전 구간 농도구배를 갖는 리튬이차전지용 층상계 양극소재의 전이금속 조성 변형에 의한 전기화학적 특성 개선에 관한 연구

Abstract

리튬이온 2차 전지는 높은 에너지 밀도와 뛰어난 수명특성 및 율속 성능으로 인해 전기자동차 (EVs, PHEVs) 의 전력원으로 주목받고 있다. 그러나 전기자동차의 상용화를 위해서는 좀 더 높은 에너지 밀도 및 향상된 안정성을 갖는 전지가 요구되며, 이를 위해서 고용량을 갖는 양극 활물질 개발이 지속적으로 이루어지고 있다. 근래에 들어서, LiCoO2의 우수한 율속 성능, LiNiO2의 높은 용량, 그리고 Mn4+로 인한 구조적 안정성이 결합된 층상형태의 Li[Ni1−x−yCoxMny]O2 물질이 높은 관심을 받고 있으며 특히 높은 방전용량을 발현하는 Ni 과량의 조성의 Li[Ni1-xMx]O2 (M=전이금속, x ≤ 0.2) 물질은 PHEVs 와 EVs 용도의 배터리에 사용될 가장 유력한 양극 활물질로 조명받고 있다. 그러나, Ni 과량 물질은 충전된 상태의 전극에서 발생되는 산소 발생 및, 불안정한 Ni4+가 쉽게 NiO-like phase로 상변이가 일어남으로 인한 불안정한 열 안정성 및 수명특성 저하의 문제를 가지고 있다. 이러한 문제를 해결하기 위하여 선양국 교수 연구팀은 전구간 농도구배 (FCG)를 갖는 형태의 양극 활물질을 개발하였다. FCG 양극 활물질은 구형의 2차입자의 내부에 Ni 과량의 조성이, 외부에 Mn 과량 조성이 배치된 형태로, 높은 용량, 안정적인 수명특성 및 우수한 안정성을 갖는다. 본 논문에서는 FCG 타입의 양극 활물질의 최적화를 위한 Mn 조성 및 표면의 유지구간에 대해 논의하고, 이어서 FCG 및 Ni-rich 계열의 양극활물질이 전기화학적 테스트 중에 생성되는 구조적인 변화에 대하여 논의하였다. 제 1장에서는, 2차입자의 중심에서 표면의로 Ni 조성이 감소하고, Co 조성이 증가, Mn 조성은 고정되어있는 형태의 FCG 타입 양극활물질을 공침법을 통해 제작하였다. 조성 최적화를 위해, Mn 조성이 20, 25, 33% 을 갖는 3종류의 FCG 타입의 양극 활물질을 비교하였고, 통상적인 층상형 양극 활물질과 같이 Mn 조성이 높을수록 우수한 수명특성 및 열 안정성을 보이나 낮은 방전용량을 구현하는 경향을 보였다. 결과적으로 25% 의 Mn 조성을 같는 FCG Li[Ni0.59Co0.16Mn0.25]O2 물질이 4.3 V, 25 ℃ 조건에서 188 mAh g−1의 방전용량 및 96%의 수명특성으로 가장 최적화 된 것으로 보인다. 제 2장에서는, 마찬가지의 FCG 형태의 활물질의 2차입자 표면에 단일조성으로 이루어진 표면층의 두께에 따른 전기화학 및 구조적 특성변화를 평가하였다. 단일조성의 표면층이 두꺼울수록 1차입자의 두께가 증가하는 현상을 보였으며, 안정적인 수명특성을 보였다. 반면, 표면층의 두께가 증가할수록 율속특성 및 저온에서의 성능이 저하되는 현상을 보였다. 제 3장에서는, 막대 형태를 갖으며 약 2.5μm 의 길이의 1차입자가 구형의 2차입자의 지름방향으로 나열되어있는 형태의 FCG Li[Ni0.59Co0.16Mn0.25]O2 양극 활물질을 이용해 full cell 제작 후 2500 사이클링을 진행, 83.3%의 수명특성을 보였다. 이후 분석을 통해 장기 사이클링이 진행되면서 활물질 입자 내부에 microstrain이 발생, 결과적으로 crack이 형성되는 현상이 발견되었다. 이러한 결과로 미루어 볼 때 격자상수의 부조화에 따른 microstrain 억제를 통하여 FCG 형태의 양극활물질의 성능 개선이 이루어질 것으로 전망하였다. 제 4장에서는. 공침법을 이용하여 Li[Ni0.90Co0.05Mn0.05]O2, Li[Ni0.95Co0.025Mn0.025]O2 , LiNiO2을 합성하였고, 2.7V에서 4.3V 에서의 0.1C 테스트에서 각각 221 mAh g-1, 230 mAh g-1, 240 mAh g-1의 용량을 나타냈으며, 100사이클에서 초기용량대비 70%이상의 수명특성을 보였다. 지금까지 보고된 바 없는 높은 가역용량은 구조 내에서 과량의 리튬이온과 cation mixing으로 생성된 rock-salt 상으로 인한 것으로 보인다. 생성된 rock-salt 상은 전기화학적으로 비활성구조이나, 과량의 리튬으로 인해 리튬이온의 이동이 가능하며, 리튬이온이 탈리된 상태에서 구조를 안정화 시킨다.| Rechargeable lithium-ion batteries has been received great attention as the power source in plug-in hybrid vehicles (PHEVs) and electric vehicles (EVs) with high energy density, long cycle life, and excellent rate capability. However, commercialization of these batteries for the automobile industry requires further improvements in energy density and safety. In order to meet these requirements, considerable efforts have been focused on finding new high-capacity electrode materials, especially cathode materials. At the present time, special attention has been focused on the series of layered materials, Li[Ni1−y−zCoyMnz]O2, which combines the rate performance of LiCoO2, the high capacity of LiNiO2, and the structural stabilization imparted by the presence of Mn4+. Particularly, the Ni-rich layered composite oxides Li[Ni1-xMx]O2 (M = transitionmetal, x ≤ 0.2), are considered the most promising cathode materials for PHEVs and EVs because they offer higher capacity than existing cathode materials. However, the Ni-rich materials have poor thermal stability due to oxygen release from the charged electrode, which can lead to a severe thermal runaway with explosion. Moreover, the unstable Ni4+ easily reduce to inactive NiO-like phase, resulting in high solid–electrolyte interfacial impedance and poor cycle life of the cell. To overcome these problems of Ni-rich layered materials, Sun et al. have developed functional lithium nickel-cobalt-manganese oxide cathode materials which have full concentration gradient (FCG) structure. These materials are composed of a Mn-rich outer surface providing excellent safety and a Ni-rich center delivering a high capacity. These material shows high capacity at high voltage cycling, outstanding cycle life, and excellent safety performance. In this dissertation, optimization of layered type FCG materials was reported in terms of Mn concentration and outer layer thickness. Also, structural change of FCG and conventional Ni-rich cathode materials was discussed. In chapter 1, Li[NixCoyMn1−x−y]O2 cathode materials were synthesized with varying concentration gradients of Ni and Co ions from the particle center to the surface with fixed Mn concentrations. In particular, the Mn concentration (20, 25, and 33 mol %) was controlled to optimize electrode performance. The average chemical compositions of lithiated products were Li[NixCo0.16Mn0.84−x]O2 (x = 0.64, 0.59, 0.51). These cathode materials with concentration gradients followed the general performance trend of conventional layered materials; an increase in Ni content improved the capacity, whereas a higher amount of Mn delivered better capacity retention and thermal properties at the expense of capacity. As a result, we determined an optimal level of Mn concentration among the tested FCG cathodes, which maximized the discharge capacity of 188 mAh g−1 and had an excellent capacity retention of 96% over 100 cycles operated up to 4.3 V at 25 ℃, with a composition of FCG Li[Ni0.59Co0.16Mn0.25]O2. In chapter 2, Full concentration gradient (FCG) layered cathode materials Li[Ni0.6-xCo0.15+xMn0.25]O2 (x = 0, 0.01, and 0.04) with different outer layer thicknesses are synthesized via a specially developed co-precipitation method. In the FCG cathode, the nickel concentration decreases linearly and the cobalt concentration increases from the center to particle surface throughout the particle at a fixed composition of Mn. The thickness of the FCG primary particle increases in the radial direction with an increasing outer layer thickness of the secondary particles and significantly affects the electrochemical performance. An increase in the stable outer layer thickness improves the cycle performance of the FCG materials at the expense of reversible capacity, whereas the rate capability and low temperature performance are significantly deteriorated by increasing outer layer thickness. All of the FCG materials exhibit superior electrochemical properties compared to the conventional cathode Li[Ni0.58Co0.17Mn0.25]O2 due to the unique microstructure of the FCG cathode. In chapter 3, The synthesized full concentration gradient (FCG) material with a fixed manganese composition has an average composition of Li[Ni0.59Co0.16Mn0.25]O2 and is composed of rod-shaped primary particles whose length reaches 2.5 μm, growing in the radial direction. Electrochemical characterization demonstrated that a full cell with this cathode can be continuously operated for 2500 cycles with a capacity retention of 83.3%. Electron microscopy and high-resolution X-ray diffraction were employed to investigate the structural change of the cathode material after this extensive electrochemical testing. It was found that microstrain developed during the continuous charge/discharge cycling, resulting in cracking of nanoplates. This finding suggests that the performance of the cathode material can be further improved by optimizing the concentration gradient to minimize the microstrain and to reduce the lattice mismatch during cycling. In chapter 4, Via co-precipitation method, we synthesized Li[Ni0.90Co0.05Mn0.05]O2, Li[Ni0.95Co0.025Mn0.025]O2, and LiNiO2 which delivered 221, 230, and 240 mAh g−1, respectively, when cycled from 2.7 to 4.3 V vs. Li0/Li+ at 0.1 C and retained ∼70% of the initial capacity after 100 cycles. To date, such high reversible capacities are not yet to be reported from the Ni-rich Li[Ni1−x−yCoxMny]O2 cathodes. The observed high capacities were attributed to the presence of a rock salt phase from severe cation mixing and excess Li ions in the host structure. It is believed that the rock salt phase stabilized the host structure in the delithiated state while the excess Li allowed the Li ions percolated through the rock salt phase which would be electrochemically inactive otherwise.; Rechargeable lithium-ion batteries has been received great attention as the power source in plug-in hybrid vehicles (PHEVs) and electric vehicles (EVs) with high energy density, long cycle life, and excellent rate capability. However, commercialization of these batteries for the automobile industry requires further improvements in energy density and safety. In order to meet these requirements, considerable efforts have been focused on finding new high-capacity electrode materials, especially cathode materials. At the present time, special attention has been focused on the series of layered materials, Li[Ni1−y−zCoyMnz]O2, which combines the rate performance of LiCoO2, the high capacity of LiNiO2, and the structural stabilization imparted by the presence of Mn4+. Particularly, the Ni-rich layered composite oxides Li[Ni1-xMx]O2 (M = transitionmetal, x ≤ 0.2), are considered the most promising cathode materials for PHEVs and EVs because they offer higher capacity than existing cathode materials. However, the Ni-rich materials have poor thermal stability due to oxygen release from the charged electrode, which can lead to a severe thermal runaway with explosion. Moreover, the unstable Ni4+ easily reduce to inactive NiO-like phase, resulting in high solid–electrolyte interfacial impedance and poor cycle life of the cell. To overcome these problems of Ni-rich layered materials, Sun et al. have developed functional lithium nickel-cobalt-manganese oxide cathode materials which have full concentration gradient (FCG) structure. These materials are composed of a Mn-rich outer surface providing excellent safety and a Ni-rich center delivering a high capacity. These material shows high capacity at high voltage cycling, outstanding cycle life, and excellent safety performance. In this dissertation, optimization of layered type FCG materials was reported in terms of Mn concentration and outer layer thickness. Also, structural change of FCG and conventional Ni-rich cathode materials was discussed. In chapter 1, Li[NixCoyMn1−x−y]O2 cathode materials were synthesized with varying concentration gradients of Ni and Co ions from the particle center to the surface with fixed Mn concentrations. In particular, the Mn concentration (20, 25, and 33 mol %) was controlled to optimize electrode performance. The average chemical compositions of lithiated products were Li[NixCo0.16Mn0.84−x]O2 (x = 0.64, 0.59, 0.51). These cathode materials with concentration gradients followed the general performance trend of conventional layered materials