이차전지용 Ni-rich 층상형 산화물 및 탄소-황 복합체 양극에 관한 연구

Abstract

최근 리튬이온전지는 소형전자기기에 광범위하게 적용되어 왔고, 근래에 들어 ESS (energy storage system)나 HEV (hybrid electric vehicle), EV (electric vehicle)를 위한 중대형전지로 그 영역을 넓혀가고 있으나 아직 충분한 성능을 만족시키지 못 하고 있다. 전력의 효율적인 저장 및 공급, 전기자동차의 주행거리 보장을 위해서는 에너지밀도를 획기적으로 향상시킬 필요가 있다. 현재의 리튬이온전지는 에너지밀도가 150 Wh kg-1 정도로 제한적이며, 기존 소재보다 저가격, 풍부한 원료, 친환경적인 요소가 필수적이다. 이러한 요구조건을 만족할 수 있는 전극 소재로는 Ni-rich 층상형 양극 소재가 있다. 또한, 좀 더 높은 에너지밀도를 가진 중대형전지를 개발하기 위해서는 소듐황전지와 같은 가역용량이 월등히 높은 전지용 소재 개발이 시급하다. 본 논문에서는 합성법 최적화와 표면개질법을 통한 고용량, 장수명을 가진 Ni-rich 층상형 양극 소재에 대해서 논의하고, 이어서 차세대리튬이차전지로서 상온에서 구동 가능한 소듐황전지를 위한 양극 소재와 전지구성 연구에 대해 논의하였다. 제 2장에서는 공침법을 이용해 Li(Ni1-xMnx)O2 (x = 0.1, 0.2, 0.3, 0.4, 0.5) 양극 소재를 제조하고, 전기화학특성 및 구조적 분석 결과를 논의하였다. 제조된 양극 소재는 균일한 구형의 2차 입자를 가지며, 평균 10 ㎛의 크기를 나타내었다. XRD 분석을 통해 각 양극 소재들은 불순물이 없이 α-NaFeO2 구조를 가진 것을 확인하였다. 제조된 소재들 중 Li(Ni0.9Mn0.1)O2 양극 소재는 가장 낮은 정도의 cation mixing을 보였다. Li(Ni0.9Mn0.1)O2 양극 소재는 0.1 C에서 212.3 mAh g-1의 방전용량을 보였고, 10 C의 높은 전류밀도 조건에서도 160 mAh g-1의 용량을 보였다. 또한, 고전압 및 고온의 충방전 조건에서도 우수한 가역용량을 보였다. 장기간의 충방전 이 후, 전극의 XRD 및 Rietvelt refinement 결과에서 상대적으로 낮은 cation mixing 정도를 보여, 최적화된 합성조건을 통해 구조적 안정성이 우수한 양극 소재가 제조되었음을 확인하였다. 제 3장에서는 건식 볼밀링법을 통해 LiNi0.8Co0.15Al0.05O2 양극 소재의 표면을 Ni3(PO4)2로 코팅하고 향상된 전기화학특성 등을 논의하였다. SEM 및 TEM을 통해 코팅 전후의 형상변화를 분석한 결과, Ni3(PO4)2가 LiNi0.8Co0.15Al0.05O2 양극 소재 표면에 고르게 코팅된 것을 하였다. 양극 소재 표면의 TEM 분석을 통해 확인된 코팅층의 두께는 10-20 nm인 것을 확인하였다. 코팅된 양극 소재는 코팅 전에 비해, 고온 충방전 실험에서 향상된 수명을 보였고, 7 C 이상의 조건에서 출력 특성도 향상되었다. 장기간의 충방전 이후, 코팅 전의 양극 소재 표면은 열화가 심하게 발생하였으며, 이는 전기화학특성 열화의 한 요인으로 보인다. 또한, 충방전 후의 양극에 대한 XRD 분석을 통해서 코팅 이후 구조가 안정하게 유지됨을 확인하였다. 따라서, Ni3(PO4)2 코팅이 Ni-rich 양극 소재의 고온 충방전 특성 및 출력 특성 향상에 효과가 있음을 알 수 있었다. 제 4장에서는 hollow carbon sphere-sulfur (HCS-S) 양극 소재로 구성된 상온 구동 가능한 소듐황전지를 다루었다. 이를 위한 음극 소재와 전해질 소재로는 각각 Sn-C 음극 소재와 TEGDME 기반의 전해질이 사용되었다. hollow carbon sphere는 내부가 비어있는 구조를 가지고 있으며, 황 함침이후 내부가 황으로 균일하게 채워진 것을 TEM 사진을 통해 확인하였다. HCS-S 양극 소재는 0.1 C에서 1000 mAh g-1의 용량을 보였다. 이 후의 사이클에서 용량은 약 600 mAh g-1, 평균전압은 약 1.3 V를 나타내었다. Sn-C 음극 소재는 50 mA g-1의 전류밀도에서 약 180 mAh g-1, 평균전압은 약 0.3 V를 나타내었다. 각 HCS-S 양극 소재, Sn-C 음극 소재를 TEGDME4NaCF3SO3 전해질로 구성된 소듐이온황전지는 기존의 고온구동 소듐황전지와 달리 상온에서 구동가능하며, 550 mAh g-1의 용량과 1.0 V의 평균 전압을 나타내었다.| Recently, Li-ion batteries have dominated power sources for portable electric devices, and have gradually expanded their applications to the field of green transportation and stationary energy storage system. However, energy density of Li-ion batteries have to be increased to drive electric vehicles and to support renewable energy plants. Generally, the energy density of the conventional Li-ion batteries is about 150 wh kg-1 due to limited capacity of electrode materials. Other important issues on the mid-larage scale batteries are low cost, abundance and being environmentally friendly. In oder to meet above requirements, development of alternative high capacity cathode materials, such as Ni-rich layered oxides, is mandatory to replace the conventional cathode materials. Moreover, advanced electrode materials for battery with new chemistry must be developed such as cathode materials for Na-sulfur battery, which operates according to the conversion reaction of sulfur. Depend on the chemical composition and synthetic conditions, the Ni-rich cathode materials shows various electrochemical and structural properties. Furthermore, instability of interface between the surface Ni-rich cathode materials and electrolyte under severe conditions must be controlled. In the case of Na-S battery, intrinic insulating characteristic of sulfur causes low utilization of active material in the electrode. In addition to this, intermediate polysulfide dissolution during cycling lead to capacity fade and lowering efficiency of Na-sulfur cells. In this dissertation, high capacity Ni-rich layered oxides for Li-ion batteries and a strategy of hollow carbon sphere-sulfur composite for Na-sulfur batteries were discussed. In chapter 2, Li(Ni1-xMnx)O2 (x = 0.1, 0.2, 0.3, 0.4, 0.5) were synthesized by co-precipitation method. The cathode materials have regular, homogeneous morphology of spherical secondary aggregates with average size of ∼10 ㎛ for all compositions. X-ray diffraction patterns of all the samples showed impurity-free single phase of α-NaFeO2 type structure. The Li(Ni0.9Mn0.1)O2 showed the lowest cation mixing in the Li layers among samples and exhibited exceptionally high rate capacity (approximately 160 mAh g-1 at 10 C-rate) at 25 ℃ and high discharge capacity upon cycling under a severe condition, in the voltage range of 2.7-4.5 V at 55 ℃. The cation mixing in Li(Ni0.9Mn0.1)O2 increased slightly even after the extensive cycling at the elevated temperature, which is ascribed to the structural integrity induced from the optimized synthetic condition using the coprecipitation. In chapter 3, the surface of LiNi0.8Co0.15Al0.05O2 cathode materials were coated with Ni3(PO4)2 via simple ball milling. The morphology of the Ni3(PO4)2-coated LiNi0.8Co0.15Al0.05O2 particle was characterized using SEM and TEM analysis. The thickness of Ni3(PO4)2 coating layer is determined as approximately 10-20 nm by TEM. The Ni3(PO4)2-coated LiNi0.8Co0.15Al0.05O2 cell showed improved cycling life and rate capability. Scanning electron microscopy of extensively cycled particles confirmed that the particle surface of the Ni3(PO4)2-coated LiNi0.8Co0.15Al0.05O2 remained relatively undamaged, whereas pristine particles were severely serrated. EIS analysis revealed that increase of the charge transfer resistance during cycling was significantly suppressed by Ni3(PO4)2 coated LiNi0.8Co0.15Al0.05O2 cell. The stabilization of the host structure by Ni3(PO4)2 coating was also verified using X-ray diffraction. In chapter 5, hollow carbon sphere-sulfur (HCS-S) composite cathode material was developed for room temperature sodium-ion sulfur battery combined with Sn-C anode and TEGDME4NaCF3SO3 electrolyte. The hollow carbon sphere (HCS) showed hollow structure in its core. The hollow void in HCS was filled with sulfur after sulfur impregnation process. The enhanced morphology of the electrodes, the high values of conductivity and of sodium transference number, as well as the good stability of the electrolyte are unique properties that are expected to allow the development of a new sodium ion cell. HCS-S composite cathode material showed the initial discharge capacity of 1000 mAh g-1 at 0.1 C. In the following cycles, reversible capacity was stabilized to 600 mAh g-1 with average voltage of about 1.3 V. Sn-C composite anode material showed reversible capacity of 180 mAh g-1 and average voltage of 0.3 V. In deed, the results reported in this work show that this cell can provide a remarkable capacity of 550 mAh g-1 with average voltage of 1.0 V and an expected theoretical energy density of 550 Wh kg-1.; Recently, Li-ion batteries have dominated power sources for portable electric devices, and have gradually expanded their applications to the field of green transportation and stationary energy storage system. However, energy density of Li-ion batteries have to be increased to drive electric vehicles and to support renewable energy plants. Generally, the energy density of the conventional Li-ion batteries is about 150 wh kg-1 due to limited capacity of electrode materials. Other important issues on the mid-larage scale batteries are low cost, abundance and being environmentally friendly. In oder to meet above requirements, development of alternative high capacity cathode materials, such as Ni-rich layered oxides, is mandatory to replace the conventional cathode materials. Moreover, advanced electrode materials for battery with new chemistry must be developed such as cathode materials for Na-sulfur battery, which operates according to the conversion reaction of sulfur. Depend on the chemical composition and synthetic conditions, the Ni-rich cathode materials shows various electrochemical and structural properties. Furthermore, instability of interface between the surface Ni-rich cathode materials and electrolyte under severe conditions must be controlled. In the case of Na-S battery, intrinic insulating characteristic of sulfur causes low utilization of active material in the electrode. In addition to this, intermediate polysulfide dissolution during cycling lead to capacity fade and lowering efficiency of Na-sulfur cells. In this dissertation, high capacity Ni-rich layered oxides for Li-ion batteries and a strategy of hollow carbon sphere-sulfur composite for Na-sulfur batteries were discussed. In chapter 2, Li(Ni1-xMnx)O2 (x = 0.1, 0.2, 0.3, 0.4, 0.5) were synthesized by co-precipitation method. The cathode materials have regular, homogeneous morphology of spherical secondary aggregates with average size of ∼10 ㎛ for all compositions. X-ray diffraction patterns of all the samples showed impurity-free single phase of α-NaFeO2 type structure. The Li(Ni0.9Mn0.1)O2 showed the lowest cation mixing in the Li layers among samples and exhibited exceptionally high rate capacity (approximately 160 mAh g-1 at 10 C-rate) at 25 ℃ and high discharge capacity upon cycling under a severe condition, in the voltage range of 2.7-4.5 V at 55 ℃. The cation mixing in Li(Ni0.9Mn0.1)O2 increased slightly even after the extensive cycling at the elevated temperature, which is ascribed to the structural integrity induced from the optimized synthetic condition using the coprecipitation. In chapter 3, the surface of LiNi0.8Co0.15Al0.05O2 cathode materials were coated with Ni3(PO4)2 via simple ball milling. The morphology of the Ni3(PO4)2-coated LiNi0.8Co0.15Al0.05O2 particle was characterized using SEM and TEM analysis. The thickness of Ni3(PO4)2 coating layer is determined as approximately 10-20 nm by TEM. The Ni3(PO4)2-coated LiNi0.8Co0.15Al0.05O2 cell showed improved cycling life and rate capability. Scanning electron microscopy of extensively cycled particles confirmed that the particle surface of the Ni3(PO4)2-coated LiNi0.8Co0.15Al0.05O2 remained relatively undamaged, whereas pristine particles were severely serrated. EIS analysis revealed that increase of the charge transfer resistance during cycling was significantly suppressed by Ni3(PO4)2 coated LiNi0.8Co0.15Al0.05O2 cell. The stabilization of the host structure by Ni3(PO4)2 coating was also verified using X-ray diffraction. In chapter 5, hollow carbon sphere-sulfur (HCS-S) composite cathode material was developed for room temperature sodium-ion sulfur battery combined with Sn-C anode and TEGDME4NaCF3SO3 electrolyte. The hollow carbon sphere (HCS) showed hollow structure in its core. The hollow void in HCS was filled with sulfur after sulfur impregnation process. The enhanced morphology of the electrodes, the high values of conductivity and of sodium transference number, as well as the good stability of the electrolyte are unique properties that are expected to allow the development of a new sodium ion cell. HCS-S composite cathode material showed the initial discharge capacity of 1000 mAh g-1 at 0.1 C. In the following cycles, reversible capacity was stabilized to 600 mAh g-1 with average voltage of about 1.3 V. Sn-C composite anode material showed reversible capacity of 180 mAh g-1 and average voltage of 0.3 V. In deed, the results reported in this work show that this cell can provide a remarkable capacity of 550 mAh g-1 with average voltage of 1.0 V and an expected theoretical energy density of 550 Wh kg-1.