Correlation of Electrochemical Performance with Lithium Environments and Cation Dynamics in Li-2(Mn1-yFey)P2O7 using Li-6 Solid-State NMR

Li-6 solid-state nuclear magnetic resonance (ssNMR) is used here to evaluate a series of Li2Mn1-yFeyP2O7 cathode materials in an effort to quantify ion exchange rates and diffusion pathways. Magic angle spinning (MAS) NMR of the series of mixed metal pyrophosphates reveals a trade-off between electrochemical performance and well-resolved NMR spectra resulting from the change in electronic structure of the transition metal redox center. In addition, 1D Li-6 selective inversion NMR is employed to characterize Li ion dynamics in the fully Mn substituted member of the pyrophosphate series, where three of the four unique Li resonances are well resolved and labeled AB, C, and D, with AB corresponding to Li ions within one tunnel, and C and D Li ions residing in another. Despite limited inversion efficiency it is found that the utility of this experiment is not compromised so long as the initial magnetization conditions are well-defined. Initial fitting procedures involved the inclusion of all possible exchange pairs, a process which gave rise to consistently negative rate constants for C-AB or D-AB exchange, suggesting negligible exchange between these Li ions. Upon limiting the exchange model to ion exchange processes between the pairs of high and low frequency sites, rate constants of 45 +/- 25 and 100 +/- 30 Hz were obtained for C-D exchange at room temperature and 350 K respectively. Ion exchange pathways that are revealed by the exchange experiments imply limited mobility across distinct two-dimensional tunnels and slow exchange for within-tunnel ions. These exchange results provide corroboration for the geometrically determined site assignment in the 1-D spectrum, as well as support the notion of limited ion mobility in the Mn-phase resulting in poor electrochemical capability.