Capacity Fade Analysis of Sulfur Cathodes in Lithium-Sulfur Batteries.

Jianhua Yan,Xingbo Liu,Bingyun Li

doi:10.1002/advs.201600101

Jianhua Yan, Xingbo Liu + Show 1 more

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https://doi.org/10.1002/advs.201600101

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Abstract

Rechargeable lithium–sulfur (Li–S) batteries are receiving ever‐increasing attention due to their high theoretical energy density and inexpensive raw sulfur materials. However, their rapid capacity fade has been one of the key barriers for their further improvement. It is well accepted that the major degradation mechanisms of S‐cathodes include low electrical conductivity of S and sulfides, precipitation of nonconductive Li2S2 and Li2S, and poly‐shuttle effects. To determine these degradation factors, a comprehensive study of sulfur cathodes with different amounts of electrolytes is presented here. A survey of the fundamentals of Li–S chemistry with respect to capacity fade is first conducted; then, the parameters obtained through electrochemical performance and characterization are used to determine the key causes of capacity fade in Li–S batteries. It is confirmed that the formation and accumulation of nonconductive Li2S2/Li2S films on sulfur cathode surfaces are the major parameters contributing to the rapid capacity fade of Li–S batteries.

Highlights

Rechargeable lithium–sulfur (Li–S) batteries are receiving ever-increasing the fairly complex chemistry occurring attention due to their high theoretical energy density and inexpensive raw sulfur materials
Our former studies demonstrated that binder-free cathodes could reach much higher capacities compared to PVDF-based cathodes, which limits the access to active sulfur materials and reduces the capacity of Li–S batteries because of the blockage of pores in sulfur cathode caused by PVDF.[21]
Www.MaterialsViews.com batteries with 12 μL mg−1 electrolytes had the fastest rate of capacity fade between the second and the 100th cycle. These results indicated that the amounts of electrolytes had significant influence on capacity and capacity fade of sulfur cathodes

Summary

Theoretical Capacity Fade Analysis of Li–S Batteries

It is known that sulfur undergoes a multistep reaction during each discharge and charge process. A reduced polarization during the charge process caused by the dissolution of solid Li2S2/ Li2S is verified by the small peak that is circled as point 2 (Figure 1).[4] The upper charge plateau indicates the oxidation reactions from the soluble long-chain polysulfide species to solid sulfur. During the charge/discharge cycling, soluble polysulfide species move freely through the separator to the Li-anode and multiple concurrently parasitic reactions take place simultaneously.[9d,17] For example, the soluble polysulfide species can react with Li-ions in the electrolyte and generate insoluble Li2S, as shown in Equation (7). Where Q1 is the capacity fade due to the loss of sulfur into the liquid electrolyte; Q2 is the capacity fade due to the precipitation of nonconductive Li2S2/Li2S films onto the surfaces of both electrodes that form passivation layers, which inhibits further lithiation/delithiation; Q3 is the capacity fade due to the incomplete conversions from Li2S2 to Li2S in discharge, and from long-chain polysulfide species to elemental sulfur in charge. Our former studies demonstrated that binder-free cathodes could reach much higher capacities compared to PVDF-based cathodes, which limits the access to active sulfur materials and reduces the capacity of Li–S batteries because of the blockage of pores in sulfur cathode caused by PVDF.[21]

Long-Cycle Performance

Sulfur Reaction Kinetics

Conclusion

Experimental Section