With electromobility also the search for alternatives to the classic lithium-ion batteries ride. A cuisant candidate for this is the lithium-sulfur battery. To find out why this type battery has not yet achieved its maximum possible capacity and service life, a measuring system was developed in the Physikalisch-Technische Bundesanstalt (PTB), which can be used in the current operation of the battery.
Thus, a possible reason for the unwanted aging was determined: polysulfide, chain-shaped molecules made of lithium and sulfur, which enrich the minus pole, so that less and less lithium and sulfur is available for energy storage. The movement and enrichment of the polysulfides was observed with two state-of-the-art analysis methods with X-ray of the synchrotron radiation source BESSY II in Berlin molecular specifically and assigned to the respective state of charge.
The measurements suggest that the development and use of polysulfide-impermeable separators in such batteries can increase the life. The results are published in the Journal of Materials Chemistry.
Powerful, rechargeable batteries (accumulators, short batteries) comes a key role in the context of the energy transition, z. B. As a stationary buffer for energy from renewable energy sources or in electro cars for displacement fossil energy carrier. For these fields of application, the current lithium-ion batteries come into their limits regarding capacity and service life. In addition, expensive and toxic raw materials are often used, which are partially degraded under questionable conditions.
Why sulfur would be a good alternative
Therefore, alternative, environmentally friendly battery types with higher capacity and prolonged life are needed to which potentially belongs to the lithium-sulfur battery. Such a battery cell with lithium as minus pole (anode) material and sulfur as positive pole (cathodes) material has several advantages: sulfur is inexpensive, environmentally friendly and abundant. And the theoretical energy density of such a cell is due to the lightweight elements at up to 2500 Wh / kg, which is significantly higher than in lithium-ion batteries. But so far, only about a quarter of the theoretically achievable energy density could be realized, and the batteries of this kind aging are too fast, so that the at least 1000 charging cycles required by the industry can not yet be achieved.
In the search for reasons for the rapid decline in capacity, the polysulfides were in focus. Polysulfides are chain-shaped molecules that consist of lithium and sulfur, so exactly those elements responsible for energy storage in this battery type. When the polysulfides dissolve in the electrolyte, the proportion of lithium and sulfur is lost for energy storage, and thus the capacity decreases. They form during the battery operation on the positive pole, dissolve in the electrolyte and hiking to the minus pole.
When recharging, you must go back to the positive pole; But that does not work completely. The polysulfides rich with increasing cycle number on the minus pole. On the positive pole is thus less and less sulfur available, which is reflected in decreasing capacity. With the method developed in the PTB, molecular-specific molecular specifically could now be detected, in which loading and unloading state is like many polysulfides in the electrolyte on the two poles. For this purpose, the scientists at the Synchrotron radiation source BESSY II in Berlin set the NEXAFS absorption fine structure analysis (NEXAFS) as well as reference sample-free quantification with X-ray fluorescence analysis (RFA) for the sulfur element. The methods are very accurate, traceable to the international unit system (SI) and come without reference material.
New strategies for cell design required
In addition to the percentage loss of the cathodic (ie plus pole) active material sulfur for various charging states, the scientists could determine the change in the molecular lengths of the polysulfides, which significantly influences both solubility and reactivity. By examining both electrode pages, the shuttle effect, ie the movement of the polysulfides between the electrodes, and in particular the accumulation of the minus pole for progressive cycle number could be observed. These time-resolved measurements in the current operation of the cell (operando mode) enable allocation of changes to atomic level to the electrical properties of the battery.
The measurements showed that not primarily the formation of polysulfides, but their movement and deposit on the minus pole for the decline in cell capacity is responsible. This leads to new strategies in cell design, for example for the use of polysulfide-impermeable separators.
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