The Electrochemical Properties of Porous Carbon for the Li-ion and Li-Sulfur Batteries

Abstract

The role of the energy storage in the society is becoming increasingly important. Until recently, the applications of energy storages have been generally limited to portable devices, and the energy storage companies have been unable to fully tap into the markets with great potential, such as electric vehicles and storages for residences. The market conditions, which favor the economically more viable fossil fuel, have been a formidable obstacle. However, today, the society seeks cleaner energy, which could replace the conventional fossil fuel, in order to provide a more sustaining environment for the next generations. Various factors have contributed to the change. For instance, the societies are alerted by the abnormal weather conditions such as El Niño and dust bowl that wreak havoc to cities and cause much dismay to its inhabitants. Moreover, the monopoly of oil producing countries that causes lopsided energy dependence for the industrialized countries have created a general consensus that energy independence is not an option, but a priority. The recent scandal involving Volkswagen and their complicit in the manipulation of the public regarding the mileage and emission of their diesel cars has further fueled the societys interest in the electric vehicles. While such attentions are highly encouraging for the engineers, scientists, and corporations involved in the energy storage sector, the means to provide suitable energy storages for the needs of the society is yet to be realized. In the case of lithium-ion battery (LIB), which is already commercialized by several companies in the world, there is still much room for advancement regarding its specific capacity and the rate capability in order to meet the requirements of energy intensive applications such as electric vehicles and storages for residences. This is not to say that there has been lack of effort in attempts to improve the aforementioned limitations. On the contrary, the energy storage is one of the most popular research topics in the academia and numerous papers are published every year. The real challenge, I believe, is maintaining the effort to truly understand science of the electrochemical system using both novel and conventional techniques, despite the unrealistic standards set by institutions and governments that favor reports of new materials with astounding cyclability and rate performance in high impact factor journals. Such limitations have encouraged the researchers to seek for quick reports on novel design of materials and their performance, which deserves some merit, but they often lack the depth for further advancement. Often, a publication means the end to the research regarding that material for many engineers and scientist. In this regards, I believe I have tried to evade such tendencies and made an effort to truly understand the electrochemical phenomena occurring within the porous carbon structures. The first chapter of the thesis introduces the general overview of the Li-ion and Li-S batteries. The part includes the discussion of the critical issues that are under investigation in academia regarding each electrochemical system. Furthermore, a part explaining the overall objective of the dissertation is included. The second chapter of the thesis discusses about the activation of micropore-confined sulfur within hierarchical porous carbon for lithium-sulfur batteries. Hierarchical porous carbon is often used in Li-S batteries due to the widely perceived benefits regarding the wide range of pore sizes. However, such notions are based solely on demonstrations of improved cyclic performances, and specific evidence to prove the utilization of the pores is yet to be found. Herein, we report, for the first time, the evidence for gradual activation of micropore-confined sulfur within porous carbon structures. By systematic comparison of microporous and hierarchical porous structures, we show that at sufficiently low current, sulfur infused hierarchical porous structures display a slowly activated and reversible reaction at 1.75 V vs Li/Li+ during discharge. This is in addition to the conventionally reported two voltage plateau at 2.3 and 2.1 V. Furthermore, the effects of LiNO3 decomposition on the system and the electrochemical mechanism behind the activation process is elucidated. Overall, the findings supplement the currently known electrochemical mechanisms occurring within porous structures and pave the way for more efficient utilization of hierarchical porous structures for applications in Li-S batteries. The third chapter of the thesis discusses about the improved electrochemical performance of a three-dimensionally ordered mesoporous carbon based lithium-ion battery using vinylene carbonate as an electrolyte additive. Within this study, the effects of vinylene carbonate (VC) as an electrolyte additive on a 3-dimensionally ordered mesoporous carbon (3DOmC) based lithium ion battery (LIB) are investigated. Our investigation reveals an optimal concentration of VC, which improves the discharge specific capacity at the 100th cycle to 844.3 from 684.3 mAh g-1, and improves the first cycles Coulombic efficiency to 32.4 from 23.7%. The improvements are revealed to be a result of the reduced charge transfer and solid electrolyte interface (SEI) resistance, enabling better permeability of Li ions. This work demonstrates that VC is a viable electrolyte additive in improving the performance of a non-graphitic carbonaceous material. The studies mentioned above addresses the most critical issues in each system and provides a stepping stone for further research to establish ideal solutions. We hope to see the fruition of hard work in the near future.