Surface and interface modifications for improving reversibility of metal electrodes

Abstract

With the development of secondary battery technology, the battery usage area of our society has gradually expanded. In particular, lithium secondary batteries have popularly been used in most recent IT and mobile devices. However, with the development of lithium secondary batteries to their capacity limits, there is a growing interest in developing new types of batteries with higher energy densities and lower prices, which are called as ‘next generation batteries’. In addition, as the need for development of electric vehicles and the smart grid industry has recently increased, the research on next-generation batteries is also more accelerating. One method for realizing a secondary battery having a high energy density is to use a metal electrode as anode. Since the metal has a relatively low redox potential and a high theoretical capacity, it is possible to 2 or 3-times increase the energy density of the battery, by simply replacing the graphite anode with a metal foils, whose type of next generation battery is ‘metal anode battery’. In addition, in order to realize other types of next generation batteries such as metal-sulfur and metal-oxygen batteries, the use of metallic anode is more essential because sulfur or oxygen cathode doesn’t include ion sources. However, some metallic anodes such as lithium (Li) and sodium (Na) have a critical problem, which is inhomogeneous plating/stripping behaviors during battery cycling, so it is impossible to directly use them as anode materials at the current technology levels due to low durability and safety issues. Furthermore, due to high reactivity of Li and Na metals with liquid electrolytes, the failure of metallic electrode can be accelerated by forming a thick porous layer on the surface of metallic electrode, consisting of solid electrolyte interphase shells and dead Li or Na. Therefore, in order to successively apply these metallic anodes to Li or Na secondary batteries, it is definitely needed to stabilize those metal under electrochemical operating conditions. In this thesis, in order to alleviate these critical problem of metallic electrodes, the various methods to suppress inhomogeneous Li or Na electrodeposition and to induce improved reversibility of metallic electrodes are dealt with. Chapter 2 include the mechanical stabilization of Na metal electrode by introducing composite protective layer (CPL) on the surface of Na electrode. Especially, by controlling the ratio of PC in the CPL, the modulus of CPL is also controlled for confirming the critical points of mechanical properties of CPL for suppressing Na dendritic growth. Chapter 3 deals with the facet modulation of Cu current collector for inducing uniform Li electrodeposition on heterogeneous Cu substrate. By aligning the surface crystallinity of Cu current collector, the fabrication of uniform thin-film Li metal electrode can be possible with an improved cycling durability of Li metal electrode. Finally, on chapter 4, CNT-Na composite electrode was fabricated by a simple rolling and folding method for leading to uniform Na electrodeposition and facile utilization of Na metal electrode. Due to electronic and ionic conductivities of embedded sodiated-CNT network, the nucleation mode of Na can be changed to make more Na nuclei density with high uniformity and the reversibility of composite electrode is also highly improved. The effectivity of all above researches are evaluated and discussed in the systems of metal anode applied Li or Na secondary batteries with intercalation-based cathode.