Lithium Nitrate in Rechargeable Lithium-Sulfur Batteries: Catalytic Feature of Redox Shuttle Suppression

Abstract

A big breakthrough in the study of rechargeable Li-S batteries was the discovery of LiNO3 additive in 2008, which effectively suppresses LiPSs redox shuttle and enables real rechargeable Li-S batteries. The role of LiNO3, according to conventional understandings, was to react with lithium anode in a controlled way to form a LiNxOy passiviation layer on the anode which prevents the reduction of polysulfide thereon. This understanding, however, is seeing more challenges. Recently, Dr. Sheng S. Zhang of US Army Research Lab detected progressive reduction of LiNO3 on Li anode accompanied by the gradual increase of internal resistance in Li-S batteries. This finding is a direct challenge to the claim that LixNOy layer on anode prevents the LiPSs reduction thereon, as LiPSs with higher reduction voltage (2.1 V) are more easily to be reduced than LiNO3 (1.7 V). The unclear mechanism of redox-shuttle suppression and limited understanding on the impact of LiNO3 have hampered the development of Li-S batteries with long-term cycling stability, despite huge efforts and encouraging progress made in the engineering of nanostructured sulfur cathode reported in a large number of high impact journal publications.In this manuscript, we revisit some key factors that influence LiPSs redox shuttles, clarify the roles of LixNOy passivation layers, and propose a new mechanism that redox shuttle suppression in the presence of LiNO3 is due to strong binding between the soluble high-order LiPSs and nitrate (NO3 –) anions adsorbed on carbon substrate, which promotes the oxidation of polysulfides to sulfur on charging. This is supported by density functional theory (DFT) calculations and NO3 –-induced low self-discharge rate in Li-S batteries. As the progressive reduction of LiNO3 compromises long-term cycling stability of Li-S batteries by gradual increase of the internal resistance, we propose the substitution of soluble LiNO3 with solid oxide catalyst of good polysulfide oxidation reaction (PSOR) activity, to which ruthenium oxide was found as a preferable choice. This new understanding on the role of LiNO3 is opening a new research path to the development of nanostructured polysulfide oxidation catalysts with fine-tuned composition and morphology to boost the performance of Li-S batteries holistically.