Development of Stabilization Method for Alkali Metal Anodes

Abstract

From cell phones to electric vehicles, lithium-ion battery (LIB) with various advantages (low self-discharge, high operation voltage, high energy density, and no memory effect) has made our life comfortable. However, LIB technology is similar with when it was commercialized while developed technology in other parts dramatically increased the energy consumption. Therefore, developing the new energy storage system that can fulfil the energy consumption is highly required. As the next-generation secondary batteries, metal-sulfur, metal-air, and all-solid-state are promising candidate that can exceed the limited energy density of LIBs. In these systems, alkali metals (Li, Na) with low reduction potential (Li: −3.04 V, Na: −2.71 V vs. SHE) and high theoretical capacity (Li: 3860 mA h g−1, Na: 1166 mA h g−1) are employed. However, there are severe problems that hinder the practical use of alkali metal anodes: high reactivity, dendritic growth, severe volume change. These problems induce poor electrochemical performance and even possibly generate short circuit. Therefore, developing proper methods that can stabilize alkali metal anode and suppress the dendrite formation is necessary for commercialization. In Chapter 2, mesoporous carbon was applied as host material for Li metal anode. By increasing the Sand’s time with mesoporous material, Li metal was effectively confined in the mesopores and improved the cycle performance of Li metal anode. Furthermore, Li-plated mesoporous carbon as an anode in lithium-sulfur battery (Li-S) full cell. In Chapter 3, a polymer film composed of sodium alginates (Na-Alg), which is an natural biopolymer from brown algae, and poly(ethylene oxide) (PEO) was applied as a separator. Na-Alg sustained a film structure and PEO allowed Li-ion diffusion by absorbing liquid electrolyte. Without any use of additives in commercial carbonate electrolyte, this method extended the cycle life of Li metal symmetric cell to 1000 h with overpotential < 10 mV. The polymer film also increased the cycle life of a full-cell with LiNi0.6Co0.2Mn0.2O2 cathode. In Chapter 4, fluorine-free Na-electrolyte using NaBH4 was used in the electrolyte for Na metal anode. Even without fluorine which can help the formation of stable SEI with NaF, NaBH4 dissolved ether electrolyte stabilized Na metal anode by showing cycle life of 2000 hours in symmetric cell. Also, Na metal was smoothly plated without showing micro-meter scale dendrites. The improved Na metal anode stability can be due to stable SEI formation, which will be analysed in further studies. In this thesis, alkali metal anode stabilizing methods were studied in various points of view: electrode, separator, and electrolyte. Since every component in battery can affect the electrochemical performance, understanding each components and combining the effective method can accelerate the practical use of alkali metal anode. Also, this study suggests watching every sides of the system can be efficient strategy to solve the problems.