CHEMICAL SCIENCE

Chinese scientists have discovered lithium polysulfide cations in lithium-sulfur batteries


On August 11, 2022, Professor Zhang Qiang of the Department of Chemical Engineering of Tsinghua University and Li Boquan, associate researcher of the Institute of Frontier Interdisciplinary Sciences of Beijing Institute of Technology, published a research paper entitled “Cationic lithium polysulfides in lithium–sulfur batteries” at Chem, which systematically studied the existence and reaction behavior of lithium polysulfide in lithium-sulfur batteries.

Professor Zhang Qiang and Associate Researcher Li Boquan are the corresponding authors of the paper; Song Yunwei and Shen Liang of Tsinghua University are the co-first authors of the paper.

Lithium-sulfur batteries are considered to be one of the most promising next-generation electrochemical energy storage systems because of their ultra-high theoretical energy density of 2600 Wh kg−1, low cost and environmental friendliness of sulfur cathode. Lithium polysulfide (Li2Sn) is an important intermediate in the reaction of lithium-sulfur batteries, and its reaction behavior directly determines the actual performance of lithium-sulfur batteries. However, the current real form of lithium polysulfide in the electrolyte, including its chargability and dissociation/association behavior, still lacks clear experimental evidence to identify. The conventional wisdom is that lithium polysulfide, like common lithium salts, is sufficiently dissociated in the electrolyte and exists mainly in the form of polysulfide anions (Sn2−). In the research process of lithium-sulfur batteries, this understanding guides many strategies such as cathode material design, electrolyte regulation, and lithium anode protection.

Recently, Professor Zhang Qiang of the Department of Chemical Engineering of Tsinghua University and Li Boquan, associate researcher of the Institute of Frontier Interdisciplinary Sciences of Beijing Institute of Technology, conducted a systematic study on the existence and reaction behavior of lithium polysulfide in lithium-sulfur batteries. The study identified for the first time the lithium polysulfide in lithium-sulfur batteries in the form of cations. Kinetic tests further show that the formation of polysulfide cations is not conducive to battery performance. Therefore, reducing the lithium salt concentration to inhibit the formation of polysulfide cations can effectively reduce the positive polarization and inhibit the negative side reaction, guiding the construction of a high energy density lithium sulfur soft pack battery. This work reveals the real existence and reaction behavior of lithium polysulfide in lithium-sulfur batteries, updates the basic understanding of lithium-sulfur chemistry, and has important guiding significance for the rational design of electrolyte and electrode materials for high-performance lithium-sulfur batteries.

1. Identification of lithium polysulfide cations

Electrospray ion mass spectrometry (ESI-MS) shows that after adding additional lithium salts (LiTFSI) to the Li2S6 solution, mass spectrometry gives a significant and well-defined characteristic peak of lithium polysulfide cation Li3S6+. In addition, the variable temperature 7Li NMR t1 relaxation time test showed that the added additional Li+ tended to bind to Li2S6, thus exhibiting a characteristic coordination environment. Correspondingly, the ultraviolet–visible spectrum shows that this interaction significantly alters the electronic structure of the polysulfide chain. These experimental results effectively demonstrate that in the electrolyte with lithium salts, Li2S6 has a tendency to associate free Li+ to generate Li3S6+ cations.

Figure 1: Identification of lithium polysulfide cations in the electrolyte

Further, the reaction equilibrium constant for generating Li3S6 + cations can be quantitatively fitted from the conductivity data as K2 = 4.70×102. This suggests that lithium polysulfide has a strong tendency to bind additional lithium ions. The distribution of different sulfur-containing species in the electrolyte containing lithium salts was calculated by the equilibrium constant, and the content of lithium polysulfide cations was found to be dominant.

Figure 2: Quantitative analysis of lithium polysulfide cations.

2. Kinetic effects of lithium polysulfide cations

Kinetic tests showed that lithium polysulfide cations showed slow adsorption and redox kinetics at the positive electrode, while the stability of the metal lithium anode was reduced, and the side reaction with lithium metal was more serious. This phenomenon is attributed to the electrostatic repulsion or adsorption effect caused by the positive charge of lithium polysulfide cations, thereby inhibiting or promoting its adsorption on the positive/negative electrode.

Figure 3: Kinetic effects of lithium polysulfide cations.

3. Inhibit lithium sulfide cations to improve lithium-sulfur battery performance

Based on the above results, this work proposes a strategy to reduce the lithium salt concentration to inhibit the formation of lithium polysulfide cations to improve battery performance. The results show that reducing the lithium salt concentration can improve the magnification performance of the battery, and effectively improve the discharge capacity and energy density of the practical soft pack battery.

Figure 4: Relationship between the formation of lithium polysulfide cations and the performance of lithium-sulfur batteries。 (Source: Science Network)

Related paper information:https://doi.org/10.1016/j.chempr.2022.07.004



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