Nanostructured Tin-based Alloys Composites using Deep Eutectic Solvents as Electrolytes

Metal and alloys electrodeposition from aqueous electrolytes is restricted due to the narrow electrochemical window and hydrogen evolution. To overcome these disadvantages, over the past years, ionic liquids (ILs) and deep eutectic solvents (DES) based on choline chloride have been successfully applied for the electrodeposition of different metals. Tin (Sn) layers applied to automotive or decorative plating are thought of as ecological alternatives to exchange lead and nickel/chromium coatings. Over the past few years, the attention drawn by metallic alloys and composites, namely Sn alloys (nickel, indium, copper, zinc...) and Sn-carbon materials composites, has increased due to the possibility of applying these materials as anodes for lithium-ion batteries. This review will highlight the leading research regarding the electrodeposition of Sn and several alloys and carbon composites, emphasizing the morphological changes of the alloy combinations using DESs as electrolytes. Author


Introduction
The electrodeposition of metals and metallic alloys has been used for extensive range of applications, such as electroplating (Zhang and Hoshino 2014), electrowinning (Schlesinger et al. 2011b) and electrorefining (Schlesinger et al. 2011a). The electrodeposition technique is used in various domestic components to technological devices to improve the appearance and also to protect against corrosion (Sobha Jayakrishnan 2012). Metals and alloys electrodeposition process relies on the metal ions reduction from the electrolyte solutions on the top of the surface to be coated (Tarditi, Bosko, and Cornaglia 2017). Figure 1 presents the principle behind the electroplating process. An electrodeposition cell consists of an anode (positive charge), a cathode (negative charge), an electrolytic bath, and a current/voltage source (Tarditi, Bosko, and Cornaglia 2017). Reduction and oxidation reactions occur at the cathode and anode, respectively, where the cathode is the conducting substrate on which the electrodeposition occurs, and the anode can be either soluble or inert. The overall reactions (Tarditi, Bosko, and Cornaglia 2017) taking place during electrolysis can be represented as: at the cathode (M Z+ (aq) + ne - M(m), at the soluble anode (M'(m)  M' Z+ (aq) + ne -), and at the insoluble anode (H2O(l) 2H + (aq) + ½ O2(g) + 2e -), where M and M' can be the same or different materials.  Tarditi, Bosko, and Cornaglia (2017), with permission from Elsevier) The electrodeposition method applied by the industrial sector is mainly based on aqueous systems as a result of the high solubility of electrolytes and metal salts present in water, which can lead to solutions with high conductivity (Smith, Abbott, and Ryder 2014). However, water presents a narrow potential window, making it not ideal for the deposition of metals with low current efficiencies. To overcome this problem, ionic liquids (ILs) are drawing much attention as an alternative for aqueous solutions in many electrochemical processes. This is mainly due to the wide potential window, high ionic conductivity and thermal stability, alongside low vapor pressure (Endres, Abbott, and MacFarlane 2008).

Electrolyte -Which One Suits Best for Electrodeposition?
Ionic liquids (ILs) were firstly presented by Habboush and Osteryoung (1984) for the replacement of aqueous baths, reducing the effect caused by hydrogen evolution. The main difference between these two electrolytes is their internal composition. ILs are entirely composed of ions, making some of them very viscous at room temperature, not being very practical to use. Also, ILs present high costs and are very susceptible to the presence of water (López-León, Ortega-Borges, and Brisard 2013). There has been an increase in research about ILs throughout the years due to the necessity to improve the performances of the ionic solvents and developing new technologies. Up to now, there has been a continuous increase in studies about this matter, with almost 5000 new research publications each year. More recently, Abbott et al. (2007a) introduced an alternative to ILs for metal electrodeposition, known as deep eutectic solvents (DESs). The first study regarding DESs was presented by the same author (Abbott et al. 2001), in which different quaternary ammonium salts were heated with zinc chloride, and the freezing points of the obtained liquids were measured. DESs are made through the complexation of a quaternary ammonium salt with a metal salt or with a hydrogen bond donor (HBD). The most common quaternary ammonium salt-based DES are those with amides, glycols, or carboxylic acids as HBDs (Abbott et al. 2001). DESs are considered as a new class of ILs because they present similar properties to ILs (Khandelwal, Tailor, and Kumar 2016;Lee and Row 2016), showing potential for the electrodeposition of metals and alloys (Abbott et al. 2007b;Abbott et al. 2008;Abbott et al. 2015;Anicai, Florea, and Visan 2011;Endres, Abbott, and MacFarlane 2008). The main difference between ILs and DESs is that the reduced melting points in DESs are achieved by the addition of HBDs, instead of changing the structure of the ions, in the case of the ILs (Chen et al. 2017). Figure 2 presents some examples of halide salts and HBDs. The main focus in this review will be given to the mixtures of choline chloride salt (ChCl) with ethylene glycol, malonic acid, and urea as HBDs for Sn and Sn-alloys electrodeposition.

Tin Electrodeposition
Tin coatings have been recognized as surface protectors from air oxidation (Walsh and Low 2016b). The brightness of tin coatings also provides decorative finishes, appealing to the eye and is one of the few metals that are suitable for contact with food while cooking. Tin electrodeposition presents a long history, and several authors dedicated their attention to it (Gabe 2014;Jordan 1995;Walsh and Low 2016b). Albeit the continuous increase in concern about health and environment tin is considered as an ecological alternative to replacing lead or nickel-chromium coatings (Han, Liu, and Ivey 2009;Jaén et al. 1991;Jellesen and Møller 2005). The melting point of tin is 223 °C, not making it easy to be used in its pure state (Smith, Abbott, and Ryder 2014). Combining Sn with other metals (e.g., nickel), forming alloys, makes them suitable for different applications, from surgical instruments, printed circuits boards, to electrical connectors, representing a substitute to chromium due to their excellent anticorrosive properties. Several research groups have been working with Sn alloys. This chapter will be dedicated to present the main results, published so far, regarding the electrodeposition of different Snalloys (nickel (Ni), indium (In), copper (Co), and zinc(Zn)) and Sn-carbon composites in DESs.

Sn and Sn alloys electrodeposition in DESs
This subsection is going to be focused on the electrodeposition of Sn alloys in DESs with electrochemical and morphological characterization. It will be mentioned the latest research regarding this matter, and with some alloys being addressed. The alloy electrodeposition can be a very complicated process. According to several authors (Endres, Abbott, and MacFarlane 2008;Smith, Abbott, and Ryder 2014;Walsh and Low 2016b), alloy coatings require a potential that is considered negative regarding the reduction potential for the most cathodic metals. Though the fact that the concept looks simple, it is moderately challenging to control the alloy composition during the electrodeposition process. This is most probably due to the significant difference in the redox potentials of the two metal ions or due to the differential deposition kinetics of the two metals (Endres, Abbott, and MacFarlane 2008).

Sn-Ni alloy electrodeposition
The electrodeposition of Sn-Ni alloy was studied in different research papers using different DESs ( The obtained Sn and Sn-Ni alloy deposits present good adherence and uniformity, which is shown in Figure 3 through the SEM analysis, revealing distinct morphologies. Sn deposit present bigger nuclei, compared to Sn-Ni alloy, indicating the significant change in morphology when more than one metal is being electrodeposited. Different HBDs are a case of study regarding its effect on metal electrodeposition. In the same study, Anicai et al. (2013) presented some results regarding the use of malonic acid as HBD.
The use of malonic acid as HBD showed good adherence, with an increase in the brightness of the coating and better corrosion performance. Very recently, it was presented by Rosoiu et al. (2019) for the first time, Sn-Ni alloy electrodeposition by pulse current using ethylene glycol as HBD. In this work, it was compared the efficiency between pulse and direct current. It was established that the pulse current electrodeposition method leads to a diminution of the crystallite size, through the study by Xray diffraction (Figure 4), showing an enhancement in the mechanical properties when compared to the direct current method.

Sn-In alloy electrodeposition
Indium (In) is a metal with high technological importance, finding different applications in flat panel display devices, semiconductors, and solar cells (Schwarz-Schampera and Herzig 2002). The electrodeposition of In in aqueous electrolytes was studied, but the hydrogen gas evolution leads to shallow current intensity (Piercy and Hampson 1975;Walsh and Gabe 1979 Meinshausen et al. 2016). In addition to being used as lead-free, Sn-In alloys are also used as protective and wear-resistant coatings (Kim et al. 2018;Lai, Yang, and Yuan 2017). Anicai et al. (2019) presented several experimental results regarding Sn-In alloy electrodeposition using DES as electrolyte with ethylene glycol as HBD. The In content found to increase with an increase of Sn: In molar concentration and increase in temperature. SEM analysis ( Figure 5) of the alloy coatings presented irregular particles using direct current, while at pulse current, a more homogeneous and compact deposit was obtained. Pulse current also allowed to reach higher currents, and due to that, the amount of In was higher. Corrosion tests were also performed. After 360 hours of continuous immersion in chloridecontaining aggressive medium, there was no change on the surface of the coatings. This shows that Sn-In alloy presents good characteristics against corrosion, being excellent for different electronic applications, using DESs as an electrolyte.

Sn-Cu alloy electrodeposition
Abbott et al. (2016) studied the Sn-Cu alloy electrodeposition in DESs (ethylene glycol and urea as HBDs). This study was followed with the electrochemical quartz crystal microbalance (EQCM), showing the real-time monitoring of alloy composition. They were able to successfully deposit Sn-Cu alloys from DESs. EQCM analysis presented that the alloy composition is not homogeneous throughout the alloy film. This research paper was more focused on the study of the amount of metal present in each step of the electrodeposition, showing that some brighteners that work in aqueous solutions for copper electrodeposition are also useful in DESs.

Sn-carbon composites electrodeposition
The utilization of carbon materials in electrodeposition for the formation of metal composites are becoming of great interest (Brandão et al. 2019;Pereira et al. 2017;Tocoglu et al. 2014;Wang et al. 2001;Xu et al. 2013a;Xu et al. 2013b;Zhu et al. 2011). Carbon nanotubes (CNTs) are made of graphene sheets rolled with a cylindrical shape, showing excellent properties such as high mechanical strength. Reinforcing metal deposits with CNTs or graphene presents high potential due to the extraordinary specific stiffness and strength of these carbon materials, representing a remarkable opportunity to develop new materials. This is an area that presents an exponential growth regarding the research interest, with an increasing number of articles being published each year, according to the Science Direct website. However, the incorporation of carbon materials, namely CNTs, into metal matrixes is still a challenge. Some research groups have presented successful results in the preparation of CNTs

Future Regarding Tin Electrodeposition
Sn electrodeposition on different substrates needs to be continuously studied to present better performance regarding the various applications. Several topics need to be considered in the future (Jordan 1995; Walsh and Low 2016a; Walsh and Low 2016b):  Size increase of the surface to be deposited;  Evolution of nucleation and growth morphology of the deposit, especially regarding the whisker growth;  In-situ analysis through Raman spectroscopy to study the mechanisms of nucleation and growth of tin deposits.

Application of Tin as Anode for Lithium-ion Batteries
The development of high-performance rechargeable lithium batteries is of the highest importance for portable electronics and electric vehicles. This stimulates the continuous search for new and innovative electrode materials with huge capacities, long life, and higher rate capability (C. . Graphite is being considered as the most commonly used anode material in Li-ion batteries due to the low cost, accessibility, and durability (Shafiei and Alpas 2011). However, there has been a growing interest in using metals, intermetallic, and alloys as alternatives. Sn film anode presents a higher value of theoretical capacity (962 mAh.g -1 ) compared to graphite (372 mAh.g -1 ) (Tarascon and Armand 2001). Metals with foam structure have been used as current collectors to improve the electrochemical performance of Sn structure. Pores can accommodate the volumetric expansion but also allows fast reactions caused by its high surface area (Nam et al. 2012). Metal foam presents several advantages, such as easy preparation and thickness control, and large surface area (Shin and Liu 2005). Ni foam is a low-density permeable material where 75 to 95% of its volume consists of void spaces (Chen and Xue 2014). The development of rechargeable lithium batteries is of great importance for portable electronic devices and electric vehicles (J. . Tin anode presents a massive drawback as it undergoes significant volume changes during cycling, leading to mechanical stress and loss of electrical contact and lower cycle life (Shafiei and Alpas 2011). To overcome this problem, the use of alloy materials has been tested. Different tin alloys (Sn-Ni-P, Sn-Ni, and Sn-Co) have been considered (Cheng and Shi 2005;Mukaibo, Momma, and Osaka 2005;Naille et al. 2007;Xia, Lin, and Li 2008), with different electrochemical activities versus Li/Li + , showing promising results with reducing volume expansion. Another approach is developing composite anode materials by mixing active elements with carbon materials, which was presented by several researchers (Arai and Fukuoka 2016; Choi and Yoon 2015; Dhanabalan et al. 2013;Polat et al. 2014;Tocoglu et al. 2014;Wang et al. 2009;Zuo et al. 2018), that may lead to an increase in conductivity and a higher control in the volume change. Uysal et. al (2014) presented the application of Sn-Ni/MWCNTs nanocomposites as anodes for Li-ion batteries, in which the effect of the molar ratio of Sn: Ni was studied. Results presented that increasing the co-deposited Ni amount, a substantial impact on the electrochemical performance is observed.
Recently, Ahn et al. (2019) used electrochemical co-deposition to develop a silicon-based anode using tin as supporting materials, named Sn/Si/O/C composite for application on lithium-ion batteries. Figure 8 presents the Sn/Si/O/C composite using SiCl4 and SnCl2 as silicon and tin sources, with propylene carbonate as an electrolyte. Sn composite presents a whisker structure, which can act as an electron pathway, offering an improved electrochemical high rate performance, as presented in Figure 9. The electrochemical performance of Sn/Si/O/C composite shows that this material has excellent potential as an anode for application in high energy density lithium-ion batteries.

Conclusions
This review has revealed that Sn alloy coatings deposited from DESs are becoming of great interest for different applications, namely as anodes for lithium-ion batteries. DESs and ILs present many similarities regarding the physical properties, making DESs an excellent substitute for ILs. However, the difference regarding the chemical properties between these two liquids means that DESs can be used in different applications, compared to ILs. It was reviewed some Sn alloys coatings and Sn-carbon composites developed in DESs and the morphological characteristics to understand the alloying effect as well as the presence of carbon materials on the Sn metallic matrix. The electrodeposition of metal alloys and composites using DESs as electrolytes are becoming of great interest since it can be used as anodes for lithium-ion batteries. Even though DESs is still an immerging class for many applications, it presented some exciting results when applied for lithium-ion batteries.
It is necessary to continue the research regarding metal electrodeposition using DESs, since there is still lack of information regarding their characteristics. Only a small number of DESs formulation have been studied so far. With the increasing interest in using these type of electrolytes, there is a huge potential to expand the study to other type of salts and HBDs, and further increase the applications of these solvents.