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Feasible material design of Si anodes for high-energy and advanced Li-ion batteries

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Title
Feasible material design of Si anodes for high-energy and advanced Li-ion batteries
Other Titles
고 에너지, 그리고 발전된 리튬이온전지를 위해 실현가능한 실리콘 음극소재 설계
Author
Lee, Yoonkwang
Advisor
Cho, Jaephil
Keywords
Li-ion battery anode silicon material design
Issue Date
2020-08
Publisher
Graduate School of UNIST
Abstract
The unprecedentedly expanded use of energy storage devices has accentuated the requisition of improved energy density and long-life span lithium-ion batteries (LIBs). Though the commercial graphite demonstrates low working potential (versus Li+/Li), exceptional initial Coulombic efficiency, electric conductivity, and cycle stability, unfortunately, commercially applied graphite anodes are imminent their theoretical capacity (372 mAh g–1). A variety of anodes candidates alternating the graphite with higher capacity have been studied to overcome its capacity limit. Furthermore, explosive demands for sustainable electromobility with high-energy density have motivated tremendous researchers to improve next-generation anode materials, but high gravimetric and volumetric capacity anodes retain obvious limits in the stability of the cell properties. In terms of theoretical gravimetric capacity, silicon has been illuminated as the most promising candidate for the specific capacity, which surpasses more than 10 times comparing to that of graphite. Besides, silicon exhibits low operation voltage, low cost and abundant in the earth. Despite these outstanding advantages, the application of Si as practical anodes still has been avoided by its fundamental problem of volume changes during the charge and discharge procedure. These extreme volume changes could cause material fracture, resulting in electrical contact loss from the current collector and formation of fresh surface to electrolyte. The fresh surface will be provided to reactive site of solid electrolyte interphase (SEI) formation, ascribing excessive electrolyte consumption. Recently, small quantity of Si applied batteries are produced with improved properties. And higher quantity of Si applied batteries has been commercialized in specific application such as power tools. For these reasons, a breakthrough strategy is required to utilize Si-dominant practical anodes. Previously reported void and nano engineering strategies have inherent problems in battery operation and commercial aspects. Enlarged particle size, densified morphology, high tap density, and low specific surface area should be contemplated for the rational active material design. In terms of post-generation technologies, lithium metal anodes have been perceived due to its highest theoretical gravimetric (3,860 mAh g–1) and volumetric (2,061 mAh cm–3) capacity with the lowest potential (–3.04 V, SHE). Unfortunately, there are impregnable challenges to be solved by worldwide researchers to introduce the lithium metal anodes in commercial battery. Effortless dendritic Li formation causes fatal safety issues, accompanying the low cycling efficiency. To suppress the dendrite formation and stabilize the cyclability, vigorous efforts and various strategies have been devoted lately; electrolyte and additive improvement, protective materials to inhibit the dendrite formation, host materials for rational Li deposition, and lithiophilic materials addition for selective Li deposition. Nonetheless, the performance of lithium metal battery is still in the infancy level to be applied in practical batteries, remaining huge gap between academic state and commercial requirements. In this dissertation, I describe brief interpretation of lithium ion batteries including principles and components of lithium ion batteries in chapter 1. Then, I briefly introduce the current and next generation anode materials for high energy density lithium ion batteries; Si-based anodes and Li metal anodes. In chapter 2, I proposed high gravimetric and volumetric capacity composite anodes for next-generation lithium-ion batteries. I emphasized that nano and void-engineering strategies had showed intrinsic limit in fabrication of practical electrode condition. Achieving high electrode density is particularly paramount factor in terms of the commercial feasibility that the tap density of active material was enhanced to 1.1 g cm-3. I introduced micron-sized double passivation layered Si/C design with restrictive lithiation state, based on finite element method calculation. The structure integrity was demonstrated under the industrial electrode fabrication with electrode density (1.6 g cm–3), areal capacity (>3.5 mAh cm–2), and electrode composition (additives < 4 wt%). Such design takes advantages in long-term cycling performances even at high gravimetric capacity (1400 mAh g-1), withstanding the induced stress from Li insertion upon repeated cycling. We performed the 1 Ah pouch-type full-cell evaluation with high mass loading and electrode density (~3.75 mAh cm–2 and ~1.65 g cm-3), which demonstrates superior cycle stability without rapid capacity drop during 800 cycles. In chapter 3, I briefly reviewed lithium metal anodes, and described this work to overcome the current issue. We synthesized ion and electron conductive carbon structure, which contains enough space to accommodate metallic Li during plating process. Li is intensely light metal denoting 0.534 g/cm3 density that the required dense lithium thickness is 14.6 μm for areal capacity (≥ 3 mAh/cm2). The interconnected pore structure containing 500 nm pore and showing 0.2 g/cm3 tap density takes merits in expanded potential area for Li deposition. To incorporate lithiophilic surface layer, thermal decomposed Si nano-layer was deposited uniformly with 3 nm. The lithiophilic lithiated Si (LixSi) alleviated the polarization that it could induce planar Li nucleation and continuous Li accumulation at inner pore and outer surface region, followed by dense and smooth Li plating in electrode. The volume expansion of the electrode at lithiated state of 3 mAh/cm2 was just 30% from 27 μm to 35 μm because of the adequate pore volume. And lithiophilic layer induced electrode exhibited prolonged lifespan and high power at high current density comparing to carbon frame without the Si layer.
Description
Department of Energy Engineering (Battery Science and Technology)
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