Carbon- and Binder- Free Cathode for Lithium Oxygen Batteries

Abstract

Unstable oil prices and the effects of global warming have forced us to look for alternative energy storage and conversion systems. So battery industry especially lithium ion battery (LIB) have been developed so fast. A battery is usually made up of an anode on one side, a cathode on the other, and an electrolyte (and separator) in between. For a LIB, lithium ions move from the anode to the cathode through the electrolyte, creating a chemical reaction that allows electrons to be harvested along the way. And very recently portable electronic devices and electric vehicle (EV) have been developing at a rapid pace, and these progress demand much high energy and power density. So metal-air batteries have been shed light on due to their high energy density and extremely high power compared to those of other conventional batteries. A metal air battery is a battery that could use a metal - lithium, aluminum, iron, or zinc etc.- for the anode, air (technically oxygen) as the cathode and electrolyte. Many people have interests in the metal air batteries because oxygen is abundant in nature, free, and doesn’t require a heavy casing to keep it inside a battery cell. Among metal-air batteries, metals such as Li, Al, Fe, and Zn, Zinc-air and Li-O2 batteries in particular have potential for use as alternative energy storage devices. Although other metals like Al can show much high voltage and power, Zn has various advantages such as low cost, abundance, low equilibrium potential, environmental benignity. The theoretical specific energy density of Zn-air batteries is 1084 Wh · kg-1. And Li-air (or Lithium Oxygen) battery; Although li metal is explosively reactive with water, the lithium Oxygen (Li-O2) battery has attracted interest because of its extremely high theoretical energy density, 11,140 Wh · kg-1 (excluding O2) and power density is 3505 Wh · kg-1, which is about eight times larger than that of conventional rechargeable lithium-ion batteries. The Zn-air and Li-O2 battery, however, have many problems in the case of Zn air battery; ohmic loss, carbon dioxide absorption and zinc dendrite formation and in the case of Li-O2 battery; a low current density, instability of nonaqueous electrolytes, and poor cycle ability etc. Moreover, carbon cathode can lead to the inevitable reactions between the discharge product Li2O2. In addition, several recent studies have reported about binders including PVDF which are necessary to make a carbon electrode also react with chemically generated LiO2. So in this PhD thesis, I studied on the problems of Li-O2 battery. And the possibility of carbon- and binder free cathodes for the Li-O2 battery has been studied. Gold (Au) and Silver (Ag) nanoparticles coated Ni nanowire substrate were used as electrodes (Au/Ni, Ag/Ni electrode) for Li-O2 battery. The Au/Ni electrode demonstrates improved capacity of ~ 600 mAh g-1Au. More importantly, it exhibited improved cyclability over 200 cycles at full discharge and charge condition between 2.3 V and 4.3 V. Meanwhile, since Ag is not electrochemically stable as much as Pt and Au at high voltages. It is needed to be found proper electrolytes for Ag/Ni electrode. The stability and performance of different electrolyte solvents were investigated such as 1,2-dimethoxyethane (DME), diethylene glycol dimethyl ether (DEGDME), tetraethylene glycol dimethyl ether (TEGDME), dimethylformamide (DMA), dimethyl sulfoxide (DMSO) or N-methyl-2-pyrrolidone (NMP). It was found that the NMP based electrolyte exhibits superior electrochemical properties. The Ag/Ni electrode with NMP/1M LiTFSI delivers a capacity of 473 mAhg-1Ag at 100 mAg-1Ag under between 2.3 V and 3.8 V and shows stable cycling performance until 35th with 300 mAhg-1Ag cut off condition at 100 mAg-1Ag.