An Improved Understanding of Li⁺ Hopping Pathways and Rates in Li₃Fe₂(PO₄)₃ Using Selective Inversion ⁶Li NMR Spectroscopy

Abstract

6 Li selective inversion NMR experiments are used to reveal Li ion exchange rates and energy barriers for Li ion hopping in monoclinic Li 3 Fe 2 (PO 4 ) 3 . The three crystallographically unique Li sites in this material are well resolved by magic-angle spinning, thus allowing for the examination of all three exchange processes. We have revisited this material using selective inversion to probe dynamics, and energy barriers over the temperature range 268−397 K are found to be 0.37 ± 0.07, 0.53 ± 0.02, and 0.52 ± 0.03 eV for the three unique exchange pairs. The results presented here are consistent with the known Li 3 Fe 2 (PO 4 ) 3 crystal structure. The selective inversion experiment is more robust than 2D EXSY for the determination of energy barriers by NMR; this can be attributed to the efficiency of the onedimensional technique, and an exchange model that accounts for multisite exchange and fast spin−lattice relaxation. Moreover, bond valence sum density maps provide a meaningful depiction of lithium ion diffusion pathways in this material that complement the NMR results.

Temperature-driven structural evolution of carbon modified LiFePO4in air

“…Finally, the peaks at 3.4 V disappears for the 400 ˚C sample, and only the 2.7 V redox 10 remains, suggesting a full evolution of LiFePO 4 phase. 28 This CV results correlate the structural and spectroscopic analyses well. Figure 5b shows their galvanostatic discharge curves during the first cycle.…”

Understanding Surface and Interfacial Chemistry in Functional Nanomaterials via Solid‐State NMR

“…SSNMR offers a collection of methods that are capable of probing jump rates, activation energies, diffusion coefficient, and concentration distribution, etc. Thus, SSNMR techniques, including 7 Li NMR lineshape analysis, spin–spin‐ and spin–lattice‐relaxation NMR, 2D exchange NMR, and pulse‐field‐gradient (PFG) NMR, as well as NMR imaging, have been developed to study the ion dynamics at interfaces in batteries.…”

Correlation of Electrochemical Performance with Lithium Environments and Cation Dynamics in Li2(Mn1–yFey)P2O7 using 6Li Solid-State NMR

“…In contrast, the asymmetric electron configuration of Fe 2+ (t 2g The use of SI NMR as a method for characterizing ion dynamics in Li ion cathode materials has been extremely useful in determination of multisite exchange pathways. 23,24,37 Traditional 2D exchange spectroscopy has proven difficult when relaxation times are very fast, and a lack of sufficient resolution can further muddle 2D EXSY spectra. As such, SI was employed as a method for obtaining rate constants for exchange between the four crystallographically unique sites in the Mn pyrophosphate material.…”

“…This is in contrast to work with other phosphates that have enjoyed sufficient siteresolution to achieve near-perfect selectivity. 23,25 Fortunately, this can be accounted for in the relaxation matrix used to fit the experimental data by including the initial attenuated intensity of the affected peaks in the fit. In fact, upon testing various selective pulse lengths and shapes, the extent to which nearby sites were affected by the selective pulse had little effect on the resulting rate constant obtained from the fit.…”

“…One-dimensional SI has been advantageous for studying other Li materials exhibiting fast relaxation such as Li 2 VPO 4 F, 23 LiVOPO 4 , 31 and Li 3 Fe 2 (PO 4 ) 3 , 25 where the latter in particular demonstrated relatively fast ion dynamics relative to nuclear relaxation times. Li 2 MnP 2 O 7 suffers both from short spin lifetimes and slow Li ion exchange rates, making the separation of both phenomena particularly challenging.…”