Density Functional Theory-Based Bond Pathway Decompositions of Hyperfine Shifts: Equipping Solid-State NMR to Characterize Atomic Environments in Paramagnetic Materials

Solid-state nuclear magnetic resonance (NMR) of paramagnetic samples has the potential to provide a detailed insight into the environments and processes occurring in a wide range of technologically-relevant phases, but the acquisition and interpretation of spectra is typically not straightforward. Structural complexity and/or the occurrence of charge or orbital ordering further compound such difficulties. In response to such challenges, the present article outlines how the total Fermi contact (FC) shifts of NMR observed centers (OCs) may be decomposed into sets of pairwise metal–OC bond pathway contributions via solid-state hybrid density functional theory calculations. A generally applicable “spin flipping” approach is outlined wherein bond pathway contributions are obtained by the reversal of spin moments at selected metal sites. The applications of such pathway contributions in interpreting the NMR spectra of structurally and electronically complex phases are demonstrated in a range of paramagnetic Li-ion battery positive electrodes comprising layered LiNiO2, LiNi0.125Co0.875O2, and LiCr0.125Co0.875O2 oxides; and olivine-type LiMPO4 and MPO4 (M = Mn, Fe, and Co) phosphates. The FC NMR shifts of all 6/7Li and 31P sites are decomposed, providing unambiguous NMR-based proof of the existence of local Ni3+-centered Jahn–Teller distortions in LiNiO2 and LiNi0.125Co0.875O2, and showing that the presence of M2+/M3+ solid solutions and/or M/M′ isovalent transition metal (TM) mixtures in the olivine-type electrodes should lead to broad and potentially interpretable NMR spectra. Clear evidence for the presence of a dynamic Jahn–Teller distortion is obtained for LiNi x Co1–x O2. The results emphasize the utility of solid-state NMR in application to TM-containing battery materials and to paramagnetic samples in general.