Single Crystal Nuclear Magnetic Resonance Studies of LiMnxFe1-XPO4

LiMPO₄ are orthorhombic crystals, belonging to the olivine family and exhibit exceptionally high applicability for electrochemical energy storage in lithium-ion secondary batteries due to their charge storage capacity, chemical and thermal stability, and cost. Furthermore, mixed transition metal ion compounds Li(M,M’)PO₄ are particularly promising cathode materials because the redox voltage increases as from Fe to Mn, Ni, Co ¹and the mixed compounds can be tailored for specific applications.

The aim of these studies is to better understand the key role played by the transition metals such as Fe, Mn and particularly their mixtures in determining the properties of these materials. Single crystal ⁷Li and ³¹P NMR studies are used to obtain precise information on the magnetic environments of the lithium ions and phosphate groups which may ultimately be correlated with processes and structural features that determine the electrochemical behavior of these olivine phosphates.

By measurements of ⁷Li and ³¹P NMR frequencies, i.e. ν = B₀∙σ∙Ι, where B₀ is the applied field and Ι is the nuclear spin, with crystal orientation, it is possible to determine interaction tensor components {σ_ij} giving rise to the lithium and phosphorus shifts.  

The series of the single crystals of LiMPO₄ were prepared by a reciprocal salt flux growth technique, which allows the facile preparation of single crystals up to 1 cm across with well-developed facets at relatively low temperatures. Crystals for the NMR measurements present their natural faceting².

X-ray diffraction measurements were performed in order to identify the principal crystallographic axes [100], [010] and [001] of the samples. Subsequently the crystals were fixed to a rigid insert in the NMR probe, which was mounted onto a goniometer and inserted between the gap of a permanent magnet (B₀=2.1T). The use of a relatively low field strength allowed for acquisition of the entire rotation pattern within the spectrometer bandwidth. Rotations were made with the goniometer shaft perpendicular to applied magnetic field, in this way it was possible to generate 24 spectra, one every 15^o, for each crystallographic axis [100],[010] and [001].

Previous studies³ showed that in the case of ³¹P, crystals exhibit large (paramagnetic) shifts on the order of 3000ppm, primarily due to the hyperfine interactions between the unpaired electron spins of the paramagnetic ion and the phosphorus nuclei.

The ⁷Li resonances are also strongly affected by the paramagnetic susceptibility though not to the same extent as ³¹P. However in addition, there is a nuclear quadrupole interaction which splits the resonance and provides information concerning the electric field gradient

In addition to the endpoints (Fe and Mn analogues), we present an NMR analysis of the mixed transition metal ion compounds LiFe_xMn_1-xPO₄ , where x = 0.25, 0.5, and 0.75 in order to shed some light on the degree of cation disorder between the lithium, phosphorus, and transition metal sites that can result in substantial degradation of cathode performance particularly with regard to reduced rate capability.

This research was supported by the U.S. Department of Energy

References:

¹ K. Wang, A. Maljuk, C.G.F. Blum, T. Kolb, C. Jähne, C. Neef, H.J. Grafe, L. Giebeler, H. Wadepohl, H.P. Meyer, S. Wurmehl, R. Klingeler, Journal of Crystal Growth, Volume 386, 16(2014)

² Y. Janssen, D. Santhanagopalan, D. Quian, M. Chi, X. Wang, C. Hoffmann, Y.S. Meng, P.G. Khalifah, Chem. Mater., DOI: 10.1021/cm4027682 Publication Date (Web): October 10, 2013

³ P.E. Stallworth, R. Samueli, P. Sideris, D. Vaknin, S.G. Greenbaum, Advances in Inorganic Phosphate Materials: Ceramic Transactions, Volume 233 (eds I. Belharouak and V. G. Pol), John Wiley & Sons, Inc., Hoboken, NJ, USA (2012)