차세대 배터리용 리튬 공기 이차전지의 효율 향상에 대한 연구

Abstract

therefore, such devices should technically be called lithium-oxygen batteries. Consequently, to achieve the next step for a lithium-air battery, it is critical to understand how the reaction chemistry of such a battery is affected by non-O2 components of ambient air (such as N2, Ar, and CO2). Despite their abundance in ambient air, N2 and Ar, a noble gas, barely react with Li electrochemically within the cathode potential range, whereas CO2 can potentially react with Li to yield Li2CO3 within the cathode voltage range. It was found that the presence of CO2 significantly affects the discharge product of a Li-air cell and that the reaction pathways can be leveraged by changing the electrolyte.

Exploring a new chemistry in metal-gas systems plays a key role in creating new possibilities in the development of an ultra-high-energy-density battery. In Chapter 6, I shed new light on conventional Li-SO2 batteries, which have been widely used for military devices because of their wide operating temperature range, high energy density, and long shelf life. Although Li-SO2 batteries have been used as primary batteries to date, the feasibility of a rechargeable Li-SO2 battery is demonstrated here based on the reversible formation and decomposition of the solid product, Li2S2O4. The charge polarization is markedly lower than that of a Li-O2 cell even without a catalyst. Consequently, the observed energy efficiency of the Li-SO2 system is significantly better than that of the Li-O2 system.

The development of a new next-generation battery with high energy density lies at the heart of the emerging electric vehicle market. Among several candidates, the metal-gas battery system is particularly promising because it offers exceptionally high energy density at a level unattainable by conventional lithium-ion batteries. Especially, great interest has emerged on Li-O2 battery with a hope for ultra-high-energy-density batteries. The Li-O2 battery can deliver the highest theoretical energy density among any battery type, because the reaction between lithium and oxygen occurs directly on the electrode surface, without any heavy transition metals or crystal framework, such as LiCoO2 or LiFePO4. However, Li-O2 batteries have in the past exhibited poor rechargeability and low power capability. While various attempts have been made to resolve these problems, the limitations of poor cyclability and rate capability still remain as critical drawbacks of the Li-O2 battery. Accordingly, recent research on Li-O2 batteries has mainly focused on the enhancement of overall electrochemical properties. In each chapter, I proposed novel approaches for enhancing electrochemical properties of Li-O2 batteries and introduced new types of metal-air systems such as Li-O2/CO2 battery and Li-SO2 battery. I believe that the several approaches presented here can be used to guide the development of a new metal-gas system and will bring fresh insight to further the development of Li-air batteries. The content of each chapter is summarized as shown below.

Chapter 2 deals with the controlled porous framework of the woven carbon nanotube (CNT) sheet electrode as a new carbon cathode for Li-O2 battery. I designed hierarchical carbon electrodes with highly aligned CNT fibrils and demonstrated that the electrode significantly enhanced the cyclability and rate capability of the Li-O2 battery. The controlled framework of the woven CNT sheet electrode enabled effective formation and decomposition of lithium peroxide, resulting in enhanced cyclability. The high mechanical strength, conductivity, and flexibility of the CNT fibrils contributed to the enhanced performance.

In Chapter 3, I proposed a new air-electrode design that incorporates catalysts in the hierarchically porous framework. A simple process of catalyst loading could successfully retain the optimal air pathways of the hierarchical carbon electrode and provide the condition for effective catalytic activity. The new electrode can deliver the cycle performance over 100 cycles, with 1,000 mAh g–1 at a high current rate of 2 A g–1. Additionally, I demonstrated how the Pt catalyst affected the morphology of the discharge products, which resulted in the enhanced cyclability of the Pt/CNT electrode.

Chapter 4 introduces a novel Li-O2 battery using a soluble catalyst combined with a hierarchical nanoporous air-electrode. I combined a hierarchical nanoporous air-electrode with the LiI redox mediator as a soluble catalyst. A cross-woven CNT fibril sheet provided homogeneous macro- and microscale pores to facilitate the rapid transport of both the reaction ions and the catalyst. The porous three-dimensional network structure of the air-electrode provided a rapid highway for the soluble catalyst, which resulted in a markedly increased energy efficiency and enhanced cyclability. This will spur discussion of the optimal design of electrode architecture and the catalyst activity in Li-O2 batteries and will attract broad interest in the field of energy storage and conversion.

Chapter 5 deals with the effect of CO2 on the Li-air battery cell systems based on quantum mechanical (QM) calculations coupled with experimental verification. Up until now, most studies of lithium-air batteries were focused on the battery mechanism/operation in a pure oxygen environment