Sodium ion batteries have experienced a renaissance in recent years owing to the low cost and high abundance of sodium relative to lithium resources.¹ Despite this renewed attention, there remains a need to realize sodium electrode materials that are suitable for commercial applications.  In light of this, there is significant motivation to identify and analyze various properties of novel electrode materials, including structural and ion diffusion changes that occur during the electrochemical cycling process.

Solid-state nuclear magnetic resonance (ssNMR) is implemented here in an attempt to characterize structure and site-specific ion mobility in the fluorophosphate family of cathode materials (Na₂MPO₄F), as they are known to exhibit high thermal and electrochemical stability.^2,3 Ex situ ²³Na NMR studies at fast magic angle spinning (65 kHz) allow for sufficient spectral resolution to observe local changes at unique Na environments in these paramagnetic electrode materials.  

Na atoms housed in the two crystallographically unique positions in the layered Na₂FePO₄F phase are distinguishable via this technique (Figure 1a), offering the opportunity to probe local properties in the pristine phase. Further, by incorporation of this material into a sodium ion cell, the changes to the ²³Na NMR spectrum can be correlated ex situ to structural changes directly resulting from the electrochemical (de)intercalation process.  The appearance and relative increase of two new Na peaks in the NMR spectrum (Figure 1b) upon electrochemical desodiation of the material suggests that Na atoms in both crystallographic environments may in fact be mobile despite an apparent lack of chemical exchange between sites.  These new Na resonances can be assigned to the partially oxidized variant of the pristine material, where the dramatic shift in peaks in the NMR spectrum is attributed to the change in electron configuration of the transition metal from Fe²⁺ to Fe³⁺.  Ongoing efforts to quantify the diffusion of Na ions and characterize the mechanism of electrochemical desodiation will be presented.

References:

(1)      Palomares, V.; Serras, P.; Villaluenga, I.; Hueso, K. B.; Carretero-González, J.; Rojo, T. Energy Environ. Sci. 2012, 5(3), 5884.

(2)      Ellis, B. L.; Makahnouk, W. R. M.; Rowan-Weetaluktuk, W. N.; Ryan, D. H.; Nazar, L. F. Chem. Mater. 2010, 22(3), 1059.

(3)      Ellis, B. L.; Makahnouk, W. R. M.; Makimura, Y.; Toghill, K.; Nazar, L. F. Nat. Mater. 2007, 6 (10), 749.