How does the formation of polysulfides reduce the capacity of the system?
In a room temperature Na-S battery, the sodium metal anode and sulfur cathode are separated by a porous polymer separator. The sulfur (exists as S8 - note that sulfur can exist in several allotropes such as S6, S8 etc - similar to carbon) cathode can accept 0.5 electron per S atom or accept 1 electron per S atom - thus producing polysulfides such as S42-, and S22-. These polysulfides (Sn2-) that formed during the discharge could diffuse through the porous separator and reach the sodium anode and form corrosion products. This is a parasitic reaction and does not contribute to energy storage/dissipation. Therefore, it reduces the capacity of the battery. The polysulfides can diffuse back to the cathode as Sn-x2-. The transport of polysufides through the separator between sulfur cathode and sodium anode is called shuttle effect. You can prevent this shuttling by using ion-selective membranes.
For a better description, you can refer to: Y. V. Mikhaylik and J. R. Akridge, J. Electrochem. Soc., 2004, 151, A1969–A1976. This paper is on Li-S battery, but the mechanism is applicable to Na-S battery.
In a room temperature Na-S battery, the sodium metal anode and sulfur cathode are separated by a porous polymer separator. The sulfur (exists as S8 - note that sulfur can exist in several allotropes such as S6, S8 etc - similar to carbon) cathode can accept 0.5 electron per S atom or accept 1 electron per S atom - thus producing polysulfides such as S42-, and S22-. These polysulfides (Sn2-) that formed during the discharge could diffuse through the porous separator and reach the sodium anode and form corrosion products. This is a parasitic reaction and does not contribute to energy storage/dissipation. Therefore, it reduces the capacity of the battery. The polysulfides can diffuse back to the cathode as Sn-x2-. The transport of polysufides through the separator between sulfur cathode and sodium anode is called shuttle effect. You can prevent this shuttling by using ion-selective membranes.
For a better description, you can refer to: Y. V. Mikhaylik and J. R. Akridge, J. Electrochem. Soc., 2004, 151, A1969–A1976. This paper is on Li-S battery, but the mechanism is applicable to Na-S battery.
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