State of the Art of Batteries of the 4th Generation

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” (4th 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 [1-2]. Moreover, insulating and insoluble polysulfide discharge product (Li2S) can precipitate on the surface of electrodes, avoiding further electrochemical reaction. As a result, the specific capacity of the battery decreases considerably, especially in the first cycles. In order to optimize the performance of the cell, it is highly important to understand the degradation processes of sulfur cathode under operating conditions.In the present work, x-ray diffraction (XRD) and electrical impedance spectroscopy (EIS) were measured during cycling. The formation of crystalline products was monitored in-situ and semi-quantitatively determined by XRD- analysis. Impedance measurements were performed during the first cycle at a frequency range of 1 MHz to 60 mHz. Impedance spectra were investigated at several states of charge distinguishing the impedance contributions of the different physical/chemical phenomena occurring in the battery. Furthermore, the aging during cycling was studied until fifteen cycles. Our results shown in figure 1 provide new insights into the discharging and charging processes of Li-Sulfur batteries by means of XRD and EIS characterization.