In a lithium sulfur cell, the reduction of sulfur to lithium sulfide is a critical series of reactions that provides a large theoretical capacity of 1,672 mAh/g sulfur. One of many challenges in this system is the solubility of generated lithium polysulfides during the charge/discharge process. These polysulfides derived from the reduction of elemental sulfur are soluble in organic electrolytes, and can be reduced at the anode, causing an undesired reaction. Polysulfide species can also accumulate at the surface of the cathode and be further reduced to lower-order polysulfides such as Li2S2 or Li2S. The insulating nature of these lower-order polysulfides blocks the electron pathway on the carbon cathode.
LiNO3 has been studied as a shuttle inhibitor for protecting lithium metal surfaces from unwanted chemical attack of polysulfides. The precipitation of the LiNO3 forms a stable passivation film, a solid electrolyte interface (SEI), on the surface of the lithium anode. This SEI film not only protects the anode, but also prevents the dissolved polysulfides from reduction at the lithium anode surface, resulting in improving cycling efficiency of the lithium anode.
For long cycle life and good reversibility of the lithium sulfur cells, the use of LiNO3 is to be implemented as a co-salt rather than an additive. The use of lithium fluoride as an additive in the electrolyte formulation to reduce higher-order polysulfides to lower-order polysulfides limits the lower-order polysulfide accumulation at the surface of the cathode.
This work was done by Dr. Surya Moganty of NOHMs Technologies, Inc. for Glenn Research Center.