Logo der PTB - Messen, Forschen, Wissen
Gateway for politics

Aging of lithium-sulfur batteries

Insight into atomic processes in various charging states

PTBnews 3.2021
27.09.2021
Especially interesting for

electromobility

smart grids

manufacturers of mobile devices

battery manufacturers

At present, lithium-sulfur batteries cannot reach their theoretical maximum capacity and lifetime. According to time-resolved, species sensitive and traceable measurements performed for sulfur at the BESSY II synchrotron radiation source, the accumulation of polysulfides at the anode could be the main reason for these shortcomings.

Lithium-sulfur batteries could be an environmentally friendly alternative to conventional lithium-ion batteries. With lithium as the anode and sulfur as the cathode material, they could offer enhanced capacity and longer lifetimes. In addition, sulfur is abundant, making it inexpensive, and more environmentally friendly. Moreover, with sulfur being a lighter element than the heavy metals such as mangane, nickel and cobalt which are used in conventional lithium-ion batteries, the theoretical energy density of a Li/S cell is considerably higher than that of lithium-ion batteries. Whereas conventional lithium-ion batteries reach 220 Wh/kg, LI/S cells can theoretically reach 2500 Wh/kg. Only 25 % of this energy density has, however, been achieved up to date.

In addition, such batteries age fast and cannot yet attain the minimum number of 1000 charge-discharge cycles required by industry.

industry. One of the reasons for this fast decrease in their capacity was assumed to be the formation of polysulfides. Polysulfides are chain-like molecules consisting of lithium and sulfur – i.e., of those elements that ensure energy storage in cell chemistry. When polysulfides dissolve in the electrolyte, they are no longer available to store energy, which leads to a decrease in capacity. Polysulfides are formed at the cathode while the battery is in operation. They then dissolve in the electrolyte and move on towards the anode. When the battery is being recharged, they accumulate at the anode in ever greater numbers with each charging cycle.

The investigations carried out at PTB have shown for the first time how the polysulfide molecules move between the electrodes and, in particular, how they accumulate at the anode with each new charging cycle. The time-resolved measurements performed on the cell in operation (operando analysis) allow changes at the atomic level to be attributed to the electric properties of the battery. In addition, the change in the polysulfide molecule length, which has a significant influence on solubility and reactivity, was also determined.

The measurements were carried out in Berlin, at the BESSY II synchrotron radiation source. They used near edge X-ray absorption fine structure analysis (NEXAFS) as well as quantification without a reference sample by means of X-ray fluorescence analysis (XFA) to analyze sulfur in the dissolved polysulfides. These procedures are very accurate, traceable to the SI and can dispense with reference material.

The results have shown that it is not primarily the formation of polysulfides, but rather their moving and accumulating at the anode that are responsible for the capacity decrease. This leads to new strategies for cell design, such as the use of polysulfide-impermeable separators.

Contact

Claudia Zech
Department 7.2
X-ray Metrology with Synchrotron Radiation
Phone: +49 30 3481-7179
Opens local program for sending emailclaudia.zech(at)ptb.de

Scientific publication

C. Zech, P. Hönicke, Y. Kayser, S. Risse, O. Grätz, M. Stamm, B. Beckhoff: Polysulfide driven degradation in lithium-sulfur batteries during cycling – quantitative and high time-resolution operando X-ray absorption study for dissolved polysulfides probed at both electrode sides. J. Mater. Chem. A 9, 10231–10239 (2021)
Opens local program for sending emailhttps://doi.org/10.1039/D0TA12011A