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Performance Degradation Mechanism of High Nickel Cathode Materials for Lithium Ion Batteries at High Rate Charge Revealed by Transmission Electron Microscopy

  • Minji Jo (조민지, 대학원 신소재공학과 신소재공학전공)

초록/요약

After a successful commercialization of lithium ion batteries (LIBs) for small portable devices, higher energy-density LIBs are in demand for larger scale applications such as electric vehicles (EVs) and energy storage system (ESS). High nickel layered oxides have served as a cathode of choice due to their high energy density. However, there are problems of rapid capacity fade due to the low structural stability. In order to design an alternative cathode material with exquisite chemical composition, an in-depth analysis of the degradation mechanism of the present materials in terms of structural and chemical evolution should be conducted in advance. In this work, we focused on the degradation mechanism of the cathode materials in high rate charging which is currently a major challenge in EV industry. One of the main causes of such rapid reduction in capacity of the high nickel layered cathode materials is a cation mixing phenomenon which causes a crystal structural evolution. In first research theme, the different cation mixing mechanism depending on the charging process utilized in EV industry of LiNi0.83Co0.11Mn0.06O2 (NCM83) under high rate charge condition is investigated by tracing the crystallographic and electronic structural changes by transmission electron microscopy (TEM). In second research theme, the phenomenon and origin of chemo-mechanical microcracks in primary particles of LiNi0.835Co0.15Al0.015O2 (NCA83) are investigated. And we analyzed that this microcracks in high nickel (Ni > 80%) materials are induced by the inhomogeneity under fast charging condition. This work was conducted by comparing with the degradation mechanism of LiNi0.80Co0.15Al0.05O2 (NCA80) due to the fine difference in Ni/Al content of 3.5%. These in-depth analysis of performance degradation mechanism of high nickel layered materials will provide a guidance for developing next-generation cathode materials with improved electrochemical and safety characteristics.

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초록/요약

고니켈계 층상계 산화물은 높은 에너지 밀도 덕에 대부분의 전기차에서 양극소재로 채택되어왔다. 그러나 낮은 구조적 안정성으로 인해 용량이 빠르게 감소하는 문제가 있다. 정교한 화학적 조성으로 높은 용량을 구현하면서도 사이클 특성이 좋은 양극재를 설계하려면 구조·화학적 측면에서 재료의 열화 현상에 대한 심층 분석이 선행되어야 한다. 해당 논문에서는 현재 전기차에서 직면하고 있는 문제인 고속 충전 시 양극재 열화 메커니즘에 초점을 맞춰 연구를 진행했다. 이러한 고니켈계 층상계 양극재가 급격한 용량 감소를 보이는 주요한 원인 중 하나는 결정 구조적인 변화를 초래하는 양이온 혼합 현상이다. 첫 번째로, LiNi0.83Co0.11Mn0.06O2에 대해 전기차 고속 충전 방식에 따라 유도되는 두 가지 다른 양상의 양이온 혼합 메커니즘을 투과전자현미경(TEM)을 이용해 결정학적 및 전자적 구조 변화를 추적하며 조사하였다. 두 번째 주제에서는 고속 충전 조건에서의 비균질성에 의해 유도된 LiNi0.835Co0.15Al0.015O2 (NCA83)의 일차 입자의 화학적∙기계적 미세 균열 (microcrack) 현상 및 기원에 대한 분석을 진행하였다. 이는 LiNi0.80Co0.15Al0.05O2과 비교∙분석하여 3.5%의 미세한 Ni/Al 함량 차이에 따라 전지의 수명에 미치는 영향 및 원인을 분석하였다. 이러한 고니켈계 양극소재의 성능 저하 메커니즘에 대한 심층적인 분석은 전기화학적·사이클 특성이 개선된 차세대 양극재 개발에 지침을 제공할 것이라 예상한다.

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목차

Contents
Abstract ........................................................................................................... Ⅰ
List of Figures ................................................................................................. Ⅱ
List of Tables .................................................................................................. Ⅲ
Chapter Ⅰ. Introduction .................................................................................... 1
1.1 Lithium Ion Batteries................................................................................. 1
1.2 High Nickel Layered Cathode Materials.................................................. 3
1.3 Transmission Electron Microscope Analysis ......................................... 6
Chapter Ⅱ. Performance Degradation of NCM Cathode Induced by Periodic Cation Mixing ................................................................................................... 11
2.1 Experimental Details............................................................................... 11
2.1.1 Electrochemical Measurements....................................................... 11
2.1.2 Materials Characterization............................................................... 12
2.2 Results and Discussion........................................................................... 13
2.3 Conclusion............................................................................................... 41

Chapter Ⅲ. Investigating the Chemo-mechanical Breakdown of NCA Cathodes Originated by Inhomogeneity.......................................................... 43
3.1 Experimental Details............................................................................... 43
3.1.1 Electrochemical Measurements....................................................... 43
3.1.2 Materials Characterization............................................................... 44
3.2 Results and Discussion........................................................................... 45
3.3 Conclusion............................................................................................... 72
Reference ........................................................................................................ 73

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