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Advanced science: one step reaction -- high capacity and long cycle life room temperature sodium sulfur battery

wallpapers News 2020-08-18

room temperature sodium sulfur (RT Na-S) batteries have attracted much attention in recent years due to the advantages of sulfur cathode materials such as high theoretical specific capacity abundant natural storage capacity. However due to the poor conductivity of sulfur the dissolution of sodium polysulfide in electrolyte other problems the capacity of RT Na-S battery decays rapidly the coulomb efficiency is low the self discharge is serious which limits the practical application of RT Na-S battery. In recent years it has been reported that sulfur / carbon composites were used as cathode materials for RT Na-S batteries to improve electrode conductivity inhibit the dissolution of polysulfides. However the formation of polysulfides the capacity degradation caused by polysulfides are still the biggest challenges in the research development of RT Na-S batteries. Theoretically one-step Na-S reaction with Na2S as the only reaction product without polysulfide formation will be the most ideal design of RT Na-S battery which not only can obtain high specific capacity long cycle life but also can greatly improve self discharge.

Xia Hui of Nanjing University of science technology Yan Chenglin of Suzhou University their collaborators have realized the one-step reaction of RT Na-S battery by using ultra microporous confinement. It is found that only two-dimensional Na2S molecules can be formed in the ~ 0.5 nm micropore after s enters the carbon micropore in the form of small sulfur molecules due to the limited size of the micropore. The researchers of

used coffee dregs as raw materials carbonized activated to obtain porous carbon materials with the main body of 0.5nm slit type micropores. When the carbon material was used as the sulfur carrier 40 wt% sulfur (acc-40s) was loaded sulfur mainly entered into the micropore in the form of small sulfur molecules (S2-4). The sulfur carbon composite was applied to RT Na-S battery. It was proved by in situ UV Vis spectroscopy measurements density functional theory (DFT) calculation that the formation of polysulfide intermediate was inhibited during the whole charging discharging process only one reaction product appeared in the system namely Na2S. The results show that when the micropore size is less than 0.75 nm the formation of even the smallest polysulfide na2s2 molecule is thermodynamically unstable only the two-dimensional molecular structure of Na2S molecule can be formed freely indicating that one-step reaction from s to Na2S can be effectively realized by micropore confinement. The reaction mechanism enables acc-40s electrode to obtain a high reversible specific capacity of 1492 MAH g-1 a long cycle life (95% capacity retention after 2000 cycles) in RT Na-S battery at 0.1C current density. What's more RT Na-S battery with acc-40s electrode also shows very low self discharge its capacity daily decay rate is only 0.17% which makes it have potential application prospect in large-scale energy storage. This work provides a new idea for the design of porous carbon carrier materials for sulfur positive electrode of room temperature sodium sulfur battery provides a theoretical basis for inhibiting the formation of polysulfides especially the strategy of micropore confinement.

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