PPY based anode binder for lithium ion battery application

While the progress of science and technology, application of next-generation energy storage systems has been increased. Energy storage systems have been used from electronics to electric cars. These systems must have high energy and power densities with cost efficiency. Lead acid batteries, nickel metal hydroxide batteries and a number of other well-known batteries were used as traditional energy storage devices. In the last 20 years, LIBs have achieved great success in consumer electronics and electric vehicles. Since LIB are lighter and smaller than the other rechargeable batteries and have higher energy, power and cycling life, they can be used in technological devices such as phones, computers and digital video cameras. LIB is a kind of battery which the lithium ions move between the anode and cathode with help of an electrolyte during the charge and discharge process. The main fundamental parts of lithium ion batteries are anode as negative electrode, cathode as positive electrode, electrolyte and separator. Electrolyte provides a transferring medium for ions. Anode is oxidized during the electrochemical reaction, gives electrons to the external circuit, and cathode that is reduced during the electrochemical reaction and takes electrons from external circuit. Generally, polymer, water and glass and ceramic based electrolytes are used in LIB. LiPF6 salt which is dissolved in organic solvents such as ethylene carbonate, diethyl carbonate and/or dimethyl carbonate is the most widely used electrolyte as commercial product. However there are intense studies on solid electrolytes or polymer electrolytes. Seperators are categorised as, micro porous, ion exchange membrane and polymer based seperators. Generally seperators are made from the polymeric electrolytes or porous plastic films such as polypropylene (PP) and polyethylene (PE). The cathode materials are metal oxide structures that can react with lithium reversibly to form a host-guest compound. For cathode materials, olivine (one dimensional),layered (two dimensional) and spinel (three dimensional) structure are choosen. The most important examples that have been commercialized among these structures are lithium cobalt oxide (LCO), lithium iron phosphate (LFP) and nickel manganese cobalt (NMC) composites. In 1970s, lithium metal used as first anode active material. However, it was discontinued with lithium metal because of sudden warming and exothermic reaction in use. In other studies and commercial products, carboneous anode, especially graphite were used because of their good cycleability and low stable discharge voltage plateau. Graphite carbons which have good cycling ability and low stable discharge voltageu plateucan be used as active material in anodes. However, graphite has low Li atom density, thus ıt has low theoretical capacity as 372 mAh/g. In order to get higher capacity, different anode active materials are investigated. Si is the most promising atom for higher theoretical capacity as 4200 mAh/g and low discharge voltage. Also, Si is encouraging alternative anode active material because of abundant resource, low cost, low toxicity, high safety and environmental compatibility. During the charge-discharge process, performance of Si decreases because of 310% volume expansion and semiconductivity of Si. In addition of, Si causes pulverization, loss of electrical contact between Si particles and current collector, and formation of an unstable solid electrolyte interphase during charge-discharge process. In order to profit from advantages of Si, anode materials have been studied for tolerating these defects by improving the chemical composition and the mechanical strength tolerance. For preventing this decreasing, Si active material is used with conductive and flexible polymer binders. Because of the good enviromental stability and high conductivity, conducting polymers such as PPy, poly(3,4-ethylenedioxythiophene) (PEDOT) and polyaniline (PANi) have been used in electronic, both anode and cathode electrode materials in batteries and supercapacitors. In litrature, PPy, PANi, PPr, PAA, PF and PFi are used as polymeric binders for anodes. This conducting polymers exhibited good charge/discharge properties, indicating clearly that the conducting polymer could work as a conducting matrix for batteries. Since the polymer could form a conducting elastic matrix, which provides a conducting backbone for the electrode, it could also be used as a flexible host matrix of Si particles to prevent the deformation of the cell with the large volumetric changes during lithation and delithation process. In addition to these polymers, carbon nanotubes, carbon black and acetylene black are used in order to increase conductivity and stability. In this study, colloidal conductive polymers were used for polymeric binder in anode. First study about colloidal PPy was referenced from Baker's method. In this method, Py was polymerized in ethanol, water and sulphuric acid mixture under nitrogen atmosphere by using potassium persulfat at 80°C and the colloidal product was obtained at after 3 h. In another study the colloidal PPy was prepared by the oxidation of Py with potassium persulfat in acidic, 25% aqueous and ethanol at 95°C. PPy was prepared as a colloidal solution also by oxidizing in an aqueous solution of methylcellulose as template. The PPy-tin (IV) oxide and PPy-silica nanocomposite colloids by using iron (III) chloride and ammonium persulfate as oxidation reagents had also been investigated. In most studies, colloidal PPy was synthesized with the steric stabilizers using ferric (III) chloride as the initiator. This steric stabilizers are utilized from commercial polymers such as poly(ethylene oxide), poly(N-vinyl pyrrolidone),poly(vinyl alcohol), and partially hydrolysed poly(viny1 acetate) (PVA). Colloidal PPy which was synthesized easily in one step was used as polymeric binder for Si anode with/without commercial polymeric additives for increasing performance of Si anode in LIB. In this study, colloidal PPy was synthesized by chemical oxidation reaction of Py with CAN in DMF with/ without polyurethane and used for the first time as a polymeric binder with Si active material. This colloidal PPy was obtained easily in one step. Optimum conditions of polymerization were determined by using different concentration of monomer and oxidant. For understanding mechanism of polymerization, different monomers such as N-methyl pyrrole, carbazole, N-ethyl carbazole and thiophene and different oxidant as iron (III) chloride were used. Also, for understanding polymerization mechanism, Py was polymerized electrochemically in the presence of DMF. For electrochemical polymerization, platin electrode as working electrode, silver wire as referance electrode, platin wire as counter electorde and tetrabutyl ammonium tetrafluoroborate as electrolyre were used in experiments. By reducing the size of chemically synthesized PPy particles which obtained by convenient method to the size of colloidal particles(1nm-1000nm), SiNP were included in Col-PPy more effectively. Because, colloidal PPy which consists particles in nano sizes, surrounds Si nanoparticles more effectively, and crate porous structure, as a result more efficient matrix sturucture was created in anode. Performance of Si anode was increased by using colloidal PPy due to the nano size polymer and solvent addition during slurry preperation. This also reduced the amount of solvent added , obtained porous Silicon nanoparticles/collodial-PPy anode became more effective and helped to prevent the crack generation and delamination during lithation and delithation because electrodes were obtained with this polymer. Silicon nanoparticles/collodial-PPy exhibited enhanced electrochemical performance, including high capacity retention and superior rate capability without any conductive additive. The improved performance with more efficient matrix structure in Silicon nanoparticles/collodial-PPy anode was credited to the stable PPy shell with high conductivity. The starting materials and the synthesis processes were both facile for large-scale production, which may be extended for manufacturing of the next generation Li-ion batteries. Colloidal PPy was characterized by viscosity, centrifuge, conductivity, ATR-FTIR and UV-Visible and H-NMR spectrophotometric measurements. The redox behaviour of colloidal PPy was investigated by LSV and CV and its capactive behaviour was investigated by EIS measurements. Optimum monomer/oxidant ratio was determined as 0.086/0.01 and the reaction time as 24h as a result of characterization. EIS measurements of colloidal PPy suggested that if one would like to use this silicon nanoparticle/colloidal-PPy as an anode at LIB, it would be better to obtain cololidal PPy by appling potential in the range of 0.2V and 0.6V. For using as polymeric binder in anode, colloidal PPy solution was mixed with silicon nanoparticles to obtain silicon nanoparticle/colloidal-PPy binder and used successfully prapered the silicon nanoparticle/colloidal-PPy anode as alone and together with polymeric additive such as PVDF and PVP .Than the electrodes were used to assemble the coin cells. As a counter electrode, the Li metal was used. All the coin cells were assembled in the Ar-filled glovebox. PVDF and PVP polymeric additives were used to improve the cyclability and capacity in Si-anodes. These Si-anodes were further investigated with cycling, c-rate tests in half cells and SEM images before and after cycling. The results of cycling test were compared with literature test results. As an anode for Li-ion batteries, silicon nanoparticle/colloidal-PPy exhibited high reversible capacity (2371 mAh/g at C/10 (0.42 A/g)) after 100 cycles and good cycling performance (reversible capacity of 770 mAh/g after 200 cycles at C/3 (1.4 A/g)). At the end of this study, it is considered that colloidal PPy are used for Si based anode in LIB application. It is believed that these colloidal polymers and polymeric additive and the synthesis processes are appropriate for commercial production, which may be extended for manufacturing of the next generation Li-ion batteries.