Low Dimensional Nanostructured Materials for Rechargeable Metal-Ion Batteries

Abstract

Interest in energy-functional nanomaterials for electrical energy storage has grown sustainably due to the rapid increase in demand for electrical energy. This thesis has suggested various approaches to improve the electrochemical activity for energy storage. The utility of nanomaterials which are composited with other inorganic materials or changed the interlayer property are deeply studied in the field of anode materials for sodium-ion battery (SIB). The crystalline change and intimate mixed nanocomposite with graphene or other inorganic materials are evaluated as the advanced anode material of lithium-ion battery (LIB). Finally, the nanosheets with small pores and high electrical conductivity are synthesized and verified the possibility as the electrocatalytic material. This thesis consists of 8 chapters. In chapter I, the general introduction about the basic principles how to convert electrical energy to chemical energy and the energy storage equipment on each methodes are provided. The possibility and importance of 2D nanosheets as energy storage material are also explained in this chapter. In chapter II, the usefulness in composite formation between the semiconducting metal oxide and metal sulfide is investigated in exploring new efficient NIB electrode materials. The heat-treatment of TiO2(B) under CS2 flow yields an intimately-coupled TiO2(B)TiS2 nanocomposite with intervened TiS2 domain, since the reaction between metal oxide and CS2 leads to the formation of metal sulfide and CO2. The negligible change in lattice parameters and significant enhancement of visible light absorption upon the reaction with CS2 underscore the formation of conductive metal sulfide domains. The resulting TiO2(B)TiS2 nanocomposites deliver greater discharge capacities with better rate characteristics for electrochemical sodiationdesodiation process than does the pristine TiO2(B). The 23Na magic angle spinning nuclear magnetic resonance analysis clearly demonstrates that the electrode activities of the present nanocomposites rely on the capacitive storage of Na+ ions and the TiS2 domains in TiO2(B)TiS2 nanocomposites play a role as mediators for Na+ ions to and from TiO2(B) domains. According to the electrochemical impedance spectroscopy, the reaction with CS2 leads to the significant enhancement of charge transfer kinetics, which is responsible for the accompanying improvement in electrode performance. In chapter III, the effect of the chemical environments of the interlayer Na sites of layered titanate is observed to explore an effective way to improve its electrode functionality for SIB. The interlayer sites are finely controlled by the intercalation of n-alkylamine with various alkyl chain lengths. The n-alkylamine intercalation via ion-exchange and exfoliationrestacking routes allows the modification of in-plane structures of layered titanate to be tuned. Among the present n-alkylamine-intercalates, the n-pentylamine-intercalated titanate shows the largest discharge capacity with the best rate characteristics, underscoring the critical role of optimized intracrystalline structure in improving the SIB electrode performance of layered titanate. The creation of turbostratic in-plane structure degrades the SIB electrode performance of layered titanate, indicating the detrimental effect of in-plane structural disorder on electrode activity. 23Na magic angle spinning nuclear magnetic resonance spectroscopy demonstrates that the n-alkylamine-intercalated titanates possess two different interlayer Na+ sites near ammonium head groups/titanate layers and near alkyl chains. The intercalation of long chain molecules increases the population of the latter site and the overall mobility of Na+ ions, which is responsible for the improvement of electrode activity upon n-alkylamine intercalation. In chapter IV, the effect of the amorphous structure and nanocrystalline nature of metal oxide on its anode performance in LIBs is investigated with two nanocrystalline and one well-crystallized layered manganese oxides. X-ray amorphous manganese oxide nanocrystals are synthesized by soft-chemical redox reactions using reducing agents of KBH4 and LiI at room temperature, whereas well-crystallized layered manganese oxide is obtained by solid state reaction at elevated temperature. Although both of the amorphous manganese oxides lack a long-range structural order, they are crystallized with a layered MnO2-type local structure, which is nearly identical to the crystal structure of the well-crystallized K0.45MnO2. In comparison with the well-crystallized K0.45MnO2, both the amorphous manganese oxides commonly possess smaller particle sizes with larger surface areas and better homogeneity of composite structure. The amorphous manganese oxide nanocrystals show better anode performance with greater discharge capacity for LIBs than does the well-crystallized K0.45MnO2, which is attributable to the greater surface area, higher structural and electrochemical stability, more homogeneous composite structure, and better charge-transfer characteristics of the amorphous materials. In chapter V, evolution of the chemical bonding nature and electrochemical activity of indium selenide upon the composite formation with carbon species is systematically investigated. Nanocomposites of In4Se2.85@graphene and In4Se2.85@carbon-black are synthesized via a solid-state reaction between In and Se elements, and the following high energy mechanical milling of In4Se2.85 with graphene and carbon-black, respectively. The high energy mechanical milling (HEMM) of In4Se2.85 with carbon species gives rise to a decrease of particle size with a significant depression of the crystallinity of In4Se2.85 phase. In contrast to the composite formation with carbon-black, that with graphene induces a notable decrease of (InSe) bond covalency, underscoring significant chemical interaction between graphene and In4Se2.85. Both the nanocomposites of In4Se2.85@graphene and In4Se2.85@carbon-black show much better anode performance for LIBs with larger discharge capacity and better cyclability than does the pristine In4Se2.85 material, indicating the beneficial effect of composite formation on the electrochemical activity of indium selenide. Between the present nanocomposites, the electrode performance of the In4Se2.85@graphene nanocomposite is superior to that of the In4Se2.85@carbon-black nanocomposite, which is attributable to the weakening of (In-Se) bonds upon the composite formation with graphene as well as to the better mixing between In4Se2.85 and graphene. In chapter VI, the usefulness of the intimately coupled nanocomposite of exfoliated nanosheet and metal cation as the precursor of new advanced electrode material is provided. The nanocomposite of Fe2O3 and Mn3O4 is synthesized by an electrostatically-derived self-assembly between exfoliated MnO2 nanosheets and Fe cations, which is followed by heat-treatment at elevated temperature. The as-prepared Felayered MnO2 nanocomposite experiences phase transformations into Fe-substituted Mn3xFexO4 nanoparticle at 450 oC and Fe2O3Mn3O4 nanocomposite at 650 oC. The Fe2O3Mn3O4 nanocomposite shows better performance as anode material for LIBs than the Fe-substituted Mn3xFexO4 nanoparticle, indicating the beneficial effect of composite formation on the electrode performance of 3d metal oxide. In chapter VII, the merits of the holey 2D nanosheet morphology as the metal oxide electrocatalysts are studied. The holey 2D nanosheets of low-valent Mn2O3 are synthesized by thermally-induced phase transition of exfoliated layered MnO2 nanosheets. The heat-treatment of layered MnO2 nanosheets at elevated temperatures leads not only to transitions to low-valent manganese oxides but also to the creation of surface hole in the 2D nanosheet crystallites. Despite distinct phase transitions, highly anisotropic 2D morphology of the precursor MnO2 material remains intact upon the heat-treatment whereas the diameter of surface hole becomes larger with increasing heating temperature. The obtained holey 2D Mn2O3 nanosheets show promising electrocatalyst performances for oxygen evolution reaction, which are much superior to that of non-porous Mn2O3 crystal. Of prime importance is that this material shows much better catalytic activity for LiO2 batteries than does non-porous Mn2O3, underscoring the critical role of porous 2D morphology in this functionality. In chapter VIII, the simple and effective way to synthesize holey titanium oxynitride 2D nanosheet with high electrical conductivity and excellent electrocatalytic activity are provided. Holey titanium oxynitride 2D nanosheets are synthesized by the heat-treatment of exfoliated layered titanate nanosheets at 700900 oC under ammonia gas flow. The heat-treatment induces a phase transition from lepidocrocite-structured titanate to rocksalt-structured TiO1-xNx with the creation of surface holes in 2D nanosheet crystallite. An elevation of heating temperature leads to the expansion of unit cell and the enlargement of surface holes whereas the highly anisotropic 2D nanosheet morphology of the precursor remains unchanged. The calcined materials show mesoporous stacking structure of 2D nanosheets having thickness of ~30 nm and much larger lateral size of ~0.1-3 um. The formation of highly conductive TiO1xNx is obviously evidenced by remarkable color change to dark blue and the increase of light absorptivity. X-ray photoelectron spectroscopic analysis clearly demonstrates the incorporation of nitrogen element into titanium oxide lattice and the decrease of Ti oxidation state with the partial formation of Ti3+ ion. The obtained holey TiO1xNx 2D nanosheet displays promising electrocatalytic activity for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). In chapter IX, the overall conclusion of these researches and the future scope about the research of energy-xfunctional nanomaterials are provided.;급격한 전기 에너지 수요와 맞물려 전기 에너지 저장을 위한 에너지 기능성 나노 소재에 대한 관심은 지속적으로 높아지고 있다. 본 학위논문은 나노 소재 전극 물질의 전극 활성을 향상시키기 위한 다양한 접근 방법을 제시하였다. 전기전도성이 큰 다른 무기 물질과 컴포짓하거나 층상 물질의 층간 성질을 변화시켜 소듐 이온 전지의 음극재로 활용하였다. 또한 그라핀이나 이종의 무기 물질과 컴포짓하거나 비정질 구조를 형성하여 리튬 이온 전지 음극재로서의 활성 차이를 평가하였다. 마지막으로 포어를 가지는 나노시트 형태의 무기물질을 합성하여 전기 촉매 로서의 가능성을 평가하였다. 본 논문은 총 9장으로 구성되어 있으며 1장에서는 에너지 저장 장치 및 전기 에너지를 화학 에너지로 바꾸는 원리에 대해 기술하였다. 또한 에너지 저장 소재로서 이차원 나노 소재가 가지는 가능성과 중요성에 대해 서술하였다. 2장에서는 티타네이트 나노 와이어와 티탄 황화물의 컴포지트를 소듐 이온 음극재로서 평가하였다. 티탄 황화물은 CS2 기체를 흘려주며 고온에서 열처리하는 과정을 통해 티타네이트 나노 와이어 표면에 미량 형성된다. 티탄 황화물이 형성된 컴포지트는 티타네이트 나노 와이어보다 향상된 전극 성능을 가진다. 전극 성능 향상의 원인은 티탄 황화물의 높은 전도도와 티탄 황화물의 캐패시티브한 이온 저장 능력 그리고 티타네이트 나노 와이어로의 소듐 이온 전달 중재 역할에 의한 것으로 파악하였다. 3장에서는 층상 티타네이트에 다양한 길이의 알킬 아민을 삽입하여 층간 거리를 변화시킨 후 소듐 이온 전지 음극재로서의 성능 변화를 관찰하였다. 알킬 아민이 삽입된 층상 티타네이트는 층간의 세슘 이온을 프로톤으로 치환한 후 다시 알킬 아민을 삽입하는 방법으로 합성하였다. 층간 거리가 넓어지면 소듐 이온이 들어갈 수 있는 구역은 넓어지나 무극성의 알킬 체인에 의해 소듐 이온의 이동이 제한될 수 있다. 따라서 적절한 길이의 알킬 아민이 삽입된 경우 소듐이 들어가기에 최적화된 화학적 환경이 조성되어 가장 좋은 소듐 전극 특성을 가지는 것을 입증하였다. 4장에서는 망간 산화물 비정질 나노 물질이 결정질 물질에 비해 리튬 이온 전지의 음극 물질로 좋은 성능을 가질 수 있다는 것을 증명하였다. 비정질 물질의 국부구조는 결정질 물질의 구조와 같이 층상 구조를 가진다는 것을 EXAFS 분석을 통해 확인하였다. 합성한 비정질 물질은 큰 비표면적, 구조적 안정성, 좋은 전하 전달 특성으로 인해 리튬 이온 배터리의 음극재로서 높은 용량을 가질 수 있다는 것을 확인하였다. 5장에서는 볼밀을 통한 인듐 셀레나이드와 카본 물질 컴포지트의 결합 특성과 전기화학적 특성에 대해 기술하였다. 컴포지트 물질은 볼밀 과정을 통해 입자 크기와 결정성이 크게 줄어드는 것을 확인하였다. 또한 그라핀과 컴포지트한 경우 (InSe) 결합의 공유성이 줄어든다는 것을 확인하였다. 리튬 이온 전지 음극 성능 측정을 통해 볼밀 과정을 통해 합성한 금속 찰코게나이드-카본 나노컴포지트가 좋은 전극 물질이 될 수 있음을 증명하였다. 6장에서는 박리화된 나노시트와 금속 양이온을 빠르게 컴포짓한 물질을 열처리하여 전극 물질로 사용할 수 있음을 확인하였다. 이를 위해 망간 산화물 나노시트와 철 2가 양이온을 정전기적 방법으로 재조합한 후 온도를 다르게 조절하며 열처리하여 철이 치환된 Mn3-xFexO4와 Fe2O3-Mn3O4 나노컴포지트를 형성하였다. Fe2O3-Mn3O4는 Mn3-xFexO4에 비해 좋은 리튬 이온 배터리 음극 성능을 보였다. 7장에서는 포어가 있는 금속 산화물 나노시트를 합성하였으며 이러한 형태가 전기화학 촉매 및 리튬-공기전지의 전극 촉매로서 유용하다는 것을 확인하였다. 층상 망간 산화물 나노시트를 높은 온도에서 열처리하면 낮은 산화수의 망간 산화물이 형성될 뿐 아니라 이차원 나노시트 구조에 포어가 형성되는 것을 확인할 수 있었다. 이렇게 얻은 포어가 있는 Mn2O3 나노시트는 벌크 Mn2O3에 비해 전기 촉매 활성 및 리튬 공기 전지에서의 활성이 크게 증가하였다. 8장에서는 나노시트를 출발 물질로 하여 포어가 있으면서 전기전도성이 높은 나노 시트를 합성할 수 있으며 이를 전기 촉매로 활용할 수 있음을 보였다. 티타네이트 나노시트를 암모니아 분위기에서 열처리하여 표면에 포어가 있는 포러스한 나노시트를 합성하였으며 이것의 전기 촉매 활성을 확인하였다. 마지막으로 9장에서는 본 논문에서 보고한 연구의 결론과 전극 기능성 물질 로서의 나노 물질 연구에 대한 전망을 제시하였다.