NMR Spectroscopic Investigation of LiNO3-Induced SEI Modification in Li-S Batteries: A Concentration-Dependent Study

Abstract

The growing demand for sustainable energy has intensified efforts to develop safer, high-performance batteries. Lithium metal offers exceptional energy density but its use is limited by safety concerns and short cycle life. Electrolyte additives such as LiNO3 are known to enhance battery performance, yet their specific mechanism remains unclear. Nuclear magnetic resonance (NMR) spectroscopy provides a powerful and non-destructive means to probe Li metal batteries, offering a unique insight into the Li species and interfacial processes. Among the different NMR methods, operando 7Li NMR measurements of the Li-S battery enable time-resolved and quantitative monitoring of electrochemical Li metal deposition, thereby linking electrochemical performance to changes in the metallic Li environment. Complementarily, ex situ dynamic nuclear polarization (DNP) NMR experiments on Li metal microstructures provide detailed structural information about the interface between the metal and the solid-electrolyte interface (SEI). Together, these approaches provide a comprehensive picture of both the dynamic and structural aspects governing Li metal anode behavior. In this work, we systematically investigate the influence of LiNO3 concentration in the ubiquitous Li-S electrolyte, 1 M LiTFSI DOL:DME, on controlling anode performance and interfacial processes using a combination of operando 7Li NMR and ex situ DNP NMR spectroscopy. DNP NMR spectroscopy reveals that LiNO3 distinctly modifies the inner SEI, correlating with improved cell performance. In contrast, operando 7Li NMR shows that increasing LiNO3 concentrations only marginally affect Li deposition. Together, these results demonstrate that while LiNO3 enhances Li metal anode behavior in Li-S batteries, higher additive levels do not yield additional benefits. This combined NMR approach provides new insight into interfacial processes and supports rational electrolyte design for high-performance Li-S batteries.