Selective NMR Observation of the SEI–Metal Interface by Dynamic Nuclear Polarisation from Lithium Metal

Li metal anodes represent the ultimate in energy density for battery materials but are plagued with safety issues caused by dendrite formation during lithium plating. To mitigate this, it is critical to understand the solid–electrolyte interphase (SEI) layer which mediates the transport of lithium to the metal surface. Significant strides have been made to study the SEI with NMR spectroscopy, which can give detailed information on local structure and dynamics; however, as a bulk technique, it is challenging to selectively observe the SEI using NMR, especially in the presence of electrolyte.<br/>Dynamic nuclear polarisation is a promising approach to enhance the signal in NMR experiments, by exploiting the ~10³ times greater gyromagnetic ratio of paramagnetic electrons to hyperpolarise the nuclear spins. Previously the SEI has been studied by exogenous DNP, whereby organic radicals are added to the system which is then cooled to cryogenic temperatures (100 K or below). This approach has several limitations: (i) the addition of organic radicals may alter the nature of SEI; (ii) the organic radicals are external to the SEI so cannot be readily used to probe the SEI–metal interface; (iii) exogenous DNP is not selective to the SEI and can also enhance any impurities in the system; and (iv) the use of cryogenic temperatures means that the experiments cannot be coupled with in-situ electrochemical cycling or used to study Li⁺ dynamics.<br/>In this work, the Pauli paramagnetism of the lithium metal itself was exploited to hyperpolarise nearby nuclear spins via Overhauser DNP and investigate the lithium microstructures and SEI of lithium metal batteries [1]. Since the lithium metal is in direct contact with the SEI, this allows the buried SEI–metal interface to be selectively and non-destructively probed, which would be extremely challenging via other techniques. Furthermore, since the electron relaxation in lithium metal is temperature independent, the experiments can be performed at room temperature, paving the way for the investigation of dynamic processes and in-situ methodologies.<br/>First, significant hyperpolarisation of the ⁷Li metal signal was demonstrated at high magnetic field, as required for high-resolution experiments. The hyperpolarisation was then harnessed to selectively enhance, and thus identify, the diamagnetic components of the SEI via the ⁷Li, ¹H and ¹⁹F NMR spectra. The relative DNP enhancements allow the proximity of different species in the SEI to the metal surface to be inferred, while double resonance experiments (⁷Li→¹H/¹⁹F cross polarisation and ⁷Li{¹⁹F} rotational echo double resonance) were further used to reveal minor components such as polymeric organic species and LiF. The chemical composition and structure of the SEI layer is highly dependent on the electrolyte system used, so we compared samples prepared from electrolytes with and without the common additive fluoroethylene carbonate (FEC). This showed that the addition of FEC reduces the amount of trapped ethylene carbonate in the SEI and promotes the formation of poly-vinylene carbonate species at the expense of poly-ethylene oxide-like species. Overall, we present a new technique to study the lithium–SEI interface, which is of key interest for the development of safe lithium metal batteries.<br/>1. Hope, M. A.; Rinkel, B. L. D.; Gunnarsdóttir, A. B.; Märker, K.; Menkin, S.; Paul, S.; Sergeyev, I. V.; Grey, C. P. Nat. Commun. 2020, 11 (1), 2224.