Fiziksel buhar biriktirme yöntemiyle katı hal lityum iyon pil bileşenlerinin geliştirilmesi = Development of solid state lithium-ion battery components by physical vapor deposition method

Abstract

Son birkaç yüzyıldaki küresel enerji talebi, karbondioksit emisyonları nedeniyle küresel iklim değişikliğine yol açan fosil yakıtların yanma reaksiyonu ile karşılanmıştır. Teknolojik gelişmenin yanı sıra artan nüfusun artan enerji ihtiyaçlarını karşılamaya devam edebilmesi için daha sürdürülebilir bir enerji üretim yöntemi benimsenmelidir. Elektrokimyasal enerji depolama ve dönüştürmeye dayalı yöntemler kullanılarak cep telefonlarının, taşınabilir elektronik cihazların, robotik elektrikli süpürgelerin ve dizüstü bilgisayarların artan ihtiyaçlarına daha sürdürülebilir ve çevre dostu bir alternatif olarak enerji sağlanmaktadır. Ancak mikro ve nanoelektromekanik sistemler gibi daha küçük boyutlu cihazlar üretilmek isteniyorsa daha küçük boyutlu ve yüksek güç yoğunluğuna sahip bataryalar geliştirilmelidir. İnce film piller, daha küçük boyutlu cihazlar için en umut verici aday ve alternatif olarak göze çarpmaktadır. Bu tür pillerin, yüksek enerji yoğunluğu, mükemmel güvenlik, artan çevrim ömrü ve çok çeşitli çalışma sıcaklıkları gibi çeşitli avantajlara sahip olduğu bilinmektedir. Yaygın olarak kabul görmüş olmasına rağmen, yeni nesil pil malzemeleri pahalı olması, toksik bileşenler içermesi ve güvenlik kaygıları nedeniyle bazı sınırlamalara sahiptir. Aynı zamanda lityum iyon pillerde kullanılan sıvı elektrolitin oldukça yanıcı ve parlayıcı bir malzeme olduğu bilinmektedir. Bu nedenlerle mevcut yöntemler, gelecekteki uygulamalar için öngörülen depolama gereksinimlerini karşılayamamaktadır. Çalışmaların çoğu genel olarak, yeni malzemelerin geliştirilmesi ve araştırılması için büyük ölçekli üretimle gerçekleştirilen geleneksel sentez tekniklerine dayanmaktadır. Bu çalışmanın odak noktası ise ince film biriktirme yöntemi olarak radyo frekanslı (RF) sıçratma yöntemi kullanılarak lityum iyon pillerde kullanılan hücre bileşenlerini üretmek ve kaplama parametrelerini optimize etmektir. Mevcut teknikle hücre bileşenlerinin üretim parametrelerinin optimizasyonu, lityum iyon pillerin seri üretimi için geleneksel süreçlerin geliştirilmesine yardımcı olmaktadır. Aynı zamanda taşınabilir elektronikler için gelecekte tamamen ince film yöntemiyle üretilen pillerin yolunu açmaktadır. Mevcut çalışmada lityum iyon pillerin temel bileşenleri olan anot, katot ve elektrolit ince film olarak biriktirilmiş yarı hücreler vasıtasıyla bu malzemelerin elektrokimyasal testleri gerçekleştirilmiştir. Altlık malzemesi olarak kullanılan Si-wafer, Cu-folyo ve paslanmaz çelik üzerinde Si ince filmlerin biriktirilmesi 2 inç'lik çapa sahip ticari bir hedef malzemesi kullanılarak gerçekleştirilmiştir. Böylece anot malzemesi olan Si için yapısal karakterizasyon için Si-wafer, elektrokimyasal karakterizasyon için de paslanmaz çelik uygun altlık malzemesi olarak belirlenmiştir. Bu bilgiler ışında katot olarak LiCoO2, elektrolit olarak ise LIPON ince filmler benzer altlık malzemeleri üzerinde test edilmiştir. Katot ve elektrolit üretimi için kullanılan hedef malzemelerin üretiminde 2 inç'lik çapa sahip paslanmaz çelik kalıp kullanılmıştır. Her malzeme özelinde altlık ısıtma, altlık temizleme, RF gücü gibi uygun deneysel parametreler belirlenerek üretime aktarılmıştır. RF manyetik sıçratma ile ince filmlerin biriktirilmesi, bileşenler arasındaki sitokiyometrik oranı elde etmek için her zaman belli bir gaz karışımında gerçekleştirilmiştir. Tüm bileşenler için biriktirme işleminin tamamlanması akabinde bileşenlerin yapısal özellikleri çeşitli karakterizasyon yöntemleriyle test edilmiştir. Yüzey profilometresi, taramalı elektron mikroskobu (FESEM), atomik kuvvet mikroskobu (AFM), x-ışınları difraksiyonu (XRD), raman spektroskopisi, x-ışınları fotoemisyon spektroskopisi (XPS) yapısal karakterizasyon yöntemleri olarak yer almaktadır. Si ve LiCoO2 ince filmlerle üretilen yarı hücrelere çevrimsel voltametri (CV), elektrokimyasal empedans spektroskopis (EIS) ve galvanostatik şarj/deşarj testleri uygulanmıştır. Si ince filmler için deşarj kapasitesinin 20 çevrim sonunda 2440 mAh g-1 olduğu görülmüştür. %10'luk O2 atmosferinde üretilen LiCoO2 ince filmlerin deşarj kapasitesi ise 100 çevrim sonrası 13 µAh/cm2'dir. Ayrıca tersinir reaksiyonlar sırasında silisyumun lityum ile etkileşimi sonucu ortaya çıkan genleşmelerle yapısındaki değişimler mevcut pil tasarımlarıyla engellenmiştir. Bu çalışmada kullanılan yöntem, ince film mikro batarya üretiminde ve ölçeklemede oldukça etkili bir yöntem olarak dikkat çekmektedir.

Global energy demand over the past few centuries has been met by the combustion reaction of fossil fuels, which has led to global climate change due to the depletion of reserves and carbon dioxide emissions. A more sustainable method of energy production must be adopted so that technological development, as well as the growing population, can continue to meet the growing energy needs. Recently, electrochemical energy storage and conversion are being considered a more sustainable and environmentally friendly alternative to provide energy to the growing needs of mobile phones, portable electronic devices, robotic vacuum cleaners and laptop computers. However, if it is desired to produce smaller-sized devices such as micro and nanoelectromechanical systems, batteries with smaller sizes and high-power density should be developed. In this case, thin-film batteries are the most promising candidates and alternatives for smaller-size devices. These batteries have various advantages including high energy density, excellent safety, increased cycle life, and a wide range of operating temperatures. In the current studdy, innovative and safe electrode designs have been developed in order to create smaller sized lithium-ion batteries. Produced thin film electrodes have been tested with various characterization methods. Anode, cathode and electrolyte materials, which are the most basic components in a battery cell, are produced as thin films and an innovative approach to battery design has been revealed. As a thin film deposition method, radio frequency magnetron sputtering method, which is one of the most suitable methods for the accumulation of more homogeneous and uniform films, was selected. 2-inch of commercial silicon target was used on the anode side as a target material for deposition. For electrolyte and cathode deposition, the production of target materials was carried out using 2-inch molds. Commercial LiCoO2 and synthesized Li3PO4 powder were used in the production of target materials for cathode and electolyte production, respectively. The appropriate gas atmosphere, vacuum conditions, substrat materials and substrat heating temperature are impotant parameter in radio frequency magnetron sputtering method. By changing some of the thin film production parameters listed above, the effect of these changes on the structure and electrochemical properties of the thin film electrodes was examined in detail. 2 inch commercial silicon target materials were used in the deposition process of nano-sized silicon thin films. The deposition process was carried out at a working pressure of 10-2 Torr, using RF power of 100 W, under conditions of 100% Ar gas atmosphere. For the deposition process, si wafer, copper foil and stainless steel substrate materials were used, and the differences in the accumulation mechanism of the material were well noted. The thickness information and 3D topography of the thin-film silicon electrodes were obtained by surface profiler. Using the thickness information for deposited thin films as a single crystal, it becomes possible to determine the hypothetical weight of thin films. Since silicon thin films consist of a single element and its density (2,285 g/cm3) is relatively easily determinable, it has become possible to calculate an active material weight (8,06.10-5) in the deposited film. With the current method, the deposition rate was also determined (2,5 nm/min) and the deposition conditions are optimized with this information. Field emision scanning electron microscop (FESEM) images of thin films that manufactured in various substrat materials have been included in the current study as well. The FESEM image of the grains in the coating zones, the cross-sectional images and the results of the elemental analysis were also discussed separately in this part. From the deposited thin films, an average grain size of 20 nm and a coating thickness of 190 nm were obtained. In the deposition process performed on copper foil, it was seen that the grains were clustered, while in the deposition process performed on stainless steel, epitaxial growth occurred during deposition. Atomic force microscopy (AFM) images results from the same samples are consistent with FESEM images and some parameters related the surface roughness were obtained by using AFM test. The results of roughness measurements subjected on copper foil are higher than measurements using other substrate materials. The test results of X-ray diffraction (XRD) and Raman spectroscopy were also proved the existence of silicon thin films deposited on the surface. Produced half cells from the silicon thin film electrodes have been subjected to a number of electrochemical tests such as cyclic voltametry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge. CV analysis results have be proved the anodic and cathodic reactions that occur on the surface and the potential values of the reduction/oxidation zones were determined by this method. The results of the EIS analysis were used to comprehend the resistance factors in the battery cell. Thus, information on solution (electrolyte) resistance (Rp= ~10-3 Ω), charge transfer resistance (Rct=2000 Ω) and warburg diffusion (Wdif) tail have be seen on Nyquist diagram. 2-inch stainless steel molds are used in the production of target materials during the deposition of LiCoO2 thin films. Pressed target pellets were sintered at a certain temperature and mechanically reinforced targets were obtained by this way. 24-hours at 950 °C was noted as the most favorable condition for sintering. Using the produced target materials, deposition was carried out at an ambient pressure of 5.10-4 Torr and RF power of 125 W was applied on the target. The composition of the gases in the working environment was determined as the exchange parameter and the coating was carried out at 0%, 3% and 10% oxygen gas partial pressure. 250 °C substrat heating was applied to si wafer and stainless steel substart material.and thin films were annealed at 750 °C for 2 hours. The structural information of the samples produced by changing the gas content has been received by FESEM and AFM method. Increasing the oxygen partial pressure has been caused a decrease in the rate of depositon. +Under all deposition conditions, LiCoO2 thin films have been shown grain size of 30-60 nm. Cross-sectional images and 3D AFM images have been shown colonial growth in LiCoO2 thin films clearly. The Eg and A1g peaks obtained from Raman spectroscopy were represented some vibrational modes in the bonds made by cobalt and oxygen atoms. The structural characterization section is completed with XRD analysis and the intensity of the orientation in the (003) decreased while the intensity of the orientation in the (101) increased. Thus, it has been proven that perfect LiCoO2 thin films have been produced and the final structure is more prone to Li+ ion diffusion. Electrochemical tests Li/Li+ vs. revealed the material's cycling ability, capacity, resistance elements and electrochemical reaction voltage peaks. Specific capacity of 2,1 μAh/cm2 (100% Ar atmosphere), 13 μAh/cm2 (3% O2 atmosphere) and 29 μAh/cm2 (10% O2 atmosphere) were achieved at the end of 100 cycles from LiCoO2 thin films. At the same time, EIS results obtained in 1st and 100th cycles from LiCoO2 thin films that produces in 10 % O2 atmosphere have been presented charge transfer resistance (Rct) of 425 Ω and 1985 Ω. In the final part of this thesis, the production of lithium phosphorus oxynitride (LIPON) thin films as electrolyte have been emphasized. The production of Li3PO4 powders, which are used as the target material for the production of LIPON thin films, was carried out by chemical precipitation method. Li3PO4 target materials was produced with a diameter of 2 inches similar with the previous stage. XRD patterns of Li3PO4 powders coincide with the reference code of JCPDs: 96-901-2501. Thermal analysis and XRD analysis after sintering at different temperatures were applied to determine the sintering characteristic of the material. As a result of the thermal analysis process, it was seen that the structure of the material changed after 600 °C. Low temperatures and long-term sintering should be chosen for sintering conditions of the target Li3PO4. XRD peaks obtained after the sintering process carried out at different temperatures also prove the phase transformations on Li3PO4 that take place at higher temperatures. Thin film coating was carried out at an ambient pressure of 10-4 Torr and RF power of 100 W. During the 2 hour of coating process, 75% Ar, 20% N2 and 5% O2 gas mixture was applied and silicon wafer and coated samples from previous stages was used as substrat materials. The results of FESEM and EDS analysis from the surface have been clearly shown both the elemental composition in the deposited regions and the distribution of the grains. Cross sectional images gave information about how to replace present battery design with 3D battery design. With the XPS results obtained from the electrode and electrolyte surface, it has been proven that thin films were produced in the right composition. The designed Si and LiCoO2 3D batteries were electrochemically characterized and a capacity of 97 mAh/g and 8,5 μAh/cm2 was obtained as a result of the 20th cycle. The method used in this thesis was found to be a highly effective method in thin-film micro battery production and scaling.