Solid-State NMR Investigations on the Sources of Additional Capacities Seen in Metal Oxide/Fluoride Electrodes for Lithium Ion Batteries

Metal fluorides/oxides (MF_x/M_xO_y) are promising electrode candidates for lithium-ion batteries (LIBs) that operate via conversion reactions.¹ These conversion-type reactions are associated with much higher energy densities as compared with the reactions based on intercalation chemistry that are typically used in commercially available LIBs. These metal salts (MF_x/M_xO_y) can also exhibit significant additional reversible capacity beyond the theoretical estimate based on the maximum electron transfer from available redox metal centers.^{2 3 4 5 6}  However, the difficulty in characterizing structures at the nano-scale, particularly at buried interfaces, has meant that the mechanism accounting for the additional capacity remains unclear.  This study employs high-resolution multinuclear (¹H, ⁶Li/⁷Li, ¹⁷O, ¹⁹F) multidimensional solid-state NMR techniques, complemented by real-time detection using X-ray based methods in order to characterize two model systems, RuO₂ and TiF₃. The results illustrate the origin of additional capacities found in metal oxides/fluorides used as electrodes in LIBs. ⁷

Changes in the long-range structure and valence of Ruthenium in the RuO₂/Li battery system as a function of the state of charge have been followed by real-time high-resolution X-ray diffraction and absorption techniques. Variations in short-range atomic distances were probed with pair distribution function analysis and near edge X-ray absorption fine structure refinement. In-situ X-ray based techniques prove to be valuable in illustrating the reaction mechanism of the classic conversion reactions (xLi + RuO₂ = Li_xRuO₂, Li_xRuO₂ + (1-x) Li = LiRuO₂, and LiRuO₂ + 3Li = Ru + 2Li₂O), yet provide little information regarding the cause of additional capacities in the low voltage region (< 0.8 V).  A comprehensive solid-state NMR characterization protocol has been developed to identify and quantify all the chemical phases formed over the whole electrochemical process. In particular, NMR results have shown that in the RuO₂/Li system a major contribution to the extra capacity is due to the generation of LiOH and its subsequent reversible combination with extra Li to form Li₂O and LiH (Figure). First-principles calculations have been employed to examine various mechanisms for additional Li storage in the RuO₂/Li system.

To generalize the findings from the RuO₂/Li system, a similar study has been conducted on TiF₃/Li batteries. This study shows that the additional capacity seen in TiF₃/Li batteries is also mainly from the reversible formation of solid-electrolyte-interphase (SEI), likely catalyzed by Ti metal nanoparticles. In addition, the homogeneous distribution of transition metal nanoparticles within the LiF matrix facilitates reversible extraction of Li from LiF.      

This study illustrates the characterization of the source of additional capacities in metal oxide (RuO₂) and metal fluoride (TiF₃) conversion-type electrodes for LIBs and provides a comprehensive understanding of the chemical phases and the spatial distribution of these phases at the electrode and electrolyte interface. This study also demonstrates a solid-state NMR protocol with which to study the amorphous SEI that exists in essentially all battery systems. 

Figure. Schematic illustration of the source for additional capacities seen in RuO₂electrodes for LIBs.

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