¹⁹F and³¹P Solid-State NMR Characterization of a Pyridine Pentafluorophosphate-Derived Solid-Electrolyte Interphase

Abstract

Solid-state NMR spectroscopy was used to characterize the solid-electrolyte interphase (SEI) formed on the graphite electrode surface from the electrolyte additive pyridine pentafluorophosphate (PPF). The PPF-derived SEI was prepared in a full Li(Ni 0.4 Mn 0.4 Co 0.2 )O 2 /graphite cell, rather than in a coin cell or using a bench-top synthesis approach. 19 F and 19 F→ 31 P crosspolarization measurements provide direct evidence that F atoms and P-F bonds are present at the graphite surface. By comparing rinsed and unrinsed samples, an insoluble SEI component was differentiated from residual LiPF 6 from the dried electrolyte. The SEI species contains F and P chemical environments that are similar to, but distinct from, the additive starting material and the lithium hexaflurophosphate (LiPF 6 ) electrolyte salt used in this work. The results are consistent with the previously proposed formation of a dilithium 4,4'-bipyridine-N,N'-bis(pentafluorophosphate) salt. LiF was also observed and is attributed to decomposition of the PPF-derived surface layer and/or the LiPF 6 electrolyte salt. There is an increasing demand for longer-lasting and higher energy lithium-ion cells. Sacrificial electrolyte additives are a practical route to forming passive solid-electrolyte interphase (SEI) layers that limit electrolyte decomposition during cell storage and operation, which in turn can lead to improved cell performance and lifetime. [1][2][3] In recent years, significant steps have been made toward understanding the underlying chemistry of several electrolyte additives, including organic carbonate additives, 4-9 sulfur-containing additives, 6,10,11 and additives containing a Lewis acid-base adduct.6,12-14 Pyridine pentafluorophosphate (PPF, Figure 1a) belongs to this last category and has been shown to improve charge capacity retention after cycling at high temperature and high cell voltage.By combining several experimental techniques with density functional theory (DFT) calculations, a proposed pathway for SEI formation on the negative electrode from PPF has recently been proposed (Figure 1b). 13 It was suggested that during the formation cycle, dilithium 4,4'-bipyridine-N,N'-bis(pentafluorophosphate) salt, herein abbreviated as Li 2 (PPF) 2 , is produced and incorporated into the negative electrode SEI. However, the direct characterization of the PPF-derived SEI component(s) remains a significant challenge. This work explores the use of 19 F and 31 P solid-state nuclear magnetic resonance spectroscopy (ssNMR) as a complementary method to examine the chemical environment of the F and P atoms incorporated into the SEI.Whereas the sensitivity of ssNMR may be correctly anticipated to pose experimental challenges, the PPF additive is appealing because it is rich in both F and P atoms. These atoms are both desirable for the practical reasons of the high natural abundance of the NMR-active 19 F and 31 P isotopes (∼100%), their relatively high gyromagnetic ratios (γ 19F = 40.052 MHz T −1 , γ 31P = 17.235 MHz T −1 ), and because they ...

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“…The 19 F distortionless enhancement by polarization transfer (DEPT) SSNMR spectra carried out at two spinning speeds show a series of resonances centered around −165/170 ppm corresponding to the fluorine atoms of the −OC 6 F 5 moiety, with overlapping resonances with spinning sidebands (Figures S2 and S3). This is complementary to the 19 F DEPT SSNMR patterns previously reported for sMAO­(PFP) and also for a variety of 19 F systems. ,, The 27 Al direct polarization (DP)–MAS SSNMR spectrum shows a series of broad resonances between −245 and 350 ppm because of the quadrupolar aluminum nucleus ( I = 5/2) (Figure S4).…”

“…F systems.15,21,22 The27 Al direct polarization (DP)−MAS SSNMR spectrum shows a series of broad resonances between −245 and 350 ppm because of the quadrupolar aluminum nucleus (I = 5/2) (Figure S4). Polymerization Studies.…”

Development, retainment, and assessment of the graphite-electrolyte interphase in Li-ion batteries regarding the functionality of SEI-forming additives

“…Distinctive signatures, such as chemical shift differences between the dilute and the concentrated stages of graphite lithiation in 7 Li NMR spectra, detect phase changes used for a detailed study of the lithiation process during SEI formation ( Krachkovskiy et al., 2018 ; Lorie Lopez et al., 2017 ). Targeting particular elements, NMR spectroscopy could be implemented to characterize the formed SEI on graphite anode; however, a low signal-to-noise ratio of measurement may cause some limitations ( Hall et al., 2017 ; Wang et al., 2019 ). Vibrating spectroscopy including Fourier transform infrared spectroscopy (FT-IR) and Raman analysis could provide valuable surface information in order to investigate SEI composition ( Li et al., 2021 ; Nanda et al., 2019 ) but several shortcomings such as weak vibration signals in thin films, hard peak detection due to the inhomogeneous nature of the SEI, and appearance of similar functional groups from different components may limit the precision of measurement ( Verma et al., 2010 ).…”

Probing Solid-Electrolyte Interphase (SEI) Growth and Ion Permeability at Undriven Electrolyte–Metal Interfaces Using 7Li NMR

“…Recently, NMR spectroscopy and MRI techniques have been developed to study electrochemical processes in situ and derive ion mobilities, monitor electrode reactions, and observe microstructure formation. − The SEI has also been studied with solid-state NMR in destructive analyses by scraping off the layer from an electrode or grinding up the electrode and performing multinuclear magic-angle spinning experiments. − To the best of our knowledge, this technique, however, has not been used for metallic lithium electrodes. Static spectra of the SEI are relatively featureless, due to the inability to resolve broad spectral features .…”