High performance flexible/weavable lithium ion batteries enabled by flexible component, enhanced adhesion and elaborate structure

Abstract

Beyond the age of mobile device represented by smartphones and laptop, the smart wearable device age is closely approaching with the emergence of new areas application such as health care, medical, cutting edge military system and portable smart electronics. Furthermore, besides the original intended use of the wearable device, it is also regarded as a fashion items so it must satisfy esthetic aspect as well. With the development of a various electronics along the mobile device age, a lithium ion battery industry boomed as well. On the other hand, the smart wearable devices are realized by a wearable energy storage device capable of driving them. However, a conventional shapes of currently existing batteries are cylinder, square, or pouch shape. because of such structural limitation, electrochemistry performance, mechanical flexibility, and safety can’t be secured, thus it is considered a first issue to be solved for a commercialization of the smart wearable electronics. To overcome the structural limitation of a battery, a research on development of a 1-D and 2-D battery is briskly being conducted. But applying it to the wearable device has a problem of being too low capacity and limited flexibility and especially, it has an electrochemical characteristic limitation with bending and stretching which is a key element of a wearable battery. In this thesis, the electrochemical properties of graphene were clarified with various analysis and, based on them, I also introduce an enhanced adhesion, elaborate structure, and nano-sized active material synthesized with graphene template to enhance the electrochemical performance and the flexibility of the wearable battery.

In Chapter 2, we design the reduced RGO film for a lithium-ion battery cathode, and deals with the electrochemical analysis of the RGO through a carbon to oxygen (C/O) ratio. with measuring of the C/O ratio according to reduction temperature, on after deals with the electrochemical characterization of a RGO film created in correspondence to the ratio. The capacity of as-prepared electrode increased as ratio of oxygen functional group increased but also affected electrical conductivity. Based on these analysis, it will discuss the relationship of the oxygen functional group and faradaic redox reaction.

In Chapter 3 deals with an electrode in the form of LiFePO4 supported by the RGO. We synthesized a nano-sized LiFePO4 through in-situ polymerization method. Also, by directly supporting the active material on the graphene oxide, which facilitate contact between active material and conducting networks, we improved ion conductivity and electrical conductivity. This chapter demonstrated the synergy effect of the nano-sized active material and the graphene oxide template through electrochemical analysis of the electrode.

In Chapter 4 proposed a 1-D weavable battery which is a form of new flexible battery. Compared to the ordinary 2-D flexible battery, it is lighter by utilizing carbon fiber as current collector, and the cathode and anode can directly contact by porous separator coating which helps increase the energy density. In addition, by using the nano-sized active material as an electrode tackled in the previous chapter, we enhanced the electrochemical characteristic and minimized the strain caused by the external physical transformation.

In Chapter 5 deals with an alternative of solving areal capacity issue which is a drawback of 2-D flexible battery by using a conductive textiles as current collector. We increased the conductivity and the flexibility of the battery by applying the nano-sized active material that was synthesized in the previous chapter. We replaced the slurry casting method with roll-pressing procedure to tightly integrate the active material and the conductive textiles. The resulting electrodes show high areal capacity and flexibility with tolerance to mechanical stress.

This study covers about from the active material of the lithium-ion battery to various flexible batteries applying the tailor-made active material. By collaborating with the new applications, these wearable batteries will become an additional option for main power supply of the smart wearable electronics.