Lithium-sulfur battery charging and discharging working principle

Lithium-sulfur battery introduction

Lithium-sulfur batteries are different from lithium-ion batteries, fuel cells, air batteries, etc. It is a battery with the closest principle and the traditional battery principle. The cathode material is generally composed of sulfur and highly conductive materials. Mainly because sulfur itself is not conductive, the yellow and black dots in the above figure are a mixture of sulfur and carbon, so this means that sulfur as a positive electrode must be added with a conductive agent, and it is highly conductive, which reduces the sulfur of the positive electrode. Energy density (conductive agent accounts for the weight but does not produce energy); the negative electrode uses lithium flakes, which is highly active. As a negative electrode, there is nothing to say, but when used, pay attention to safety. High activity often means this danger exists; then the electrolyte is mainly Some lithium salt solutions, electrolytes and esters commonly used in lithium-ion batteries, lithium and sulfur are generally used in ethers, here is also a very particular place, the electrolyte will contact with the positive electrode, then it involves sulfur and Whether the positive electrode product is directly dissolved in this, which causes a decrease in battery cycle performance; and a separator.

Let us look at the charge and discharge performance of lithium-sulfur batteries:

From the discharge curve, there are two discharge platforms for lithium-sulfur batteries. The high-voltage platform is about 2.4V, and the low-voltage platform is about 2.1V, but the capacity is very high it is easy to 1000+mAh/g. You can also see from this picture. There are many intermediates in this process, Li2S8, Li2S6, Li2S4. . These intermediates are often in the way, their presence brings many problems to the sulfur positive electrode, such as the shuttle effect, the solubility problem, and the final product is an electronic insulator, which reduces the kinetic rate of the reaction, making the battery The rate performance decreases, the density of sulfur is larger than the product Li2S, that is to say, Li2S is more fluffy than S, so the volume is inevitably expanded, which is also an inevitable problem.

Here, we should be able to understand the problems encountered in lithium-sulfur batteries. The current research is basically focused on these issues, to achieve high energy density, improve the content of positive S, enhance cycle stability, and safety. By the way, in the course of this research, the damage of the equipment is really big, and it can't be hurt.

Lithium-sulfur battery charging and discharging working principle

A typical lithium-sulfur battery generally uses elemental sulfur as the positive electrode and lithium metal plate as the negative electrode. The reaction mechanism is different from the ion-deintercalation mechanism of the lithium ion battery, but the electrochemical mechanism.

Lithium-sulfur batteries use sulfur as the positive reaction substance and lithium as the negative electrode. When the discharge is negative, the negative electrode reacts to lithium, and the electron loses to become lithium ion. The positive electrode reacts with sulfur to react with lithium ions and electrons to form sulfide. The potential difference between the positive electrode and the negative electrode is the discharge voltage provided by the lithium-sulfur battery. Under the action of the applied voltage, the positive and negative electrodes of the lithium-sulfur battery react in reverse, which is the charging process. According to the unit mass of elemental sulfur completely changed to the amount of electricity that can be supplied by S2-, the theoretical discharge mass ratio of sulfur is 1675 mAh/g. Similarly, the theoretical discharge mass ratio of elemental lithium is 3860 mAh/g. The theoretical discharge voltage of a lithium-sulfur battery is 2.287V when sulfur is completely reacted with lithium to form lithium sulfide (Li2S). The theoretical discharge mass specific energy of the corresponding lithium-sulfur battery is 2600 Wh/kg.

The charging and discharging reactions of the sulfur electrode are complicated, and as of 2013, there is no clear understanding of the intermediate products produced by the sulfur electrode in the charging and discharging reactions. The charge-discharge reaction of the lithium negative electrode and the sulfur positive electrode is as shown in the formula (1-1) to the formula (1-4), and the discharge process of the sulfur electrode mainly comprises two steps corresponding to two discharge platforms. The formula (1-2) corresponds to the chain structure of Sn2-(3≤n≤7) ions in the ring structure of S8, and combines with Li+ to form Li2Sn, which corresponds to a discharge near 2.4-2.1V on the discharge curve platform. The chain structure of the formula (1-3) corresponding to the Sn2- ion becomes S2- and S22- and combines with Li+ to form Li2S2 and Li2S, which corresponds to a longer discharge platform near the 2.1-1.8V in the discharge curve, the main discharge area of lithium-sulfur batteries. YuanLixia et al. studied the electrochemical reaction process of sulfur positive electrode in lithium-sulfur battery. They believe that the potential range of 2.5-2.05V during discharge corresponds to the reduction of elemental sulfur to form soluble polysulfide and polysulfide, and the potential interval of 2.05-1.5V corresponds to the reduction of soluble polysulfide to form lithium sulfide solid film. It covers the surface of the conductive carbon substrate. During charging, Li2S and Li2S2 in the sulfur electrode are oxidized S8 and Sm2-(6≤m≤7), and cannot be completely oxidized to S8. The charging reaction corresponds to a charging platform near 2.5~2.4V in the charging curve.

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