미세구조 조절을 통한 소듐이온전지용 O3-type 층상계 Na[NixCoyMnz]O2 양극 소재의 전기화학 성능 향상 연구

Abstract

Energy production and storage technologies have attracted a great deal of attention for day-to-day applications. In recent decades, advances in lithium-ion battery (LIB) technology have improved living conditions around the globe. LIBs are used in most mobile electronic devices as well as in zero-emission electronic vehicles. However, there are increasing concerns regarding load leveling of renewable energy sources and the smart grid as well as the sustainability of lithium sources due to their limited availability and consequent expected price increase. Therefore, whether LIBs alone can satisfy the rising demand for small- and/or mid-to-large-format energy storage applications remains unclear. To mitigate these issues, recent research has focused on alternative energy storage systems. Sodium-ion batteries (SIBs) are considered as the best candidate power sources because sodium is widely available and exhibits similar chemistry to that of LIBs. Recently, sodiated layer transition metal oxides, phosphates and organic compounds have been introduced as cathode materials for SIBs. Among the candidate materials, O3-type layered structured transition metal oxides most widely explored and developed based on the success of using layered LiMO2 cathodes in LIBs. However, developing a O3-type layered cathode that is capable of delivering a high capacity with stable cycle retention and thermal stability remains a formidable challenge due to the critical issues as follows. First, sluggish mobility of Na+ ions due to the large ionic size (1.02 Å) is still a challenge to fully utilize their theoretical capacity. Second, the charge/ discharge curves of typical O3-type cathode materials usually involve several voltage plateaus and steps, which reflect multiple phase transformations occurring along with TMO2 slab gliding, which lead to the gradual capacity fading during prolonged cycling. Finally, another capacity degradation mechanism is the oxidative electrolyte decomposition above 4V (especially in high nickel composition compound) and the subsequent formation of HF in the acidic electrolyte solution containing NaPF6 salt, which has been shown to restrict the reversibility of Na+ ions during the charge-discharge process. This study focused on developing the advanced O3-type Na[NixCoyMnz]O2 (x+y+z=1) cathode materials for high performance sodium-ion batteries. First, we systematically investigated the role of each transitionmetals in O3-type Na[NixCoyMnz]O2 (x=1/3, 0.5, 0.6, and 0.8) cathode via electrochemical property characterization, structural analysis, and thermal stability testing. In chapter 1, a comprehensive study of Na[NixCoyMnz]O2 (x=1/3, 0.5, 0.6, and 0.8) cathodes is carried out to determine the optimal composition as the electrochemical, structural, and thermal properties in O3-type layered cathodes are strongly dependent on the transition metal composition. Briefly, an increase of the Ni fraction resulted in an increasingly higher capacity but is accompanied by progressively poor capacity retention. On the other hand, the Co metal played an important role in stabilizing the structure, while the Mn content contributed to enhancing the capacity retention and thermalstability. The present study highlights the importance of appropriately balancing the transition metal composition in a layered Na[NixCoyMnz]O2 cathode. Furthermore, this work provides a design guideline for developing an ideal Na[NixCoyMnz]O2 cathode with both high capacity and optimal cycle retention in addition to thermal stability. Based on fundamental test results, we have designed the new concept that having spoke-like nanorod assemblies arranged in spherical secondary particles by varying the chemical composition from Ni-rich core to Mn-rich shell of the structure. In Chapter 2, we present a composition-graded cathode with average composition Na[Ni0.61Co0.12Mn0.27]O2, which exhibits excellent performance and stability. Inaddition to the concentration gradients of the transition metalions, the cathode is composed of spoke-like nanorods assembled into a spherical superstructure. Individual nanorod particles also possess strong crystallographic texture with respect to the center of the spherical particle. Such morphology allows the spoke-like nanorods to assemble into a compact structure that minimizes its porosity and maximizes its mechanical strength while facilitating Na+-ion transport into the particle interior. Micro-compression tests have explicitly verified the mechanical robustness of the composition-graded cathode and single particle electrochemical measurements have demonstrated the electrochemical stability during Na+-ion insertion and extraction at high rates. Such particles are very robust and mechanically stable. This robustness also promotes electrochemical and thermal stability. The low porosity and preferential crystallographic orientation facilitate excellent transport properties for Na+ ion insertion/deinsertion processes. In addition, electrochemical reaction based on Ni2+/3+/4+ is readily available to deliver a high discharge capacity and excellent rate capability. Finally, to mitigate surface deterioration include reducing the exposed area of the active material, I proposed simple practical strategy for resolving the degradation pathways of cathode surface and enhancing the battery performance of O3-type cathodes by using a coating of nanosized inert materials. In Chapter 3, a surface-modified O3-type Na[Ni0.6Co0.2Mn0.2]O2 cathode was synthesized by Al2O3 nanoparticle coating using a simple dry ball-milling route. The nanoscale Al2O3 particles (~15 nm in diameter) densely covering the spherical O3-type Na[Ni0.6Co0.2Mn0.2]O2 cathode particles effectively minimized parasitic reactions with the electrolyte solution while assisting Na+ migration. The proposed Al2O3 coated Na[Ni0.6Co0.2Mn0.2]O2 cathode exhibited a high specific capacity of 151 mA h g-1, as well as improved cycling stability and rate capability in a half cell. Furthermore, the Al2O3 coated cathode was scaled up to a pouch-type full cell using a hard carbon anode that exhibited a superior rate capability and capacity retention of 75 % after 300 cycles with a high energy density of 130 Wh kg-1. Inaddition, the postmortem surface characterization of the cathodes from the long-term cycled full cells helped in identifying the exact mechanism of the surface reaction with the electrolyte and the reason for its subsequent degradation and showed that the nano-scale Al2O3 coating layer was effective at resolving the degradation pathways of the cathode surface from hydrogen fluoride (HF) attack. Based on my research results on electrochemical performances of O3-type layered oxide cathode materials for sodium-ion batteries, I could conclude that it is not possible to develop an ideal cathode material having both high capacity and optimal thermal safety by simply changing the composition. It is necessary to balance the physico-chemical role of each transition metal in an advanced microstructure design. Furthermore, understanding the surface chemistry behind the electrochemical activity is also very important to address the practical issues of current SIBs. Although the practical use of an SIB based on unique cathode design and surface modification will require further work, it is believed that the results presented here will be helpful for understanding the SIB system and further developing commercial SIBs.