title: Spin-transfer pathways in paramagnetic lithium transition-metal phosphates from combined broadband isotropic solid-state MAS NMR spectroscopy and DFT calculations.
abstract: Substituted lithium transition-metal (TM) phosphate LiFe(x)Mn(1-x)PO(4) materials with olivine-type structures are among the most promising next generation lithium ion battery cathodes. However, a complete atomic-level description of the structure of such phases is not yet available. Here, a combined experimental and theoretical approach to the detailed assignment of the (31)P NMR spectra of the LiFe(x)Mn(1-x)PO(4) (x = 0, 0.25, 0.5, 0.75, 1) pure and mixed TM phosphates is developed and applied. Key to the present work is the development of a new NMR experiment enabling the characterization of complex paramagnetic materials via the complete separation of the individual isotropic chemical shifts, along with solid-state hybrid DFT calculations providing the separate hyperfine contributions of all distinct Mn-O-P and Fe-O-P bond pathways. The NMR experiment, referred to as aMAT, makes use of short high-powered adiabatic pulses (SHAPs), which can achieve 100% inversion over a range of isotropic shifts on the order of 1 MHz and with anisotropies greater than 100 kHz. In addition to complete spectral assignments of the mixed phases, the present study provides a detailed insight into the differences in electronic structure driving the variations in hyperfine parameters across the range of materials. A simple model delimiting the effects of distortions due to Mn/Fe substitution is also proposed and applied. The combined approach has clear future applications to TM-bearing battery cathode phases in particular and for the understanding of complex paramagnetic phases in general.
authors.affiliation: Centre de RMN à Très Hauts Champs, UMR 5280 CNRS/Ecole Normale Supérieure de Lyon/UCB, Lyon 1, 69100 Villeurbanne, France.
authors.forename: Raphaële J
authors.initials: RJ
authors.lastname: Clément
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Pell
authors.forename: Derek S
authors.initials: DS
authors.lastname: Middlemiss
authors.forename: Fiona C
authors.initials: FC
authors.lastname: Strobridge
authors.forename: Joel K
authors.initials: JK
authors.lastname: Miller
authors.forename: M Stanley
authors.initials: MS
authors.lastname: Whittingham
authors.forename: Lyndon
authors.initials: L
authors.lastname: Emsley
authors.forename: Clare P
authors.initials: CP
authors.lastname: Grey
authors.forename: Guido
authors.initials: G
authors.lastname: Pintacuda
mesh_terms: D008094:Lithium / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D010710:Phosphates / Q000737:chemistry*; D010759:Phosphorus Isotopes; D011789:Quantum Theory; D028561:Transition Elements / Q000737:chemistry*
chemical_list: D010710:Phosphates; D010759:Phosphorus Isotopes; D028561:Transition Elements; D008094:Lithium