Novel electrospun anode materials for Lithium ion batteries
This work on batteries is done mainly by focusing on synthesis of novel anode materials and to study their features as an efficient energy storage device. We are familiar with different types of batteries, of them Lithium ion batteries hold the top position. It's application in various fields such as mobile phones, laptop and even as energy storage device in vehicle make them more of a matter of discussion. So, the thought of implementing new materials into the place of electrodes instead of conventional materials may add up to its applicabilty to various fields. The material that is having a high specific capacity can enhance the overall performance of the battery. Therefore, we are synthesizing materials through an efficient fiber synthesizing technique, namely, electrospinning, and they are used in Lithium ion battery as cathode. Once complete lithiation of the material is done, it can be used as Li source(Anode) in another battery. Also, an alloying mechanism is adopted in the working of battery. Sn and Sb mixed in a suitable polymer in DMF serves as the precursor solution for electrospinning. Morphological studies as well as electrochemical studies reveal the capability of the composites as excellent electrode materials. Another material has also been synthesized through electrospinning, which is a Cobalt oxide material in CNF is also a promising substance as an electrode.
Keywords: Electrospinning, Sn and Sb, specific capacity, Cobalt oxide, CNF
|LIB's||Lithium ion batteries|
|CNF||Carbon Nano Fibers|
In the current world scenario of exponentially rising need for renewable energy sources, the application of electrochemical energy as an equally replaceable energy source is seemingly accepted. The present fuel economy based on fossil fuels is at serious risk due to an array of factors. The urgency for energy renewal requires the use of clean energy sources at a much higher level than that presently in action. Electrochemical systems, such as batteries and super capacitors1, that can effectively store and deliver energy on demand provide power quality and efficiency. Though photovoltaics2 is a relatively new field, it has become a rapidly growing and increasingly significant renewable alternative to conventional fossil fuel electricity generation in terms of its applications. The role of batteries in Renewable Energy Power plants (REP’s3) is attributed to their content in energy efficiency and life time. Their applicability in daily life starts from tiny batteries in mobile phones, cameras and laptops from larger batteries in electric vehicles. Thus, in this discussion we are dealing with batteries, especially their material aspect through what we are able to bring about changes in their capability as an efficient energy source.
A battery is a device that converts the chemical energy present in its active materials directly into electrical energy by means of an electrochemical redox reaction. An electrochemical battery consists of i) A Cathode ii) An Anode and iii) Electrolyte. The anode or the negative electrode is the one which gives up electrons to the external circuit and gets oxidized during the reaction. Cathode is the positive electrode that accepts electrons from the medium for the transfer of charge, as ions, inside the cell between the anode and the cathode. It is typically a liquid, water or any organic solvents, with dissolved salts or acids or alkali to impart ionic conductivity. Metals are commonly used as anode materials. For example, Zn, Li (Being the lightest metal, with a high value of electrochemical equivalence, it can be used as suitable and compatible electrode). The necessary conditions for a material to be a good anode include its efficiency as a reducing agent, stability, ease of fabrication, conductivity and coulombic output. Meanwhile the cathode must hold some other features like it should have a useful working voltage, should be stable when in contact with the electrolyte and it must be an efficient oxidising agent. Metallic oxides are the commonly used cathode materials. The electrolyte must have good conductivity and it should be non-reactive with the electrode materials. And it must not be electronically conducting as this would cause internal short circuiting. That is, it should conduct ions but not electrons,
The symbol used for representing a battery is as shown above. The negative end represents the anode while the positively charged end shows the cathode. The chemical reactions in a battery causes a build-up of electrons at the anode end, thus resulting in an electrical difference between the anode and the cathode. So, the electrons from the anode moves to the cathode and causes production of electricity. The electrolyte in the system helps this movement of ions internally. This electrochemical process occurring for a long time causing limited amount of power available in the battery. When we recharge the battery, by applying an external potential from outside, the direction of flow of ions can be changed; then the electrochemical processes happen in reverse and the cathode and anode are restored to their original states.
In the next sessions we will discuss more deeply about primary and secondary batteries as the work is upon the anode materials for Lithium ion batteries.
Primary and secondary batteries
Batteries are broadly classified into two types; primary and secondary. Primary or non-rechargeable battery is the one that can only be used once, usually called as “Use once and throw”. It only runs for a complete discharge cycle. As whole of the active material is consumed during its life time, it becomes useless. These are light weight in nature and usually much inexpensive. Since they are portable, they can be handled easily and are used in toys, watch and in photographic equipments. Whereas a secondary battery can be recharged electrically after discharge, to their original condition by passing current through them in the opposite direction to that of the discharge current. They are otherwise called “Storage batteries or Accumulators”. Secondary batteries are characterized by high energy density, high discharge rate, flat discharge curves and good low-temperature performance. Anyway, capacity of the secondary battery that is lost on standing can be restored by recharging. The oldest type of secondary battery is Lead-Acid battery, with an emf of 2V per cell, they are commonly used as storage battery in vehicles. But as Ni-Cd batteries were discovered, they began to replace the Lead-Acid batteries in small electrical appliances. Then came Nickel Metal Hydride battery due to the harmful effects caused by the Cd4 present in the other. Li-ion batteries, as one of the most advanced rechargeable batteries, are attracting much attention in the past few decades. They are highly advanced as compared to other commercial rechargeable batteries. In our discussion, the major role is assigned to Lithium ion batteries with lithiated electrode materials being our centre of attraction.
Lithium ion batteries (LIB's)
Amidst various secondary batteries, batteries based on lithium chemistry are very attractive as they provide the best opportunity to develop high energy density batteries. This is due to the fact that lithium is most electropositive (-3.04 V versus the SHE) and the lightest (equivalent weight = 6.94 g/mol specific gravity 0.53 g/cm3). Apart from the metal-ion rechargeable batteries, which function predominantly based on the conventional intercalation storage process, various other types of battery chemistries comprising purely of elemental electrodes (both anode and cathode) have gained importance in recent times. LIB's are the primary energy storage devices in communication, transportation and renewable energy sectors5. In commercial LIB's, the usual positive electrode materials are metal oxides or phosphates (LiCoO2)6 and the negative electrode is graphite. Both the positive and negative electrode materials react with Li via an intercalation mechanism. Li ions or atoms resides in interstitial sites within the host lattice and the insertion/ extraction of Li results in only small and irreversible structural changes in the host material. In a Li-ion cell, the cathode is the source of Lithium. In charging process, Li+ ions are removed from the cathode and get stored in the anode. Electrolyte serves as the ion transport medium from one electrode to the other. As a result of this, excess negative charges (electrons) move from the cathode to the anode through the external circuit to maintain electro-neutrality. Therefore, the voltage difference between cathode and anode increases while charging reaching the higher cut-off voltage of the battery. In case of discharging, the stored lithium-ions in the anode return back to the cathode and voltage difference between cathode and anode decreases till it reaches the lower cut-off voltage of the battery. One charging followed by one discharging step is termed as one cycle.
But the specific capacity of these materials is low due to the limited number of intercalation sites for Li ions within the host lattice. These Li-ion batteries are having a higher energy density than most other types of rechargeables. 3.7 V for Li ion vs. 1.27 V for a NiMH or NiCd battery. And, also once they are charged, they will retain their charge for a longer time than other types of batteries. Although these are smaller or lighter having a high voltage, these are more expensive than similar capacity batteries. Here, graphite acquires the position of anode due to its long cycle life, abundant material supply, relatively low cost and importantly, its lithium insertion potential close to zero vs. Li/Li+. The initial commercialisation of Lithium ion battery was achieved in 1991 using LiCoO2 anode, carbon as cathode and a liquid electrolyte. Most of the Lithium ion batteries use Co based anode materials. But here we are synthesizing novel anode materials other than conventional carbon electrode for Li-ion battery in order to increase the specific capacity since Co is expensive and toxic. Electrode materials for Lithium ion battery can be classified into different categories based on the mechanism of Lithium ion storage: Intercalation, alloying and conversion. Both intercalation and alloying lead to larger structural deformations. Even then the capacity of the alloyed materials is much more than that of intercalated ones and through the introduction of alloying particles into a carbon nanofibrous material, the structural abnormalities are not much affected in case of alloying anodes. To increase the energy density and safety of the battery at high current densities, materials supporting higher number of exchangeable Lithium at higher redox voltage are needed. The conversion mechanism happens when Li ions are inserted into the nanosized binary compounds as MX (M stands for Fe,Co,Cu,Sn,… and X for O,S,F,…) and causes the reduction of M cations to M0 and formation of LiX8. Graphite is a commercial anode material for Li-ion batteries with a limited theoretical capacity of 372 mAh/g9, but fails to meet the fast-growing requirements of high-power Li-ion batteries.
One of the most important electroactive elements of a battery system being the electrolyte has to be chosen carefully. In general, it is an organic solvent or a mixture of organic solvents, with optimized dielectric constant, viscosity and chemical nature. Generally, the Lithium aprotic solvents like LiPF6 are dissolved in organic alkyl carbonate solvent mixtures (here EC, Ethylene Carbonate and DMC, Di Methyl Carbonate). Figure 4 shows the chemical structure of those chemical compounds. The electrolyte must be ionically conducting, not electronically.
Lithium sulfur batteries
Li-Sulfur (Li-S) batteries are one of a kind, using a pro-lithiated anode and a sulfur cathode. While, Lithium-Sulfur batteries offer a very high theoretical specific energy, ~2600Wh/Kg10, the real performance is proving to be limited; directly related to the role of electrolyte. However, Sulfur is a non-toxic inexpensive material which is abundant in nature, it is used for some applications. It has a high theoretical and experimental specific energy. Therefore, Li-S battery is the most capable energy storage option for electrical vehicles and power grid. Anyway, it has life cycle issue to prevent its usage in electrical vehicles. Li-S battery need to improve life cycle issue in order to work with current battery applications. This battery has complex chemistry and process to charge and discharge Li-S cell. The cost and manufacturing process of Li-S batteries will be one of the major challenges for commercial transformation of the concept. A usual Li-S battery uses Sulfur cathode and Lithium anode separated by a suitable electrolyte. As the battery discharges, lithium metal is dissolved at negative electrode to produce lithium ions and electrons travel towards positive electrode and start reaction with sulfur at the cathode. The process will produce Lithium-polysulfide and energy gets released during this reaction. The lithium-polysulfide halfway transforms to lithium sulfide (Li2S) as end product of Lithium-Sulfur discharge. A reverse process happens during battery charge. While charging lithium-sulfur battery, a reverse reaction occurs from lithium sulphide product. The lithium ions separate from lithium sulfide and attract the negative terminal on the lithium metal. These reactions occurring inside this battery can be represented as a figure with Polysulfide shuttle.
Sulfur has the high theoretical capacity of 167211 mAhg-1, which is 10 times higher than that of conventional cathode materials (such as LiCoO2 and LiFePO46) for LIB's. Therefore, higher energy density can be achieved using sulfur as cathode. Li-S cell can produce 2.15 V during chemical reaction between Lithium and Sulfur. However, LIB's use Carbon-sulfurous cathode and Lithium metal as anode at some cases. Carbon-sulfur is actually waste product of petroleum process. The Li-S batteries can be produced with more density compared to the Li-ion batteries because sulfur has low atomic weight. Therefore, the Li-S batteries have more storage capacity compared to the Li-ion batteries. The electrical energy output of a single Li-S cell is approximately 1.8 V if it is fully charged. The major issue of Li–S battery is low electrical conductivity of sulfur cathode needing an extra mass for a conducting agent and the current research focus is to find highly conductive cathodes.
As the present discussion is only concerned about anode materials, we restrict our discussion on various kinds of anode materials for Li-ion and Li-Sulfur batteries. The negative side of implementing pure metallic Lithium as anode is the disastrous growth of dendrites of Lithium which will lead to short circuit in the system. Micrometer-sized graphite particles is the most opted anode material for commercialised Lithium-ion batteries. In them, Lithium is inserted into graphite at a potential less than 0.1 V versus Li+/Li. But issues like SEI formation on battery surface will open ways to search for novel anode materials which can almost satisfy the conditions for an anode material. Generally, anode materials for batteries can be broadly classified into Carbonaceous and Non-Carbonaceous materials. Recent researches are focusing on the development of Graphite-like carbonaceous alternatives and non-carbonaceous anode materials for Lithium-ion batteries. Now, our focus turn into some important carbonaceous materials adopted as anode materials in these batteries.
Carbonaceous anode materials
The very first commercialised anode material for Li ion battery is graphite. It has a theoratical capacity of 372 mAhg-1 and form LiC6 after complete lithiation in accordance with the following reaction:
Carbonaceous materials have large variations in crystallinity, chemical composition and texture depending on their preparation, processing and precursor materials. The amount and nature of Li accommodation depends on a combination of several factors such as surface and crystallinity12, particle size13 and even binder properties14. Graphite, support reversible intercalation of Li ions at potentials close to that of Lithium. And the reversible capacity of graphite can be raised by improving their degree of crystallinity15 even though there occurs a large irreversible capacity loss due to the electrolyte decomposition resulting in the formation of a Solid Electrolyte Interface (SEI)16 on the surface. (The Solid Electrolyte Interface is a thin layer composed of inorganic and organic products deposited on the anode surface during the first discharge cycle due to electrolyte reduction and other surface reactions in a cell containing a carbonaceous anode). The energy storage mechanism in the anode materials can be of three types: Intercalation, alloying or conversion. Intercalation process is a mechanism of energy storage where ions are electrochemically inserted into specific areas inside the crystal structure. Electrochemical insertion usually does not result in any significant alteration in the structure of the host compound. In the reverse process, the inserted ions are released from the host. Due to retention in the structural integrity during intercalation/de-intercalation mechanism, long-term cyclability of Li-ion cell is highly probable. Commercial anode (graphite) and cathode (LiCoO2) are well known examples of electrodes which operate via intercalation-de-intercalation mechanism. Inspite of stable cyclability and less structural losses during successive charge-discharge cycles, intercalation method of storage predominantly results in low specific capacity. So, the next type of anodes come by alloying. Lithium can form alloy with elements such as Al, Si, Ge, Sn, Sb etc. Usage of alloying electrodes in lithium-ion battery results in drastic increase in the number of exchangeable lithium and specific capacity Although the alloying form of storage provides high specific capacity however, the cycling stability of these compounds is unsatisfactory This is mainly due to the fact that alloying elements undergo a drastic volume change (~300-400%)17 during discharge-charge cycling. Due to large volume expansion and contraction, very high strain develops in the electrode materials and as a consequence the particles break down into smaller crystallites and generate large number of grain boundaries. This predominantly results in disruption of the electron pathways leading to increased internal resistances and severe capacity fade. An important and effective strategy to reduce this is to confine the alloying element inside porous carbonaceous host where the core of the porous structure is usually partially filled. Carbon host not only acts as an electronic channel but also serves the purpose of accommodating volume changes occurring during repeated charge and discharge cycles. In case of SnSb, both Sn and Sb are electroactive towards lithium. Anyways, the alloying reaction of Li/Sb and Li/Sn take place at different voltages. In the conversion mechanism, during the lithiation step the transition metal-oxide or fluoride or sulfide reacts with Li+ and get transformed to nanometer-sized metal particles and Li- oxide or fluoride or sulfide. During the delithiation step, the initial transition metal-oxide or fluoride or sulfide is reformed. Although conversion method delivers high capacity, cyclability of these materials are generally poor. Apart from this, polarization and hysteresis in the galvanostatic charge discharge voltage profiles are some of the major issues. Thus, we still have to wait for the day when converted type of anode materials are able to be in an adaptable condition in batteries.
Importance of morphology
While we adopt Carbon containing materials to be electrodes, our selection criteria will be limited to nanostructured materials due to many reasons. One of the most important cause is, while large particles may deliver high storage capacity and long cycle life, it may be unable to perform at high current rates18. Reduced electrode particle size will considerably improve the rates of insertion/deinsertion and hence battery power density. It is because of the novel properties presented by the nanostructures like decreased size but increased surface area, unique shape and they are expected to exhibit characteristics superior to their bulk counterparts19. In effect, use of nanoparticular materials can mitigate the problem of slow diffusion by limiting the diffusion distance to the radius of the nanoparticle. Carbonanofiber or carbonanowires like structures do good to the function since they are unlike carbon nanotubes, which permit Li insertion only through open ends, Li ions can also get inserted through surface defects in the fiber walls.
And finally, we are getting introduced to a fiber production technique using which we are electrochemically obtaining novel alloyed anode materials.
Introduction to electrospinning
Electrospinning is the method for producing useful 1-D nanostructures with uniform diameters containing the metal particles embedded on it. The process is based on the electrostatic interaction between the particles of the solution and the ground collector.
The method of electrospinning is found useful to synthesize fully Li-alloy based anode in rechargeable Li-ion batteries. In this work, we are synthesizing Sn-Sb alloy arranged inside Carbon nanofibers. Among various Sn-alloys, Sn-Sb20 is considered good because a stepwise insertion mechanism into the components can relieve the volume changes and improve the mechanical stability of the electrodes thus ensuring better electrochemical properties than the single-Sn phase. Thus, when one component reacts with Lithium the other will take on the role of buffer and vice versa. In this study, Sn and Sb is used in a particular composition in such a way that Sn is taken 3 times than Sb. It can be seen that the combination of both will be resulting in a varying outcome as in the case of energy density and cycle life. More details on electrospinning (Method and materials) can be seen in the next chapter.
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In this chapter, the major focus is on the different instrumental methods we have adopted in order to complete the work. It includes the electrospinning method that is the method of synthesis of nanofibers, as well as structural and morphological characterisation techniques including PXRD, SEM, EDS, FTIR, CV and Galvanostatic cycling.
The electrospinning method of synthesis is a versatile tool for the generation of 1 dimensional nanostructure with uniform diameters containing the metal particles embedded in it. This process can be carried out from a polymer melt1 or from a polymer solution. The electrospinning of polymer solutions into fibers has drawn a lot of attention in recent years for applications in different fields, namely tissue engineering, filtration, drug delivery, and bio sensing, among others. This method uses electric forces to draw charged threads of polymer melts upto fiber diameters in the order of some hundred nanometers.
The electrospinning setup consist of four major parts:
- A metallic needle with a blunt tip
- A high voltage source
- A metallic drum collector covered by an aluminium foil
- A syringe filled with a viscous polymeric solution.
If we apply adequately high voltage to a single droplet, the electrostatic repulsion overcomes the surface tension and the droplet becomes stretched as a result of the charging of the liquid surface. At this point, a liquid jet is ejected through the needle tip. This point of eruption of the liquid is called Taylor cone2. The jet typically undergoes a whipping motion enabling significant thinning of the diameter. While traveling to the collector, the solvent evaporates from the jet solution and charged fibers eventually get accumulated on the collector. Several factors are known to influence the diameter of the nanofibers and the final nanostructure including the intrinsic properties of the spinning solution such as electrical conductivity, concentration, surface tension and viscosity as well as operating conditions such as the flow rate, applied voltage/electric field intensity, tip-collector distance and humidity3.
The diameter of the fibers thus produced is influenced by the viscosity and generally increases with increasing viscosity of the solution according to the equation:
where η is the viscosity of the spinning solution and it depends upon polymers. k is an arbitrary constant. Furthermore, the viscosity and the surface tension of the solution must be adequate in order to get smooth fibers. An intermediate viscosity is the appropriate measure as if it will neither become particles due to very low viscosity nor show hardness to form smooth fibers. Another parameter which decides the morphology of the fibers is humidity. If the environment is humid, certainly it will lead to production of pores on the surface of the polymer mat as water will get condensed upon the surface. Even the distance of separation between the drum collector and the needle tip matters, since the formation of fine fibers is resulted from the fixing of distance in accordance with the applied voltage. Because of the structural features of the electrospun materials, they are being applied to various fields.
Powdered X Ray Diffraction (PXRD)
X Ray Diffraction is a rapid analytical tool used for the phase identification of a crystalline material. Discovered by Max Von Laue, in 1912, is based on the constructive interference of monochromatic X Rays and crystalline sample. The X Rays are generated by a cathode ray tube that produce a monochromatic radiation that will be directed towards the sample. The interaction of the incident rays with the sample causes constructive interference and a diffracted ray will be emerging when conditions satisfy Bragg’s Law: nλ=2d sin θ (where n is the order, λ is wavelength of the monochromatic ray and d is the lattice spacing in the crystal). This law shows the relationship between the wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a crystalline sample. The measurements are done later by detecting the diffracted X-rays, processed and counted. By scanning the sample through a range of 2θ angles, all possible directions of diffractions of the lattice should be attained due to the random orientation of the powdered material. It can be effectively applied to analyse the surface of the SnSb polymer material that we have synthesized. As X Ray diffraction studies is common for crystal structure and atomic spacing studies now, it has applied here as a material characterization technique. PXRD patterns were collected in a 2θ configuration using a Philips X’Pert Pro diffractometer.
Thermo Gravimetric Analysis (TGA)
This technique has been used to estimate the weight loss of the sample after its complete carbonization. Here, the mass of a substance is monitored as a function of temperature or time as the sample specimen is subjected to a controlled temperature program in a controlled atmosphere. The plot thus obtained is called a Thermogram. That point in the thermogram from which the inclination starts indicates decomposition of the substance.
Once the decomposition is complete, the weight remains constant again and from the plot we can find out the percentage composition of the material actually present in it. Here, the carbonization of the SnSb carbon nanofiber will give the weight loss of the substance either in percentage weight of carbon or the remaining weight of SnSb.
Scanning Electron Microscopy (SEM)
SEM studies were done in order to understand the surface topographical features of the as-synthesized samples. The basic working principle of a scanning electron microscope involves a beam of electrons generated from a suitable source, generally a tungsten filament or a field emission gun, which will get collided with the surface of the specimen, then the nature of the secondary electrons emitted from the surface is examined to get an idea of the surface. The chemical composition of the nanostructured electrode material was characterized by SEM.
On magnification of the particle surface, the morphological features can be thoroughly studied as the percentage of various elements present even the extent of carbonisation is identified in our sample. The combination of higher magnification, greater resolution, compositional and crystallographic information makes this characterization technique one of the most advanced equipment in this field.
In this technique, electromagnetic radiation in the Infrared region is used for the determination and identification of molecular structure as well as the studies associated with the functional groups present in them. The necessary condition for a molecule to be IR active is that it must have a change in dipolemoment related with its vibrational mode. FTIR spectrophotometers are used to measure the IR activity of the as synthesized samples. Whenever the frequency of the IR radiation matches with the vibrational frequency of a bond or a collection of bonds, absorption band will be obtained. Using this principle, the major bands in the samples are examined and the presence of different groups are identified.
Cyclic voltammetry is a powerful as well as popular electrochemical technique commonly employed to investigate the oxidation and reduction of a molecular species. It can be usually employed as an investigation tool to characterize new electrode systems. CV is performed by cycling the potential of the working electrode over a potential window and measuring the resulting current. Voltammograms are the plots of Current versus Voltage.
The system shown above consist of an electrolysis cell, a potentiostat, a current to voltage converter and a connected data acquisition system. The potential of working electrode being linearly varied with time, meanwhile the reference electrode is maintained at a constant potential. Comparatively being a faster technique, it gives a much lower signal to noise ratio.
As the name suggests, a constant current is applied between the working and the counter electrodes and the cell voltage is measured as a function of time. This is an easy way to obtain the capacity of the active material during charge and discharge using different cycling rates. We are mainly concentrating on galvanostatic charge/discharge cycling studies on cells containing alloying electrode materials with respect to Li metal as a reference at room temperature using a multi-channel galvanostat instrument. From the known time, the specific charge/discharge capacity can be estimated for the material.
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SnSb electrospun nanofibers as an anode material in a Lithium ion battery
A crucial problem associated with the usage of Group IV elements (eg; Tin) based electrode materials is the drastic volume changes occurring during repeated alloying-dealloying of Lithium. The elements of Group IV have the potential to form alloy with Li and can store more than 4 Li+ during charge-discharge process. Sn is always a chosen candidate for being the electrode material as it increases the energy density and safety of the battery at high current densities. But the potential risk of using bare Sn material is their enormous volume change during the electrochemical cycle which tend them to easily undergo a serious volume effect1. Thereby we can use a carbonaceous matrix to reduce the expansion of Sn-based electrode material during electrochemical cycling. In this chapter, we are taking into account an alloyed material, ie, Sn being alloyed with Sb, which is redox active at a potential higher than that of Sn, that will reduce the Lithium uptake amount of Sn in SnSb compared to that in bare Sn. Therefore, the volume changes in SnSb will be expectedly much lower than that in pure Sn anode material. And also, as Sn and Sb are electrochemically active, they both can contribute to capacity of the electrode material. The composition plays a very crucial role as specific capacities and cyclability of SnSb is observed to depend on the variable percentage of Sn. That particular composition upon which this work has been carried out is Sn 75-25 Sb and the results are in compatible with the expected outcomes. Excess Sn in SnSb leads to some interesting implications in terms of electrochemical properties and enhances the performance of the material compared to the reported SnSb with Sn and Sb in equimolar concentrations2.
Synthesis of materials
The SnSb-Carbon nanofiber composites are synthesized using electrospinning method in which the electrostatic forces are made use in order to get 1-D nanofibers. The electrostatic force of attraction between the needle tip and the drum collector produces a nanofibrous mat containing the material. Firstly, 0.4g of Polyacrylo Nitrile (PAN, MW=15k, provided by Aldrich) is dissolved in4 mL of Dimethylformamide (DMF, Spectrochem Pvt. Ltd) by stirring the solution for 8 hrs at 70 ̊ C Separately another solution is prepared by dissolving 0.142g of SnCl2 and 0.057g of SbCl3 (3:1 ratio) in 4 mL DMF by stirring for 2 hrs. The two solutions are mixed together and stirred for 8 hrs to obtain a solution for electrospinning. The mixed precursor solution is loaded into a 5mL syringe, and the needle of the syringe is connected to a high voltage power supply of the electrospinning instrument (Physics equipment Co. Chennai. India). Flow rate of the solution is maintained at 0.25mL/h, and the voltage difference between the tip of the needle and the metallic drum collector covered with Aluminium foil is kept at 18 kV. The distance between the needle and the collector is set as 15 cm. A Taylor cone is formed at the tip of the needle due to the electrostatic attraction between the needle tip and the drum collector that is rotating at a speed of 2000 rpm. This results in the formation of a white polymer mat composed of PAN and the metal chlorides. This polymer mat is removed from the Aluminium sheet and stabilized in a tubular furnace at 280 ̊ C in air for 3 hrs at a heating rate of 5 ̊ C/min. This is then heat-treated at 700 ̊ C for 3 hrs at the same heating rate in a reducing atmosphere of Ar/H2 to obtain nanoparticles embedded graphitic carbon fibers.
Electrode casting, battery assembly and electrochemical testing
Electrode preparation: The electrodes are prepared by mixing appropriate amounts of nanostructured electrochemically active materials (SnSb compound here) with an electronic conductor (Glass wool) and binder (Poly Vinylidene fluoride) in a solvent (usually cyclopentanone) to form a homogeneous slurry. This slurry was then spread onto a suitable thin metal (Copper or Nickel foam) which act as the current collector. These coated electrodes were subsequently dried at temperatures above 120 ̊ C in a vacuum oven. And then the electrodes were pressed at a particular pressure to enhance the contact between the electrochemically active materials and the collector. Lithium metal was used as counter electrode for the procedure.
Battery Assembly: That laboratory cells used for conducting the electrochemical measurements are Swagelok cells (Figure 13). Since Lithium and Lithium salts are very sensitive to atmospheric conditions, the electrode assembly have necessarily to be done inside an Argon-filled glove-box with oxygen and water levels less than 0.1ppm. The battery assembly consists of the active material as working electrode, both the counter electrode and reference electrode be Li metal separated by a separator soaked in electrolyte (1M LiPF6 in EC and DMC). The condition for a good separator is that it must be ionically conducting as well as electronically insulating inside the battery. The most chosen one is glass wool.
These cells are made of stainless steel. The order of arrangement of different components in the cell follows as: the counter electrode is mechanically pressed onto a cylinder and then the cylinder-Li is pressed with a spring against the working electrode (cast on a Ni foam current collector). We put a glass wool separator soaked with the electrolyte material between the two electrodes. The cell is tightened and pressurized and kept inside the glove box for over 5 hrs before carrying out the electrochemical measurements.
Structural and morphological characterization
The structural and crystallographic phase analysis of the as synthesized SnSb in carbon nanofiber was done using Powder X-ray Diffraction (PXRD) (XRD, Philips X’pert Pro Diffractometer, nickel filtered Cu Kα radiation, λ = 1.5418 Å, Voltage 40 kV; Current 30 mA; cu-filtered Kα radiation as the X-ray source, range of 2θ = 20°-80 ̊). A Mettler Toledo thermogravimetric system is used to perform thermogravimetric analysis (TGA) in the room temperature (30˚C) to 800 ˚C with a heating rate of 5˚C/min under oxygen atmosphere. A Perkin Elmer Fourier transform infrared spectrometer with Spectrum 2000 software is used to record Fourier transform infrared spectra (FTIR). Scanning Electron Microscopy (SEM) was done with FEI FEG 250 ESEM machine. Energy Dispersive X-Ray Spectroscopy (EDS) was done along with the SEM characterization with FEI FEG 250 ESEM machine.
Electrode preparation and Electrochemical Properties
The electrospun SnSb nanofiber material were mixed with a binder (PVdF) at a ratio of 90:10 in Cyclopentanone solvent without adding any further Carbon black. Cleanly cut Ni foam pieces were poured with this slurry and kept for overnight drying. Later it was pressed under at 150 Kg/cm2. The electrolyte used was 1M LiPF6 in a mixture of EC-DMC and Li metal foil (99.9%, Aldrich) was used as counter and reference electrode and glass wool as separator. All cell assembly was done at 25°C in a glove box (MBraun) under Ar (O2 & H2O < 0.5 ppm). Cyclic voltammetry of the sample is carried out in CH-Instruments (CH608C) in the voltage range of (0.05-2.0) V, first at a scan rate 1mV/s and secondly at a scan rate of 0.5 mV /s. Galvanostatic charge/discharge cycling (Arbin instruments, MSTAT) were performed at 0.2A/g (for cell1 over the potential window 0.05 to 2V) and for cell2 at a 0.1 A/g current density upto complete lithiation and discharging.
RESULTS AND DISCUSSION
Structural and morphological characterization of SnSb-CNF
Powder X-Ray Diffraction
Figure 14 shows the powder-X-ray diffraction pattern of the as-synthesized SnSb fibers with a particular molar composition of Sn encapsulated inside carbon nanofiber in the range 2θ =20˚-80˚.
From literature it can be seen that the powder X-ray diffraction pattern of SnSb-75-25-CF is a combination of diffraction peaks of SnSb and Sn, indicating an intimate mixture of Sn and SnSb3. The sharp peaks appearing for the sample implies that the sample is generally in crystalline nature. The Carbon present in the SnSb-CNF does not show up in the pattern because it is in the amorphous form. This is evident from the broad background observed in the Powder X-Ray diffraction pattern. The peaks marked by * are obtained from the JCPDS no. 00-001-0830 which refers to the Sn alloyed carbon nanofiber material and the second set of peaks marked by # implies the Sn 50-50 Sb with JCPDS 00-001-0926 that implies the presence of Sn and Sb composite in the sample.
To investigate the thermal stability of the as-synthesized sample, thermogravimetric analysis (TGA) is carried out under oxygen atmosphere in the temperature range (0−750°C) (Figure 14). The percentages of carbon present in the samples can be determined from the weight loss around 410°C. The carbon content in the samples is found to vary in the range of 61% implying the SnSb contents to be in the range of 39−40%.
The carbonization process gives the amount of carbon content in SnSb nanofibers and it was thoroughly investigated as it is only a single-step process in SnSb-C fibers.
The spectrum for PAN displays characteristic band at 2250 cm-1 that can be assigned to the stretching of the nitrile group. The band at 1663 cm-1 is assigned to C=O stretch while the absorption bands in the range (1000-1500) cm-1 is due to C-H stretching4.
The prominent peak appearing in the plot of PAN present in the sample is at a position of 1580 cm-1, that can be assigned to the presence of PAN present in it irrespective of whether it is carbonized or annealed. As we can see after stabilization at 280°C, there is the disappearance of two prominent peaks at 1200 and 1450 cm-1. But the major peak is retained there indicating the presence of the PAN. Followed annealing at 700°C makes the major peak be broadened because then the appearance of SnSb as prominent component occurs while the major peaks of the polymer is almost fading away in the diagram. And it implies the formation of new SnSb carbon nanofibers.
Scanning Electron Microscopy and EDS measurements
To investigate the morphology of the samples, SEM and EDS characterizations have been carried out. Figure 16 represents the two class of samples we have investigated under the methods.
The main difference between Figures is that the first one gives the fine fiber structure of SnSb Carbon nanofibers which is only underwent a thermal treatment merely at 250°C. Whereas the second gives the SEM image of the fibers after carbonization of PAN to carbon nanofibers after a heat treatment at 700°C. It is to be noticed here that the carbonization results in the breaking of long nanofibers into shorter fibers with nearly the same thickness. Although the thickness values vary in the range of 130-230 nm in both the forms, it can be found out that the sample don’t have a uniform diameter. The further analysis on the surface features was done using EDS measurements which produced the exact percentage of components present in the material and also the prominent component in the active material was obtained according to our objective. As we have seen, the carbonized material has lost its fibrous structure after annealing and what is remaining is only some spherical structures along with a cluster of crushed particles.
This EDS data reveals the fact that Sn is having a high percentage over Sb on the spherical surface. We can analyse this with a table containing the weight percentage.
The EDS images produced thus gives some interesting results that were lacked in the SEM analysis. The projected portion in the figure 19 is a spherical portion that is covered by the material composite. There we can see the uniform dispersion of all of the major components as this is an important feature of all electrospun materials. And the presence of Oxygen as a component may be due to the stabilization procedure carried out in open air atmosphere.
Cyclic voltammetry and Galvanostatic charge-discharge cycling are used to estimate the electrochemical properties of the as-synthesized material.
Sn 75-25 Sb CNF versus Li+/Li
Figure 20 shows the cyclic voltammograms for the first 3 cycles of the SnSb-Carbon nanofiber composite electrode run at two different voltage values. More clear oxidation-reduction peaks are obtained when the voltage supply is minimized. From the plot, the first reduction peak is appeared around 1.1V in all the three scans which corresponds to the formation of a Solid Electrolyte Interphase on the carbon surface.
As we can see from the plot, during delithiation process, SnSb is restored. The shoulder peaks in the figure can be explained as follows: There are two pairs of redox pairs along with some other reduction peaks that implying the Li-Sb and Li-Sn alloying-dealloying reactions. For this composite, at first, a LixSnSb phase will be formed which then reacts with more Li+ ions to produce Li3Sb. As the next step, Lithium alloys with Sn. Lithium insertion into carbon may occur in the lower voltage regions indicated by strong reduction peaks ranging from 0.6 to 1.1V.
Figure 21 represents the galvanostatic charge and discharge profiles for 25 cycles at a constant current density (0.2 Ag-1).
During the potential range 0.05 to 2V, the voltage profiles are given above for 25 cycles. The observed voltage plateaus do good agreement with the CV. An initial discharge and charge capacities for the SnSb-CNF composite electrode were 2038 mAhg-1 and 837.7 mAhg-1. In the second cycle, both these values are found to be decreased to 833 and 753 mAhg-1. But, most of the cycles, have a high reversible discharge capacity as can be seen in the 10th (605 mAhg-1) and 25th (515 mAhg-1) cycles.
After 27 cycles, the discharge capacity of SnSb-C composite was found to be 498 mAhg-1. It corresponds to a capacity retention of 59% with respect to 2nd discharge capacity. This exceptional cyclability originates from the configurational influences of the material inside a carbon nanofiber matrix. The presence of a carbon fiber coat upon the surface of SnSb particles provide sufficient void space to accommodate the volume changes during the alloying-dealloying process.
Complete lithiation of the cell
Another cell made from the same composition of material was made to discharge completely and a resulting discharge cycle is studied well at a current density of 0.1 Ag-1. It follows the usual trend of a discharge curve and this electrode can now be used as an active lithiated electrode material for some other battery assemblies like Li-S battery.
In conclusion, this chapter was dealt with some alloying anode material for Lithium-ion battery, namely SnSb metallic alloy incorporated to Carbon nanofiber. The material has been successfully synthesized through electrospinning method. We used a particular composition of precursors (Sn 75-25 Sb) in order to counteract the detrimental effect caused by volume change issues. Alloying has been found a very adaptable method for electrode manufacture since they helped in retaining the capacity of the battery even after a continuous number of cycles. And at last, the completely discharged electrode material can be run as an active anode material for some other batteries.
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2. Konda Shiva, Hongahally B. Rajendra, and Aninda J. Bhattacharyya. ChemPlusChem 2015, 80, 516 – 521
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Cobalt incorporated Carbon nanofibers as Active electrode material for Li-ion batteries
In the previous session we investigated the advantages of replacing conventional electrode material for Lithium-ion battery by an alloying material. In this chapter we pay our attention to transition metal oxides which possess much higher theoratical capacities than carbonaceous materials1. Of them, Co3O4 has been considered a suitable choice due to its high theoratical capacity of 890 mAhg-1. On further investigation, it has been found that the Lithium insertion mechanism in Co3O4 is a reversible- conversion mechanism2 which suggest the storage of 8 moles of Lithium per mole of Co3O4. The composite material for electrode is synthesized through the most adaptable method for synthesizing 1-D nanostructures, electrospinning method. The aforesaid method will yield Co oxide nanoparticles contained within the carbon nanostructure skeleton. Different carbon allotropes have been chosen as the supporting matrix for high-capacity nanoparticle materials, the most preferable one for us because of the easiness of synthesis as well as incorporation is carbon nanofibers. After morphological and compositional characterization, the material will be casted into electrode and its electrochemical properties can be studied by accommodating them in the Lithium-ion battery.
Synthesis of materials
The precursor solution for electrospinning was prepared by dissolving 0.4 g of PAN (Polyacrylo nitrile, MW=15k, Aldrich) in a 4mL of DMF (Dimethyl Formamide). Another solution was prepared by adding 0.3 g of Cobalt(II) Acetate Tetrahydrate (Co(OAc)2.4H2O, provided by Aldrich) into 4mL DMF in another container. These two solutions were mixed and stirred constantly for 8hrs. This precursor solution is then loaded into a 5 mL syringe, and the needle of the syringe is connected to a high voltage power supply of the electrospinning instrument (Physics equipment Co. Chennai. India). Flow rate of the solution is maintained at 0.25 mL/h, and the voltage difference between the tip of the needle and the metallic drum collector is kept at 18 kV. The distance between the needle and the metallic collector is kept at 15 cm. Because of the electrostatic interaction between the needle tip and the metallic drum rotating at a speed of ~2000 rpm, a Taylor cone will be generated at the needle tip and a light-pink coloured nanofiber mat will be formed afterwards. The fiber mat was first stabilised at 250°C for almost 90 minutes and then annealed at 650°C for 1 hr in N2 atmosphere. The resultant carbonized material was post-heat treated for 15 minutes in order to get Co oxide incorporated carbon nanofibers.
Structural and Morphological characterisation
The structural and crystallographic phase analysis of the as synthesized Cobalt oxide particles in carbon nanofiber was done using Powder X-ray Diffraction (PXRD) (XRD, Philips X’pert Pro Diffractometer, nickel filtered Cu Kα radiation, λ=1.5418 Å, Voltage 40kV; Current 30 mA; cu-filtered Kα radiation as the X-ray source, range of 2θ=200-800). Scanning Electron Microscopy (SEM) was done with FEI FEG 250 ESEM machine. Energy Dispersive X-Ray Spectroscopy (EDS) was done along with the SEM characterization with FEI FEG 250 ESEM machine. More surface analysis was carried out using Transmission Electron microscopic technique.
Results and Discussion
Powder X-Ray Diffraction
Figure 24 shows the powder X Ray Diffraction pattern of the as-synthesized Cobalt Oxide carbon nanofiber material in the range 2θ=15-70. From the pattern the following deductions can be summarised: It shows prominent peaks at 2θ= 18.99, 31.27, 36.84, 55.6, 59.35 and 65.2 corresponding to the (111), (220), (311), (422), (511) and (440) planes indicating the presence of Co3O4 (JCPDS no. 078-1970). This pattern is an evident for the fact that heat treatment conditions were well-optimized to produce pure Co3O4 fibers without burning carbon. And also, some other peaks are spotted at 2θ= 42.61 and 61.84 corresponding to the formation of CoO in the composite material. As a part of the carbonization reaction, a small quantity of CoO is also produced which give their characteristic peaks at the indicated 2θ values as above (JCPDS: 01-075-0418).
Scanning Electron Microscopy
Figure 25 shows the SEM images of stabilized and annealed cobalt oxide material. Although the material is seen to be pulverized it can be assumed as because of the carbonisation. The experimental data gives the percentage of distribution of various components over the surface of Carbon nanofibers.
Transmission Electron Microscopy
The TEM images of the material shows that Cobalt oxide particles are embedded inside the carbon nanofibers. The magnified image is the result of the electrospinning process as it causes a uniform distribution of particles within the fibers.
Conclusion and future prospect
In this chapter we dealt with the same electrospinning method for synthesis of electrode material, but another promising anode material with high theoretical capacity was produced. The Co nanoparticle embedded in Carbon nanofibers can be tested as electrode material in Lithium ion, Lithium sulfur and Zn ion batteries since they are expected to have higher cyclability and even high capacity than conventional electrode materials. And the Co incorporated electrode materials are already in use in Lithium ion batteries. As a continuation of this work, the above characterized composite material can be accommodated as anode in Zinc-ion batteries as well as cathode in Lithium ion batteries.
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First of all, I would like to thank God for giving me such an opportunity to work in India's one of the top Science institutes. I am extremely greatful to my Guide Prof. Aninda J. Bhattacharya for his support and conern in my work. It was an honour for me working under such a great scientist of India. I am using this space to express my admiration and love towards my Co-Guide Sweta M. George; 3rd year Ph. D student, SSCU, for guiding me this two months, spending her valuable time for me. I remember all my teachers, proffessors and my parents in this occasion.