High energy density cathode active materials for lithium-ion batteries
Başlık çevirisi mevcut değil.
- Tez No: 523361
- Danışmanlar: Prof. KUZHIKALAIL M. ABRAHAM
- Tez Türü: Doktora
- Konular: Biyokimya, Kimya, Kimya Mühendisliği, Biochemistry, Chemistry, Chemical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2015
- Dil: İngilizce
- Üniversite: Northeastern University
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 173
Özet
Özet yok.
Özet (Çeviri)
Renewable energy sources such as solar energy, wind and hydroelectric power are increasingly being developed as essential energy alternatives to alleviate the deleterious effects of greenhouse gases in the globe. Large scale energy storage is an indispensable component of renewable energy sources and in this context, Li-ion batteries (LIBs), due to their high energy and power densities and long cycle life, have spurred great interest. Current Li-ion battery technology employs lithium cobalt oxide, LiCoO2, or one of its congeners, in which some of the Co is substituted with Ni and/or Mn as cathode active material. The deficiencies of LiCoO2 include: i-) low capacity with only 0.5 mole of Li+ is being reversibly used in the battery leading to 140 mAh/g discharge capacity at low to medium rates, ii-) high cost, and iii-) environmental concerns arising from the harmful physiological effects of Co metal. In order to overcome these deficiencies of LiCoO2, Li-rich layered metal dioxides, also known as layered-layered lithiated metal oxide composite compound, formulated as xLi2MnO3.(1-x)LiMO2 (M=Mn, Ni or Co), have been proposed recently. This dissertation presents an account of investigations leading to advanced materials which overcome the deficiencies of this class of high energy density Li-ion battery cathodes. Chapter 1 discusses the fundamental aspects of generic battery systems and elaborates on the current state of the art of rechargeable Li batteries. Chapter 2 deals with the discovery of the material 0.3Li2MnO3.0.7LiNi0.5Co0.5O2 (LLNC) that allowed us to conclude which segment of the lithium rich layered composite metal oxide is responsible for structural transformation from the layered to spinel phase during charge/discharge cycling. The crystal structure and electrochemistry of this new cathode active material in Li cells have been compared to that of 0.3Li2MnO3.0.7LiMn0.33Ni0.33Co0.33O2 (LLNMC). In LLNC, the removal of Mn from the LiMO2 (M=transition metal) segment allowed us to determine the identity of the manganese oxide moiety in it that triggers the layered to spinel conversion during cycling. The new material LLNC resists the layered to spinel structural transformation under conditions in which LLNMC does. X-ray diffraction (XRD) patterns revealed that both compounds, synthesized as approximately 300 nm crystals, have identical super lattice ordering attributed to Li2MnO3 existence. Using X-ray absorption (XAS) spectroscopy we elucidated the oxidation states of the K edges of Ni and Mn in the two materials with respect to different charge and discharge states. The XAS data along with electrochemical results revealed that Mn atoms are not present in the LiMO2 structural segment in LLNC. Electrochemical cycling data from Li cells further revealed that the absence of Mn in the LiMO2 segment significantly improves the rate capabilities of LLNC with good capacity maintenance during long term cycling. Removing the Mn from the LiMO2 segment of lithium rich layered metal oxides appears to be a holistic strategy for improving the structural robustness and rate capabilities of these high capacity cathode materials for Li-ion batteries. Chapter 3 examines the effect of alkali ion doping (Na+) into the cathode material of the composition 0.3Li2MnO3.0.7LiMn0.33Ni0.33Co0.33O2 (LLNMC). The 5 wt. % Na doped material, formulated as 0.3Li2MnO3.0.7Li0.97Na0.03Mn0.33Ni0.33Co0.33O2, was compared to its counterpart without Na doping. The discharge rate capability of the LLNMC was greatly improved at both room temperature and 50 0C with the Na doping. The Na doped material exhibited significantly higher electronic conductivity than its un-doped analog as evidenced by dc electronic conductivity data and AC impedance of Li cells. Charge/discharge cycling results of Li cells containing these cathode materials at 50 0C indicated that the voltage decay of LLNMC, accompanied by a layer to spinel structural conversion, was mitigated with Na doping. X-ray diffraction analysis revealed that ionic exchange of Na occurs upon contact of the cathode material with the electrolyte and produces a volume expansion of the crystal lattice which triggers a favorable metal (probably Ni) migration to Li depleted regions during electrochemical oxidation of Li2MnO3 in the first charge. X-ray absorption near edge spectra (XANES) data showed that Na doped NMC has better Ni reduction efficiency to provide higher rate capability. Extended X-ray absorption fine spectra (EXAFS) data supported this conclusion by showing that in the case of Na doped LLNMC, the structure has a larger crystal cage allowing for better metal migration into the Li depleted regions located in the layered unit cell of C2/m space group. XANES of Mn K-edge supported by pre-edge analysis also revealed that during charging of the electrode, the Na doped NMC was oxidized to a higher Mn valence state compared to its undoped counterpart. The results of a comprehensive electrochemical and structural investigations of a wide range of lithium rich layered metal oxide cathode active materials, xLi2MnO3.(1- x)LiMnaNibCocO2 (where x, a, b and c vary) are reported in Chapter 4. In order to identify the cathode material having the optimum Li cell performance we first varied the ratio between Li2MnO3 and LiMO2 segments of the composite oxides while maintaining the same metal ratio residing within their LiMO2 segments. The materials with the overall composition 0.5Li2MnO3.0.5LiMO2 containing 0.5 mole of Li2MnO3 per mole of the composite metal oxide were found to be the optimum in terms of electrochemical performance. The electrochemical properties of these materials were further tuned by changing the relative amounts of Mn, Ni and Co in the LiMO2 segment to produce xLi2MnO3.(1-x)LiMn0.50Ni0.35Co0.15O2 with enhanced capacities and rate capabilities. The rate capability of the lithium rich compound in which x=0.3 was further increased by preparing electrodes with about 2 weight-percent multiwall carbon nanotube in the electrode. Lithium cells prepared with such electrodes were cycled at the 4C rate with little fade in capacity for over one hundred cycles. In Chapter 5, the results of a new synthesis technique, called self-ignition combustion (SIC), that dramatically enhanced the rate capabilities of a lithium rich layered metal oxide compound we prepared are discussed. In this chapter, we report a high rate Li-rich layered manganese nickel cobalt (MNC) cathode material of the composition 0.5Li2MnO3.0.5LiMn0.5Ni0.35Co0.15O2, termed SIC-MNC cathode material for Li-ion batteries with discharge capacities of 200, 250, and 290 mAh/g at C, C/4 and C/20 rates, respectively. This high rate discharge performance combined with little capacity fade during long term cycling is unprecedented for this class of Li-ion cathode materials. The exceptional electrochemistry of the Li-rich SIC-MNC in Li-ion cells is attributed to its open porous morphology and high electronic conductivity. The structure of the material investigated by means of X-ray diffraction, High Resolution Transmission Electron Microscopy (HRTEM) and X-ray absorption spectroscopy combined with electrochemical data revealed that the porous morphology was effective in allowing electrolyte penetration through the particle grains to provide high Li+ transport in tandem with high electronic conductivity for high rate discharge. Extended cycling behavior and structural phase transition of the new material were further examined through Field Emission Scanning Electron Microscopy (FESEM), XRD, XAS and HRTEM. The new SIC-MNC cathode represents the long sought after next generation cathode material for Li-ion batteries with specific energy exceeding 400 Wh/kg or energy density over 1000 Wh/l. The conclusion and future directions are presented in Chapter 6.
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