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The cathode material in a lithium (Li) battery determines the system cost, energy density, and thermal stability. In anode-free batteries, the cathode also serves as the source of Li for electrodeposition, thus impacting the reversibility of plating and stripping. Here, we show that the reason LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes deliver lower Coulombic efficiencies than LiFePO4 (LFP) is the formation of tortuous Li deposits, acidic species in the electrolyte, and accumulation of “dead” Li0. Batteries containing an LFP cathode generate dense Li deposits that can be reversibly stripped, but Li is lost to the solid electrolyte interphase (SEI) and corrosion according to operando 7Li NMR, which seemingly “revives” dead Li0. X-ray photoelectron spectroscopy (XPS) and in situ 19F/1H NMR indicate that these differences arise because upper cutoff voltage alters electrolyte decomposition, where low-voltage LFP cells prevent anodic decomposition, ultimately mitigating the formation of protic species that proliferate upon charging NMC811. 

The performance of Li metal batteries is tightly coupled to the composition and properties of the solid electrolyte interphase (SEI). Even though the role of the SEI in battery function is well understood (e.g., it must be electronically insulating and ionically conductive, it must enable uniform Li+ flux to the electrode to prevent filament growth, it must accommodate the large volume changes of Li electrodeposition), the challenges associated with probing this delicate composite layer have hindered the development of Li metal batteries for practical applications. In this review, we detail how nuclear magnetic resonance (NMR) spectroscopy can help bridge this gap in characterization due to its unique ability to describe local structure (e.g., changes in crystallite size and amorphous species in the SEI) in conjunction with ion dynamics while connecting these properties to electrochemical behavior. By leveraging NMR, we can gain molecular-level insight to aid in the design of Li surfaces and enable reactive anodes for next-generation, high-energy-density batteries.