Porosity generation and optimization of silicon-based anodes for high energy density lithium ion batteries

Energy storage systems are widely used to regulate energy grids and to rehabilitate the energy quality taken from grid by increasing grid realibility. However, at present as a result of the clean energy tendency, developments of the energy storage systems have come to forefront to be able to use for electric/hybrid vehicles and renewable energy sources. Thus, renewable energy sources such as wind energy and solar energy which are not sustainable will be stored when they are available. Then, this stored energy can be used when the energy sources has interruption. This will make it charming by providing the high realibility for renewable sources. Energy storage technologies are in transformation that power (watt) and energy (watt-hour) can be provided separetly while it is not possible to produce them enough at the same time. Supercapacitors can provide high power only for seconds and fuel cells can store limited energy in peak powers. This is a big challenge for the modern advanced technologies (smart phones, wearable computer, electrical vehicles etc.) which need high energy and power. Lithium ion batteries (LIB) are the very promising solution for the high energy and power applications. Because LIBs have high energy and power density and light weight when it is compared to any another rechargable batteries. These properties are the reason to use LIBs in mobile phones, laptops, digital camera/video camera and, other electronic equipments. The other advantages of LIBs, when it is compared to lead acid and metal hydride batteries, are high energy efficiency, no memory effect, and relatively long cycle life. Lithium ion batteries are the energy storage systems which have widely application range. From consumer electronic like; smart phones, tablets, portable computers etc. to electric vehicle, from aerospace applications to military usage, they can be used as an essential equipment in social and political area. So, it has been critical field with a big competition as a result of the importance of the indigenization. Today, nearly all the lithium ion batteries used in consumer electronics, have been produced in Asia and for the electrical vehicles, new technologies are needed. The Unites States of America has been planned to be one of the leader as an electrical vehicle producer with considering the importance of being the local LIBs procuder is critical and they needed long-term plans to be in that competitive market and to be successful. In our country, a couple of universities, institutes and business firms has started to study on LIBs technologies. The main components of lithium ion batteries are electrolyte, which provides a transferring medium for charged structures such as ions; anode, that 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. LiPF6 salt is dissolved in organic solvents such as ethylene carbonate, diethyl carbonate and/or dimethyl carbonate and it is the most widely used electrolyte which is commercially available, however there are intense studies on solid electrolytes or polymer electrolytes. The cathode materials are metal oxide structures that can react with lithium reversibly to form a host-guest compound. 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. At present, graphite is commercially used anode material while silicon, tin, and germanium are widely studied. The theoretical capacities are 4200 mAh/g, 1600 mAh/g, and 999 mAh/g, respectively. The fact that graphite anode active material is currently the most widely used and commercialized product, however, its limited energy density makes high-capacity silicon (Si) based materials as an alternative. Present commercial anodes are graphite for LIBs. But limited energy density of graphite makes high capacity Si based materials an alternative electrode material. When the theoretical capacity of graphite, 372 mAh/g, is compared to Si based anode electrodes, 4200 mAh/g, it seen that Si based anodes have 11 times higher capacity than graphite. However, Si based anodes have some challenges which need to be overcome. The most important one is the volume change of Si (300% volume change) at the charge and discharge steps. As the result of this volumetric change, anode material has cracks, electrical conductivity is failed, ionic transfer, and the capacity decreases. The problem of silicon has been tried to solve by developing silicon based composites, synthesizing porous silicon structures, studying polymer binder solutions, or by developing various production methods. Here, prospective solutions were presented with optimized and developed methods for silicon based anode active materials and by using these methods with various polymer binders. Firstly, the calendaring process, which is an important step in anode production, was carried out for different anode porosity percentages and the most suitable ratio was tried to be found. For the c-SiO/PFM composition, 15% calendering was found as the most suitable ratio and it also studied with the Si/PAA and Si/nPAA compositions, which is a different material and polymer composition. Structural and electrochemical analyses of the obtained anodes are made and the presented contribution is explained. Pycnometry was also used to show the electrode density and pore volume change after calendering process. Secondly, porous Si based anode was produced by a developed method. Here the main purpose is; to use the Si-based anodes for long cycles with high columbic efficiency. Today, the capacity of commercially available carbon-based anodes is not sufficient for our daily applications (mobile phones and computers) and for electric vehicles which are planned to have widespread use in the future. For this reason, it is necessary to develop high specific capacity and high energy density anodes. Here, on the other hand, it is desired to provide an anode design which can meet these requirements. The c-SiO/PFM composition is desired to be rendered porous form by adding and removing different ratios of the salt to the structure. These porosites, which are created in the anode structure, prevent the deterioration of the integrity of the structure by allowing the volumetric expansion which happens when the lithium enters into the silicon structure. Thus, more stable electrode results can be obtained over longer cycles, even though the high loading active material is coated. In the completed study, the optimum salt rate added and removed from the electrode structure was determined as 30% by weight and the study was also carried out for the Si/PPy anode composition. For the characterization of the porous electrode, SEM, EDS, AFM, pycnometry and mercury porosimeter were used. As a result of calendering optimization, the optimum calendering ratio was found as 15%.The c-SiO/PFM anode composition reached a loading capacity of 3,5 mAh/cm2 and a specific capacity of 1250 mAh/g after 100 cycles with 15% calendering. When the same calendaring ratio was applied to the Si/PAA and Si/nPAA anodes, the electrodes exhibited a stable cycle performance and gave a specific capacity of 511 and 1370 mAh/g at the end of the 100th cycle, respectively. Pycnometry results for c-SiO electrode showed increasing electrode density while pore volume was decreasing after increasing calendaring ratio. In the porosity formation study we conducted, c-SiO/PFM anode working with 10% NaCl gave 3 mAh/cm2 loading capacity and a specific capacity of 1000 mAh/g after 100 cycles. The c-SiO/PFM anode operated with 30% NaCl gave 4 mAh/cm2 loading capacity and a specific capacity of 750 mAh/g at 100 cycles. The method was also applied to the Si/PPy anode composition using 30% NaCl. With the Si/PPy anode operating with 30% NaCl, it was reached a specific capacity of 1100 mAh/g at 100 cycles. Structure characterization results also confirmed the porous electrode formation. Under SEM top view and cross section analysis, pores were observed. EDS analysis showed the presence of Cu particles on the electrode surface after washing. AFM results indicated higher roughness value (Ra) after porosity generation. Results of pycnometry and mercury porosimeter were also claimed increasing pore volume and porosity after electrode washing process which provided pores on the electrode structure. Considering the studies and results presented in this thesis, it is understood that the generated method and performed optimization from this thesis can shed light to commercial usage of Si based materials for lithium ion battery applications.