Simple and Economic Methods to Improve Li-S Battery Performance

Abstract

Lithium ion batteries (LIBs) have been widely used as main power sources for portable electronic devices such as cellular phones and laptop computers. However, the energy density of LIBs cant satisfy the continuous demand for increased energy density requirement, especially for electric vehicles (EVs) and energy storage systems (ESSs). One of the most promising candidates for next generation batteries is the lithium-sulfur (Li-S) battery, which has theoretical specific capacity of 1672 mAh g-1. However, there are still many obstacles for its practical application. First, the sulfur has extremely low electrical and ionic conductivities. It leads to limited sulfur utilization for electrochemical reaction. And a large amount of conductive agent is normally used to compensate the limited conductivities, ultimately the sulfur content in cathode is very low. Second, the poor cycle performance. There are many factors responsible for cycle fading. The main two factors are the soluble reaction intermediates (polysulfides) and the volume change during charge/discharge processes. The dissolution of polysulfides leads to continuous active material loss from cathode, and the volume change can induce the increase of cathode impedance from the structure collapse. Many approaches for advancing Li-S batteries have been reported during past few decades. One of the most effective and popular methods of overcoming the problems mentioned above is based on the infiltration of sulfur into carbonaceous materials with high specific surface area and large pore volumes. The high surface area not only increases the electronic contact area between carbon and sulfur, but also provides more reaction sites. Furthermore, the pores in carbonaceous materials can physically trap dissolved polysulfides and accommodate the volume change during cycling, which is beneficial for maintaining electronically conductive networks. Another method is introducing conductive and sulfiphilic surface (e.g., functionalized carbon, metal oxide, metal sulfide or metal carbide) that effectively adsorbs sulfur species. However, there are little researches on practically available methods. In this dissertation, I will introduce two economic and simple methods for advancing Li-S batteries. In first part of chapter 1, I generally discuss LIBs. An overview of basic knowledge of batteries and materials currently being used for the cathode and anode of LIBs are introduced. In addition, a general introduction about Li-S batteries is followed. I also briefly summarized recent issues on Li-S batteries. In chapter 2, I introduce a powerful and economical method through a very simple electrochemical control to improve the cycle life performance. Several discharge and charge cycles at high-potential regions (i.e., above 2.2 V vs. Li/Li+) before cycling result in the dissolution of a certain amount of polysulfide into the electrolyte, as well as the redistribution of sulfur on the cathode. Ultimately, these processes significantly enhance the cycle retention of Li-S battery. This approach is based on an electrochemical technique refer to as activation-cycling. It is not only very simple but also can apply to all kinds of sulfur cathodes. In particular, cycling stability of bare sulfur cathode, which normally is considered to be poor and difficult to be improved without materials modifications, can be simply enhanced by activation cycling. I analyze the effects of this process in terms of dissolved polysulfides, and sulfur redistribution in cathode structure by the electrochemical impedance spectroscopy (EIS) and scanning photoelectron microscopy (SPEM) analysis. In chapter 3, I will report on the use of a redox mediator as an electron-hole transfer agent between the solid electrode and polysulfides in the electrolyte. This novel approach successfully realized the high performance Li-S battery for ultra-high sulfur content (80 wt%) cathode. The effective cathode conductivity is increased by introducing a redox mediator (cobaltocene) into the electrolyte. It has a redox potential within the region of polysulfide reduction. I confirmed that cobaltocene enabled Li2S nucleation and growth not only on the conductive carbon surface but also in the electrolyte. The redox mediator acts as an electron transfer agent: it is reduced at the cathode and then oxidized by the polysulfides remote from the conductive surface to produce Li2S. Taken together, this unified mechanism allows sufficient Li2S formation with a very low amount of conductive agent in the cathode. It is confirmed by electrochemical method, scanning electron microscope (SEM), X-ray absorption near edge structure (XANES) and in-situ XRD studies.