전이금속의 농도구배를 통한 리튬이차전지용 층상계 양극활물질의 미세구조제어에 관한 연구

Abstract

지난 20년 동안 리튬이온배터리는 휴대용 전자기기에서 부터 에너지 저장 시스템에 이르기까지 주된 전력 원으로 사용되고 있다. 높은 에너지 밀도와 장기 수명, 열적안정성을 만족시키는 배터리는 일상생활을 살아가는데 있어서 없어서는 안 될 요소이다. 특히 배터리를 구성하고 있는 많은 요소들 중에서도 양극 물질의 역할은 안정성과 결부되기 때문에 매우 중요하다. 과충전 상태에서는 큐빅 스피넬 구조를 형성하게 되고 이것은 배터리의 안정성을 위협하는 격렬한 발열반응을 동반하게 된다. 이번 연구를 통해서 코어쉘 구조와 농도구배 구조를 이용하여 높은 에너지 밀도와 장기수명, 우수한 열적 안정성을 가지는 양극 활물질을 합성하고자 하였다. 제 1장에서는 공침법을 이용하여 Li[NixCoyMnz]O2 (x = 1/3, 0.5, 0.6, 0.7, 0.8 and 0.85)을 합성하였다. Li[NixCoyMnz]O2의 전기화학적 열적안정성 특성은 조성에 의존하는 경향을 보였으며 Ni함량이 증가할수록 용량과 잔류리튬의 양이 증가하는 반면 수명특성과 안정성이 저하하는 경향을 보였다. 제 2장에서는 CS와 CSG의 구조 및 전기화학적, 열적안정특성을 비교 평가하였다. 나노로드의 밀접한 접촉형태는 전기 전도성을 향상시켰다. 그결과 CSG 의 경우 60 ℃, 4.5 V 조건에서 150 싸이클 동안 85.5 %의 우수한 수명특성을 보였다. 열적특성 또한 CSG 의 경우 발열픽은 279.4 ℃에 발열량은 751.7 J g-1 으로 CS 대비 우수한 열안정성을 보였다. 제 3장에서는 공침법을 이용하여 내부에는 니켈과량 조성이, 외부에는 망간과량 조성을 가지는 농도구배 구조를 합성하였다. BET를 통해 파티클 내부의 표면적 및 기공을 확인해본 결과 농도구배를 통해 나노로드 형태를 가지는 파티클이 더 조밀하게 합성이 진행된 것을 확인하였다. 그 결과 FCG 물질이 전기화학적, 열적안정성 특성에서 더욱 우수한 특성들을 보였다. 제4장에서는 공침법을 통해 두개의 기울기의 농도구배를 가지는 Li[Ni0.65Co0.13Mn0.22]O2 양극물질의 합성을 진행하였다. 전기화학적 특성을 살펴본 결과 30 ℃, 4.3 V 조건에서 200 mAh g-1의 방전용량과 1500 싸이클 동안 88 %의 우수한 수명특성을 보였다. NCA와 CC의 결과와 비교 하였을 때 수명특성, 율특성, 열적안정성 특성에서 가장 우수한 특성을 보였다. TEM을 통해 싸이클 이후의 TSFCG와 CC, NCA를 비교해 본 결과 TSFCG의 경우 구조적으로 안정성을 유지하였지만, NCA의 경우 싸이클 동안 거의 부서진 형태를 보였다.|As observed over the past 2 decades, LIBs are a leading power source for portable devices to energy storage systems. Batteries that satisfy the high energy density, long cycle life, and good thermal stability are indispensable for supporting daily life. Among components constituting the batteries, the roles of cathode materials are very important, in particular, due to safety concerns. In a deeply charged state, oxygen evolution from the host structure is unavoidable to form a cubic spinel structure, where a violent exothermic reaction accompanies the reaction, threatening safety. This study introduces the development of cathode-active materials that meet the high energy density, long cycle life, and good thermal stability achieved by the introduction of microscale spherical core−shells to full concentration gradient particles supported by long nanorods distributed radially. In chapter 1, Li[NixCoyMnz]O2 (x = 1/3, 0.5, 0.6, 0.7, 0.8 and 0.85) are synthesized by a coprecipitation method. The electrochemical and thermal properties of Li[NixCoyMnz]O2 are strongly dependent on its composition. An increase of the Ni content results in an increase of specific discharge capacity and total residual lithium content but the corresponding capacity retention and safety characteristics gradually decreased. In chapter 2, The structure, electrochemistry, and thermal stability of concentration gradient core–shell (CGCS) particles with different shell morphologies were evaluated and compared. Intimate contact among the nanorods is likely to improve the resulting electric conductivity. As a result, the CGCS Li[Ni0.60Co0.15Mn0.25]O2 with the nanorod shell retained approximately 85.5 % of its initial capacity over 150 cycles in the range of 2.7–4.5 V at 60 ℃. The charged electrode consisting of Li0.16[Ni0.60Co0.15Mn0.25]O2 CGCS particles with the nanorod shell also displayed a main exothermic reaction at 279.4 ℃ releasing 751.7 J g-1 of heat. In Chapter 3, Full concentration gradient (FCG) cathode material having nickel-rich core and nickel-deficient outer layer was synthesized via co-precipitation method. The BET surface area measurement demonstrated the presence of densely agglomerated primary particles in the FCG cathode. Owing to the unique spatial distribution of the cations and particle morphology, the FCG cathode delivered higher discharge capacity, superior cycle stability and excellent thermal stability compared to the CC. In Chapter 4, Li[Ni0.65Co0.13Mn0.22]O2 cathode with two-sloped full concentration gradient (TSFCG), is synthesized via a specially designed batch-type reactor. The cathode delivers a discharge capacity of 200 mAh g−1 (4.3 V cutoff ) with excellent capacity retention of 88 % after 1500 cycles in a full-cell configuration. The TSFCG cathode exhibits the best cycling stability, rate capability, and thermal stability of the three electrodes. Transmission electron microscopy analysis of the cycled TSFCG, CC, and NCA cathodes shows that the TSFCG electrode maintains both its mechanical and structural integrity whereas the NCA electrode nearly pulverizes due to the strain during cycling.; As observed over the past 2 decades, LIBs are a leading power source for portable devices to energy storage systems. Batteries that satisfy the high energy density, long cycle life, and good thermal stability are indispensable for supporting daily life. Among components constituting the batteries, the roles of cathode materials are very important, in particular, due to safety concerns. In a deeply charged state, oxygen evolution from the host structure is unavoidable to form a cubic spinel structure, where a violent exothermic reaction accompanies the reaction, threatening safety. This study introduces the development of cathode-active materials that meet the high energy density, long cycle life, and good thermal stability achieved by the introduction of microscale spherical core−shells to full concentration gradient particles supported by long nanorods distributed radially. In chapter 1, Li[NixCoyMnz]O2 (x = 1/3, 0.5, 0.6, 0.7, 0.8 and 0.85) are synthesized by a coprecipitation method. The electrochemical and thermal properties of Li[NixCoyMnz]O2 are strongly dependent on its composition. An increase of the Ni content results in an increase of specific discharge capacity and total residual lithium content but the corresponding capacity retention and safety characteristics gradually decreased. In chapter 2, The structure, electrochemistry, and thermal stability of concentration gradient core–shell (CGCS) particles with different shell morphologies were evaluated and compared. Intimate contact among the nanorods is likely to improve the resulting electric conductivity. As a result, the CGCS Li[Ni0.60Co0.15Mn0.25]O2 with the nanorod shell retained approximately 85.5 % of its initial capacity over 150 cycles in the range of 2.7–4.5 V at 60 ℃. The charged electrode consisting of Li0.16[Ni0.60Co0.15Mn0.25]O2 CGCS particles with the nanorod shell also displayed a main exothermic reaction at 279.4 ℃ releasing 751.7 J g-1 of heat. In Chapter 3, Full concentration gradient (FCG) cathode material having nickel-rich core and nickel-deficient outer layer was synthesized via co-precipitation method. The BET surface area measurement demonstrated the presence of densely agglomerated primary particles in the FCG cathode. Owing to the unique spatial distribution of the cations and particle morphology, the FCG cathode delivered higher discharge capacity, superior cycle stability and excellent thermal stability compared to the CC. In Chapter 4, Li[Ni0.65Co0.13Mn0.22]O2 cathode with two-sloped full concentration gradient (TSFCG), is synthesized via a specially designed batch-type reactor. The cathode delivers a discharge capacity of 200 mAh g−1 (4.3 V cutoff ) with excellent capacity retention of 88 % after 1500 cycles in a full-cell configuration. The TSFCG cathode exhibits the best cycling stability, rate capability, and thermal stability of the three electrodes. Transmission electron microscopy analysis of the cycled TSFCG, CC, and NCA cathodes shows that the TSFCG electrode maintains both its mechanical and structural integrity whereas the NCA electrode nearly pulverizes due to the strain during cycling.