Novel Organic Cathode for Li Based Battery Using Pillar[5]Quinone/Carbon/AgNWs
Authors: Yoo Gayeong , Gong Yong Jun , Lee SeongHun , Cho Jinil , Yoo Jeeyoung , Kim Youn Sang 
Published: February 2020

Rechargeable lithium based batteries have been widely used for portable devices such as cellular phone and laptop computer. Those electronics are more and more being multi-functionalized, and the energy storage devices for them are required much higher energy density. The energy storage system loading high energy density will also dominate the market of large scale power systems, in the rapid development of electric vehicles. Therefore, lithium ion battery with high specific capacity became an essential factor. Although the battery can offer a high gravimetric capacity of 3862 mAh g-1 by using lithium metal as the anode part, most of cathode materials which are represented as lithium (cobalt, nickel, manganese, aluminum) oxide are mainly restricted in capacity. Thus, the cathode is the most important aspect to offer the high capacity. Commonly used inorganic lithium metal oxide materials as the cathode part have drawbacks such as their low theoretical specific capacities (<170 mAh g-1), structure stability and their unstable trend of price. Also, the cathodic active materials have a disadvantage in that a high-temperature required for manufacturing process and the side reaction is not environmentally friendly.

To overcome the disadvantages, the organic compounds have recently been considered as promising candidates for the next generation of energy storage systems. They have many obvious merit compared to the inorganic materials in high theoretical capacity (>400 mAh g−1), safety, sustainability, environmental friendliness and low cost. In addition, they are electroactive toward lithium metal as well as practically any metals like magnesium, zinc, aluminum owing to their redox electron reactions. Especially, Pillar[5]quinone (P5Q), containing five quinone units linked by methylene bridges at para positions, can not only implement a high theoretical capacity of 446 mAh g−1 but also realize so effective for the use of the active sites that it is able to favorable to Li ion uptake. However, those kinds of organic materials with small molecular weight have two critical problems. First, they are easy to dissolve in aprotic electrolyte such as carbonate-based solvent, leading to poor cyclability and rapid capacity fading. Second is that the low conductivity of organic molecules limits their rate performance.

In the effort of solve the problems, we fabricated the cathode using the P5Q enveloped in carbon (e.g. MWCNTs and CMK-3) and silver nanowires (AgNWs) with simple vacuum filtration method. At first, P5Q was obtained after synthesizing 1,4-dimethoxypillar[5]arene and the products obtained by each step were identified by using 1H, 13C NMR and FT-IR spectra. Carbon and AgNWs act as a framework, allowing to trap the active material. Furthermore, they perform a current collector, providing the conductive pathway of electrons for fast charging/discharging the battery. It doesn’t need binders and aluminum foil which is components of traditional slurry type of cathode. As the result, not only constrains the solubility in organic electrolyte, preparing the electrode synthesized with P5Q/carbon/AgNWs nanocomposite gives the enhanced conductivity. The content of each components was verified by Thermogravimetry Analysis(TGA). Suggested cathode has super-lightweight and an extremely low resistance of 2~3 ohm cm. Also, it demonstrates a specific discharge capacity of about 420 mAh g−1 at 0.1C. and an average operation voltage of 2.8 V when applying Li metal battery.

For obtaining a high specific initial capacity, ether-based solvents (1,3-dioxolane/dimethoxyethane=1:1) containing high concentration Li salt (Bis(trifluoromethane)sulfonimide lithium salt) were used instead of carbonate-based electrolytes, achieving the reversible cycles. Moreover, the electrolyte was optimized by adding LiNO3 additive, preventing the formation of a protective layer on the surface of Li anode as well as the dissolution of active material (P5Q) in the electrolyte with high Li salt concentration.

Likewise, the proposed battery concept reveals a high possibility of future battery and novelty by optimizing the electrolyte and modifying the cathode. Those methods can be applied at other organic cathode for Li based battery to reach their theoretical capacity and improve the cycle performance.