Interface control by ion-permselective polymers for lithium secondary batteries = 이온 선택투과성 고분자 막을 이용한 리튬 이차전지 계면 제어

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The development of lithium ion batteries (LIBs) is the great success of modern electrochemistry because it newly opens and moving toward a ubiquitous society as the main power source for various portable IT devices and electric vehicles. However, current LIBs comprised of carbon anodes and transition metal oxide cathodes have limited specific energy densities of $~250 Wh kg^{-1}$, not fully satisfying the gradually growing needs for fu-ture. Therefore, alternative rechargeable battery systems beyond LIBs are being extensively investigated. Among various systems, the Li?S battery is considered to be a promising candidate for post LIBs, because the high theoretical specific capacities of sulfur $(1675 mAh g^{-1})$ and Li metal $(3864 mAh g^{-1})$ give the cell a theoretic energy density of $2500 Wh kg^{-1}$ which is much higher than that of LIBs. Moreover, the non-toxicity, low price and abundance of sulfur hold the possibility of reduced cost. However, despite such promise, the commercialization of Li-S batteries has yet to succeed even after sustained effort spanning several decades. Firstly, the insulating nature of sulfur means that a large amount of conducting agents (mostly carbons) are necessary, thus decreasing the total energy density of the batteries. Moreover, polysulfides (PS) shuttle from the dissolution of PS into the electrolyte followed by irreversible reactions both on Li metal anode and sulfur cathode eventually results in poor capacity retention and the low coulombic efficiency of Li-S batteries. Secondly, inhomogeneous Li electrodeposition on the Li metal surface during the repeated charge/discharge cycling results in dendritic/mossy Li growth on the Li metal anode. The growth of the dendritic/mossy Li accelerates electrolyte decomposition and results in a low coulombic efficiency of the cell. Moreover, the Li dendrite can lead to sudden cell failure or fire/explosion owing to short-circuiting. To alleviate the problems both from the sulfur cathode and the Li metal anode, this study mainly focused on the ion-permselective property of the polymers. It is because most of the problems are related with the transport property of the Li and PS ions, therefore ion-permselectivity can significantly affect the overpoten-tials from concentration gradient and solid-electrolyte interface layers, and overall cell performance. This study mainly includes the application of functional ionomers which have fixed anions on the side chains, thus improving the electrolyte properties including single ion passage and stabilization of SEI layer. Chapter 2 deals with the representative perfluorinated ionomer, Nafion, enveloped sulfur cathode structure which suppressed PS anions dissolution to the bulk electrolyte, resulting in the improvement of cycle retention properties. Chapter 3 deals with the functional monomers which can in situ polymerized both on the Li metal anode and sulfur cathode, thereby inhibiting the PS ion passage and undesired reactions by Donnan exclusion principle. On chapter 4, Nafion thin protection layer is introduced on the Li metal anode, thus leading to uni-form distribution of Li ions and stable working of the Li metal anode during the repeated cycles. All of the above ionomers have single ion conducting properties (for Li cations or PS anions), and their effects to the Li?S cell operations are evaluated and discussed.
Park, Jung-Kiresearcher박정기researcher
한국과학기술원 :생명화학공학과,
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학위논문(박사) - 한국과학기술원 : 생명화학공학과, 2016.2 ,[xii, 138 p. :]


Lithium-sulfur batteries; Ionomers; Protection layer; Donnan exclusion; Transference number; 리튬-황 이차전지; 이오노머; 보호막; 도난 배제; 운반율

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