Production of sn based anodes via electrochemical method and their evaluation of usage of Sn based anodes in lithium ion batteries

In today, there is a need for environmentally sensitive energy storage with a high energy density, lightweight, long life and short-term rechareability. With these features in today's standarts, batteries have started to be widely used in electric vehicles, health, military areas. Batteries are called current collectors, which convert electrical energy into chemical energy and, if necessasy, convert chemical energy back to electrical energy. Secondary batteries are often prefered in recent years due to their ability to be recharged. These batteries are mainly silver-zinc, nickel-zinc, nickel-hydrogen, nickel-cadmium, nickel-metal hydride and lithium ion batteries. Lithium ion batteries are distinguished among the alternatives by their long life, their wide operating temperature range, their ability to charge quickly, their low memory effect and their high energy density. Lithium ion batteries contains anode, cathode, seperator and electrolyte. However, since an anode and cathode primarily effect battery capacity, studies have focused on anode and cathode. Lithium metal is used as the first anode material in lithium ion batteries. However, the fact that lithium metal has a flammable property in the case of air and water contact limits the use of this metal as an anode. Graphite is first commercial anode because it is abundant and cheaper in natüre. With the progress of the technology, the storage capacity of graphite with a theoretical discharge capacity of 372 mAh/g is insufficient. In this contex; scientists search a new metal-based (LixM) anode material, which is capable of forming an alloy with Li+ and has a theoretical capacity higher than graphite. The main anode materials of new generation can be said as Sn, Al, Sb, Si, Mg. Among these materials, Sn is predominant among others due to its high discharge capacity. (4.4Li+ per unit Sn atom). However; Sn-based anodes have various disadvantages lithium ion battery applications as well as these advantages. According to this; due to the lithium ions entering the structure after the 1 st discharge of the Sn-based anodes, the volüme change of 260% occurs in structure. Accordingly, the tin electrode peels and removes from substrate. On the other hand, electron exchange can not be take place efficiently during the alloying reactions due to low electrical conductivity of Sn (9.17 × 106 Sm-1), resulting in low capacity( electrical conductivity of C=1,00×108 Sm-1). Furthermore, due to the fact that Sn has a low Li+ diffusion coefficient (10-16-10-13cm2s-1), the input and output of lithium to the structure is slow. This leads to a decrease in the Culombic efficiency. Finally, since the grain boundary diffusion of Sn-based materials at room temperature is higher than the volumetric diffusion, the excess Sn particles in the grain boundaries form a whisker. These formations constitute an obstacle for electron transitions and reduce reaction efficiency. Various approaches have been adopted to solve these problems in Sn based anode materials. These approachs are mainly composed of two main headings: compositional and morphologically. In compositional approaches, it is mainly aimed at increasing the electrical conductivity of the electrode by adding another metal into the structure. At the same time, this metal addition is expect to form intermetallic phases in the structure, and these phases are expected to cause buffering effect in the face of volumetric expansion problem of Sn. For this purpose; Ni, Cu, Ag, Co and Fe are at the beginning of the metals added to the structure. In the other improvements made on morphology; the grain size of electrode material is reduced or the porous structure is made in the structure. The purpose of these morphological approaches is; to provide more void space for volume expansion in the struture and to make Li+ input output more confortable. In this study; Sn, M/Sn/M and porous M/MSn/M multilayered films will be produced in order to solve problem experienced in Sn-based anode materials (M: Cu, Ni). The reason for using Cu and Ni in the Sn-based anode structure is that they have inactive behavior exhibits against lithium and high electrical conductivity. In three layer M/Sn/M anode structure; it is aimed to increase adhesion of Sn to the substrate and increases electrical conductivity of the electrode with the bottom layer. It is intended to provide mechanical tolerance against volumetric expansion during cycling with the upper layer. In the M/MSn/M structure, it is aimed to increase the electrical conductivity of Sn by metal addition in the middle layer and to make reaction more efficient. It is desired to avoid the harmful effects of volumetric expansion during the cycle by providing sufficient free space in the structure with porous M/MSn/M structure. The samples were produced by electrochemical coating method. The electrochemical coating method has been chosen because it supllies rapid production even at low temperatures, production is cheap and easy to apply. Chemical dealloying technique is used for porous anode production. In addition, with the aim of observing the effect of heat treatment on electrochemical performance; heat treatments were applied to CuSn and NiSn based specimens at 200ºC and 215ºC for 24 hours, respectively. The pre-test and post-test surface SEM images, XRD analyzes, 50 cycle capacity-cycle curves, 3 cycle cyclic voltameters and Nyquist curves of the samples were taken. At the conclusion of the results; the single-layer Sn film failed after the 2cycle with a sudden drop to 40 mAsa /g capacity values. On the other hand, the best electrochemical performance in CuSn based anodes was obtained from the porous-annealed Cu/CuSn/Cu film with a discharge capacity of 395mAh/g after 50 cycles. For NiSn based anode films, the best electrochemical performance was obtained with the porous-annealed Ni/NiSn/Ni film with discharge capacity of 445 mAsa/g after 50 cycles. As a result; the multi-layer structure suppresses the volumetric expansion, enhances the ability of the Sn-based films to adhere to the substrate and the electrical conductivity. The deleterious effects of the volumetric expansion are eliminated with the porous structure. Intermetallics formed by heat treatment in the structure have a buffering effect against the volumetric expansion. Accordingly, it has been found that the multi-layered porous structure and heat treatment provide capacity maintenance and improvement at capacity.