Post Li-ion batteries

Kurzfassung

In the last decades, the investigation of new secondary cells has been increased considerably, because high energy density rechargeable batteries are supremely demanded for different applications, such as consumer electronic, electro mobility and renewable energy storage. Very promising battery systems are the so called “Post Li-ion batteries” (4. generation batteries) with metal anodes: metal-sulfur and metal-air (oxygen) batteries, in particular Li-sulfur and Li-air batteries. Li-sulfur battery is a promising system, due to its high theoretical capacity (1675 mAh/gsulfur), energy density (2500 Wh/kg), the low cost and non-toxicity of sulfur. Nevertheless, some of the drawbacks of lithium-sulfur batteries are the poor rechargeability and high self-discharge rates. Due to the low electrical conductivity of sulfur, electrical conductive material has to be added in order to encourage the electrochemical reaction. Furthermore, polysulfides of high order (Li2Sn with 2 ≤ n ≤ 8) dissolve in the electrolyte and can diffuse to the anode and react directly with lithium metal. This so-called shuttle mechanism causes irreversible loss of sulfur. Moreover, insulating and insoluble polysulfide discharge product (Li2S) can precipitate on the surface of electrodes, avoiding further electrochemical reaction. Metal-oxygen cells exhibit highest theoretical energy densities. Among them, the Li-O2 system offers highest theoretical gravimetric energy density (11680 Wh/kg) however, anodes of highly abundant elements, such as Al, Si or Zn, offer several advantages over lithium. Apart from the availability, these latter metals are safer and more stable, allowing the battery processing in air. State-of-the-art metal-oxygen cells mainly suffer from severe cyclic aging and low Coulombic efficiency. To overcome these challenges, the underlying complex electrochemical reactions between the electrolyte and the electrocatalyst and the electrodes have to be understood. One of the major challenges is the development of suitable electrolytes and electrocatalysts for the oxygen reduction and gas evolution reactions. One of the major limiting factors on performance and round-trip efficiency is the catalyst used. Today´s Lithium-air batteries still suffer from high charge- and discharge overpotentials caused by insufficient catalysts. The major goal is to reduce overpotentials by using bifunctional catalysts catalyzing both the oxygen reduction reaction (ORR) as well as the oxygen evolution reaction (OER). Noble metal catalysts show good performance on both reactions but with increasing prices of noble metals metal oxides have drawn attention recently. In the presentation new results from the production and characterization of Lihium-sulfur batteries and bifunctional cathodes for Li-air batteries will be shown.