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Title

Structural Studies of Degradation Mechanism of Lithium Rich Manganese, Nickel, and Iron Based Cathodes

Creator

Aryal, Shankar

Date

2018

Description

Layered oxide compounds are superior with respect to discharge voltage and discharge specific capacity compared to other families of cathodes....
Show moreLayered oxide compounds are superior with respect to discharge voltage and discharge specific capacity compared to other families of cathodes. Therefore, LiCoO2 and LiMnxNiyCozO2, are the most commonly used cathodes since the commercialization of lithium ion battery. Recently, Li rich Ni, Mn, and Co oxide composite cathodes have been introduced with some improvements. As Co is toxic and expensive, attempts have been made to replace Co with cheap and environmentally friendly Fe. This dissertation reports that comparable discharge specific capacity and discharge voltage can be achieved by replacing Co with Fe and optimizing the composition of Mn, Ni, and Fe. However, the capacity and voltage fading on cycling are still remaining challenges. Structural change on electrochemical cycling is the main reason behind this fading. X-ray absorption spectroscopy (XAS), the specific element probe technique to study local structure and X-ray diffraction (XRD) to study the crystallographic phase information are utilized to understand the degradation/aging mechanism. A series of Li rich Mn, Ni, and Fe oxide composite cathode materials Li1.2Mn(0.30+x)Ni(0.40-x)Fe0.10O2 for x = 0, 0.05, 0.10, 0.15, 0.20 and 0.25 were prepared using a sol-gel synthesis method. Rhombohedral and monoclinic crystal phases are found in Li rich Mn, Ni, and Fe composite oxide materials, but pure rhombohedral phase cannot be obtained without excess Li in the stoichiometric LiMO2 form. The pure monoclinic phase Li2MnO3 is also synthesized to confirm its presence in the composite oxide cathodes. Particle size and surface morphology are studied with scanning electron microscopy. The composite cathodes are cycled to over 100 cycles at 0.3C, for C = 250 mAhg-1 rate. XAS before and after 100 electrochemical cycles of Li rich Mn-Ni-Fe based cathodes is reported for the first time. The determination of fractional contents of monoclinic and rhombohedral phases in the composite oxide cathodes is not possible by powder XRD analysis, however, Li2MnO3 content decreases on decreasing Mn content and on increasing Ni content. The composition with higher Ni content has a higher degree of cation mixing. The synergistic effect of rhombohedral and monoclinic phases in Li rich Mn, Ni, and Fe based cathode is critical for stable electrochemical performance. The Li1.2Mn0.50Ni0.20Fe0.10O2 cathode showed the most stable cyclability performance (194 mAhg-1 first discharge capacity with 94 % capacity retention after 100 cycles at 0.3C rate) however, Li1.2Mn0.40Ni0.30Fe0.10O2 (220 mAhg-1 first discharge capacity with 57 % capacity retention) and Li1.2Mn0.55Ni0.15Fe0.10O2 (241 mAhg-1 first discharge capacity with 68 % capacity retention) cathodes showed higher 1st discharge capacity but poor cyclability under the same charge/discharge cycling.The XAS at Mn K-edge is used to explain the mechanism of Li2MnO3 activation for the improved electrochemical performance of Li rich Mn, Ni, and Fe oxide composite cathode, however Li2MnO3 contributed differently in different compositions. Synchrotron XRD and XAS measurements probed the lattice size expansion, which decreases the chemical potential of Li ions in the cathode on cycling leading to lower discharge voltage after cycling.
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Title

IN SITU X-RAY ABSORPTION SPECTROSCOPY STUDY OF TIN-BASED GRAPHITE COMPOSITE ANODES FOR LITHIUM-ION BATTERIES

Creator

Ding, Yujia

Date

2019

Description

Sn-based anode materials such as Sn, SnO2, Sn4P3, and SnS2 that exhibit large theoretical capacities are promising alternatives to traditional...
Show moreSn-based anode materials such as Sn, SnO2, Sn4P3, and SnS2 that exhibit large theoretical capacities are promising alternatives to traditional graphite anodes for Li-ion batteries. However, their capacities fade drastically in a few cycles due to substantial volume changes during the lithiation/delithiation process resulting in cracking and pulverization of the electrode. A graphite matrix is introduced by high-energy ball milling to obtain a graphite composite and enhance the electrochemical performance. Indeed, Sn4P3/graphite composite exhibits a reversible capacity of 651 mA h g-1 in the 100th cycle, and SnS2/graphite composite shows 591 mA h g-1 in the 50th cycle.To obtain a better understanding of the improved performance of the composite materials and the reason for the more gradual capacity fading, in situ EXAFS is used to investigate these mechanisms using in situ coin cells and in situ vacuum-sealed pouch cells. The collected EXAFS data were analyzed by modeling to extract detailed local environment changes during the lithiation/delithiation process.In the crystalline phases of Sn-based materials, the conversion reaction forming metallic Sn is partially reversible and partially irreversible, and the subsequent alloying/dealloying reaction forming LiSn alloys is reversible. Introducing the graphite matrix increases electrical conductivity and prevents aggregation of intermediate Sn clusters. The graphite matrix also plays a significant role in transforming composites into highly dispersed amorphous phases. These amorphous phases, formed in the first few cycles of Sn4P3/graphite and SnS2/graphite composites, exhibit excellent reversibility in both conversion and alloying/dealloying reactions, which is the main reason for the significant improvements in electrochemical performance. The slow growth of metallic Sn clusters and the slight reduction in amorphous phases result in gradual capacity loss over long-term cycling. Introducing the graphite matrix and creating highly dispersed composite samples are the successful strategies that can be scaled up to develop new battery materials in the future.
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