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SOCl2 additive for lithium anode protection and lithium-sulfur battery stabilization

The lithium metal anode is the darling of a new generation of high-energy batteries with the lowest redox potential (-3.04V vers S.H.E.), ultra-high theoretical specific capacity (3860mAh/g) and the lowest bulk density (0.534g/cm3). However, metal lithium anodes have interface problems such as dendrite growth, infinite volume expansion, and electrode powdering in electrochemical reactions. These problems not only cause electrochemical performance dysfunction, but also cause safety accidents such as battery short circuit and thermal runaway. In recent years, researchers have tried to suppress the growth of lithium dendrites by using various strategies such as solid electrolyte, electrolyte component optimization, and artificial SEI membrane construction. Among them, the optimization of the electrolyte composition is popular because of its simple operation, almost no influence on energy density, and low cost. Generally, the role of the electrolyte additive is to assist in the production of a strong SEI film to passivate the metallic lithium surface. However, in the electrochemical reaction process, the additive is also continuously consumed, and the task of finding an additive that is beneficial to the long-term electrochemical cycle is very urgent.

Lithium-sulfur batteries are an important member of the lithium-metal battery family, and they are favored in many energy storage devices due to their high theoretical specific capacity and high energy density. Like lithium metal, the industrialization of lithium-sulfur batteries has also encountered many bottlenecks, mainly due to the low electrical conductivity of sulfur, the volume expansion and the shuttle effect of polysulfides. The current research focus of lithium-sulfur batteries focuses on the modification and optimization of sulfur-containing cathode materials. For example, a porous carbon material is used as a sulfur storage carrier, which not only improves the overall electrical conductivity of the positive electrode, but also buffers volume expansion and suppresses the shuttle effect. However, the electrochemically inert carrier material reduces the energy density of the battery and does not improve the stability of the lithium negative electrode interface, so the effect on the long-term cycle performance of the lithium-sulfur battery is limited.
 
[Introduction]
Recently, Prof. Huo Fengwei from the Advanced Materials Research Institute of Nanjing University of Technology, together with Professor Wang Jiulin from Shanghai Jiaotong University and Professor Wang Chao from Jiangsu Normal University, published the title “Desiging Li-protectivelayer” in the internationally renowned energy journal Energy Storage Materials. A research article on via SOCl2 additive for stabilizing lithium-sulfur battery. They use simple thionyl chloride (SOCl2) as an electrolyte additive, and at the same time achieve the stability of the metal lithium anode interface and the long-term cycle performance of the lithium-sulfur battery. The thus assembled Li-S battery has a discharge specific capacity of up to 2202.3 mAh/g at a current density of 400 mA/g (the portion exceeding the theoretical capacity comes from the decomposition compensation of the additive). This work provides a new idea for the subsequent research of metal lithium anode and lithium sulfur battery.
 
 [depth analysis]

1. How does the SOCl2 additive achieve the dual function of stabilizing the interface between the metal lithium anode and improving the stability of the lithium-sulfur battery?

This begins with the chemistry of SOCl2. One application example of a lithium negative electrode in a high specific energy battery is a lithium-thionyl chloride primary battery in which thionyl chloride serves both as an electrolyte component and as a positive electrode active material. When lithium is contacted with thionyl chloride, a chemical reaction occurs to produce inorganic substances LiCl and S and a small amount of SO2 (dissolved in an excess of thionyl chloride). The authors used this property of thionyl chloride to select it as a bifunctional electrolyte additive. In this paper, the author adds 2% by volume of thionyl chloride to the traditional carbonate electrolyte, and solves the problem of both sides of the positive and negative electrodes by using the reaction. The inorganic substances such as LiCl and Li2SO3 formed by the reaction are insoluble in the electrolyte, so that a uniform and dense artificial SEI film is formed at the interface of the metal lithium negative electrode.

LiCl has a high similarity with the widely studied LiF (SEI membrane component which is very effective on the surface of lithium metal anode). It not only has certain ion conductance, but also regulates the uniform diffusion of lithium ion current at the interface of the negative electrode. The physical barrier blocks direct contact of the electrolyte with the lithium negative electrode. The other reaction product S is dissolved in the electrolyte (the electrolyte changes from transparent to yellow), and an additional capacity is gradually diffused to the positive electrode during discharge. By SEM characterization, it can be clearly observed that the surface of the lithium negative electrode soaked in the electrolyte containing thionyl chloride changed from the initial smooth flatness to the dense and ordered laminated structure.

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Figure 1 Schematic diagram of the action of thionyl chloride additive
 
Second, how does the author prove the role of thionyl chloride additives?

 In order to investigate the stability of the metal-lithium negative interface, the authors assembled a Li/Li symmetric battery to observe the change in polarization voltage at a specific current density (3.8 mA/cm2) and a specific capacity (1.9 mAh/cm2). The symmetrical cell polarization voltage circulating in the electrolyte containing the thionyl chloride additive can be stabilized for a longer period of time. SEM found that the surface of the lithium negative electrode was uniformly and densely deposited after the cycle, which confirmed that the inorganic SEI film formed by the reaction had the effect of regulating the flow of lithium ions to make it evenly distributed at the interface. On the surface of the blank metal lithium negative electrode, the surface of the lithium negative electrode was found to be severely damaged on the SEM photograph, which is undoubtedly the result of the rupture of the fragile primary SEI film.

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Figure 2 a) Comparison of cycle performance of symmetric batteries in two electrolytes; bf) SEM photograph of the surface of lithium anode after 100 cycles of two electrolytes; g) EDX of lithium anode surface after 100 cycles of two electrolytes Signal curve
 
For the effect of thionyl chloride in lithium-sulfur battery, the author also comprehensively analyzed through various means. The authors analyzed the morphology and composition of the surface of the lithium metal negative electrode after disassembling the Li-PAN@S battery after circulating for 20 weeks at a current density of 400 mA/g. The results of the morphology analysis were similar to those of the Li/Li symmetric battery. Surface XPS analysis showed that Li, P, C, O, and F existed on both negative electrode surfaces, but Cl was only present on the surface of the negative electrode using thionyl chloride additive, indicating that the LiCl artificial SEI film on the negative electrode surface after 20 cycles Still stable. The results of the peaks of Li 1s, Cl 2p, S 2p and F 1s further reveal that the surface composition of the anode after electrochemical cycling includes: LiCl, Li 2 SO 4 , Li 2 SO 3 , Li 2 CO 3 , LiF and the like. The in-situ sturdy SEI film not only protects the surface of the negative electrode, but also improves the electrochemical performance of the S positive electrode.

The cyclic voltammetry curves of the blank battery and the battery using the thionyl chloride additive are basically the same (the redox peak position does not change), which indicates that the addition of thionyl chloride does not cause the electrochemical reaction mechanism to change, and no additional pair is introduced. reaction. After the addition of thionyl chloride, the discharge capacity was increased to 2202.3 mAh/g in the first week, and the capacity retention rate was still 70% after 200 weeks of deep cycle at a current density of 400 mA/g. The test of rate performance shows that the polarization voltage (platform voltage difference) does not increase significantly with the increase of current density, which is closely related to the good kinetics of artificial SEI film.

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Figure 3 XPS analysis of lithium metal surface after 100 weeks of cycling
 
Third, there is now a new problem: Since thionyl chloride can compensate for the capacity of lithium-sulfur batteries, how to clarify its contribution to total capacity?

The authors creatively designed the experiment to produce an electrode (called a blank electrode) containing no active sulfur and containing only conductive carbon and a binder in both electrolytes. The blank electrode has a reversible capacity of less than 10 mAh/g in a common electrolyte and a reversible capacity of up to 365 mAh/g in an electrolyte containing thionyl chloride. This means that the extra capacity in the lithium-sulfur battery does come from thionyl chloride. So the more thionyl chloride added to the electrolyte, the better the electrochemical performance of the lithium-sulfur battery? The authors conducted a further set of control experiments and found that the addition of more thionyl chloride did increase the discharge capacity, but it reduced the stability of the cycle. This may be due to the irreversible deterioration of the entire electrolyte system due to the addition of too much thionyl chloride. Therefore, the effect of additives on the properties of the electrolyte is also an important factor to consider when selecting additives.

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Figure 4 Electrochemical performance of lithium-sulfur battery
 
[summary and expansion]

In general, this article is inspired by the principle of lithium-thionyl chloride primary battery. It is creative to introduce thionyl chloride additive into lithium-sulfur secondary battery, which improves the electrochemical of metal lithium anode and lithium-sulfur battery to some extent. performance. In fact, there are still quite a few lithium primary batteries with excellent performance in medical and military fields, such as lithium iodine batteries and lithium-manganese dioxide batteries. The editor believes that the basis of these lithium primary battery applications lies in its high energy density, but the reason for limiting its use as a secondary battery is that it has its own system in addition to the lithium negative interface problem. In recent years, improvements in these battery systems have rarely been reported. On the one hand, with the deepening of metal lithium anode research, we can try to improve these primary batteries into secondary batteries; on the other hand, we can also get inspiration for the new generation battery system from the system as in this paper. .

Another hidden idea in this paper is the targeted induction of the target artificial SEI membrane components, which is generally involved in the article on electrolyte component optimization. In brief, previous studies on the composition of SEI membranes have made it clear that SEI membranes containing LiF, LiCl, Li3N and some polymers are beneficial for the improvement of the interface problem of metallic lithium anodes. Therefore, in the process of optimizing the electrolyte composition, it is possible to select components which can generate these substances by chemical or electrochemical reaction inside the battery system, such as FEC, nitrogen-containing ionic liquid, electrochemical polymerization additive, etc., to induce induced generation. Target artificial SEI membrane. Considering that electrolyte composition optimization is easy to operate and highly compatible with existing processes, this strategy will be widely used in the future.

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