NMR spectroscopic investigation of LiNO₃-induced SEI modification in Li-S batteries: a concentration-dependent study

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, operando⁷Li 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 operando⁷Li 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, operando⁷Li 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.