Grafen/kalay esaslı nanokompozitlerin lityum iyon pillerde anot malzemesi olarak kullanımının incelenmesi
Investigation of the use of graphene/tin based nanocomposides as anode material in lithium ion batteries
Yazar: SELİN ALP
Danışman: PROF. DR. REHA YAVUZ
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Kimya Mühendisliği Ana Bilim Dalı / Kimya Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2022
Konu: Enerji = Energy
Dizin: Batarya = Battery ; Grafen = Graphene ; Karakterizasyon = Characterization ; Lityum iyon pil = Lithium ion battery

Özet:
Dünya nüfusundaki artış ve sürekli gelişim içerisinde bulunan teknolojiye bağlı olarak enerji ihtiyacı da artmaktadır. Özellikle taşınabilir elektronik cihazlar, otomobiller için sürdürülebilir enerji kapsamında depolanma teknolojileri gelişmektedir. Elektrikli otomobiller, hibrit otomobiller, dizüstü bilgisayarlar ve cep telefonları gün geçtikçe yaygınlaşmakta ve şarj/deşarj süreleri de daha önemli hale gelmektedir. Tüm bu gelişmelerle birlikte çevrenin korunması da önemli olmaya devam etmektedir.
Lityum iyon piller, endüstride enerji güvenilirliğini arttırarak mevcut kaynakların verimli kullanılmasını sağlamakta olup, bu alandaki çalışmalar ve yatırımlar gün geçtikçe artmaktadır. Bu nedenle, lityum iyon pilin maliyet, çevrim ömrü ve güvenlik bakımından iyileştirilmesi amacıyla, mevcut pillerin özellikleri geliştirilerek yüksek enerji yoğunluklu ve uzun çevrim sayısına sahip anot malzemeler konusunda ağırlıklı  olmak üzere muhtelif çalışmalar gerçekleştirilmektedir.
Lityum iyon piller, sahip oldukları yüksek enerji/güç yoğunluğu, uzun ömür ve düşük maliyet gibi nedenlerle, enerji depolama sistemlerinde taşınabilir enerji kaynağı olarak tercih edilmektedir. Lityum iyon pillerin performansı, pillerde kullanılan bileşenlerin çeşidi ve verimliliğine bağlı olarak değişim göstermektedir. Pil kapasitesinin artırılması, şarj-deşarj sürelerinin verimli hale getirilmesi için anot ve katot kısımlarında çeşitli malzemeler kulanılmak suretiyle deneysel çalışmalar gerçekleştirilmektedir. Lityum iyon pillerde en yaygın olarak kullanılan anot malzemesi grafendir. Silisyum (Si), kalay(Sn) gibi anot niteliğine sahip diğer bazı malzemeler ile karşılaştırıldığında, daha az spesifik kapasiteye sahip olmasına rağmen grafit, yapısal olarak daha kararlı ve uzun ömürlü olduğundan anot malzemesi olarak kullanımda tercih edilmektedir. Ancak grafitin enerji kapasitesinin oldukça sınırlı olması nedeniyle, grafitin doğrudan anot malzemesi olarak kullanılacağı lityum iyon pillerin enerji yoğunluğu, taşınabilir elektronik cihazların enerji ihtiyacını karşılayamaz nitelikte olmaktadır. Grafen ise grafitten elde edilen olağanüstü özelliklere sahip bir malzeme olup, kimyasal olarak kararlı ve yüksek elektrik iletkenliğine sahiptir. Bunun yanı sıra iyi mekanik ve ısıl iletkenlik özelliklerine sahip olması nedeniyle, kompozit anot malzeme sentezleme çalışmalarında baz elektrot olarak tercih edilmektedir.
Tez çalışması kapsamında, İstanbul Teknik Üniversitesi – Enerji Enstitüsü, Malzeme Üretim ve Hazırlama Laboratuvarı'nda grafitten grafen (rGO) elde edilerek, kalay oksit katkılandırma ile farklı oranlarda grafen-kalay oksit nanokompozit anot malzemeleri üretilmiş ve pil yapımında kullanılmak suretiyle de söz konusu malzemelerin kapasiteleri incelenmiştir. Elde edilen malzemeler, taramalı elektron mikroskopu (SEM), termogravimetrik analiz (TGA)  ve X-ışını difraksiyonu (XRD) yöntemleriyle karakterize edilmiştir. Ayrıca, malzemelerin pil performansını değerlendirmek amacıyla sentezlenen malzemelere şarj-deşarj kapasite ölçümü, dönüşümlü voltametri ve elektrokimyasal empedans spektroskopisi (EIS) yöntemleri uygulanmıştır.

Abstract:
Energy demand is increasing because of growing world population and continual improved technology. Storage technologies are developing within the scope of sustainable energy, especially for portable electronic devices and automobiles. Electrical automobiles, hybrid vehicles, laptops and mobile phones become widespread progressively and their charging-discharging time gains importance. Protection of the environment also continues to being important with all these improvements.
Lithium-ion batteries increase energy reliability in the industry and ensure the efficient use of existing resources, and studies and investments in this area are increasing day by day. As a result of this situation, in order to improve the lithium-ion battery in terms of cost, cycle life and safety, the properties of existing batteries are developed and anode materials with high energy density and long cycle numbers are studied.
Lithium ion batteries are preffered as portable energy source in energy storage systems due to their high energy/power density, long life and lower cost. Performance of lithium ion batteries changes according to components' type and components'efficiency used in batteries. Experimental studies are done with various materials in anode and cathode materials to make charging-discharging time efficient and to increase capacity of battery. 
In lithium ion batteries which uses graphitic carbons as anode material, the morphology and chemistry of graphite surface have a significant influence on the formation of solid electrolyte interphase (SEI), irreversible charge loss and the overall electrochemical anode performance. Therefore, it is possible to improve the battery performance with the modifications on the morphology and surface of graphite anode material.
Graphitic carbons principally possess two different kinds of surfaces, basal plane and prismatic (edge) surfaces. 
Basal plane surfaces are homogeneous and smooth which consist only of carbon atoms. On the contrary, prismatic surfaces are heterogeneous and beside carbon, containvarious surface groups, mostly oxygen containing. The transport of lithium ions during charge/discharge operation takes place via the prismatic surfaces rather than the basal plane surfaces. 
Therefore, oxidative modifications on the surface of graphite provide more inert basal plane which make the differentiation of anode performance by the differentiation of the structure and the chemistry of prismatic surfaces. Along with the surface modifications, Li+intercalation is improved by stabilisation of graphite surfaces and the decomposition of electrolyte is prevented.
The most used anode material in lithium ion batteries is graphite. Inspite of its lower specific capaticy in comparision like Si, Sn metals, graphite is preffered because of its structural stability and long life. But because of the limit in the energy capacity of graphite, energy density of lithium ion batteries can not satisfy energy requirement of portable electronic devices. 
Eventhough, metallic materials like silicon, tin, borom etc. have higher specific capacity, due to severe volume changes during charge/discharge, graphite is preferred more compared to metallic materials. 
However, specific capacity of graphite needs to be improved which is only 372 mAh/g. Intensive researches continue on developing new anode materials with graphite and metals to improve cyclability and specific capacity of lithium ion batteries.
Beside, graphene is a miracle material synthesized from graphite, chemically stable and has high electrical conductivity. Furthermore, based on its good mechanical properties and thermal conductivity, it is preffered as base electrode in studies of composite anode material synthesis.
In this thesis study, graphite was synthesized by the Modified Hummers method, which is one of the methods suitable for large-scale production, and converted into graphite oxide structure. Graphene (rGO) was obtained by thermal reduction of the synthesized graphite oxide material. 
Graphene is synthesized from graphite and it is mixed with a metal to get graphene-metal nanocomposites as anode material, and it is investigated by usage in battery application in Istanbul Technical University- Energy Institute, Material Production and Preparation Laboratory. The obtained materials were characterized by scanning electron microscope (SEM), thermogravimetric analysis (TGA) and X-ray diffraction (XRD) methods. In addition to these characterization methods, charge-discharge capacity measurement, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) methods were applied to the synthesized materials in order to evaluate the battery performance of the materials. 
The fact that a very high amount of mass loss was observed as a result of TGA of the graphite oxide material produced by the modified Hummers method revealed that the graphite structure was significantly oxidized. According to the XRD results, a graphitic peak was observed at 2θ = 26.5° in the (002) plane in the XRD pattern of pure graphite, and the distance between the layers was determined to be 3.34 Å. After the oxidation process of graphite, it was observed that the distance between the layers increased from 3.34 Å to 8.4 Å due to the inclusion of oxygen-containing functional groups between the graphite layers, and a graphitic peak was formed at 2θ=10.5°. 
The detection of the ID/IG ratio of 0.90 in the Raman pattern of graphene samples synthesized by the thermal reduction method indicates that the structural defects have increased and the number of layers of the obtained graphene samples has decreased.
Tin oxide was synthesized by hydrothermal process using SnCl2.2H2O material at 200 °C for 4 hours. A sharp tin oxide peak was observed at 2θ=30° in the XRD spectrum of tin oxide, and the average crystal size was determined as 10.24 nm in accordance with the Debye-Scherrer equation. In the synthesis of graphene/tin oxide composites, the graphene/tin oxide ratio was chosen as 5/1 : 5/4 : 1/1 : 3/4 : 1/2 : 1/4 and the average crystal size of the obtained nanocomposites was the most intense peak (110) plane was determined as 8.71, 12.2, 15.24, 15.25, 15.26, 12.2 nm, respectively. 
Graphene, tin oxide and graphene/tin oxide nanocomposites were coated with carbon and the average crystal sizes of the obtained materials were determined as 12.18, 10.17, 11.02 nm, respectively.
The charge-discharge profiles of the graphene electrode were obtained at a potential range of 0.01-3.00 V, at a current density of 50 mA/g, and the charge and discharge capacity in the first cycle was determined as 390 mAh/g and the coulombic efficiency was 94%.
EIS measurements of the graphene electrode were carried out at room temperature in the frequency range of 0.01 Hz to 100 kHz, and when the electrochemical impedance spectra of graphene were examined after the 1st, 2nd, 3rd, 4th and 5th cycles, it was found that the diameter of the impedance spectra increased as the number of cycles increased. This is an indication of the loss of contact in the electrode and a decrease in the capacity of the electrode with the increasing number of cycles.
When the charge and discharge profiles of the tin oxide anode are examined, the charge and discharge capacities in the first cycle are determined as 245 and 304 mAh/g, and the coulombic efficiency is 80%, respectively.
When the EIS measurements of the tin oxide anode electrode were examined, it was observed that the charge transfer resistance had high values due to a contact resistance arising from the solid-electrolyte interface. Due to the fact that this resistance decreased significantly after the first charge-discharge process between 0 and 3 V, electron exchange accelerated.
In the synthesis of graphene/tin oxide composites, charge-discharge profiles of the anode materials prepared with a graphene/tin oxide ratio of 5/1 : 1/1 : 3/4 : 1/2 : 1/4 were obtained. It has been observed that a decrease in specific capacitance values occurs depending on the increase in the number of cycles for each composite anode. The first cycle coulombic efficiencies were found to be 78%, 31%, 50%, 36% and 34%, respectively.
In addition, it has been revealed that the battery prepared with graphene/tin oxide:1/4 composite anode has a higher specific capacity value than other composite anode materials, with a capacity of 857.67 mAh/g in the first cycle.
When the EIS measurements of the graphene/tin oxide composites anode electrodes were examined, it was observed that the charge transfer resistances had high values due to the formation of contact resistances originating from the solid-electrolyte interface.