Understanding on structural deterioration mechanism and limited electrochemical revresibility of layered-structured Co-rich materials

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Understanding on structural deterioration mechanism and limited electrochemical revresibility of layered-structured Co-rich materials
Other Titles
층상구조 코발트리치 물질의 구조열화 메커니즘과 한정된 전기화학적 가역성에 대한 이해
Cho, Woongrae
Cho, Jaephil
Issue Date
Graduate School of UNIST
Layered-structured lithium metal dioxide materials (LiMO2, M = Co, Ni, Mn) are most generally utilized cathode materials for lithium ion batteries, thanks to their high theoretical capacity (> 270 mAh g-1) and stable cycle life. Among them, Co-rich material, especially LiCoO2 (LCO), has been firstly commercialized because of high volumetric energy density, good rate capability and easiness of synthesis process. However, Co-rich materials are not considered as candidates for high energy density material owing to restricted structural reversibility and consequent low practical capacity. Researchers tried to overcome the reversibility issue in two ways, surface stabilization and enhancing bulk structural stability. One of the most popular way to stabilizing the surface of Co-rich material is coating method, which is considered as an effective and practical solution. Lots of compounds have been utilized as coating materials and a variety of synthesis processes were introduced to achieve stable and homogeneous coating layer. This led to significant development in coating method and electrochemical performances, even in industrial field. On the other hand, in case of bulk structural stability, it could not receive enough attention compare to coating. Therefore, fundamental understanding about structural deterioration mechanism and rational solution design based on it is required to overcome irreversible phase transition issue of Co-rich materials. This dissertation is consisted of three chapter. Chapter 1 is an introduction part for cathode active materials for high-energy-density lithium ion batteries (LIBs) and research trend to achieve it. Especially, it mainly deals with Co-rich materials and Ni-rich materials among layered-structured metal oxide materials in order to compare two different approaches for high energy density. Furthermore, this part also covers previously suggested fundamental understandings for structural deterioration mechanisms of Co-rich materials and Ni-rich materials, respectively. In chapter 2, investigation on lithium vacancy generation during electrochemical charge/discharge process and nickel doping on LCO is suggested. Furthermore, deeper understanding on connecting link between structural deterioration and electrochemical reversibility is suggested based on a combination of ex-/in-situ structure analysis tools, half-cell test, and Galvanostatic intermittent titration technique (GITT). In chapter 3, generation of cation vacancies in Co-rich material is suppressed by introducing sodium and iron as dopants for a new generation structural stabilization technique. Structural miscibility of α-NaFeO2 and absence of cation migration phenomenon leads to successful suppression of cation vacancies and enhancement of long-term structural stability. I would like to suggest a new way to achieve high energy density cathode material by extending electrochemical reversibility of layered-structured Co-rich material.
Department of Energy Engineering (Battery Science and Technology)
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