306799@gmail.com 2 Department of Chemistry and Earth Sciences, College of Arts and Sciences, Qatar University, Doha P.O. Box 2713, Qatar; yd1601559@student.qu.edu.qa * Correspondence: merabet@qu.edu.qa (H.M.); nasr.bensalah@qu.edu.qa (N.B.) Abstract: Silicon and silicon nitride (Si 3 N 4 ) are some of the most appealing candidates as anode materials for LIBs (Li-ion battery) due to their favorable characteristics: low cost, abundance of Si, and high theoretical capacity. However, these materials have their own set of challenges that need to be addressed for practical applications. A thin film consisting of silicon nitride-coated silicon on a copper current collector (Si 3 N 4 @Si@Cu) has been prepared in this work via RF magnetron sputtering (Radio Frequency magnetron sputtering). The anode material was characterized before and after cycling to assess the difference in appearance and composition using XRD (X-ray Powder Diffraction), XPS (X-ray Photoelectron Spectroscopy), SEM/EDX (Scanning Electron Microscopy/ Energy Dispersive X- Ray Analysis), and TEM (Transmission Electron Microscopy). The effect of the silicon nitride coating on the electrochemical performance of the anode material for LIBs was evaluated against Si@Cu film. It has been found that the Si 3 N 4 @Si@Cu anode achieved a higher capacity retention (90%) compared to Si@Cu (20%) after 50 cycles in a half-cell versus Li + /Li, indicating a significant improvement in electrochemical performance. In a full cell, the Si 3 N 4 @Si@Cu anode achieved excellent efficiency and acceptable specific capacities, which can be enhanced with further research. Keywords: Li-ion batteries; silicon-based anode; thin film; RF sputtering; performance 1. Introduction Due to the continuous need for high-performance batteries, research interests in energy storage materials have become more significant as we move towards sustainable energy applications. Li-ion technology has remained at the forefront of energy research due to lithium’s high, variable discharge rate and specific capacity, as well as its small size [1] and relatively long cycle life, among others. The concept of Li-ion technology was originally proposed by Armand [2] as a method of improving the safety aspects of Li-based batteries through the use of intercalated electrodes. Today, the most commonly used Li-ion battery (LIB) involve intercalation materials, typically consisting of a cathode in the form of Li x M y O 2 (M = Co, Ni, or Mn) [3,4] and a carbon material (i.e., graphite) [5] as the anode. Layered lithium metal oxides provide ≈170 mAh/g, whilst graphite has a low theoretical specific capacity of 372 mAh/g. This combination provides a specific energy of ≈150 Wh/kg [6], making it suitable enough for the small-scale applications it is found in today. However, large-scale applications, such as energy storage systems and electric vehicles, demand batteries with a much higher capacity and longer cycling capabilities [7]. Silicon as an anode material for LIBs presents a more attractive set of attributes that make it the best alternative to graphite. Aside from being the second most abundant element in the Earth’s crust, silicon is considered to be environment-friendly and cost- efficient [8,9], as opposed to lithium. Furthermore, Si offers a high specific capacity of about 4200 mAh/g based on the formation of Li 22 Si 4 at high temperatures [8,10,11], which is over 10 times greater than that of graphite. This is due to the ability of silicon to Materials 2021, 14, 2824. https://doi.org/10.3390/ma14112824 https://www.mdpi.com/journal/materials